Post by Bill Von Sennet on Aug 22, 2008 22:30:37 GMT -5
Tom Goodrick
Fs Piloting Skills
« on: Mar 11th, 2007, 7:00pm »
We will discuss piloting skills both from the point of view of general flight mechanics applicable to real aircraft to specific issues for FS aircraft and controls. We will advocate use only of FS aircraft to study and practice these methods.
For example, in the case of every aircraft from a J-3 Cub to a 747, the pilot uses a power setting and an airspeed to control the airplane in the vertical plane - that is, climbing, cruising and descending. Some people think it can't be that simple but, indeed, this is what real pilots do. Of course there are some practical differences between the Cub and the 747. The speeds and power for a Cub are simple while those for the 747 depend on the instantaneous weight and atmospheric conditions. When you set power and trim for a particular indicated airspeed, the plane will fly with a corresponding vertical speed. You don't have to look at vertical speed but in some cases you need to for reasons of comport. Also you may need to fly a particular glide path as when using an Instrument Landing System (ILS). You will generally fix airspeed and adjust power to get either the descent rate or the path in line with a requirement. The pitch trim adjusts the speed and the throttle adjusts the vertical speed.
You might ask why I did not mention the joystick or yoke. That is because you only need to use this control to make a turn, to rotate for takeoff or to make a flare for landing.
« Last Edit: Mar 11th, 2007, 7:02pm by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #1 on: Mar 11th, 2007, 9:30pm »
Why is indicated Airspeed so important to the proper control of an aircraft?
This can be explained mathematically with some difficulty and complexity. I have seen and have used the math. But it is a bit messy so most people don't want to see it. For those who do want to see the math in as straight-forward a fashion as possible, find the book "Introduction to Aircraft Performance, Selection and Design" by Francis J Hale of North Carolina State University. I had other books in college that were much more obtuse. This one I picked up at Auburn University while waiting for a football game to start. (My youngest son was a student there.)
The reason Indicated airspeed is vital to the control of an aircraft is that only indicated airspeed relates directly to the pressure causing the forces and moments acting on an aircraft. It is also directly related to the angle of attack from which the lift and drag can be separately determined. Thus, if you fly at certain indicated airspeeds, you will be assured of safe and efficient operation. let's go just a little into the fundamental physics on which this is based. Most normal flight is conducted very near steady state and very close to straight and level. In this condition, lift equals weight. Based on the equation used to calculate lift, this gives an equation in which airspeed is directly related to the lift coefficient which is, in turn, directly proportional to the angle of attack. Now within this equation is a factor called the dynamic pressure which is related to the pressure that can possibly act on a surface because of air moving near the surface. The instrument that measures airspeed actually senses this dynamic pressure. Now dynamic pressure can relate to true airspeed using the local air density or to indicated airspeed using the standard men sea level density. This is because in the airspeed instrument, the standard mean sea level density is used in the calibration. Thus a measurement of density is not required for the airspeed indicator to work. With this relation, the indicated airspeed is directly related to the aerodynamic forces regardless of altitude. We can use the indicated airspeed to tell when a plane is about to stall at any altitude.
Thus, in summary, we use indicated airspeed as an indicator of both dynamic pressure and angle of attack. we don't need to know those other parameters as long as we operate within limits of indicated airspeed specied for the airsraft. The low limit would be the stall speed and the high limit would be VMO or max operating airspeed at which a simple upset could tear off the wings. The bad things that can happen - stall and wings falling off - are also related to the gross weight of the aircraft. These speeds are also tied to the weight. But we can use the speeds given at max gross weight which are higher than the speeds that would apply to lesser weights and be safe because the speeds for lesser weights would be lower. Thus if you test for stall in a plane where clean stall occurs at 65 KIAS, you may find it stalls at only 60 knots. But if you stay above 65 KIAS all the time to avoid stall, you won't stall at the lighter weight or the heavy weight.
Don't think that True Airspeed is never used. It is always used as the spec for cruising speed. When somone says "The cruise speed of the X-97 is 400 knots, they mean the cruise speed is 400 KTAS. That is the speed at which it moves through the air at cruise. You must subtract the wind vector to get the ground speed. (Or, you can peek at the Garmin.) True airspeed will always be faster than Indicated airspeed. salesmen love to use True airspeed. It does have an important technical aspect in that it is used to determine the Mach number of jets. The Mach number is a ratio of True airspeed to the local speed of sound - the speed of sound at the same position and altitude. Stability, controllability and performance of jets depends on Mach number once you are above 30,000 ft.
Indicated Airspeed for takeoff is chosen as 1.3 times the clean stall speed at gross weight. For jets where a very large percent of the gross weight is in fuel which can change, the safe takeoff speed V2 is calculated based on the actual takeoff weight. it varies with the square root of the weight ratio. Commercial pilots are required to make this calculation for every takeoff. I have saved the FS jet pilot the trouble by including a calculation made by the computer and displayed on the panel next to the airspeed indicator when the aircraft is on the ground.
Indicated Airspeed for landing is also based on 1.3 time stall speed but in this case the configuration used is flaps down and the weight is maximum landing weight (normally much less than the max takeoff weight meaning a jet must fly and burn fuel before landing if it took off near max gross weight). This airspeed for landing is called Vref and is flown during final approach with full flaps. Again, I made a gauge that calculates and displays this for jets based on actual weight. (A fancy computer with weight sensors used on the ground and fuel-on-board calculations makes this Vref calculation. The result is displayed next to the airspeed indicator when the landing gear is lowered.
Unfortunately, Microsoft has thoroughly screwed up the matter of Indicated and True airspeed. They specify that true airspeed is to be used for stall conditions. They also say that using indicated airspeed rather than true airspeed diminishes the realism. They have some ignorant employees. At least they were kind enough to let us use indicated airspeed. Make sure you have that set under "Realism."
« Last Edit: Mar 11th, 2007, 9:32pm by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #2 on: Mar 12th, 2007, 9:27pm »
Use of controls and control sensitivity are areas in which FS differs from the real world. The physical connection to the aircraft for the pilot is rather different in the sim. You must understand this and be prepared to make adjustments that will suit you with your equipment. There are also control procedures that real pilots use that are not what some simmers expect.
Of course the first thing you must add is a control input device. In theory you can use the keyboard for everything. But this does not work well. You cannot input more than one control at a time using the keyboard. Flying requires that you do this in several instances - pitch inputs at the same time roll and yaw inputs are made. A good control interface can also reduce the unnatural aspects of sim flying. You need to be able to look outside and back at the panel. You must always be able to read the panel instruments accurately though you must also be able to look out and spot the runway as you line up for an approach.
The controller must have axes for pitch and roll. I like a stick because I first learned to fly an airplane with a stick. Now on FS i fly many aircraft that in reality have yokes. But many aircraft do have sticks which are making a comeback to some extent. A stick is particularly well-suited if you want to do aerobatics. The stick will still work well for normal flying. A yoke is not too good for aerobatics. The best answer is to get one of each and use them as appropriate. (This may be a hard sell to the wife.
Rudder pedals are important. As a student pilot you would be taught very early to use the rudder properly in making turns. The pedals are also used to steer on the ground. Most prop aircraft have a storng left yaw tendency when you add full power for takeoff. this is normally counteredt with rudder. If you have to use the stick or yoke for this it makes things difficult. It is unnatural. Similarly, many aircraft should be slipped a little on final approach, expecially in a crosswind. This requires that the roll be set opposite to the yaw - not possible without rudder pedals.
You will need to have a good and quickly effective throttle control. I am happy sing the keyboard for this though I have a control on the joystick I could use. My reason is I can make very precise control inputs with the keys which I need to set the power exactly. i have learned to do this quickly. Most throttle controls built into joysticks or yokes seem to me to be very cheap. people have reported problems flying airplanes I fixed up. Then we find it is a noisy throttle control. Ther are good separate throttle controls and even quadrants with throttle, prop and mixture levers for one or two engines. These look great but they are expensive. Still, I might pop for one of them one day - before I pop for a sim upgrade!
A good controller should also have extra buttons and switches that you can define as needed. I like a button for nose-up and for nose-down trim. It is very realistic. I use mine all the time when flying. I also have a hat switch I use to quickly look left or right. To look half left or half right, I use the number keys on the right numpad. I also use keys for landing gear, spoilers and pairs of F keys for prop, mixture, flaps and brakes. I have changed many of the key assignments so these lie in order across the top of my keyboard. I made a simple paper label that I tape across the top of the keyboard F1 through f4 are the throttle keys as set by defult.
These controlelrs must first be calibrated using the Windows calibration scheme. You may also have a disk or Cd furnished with the controller that should be run before calibration is attempted. It will set drivers and some special Windows calibration software.
Once the initiial calibration is completed, start FS and go flying, using an aircraft with which you are very familiar. This is no time to try a fancy new aircraft. Turn off the wind using Clear Weather. While doing this indoctrination flight, check the control sensitivities and null widths set within FS.
The "joystick" control sensitivity and null width can only be set within FS itself. Go to Options, Select Controls and then Sensitivities. Click on Advanced. Bear in mind what works for me may not be to your liking. But you will probably want something other than the default settings.
When making these changes, test with a very familiar aircraft that you have flown a lot. These adjustments are meant to take out peculiarities in your controller that are not related to any specific aircraft.
Note first that your control device should be shown in the window. Mine says "CH Flightstick Pro". next there are sliders for sensitivity and for null width on each of the main control axes - ailerons, elevator and rudder. There could be others such as throttle if you have that set in the calibration of your device. (This is to be done after you have calibrated the device.)
My settings for sensitivity are for ailerons 50%, for elevator 25%, for rudder 33%. The settings for all null widths are 25%.
The purpose of the null width is to handle any chatter you may have in the controller - digital or analog noise. I have had several sticks go bad sending noise into the controls.
There is one more sensitivity that gives a lot of people trouble, particularly for those transitioning to rudder pedals for the first time. This is the nose wheel turn angle. It is set in the aircraft.cfg file for each aircraft in the section labeled "[contact_points]'. (Note that any .cfg file is simply a text file you can read and edit using Note Pad. tech your computer to treat these files as .txt files.) Not all files have a key telling you the components of a line in the contact_points section. Check a default aircraft like a Boeing to find the key. You can copy the key into any other aircraft .cfg file since it is just a series of remarks. On an airplane that is difficult to keep straight during takeoff, reduce the wheel angle by editing the line in the .cfg file.
I seldom set the turn angle for the nose wheel less than 20 degrees. But, often I'll find values of 60 to 90 degrees in downloaded aircraft. I always reduce them to 30 degrees or less. But you can get used to anything after a while. Often, by the time I have tested an aircraft enough to get other bugs worked out, I have learned to live with a large angle. You don't need a large angle here, even if there is one in the real aircraft.
Also, if you have an aircraft you really like and want to fly it a lot but are having some problems controlling it, it pays to look in the aircraft.cfg file for the "[flight_tuning]" section and play with the aileron, elevator and rudder sensitivity scalars. (A scalar is a number of about 1 that is a factor to the actual control parameter. 1.0 means no change, 0.9 means a 10 percent reduction and 1.1 means a 10 percent increase.)
