Post by Bill Von Sennet on Aug 22, 2008 22:21:13 GMT -5
Tom Goodrick
Understanding The Autopilot
« on: Mar 6th, 2008, 12:18am »
How Automatic is the Autopilot?
PART 1
Several questions recently have prompted me to discuss the matter of the autopilot. How automatic is it and how should it be used in FS9? I found an old article I wrote on autopilots in 2003 and it had a bunch of obsolete information. So it is time for a new article.
I looked into how the autopilot works and how you can control it in its several modes. I tried to find how you can adjust it to fit particular aircraft. In this I was mainly unseccessful. Mostly, it works. If it does not work with a particular aircraft, the solution lies in adjustment of the FD the same way we do for better handling and for realistic performance. If an aircraft works well when you fly it manually, it will work well on autopilot. Do not hesitate to use the autopilot when flying cross-country. It offers the best way of managing climb to cruise altitude, holding cruis altitude and attaining and maintaining the best speed, and flying the best track to the destination. It can also be used very well to fly vectors during a departure or an approach. In some cases it will do an ILS approach but you are better off flying approaches manually.
Part 1 is an introduction to the "hardware" you see in the panel.
Part 2 covers the altitude mode or ALT mode.
Part 3 covers the heading mode or HDG mode.
Part 4 covers the navigation or NAV mode.
Part 5 covers the SPD/MACH modes.
Part 6 covers the the approach mode and other odds and ends such as LWL, BC, and YD.
Below we will discuss the use of the Autopilot in ALT mode, HDG mode, NAV mode, SPD mode and APR mode. I'll use some examples involving the Baron 58, the Beechcraft King Air 350 and the Learjet 45. For each of these there is a download on my web site that improves the FD. For the Learjet 45, the download is especially helpful because the original FD causes it to behave poorly on autopilot. For simple Cessnas, the autopilot is of little use and isseldom necessary. But, it could be considered necessary if you fly solo IFR in any airplane. it certainly is needed in more complex aircraft to reduce the pilot's work load. Many jets are not certified to be flown without a working autopilot or must be operated low and slow if the autopilot is malfunctioning. Using the autopilot in no way reflects poorly oin your pilot abilities. It is a necessary tool when flying complex aircraft with high cruise and approach speeds. it is vital to the new reduced altitude separation rules where aircraft on opposite headings may be separated in altitude by only 1000 ft.
In FS there are several different "control heads" for the autopilots. These are units you see in the panel by which you can control the autopilot. There are the old ones that were used in singles in a pull-down image that included the radio stack. In my panels I use the more sophisticated units that were mounted in jets. I think you need the control head mounted as a permantent part of the panel because you always need to be able to see what the settings are at a glance. You cannot cover up another vital part of the panel by pulling something down on top of it. Of course thei requires a PFD display to show all the vital flight instruments in a compact manner so there is enough space to put the control head as well as the GPS map which should always be visible as well.
But you should know that the control head must work with elements in the air file for the particular aircraft. Not all the switches and settings will work with all aircraft. The gauge coding relates to settings in the air file. Some parameters are in the aircraft.cfg file. It is not clear how these work.
But can the autopilot be trusted to fly the airplane completely with no supervision? No, not in many cases. A competant pilot, fully able to take manual control at any stage, must be watching all gauges all the time. He must interrupt at any unexpected deviation from normal procedure and performance. He can guide the procedure with heading adjustments, thrust adjustments and vertical speed adjustments.
The first rule for using an autopilot is DO NOT ACTIVATE IT until you are sure you are ready. An autopilot should NEVER be turned on when the plane is on the ground. That mistake has killed many people. In FS it can mess you up so that you don't know what is happening. Of course I have done that. I have interrupted a flight while the autopilot is on, popped the airplane down to a location on the end of the runway at a faraway airport and then tried to takeoff, wondering why the plane is climbing to a stall and turning furiously to the left.
Always prepare the autopilot by setting the parameters for whatever mode you intend to use before you activate the mode and/or the autopilot. When activated, the autopilot will try very hard to do what you have commanded. make sure you are ready for that.
In my Baron, the autopilot "head" is always visible. This is the part you use to control the autopilot. On the left are two buttons AP and FD for Autopilot and Flight Director. For now we can ignore FD. We will click on or "press" AP when we want to turn the autopilot ON. To the right of FD is a window with a number in it and below that is a CRS knob. In real life you rotate the knob to change the course to be displayed. In FS you put your mouse pointer on the left or right side of the number and press the mouse button to decrease or increase the number. This applies only to the Instrument Landing System (ILS) localizer. That number agrees with the heading you will fly toward the runway. To the right of it is anothe number with a knob below labelled "HDG. This can hold the heading you want the aircraft to fly in HDG mode. Between the two numbers is a button labelled "NAV" that turns on NAV mode. To the right of the heading number is a button labelled "HDG". Press this to get into HDG mode. You can change the heading number and all other numbers on the autopilot by putting the mouse arrow on one end of the number and clicking or holding the mouse button. Below HDG is APP, another button that turns on APP mode. In the column of buttons to the right of HDG is BC (turns on "back Course", LVL (turns on the wing leveler) to keep the wings level, YD (turns on the Yaw Damper) that reduces unwanted yaw oscillations, and to the right of those buttons are MACH (setting Mach speed limit) and IAS (setting indicated airspeed speed limit). A number to the right of MACH can hold the Mach limit or the airspeed limit. Next you see ALT, a button that sets ALT mode. To its right is a number showing the altitude you want the autopilot to hold and farther to the right is a window showing the vertical rate limit you want to impose during the transition to that altitude.
Tom Goodrick
Re: Understanding The Autopilot
« Reply #1 on: Mar 6th, 2008, 12:19am »
PART 2
ALT MODE
The most basic autopilot mode is ALT mode in which the autopilot attempts to make the airplane climb or descend to a particular altitude and then maintains that altitude as long as the autopilot is ON and ALT mode is ON. To use ALT mode you must set an altitude to be held and a vertical rate limit to be used to get to that altitude. This is the key to controlling the aircraft for safe flight. The autopilot does not know what power is needed or what airspeed is safe to fly. You must set the power and monitor the airspeed. You can change the allowed vertical rate as you fly to keep the right airspeed. You can adjust the throttle to maintain the climb or to avoid over-speeding during descent or getting too slow during a climb.
How does the autopilot control climb? It simply increases or decrease the pitch trim until the vertical rate reaches the value set in the window. Then it continues to adjust the trim to maintain that rate until the altitude set in the window is nearly reached. It reduces the vertical rate slightly before reaching the selected altitude and then works to hold the altitude. I tried setting various values in the aircraft.cfg file for the pitch parameters. Nothing seemed to make a difference. I doubled them and increased them by 10x. There was no change in climb performance. I would assume that, if the FD is set for proper CG and for proper pitch trim sensitivity, the autopilot will work well during climb or descent. But it requires that you know how to adjust the proper in a reasonable way during climb or descent. You must select a reasonable vertical rate and then must set the power properly. In the case of a normally-aspirated engine like the Baron's, you must also adjust the mixture as the aircraft climbs to maintain the max fuel flow or power. When descending you must set a sensible power setting . For the Baron this would be 15 inches and 2100 rpm.
BARON
Lets do a basic climb on autopilot that is commonly done on takeoff in IFR conditions. The controller will instruct you to "Maintain runway heading and climb to 8000 feet." We will do this in the Baron while taking off from runway 18L at KHSV. While the Baron is parked, set 8000 in the altitude window and 1000 in the vertical rate window. The aircraft can safely handle this rate while accelerating and while you check that the gear is up and bring the flaps up. Later during the climb we will raise this to 1000. The FD designer has left a value in the files that the aircraft will go to if you don't have anything in the window. But it is best to think ahead and put a positive number in there the aircraft can handle. I have been unpleasantly surprised to find a negative climb rate in there several times. When you are trying to get a few hundred feet above the ground and clear of the trees, power lines and hills beyond the runway, you don't need a negative climb rate. We set the heading 180 in the HDG window because from runway 18, that is the heading we will be flying. You should also think ahead and put the number 180 in the course window and dial the freq 111.9 in the Nav 1 frequency slot. That way you will have the return ILS all set in case an emergency develops and you have to go around and land as soon as possible. The last thing you need with an emergency is to be changing numbers or even looking them up with smoke in the cockpit, etc.
