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Post by bkimoun on Dec 27, 2010 9:29:01 GMT -5
Hi all, I don't know if you have already noticed that an aircraft at trim ( hands off) in MSFS does not show any elevator deflection . Normally, the trim tab developes the necessary moment that deflects the elevator surface (by the way, trim tab and elevator surface rotate in opposite direction) and maintains the elevator at the deflection angle that trims the aircraft so that the pilot does not have anymore to pull or push the yoke (hands off) to maintain level flight. In other words, it is the pitching momentCm_de*elevator deflection that trims the aircraft and not the Cm_dt*trim tab deflection. The latter serves only as a substitution to the moment that the pilot exerts when dealing with the yoke to assure the trimming deflection of elevator. As well explained by the aerodynamic litterature, the trim tab (Cm_dt*trim tab deflection) serves to counter the aerodynamic force around the hinge( ie the hinge moment) of the elevator so that the latter can deflect by a certain angle. The role of the trim tab is then the same as pushing or pulling manually the elevator yoke. It appears unfortunately that MSFS makes a confusion between elevator hinge moment and elevator pitching moment. As one major bad consequence of this bug, if you keep the weight of aircraft constant (only payload, let's say aircraft with zero fuel and Illimitted fuel checked) and you move the cg location , let's say between 6% MAC and 36% MAC, the angle of attack alpha does not change (remains the same) and this violates the aircraft aerodynamics! Any comment about this bug?
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Post by Tom Goodrick on Dec 27, 2010 10:01:39 GMT -5
Have you looked at the "elevator pitch trim angle" while doing this? As I former pilot and a retired aeronautical engineer with 30+years of experience, I have found FS does manage these things quite well although you may not be able to see them. I normally fly with a pitch trim gauge and an angle of attack gauge, both digital, on the panel. My pitch trim gauge simply reads the pitch trim value in the sim. The AoA gauge calculates the angle of attack from velocity vectors and the pitch angle.
If you send me an email, I can send you the gauges I use so you can put them on your panel. But the codes are short so I could publish them in my next reply if you would like to do it that way.
Click the email icon under my photo to get my email address.
I cannot defend all things FS does. But for the most part the sim is technically correct. In steady flight, the angle of attack and the pitch trim must both change if the CG changes. (I also have a gauge that reads the CG position in the sim.
What program are you using to get this data? The program you are using may be incorrect.
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Post by louross on Dec 27, 2010 14:55:02 GMT -5
I'm just spiffballing off the top of my head here, but, tell me if this is right or wrong. (1) I'm flying along here in the RW, with a forward CG. All of a sudden, the CG goes way aft. To counter the change, trim nose down. The position of the elevator will change, but the angle of attack (wing) would remain the same. (2) However, I'm flying along with the CG wherever, and all of a sudden I lose a substantial amout of weight. The a/c tries to climb, so trim nose down to counter. The position of the elevator changes, and the angle of attack also decreases.
Elevator position-FS9: I looked at 3 default a/c, the C172, Lear, and KingAir. Ran the trim switches max forward and aft. No elevator change. Then checked the Dreamfleet 727, HJG DC8, and one other add-on (don't remember which one). Elevators moved to the correct position. lr.
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Post by Tom Goodrick on Dec 27, 2010 16:01:40 GMT -5
Lou your situations are: (1) I'm flying along here in the RW, with a forward CG. All of a sudden, the CG goes way aft. To counter the change, trim nose down. The position of the elevator will change, but the angle of attack (wing) would remain the same.
I would expect, and this can easily be demonstrated in the sim, the proper trim response you describe would be effective, but the angle of attack may change slightly. Various RW (Real World) aircraft have different ways of effecting trim control. Some use trim tabs, some use trimming stabolators, etc. In FS, if you are flying on autopilot, the trim will change with no change in elevator. The angle of attack will change if the trim adjustment changes the drag. Then to maintain level flight there would be a change in the angle of attack to maintain the weight at a slightly lower airspeed.
While many aircraft designers like to put all the payload at x=0, I set up several stations for the payload so it would be easy to shift a heavy weight suddenly from the front to the back and see what happens.
(2) However, I'm flying along with the CG wherever, and all of a sudden I lose a substantial amout of weight. The a/c tries to climb, so trim nose down to counter. The position of the elevator changes, and the angle of attack also decreases.
Yes, again this can be demonstrated witha number of aircraft in FS. Simply get an aircraft in level, steady flight with the autopilot and then take out most of the fuel.
Were you flying when you ran those trim switches full forward or back, Lou? If you do that while flying you'll see a change.
What the aircraft model shows has nothing to do with what the model in the sim does. The stuff you see is always just eye candy.