I find that elevator sensitivity is usually too high. Sometimes aileron is too high as well. This will improve the handling of only that particular aircraft. I find often that FD designers have over -cooked the control sensitivities, notably the elevator sensitivity. You just want to be able to raise the nose adequately on takeoff when the proper amount of pitch trim has already been set for takeoff. Don't leave so much sensitivity for elevator that you can rotate without regard to the trim setting.
Tom Goodrick
Re: Fs Piloting Skills
« Reply #3 on: Mar 12th, 2007, 10:15pm »
Pitch trim setting is a major aspect of pilot control technique that many non-pilots have no knowledge about. Pilots themselves almost forget that they use it all the time. it becomes second nature, like using rudder in a turn. But it is vital to proper control of the aircraft.
The forward or backward motion of the stick or yoke only has a limited range of control. The pitch trim must be used to augment this control. For takeoff there is usually a setting marked in real aircraft. I usually put a remark in the checklist as to what pitch trim setting to use for takeoff. Sometimes it varies with different weight or CG position. The value is in degrees. I give you a pitch trim value on the panel shown in degrees as well as weight and CG position if that is needed.
Set the trim for takeoff. The pitch control should be set for spring centering at neutral. A control that outputs forces is not necessary and not desireable in my opinion. Just spring centering is needed. After takeoff, with a slight pitch input needed to rotate, set the speed for climb using the stick or yoke. Then dial in enough pitch trim to take out the stick force so it will fly without your hand on the stick. This will work during most of the climb. I usually turn on the wutopilot at this point having set the cruise altitude and either a flight heading or with a flight plan entered I'll turn on NAV mode (set the NAV/GPS switch to GPS) and let the autopilot control pitch trim to maintain a particular rate of climb and heading or as needed to follow the path on the flight plan. I just adjust the power between climb and cruise.
If you are flying manualy, you have a lot of work to do and you will probably never do it perfectly. You must level off at cruise altitude, reduce power slowly to cruise power as the speed increases. Here you don't set a speed, you set only the power and the speed happens, depending on your weight, whether flying manually or on autopilot. But on autopilot the steady cruise condition is reached very quickly because the autopilot responds quickly and with just the right amount of conrol change to get to equilibrium flight with all forces balanced. You will probably chase the phugoid for quite a while.
The phugoid is the undulating motion that happens when the lift does not balance the weight. Though this is a pitch motion, the angle of attack remains constant during phugoid motion. The lower lift causes a slight descent which adds speed starting a levelling off with the lift greater than the weight. It then starts a slight climb. Then it will slow down with the lift less than the weight The damping of this motion depends on the drag if the controls are not changed. Clean airplanes with little drag will oscillate for a long time. The solution is quick and short pitch trim inputs when the speed is at a medium value. The autopilot does this very well.
Whenever I want to test a plane in cruise to lsee if its speed is corect, I use the autopilot. I don't waste time trying to stop the phugoid. On a draggy plane like a Cub or a Champ, the phugoid is well-damped so you can get it to fly steadily without much trouble.
When I was flying real aircraft in the real world (which is fairly bumpy), I don't think I every had the plane in steady cruise for very long. I always had a hand on the stick or yoke and was "diddling" with it continuously. I had no autopilot. I also never had the aircraft precisely "on" its specified cruise speed. It was probably close but off by a couple of knots. So don't feel bad if you can't hand fly an airplane steadily in FS. Use the autopilot like a pro would.
Tom Goodrick
Re: Fs Piloting Skills
« Reply #4 on: Mar 15th, 2007, 3:38pm »
NOTES ON USING THE MIXTURE CONTROL IN FS9
If you fly a plane in FS9 that has a normally-aspirated engine, you must learn to use the mixture control in order to climb and fly above 4,000 ft. But if you fly a tubocharged aircraft, you should do nothing except leave the mixture on full rich (fully in against the panel). If you don't know which way a plane is set up, check the aircraft.cfg file.
If the aircraft is normally aspirated, like this C172SP, you'll find this line in the [piston_engine] section:
turbocharged= 0 //Is it turbocharged? 0=FALSE, 1=TRUE
If it is turbocharged as this Mooney Bravo is, you'll find this line:
turbocharged= 1
and then you should look for this line:
fuel_air_auto_mixture= 1 //Automixture available? 0=FALSE, 1=TRUE
This has not always been the case so, if you see turbochaged=1 and fuel_air_auto_mixture=0, you should change it to =1.
The reason is that real pilots of turbocharged aircraft are taught to push the Mixture to full rich to begin and to maintain a climb at any altitude. Thus the operation will be realistic. Unfortunately, in real life, the pilot can adjust the mixture once he gets to cruise altitude where we cannot. But both the piloting technique and the performance is more realistic for tubocharged aircraft if auto mixture is used.
There is a slight departure from realism when using the mixture control with non-turbo aircraft. Real pilots (and some notes in FS documentation) are told to adjust the mixture to get a max on the EGT and then go beyond the max on the lean side. But in FS this does not work well. The thing that does work well is to adjust the mixture to see a maximum fuel flow. That coresponds to the maximum power you can get from the engine at that condition. That means you'll get good performance and will be treating the engine well for long life.
There is a big myth in aviation that pilots should lean for economy. They are probably even told to do that by the companies that own the airplane. But to lean for best economy, you would lean beyond max power to a higher temperature (EGT). This can easily cause engine wear and it does not save as much fuel as simply setting a lower throttle position would give you. There have been many cases reported where post-crash analysis showed engines were burned out when they failed by the leaning practices of the pilots.
As you climb in a normal aircraft, leaning to max fuel flow will keep you climbing as well as you can. Failure to do anything will leave you down low and slow.
Note that you cannot tell if an airplane is turbocharged by any means other than checking the aircraft.cfg file. One clue is that turbocharged engines usually have a TIT gauge for Tubine Inlet Temperature. This can cause problems if the mixture is not adjusted properly. Mooney made the Bravo and the 231 with turbocharging but the 201 and MSE did not have turbocharging. I have four versions of the Skylane, two with turbocharging and two without.
« Last Edit: Mar 15th, 2007, 3:39pm by Tom Goodrick »
Ed_Burke
Re: Fs Piloting Skills
« Reply #5 on: Mar 15th, 2007, 6:35pm »
" Real pilots (and some notes in FS documentation) are told to adjust the mixture to get a max on the EGT and then go beyond the max on the lean side."
I knew a chief engineer of an air charter business who threatened to nail any of his pilots to the wall by their dangly bits if they used that leaning method, despite it apparently getting the ok in some ops manuals. He reckoned the risk of detonation was too high and the extra fuel was engine insurance.
Peak the EGT and then drop it on the rich side was his instruction.
Ed
« Last Edit: Mar 15th, 2007, 6:38pm by Ed_Burke »
ED B
flaminghotsauce
Re: Fs Piloting Skills
« Reply #6 on: Mar 15th, 2007, 8:07pm »
I was taught that as well. Tom's method is simpler to implement, and works well for power, so I do it that way now.
Our fleet of 172's were not equally equipped, and I only recall a couple having an EGT. We were taught to adjust mix to best RPM, or peak EGT leaning to rich side.
One instructor told me to never lean during a climb, leaving a richer mixture to cool the engine until reaching cruise altitude.
"Barring all differences, they're identical!"
Tom Goodrick
Re: Fs Piloting Skills
« Reply #7 on: Mar 15th, 2007, 10:34pm »
If you try climbing on a rich mixture in FS9, you'll reach a level near 6,000 ft where the engine will not have sufficient power to climb. I don't know if this is realistic.
6000 ft is about the altitude at which a normal engine can generate no more than 75% power even at full throttle when leaned correctly.
« Last Edit: Mar 15th, 2007, 10:36pm by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #8 on: Mar 30th, 2007, 9:30pm »
Some thoughts on GLIDING that every pilot of a single-engine aircraft should know about:
Some people talk about "Glide Angle" and others talk about Glide Ratio." I have done a lot of work in this area and understand the situation. There are two problems: 1) which is better to talk about Glide angle or Glide ratio" and 2) How can you best measure glide performance?
First, it is best to use glide ratio. The two are related. The angle is the arctangent of the glide ratio. Or the tangent of the angle is the glide ratio. The reason the ratio is best to use is for practical purposes and for its strong basis in theory which leads to a simple way of testing - much simpler than trying to glide to a VOR. First we must recognize and work with the differences between indicated airspeed and true airspeed. We use indicated airspeed to set up the test. My 172 POH says the best glide comes at 80 mph. That is 69.6 knots indicated airspeed. We should use that speed to set up the test. Why? that is based on the theory of flight. For each glide speed, at a given weight (max gross is assummed here), there is a particular pair of lift and drag coefficients. There is also a particular angle of attack that pprovides these values of the coefficients but we need not bring that into the discussion. The ratio of CL/CD is the same as the ratio of the true velocity components VH/VV and is the same as the glide ratio X/H. this statement is founded in the technical definitions of lift and drag, and in the basic kinematics that speed components define the path if the ratio remains constant. Notice I did not say the speeds remain constant (although the indicated airspeed can and should remain constant).
What is practical about the glide ratio? The glide ratio is the ratio of the distance travelled, X, over the height dropped, H or X/H. If the glide ratio is 8.2, then X = 8.2H or 8.2 times the altitude. From 8,000 ft, you can go 65,600 ft which is 12.4 statute miles. There was a time I relied on this. I was flying back from Provincetown, Massachusetts across Cape Cod Bay to the area near Plymouth and wanted very much to keep my feet dry. I circled over Cape Cod and climbed to 8500 ft before setting off across all that water. That meant if the engine quit before I reached 12 miles from the shore of Cape Cod, I could turn back and glide back. When within 12 miles of Plymouth, I could relax because I could glide to the mainland.
To measure glide ratio, you don't have to sit in the plane and fly a constant KIAS for 20 minutes. Just start gliding at any altitude and measure your airspeed components accurately. I developed a "Turn Test Gauge" that is good to have. You can get it from my web site. It will give the true airspeed in decimal knots. Use my landing speed to get the vertical speed. Start a glide at 70 KIAS and smoothly adjust the trim to hold steady speed components. When the components are steady, regardless of altitude, read the KTAS, VA and the vertical rate VV in feet per minute. Convert each to feet per second:
1) multiply KTAS by 1.689 and
2) divide FPM by 60.
Then calculate the horizontal airspeed component VH: VH = SQRT(VA*VA - VV*VV) using the FPS values of each.
Then get the glide ratio: = VH/VV.
What is happening during the glide:
1. If you keep the indicated airspeed constant, then the angle of attack is constant and the values of CL and CD remain constant.
2. As you descend, the vertical speed decreases in magnitude as density increases. The true airspeed also decreases by a proportional amount so that this ratio remains constant at all altitudes from start to finish of the glide. The indicated airspeed is independent of density and altitude so it can remain constant once set up with proper trim that does not change.
In theory this ratio does not change with changes in weight. But that theory assumes the CG position is constant. Since partial loads can also change the CG and, thus, the trim, it would be good to test a couple of airspeeds and a couple of loadings, one CG forward and one CG aft.