We make the takeoff as we normally would and get the gear up. (Flaps up if you use them.) As we pass 400 feet on the radar altimeter (or 1000 ft on the baro altimeter) we punch the HDG and ALT buttons. We see the green lights and punch the AP button. (I unpunch the FD button because I don't like using it. I know what to do to fly the airplane. I don't want anything needlessly cluttering up the instruments.) I begin adjusting the power while watching the airspeed. I reduce the RPM to 2500 and then adjust the throttle looking at the power meter to set about 84%. Anything over 80% but less than 85% works for the initial climb. The power decyas rapidly as we get toward 5000 ft. I have to begin adjusting the mixture to maximize the fuel flow. Otherwise the airspeed will decay. At about 3500 ft I can set full throttle ad the power will stay in the right range though I must keep leaning the mixture a bit to keep the fuel flow and the power from declining significantly. As the aircraft reaches 5000 feet, I adjust the mixture a final time for max fuel flow and then back off on the throttle to set 75% power.
During the climb on autopilot I have to know how to keep the power properly adjusted. The procedure would be different for an aircraft with turbcharged engines like the Mooney Bravo or my Cessna 340. In that case no mixture adjustment would be required but I would have to reduce the throttle properly to keep the power level in the same range (80 to 85%). I must also watch the EGT or CHT temperatures. (I made sure before takeoff that the cowl flaps were open.) If those temperatures go high during climb, I must reduce the vertical rate to increase the airspeed and give the engines better cooling.
You can save this Baron flight situation (level at 8,000 ft) for future demonstrations. Next we will repeat the same takeoff and autopilot climb in the King Air 350 though we'll climb to 15,000 ft.
BEECH 350
Set up the King Air at KHSV 18L. (Weather should be on Clear for all test flights - no winds and no clouds). Note that the checklist for the 350 specifies flaps optional for takeoff. We will not extend the flaps. We must set 7 degrees elevator trim and use only 90% throttle for takeoff. The speed for rotation is 95 KIAS and takeoff is 105 KIAS. Set the autopilot up once again for 15000 ft altitude and 180 degree heading. This time use 1800 fpm for vertical rate as noted in the checklist for climb that you must read before takeoff). The condition levers should be at low idle. (Use the "mixture" to adjust this. It becomes the condition lever in a jet turboprop like the 350.)
Throttle to 90%, watch for 95 KIAS, rotate and let it fly off. I set the stop watch on the timer as I shoved the throttles to 90%. Just as the aircraft was climbing through 400 ft, I turned on the AP. The aircraft accelerated to 210 KIAS and climbed smoothly. I noticed that 90% throttle gave 92% power. In a turboprop, ITT is the parameter to watch on takeoff and climb. It should never exceed 800 C. The value peaked near 750 and settled down during the climb to about 640C. In 8 minutes we were level at 15000 ft. There was no fuss about the climb airspeed, the temperatures or the airspeed. The 350 is a very well-behaved aircraft. Set 80% power for cruise at 15000 ft and then save the flight for later use.
LEARJET 45
Next look under the Bombardier menu and select the Learjet 45. Put it on runway 18L at KHSV. Set the AP altitude at 15,000 ft and the heading at 180. Set the vertical rate at 4000 fpm. Do not turn on the autopilot. (Make sure it is off.) Bring up the checklist and go through it. Weight is very important in jets. While we will make a takeoff at MTOW, we will have to dump fuel if we want to land. The checklist says to make sure the weight is below 21500 lb. Yours probably is not below that weight. That is because FS always puts 100% of fuel into the center tank. Look at the page for fuel and payload. It will show someting like 715 gallons is the max fuel load you can carry this trip. Look at the fuel. Subtract the amount in the left and right tanks from 715 gallons. The result is the max you can put in the center tank. Enter that value (in gallons) into the center tank. Now you should be ready for a takeoff at MTOW. Leave the AP set for HDG and ALT with the numbers listed above in the windows. But leave the main AP switch OFF for takeoff.
The checklist says to set one notch of flaps (8 degrees) and 8 degrees of elevator trim. Do that. Note that you see 124 KIAS in the window on the left of the airspeed indicator on the PFD for V2. You will rotate at 124-10 and fly off at 124 KIAS with the present weight. The checklist says you will raise gear on positive climb and accelerate to 180 KIAS before reducing thrust to 80% and going to 4000 fpm climb. That is when you can hit Z to turn on the autopilot. Let's do that.
My takeoff was fine but the transition to climb was not smooth. I had to unset and reset both the HDG and ALT buttons after turning on the autopilot. I guess we should leave those off until we turn on the AP. Then hit AP, HDG and ALT at the same time. The airspeed should never exceed 250 KIAS until we are above 10,000 ft. Then, as the checklist indicates we go to 2000 fpm and 100% throttle. But we will stay at 2000 fpm and 80% throttle until we level off at 15,000 ft for maneuvers.
If you have trouble with any of these exercises, do them until you are satisfied. Climbing steadily in the Learjet is the toughest, especially when you go all the way to 41,000 ft. For now throttle back to 50% and save the aircraft in level flight at 15,000 ft for the next set of exercises.
Tom Goodrick
Re: Understanding The Autopilot
« Reply #2 on: Mar 6th, 2008, 12:21am »
PART 3
HDG MODE
BARON
HDG mode is the simplest turning control mode. You set a compass heading into the "HDG" window. Then the autopilot will try to turn the aircraft to that heading. Return to the Baron in level flight at 8,000 ft and make sure it really is in steady flight. Then set 090 into the HDG window. (The HDG button should already be on and a turn to the left will commence.) You are in both HDG and ATL mode as the autopilot holds altitude and turns to a heading of 090 (east). The autopilot calculates the diferents between the present heading and the desired heading, treats that difference as an "error" value and takes steps to drive that error to zero. It does this by rolling in the direction according to the sign of the difference (right for positive difference, left for negative difference). There is a max bank angle in the aircraft.cfg file that is held by the autopilot until it gets near the desired heading. As the difference diminishes to a key value, the bank angle becomes proportional to the difference. This results in a smooth roll out on the new heading. You may notice that, during the Baron's turn, its airspeed decreases slightly and the pitch trim changes slightly. This is because it loses some lift coefficient while banked (that lift is helping to turn the aircraft) and must compensate for the loss of lift by increasing the pitch trim. This will in turn slow it down slightly as drag increases in the pitch change. When the turn is completed the speed comes back and the pitch trim returns to its value before the turn was started.
The max bank angle is normally set at 25 degrees because that works for a smooth turn in most aircraft. However, I have changed it to 35 degrees in many jets because of the low turn rate at high jet speeds. But this must be checked in various conditions because it can lead to an upset condition. The turn rate depends both on bank angle and on true airspeed. Hold bank angle constant and increase airspeed and the turn rate will decrease notably. Hole true airspeed constant and increase bank angle and the turn rate will increase. But the amount of pitch-up required to maintain altitude will also increase resulting in a loss of airspeed. It gets complicated quickly so this is something to be careful about. At high altitude where jets normally cruise, this speed upset becomes very significant and can even result in the autopilot losing control.
Now with the Baron flying east at 8,000 ft, set the heading to 270 and see how well it manages both the turn and holding altitude. OK. That was easy. Now set a heading of 60 degrees and an altitude of 4000 ft. Set a vertical rate of 500 fpm. (This is the max downward rate you should use in any non-pressurized aircraft like the Baron 58.) We will give the autopilot a challenge of making both a turn and a descent at the same time. But will will have to help by adjusting the power properly. Set 20 inches and 2100 rpm. As you reach about 4500 ft, start a left turn to 360. Then when reaching 4000 ft, set 2500 RPM and 75% power which will be reached at 23.5 inches. But then remember you must adjust mixture again to get maximum fuel flow. Do this and then readjust the throttle to give 75% power. I ended up with 23.77 inches, 17.56 gph, 2500 RPM and 74.99% power.
Continue and let the aircraft get fully steady. Next set a heading of 090 and an altitude of 9,000 ft. Use 1000 fpm as a climb rate. Set full throttle. Don't forget to lean the mixture for max fuel flow every 2000 ft or so. Again, the Baron just does what you want it to do with no fuss. It is a very nice aircraft for those who need to carry more weight than a fast single will carry or who want the extra engine when flying over water, over desert, over mountains and at night and general IMC.