Here are the gauges I use when testing aircraft to see what happens. These cane be typed into to text files and then saved as XML files, placed into the Gauges folder and used. XML Gauge Files by Tom Goodrick
Angle of Attack
<Gauge Name="Digital_AoA" Version="2TG"> <Element> <Visible>(A:Circuit general panel on, bool)</Visible> <Visible>(G:Var2) 0 == (A:Avionics master switch, bool) &&</Visible> <Element> <Text X="50" Y="16" Bright="Yes" Length="5" Font="Arial" Color="%('#FF0000' '#00FFFF')" Adjust="Center" VerticalAdjust="Center" Multiline="No" Fixed="No"> <String>%((A:Vertical speed,feet per minute) 101.34 / (A:AIRSPEED INDICATED, knots) / asin 57.3 * (A:ATTITUDE INDICATOR PITCH DEGREES, degrees) + 1.5 - -1.0 *)%!4.1f!</String> </Text> </Element> </Element> <Mouse> <Tooltip>%Angle of Attack (deg)</Tooltip> </Mouse> </Gauge>
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Elevator (Pitch) Trim
<Gauge Name="Digital_ELVTRIM" Version="1.0"> <Element> <Visible>(A:Circuit general panel on, bool)</Visible> <Visible>(G:Var2) 0 == (A:Avionics master switch, bool) &&</Visible> <Element> <Text X="45" Y="15" Bright="Yes" Length="5" Font="Arial" Color="%('#FF0000' '#00FFFF' (A:ELEVATOR TRIM POSITION, radians) 0 >= ! ?)" Adjust="Center" VerticalAdjust="Center" Multiline="No" Fixed="No"> <String>%((A:ELEVATOR TRIM POSITION, radians) 57.3 *)%!4.1f!</String> </Text> </Element> </Element> <Mouse> <Tooltip>%Elevator trim</Tooltip> </Mouse> </Gauge>
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Center of Gravity %mac
<Gauge Name="Digital_CG" Version="1.0"> <Element> <Visible>(A:Circuit general panel on, bool)</Visible> <Visible>(G:Var2) 0 == (A:Avionics master switch, bool) &&</Visible> <Element> <Text X="45" Y="15" Bright="Yes" Length="5" Font="Arial" Color="%('#FF0000' '#00FFFF' (A:CG Percent, percent) 0 >= ! ?)" Adjust="Center" VerticalAdjust="Center" Multiline="No" Fixed="No"> <String>%((A:CG Percent, percent))%!5.2f!</String> </Text> </Element> </Element> <Mouse> <Tooltip>%CG Position</Tooltip> </Mouse> </Gauge>
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Total Aircraft Weight lbs (for small aircraft <100000 lbs)
<Gauge Name="Digital_WT" Version="1.0"> <Element> <Visible>(A:Circuit general panel on, bool)</Visible> <Visible>(G:Var2) 0 == (A:Avionics master switch, bool) &&</Visible> <Element> <Text X="48" Y="16" Bright="Yes" Length="7" Font="Arial" Color="%('#FF0000' '#00FFFF' (A:Total Weight, pounds) 0 >= ! ?)" Adjust="Center" VerticalAdjust="Center" Multiline="No" Fixed="No"> <String>%((A:Total Weight, pounds) 1 *)%!7.0f!</String> </Text> </Element> </Element> <Mouse> <Tooltip>%Total Weight (lbs)</Tooltip> </Mouse> </Gauge>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Nuts. This Forum removes spaces intended for indenting in the code. Not sure these will work without it.
These gauge files simply present a digital value read directly from the sim during execution and displayed on whatever background exists where they are placed on the panel. To create these gauge files for use woith FS9, just type each of them separately into text files and give them the name in the format filename.txt. Then save each again as name.xml which gives you aneditable text version and an xml version for operation in FS9. Place them in your Gauges folder in FS9.
To place a gauge on one of your panels, enter a line in the panel.cfg file, numbered in sequence with existing gauges, in the format: gauge47 = filename, xpos, ypos, width, height
Use other gauges for guidance as to where you want to place the values. You will probably not need the width and height values but they are shown in case you do. Units are the same as in the xpos and ypos values. You may wish to use PAINT to make a spot on the panel background where the values are easily read. You can label the spot where the number appears.
These gauges will show the values during ground prep and during flight.
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Post by louross on Dec 27, 2010 20:35:09 GMT -5
Tom, Okay, I'll buy all that, except the part about demostrating it in the sim, I have to take with a caveat. There are things that can be shown in FS, and also things that can be done on an instructional basis. But there are too many things that are not demonstatable in FS, so it's a case by case senario. Also, here's a question. The datum line for CG purposes is forward of the nose (RW). PMDG, the group that did the 744, says "the CG basis in FS is incorrect as they use the nose as the datum line. Therefore, they re-wrote the code for CG was in order to more accurately model their a/c." (paraphrased.) Do you have any comment on this? lr. P.S. I haven't re-done the AC500 with your new FDE yet, but thanks for sending it!