I have not done this test myself but it is a good thing to do. It should only take a few minutes. It would be interesting to do it for several single-engine aircraft because this is all we have when the engine goes quiet.
The flaw in all this neat theory is the wind. The real glide ratio is (VH-W)/VV where W is the headwind component in feet per second. This will be worst at altitude. It is the reason pilots should pick the airport most directly underneath their position and begin maneuvers to land there when the engine goes quiet.
Now I will copy this so it won't vanish when the server gets too busy.
Tom Goodrick
Re: Fs Piloting Skills
« Reply #9 on: Mar 31st, 2007, 10:34am »
There is a simple way to prove with geometry that L/D = Vh/Vv = X/H. Start by putting a dot on a piece of paper that represents the mass of the airplane. Then make a downward arrow of a certain length and an upward arrow of the same length. The downward arrow is the weight vector acting on the aircraft in a glide.Call it W. The upward arrow is the resultant, R, of all the aerodynamic forces acting on the aircraft. There are no other forces - no thrust because the engine is off. (The prop may produce drag but we lump that with the drag from the rest of the plane.)
Now draw a long straight line slanted downward. This is the glide path. Don't lable it now. It makes an angle gamma (call it what you wish) with the horizon. This is the glide angle. Put an arrow head on that line about an inch from the dot and lable it Va. That is the airspeed vector.
Next make a line going up perpendicular to the path. Make it just long enough so an arrow going back at a right angle will connect to the R vector. The upward arrow is the Lift vector, L. Make a back arrow from the mass point back in an extension of the airspeed vector and in opposition to it but only long enough to make a connection from the lift vector to the R vector in a parallelogram.
Next put in horizontal and vertical airspeed vectors from the poiint going forward to connect to the end of the airspeed vector. These are Vh and Vv. Next pick any point along the line and draw a vertical line down to a point and then draw a horizontal line from that point to intercect the glide path. This is the ground. The vertical line is the altitude H and the horizontal line is the distance X over which we will glide.
Now we have three over-lapping similar triangles: RDL, VhVvVa and pathHX. All have a right angle in one corner and the flight path angle gamma in another corner. They are similar but of different sizes. That means the leg ratios are constant.
Hence L/D = Vh/Vv = X/H. For the typical aircraft this will be about 8. For a smooth, clean sailplane it can be as high as 50. For many airliners it is about 10.
This ratio L/D is actually the ratio of the lift coefficient to the drag coefficient. It is independent of density. It plays an important part in many aspects of flight. In cruising flight it is the ratio of weight to thrust. In a climb, it influences the rate of climb for a given amount of thrust. As long as the aircraft is stable, it will naturally sit at its trim angle of attack and will generate the same pair of values of CL and CD so that ratio remains constant. The indicated airspeed will also be constant. We can use that feature to change the glide ratio if we want to. While the forces appear to be balanced, the motion is quasy-steady-state. There is a gradual deceleration as the aircraft moves into the more dense air at the low end of the glide. You control the glide ratio by picking the indicated airspeed for the glide. You want to pick an airspeed sufficiently above stall so that you can make a flair before touching the ground.
70 KIAS is the best value for a C172.
Tom Goodrick
Re: Fs Piloting Skills
« Reply #10 on: Mar 31st, 2007, 2:02pm »
It's always a little scary to climb on a soap box and say "This is the theory..." because some guy in the back row will yell out "Can you prove it?" I did some experimenting and the answer is "Yes, I can." But, of course, there is some variation in the experimental data.
I popped a C172 SP up to 10,000 feet from its position on the parking ramp and let it glide. The engine was off. It went into a stall and by the time I got it nice and steady, the altitude was 6500 ft. This was loaded at gross of 2549 lbs and the CG was at 22.03%. I did not record the trim but I got it steady at 70.52 KIAS where I saw 77.6 KTAS and -762 FPM. This gives 10.27 for L/D. I let it glide without touching it and it became quite steady. At 5500 ft it showed 70.46 KIAS, 76.4 KTAS and -682 FPM for an L/D of 11.31. At 4500 ft it showed L/D of 11.14.
I trimmed for a faster glide by setting the trim at 11.2 degrees. I got this data:
7500 ft, 75.18 KIAS, 84.0 KTAS, -832 FPM, L/D=10.18
6500 ft, 75.18 KIAS, 82.7 KTAS, -887 FPM, L/D=9.40
5500 ft, 75.08 KIAS, 81.4 KTAS, -867 FPM, L/D=9.46
4500 ft, 75.07KIAS, 80.2 KTAS, -855 FPM, L/D=9.45
I reset the load to 13.03% and 2029 lbs. At the same trim of 11.2 degrees, I saw
8500 ft, 79.92 KIAS, 90.6 KTAS, -988 FPM, L/D=9.24
Then I trimmed at 16.4 degrees and got a slower glide.
8500 ft, 70.75 KIAS, 80.3 KTAS, -887 FPM, L/D=9.12
7500 ft, 70.76 KIAS, 79.1 KTAS, -860 FPM, L/D=9.27
6500 ft, 70.75 KIAS, 77.9 KTAS, -849 FPM, L/D= 9.24
5500 ft, 70.76 KIAS, 76.7 KTAS, -836 FPM, L/D=9.24
4500 ft, 70.76 KIAS, 75.6 KTAS, -825 FPM, L/D=9.23
I checked a Piper Archer II and found similar results. At gross weight, 70.9 KIAS gave L/D=9.97. At 75.1 KIAS, I saw L/D=9.99. I have the data on a spread sheet. I recorded five sets of data at five altitudes. While the C172 glided fairly straight ahead, the Piper Archer turned steadily at about 0.5 degrees per second. A touch of rudder would have kept it straight.
Here is a quick summary of several other tests I made. I'll just give a representative glide ratio (L/D) for each aircraft. But, I did take data at four or more different altitudes in each case. They are logged on my Excel sheet.
Piper Cherokee 180 L/D=9.8
Cessna 182 L/D= 8.8
*Cessna 182 RG L/D= 12.16
*Mooney Bravo L/D= 9.68
*Velocity XL-5 L/D= 11.76
*Bonanza V35B
*Piper Malibu Mirage L/D= 12.25
* retractable gear with gear up. All are at max gross weight.
The aerodynamically clean retractable aircraft have a high L/D but are tricky to fly for this experiment because they have a long and low-damped phugoid. They oscillate in pitch all the way down rather than reaching a steady condition.
The Mirage cannot use its glide ratio if the engine stops when cruising above 15,000 ft because their first job is to get down below 12,000 ft as fast as possible without exceeding the max safe airspeed. The pressurization is lost when the engine quits.
Also, when I was testing the 182 RG, I got a taste of the real problem with some of these aircraft. I had finished a testat 4500 ft and was going to do a test at 3500 ft when the battery gave up. I lost all instruments except the airspeed and altitude so I set up for a landing on a handy road. But I could not extend the gear so I changed my target to a nice flat field. On final I found I could not lower the flaps either. The landing was good but a little fast - 70 KIAS.
« Last Edit: Apr 1st, 2007, 11:12pm by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #11 on: Apr 7th, 2007, 12:15am »
In the section on sailplanes in the FS2004 thread, I mention that I made a Glide Ratio gauge and posted it on my web site. This gauge is handy on the panel of a glider. But it does not belong on the panel of a powered aircraft. Instead, it can be placed in the panel.cfg as a pull-down gauge to use when you want to study the glide chanracteristics of an powered aircraft. To put it there just insert the floowing lines in the panel.cfg:
[Window02]
size_mm=64,48
window_size_ratio=1.0
position=2
visible=0
ident=Glide
gauge00=Digital!Glide, 0,0
The window number you assign depends on the number of other windows you have. I'd place it in a high-numbered window becaues it will seledom be used. Name the window in the top few lines of the file. Also, the name of the file can be simply "Glide" if you put it in your main Gauges Folder when you download it from my site. here I call it Digital!Glide because I have it in a sub folder of Gauges called Digital.
I used it on the C182 to get the following values:
Trim KIAS L/D
4.1 100 8.5
6.7 90 8.6
10.6 80 8.8
(The trim values are in degrees measured by my pitch trim gauge.)
Tom Goodrick
Re: Fs Piloting Skills
« Reply #12 on: Apr 13th, 2007, 3:51pm »
FUNDAMENTALS OF MEASURING AIRSPEED AND VERTICAL SPEED
To make my Glide gauge work, I simply read the vairables named "True Airspeed" and "Vertical Speed" that are made available by the Mirocoft people who designed the FS9 program. But you might ask whther there is a way it can be done on real aircraft. The answer is "Yes". For sailplanes it is done all the time.
It dawned on me that many FS fliers may not be familiar with the fundamentals of how aispeed and vertical rate are measured on an airplane. The whole basis for the validity of the Glide gauge described above is in the fact that these measurements are based on different aspects of the physics of motion through the atmosphere. Finding the true "Up" direction in an aircraft is very difficult. Nothing really does it well in a static sense. If you see a bubble in a tube, you must realize it is not showing "Up". You can dangle a nice plumb bob and not find "Down." The reason these do not work is that the aircraft is generally accelerated in all three dimensions. This means the plumb bob and the bubble will respond to a combination of gravity and acceleration that results from the motion of the aircraft. That does not detract from the use of the turn and bank indicator, in fact, it reinforces the utility of that gauge. When flying a steady, banked turn, the bubble indicates where local "down" is during the turn but that local "Down" is not True "Down" as you can easily see. It points to the floor when the pilot is flying the turn correctly. Sitting in a seat, you will feel the pull directly "Down" into the seat and yet the airplane may be banked at 45 degrees to the vertical!
So how do we measure "vertical speed?"
We measure it using a device that shows the difference between two air pressures the current pressure and the pressure that existed about 1 second before. There is a very standard and fixed gradient of air pressure in the atmosphere from the surface up to a very high altitude. This gradient, or change per unit altitude, is used to calculate the vertical rate as an aircraft climbs or descends. It is handy because it has nothing to do with the attitude of the aircraft. (It uses the pressure pushing against a port on the side of the aircraft. This can be inaccurate momentarily if you slip the aircraft significantly but is not affected by pitch attitude at zero sideslip.) You might think this method of comparing old and new pressures. Indeed there is a line in the aircraft.cfg file that enables you to set a delay in the vertical speed reading. I think it is a mistake to use that with any significant delay. In fact the reading is very responsive. I have held these devices in my hand while they were connected to a PC and have seen the change in rate as I raised and lowered my hand. I saw no indication that there was a delay. The changes at the end of thet large tube occur imediately in comparison to the pressure at the end of the small lag tube. The rate of propagation of pressure in a tube slows down with smaller diameter. This is done in tubing that is built inside the case of the vertical speed device.
How do we measure True Airspeed?