BEECH 350
Go through the same turn exercises in the Beech 350 - level from 180 to 90 and then to 270. For a level flight power setting, reduce the RPM until N2 is below 100%. You can set 80% power. Mine did those turns with ease. Now try turning to 060 and descending to 8,000 ft. Set set -1500 fpm and adjust the throttle to keep the airspeed at 230 KIAS. Level off with 70% throttle and turn to 360.
Next set 80% power, 15,000 ft with 1500 fpm and turn to 090. My aircraft does this also with no problems.
LEARJET 45
Return to the Learjet we left in level flight at 15,000 ft. i set 55% so the airspeed became steady at 280 KIAS. Now turn from 180 to 090 degrees. Now turn to 270 degrees. Mine did a good job with these. Now set the altitude at 40,000 ft, the climb rate at 4000 fpm and the throttle at 90%. As you pass 25,000 ft, turn to 360 degrees. As the airspeed drops below 300 KIAS, set the climb rate at 3500 fpm. Set 3000 fpm as airspeed drops through 295 KIAS. Continue with this adjustment as needed. Mine was climbing at only 1000 fpm from 31,000 ft upward. But remember that we started at full gross weight. We made it to 40,000 ft for cruise. Many jets cannot climb that high after a full gross weight takeoff. They must level at something like 35,000 ft and burn off some fuel before continuing. Aceelerate to Mach 0.80 and then reduce the thrust to about 80%. To optimize range in the Learjet 45, you must work up to higher altitudes as fuel burns off. The max altitude for cruise is 51,000 ft. I have never gotten the Learjet 45 that high. It always seemed that the fuel burned in climbing was not worth the reduced fuel consumption at higher altitude once the aircraft was cruising in the low 40,000 ft range.
Now we have another turn check. When the aircraft is steady at 40,000 ft. do a turn to 180 degrees. Well, this was another ho-hum for this aircraft. If you are flying a default Learjet, you might have another story to tell. Save the Flight with the Learjet steady at 40,000 ft. We can use this when talking about SPD mode.
AP TURN PARAMETERS
The parameters in the aircraft.cfg file pertaining to turns are:
max_bank=25.0
max_bank_acceleration=1.8
max_bank_velocity=3.0
These are the same for the Baron 58, the Beech 350 and the Learjet 45. They are even the same for the Boeing 737-400 and the Cessna 172SP. This seems to suggest that you don't have any reason to make adjustments to them. I fixed a problem in the Learjet 45 from the default FD files. But I did it not in the AP lines but in the pitch trim sensitivity. It seems that the trim sensitivity can make it difficult for both you and the autopilot to control the aircraft.
Tom Goodrick
Re: Understanding The Autopilot
« Reply #3 on: Mar 6th, 2008, 12:22am »
PART 4
NAV MODE
A few years ago I was warning people not to use NAV mode. I thought it did not properly correct for wind. I was wrong. It does correct for wind better than you can. (It senses and adjusts to wind changes continually as you fly if you have loaded Real Weather.) Now I use it most of the time on most cross-sountry flights in RW. I set and use HDG mode for departure and for arrival in the vicinity of the destination airport.
To use NAV mode, you must create a flight plan for the trip from the departure airport to the destination airport. It can have none or several waypoints between those two airports. (The need for waypoints can be for fuel stops or to stay out of restricted areas.) if you use ATC while flying IFR, you will be given vectors for departure and arrival. These must be flown in HDG mode. ALT mode handles all altitude assignments. But after the initial one or two vectors after takeoff, the controller will say something like navigate at your descretion or he will give a heading that matches the heading on the flight plan. Even if you are slightly off the path set for the flight plan on the GPS, you can activate NAV mode on your autopilot.
To activate NAV mode you must set two switches:
1) Set NAV/GPS to GPS.
2) Click the NAV button on the autopilot head.
When NAV mode is properly activated, the aircraft will disregard the heading number and will turn in such a way that it intercepts the path for the flight plan. You can watch this happen on the GPS map. You will still have to adjust power as needed for any climbs or descents and to enter cruise. The autopilot will head into the wind just enough to keep the flight path straight along the flight plan. In RW, the wind speed and direction change with both altitude and time. You could not possibly do as well as the autopilot in NAV mode.
When you get close to the destination, ATC will give you vectors for the approach. If not using ATC, you can give yourself vectors to get setup for the approach. If you stay with the autopilot in NAV mode too long, the aircraft will fly over the target and begin circling. That does not help you any unless you are away from the computer (shame on you!). Set the correct heading and go into HDG mode to handle the approach vectors. Also, be sure to turn the NAV/GPS switch back to NAV so you can see the ILS indications properly. If you see on the map that you should be close enough to see the ILS needles move but they do not seem right, you have probably forgotten to throw the NAV/GPS switch back to NAV.
You can do your own experiments with NAV mode. Use on your next cross-country trip. Use Real Weather too so the trip is more realistic.
NAV MODE in AIRCRAFT.CFG
These are the lines in the aircraft.cfg file that pertain to NAV mode:
nav_proportional_control=9.00
nav_integrator_control=0.25
nav_derivative_control=0.00
nav_integrator_boundary=2.50
nav_derivative_boundary=0.00
They are initially set the same for all aircraft. I have changed the first line a little for really big aircraft such as the A380 and C133. They generally work fine.
For many years, FS was ahead of the real world in navigation enhancements. We have had flight visualization for many years. That allows you to fly an ILS approach usiing an artificial display of rectangles. All you do is manage power, flaps and gear while keeping the plane flying through a series of rectangles down to the runway. This works great in very low ceilings as in fog and snow. I read recently that this is available now in general aviation aircraft for following the flight plan in NAV mode. But we cannot yet do that. We must connect the display to either NAV1 or NAV2 radios, not to the GPS. Maybe FSX can do that. The experienced pilot who described using this system (Richard Collins of FLYING), like it.
Tom Goodrick
Re: Understanding The Autopilot
« Reply #4 on: Mar 6th, 2008, 12:23am »
PART 5
SPD/MACH MODE
This mode is of use to you only if you are flying a jet like the Learjet 45. Even then it is not always necessary. But if you fly jets, you should at least learn about it. SPD mode means you want to lock in an indicated airspeed. MACH mode means you want to lock in a particular Mach number. You can do either. No, they are NOT the same thing.
We will presume that with your interest in jets, you are sufficiently experienced in flying to know about dynamic pressure - the basis for calculating all aerodynamic forces and moments acting on the airplane. By locking in an indicated airspeed, you are locking in a dynamic pressure at which the aircraft and engine(s) work well in cruise. To keep things simple, instead of talking about the dynamic pressure you are using, we talk about the indicated airspeed you are using. In jets, the fuel burn as you cruise is very significant if you are going very far. You may want to lock in an airspeed so that you are locking in a particular performance level and so you can make reasonable predictions about when you will get where you are going. You will, of course, be locking in a particular true airspeed as well since that remains a particular proportion of your indicated airspeed as long as the altitude remains constant.
As a general rule, jets fly based on KIAS below 30,000 ft and based on Mach above 30,000 ft.
MACH is new to pilots moving up to jets from lower and slower aircraft. The Mach number is the ratio of the true airspeed to the local speed of sound. By "local" I mean the speed of sound that would occur in the air imediately outside the aircraft. This varies with the temperature. The true airspeed as a proportion of the indicated airspeed varies with density. Both density and temperature vary with altitude. Here's a short table to think about.
sea level 250 KIAS 250 KTAS SoS 662 KTAS Mach 0.38
20,000 ft 250 KIAS 343 KTAS SoS 615 KTAS Mach 0.56
40,000 ft 250 KIAS 505 KTAS SoS 591 KTAS Mach 0.85
Since the proportion of true airspeed to indicated airspeed doubles between sea level and 40,000 ft while the speed of sound decreases by 11 %, it is easy for high-flying pilots to become confused about how fast they are flying! You can see that both the numerator and the denominator in the Mach number are varying as you fly upward. that is why I said that a Mach limit is not exactly a speed limit. The reason Mach becomes important is that many characteristics of the aircraft change with Mach. These affect stability and controllability, two very big concerns. Also, the effects of Mach are extremely peculiar to particular designs because they are based on the actual distribution of pressure over the body surfaces near ailerons, elevators and rudders. The jet engine performance also varies significantly with Mach number. This can be scary stuff - especially when you find the ailerons fluttering and flying off the wing because you exceeded a critical Mach number. Fortunately, for pilots of many jets, including the Learjet 45, this can be simplified by remembering one simple number - 0.80. Just set this as a limit during cruise and you will be in good shape.
I have heard jet pilots say the speed they fly depends on the temperature. Temperatures at altitude higher than normal mean a slower cruise speed. This only applies if you are flying with a Mach limit. If you hold a speed and let Mach vary a little, there would be no difference. The big question is "Can you hold speed constant rather than Mach?" Most pilots are taught to honor Mach over speed. I am not sure that is necessary for the slight variations one would encounter. I would honor fuel flow more than Mach. How does fuel flow vary?
In order to hold either a Mach limit or a speed (KIAS) limit, the autopilot will adjust the throttles. This brings up a problem of how to set a Mach limit in the AP control head. Setting a speed limit is easy. I was concerned about how you set a Mach limit and switch to Mach mode. I just did it in the LJ45 and it did not crash though its cruise was interrupted a bit. I saw only 000 in the window. There was no way to enter .80. When I clicked on the MACH button, this display changed to .00 but the throttles were suddenly pulled back. By the time I clicked the number up to .800, the aircraft had slowed considerably to M.74. There is a better way.
SETTING A KIAS LIMIT
With the aircraft flying on autopilot near the desired speed condition, just click on the numbers in the SPD window until they show the desired number. If you are flying your LJ45 at 40,000 ft, use 245. Then click on the SPD button. Be sure to set the speed value BEFORE clicking the SPD button. Don't touch the throttles because the autopilot will be controlling them. You can set a KIAS limit and then do a climb or descent. The autopilot will try hard to hold the limit. But if you have selected a speed too high to be maintained during a climb, the throttles will go to full thrust and your limit will not be held. After trying this for both a climb and a descent, I'd say you are better off just turning off SPD or MACH mode, making the climb just using altitude hold and a manual throttle adjustment, and then turning on the SPD or Mach mode after the change in altitude has been made. Trying to hold Mach or Speed does not seem advantageous.
SETTING A MACH LIMIT
The smooth way to do this is to set a SPD limit first. Pick a KIAS value close to what you are flying, enter it and turn on SPD mode. Then click on the MACH button. The current Mach number will replace the speed value you had set. This way there is no disruption in the thrust.
I tried to develop some exercises to show how this mode works. The Baron and the Beech 350 do not have this mode. It is not in their sim program. This mode - SPD or MACH - can only be used with the jets. So I tried a constant airspeed climb in the Learjet. It didn't work. I normally do a max rate climb in jets. This is similar to the optimal climb schedules produced by the theoreticians but it becomes a bit crude in practice. Above 10,000 ft where the 250 KIAS limit does not apply, you reduce the climb rate in HDG and ALT mode and let the aircraft accelerate to some speed above 300 KIAS but well below its VMO. Then you set the climb rate high - 4000 fpm for the Learjet 45. As you climb you lose airspeed so you drop the climb rate in 500 fpm increments until you are climbing at about 1000 fpm above 35,000 ft for a max gross takeoff.
Some people say you should be able to do this holding about 220 KIAS all the way. Thrust would be about 90% and the climb rate would start very high and then would decrease gradually. But it does not work out that way. Starting slow means your induced drag is high and that limits your ability to climb. I tried this in the Learjet 45. I was trying to use ALT mode as well and that may have caused some conflict. But the main culprit was the SPD mode control law that depends on thrust to control speed. In a climb in a jet, you need full thrust all the way or at least as much thrust as the engines can develop safely. (Setting 90% throttle seems to be okay.) You regulate speed not by thrust but by climb rate and the resulting pitch angle.
I tried this constant speed climb again using manual control. I did set the autopilot in HDG mode so I would not have to worry about staying on course. My intention was to just use the stick to control speed and climb rate. I held 220 KIAS after takeoff and cleanup and used 90% thrust. The vertical speed was 4000 fpm at first. But I was fighting the trim. I kept pushing it nose down and it kept coming back. With only HDG mode engaged I could not understand what the autopilot was doing. Finally I turned off the autopilot and managed fairly well to about 24,000 ft. I was hoping to go all the way to 40,000 ft. But it gets tricky in the mid 20's. Vertical rate has to drop a fair amount and the tendency is to drop it too much and then get into a phugoid. Use the autopilot in HDG and ALT mode is the most orderly way to climb a jet. You continuously reduce the set climb rate limit to keep airspeed from dropping too quickly. Your airspeed will start at about 310 KIAS and drop to about 230 KIAS at 30,000 ft. Then you can climb steadily at 1000 fpm to 40,000 ft or just above for an initial climb.
Some jets like the Gulfstreams and the Falcons have plenty of climb power so you can use higher airspeeds and climb rates longer into the climb. Still, if those are flown on international flights with full fuel, your climb will be staged - 35000, 39000, 43000 and maybe 45000. At each stage you burn off fuel. on domestic flights with full pax they will zoom right up to 45000 ft in grand style. This simply means it will take longer before you have to drop the climb rate by 500 FPM.
Tom Goodrick
Re: Understanding The Autopilot
« Reply #5 on: Mar 6th, 2008, 12:25am »
PART 6
APP MODE
APP mode is one I seldom use. I have used it occasionally in past versions of FS but not recently. There are two reasons. First You have to land most planes anyway so it is best to be in control all the way down final. The second reason is that not all aircraft can use the approach mode.
To see what happens in FS9, I set the Baron up in an interesting scenario. I put it at Albany, New York and did some VFR flying in the vicinity. There are some tall hills in that area so 2500 ft is a good minimum altitude when flying generally though you can fly the approach to KALB runway 1 down to 100 ft with the proper equipment. I turned on the Visual Flight Path and set medium size rectangles and medium length. Then I flew a downwind leg out to 10 miles and turned to intercept the ILS localizer at 30 degrees to its central direction (heading 040 to enter from the southwest). I had trouble getting the aircraft to enter the approach mode properly. I flew the approach manually. I decided to do some testing of the parameters marked "gs" in the aircraft,sfg file. I also decided to make it interesting by setting the weather to "Heavy Snows." I set up an entry point and heading for the approach and saved that Flight situation.
I tried several value changes of parameters and thought some helped. But the main difference was how I entered the approach. I had to be fairly well off the center, flying to intercept the centerline at about 30 degrees to it. In several instances the aircraft lined up properly but was high, flying well above the boxes. I had to take control early, cut the power and drop down steeply to make a landing. There was one case that worked all right. I had flown under the glideslope before turning on APP mode but speed wa already quite low. The plane lined up and flew down the center of the glideslope.
I gave up trying to find adjustments to the parameters. I put the Beech 350 and the Learjet 45 into the same situation as the Baron. They did fine. I was busy managing power, gear and flaps. But, they intercepted the localizer and flew down the the glideslope in the center of the rectangles. There was a notable crosswind and they were heading right of the track. The track was straight. Taking over manual control was very easy. I turned off the autopilot but no immediate change in control setting was required. I simply waited to get low enough to start the flare and then made a nice smooth landing.
I still do not expect to use this mode much. But if you want to try it, just be aware that you must do your own flaps, gear and power. Arm the spoiler in the jets. I recommend using the rectangles on any approach. Just keep the speed right and keep the plane centered in the rectangles. If the AP does not do this right, do it yourself.
The parameters were exactly the same for the Baron, Beech 350 and Learjet 45 yet they all worked - the last two working best. It makes you wonder what the parameters do. The Baron lands at about 86 KIAS, the Beech 350 lands at about 98 KIAS and the Learjet lands at about 129 KIAS.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~
YD Mode
The AP has a YD or Yaw Damper mode. This can be used independent of any other mode you are using, including manual control. It simply reduces the yaw excursions that may happen during bumpy air conditions. Many jets MUST be flown with YD ON above 30,000 ft for safety. Yaw instability is common at the high altitudes. A slight yaw oscillation can get larger and become troublesome.
If you turn on the YD mode and nothing seems to happen, look at the aircraft.cfg file. You should see the line:
yaw_damper_gain = 1.0
This is all you need to make the yaw damper work. I have played with the directional stability of some planes, making it worse than it normally is. The Yaw Damper makes it behave well. Some aircraft need it whenever you fly in RW.
~~~~~~~~~~~~~~~~~~~~~~~~
BC and LVL mode:
BC just means "back course". It reverses the sense of the localizer display. On an ILS this cannot be used because the glide slope points to the wrong end of the field. It is used in an approach where there is only a localizer showing where the runway is.
LVL just keeps the wings fairly level. I never use it because I want to be sure the airplane is going where I want it to go. It can turn slightly in LVL mode but not in HDG mode. Use HDG mode and "diddle" the heading numbers with your mouse to make turns.