Airspeed is measured as described above in this section but it warrents more discussion in this context. To compare with the vertical rate which is a true speed, we need a true airspeed rather than an indicated airspeed. This can be obtained two ways. In powered aircraft, the standard way is to get the indicated airspeed by measuring the difference between the pressure at the front end of a tube pointing into the relative wind and the pressure at a hole in the side of the fuselage. Bernoulli's Law says the difference between the total pressure on a streamline that stops in the end of a tube, is higher than the "static" pressure in streamline that passes along the side of the tube. The difference is called the dynamic pressure and varies with the density and the square of the speed. Indicated Airspeed is calculated and displayed based on the use of sea level density regardless of altitude. It works fine for pilots to use to keep the plane flying at a safe speed. But to get true airspeed, there must be a connection to the altimeter and a small calculation (digital or analog) that determines the true airspeed. There is another way to measure true airspeed directly. I was able to use this alternative method in my parachute testing and sailplane pilots make use of this method routinely. I utilizes the physical principle that the frequency of vortices shed from an object protruding into the flow depends directly on the true speed of the flow. All you need is an audio circuit that detects the frequency down stream of a protrusion. The device I used ws housed in a low-aspect ratio airfoil fin that stuck out from the fuselage. (We built a sloping nose fairing on the front of our instrumentation box and the checked the calibration in a wind tunnel. The fin was sticking out of the fairing on the nose. We had to check for angle of attack effects. We were able to minimize those so that we measured true total airspeed of the box hanging below the parachute.) This fin had a rectangular channel from fron to back with a diamond section cross bar. An audio signal of know and precisely controlled frequency was injected up stream of the cross piece. A small microphone picked up the sound downstream and showed us the change in frequency caused by the flow over the cross piece. It worked very well. Its readings were linear with airspeed. That device was intended for use on helicopters and V/STOL aircraft because it could measure very low airspeeds which cannot be measured by the pitot static system. Unfortunately it did not work on those aircraft because of the extremely high noise environment.
Thirty years ago I did experiments on my own that showed you could do this with a cheap microphone, tape recorder and voltmeter. With my wife driving, I would hold the mike out the window of the car and watch the voltmeter which was connected to read the average AC volts from the headphone jack on the tape recorder. I worked up a crude calibration chart. The ony problems were extraneous noises.
Having good measurements of the speed components (VT and VV), we can get the glide ratio by finding VH = SQRT (VT^2-VV^2) and then calculating VH/VV.
« Last Edit: Apr 13th, 2007, 4:00pm by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #13 on: Apr 16th, 2007, 9:30pm »
Today the wind was very gusty at many airports from the Southeat to the Northeast in the US. It made me think we need to talk about what airplanes work best in gusty winds and why. When an aircraft in cruise moves or climbs into a region of higher or lower wind, the change is slow enough so there is very little aerodynamic effect on the aircraft. The aircraft is accelerated or decelerated but at such a slow rate that the airspeed won't be seen to change. But get an airplane into gusty conditions and the airplane definitely feels the wind. Why? Read the article below. I'll have more discussion later with tables showing how different aircraft respond to wing gusts and to extreme turbulence. Then we'll pick some good airplanes for the job and go flying. I saved the RW for this morning.
EQUATIONS PERTAINING TO GUST LOAD FACTORS
(The simplified Goodrick Method.)
Here is the basic equation that I've kept in my head for years.
V = 17.16 * SQRT((W/S) / CL)_______________________________________________________{Eq 1}
where V is in KIAS, W is weight in pounds, S is wing area in square feet (= span times mean chord) and CL is the lift coefficient. The constant 17.16 comes from using sea level density for which IAS is valid and converting speed from fps to knots. This applies when the aircraft is in steady, level flight. Each of these variables takes on certain values in certain cases so we have to discuss them a little. Until all are explained, let your wonders accumulate. Don't get hung up on these. One thing you should notice soon is that W and S never appear alone. They always appear as the ratio W/S in a finished equation. This ratio is called the wing loading. But we must be careful which W we use because W can be different on different flights and at different times during the same flight. It turns out this ratio is a key to good windy flying. The higher its value, the better off you are in any wind.
If you can remember this, then the relatively straight-forward logic I'll discuss will get you any of the other useful equations with just a little algebra.
For the first manipulation, let's turn this around so it gives the value of the lift coefficient CL because most of you are wondering how we can get that. I'll show you how we can get the ball park for that value.
CL = 294.466 * (W/S) / V^2______________________________________________________{Eq2}
You could look in abook or tech report that gives CL versus angle of attack for a bunch of airfoils but we'll get a ball park for this value in the following way. Use the clean stall speed (always assummed to be at max takeoff weight) to determine the highest value CL can have, called CLmax.
We need to mention the significance of CLmax. It defines the stall condition. It is at the slowest speed at which the plane will fly steadily wihtout losing altitude though it is on the verge of falling from the sky. But, the fact that we can make the airplane fly level at least briefly at that speed (with lots of power) means that the stabilizing power of the tail is sufficient to make the airplane reach that lift coefficient. Indeed the fact is that the plane is capable of reaching that lift coefficient at ANY SPEED. That is how we find the max load factor. We assume we are flying along in steady conditions in level flight at some higher speed, with all forces nicely balanced and then something strange happens that makes the plane rotate quickly in angle of attack to a very high value. The plane can experience no greater force than when it reaches this lift coefficient. At a higher angle of attack it would stall and forces would drop.
For the Cessna 172SP this stall speed is 53 KIAS (based on our assumption in FS9 that KCAS = KIAS). W/S = 14.66 psf. Thus CLmax is 1.536. The fastest we can fly at 75% power gives us a speed of 111.8 KIAS (measured at 3500 ft but any altitude is good if 75% power can be developed). From this value we fine CL=0.345. Thus we know that, unless we do a nose dive which no good pilot would want to do, our Cessna 172SP spends its life flying with a CL between 0.345 and 1.536. Now if you are sufficiently computationally motivated, you can plug in values of CL between these extremes in Eq 1 and see what indicated airspeed results for any CL.
Now here's an eqaution from which Eq 1 was actually derived but will start with it here to go farther toward getting a load factor caused by a change in lift from either a horizontal gust or from a change in angle of attack.
The lift force F at speed V (KIAS) and any value of CL for the wing reference area, S, is:
F = (CL*S*V^2)/294.466______________________________________________________ __{Eq 3}
where the ^2 means the speed is squared and the constant is simply our friend 17.16 squared. This just happens to be the basic definition of the lift force, F. Now we play a simple trick to get a better format for load factor. We divide both sides by the weight W. Now we have
F/W = (CL / (W/S) *V^2) / 294.466________________________________________________{Eq 4}
Some folks like to add 1 to this before they call it a load factor. But if you are cruising at a value V1 and insert the value for CL that you would get from Eq 1, you will get the Value F/W = 1 meaning there is only the normal 1 g load factor applied to the aircraft. That seems good enough for me. Notice that our 'algebraic slight of hand' put the W back with the S on the right side.
But now let's see what can break the airplane. Let V1 be something like our speed at 75% (111.8 KIAS) and then put CLmax into Eq 4. You will get 4.45 g's. That might not be enough to break the 172 though it is close.
But now suppose we are flying lightly loaded. There is only one person on board with no luggage and only 40% fuel. For this case W/S is11.218 and our steady cruise speed at 75% power is 114 KIAS. Now Clmax gives F/W = 6.04 and we have TROUBLE. In a nutshell, this is the sort of thing that got Scott Crossifeld, the famous test pilot, whose Cessna 210 came apart in a thunderstorm over northeast Georgia last summer. (He had just given a lecture to some CAP cadets at Maxwell AFB on flight safety.) We don't have to say HOW this can happen. The fact is it CAN happen. The light load lets your engine pull the plane a little faster than normal. But the sudden change in CL to CLmax that CAN occur, DID occur, probably due to a little mix of Murphy's Law and Mother Nature being a witch.
Now let's go one step further and see what happens when we encounter a specific horizontal gust. We'll use V1 as our steady level flight speed again and Vg (in knots indicated) as the gust speed that Murphy's Law says we must encounter as a headwind. When hit by this horizontal gust, we assume that CL DOES NOT have time to change, and there's nothing to change W/S in the short time it takes the gust to hit. So let us take Eq 4 and divide it by itself with the 'before' condition (V=V1 and F/W=1) in the denominator and the gust load Fg/W in the numerator with V= V1+Vg. We have (Fg/W) / 1 on the left and just ((V1+Vg)^2) / (V1^2) on the right. Thus
Fg/W = (1+ Vg/V1)^2________________________________________________{Eq 5}
and we have our grand equation for the load factor due to the gust Vg at the steady speed of V1. For the Cessna 172SP at 75% power at a heavy weight, this is 1.39 for a 20 knot gust. That is enough to shake you a little bit but it won't break anything by itself. If you are holding the yoke lightly by two fingers as I used to, it can be a bit of a surprise. But if you are flying slowly at 80 knots it will be 1.56. The slower you fly the more of jolt you will feel. This process is independent of how you load the aircraft because W/S cancels out of the equation. It is simply a function of the change of speed. 20 knots added to a large speed is no big deal. But added to a small speed it can be a very big deal as far as contributing to a sudden upset is concerned.
Aeronautical engineers who don't fly generally ignore the effects of horizontal gusts. But pilots will notice it.
I'll make some more calculations for other planes. So far I have just done the Skylane which is a slight improvement over the Skyhawk. At cruise a 20 knot gust gives the Skylane a bump of 1.32 g's. But at cruise a sudden shift to max CL will give the Skylane a bump of 5.89 g's if heavy and 8.189 g's if light. You have to slow down significantly if light in bumpy air.
~~~~~~~~~~ADDENDUM~~~~~~~~~~~~~
Having lived with these equations now for a few days and having made calculations for various aircraft, I found a better way of looking at the turbulence load factor as given in Eq 4 using CLmas (which is the CL at stall).
If we write Eq 2, using stall speed VS and max weight Wmax to get the CL at the clean stall speed specified for the aircraft, we can then substitute that expression for CL in Eq 4. The result is a much handier expression for the turbulence load factor:
Fmax/W = (V/VS)^2 / (W/Wmax)__________________{Eq 6}
« Last Edit: Apr 18th, 2007, 11:27am by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #14 on: Apr 18th, 2007, 11:38am »
The two main equations from the section above, Eq 5 and Eq 6, have been used to make generalized plots of load factor. In each case the load factor is plotted against speed. But in the case of turbulence, we use the speed ratio V/VS, the ratio to the official stall speed.
It is important to note that the turbulence load factor is a max possible factor that can occur only if the angle of attack is changed to the angle that gives the maximum lift coefficient by some aspect of the turbulence. This could be a vertical gust hitting the wing or it could be a gust hitting the tail that changes the pitching moment drastically. In short we don't know exactly what happens but we know something bad happens when we find a wing or tail fin miles away from the main wreckage.
It is now clear why few aircraft ever fly at more than 2.5 times their stall speed. It is also clear you want to load up the airplane when you fly in turbulence and then you want to fly slower than your normal cruise. The "maneuvering speed" is published for most aircraft as a guide to keep you safe in turbulence. Go slower yet if you are flying light.
At landing speeds, both the effects of turbulence and the effects of horizontal gusts are the same order of magnitude. They can break the airplane only if they both happen at the same time. The most common effect of horizontal gusts is discomfort and loss of control if you are not careful.
« Last Edit: Apr 18th, 2007, 11:51am by Tom Goodrick »
Fs Piloting Skills
« on: Mar 11th, 2007, 7:00pm »
We will discuss piloting skills both from the point of view of general flight mechanics applicable to real aircraft to specific issues for FS aircraft and controls. We will advocate use only of FS aircraft to study and practice these methods.
For example, in the case of every aircraft from a J-3 Cub to a 747, the pilot uses a power setting and an airspeed to control the airplane in the vertical plane - that is, climbing, cruising and descending. Some people think it can't be that simple but, indeed, this is what real pilots do. Of course there are some practical differences between the Cub and the 747. The speeds and power for a Cub are simple while those for the 747 depend on the instantaneous weight and atmospheric conditions. When you set power and trim for a particular indicated airspeed, the plane will fly with a corresponding vertical speed. You don't have to look at vertical speed but in some cases you need to for reasons of comport. Also you may need to fly a particular glide path as when using an Instrument Landing System (ILS). You will generally fix airspeed and adjust power to get either the descent rate or the path in line with a requirement. The pitch trim adjusts the speed and the throttle adjusts the vertical speed.
You might ask why I did not mention the joystick or yoke. That is because you only need to use this control to make a turn, to rotate for takeoff or to make a flare for landing.
« Last Edit: Mar 11th, 2007, 7:02pm by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #1 on: Mar 11th, 2007, 9:30pm »
Why is indicated Airspeed so important to the proper control of an aircraft?
This can be explained mathematically with some difficulty and complexity. I have seen and have used the math. But it is a bit messy so most people don't want to see it. For those who do want to see the math in as straight-forward a fashion as possible, find the book "Introduction to Aircraft Performance, Selection and Design" by Francis J Hale of North Carolina State University. I had other books in college that were much more obtuse. This one I picked up at Auburn University while waiting for a football game to start. (My youngest son was a student there.)
The reason Indicated airspeed is vital to the control of an aircraft is that only indicated airspeed relates directly to the pressure causing the forces and moments acting on an aircraft. It is also directly related to the angle of attack from which the lift and drag can be separately determined. Thus, if you fly at certain indicated airspeeds, you will be assured of safe and efficient operation. let's go just a little into the fundamental physics on which this is based. Most normal flight is conducted very near steady state and very close to straight and level. In this condition, lift equals weight. Based on the equation used to calculate lift, this gives an equation in which airspeed is directly related to the lift coefficient which is, in turn, directly proportional to the angle of attack. Now within this equation is a factor called the dynamic pressure which is related to the pressure that can possibly act on a surface because of air moving near the surface. The instrument that measures airspeed actually senses this dynamic pressure. Now dynamic pressure can relate to true airspeed using the local air density or to indicated airspeed using the standard men sea level density. This is because in the airspeed instrument, the standard mean sea level density is used in the calibration. Thus a measurement of density is not required for the airspeed indicator to work. With this relation, the indicated airspeed is directly related to the aerodynamic forces regardless of altitude. We can use the indicated airspeed to tell when a plane is about to stall at any altitude.
Thus, in summary, we use indicated airspeed as an indicator of both dynamic pressure and angle of attack. we don't need to know those other parameters as long as we operate within limits of indicated airspeed specied for the airsraft. The low limit would be the stall speed and the high limit would be VMO or max operating airspeed at which a simple upset could tear off the wings. The bad things that can happen - stall and wings falling off - are also related to the gross weight of the aircraft. These speeds are also tied to the weight. But we can use the speeds given at max gross weight which are higher than the speeds that would apply to lesser weights and be safe because the speeds for lesser weights would be lower. Thus if you test for stall in a plane where clean stall occurs at 65 KIAS, you may find it stalls at only 60 knots. But if you stay above 65 KIAS all the time to avoid stall, you won't stall at the lighter weight or the heavy weight.
Don't think that True Airspeed is never used. It is always used as the spec for cruising speed. When somone says "The cruise speed of the X-97 is 400 knots, they mean the cruise speed is 400 KTAS. That is the speed at which it moves through the air at cruise. You must subtract the wind vector to get the ground speed. (Or, you can peek at the Garmin.) True airspeed will always be faster than Indicated airspeed. salesmen love to use True airspeed. It does have an important technical aspect in that it is used to determine the Mach number of jets. The Mach number is a ratio of True airspeed to the local speed of sound - the speed of sound at the same position and altitude. Stability, controllability and performance of jets depends on Mach number once you are above 30,000 ft.
Indicated Airspeed for takeoff is chosen as 1.3 times the clean stall speed at gross weight. For jets where a very large percent of the gross weight is in fuel which can change, the safe takeoff speed V2 is calculated based on the actual takeoff weight. it varies with the square root of the weight ratio. Commercial pilots are required to make this calculation for every takeoff. I have saved the FS jet pilot the trouble by including a calculation made by the computer and displayed on the panel next to the airspeed indicator when the aircraft is on the ground.
Indicated Airspeed for landing is also based on 1.3 time stall speed but in this case the configuration used is flaps down and the weight is maximum landing weight (normally much less than the max takeoff weight meaning a jet must fly and burn fuel before landing if it took off near max gross weight). This airspeed for landing is called Vref and is flown during final approach with full flaps. Again, I made a gauge that calculates and displays this for jets based on actual weight. (A fancy computer with weight sensors used on the ground and fuel-on-board calculations makes this Vref calculation. The result is displayed next to the airspeed indicator when the landing gear is lowered.
Unfortunately, Microsoft has thoroughly screwed up the matter of Indicated and True airspeed. They specify that true airspeed is to be used for stall conditions. They also say that using indicated airspeed rather than true airspeed diminishes the realism. They have some ignorant employees. At least they were kind enough to let us use indicated airspeed. Make sure you have that set under "Realism."
« Last Edit: Mar 11th, 2007, 9:32pm by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #2 on: Mar 12th, 2007, 9:27pm »
Use of controls and control sensitivity are areas in which FS differs from the real world. The physical connection to the aircraft for the pilot is rather different in the sim. You must understand this and be prepared to make adjustments that will suit you with your equipment. There are also control procedures that real pilots use that are not what some simmers expect.
Of course the first thing you must add is a control input device. In theory you can use the keyboard for everything. But this does not work well. You cannot input more than one control at a time using the keyboard. Flying requires that you do this in several instances - pitch inputs at the same time roll and yaw inputs are made. A good control interface can also reduce the unnatural aspects of sim flying. You need to be able to look outside and back at the panel. You must always be able to read the panel instruments accurately though you must also be able to look out and spot the runway as you line up for an approach.
The controller must have axes for pitch and roll. I like a stick because I first learned to fly an airplane with a stick. Now on FS i fly many aircraft that in reality have yokes. But many aircraft do have sticks which are making a comeback to some extent. A stick is particularly well-suited if you want to do aerobatics. The stick will still work well for normal flying. A yoke is not too good for aerobatics. The best answer is to get one of each and use them as appropriate. (This may be a hard sell to the wife.
Rudder pedals are important. As a student pilot you would be taught very early to use the rudder properly in making turns. The pedals are also used to steer on the ground. Most prop aircraft have a storng left yaw tendency when you add full power for takeoff. this is normally counteredt with rudder. If you have to use the stick or yoke for this it makes things difficult. It is unnatural. Similarly, many aircraft should be slipped a little on final approach, expecially in a crosswind. This requires that the roll be set opposite to the yaw - not possible without rudder pedals.
You will need to have a good and quickly effective throttle control. I am happy sing the keyboard for this though I have a control on the joystick I could use. My reason is I can make very precise control inputs with the keys which I need to set the power exactly. i have learned to do this quickly. Most throttle controls built into joysticks or yokes seem to me to be very cheap. people have reported problems flying airplanes I fixed up. Then we find it is a noisy throttle control. Ther are good separate throttle controls and even quadrants with throttle, prop and mixture levers for one or two engines. These look great but they are expensive. Still, I might pop for one of them one day - before I pop for a sim upgrade!
A good controller should also have extra buttons and switches that you can define as needed. I like a button for nose-up and for nose-down trim. It is very realistic. I use mine all the time when flying. I also have a hat switch I use to quickly look left or right. To look half left or half right, I use the number keys on the right numpad. I also use keys for landing gear, spoilers and pairs of F keys for prop, mixture, flaps and brakes. I have changed many of the key assignments so these lie in order across the top of my keyboard. I made a simple paper label that I tape across the top of the keyboard F1 through f4 are the throttle keys as set by defult.
These controlelrs must first be calibrated using the Windows calibration scheme. You may also have a disk or Cd furnished with the controller that should be run before calibration is attempted. It will set drivers and some special Windows calibration software.
Once the initiial calibration is completed, start FS and go flying, using an aircraft with which you are very familiar. This is no time to try a fancy new aircraft. Turn off the wind using Clear Weather. While doing this indoctrination flight, check the control sensitivities and null widths set within FS.
The "joystick" control sensitivity and null width can only be set within FS itself. Go to Options, Select Controls and then Sensitivities. Click on Advanced. Bear in mind what works for me may not be to your liking. But you will probably want something other than the default settings.
When making these changes, test with a very familiar aircraft that you have flown a lot. These adjustments are meant to take out peculiarities in your controller that are not related to any specific aircraft.
Note first that your control device should be shown in the window. Mine says "CH Flightstick Pro". next there are sliders for sensitivity and for null width on each of the main control axes - ailerons, elevator and rudder. There could be others such as throttle if you have that set in the calibration of your device. (This is to be done after you have calibrated the device.)
My settings for sensitivity are for ailerons 50%, for elevator 25%, for rudder 33%. The settings for all null widths are 25%.
The purpose of the null width is to handle any chatter you may have in the controller - digital or analog noise. I have had several sticks go bad sending noise into the controls.
There is one more sensitivity that gives a lot of people trouble, particularly for those transitioning to rudder pedals for the first time. This is the nose wheel turn angle. It is set in the aircraft.cfg file for each aircraft in the section labeled "[contact_points]'. (Note that any .cfg file is simply a text file you can read and edit using Note Pad. tech your computer to treat these files as .txt files.) Not all files have a key telling you the components of a line in the contact_points section. Check a default aircraft like a Boeing to find the key. You can copy the key into any other aircraft .cfg file since it is just a series of remarks. On an airplane that is difficult to keep straight during takeoff, reduce the wheel angle by editing the line in the .cfg file.
I seldom set the turn angle for the nose wheel less than 20 degrees. But, often I'll find values of 60 to 90 degrees in downloaded aircraft. I always reduce them to 30 degrees or less. But you can get used to anything after a while. Often, by the time I have tested an aircraft enough to get other bugs worked out, I have learned to live with a large angle. You don't need a large angle here, even if there is one in the real aircraft.
Also, if you have an aircraft you really like and want to fly it a lot but are having some problems controlling it, it pays to look in the aircraft.cfg file for the "[flight_tuning]" section and play with the aileron, elevator and rudder sensitivity scalars. (A scalar is a number of about 1 that is a factor to the actual control parameter. 1.0 means no change, 0.9 means a 10 percent reduction and 1.1 means a 10 percent increase.)