Understanding The Autopilot
« on: Mar 6th, 2008, 12:18am »
How Automatic is the Autopilot?
PART 1
Several questions recently have prompted me to discuss the matter of the autopilot. How automatic is it and how should it be used in FS9? I found an old article I wrote on autopilots in 2003 and it had a bunch of obsolete information. So it is time for a new article.
I looked into how the autopilot works and how you can control it in its several modes. I tried to find how you can adjust it to fit particular aircraft. In this I was mainly unseccessful. Mostly, it works. If it does not work with a particular aircraft, the solution lies in adjustment of the FD the same way we do for better handling and for realistic performance. If an aircraft works well when you fly it manually, it will work well on autopilot. Do not hesitate to use the autopilot when flying cross-country. It offers the best way of managing climb to cruise altitude, holding cruis altitude and attaining and maintaining the best speed, and flying the best track to the destination. It can also be used very well to fly vectors during a departure or an approach. In some cases it will do an ILS approach but you are better off flying approaches manually.
Part 1 is an introduction to the "hardware" you see in the panel.
Part 2 covers the altitude mode or ALT mode.
Part 3 covers the heading mode or HDG mode.
Part 4 covers the navigation or NAV mode.
Part 5 covers the SPD/MACH modes.
Part 6 covers the the approach mode and other odds and ends such as LWL, BC, and YD.
Below we will discuss the use of the Autopilot in ALT mode, HDG mode, NAV mode, SPD mode and APR mode. I'll use some examples involving the Baron 58, the Beechcraft King Air 350 and the Learjet 45. For each of these there is a download on my web site that improves the FD. For the Learjet 45, the download is especially helpful because the original FD causes it to behave poorly on autopilot. For simple Cessnas, the autopilot is of little use and isseldom necessary. But, it could be considered necessary if you fly solo IFR in any airplane. it certainly is needed in more complex aircraft to reduce the pilot's work load. Many jets are not certified to be flown without a working autopilot or must be operated low and slow if the autopilot is malfunctioning. Using the autopilot in no way reflects poorly oin your pilot abilities. It is a necessary tool when flying complex aircraft with high cruise and approach speeds. it is vital to the new reduced altitude separation rules where aircraft on opposite headings may be separated in altitude by only 1000 ft.
In FS there are several different "control heads" for the autopilots. These are units you see in the panel by which you can control the autopilot. There are the old ones that were used in singles in a pull-down image that included the radio stack. In my panels I use the more sophisticated units that were mounted in jets. I think you need the control head mounted as a permantent part of the panel because you always need to be able to see what the settings are at a glance. You cannot cover up another vital part of the panel by pulling something down on top of it. Of course thei requires a PFD display to show all the vital flight instruments in a compact manner so there is enough space to put the control head as well as the GPS map which should always be visible as well.
But you should know that the control head must work with elements in the air file for the particular aircraft. Not all the switches and settings will work with all aircraft. The gauge coding relates to settings in the air file. Some parameters are in the aircraft.cfg file. It is not clear how these work.
But can the autopilot be trusted to fly the airplane completely with no supervision? No, not in many cases. A competant pilot, fully able to take manual control at any stage, must be watching all gauges all the time. He must interrupt at any unexpected deviation from normal procedure and performance. He can guide the procedure with heading adjustments, thrust adjustments and vertical speed adjustments.
The first rule for using an autopilot is DO NOT ACTIVATE IT until you are sure you are ready. An autopilot should NEVER be turned on when the plane is on the ground. That mistake has killed many people. In FS it can mess you up so that you don't know what is happening. Of course I have done that. I have interrupted a flight while the autopilot is on, popped the airplane down to a location on the end of the runway at a faraway airport and then tried to takeoff, wondering why the plane is climbing to a stall and turning furiously to the left.
Always prepare the autopilot by setting the parameters for whatever mode you intend to use before you activate the mode and/or the autopilot. When activated, the autopilot will try very hard to do what you have commanded. make sure you are ready for that.
In my Baron, the autopilot "head" is always visible. This is the part you use to control the autopilot. On the left are two buttons AP and FD for Autopilot and Flight Director. For now we can ignore FD. We will click on or "press" AP when we want to turn the autopilot ON. To the right of FD is a window with a number in it and below that is a CRS knob. In real life you rotate the knob to change the course to be displayed. In FS you put your mouse pointer on the left or right side of the number and press the mouse button to decrease or increase the number. This applies only to the Instrument Landing System (ILS) localizer. That number agrees with the heading you will fly toward the runway. To the right of it is anothe number with a knob below labelled "HDG. This can hold the heading you want the aircraft to fly in HDG mode. Between the two numbers is a button labelled "NAV" that turns on NAV mode. To the right of the heading number is a button labelled "HDG". Press this to get into HDG mode. You can change the heading number and all other numbers on the autopilot by putting the mouse arrow on one end of the number and clicking or holding the mouse button. Below HDG is APP, another button that turns on APP mode. In the column of buttons to the right of HDG is BC (turns on "back Course", LVL (turns on the wing leveler) to keep the wings level, YD (turns on the Yaw Damper) that reduces unwanted yaw oscillations, and to the right of those buttons are MACH (setting Mach speed limit) and IAS (setting indicated airspeed speed limit). A number to the right of MACH can hold the Mach limit or the airspeed limit. Next you see ALT, a button that sets ALT mode. To its right is a number showing the altitude you want the autopilot to hold and farther to the right is a window showing the vertical rate limit you want to impose during the transition to that altitude.
Tom Goodrick
Re: Understanding The Autopilot
« Reply #1 on: Mar 6th, 2008, 12:19am »
PART 2
ALT MODE
The most basic autopilot mode is ALT mode in which the autopilot attempts to make the airplane climb or descend to a particular altitude and then maintains that altitude as long as the autopilot is ON and ALT mode is ON. To use ALT mode you must set an altitude to be held and a vertical rate limit to be used to get to that altitude. This is the key to controlling the aircraft for safe flight. The autopilot does not know what power is needed or what airspeed is safe to fly. You must set the power and monitor the airspeed. You can change the allowed vertical rate as you fly to keep the right airspeed. You can adjust the throttle to maintain the climb or to avoid over-speeding during descent or getting too slow during a climb.
How does the autopilot control climb? It simply increases or decrease the pitch trim until the vertical rate reaches the value set in the window. Then it continues to adjust the trim to maintain that rate until the altitude set in the window is nearly reached. It reduces the vertical rate slightly before reaching the selected altitude and then works to hold the altitude. I tried setting various values in the aircraft.cfg file for the pitch parameters. Nothing seemed to make a difference. I doubled them and increased them by 10x. There was no change in climb performance. I would assume that, if the FD is set for proper CG and for proper pitch trim sensitivity, the autopilot will work well during climb or descent. But it requires that you know how to adjust the proper in a reasonable way during climb or descent. You must select a reasonable vertical rate and then must set the power properly. In the case of a normally-aspirated engine like the Baron's, you must also adjust the mixture as the aircraft climbs to maintain the max fuel flow or power. When descending you must set a sensible power setting . For the Baron this would be 15 inches and 2100 rpm.
BARON
Lets do a basic climb on autopilot that is commonly done on takeoff in IFR conditions. The controller will instruct you to "Maintain runway heading and climb to 8000 feet." We will do this in the Baron while taking off from runway 18L at KHSV. While the Baron is parked, set 8000 in the altitude window and 1000 in the vertical rate window. The aircraft can safely handle this rate while accelerating and while you check that the gear is up and bring the flaps up. Later during the climb we will raise this to 1000. The FD designer has left a value in the files that the aircraft will go to if you don't have anything in the window. But it is best to think ahead and put a positive number in there the aircraft can handle. I have been unpleasantly surprised to find a negative climb rate in there several times. When you are trying to get a few hundred feet above the ground and clear of the trees, power lines and hills beyond the runway, you don't need a negative climb rate. We set the heading 180 in the HDG window because from runway 18, that is the heading we will be flying. You should also think ahead and put the number 180 in the course window and dial the freq 111.9 in the Nav 1 frequency slot. That way you will have the return ILS all set in case an emergency develops and you have to go around and land as soon as possible. The last thing you need with an emergency is to be changing numbers or even looking them up with smoke in the cockpit, etc.