I find that elevator sensitivity is usually too high. Sometimes aileron is too high as well. This will improve the handling of only that particular aircraft. I find often that FD designers have over -cooked the control sensitivities, notably the elevator sensitivity. You just want to be able to raise the nose adequately on takeoff when the proper amount of pitch trim has already been set for takeoff. Don't leave so much sensitivity for elevator that you can rotate without regard to the trim setting.
Tom Goodrick
Re: Fs Piloting Skills
« Reply #3 on: Mar 12th, 2007, 10:15pm »
Pitch trim setting is a major aspect of pilot control technique that many non-pilots have no knowledge about. Pilots themselves almost forget that they use it all the time. it becomes second nature, like using rudder in a turn. But it is vital to proper control of the aircraft.
The forward or backward motion of the stick or yoke only has a limited range of control. The pitch trim must be used to augment this control. For takeoff there is usually a setting marked in real aircraft. I usually put a remark in the checklist as to what pitch trim setting to use for takeoff. Sometimes it varies with different weight or CG position. The value is in degrees. I give you a pitch trim value on the panel shown in degrees as well as weight and CG position if that is needed.
Set the trim for takeoff. The pitch control should be set for spring centering at neutral. A control that outputs forces is not necessary and not desireable in my opinion. Just spring centering is needed. After takeoff, with a slight pitch input needed to rotate, set the speed for climb using the stick or yoke. Then dial in enough pitch trim to take out the stick force so it will fly without your hand on the stick. This will work during most of the climb. I usually turn on the wutopilot at this point having set the cruise altitude and either a flight heading or with a flight plan entered I'll turn on NAV mode (set the NAV/GPS switch to GPS) and let the autopilot control pitch trim to maintain a particular rate of climb and heading or as needed to follow the path on the flight plan. I just adjust the power between climb and cruise.
If you are flying manualy, you have a lot of work to do and you will probably never do it perfectly. You must level off at cruise altitude, reduce power slowly to cruise power as the speed increases. Here you don't set a speed, you set only the power and the speed happens, depending on your weight, whether flying manually or on autopilot. But on autopilot the steady cruise condition is reached very quickly because the autopilot responds quickly and with just the right amount of conrol change to get to equilibrium flight with all forces balanced. You will probably chase the phugoid for quite a while.
The phugoid is the undulating motion that happens when the lift does not balance the weight. Though this is a pitch motion, the angle of attack remains constant during phugoid motion. The lower lift causes a slight descent which adds speed starting a levelling off with the lift greater than the weight. It then starts a slight climb. Then it will slow down with the lift less than the weight The damping of this motion depends on the drag if the controls are not changed. Clean airplanes with little drag will oscillate for a long time. The solution is quick and short pitch trim inputs when the speed is at a medium value. The autopilot does this very well.
Whenever I want to test a plane in cruise to lsee if its speed is corect, I use the autopilot. I don't waste time trying to stop the phugoid. On a draggy plane like a Cub or a Champ, the phugoid is well-damped so you can get it to fly steadily without much trouble.
When I was flying real aircraft in the real world (which is fairly bumpy), I don't think I every had the plane in steady cruise for very long. I always had a hand on the stick or yoke and was "diddling" with it continuously. I had no autopilot. I also never had the aircraft precisely "on" its specified cruise speed. It was probably close but off by a couple of knots. So don't feel bad if you can't hand fly an airplane steadily in FS. Use the autopilot like a pro would.
Tom Goodrick
Re: Fs Piloting Skills
« Reply #4 on: Mar 15th, 2007, 3:38pm »
NOTES ON USING THE MIXTURE CONTROL IN FS9
If you fly a plane in FS9 that has a normally-aspirated engine, you must learn to use the mixture control in order to climb and fly above 4,000 ft. But if you fly a tubocharged aircraft, you should do nothing except leave the mixture on full rich (fully in against the panel). If you don't know which way a plane is set up, check the aircraft.cfg file.
If the aircraft is normally aspirated, like this C172SP, you'll find this line in the [piston_engine] section:
turbocharged= 0 //Is it turbocharged? 0=FALSE, 1=TRUE
If it is turbocharged as this Mooney Bravo is, you'll find this line:
turbocharged= 1
and then you should look for this line:
fuel_air_auto_mixture= 1 //Automixture available? 0=FALSE, 1=TRUE
This has not always been the case so, if you see turbochaged=1 and fuel_air_auto_mixture=0, you should change it to =1.
The reason is that real pilots of turbocharged aircraft are taught to push the Mixture to full rich to begin and to maintain a climb at any altitude. Thus the operation will be realistic. Unfortunately, in real life, the pilot can adjust the mixture once he gets to cruise altitude where we cannot. But both the piloting technique and the performance is more realistic for tubocharged aircraft if auto mixture is used.
There is a slight departure from realism when using the mixture control with non-turbo aircraft. Real pilots (and some notes in FS documentation) are told to adjust the mixture to get a max on the EGT and then go beyond the max on the lean side. But in FS this does not work well. The thing that does work well is to adjust the mixture to see a maximum fuel flow. That coresponds to the maximum power you can get from the engine at that condition. That means you'll get good performance and will be treating the engine well for long life.
There is a big myth in aviation that pilots should lean for economy. They are probably even told to do that by the companies that own the airplane. But to lean for best economy, you would lean beyond max power to a higher temperature (EGT). This can easily cause engine wear and it does not save as much fuel as simply setting a lower throttle position would give you. There have been many cases reported where post-crash analysis showed engines were burned out when they failed by the leaning practices of the pilots.
As you climb in a normal aircraft, leaning to max fuel flow will keep you climbing as well as you can. Failure to do anything will leave you down low and slow.
Note that you cannot tell if an airplane is turbocharged by any means other than checking the aircraft.cfg file. One clue is that turbocharged engines usually have a TIT gauge for Tubine Inlet Temperature. This can cause problems if the mixture is not adjusted properly. Mooney made the Bravo and the 231 with turbocharging but the 201 and MSE did not have turbocharging. I have four versions of the Skylane, two with turbocharging and two without.
« Last Edit: Mar 15th, 2007, 3:39pm by Tom Goodrick »
Ed_Burke
Re: Fs Piloting Skills
« Reply #5 on: Mar 15th, 2007, 6:35pm »
" Real pilots (and some notes in FS documentation) are told to adjust the mixture to get a max on the EGT and then go beyond the max on the lean side."
I knew a chief engineer of an air charter business who threatened to nail any of his pilots to the wall by their dangly bits if they used that leaning method, despite it apparently getting the ok in some ops manuals. He reckoned the risk of detonation was too high and the extra fuel was engine insurance.
Peak the EGT and then drop it on the rich side was his instruction.
Ed
« Last Edit: Mar 15th, 2007, 6:38pm by Ed_Burke »
ED B
flaminghotsauce
Re: Fs Piloting Skills
« Reply #6 on: Mar 15th, 2007, 8:07pm »
I was taught that as well. Tom's method is simpler to implement, and works well for power, so I do it that way now.
Our fleet of 172's were not equally equipped, and I only recall a couple having an EGT. We were taught to adjust mix to best RPM, or peak EGT leaning to rich side.
One instructor told me to never lean during a climb, leaving a richer mixture to cool the engine until reaching cruise altitude.
"Barring all differences, they're identical!"
Tom Goodrick
Re: Fs Piloting Skills
« Reply #7 on: Mar 15th, 2007, 10:34pm »
If you try climbing on a rich mixture in FS9, you'll reach a level near 6,000 ft where the engine will not have sufficient power to climb. I don't know if this is realistic.
6000 ft is about the altitude at which a normal engine can generate no more than 75% power even at full throttle when leaned correctly.
« Last Edit: Mar 15th, 2007, 10:36pm by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #8 on: Mar 30th, 2007, 9:30pm »
Some thoughts on GLIDING that every pilot of a single-engine aircraft should know about:
Some people talk about "Glide Angle" and others talk about Glide Ratio." I have done a lot of work in this area and understand the situation. There are two problems: 1) which is better to talk about Glide angle or Glide ratio" and 2) How can you best measure glide performance?
First, it is best to use glide ratio. The two are related. The angle is the arctangent of the glide ratio. Or the tangent of the angle is the glide ratio. The reason the ratio is best to use is for practical purposes and for its strong basis in theory which leads to a simple way of testing - much simpler than trying to glide to a VOR. First we must recognize and work with the differences between indicated airspeed and true airspeed. We use indicated airspeed to set up the test. My 172 POH says the best glide comes at 80 mph. That is 69.6 knots indicated airspeed. We should use that speed to set up the test. Why? that is based on the theory of flight. For each glide speed, at a given weight (max gross is assummed here), there is a particular pair of lift and drag coefficients. There is also a particular angle of attack that pprovides these values of the coefficients but we need not bring that into the discussion. The ratio of CL/CD is the same as the ratio of the true velocity components VH/VV and is the same as the glide ratio X/H. this statement is founded in the technical definitions of lift and drag, and in the basic kinematics that speed components define the path if the ratio remains constant. Notice I did not say the speeds remain constant (although the indicated airspeed can and should remain constant).
What is practical about the glide ratio? The glide ratio is the ratio of the distance travelled, X, over the height dropped, H or X/H. If the glide ratio is 8.2, then X = 8.2H or 8.2 times the altitude. From 8,000 ft, you can go 65,600 ft which is 12.4 statute miles. There was a time I relied on this. I was flying back from Provincetown, Massachusetts across Cape Cod Bay to the area near Plymouth and wanted very much to keep my feet dry. I circled over Cape Cod and climbed to 8500 ft before setting off across all that water. That meant if the engine quit before I reached 12 miles from the shore of Cape Cod, I could turn back and glide back. When within 12 miles of Plymouth, I could relax because I could glide to the mainland.
To measure glide ratio, you don't have to sit in the plane and fly a constant KIAS for 20 minutes. Just start gliding at any altitude and measure your airspeed components accurately. I developed a "Turn Test Gauge" that is good to have. You can get it from my web site. It will give the true airspeed in decimal knots. Use my landing speed to get the vertical speed. Start a glide at 70 KIAS and smoothly adjust the trim to hold steady speed components. When the components are steady, regardless of altitude, read the KTAS, VA and the vertical rate VV in feet per minute. Convert each to feet per second:
1) multiply KTAS by 1.689 and
2) divide FPM by 60.
Then calculate the horizontal airspeed component VH: VH = SQRT(VA*VA - VV*VV) using the FPS values of each.
Then get the glide ratio: = VH/VV.
What is happening during the glide:
1. If you keep the indicated airspeed constant, then the angle of attack is constant and the values of CL and CD remain constant.
2. As you descend, the vertical speed decreases in magnitude as density increases. The true airspeed also decreases by a proportional amount so that this ratio remains constant at all altitudes from start to finish of the glide. The indicated airspeed is independent of density and altitude so it can remain constant once set up with proper trim that does not change.
In theory this ratio does not change with changes in weight. But that theory assumes the CG position is constant. Since partial loads can also change the CG and, thus, the trim, it would be good to test a couple of airspeeds and a couple of loadings, one CG forward and one CG aft.
I have not done this test myself but it is a good thing to do. It should only take a few minutes. It would be interesting to do it for several single-engine aircraft because this is all we have when the engine goes quiet.
The flaw in all this neat theory is the wind. The real glide ratio is (VH-W)/VV where W is the headwind component in feet per second. This will be worst at altitude. It is the reason pilots should pick the airport most directly underneath their position and begin maneuvers to land there when the engine goes quiet.
Now I will copy this so it won't vanish when the server gets too busy.
Tom Goodrick
Re: Fs Piloting Skills
« Reply #9 on: Mar 31st, 2007, 10:34am »
There is a simple way to prove with geometry that L/D = Vh/Vv = X/H. Start by putting a dot on a piece of paper that represents the mass of the airplane. Then make a downward arrow of a certain length and an upward arrow of the same length. The downward arrow is the weight vector acting on the aircraft in a glide.Call it W. The upward arrow is the resultant, R, of all the aerodynamic forces acting on the aircraft. There are no other forces - no thrust because the engine is off. (The prop may produce drag but we lump that with the drag from the rest of the plane.)
Now draw a long straight line slanted downward. This is the glide path. Don't lable it now. It makes an angle gamma (call it what you wish) with the horizon. This is the glide angle. Put an arrow head on that line about an inch from the dot and lable it Va. That is the airspeed vector.
Next make a line going up perpendicular to the path. Make it just long enough so an arrow going back at a right angle will connect to the R vector. The upward arrow is the Lift vector, L. Make a back arrow from the mass point back in an extension of the airspeed vector and in opposition to it but only long enough to make a connection from the lift vector to the R vector in a parallelogram.
Next put in horizontal and vertical airspeed vectors from the poiint going forward to connect to the end of the airspeed vector. These are Vh and Vv. Next pick any point along the line and draw a vertical line down to a point and then draw a horizontal line from that point to intercect the glide path. This is the ground. The vertical line is the altitude H and the horizontal line is the distance X over which we will glide.
Now we have three over-lapping similar triangles: RDL, VhVvVa and pathHX. All have a right angle in one corner and the flight path angle gamma in another corner. They are similar but of different sizes. That means the leg ratios are constant.
Hence L/D = Vh/Vv = X/H. For the typical aircraft this will be about 8. For a smooth, clean sailplane it can be as high as 50. For many airliners it is about 10.
This ratio L/D is actually the ratio of the lift coefficient to the drag coefficient. It is independent of density. It plays an important part in many aspects of flight. In cruising flight it is the ratio of weight to thrust. In a climb, it influences the rate of climb for a given amount of thrust. As long as the aircraft is stable, it will naturally sit at its trim angle of attack and will generate the same pair of values of CL and CD so that ratio remains constant. The indicated airspeed will also be constant. We can use that feature to change the glide ratio if we want to. While the forces appear to be balanced, the motion is quasy-steady-state. There is a gradual deceleration as the aircraft moves into the more dense air at the low end of the glide. You control the glide ratio by picking the indicated airspeed for the glide. You want to pick an airspeed sufficiently above stall so that you can make a flair before touching the ground.
70 KIAS is the best value for a C172.
Tom Goodrick
Re: Fs Piloting Skills
« Reply #10 on: Mar 31st, 2007, 2:02pm »
It's always a little scary to climb on a soap box and say "This is the theory..." because some guy in the back row will yell out "Can you prove it?" I did some experimenting and the answer is "Yes, I can." But, of course, there is some variation in the experimental data.
I popped a C172 SP up to 10,000 feet from its position on the parking ramp and let it glide. The engine was off. It went into a stall and by the time I got it nice and steady, the altitude was 6500 ft. This was loaded at gross of 2549 lbs and the CG was at 22.03%. I did not record the trim but I got it steady at 70.52 KIAS where I saw 77.6 KTAS and -762 FPM. This gives 10.27 for L/D. I let it glide without touching it and it became quite steady. At 5500 ft it showed 70.46 KIAS, 76.4 KTAS and -682 FPM for an L/D of 11.31. At 4500 ft it showed L/D of 11.14.
I trimmed for a faster glide by setting the trim at 11.2 degrees. I got this data:
7500 ft, 75.18 KIAS, 84.0 KTAS, -832 FPM, L/D=10.18
6500 ft, 75.18 KIAS, 82.7 KTAS, -887 FPM, L/D=9.40
5500 ft, 75.08 KIAS, 81.4 KTAS, -867 FPM, L/D=9.46
4500 ft, 75.07KIAS, 80.2 KTAS, -855 FPM, L/D=9.45
I reset the load to 13.03% and 2029 lbs. At the same trim of 11.2 degrees, I saw
8500 ft, 79.92 KIAS, 90.6 KTAS, -988 FPM, L/D=9.24
Then I trimmed at 16.4 degrees and got a slower glide.
8500 ft, 70.75 KIAS, 80.3 KTAS, -887 FPM, L/D=9.12
7500 ft, 70.76 KIAS, 79.1 KTAS, -860 FPM, L/D=9.27
6500 ft, 70.75 KIAS, 77.9 KTAS, -849 FPM, L/D= 9.24
5500 ft, 70.76 KIAS, 76.7 KTAS, -836 FPM, L/D=9.24
4500 ft, 70.76 KIAS, 75.6 KTAS, -825 FPM, L/D=9.23
I checked a Piper Archer II and found similar results. At gross weight, 70.9 KIAS gave L/D=9.97. At 75.1 KIAS, I saw L/D=9.99. I have the data on a spread sheet. I recorded five sets of data at five altitudes. While the C172 glided fairly straight ahead, the Piper Archer turned steadily at about 0.5 degrees per second. A touch of rudder would have kept it straight.
Here is a quick summary of several other tests I made. I'll just give a representative glide ratio (L/D) for each aircraft. But, I did take data at four or more different altitudes in each case. They are logged on my Excel sheet.
Piper Cherokee 180 L/D=9.8
Cessna 182 L/D= 8.8
*Cessna 182 RG L/D= 12.16
*Mooney Bravo L/D= 9.68
*Velocity XL-5 L/D= 11.76
*Bonanza V35B
*Piper Malibu Mirage L/D= 12.25
* retractable gear with gear up. All are at max gross weight.
The aerodynamically clean retractable aircraft have a high L/D but are tricky to fly for this experiment because they have a long and low-damped phugoid. They oscillate in pitch all the way down rather than reaching a steady condition.
The Mirage cannot use its glide ratio if the engine stops when cruising above 15,000 ft because their first job is to get down below 12,000 ft as fast as possible without exceeding the max safe airspeed. The pressurization is lost when the engine quits.
Also, when I was testing the 182 RG, I got a taste of the real problem with some of these aircraft. I had finished a testat 4500 ft and was going to do a test at 3500 ft when the battery gave up. I lost all instruments except the airspeed and altitude so I set up for a landing on a handy road. But I could not extend the gear so I changed my target to a nice flat field. On final I found I could not lower the flaps either. The landing was good but a little fast - 70 KIAS.
« Last Edit: Apr 1st, 2007, 11:12pm by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #11 on: Apr 7th, 2007, 12:15am »
In the section on sailplanes in the FS2004 thread, I mention that I made a Glide Ratio gauge and posted it on my web site. This gauge is handy on the panel of a glider. But it does not belong on the panel of a powered aircraft. Instead, it can be placed in the panel.cfg as a pull-down gauge to use when you want to study the glide chanracteristics of an powered aircraft. To put it there just insert the floowing lines in the panel.cfg:
[Window02]
size_mm=64,48
window_size_ratio=1.0
position=2
visible=0
ident=Glide
gauge00=Digital!Glide, 0,0
The window number you assign depends on the number of other windows you have. I'd place it in a high-numbered window becaues it will seledom be used. Name the window in the top few lines of the file. Also, the name of the file can be simply "Glide" if you put it in your main Gauges Folder when you download it from my site. here I call it Digital!Glide because I have it in a sub folder of Gauges called Digital.
I used it on the C182 to get the following values:
Trim KIAS L/D
4.1 100 8.5
6.7 90 8.6
10.6 80 8.8
(The trim values are in degrees measured by my pitch trim gauge.)
Tom Goodrick
Re: Fs Piloting Skills
« Reply #12 on: Apr 13th, 2007, 3:51pm »
FUNDAMENTALS OF MEASURING AIRSPEED AND VERTICAL SPEED
To make my Glide gauge work, I simply read the vairables named "True Airspeed" and "Vertical Speed" that are made available by the Mirocoft people who designed the FS9 program. But you might ask whther there is a way it can be done on real aircraft. The answer is "Yes". For sailplanes it is done all the time.
It dawned on me that many FS fliers may not be familiar with the fundamentals of how aispeed and vertical rate are measured on an airplane. The whole basis for the validity of the Glide gauge described above is in the fact that these measurements are based on different aspects of the physics of motion through the atmosphere. Finding the true "Up" direction in an aircraft is very difficult. Nothing really does it well in a static sense. If you see a bubble in a tube, you must realize it is not showing "Up". You can dangle a nice plumb bob and not find "Down." The reason these do not work is that the aircraft is generally accelerated in all three dimensions. This means the plumb bob and the bubble will respond to a combination of gravity and acceleration that results from the motion of the aircraft. That does not detract from the use of the turn and bank indicator, in fact, it reinforces the utility of that gauge. When flying a steady, banked turn, the bubble indicates where local "down" is during the turn but that local "Down" is not True "Down" as you can easily see. It points to the floor when the pilot is flying the turn correctly. Sitting in a seat, you will feel the pull directly "Down" into the seat and yet the airplane may be banked at 45 degrees to the vertical!
So how do we measure "vertical speed?"
We measure it using a device that shows the difference between two air pressures the current pressure and the pressure that existed about 1 second before. There is a very standard and fixed gradient of air pressure in the atmosphere from the surface up to a very high altitude. This gradient, or change per unit altitude, is used to calculate the vertical rate as an aircraft climbs or descends. It is handy because it has nothing to do with the attitude of the aircraft. (It uses the pressure pushing against a port on the side of the aircraft. This can be inaccurate momentarily if you slip the aircraft significantly but is not affected by pitch attitude at zero sideslip.) You might think this method of comparing old and new pressures. Indeed there is a line in the aircraft.cfg file that enables you to set a delay in the vertical speed reading. I think it is a mistake to use that with any significant delay. In fact the reading is very responsive. I have held these devices in my hand while they were connected to a PC and have seen the change in rate as I raised and lowered my hand. I saw no indication that there was a delay. The changes at the end of thet large tube occur imediately in comparison to the pressure at the end of the small lag tube. The rate of propagation of pressure in a tube slows down with smaller diameter. This is done in tubing that is built inside the case of the vertical speed device.
How do we measure True Airspeed?