We make the takeoff as we normally would and get the gear up. (Flaps up if you use them.) As we pass 400 feet on the radar altimeter (or 1000 ft on the baro altimeter) we punch the HDG and ALT buttons. We see the green lights and punch the AP button. (I unpunch the FD button because I don't like using it. I know what to do to fly the airplane. I don't want anything needlessly cluttering up the instruments.) I begin adjusting the power while watching the airspeed. I reduce the RPM to 2500 and then adjust the throttle looking at the power meter to set about 84%. Anything over 80% but less than 85% works for the initial climb. The power decyas rapidly as we get toward 5000 ft. I have to begin adjusting the mixture to maximize the fuel flow. Otherwise the airspeed will decay. At about 3500 ft I can set full throttle ad the power will stay in the right range though I must keep leaning the mixture a bit to keep the fuel flow and the power from declining significantly. As the aircraft reaches 5000 feet, I adjust the mixture a final time for max fuel flow and then back off on the throttle to set 75% power.
During the climb on autopilot I have to know how to keep the power properly adjusted. The procedure would be different for an aircraft with turbcharged engines like the Mooney Bravo or my Cessna 340. In that case no mixture adjustment would be required but I would have to reduce the throttle properly to keep the power level in the same range (80 to 85%). I must also watch the EGT or CHT temperatures. (I made sure before takeoff that the cowl flaps were open.) If those temperatures go high during climb, I must reduce the vertical rate to increase the airspeed and give the engines better cooling.
You can save this Baron flight situation (level at 8,000 ft) for future demonstrations. Next we will repeat the same takeoff and autopilot climb in the King Air 350 though we'll climb to 15,000 ft.
BEECH 350
Set up the King Air at KHSV 18L. (Weather should be on Clear for all test flights - no winds and no clouds). Note that the checklist for the 350 specifies flaps optional for takeoff. We will not extend the flaps. We must set 7 degrees elevator trim and use only 90% throttle for takeoff. The speed for rotation is 95 KIAS and takeoff is 105 KIAS. Set the autopilot up once again for 15000 ft altitude and 180 degree heading. This time use 1800 fpm for vertical rate as noted in the checklist for climb that you must read before takeoff). The condition levers should be at low idle. (Use the "mixture" to adjust this. It becomes the condition lever in a jet turboprop like the 350.)
Throttle to 90%, watch for 95 KIAS, rotate and let it fly off. I set the stop watch on the timer as I shoved the throttles to 90%. Just as the aircraft was climbing through 400 ft, I turned on the AP. The aircraft accelerated to 210 KIAS and climbed smoothly. I noticed that 90% throttle gave 92% power. In a turboprop, ITT is the parameter to watch on takeoff and climb. It should never exceed 800 C. The value peaked near 750 and settled down during the climb to about 640C. In 8 minutes we were level at 15000 ft. There was no fuss about the climb airspeed, the temperatures or the airspeed. The 350 is a very well-behaved aircraft. Set 80% power for cruise at 15000 ft and then save the flight for later use.
LEARJET 45
Next look under the Bombardier menu and select the Learjet 45. Put it on runway 18L at KHSV. Set the AP altitude at 15,000 ft and the heading at 180. Set the vertical rate at 4000 fpm. Do not turn on the autopilot. (Make sure it is off.) Bring up the checklist and go through it. Weight is very important in jets. While we will make a takeoff at MTOW, we will have to dump fuel if we want to land. The checklist says to make sure the weight is below 21500 lb. Yours probably is not below that weight. That is because FS always puts 100% of fuel into the center tank. Look at the page for fuel and payload. It will show someting like 715 gallons is the max fuel load you can carry this trip. Look at the fuel. Subtract the amount in the left and right tanks from 715 gallons. The result is the max you can put in the center tank. Enter that value (in gallons) into the center tank. Now you should be ready for a takeoff at MTOW. Leave the AP set for HDG and ALT with the numbers listed above in the windows. But leave the main AP switch OFF for takeoff.
The checklist says to set one notch of flaps (8 degrees) and 8 degrees of elevator trim. Do that. Note that you see 124 KIAS in the window on the left of the airspeed indicator on the PFD for V2. You will rotate at 124-10 and fly off at 124 KIAS with the present weight. The checklist says you will raise gear on positive climb and accelerate to 180 KIAS before reducing thrust to 80% and going to 4000 fpm climb. That is when you can hit Z to turn on the autopilot. Let's do that.
My takeoff was fine but the transition to climb was not smooth. I had to unset and reset both the HDG and ALT buttons after turning on the autopilot. I guess we should leave those off until we turn on the AP. Then hit AP, HDG and ALT at the same time. The airspeed should never exceed 250 KIAS until we are above 10,000 ft. Then, as the checklist indicates we go to 2000 fpm and 100% throttle. But we will stay at 2000 fpm and 80% throttle until we level off at 15,000 ft for maneuvers.
If you have trouble with any of these exercises, do them until you are satisfied. Climbing steadily in the Learjet is the toughest, especially when you go all the way to 41,000 ft. For now throttle back to 50% and save the aircraft in level flight at 15,000 ft for the next set of exercises.
Tom Goodrick
Re: Understanding The Autopilot
« Reply #2 on: Mar 6th, 2008, 12:21am »
PART 3
HDG MODE
BARON
HDG mode is the simplest turning control mode. You set a compass heading into the "HDG" window. Then the autopilot will try to turn the aircraft to that heading. Return to the Baron in level flight at 8,000 ft and make sure it really is in steady flight. Then set 090 into the HDG window. (The HDG button should already be on and a turn to the left will commence.) You are in both HDG and ATL mode as the autopilot holds altitude and turns to a heading of 090 (east). The autopilot calculates the diferents between the present heading and the desired heading, treats that difference as an "error" value and takes steps to drive that error to zero. It does this by rolling in the direction according to the sign of the difference (right for positive difference, left for negative difference). There is a max bank angle in the aircraft.cfg file that is held by the autopilot until it gets near the desired heading. As the difference diminishes to a key value, the bank angle becomes proportional to the difference. This results in a smooth roll out on the new heading. You may notice that, during the Baron's turn, its airspeed decreases slightly and the pitch trim changes slightly. This is because it loses some lift coefficient while banked (that lift is helping to turn the aircraft) and must compensate for the loss of lift by increasing the pitch trim. This will in turn slow it down slightly as drag increases in the pitch change. When the turn is completed the speed comes back and the pitch trim returns to its value before the turn was started.
The max bank angle is normally set at 25 degrees because that works for a smooth turn in most aircraft. However, I have changed it to 35 degrees in many jets because of the low turn rate at high jet speeds. But this must be checked in various conditions because it can lead to an upset condition. The turn rate depends both on bank angle and on true airspeed. Hold bank angle constant and increase airspeed and the turn rate will decrease notably. Hole true airspeed constant and increase bank angle and the turn rate will increase. But the amount of pitch-up required to maintain altitude will also increase resulting in a loss of airspeed. It gets complicated quickly so this is something to be careful about. At high altitude where jets normally cruise, this speed upset becomes very significant and can even result in the autopilot losing control.
Now with the Baron flying east at 8,000 ft, set the heading to 270 and see how well it manages both the turn and holding altitude. OK. That was easy. Now set a heading of 60 degrees and an altitude of 4000 ft. Set a vertical rate of 500 fpm. (This is the max downward rate you should use in any non-pressurized aircraft like the Baron 58.) We will give the autopilot a challenge of making both a turn and a descent at the same time. But will will have to help by adjusting the power properly. Set 20 inches and 2100 rpm. As you reach about 4500 ft, start a left turn to 360. Then when reaching 4000 ft, set 2500 RPM and 75% power which will be reached at 23.5 inches. But then remember you must adjust mixture again to get maximum fuel flow. Do this and then readjust the throttle to give 75% power. I ended up with 23.77 inches, 17.56 gph, 2500 RPM and 74.99% power.
Continue and let the aircraft get fully steady. Next set a heading of 090 and an altitude of 9,000 ft. Use 1000 fpm as a climb rate. Set full throttle. Don't forget to lean the mixture for max fuel flow every 2000 ft or so. Again, the Baron just does what you want it to do with no fuss. It is a very nice aircraft for those who need to carry more weight than a fast single will carry or who want the extra engine when flying over water, over desert, over mountains and at night and general IMC.
BEECH 350
Go through the same turn exercises in the Beech 350 - level from 180 to 90 and then to 270. For a level flight power setting, reduce the RPM until N2 is below 100%. You can set 80% power. Mine did those turns with ease. Now try turning to 060 and descending to 8,000 ft. Set set -1500 fpm and adjust the throttle to keep the airspeed at 230 KIAS. Level off with 70% throttle and turn to 360.