Airspeed is measured as described above in this section but it warrents more discussion in this context. To compare with the vertical rate which is a true speed, we need a true airspeed rather than an indicated airspeed. This can be obtained two ways. In powered aircraft, the standard way is to get the indicated airspeed by measuring the difference between the pressure at the front end of a tube pointing into the relative wind and the pressure at a hole in the side of the fuselage. Bernoulli's Law says the difference between the total pressure on a streamline that stops in the end of a tube, is higher than the "static" pressure in streamline that passes along the side of the tube. The difference is called the dynamic pressure and varies with the density and the square of the speed. Indicated Airspeed is calculated and displayed based on the use of sea level density regardless of altitude. It works fine for pilots to use to keep the plane flying at a safe speed. But to get true airspeed, there must be a connection to the altimeter and a small calculation (digital or analog) that determines the true airspeed. There is another way to measure true airspeed directly. I was able to use this alternative method in my parachute testing and sailplane pilots make use of this method routinely. I utilizes the physical principle that the frequency of vortices shed from an object protruding into the flow depends directly on the true speed of the flow. All you need is an audio circuit that detects the frequency down stream of a protrusion. The device I used ws housed in a low-aspect ratio airfoil fin that stuck out from the fuselage. (We built a sloping nose fairing on the front of our instrumentation box and the checked the calibration in a wind tunnel. The fin was sticking out of the fairing on the nose. We had to check for angle of attack effects. We were able to minimize those so that we measured true total airspeed of the box hanging below the parachute.) This fin had a rectangular channel from fron to back with a diamond section cross bar. An audio signal of know and precisely controlled frequency was injected up stream of the cross piece. A small microphone picked up the sound downstream and showed us the change in frequency caused by the flow over the cross piece. It worked very well. Its readings were linear with airspeed. That device was intended for use on helicopters and V/STOL aircraft because it could measure very low airspeeds which cannot be measured by the pitot static system. Unfortunately it did not work on those aircraft because of the extremely high noise environment.
Thirty years ago I did experiments on my own that showed you could do this with a cheap microphone, tape recorder and voltmeter. With my wife driving, I would hold the mike out the window of the car and watch the voltmeter which was connected to read the average AC volts from the headphone jack on the tape recorder. I worked up a crude calibration chart. The ony problems were extraneous noises.
Having good measurements of the speed components (VT and VV), we can get the glide ratio by finding VH = SQRT (VT^2-VV^2) and then calculating VH/VV.
« Last Edit: Apr 13th, 2007, 4:00pm by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #13 on: Apr 16th, 2007, 9:30pm »
Today the wind was very gusty at many airports from the Southeat to the Northeast in the US. It made me think we need to talk about what airplanes work best in gusty winds and why. When an aircraft in cruise moves or climbs into a region of higher or lower wind, the change is slow enough so there is very little aerodynamic effect on the aircraft. The aircraft is accelerated or decelerated but at such a slow rate that the airspeed won't be seen to change. But get an airplane into gusty conditions and the airplane definitely feels the wind. Why? Read the article below. I'll have more discussion later with tables showing how different aircraft respond to wing gusts and to extreme turbulence. Then we'll pick some good airplanes for the job and go flying. I saved the RW for this morning.
EQUATIONS PERTAINING TO GUST LOAD FACTORS
(The simplified Goodrick Method.)
Here is the basic equation that I've kept in my head for years.
V = 17.16 * SQRT((W/S) / CL)_______________________________________________________{Eq 1}
where V is in KIAS, W is weight in pounds, S is wing area in square feet (= span times mean chord) and CL is the lift coefficient. The constant 17.16 comes from using sea level density for which IAS is valid and converting speed from fps to knots. This applies when the aircraft is in steady, level flight. Each of these variables takes on certain values in certain cases so we have to discuss them a little. Until all are explained, let your wonders accumulate. Don't get hung up on these. One thing you should notice soon is that W and S never appear alone. They always appear as the ratio W/S in a finished equation. This ratio is called the wing loading. But we must be careful which W we use because W can be different on different flights and at different times during the same flight. It turns out this ratio is a key to good windy flying. The higher its value, the better off you are in any wind.
If you can remember this, then the relatively straight-forward logic I'll discuss will get you any of the other useful equations with just a little algebra.
For the first manipulation, let's turn this around so it gives the value of the lift coefficient CL because most of you are wondering how we can get that. I'll show you how we can get the ball park for that value.
CL = 294.466 * (W/S) / V^2______________________________________________________{Eq2}
You could look in abook or tech report that gives CL versus angle of attack for a bunch of airfoils but we'll get a ball park for this value in the following way. Use the clean stall speed (always assummed to be at max takeoff weight) to determine the highest value CL can have, called CLmax.
We need to mention the significance of CLmax. It defines the stall condition. It is at the slowest speed at which the plane will fly steadily wihtout losing altitude though it is on the verge of falling from the sky. But, the fact that we can make the airplane fly level at least briefly at that speed (with lots of power) means that the stabilizing power of the tail is sufficient to make the airplane reach that lift coefficient. Indeed the fact is that the plane is capable of reaching that lift coefficient at ANY SPEED. That is how we find the max load factor. We assume we are flying along in steady conditions in level flight at some higher speed, with all forces nicely balanced and then something strange happens that makes the plane rotate quickly in angle of attack to a very high value. The plane can experience no greater force than when it reaches this lift coefficient. At a higher angle of attack it would stall and forces would drop.
For the Cessna 172SP this stall speed is 53 KIAS (based on our assumption in FS9 that KCAS = KIAS). W/S = 14.66 psf. Thus CLmax is 1.536. The fastest we can fly at 75% power gives us a speed of 111.8 KIAS (measured at 3500 ft but any altitude is good if 75% power can be developed). From this value we fine CL=0.345. Thus we know that, unless we do a nose dive which no good pilot would want to do, our Cessna 172SP spends its life flying with a CL between 0.345 and 1.536. Now if you are sufficiently computationally motivated, you can plug in values of CL between these extremes in Eq 1 and see what indicated airspeed results for any CL.
Now here's an eqaution from which Eq 1 was actually derived but will start with it here to go farther toward getting a load factor caused by a change in lift from either a horizontal gust or from a change in angle of attack.
The lift force F at speed V (KIAS) and any value of CL for the wing reference area, S, is:
F = (CL*S*V^2)/294.466______________________________________________________ __{Eq 3}
where the ^2 means the speed is squared and the constant is simply our friend 17.16 squared. This just happens to be the basic definition of the lift force, F. Now we play a simple trick to get a better format for load factor. We divide both sides by the weight W. Now we have
F/W = (CL / (W/S) *V^2) / 294.466________________________________________________{Eq 4}
Some folks like to add 1 to this before they call it a load factor. But if you are cruising at a value V1 and insert the value for CL that you would get from Eq 1, you will get the Value F/W = 1 meaning there is only the normal 1 g load factor applied to the aircraft. That seems good enough for me. Notice that our 'algebraic slight of hand' put the W back with the S on the right side.
But now let's see what can break the airplane. Let V1 be something like our speed at 75% (111.8 KIAS) and then put CLmax into Eq 4. You will get 4.45 g's. That might not be enough to break the 172 though it is close.
But now suppose we are flying lightly loaded. There is only one person on board with no luggage and only 40% fuel. For this case W/S is11.218 and our steady cruise speed at 75% power is 114 KIAS. Now Clmax gives F/W = 6.04 and we have TROUBLE. In a nutshell, this is the sort of thing that got Scott Crossifeld, the famous test pilot, whose Cessna 210 came apart in a thunderstorm over northeast Georgia last summer. (He had just given a lecture to some CAP cadets at Maxwell AFB on flight safety.) We don't have to say HOW this can happen. The fact is it CAN happen. The light load lets your engine pull the plane a little faster than normal. But the sudden change in CL to CLmax that CAN occur, DID occur, probably due to a little mix of Murphy's Law and Mother Nature being a witch.
Now let's go one step further and see what happens when we encounter a specific horizontal gust. We'll use V1 as our steady level flight speed again and Vg (in knots indicated) as the gust speed that Murphy's Law says we must encounter as a headwind. When hit by this horizontal gust, we assume that CL DOES NOT have time to change, and there's nothing to change W/S in the short time it takes the gust to hit. So let us take Eq 4 and divide it by itself with the 'before' condition (V=V1 and F/W=1) in the denominator and the gust load Fg/W in the numerator with V= V1+Vg. We have (Fg/W) / 1 on the left and just ((V1+Vg)^2) / (V1^2) on the right. Thus
Fg/W = (1+ Vg/V1)^2________________________________________________{Eq 5}
and we have our grand equation for the load factor due to the gust Vg at the steady speed of V1. For the Cessna 172SP at 75% power at a heavy weight, this is 1.39 for a 20 knot gust. That is enough to shake you a little bit but it won't break anything by itself. If you are holding the yoke lightly by two fingers as I used to, it can be a bit of a surprise. But if you are flying slowly at 80 knots it will be 1.56. The slower you fly the more of jolt you will feel. This process is independent of how you load the aircraft because W/S cancels out of the equation. It is simply a function of the change of speed. 20 knots added to a large speed is no big deal. But added to a small speed it can be a very big deal as far as contributing to a sudden upset is concerned.
Aeronautical engineers who don't fly generally ignore the effects of horizontal gusts. But pilots will notice it.
I'll make some more calculations for other planes. So far I have just done the Skylane which is a slight improvement over the Skyhawk. At cruise a 20 knot gust gives the Skylane a bump of 1.32 g's. But at cruise a sudden shift to max CL will give the Skylane a bump of 5.89 g's if heavy and 8.189 g's if light. You have to slow down significantly if light in bumpy air.
~~~~~~~~~~ADDENDUM~~~~~~~~~~~~~
Having lived with these equations now for a few days and having made calculations for various aircraft, I found a better way of looking at the turbulence load factor as given in Eq 4 using CLmas (which is the CL at stall).
If we write Eq 2, using stall speed VS and max weight Wmax to get the CL at the clean stall speed specified for the aircraft, we can then substitute that expression for CL in Eq 4. The result is a much handier expression for the turbulence load factor:
Fmax/W = (V/VS)^2 / (W/Wmax)__________________{Eq 6}
« Last Edit: Apr 18th, 2007, 11:27am by Tom Goodrick »
Tom Goodrick
Re: Fs Piloting Skills
« Reply #14 on: Apr 18th, 2007, 11:38am »
The two main equations from the section above, Eq 5 and Eq 6, have been used to make generalized plots of load factor. In each case the load factor is plotted against speed. But in the case of turbulence, we use the speed ratio V/VS, the ratio to the official stall speed.
It is important to note that the turbulence load factor is a max possible factor that can occur only if the angle of attack is changed to the angle that gives the maximum lift coefficient by some aspect of the turbulence. This could be a vertical gust hitting the wing or it could be a gust hitting the tail that changes the pitching moment drastically. In short we don't know exactly what happens but we know something bad happens when we find a wing or tail fin miles away from the main wreckage.
It is now clear why few aircraft ever fly at more than 2.5 times their stall speed. It is also clear you want to load up the airplane when you fly in turbulence and then you want to fly slower than your normal cruise. The "maneuvering speed" is published for most aircraft as a guide to keep you safe in turbulence. Go slower yet if you are flying light.
At landing speeds, both the effects of turbulence and the effects of horizontal gusts are the same order of magnitude. They can break the airplane only if they both happen at the same time. The most common effect of horizontal gusts is discomfort and loss of control if you are not careful.
« Last Edit: Apr 18th, 2007, 11:51am by Tom Goodrick »