Next set 80% power, 15,000 ft with 1500 fpm and turn to 090. My aircraft does this also with no problems.
LEARJET 45
Return to the Learjet we left in level flight at 15,000 ft. i set 55% so the airspeed became steady at 280 KIAS. Now turn from 180 to 090 degrees. Now turn to 270 degrees. Mine did a good job with these. Now set the altitude at 40,000 ft, the climb rate at 4000 fpm and the throttle at 90%. As you pass 25,000 ft, turn to 360 degrees. As the airspeed drops below 300 KIAS, set the climb rate at 3500 fpm. Set 3000 fpm as airspeed drops through 295 KIAS. Continue with this adjustment as needed. Mine was climbing at only 1000 fpm from 31,000 ft upward. But remember that we started at full gross weight. We made it to 40,000 ft for cruise. Many jets cannot climb that high after a full gross weight takeoff. They must level at something like 35,000 ft and burn off some fuel before continuing. Aceelerate to Mach 0.80 and then reduce the thrust to about 80%. To optimize range in the Learjet 45, you must work up to higher altitudes as fuel burns off. The max altitude for cruise is 51,000 ft. I have never gotten the Learjet 45 that high. It always seemed that the fuel burned in climbing was not worth the reduced fuel consumption at higher altitude once the aircraft was cruising in the low 40,000 ft range.
Now we have another turn check. When the aircraft is steady at 40,000 ft. do a turn to 180 degrees. Well, this was another ho-hum for this aircraft. If you are flying a default Learjet, you might have another story to tell. Save the Flight with the Learjet steady at 40,000 ft. We can use this when talking about SPD mode.
AP TURN PARAMETERS
The parameters in the aircraft.cfg file pertaining to turns are:
max_bank=25.0
max_bank_acceleration=1.8
max_bank_velocity=3.0
These are the same for the Baron 58, the Beech 350 and the Learjet 45. They are even the same for the Boeing 737-400 and the Cessna 172SP. This seems to suggest that you don't have any reason to make adjustments to them. I fixed a problem in the Learjet 45 from the default FD files. But I did it not in the AP lines but in the pitch trim sensitivity. It seems that the trim sensitivity can make it difficult for both you and the autopilot to control the aircraft.
Tom Goodrick
Re: Understanding The Autopilot
« Reply #3 on: Mar 6th, 2008, 12:22am »
PART 4
NAV MODE
A few years ago I was warning people not to use NAV mode. I thought it did not properly correct for wind. I was wrong. It does correct for wind better than you can. (It senses and adjusts to wind changes continually as you fly if you have loaded Real Weather.) Now I use it most of the time on most cross-sountry flights in RW. I set and use HDG mode for departure and for arrival in the vicinity of the destination airport.
To use NAV mode, you must create a flight plan for the trip from the departure airport to the destination airport. It can have none or several waypoints between those two airports. (The need for waypoints can be for fuel stops or to stay out of restricted areas.) if you use ATC while flying IFR, you will be given vectors for departure and arrival. These must be flown in HDG mode. ALT mode handles all altitude assignments. But after the initial one or two vectors after takeoff, the controller will say something like navigate at your descretion or he will give a heading that matches the heading on the flight plan. Even if you are slightly off the path set for the flight plan on the GPS, you can activate NAV mode on your autopilot.
To activate NAV mode you must set two switches:
1) Set NAV/GPS to GPS.
2) Click the NAV button on the autopilot head.
When NAV mode is properly activated, the aircraft will disregard the heading number and will turn in such a way that it intercepts the path for the flight plan. You can watch this happen on the GPS map. You will still have to adjust power as needed for any climbs or descents and to enter cruise. The autopilot will head into the wind just enough to keep the flight path straight along the flight plan. In RW, the wind speed and direction change with both altitude and time. You could not possibly do as well as the autopilot in NAV mode.
When you get close to the destination, ATC will give you vectors for the approach. If not using ATC, you can give yourself vectors to get setup for the approach. If you stay with the autopilot in NAV mode too long, the aircraft will fly over the target and begin circling. That does not help you any unless you are away from the computer (shame on you!). Set the correct heading and go into HDG mode to handle the approach vectors. Also, be sure to turn the NAV/GPS switch back to NAV so you can see the ILS indications properly. If you see on the map that you should be close enough to see the ILS needles move but they do not seem right, you have probably forgotten to throw the NAV/GPS switch back to NAV.
You can do your own experiments with NAV mode. Use on your next cross-country trip. Use Real Weather too so the trip is more realistic.
NAV MODE in AIRCRAFT.CFG
These are the lines in the aircraft.cfg file that pertain to NAV mode:
nav_proportional_control=9.00
nav_integrator_control=0.25
nav_derivative_control=0.00
nav_integrator_boundary=2.50
nav_derivative_boundary=0.00
They are initially set the same for all aircraft. I have changed the first line a little for really big aircraft such as the A380 and C133. They generally work fine.
For many years, FS was ahead of the real world in navigation enhancements. We have had flight visualization for many years. That allows you to fly an ILS approach usiing an artificial display of rectangles. All you do is manage power, flaps and gear while keeping the plane flying through a series of rectangles down to the runway. This works great in very low ceilings as in fog and snow. I read recently that this is available now in general aviation aircraft for following the flight plan in NAV mode. But we cannot yet do that. We must connect the display to either NAV1 or NAV2 radios, not to the GPS. Maybe FSX can do that. The experienced pilot who described using this system (Richard Collins of FLYING), like it.
Tom Goodrick
Re: Understanding The Autopilot
« Reply #4 on: Mar 6th, 2008, 12:23am »
PART 5
SPD/MACH MODE
This mode is of use to you only if you are flying a jet like the Learjet 45. Even then it is not always necessary. But if you fly jets, you should at least learn about it. SPD mode means you want to lock in an indicated airspeed. MACH mode means you want to lock in a particular Mach number. You can do either. No, they are NOT the same thing.
We will presume that with your interest in jets, you are sufficiently experienced in flying to know about dynamic pressure - the basis for calculating all aerodynamic forces and moments acting on the airplane. By locking in an indicated airspeed, you are locking in a dynamic pressure at which the aircraft and engine(s) work well in cruise. To keep things simple, instead of talking about the dynamic pressure you are using, we talk about the indicated airspeed you are using. In jets, the fuel burn as you cruise is very significant if you are going very far. You may want to lock in an airspeed so that you are locking in a particular performance level and so you can make reasonable predictions about when you will get where you are going. You will, of course, be locking in a particular true airspeed as well since that remains a particular proportion of your indicated airspeed as long as the altitude remains constant.
As a general rule, jets fly based on KIAS below 30,000 ft and based on Mach above 30,000 ft.
MACH is new to pilots moving up to jets from lower and slower aircraft. The Mach number is the ratio of the true airspeed to the local speed of sound. By "local" I mean the speed of sound that would occur in the air imediately outside the aircraft. This varies with the temperature. The true airspeed as a proportion of the indicated airspeed varies with density. Both density and temperature vary with altitude. Here's a short table to think about.
sea level 250 KIAS 250 KTAS SoS 662 KTAS Mach 0.38
20,000 ft 250 KIAS 343 KTAS SoS 615 KTAS Mach 0.56
40,000 ft 250 KIAS 505 KTAS SoS 591 KTAS Mach 0.85
Since the proportion of true airspeed to indicated airspeed doubles between sea level and 40,000 ft while the speed of sound decreases by 11 %, it is easy for high-flying pilots to become confused about how fast they are flying! You can see that both the numerator and the denominator in the Mach number are varying as you fly upward. that is why I said that a Mach limit is not exactly a speed limit. The reason Mach becomes important is that many characteristics of the aircraft change with Mach. These affect stability and controllability, two very big concerns. Also, the effects of Mach are extremely peculiar to particular designs because they are based on the actual distribution of pressure over the body surfaces near ailerons, elevators and rudders. The jet engine performance also varies significantly with Mach number. This can be scary stuff - especially when you find the ailerons fluttering and flying off the wing because you exceeded a critical Mach number. Fortunately, for pilots of many jets, including the Learjet 45, this can be simplified by remembering one simple number - 0.80. Just set this as a limit during cruise and you will be in good shape.
I have heard jet pilots say the speed they fly depends on the temperature. Temperatures at altitude higher than normal mean a slower cruise speed. This only applies if you are flying with a Mach limit. If you hold a speed and let Mach vary a little, there would be no difference. The big question is "Can you hold speed constant rather than Mach?" Most pilots are taught to honor Mach over speed. I am not sure that is necessary for the slight variations one would encounter. I would honor fuel flow more than Mach. How does fuel flow vary?
In order to hold either a Mach limit or a speed (KIAS) limit, the autopilot will adjust the throttles. This brings up a problem of how to set a Mach limit in the AP control head. Setting a speed limit is easy. I was concerned about how you set a Mach limit and switch to Mach mode. I just did it in the LJ45 and it did not crash though its cruise was interrupted a bit. I saw only 000 in the window. There was no way to enter .80. When I clicked on the MACH button, this display changed to .00 but the throttles were suddenly pulled back. By the time I clicked the number up to .800, the aircraft had slowed considerably to M.74. There is a better way.
SETTING A KIAS LIMIT
With the aircraft flying on autopilot near the desired speed condition, just click on the numbers in the SPD window until they show the desired number. If you are flying your LJ45 at 40,000 ft, use 245. Then click on the SPD button. Be sure to set the speed value BEFORE clicking the SPD button. Don't touch the throttles because the autopilot will be controlling them. You can set a KIAS limit and then do a climb or descent. The autopilot will try hard to hold the limit. But if you have selected a speed too high to be maintained during a climb, the throttles will go to full thrust and your limit will not be held. After trying this for both a climb and a descent, I'd say you are better off just turning off SPD or MACH mode, making the climb just using altitude hold and a manual throttle adjustment, and then turning on the SPD or Mach mode after the change in altitude has been made. Trying to hold Mach or Speed does not seem advantageous.
SETTING A MACH LIMIT
The smooth way to do this is to set a SPD limit first. Pick a KIAS value close to what you are flying, enter it and turn on SPD mode. Then click on the MACH button. The current Mach number will replace the speed value you had set. This way there is no disruption in the thrust.
I tried to develop some exercises to show how this mode works. The Baron and the Beech 350 do not have this mode. It is not in their sim program. This mode - SPD or MACH - can only be used with the jets. So I tried a constant airspeed climb in the Learjet. It didn't work. I normally do a max rate climb in jets. This is similar to the optimal climb schedules produced by the theoreticians but it becomes a bit crude in practice. Above 10,000 ft where the 250 KIAS limit does not apply, you reduce the climb rate in HDG and ALT mode and let the aircraft accelerate to some speed above 300 KIAS but well below its VMO. Then you set the climb rate high - 4000 fpm for the Learjet 45. As you climb you lose airspeed so you drop the climb rate in 500 fpm increments until you are climbing at about 1000 fpm above 35,000 ft for a max gross takeoff.
Some people say you should be able to do this holding about 220 KIAS all the way. Thrust would be about 90% and the climb rate would start very high and then would decrease gradually. But it does not work out that way. Starting slow means your induced drag is high and that limits your ability to climb. I tried this in the Learjet 45. I was trying to use ALT mode as well and that may have caused some conflict. But the main culprit was the SPD mode control law that depends on thrust to control speed. In a climb in a jet, you need full thrust all the way or at least as much thrust as the engines can develop safely. (Setting 90% throttle seems to be okay.) You regulate speed not by thrust but by climb rate and the resulting pitch angle.
I tried this constant speed climb again using manual control. I did set the autopilot in HDG mode so I would not have to worry about staying on course. My intention was to just use the stick to control speed and climb rate. I held 220 KIAS after takeoff and cleanup and used 90% thrust. The vertical speed was 4000 fpm at first. But I was fighting the trim. I kept pushing it nose down and it kept coming back. With only HDG mode engaged I could not understand what the autopilot was doing. Finally I turned off the autopilot and managed fairly well to about 24,000 ft. I was hoping to go all the way to 40,000 ft. But it gets tricky in the mid 20's. Vertical rate has to drop a fair amount and the tendency is to drop it too much and then get into a phugoid. Use the autopilot in HDG and ALT mode is the most orderly way to climb a jet. You continuously reduce the set climb rate limit to keep airspeed from dropping too quickly. Your airspeed will start at about 310 KIAS and drop to about 230 KIAS at 30,000 ft. Then you can climb steadily at 1000 fpm to 40,000 ft or just above for an initial climb.
Some jets like the Gulfstreams and the Falcons have plenty of climb power so you can use higher airspeeds and climb rates longer into the climb. Still, if those are flown on international flights with full fuel, your climb will be staged - 35000, 39000, 43000 and maybe 45000. At each stage you burn off fuel. on domestic flights with full pax they will zoom right up to 45000 ft in grand style. This simply means it will take longer before you have to drop the climb rate by 500 FPM.
Tom Goodrick
Re: Understanding The Autopilot
« Reply #5 on: Mar 6th, 2008, 12:25am »
PART 6
APP MODE
APP mode is one I seldom use. I have used it occasionally in past versions of FS but not recently. There are two reasons. First You have to land most planes anyway so it is best to be in control all the way down final. The second reason is that not all aircraft can use the approach mode.
To see what happens in FS9, I set the Baron up in an interesting scenario. I put it at Albany, New York and did some VFR flying in the vicinity. There are some tall hills in that area so 2500 ft is a good minimum altitude when flying generally though you can fly the approach to KALB runway 1 down to 100 ft with the proper equipment. I turned on the Visual Flight Path and set medium size rectangles and medium length. Then I flew a downwind leg out to 10 miles and turned to intercept the ILS localizer at 30 degrees to its central direction (heading 040 to enter from the southwest). I had trouble getting the aircraft to enter the approach mode properly. I flew the approach manually. I decided to do some testing of the parameters marked "gs" in the aircraft,sfg file. I also decided to make it interesting by setting the weather to "Heavy Snows." I set up an entry point and heading for the approach and saved that Flight situation.
I tried several value changes of parameters and thought some helped. But the main difference was how I entered the approach. I had to be fairly well off the center, flying to intercept the centerline at about 30 degrees to it. In several instances the aircraft lined up properly but was high, flying well above the boxes. I had to take control early, cut the power and drop down steeply to make a landing. There was one case that worked all right. I had flown under the glideslope before turning on APP mode but speed wa already quite low. The plane lined up and flew down the center of the glideslope.
I gave up trying to find adjustments to the parameters. I put the Beech 350 and the Learjet 45 into the same situation as the Baron. They did fine. I was busy managing power, gear and flaps. But, they intercepted the localizer and flew down the the glideslope in the center of the rectangles. There was a notable crosswind and they were heading right of the track. The track was straight. Taking over manual control was very easy. I turned off the autopilot but no immediate change in control setting was required. I simply waited to get low enough to start the flare and then made a nice smooth landing.
I still do not expect to use this mode much. But if you want to try it, just be aware that you must do your own flaps, gear and power. Arm the spoiler in the jets. I recommend using the rectangles on any approach. Just keep the speed right and keep the plane centered in the rectangles. If the AP does not do this right, do it yourself.
The parameters were exactly the same for the Baron, Beech 350 and Learjet 45 yet they all worked - the last two working best. It makes you wonder what the parameters do. The Baron lands at about 86 KIAS, the Beech 350 lands at about 98 KIAS and the Learjet lands at about 129 KIAS.
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YD Mode
The AP has a YD or Yaw Damper mode. This can be used independent of any other mode you are using, including manual control. It simply reduces the yaw excursions that may happen during bumpy air conditions. Many jets MUST be flown with YD ON above 30,000 ft for safety. Yaw instability is common at the high altitudes. A slight yaw oscillation can get larger and become troublesome.
If you turn on the YD mode and nothing seems to happen, look at the aircraft.cfg file. You should see the line:
yaw_damper_gain = 1.0
This is all you need to make the yaw damper work. I have played with the directional stability of some planes, making it worse than it normally is. The Yaw Damper makes it behave well. Some aircraft need it whenever you fly in RW.
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BC and LVL mode:
BC just means "back course". It reverses the sense of the localizer display. On an ILS this cannot be used because the glide slope points to the wrong end of the field. It is used in an approach where there is only a localizer showing where the runway is.
LVL just keeps the wings fairly level. I never use it because I want to be sure the airplane is going where I want it to go. It can turn slightly in LVL mode but not in HDG mode. Use HDG mode and "diddle" the heading numbers with your mouse to make turns.