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Post by Tom Goodrick on Dec 27, 2010 22:44:48 GMT -5
Re: Datum lines, it only matters if you want to compare Real World loading data with FS loading data. That is why they let you specify a datum in FS - so you can get an exact correspondence if you want one. Most of us don't care about that; we just want proper balance in a relative sense. You can translate data from one coordinate system to another yet you will still come out with a CG in the right place relative to percent mac.
I demonstrated your first situation and found the angle of attack was indeed constant. I setup the AC500 at 5000 ft with2x230lb in the front seats and 2x60 lb in the back seats (6 place). In steady flight on autopilot the Trim was 0.2 deg and the AoA was 1.2 deg. The CG was at 23.69% mac. Then I had the people exchange place really fast (!) and the Trim went to -1.5 (nose down) and the AoA was 1.2. The speed remained at 164 KIAS before and after using 75% power. The CG was at 29.20 %mac.
For dropping a weight, I assumed a 200 lb pilot with 5x160 lb skydivers. The steady flight trim at 75% at 5000 ft showed CG 21.25% mac, Trim 1.4 deg, AoA 1.4 deg (5639lb). After the five skydivers departed very quickly, there was some commotion but it settled into steady flight at CG 27.40% mac, Trim was -1.4 deg, AoA was 0.9 deg and weight was 4834 lbs. (We dropped 14% of weight.) I didn't catch the speeds. There was a change of a couple knots.
Here are some numbers for the trim change with speed. Again the AC500 at 5000 ft, 5580 lb at start.
Powr KIAS KTAS Trim AoA 75% 163 174 1.4 1.4 70% 158 169 1.8 1.5 65% 153 164 2.2 1.7 60% 149 160 2.6 1.9 55% 143 153 3.2 2.2 50% 137 147 3.9 2.5
Of course, fuel flow is proportional to power.
The sim gives good demonstrations in most cases. The only bad case I know of is where the real aircraft goes into a stall/spin. There is a basic fault in the sim that does not consider distributed lift which is necessary in the case of spin entry. We can get close if we know a lot about the dynamics we are going for. I did fairly well on the Piper Cub although numbers are hazy after 50 years.
It is fun to try different demonstrations if you think of some.
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Post by Tom Goodrick on Dec 27, 2010 22:56:58 GMT -5
There are some specific conditions that are difficult and, perhaps, impossible to match. You found one in the beta mode for Turboprops that affects engine control during taxi operations. We reached agreement on the flight power operation once I figured out you wanted to hold throttle fixed and adjust power with the prop. That was a matter of adjusting the prop pitch variation range. The sim had the capability but people were not setting it up right.
Regarding CG, there is an area where transformation between Real World and sim CG coordinates gets a little hazy. A long time ago a pilot of the Learjet 45 got on here and wanted to see if we could match his loading data. He sent me a copy of his spreadsheet that he uses in the plane. I worked a bit and came up with an exact match in FS. (I think that was FS2000.) Again just a couple years ago a pilot of CitationJet 1's got on and gave me some real data. It took me quite a while to match his data. There was something about the way the sim places the leading edge wing datum for the %mac calculation that took some work to get right so the calculations were right for all loading configurations. There was still something there that bothered me.
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Post by Tom Goodrick on Dec 28, 2010 8:33:47 GMT -5
"As one major bad consequence of this bug, if you keep the weight of aircraft constant (only payload, let's say aircraft with zero fuel and Illimitted fuel checked) and you move the cg location , let's say between 6% MAC and 36% MAC, the angle of attack alpha does not change (remains the same) and this violates the aircraft aerodynamics! Any comment about this bug? "
So, mr bkimoun, have we addressed your issue to any extent? We did the weight shift problem and found there would not be a shift in angle of attack in most cases and this does not violate any principle of aerodynamics. Remember that no weight has changed; it has just shifted position. The total lift depends on the angle of attack. With no change in weight the lift has not changed and, therefore, the angle of attack remains the same. The only condition that might cause the angle of attack to change is if the trim change caused a drag change big enough to change the airspeed. Consider the moments about the wing 1/4 chord point. The shift in CG position means the moment of the weight about the wing has changed. But the tail can effect a moment that counters this change without requiring a change in lift (acting at the wing).
In FS there is no relation between any geometric change you see - or don't see - on the model and the flight dynamics. The flight dynamics are wholly governed by two files - the aircraft.cfg and the [name].air file. The person who does the geometric design that you see is often not the same as the person who sets up the flight dynamics files. They need different capabilities. The geometric designer decides how much time he wants to spend with details such as moving control surfaces. But the flight mechanics do not depend on it in any way.
I can put the flight dynamics files for a Piper Cub in the folder of a 747 and the 747 will start behaving like a Piper Cub. (It would leave its gear down and flaps up and take off at 40 knots.)
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Post by bkimoun on Dec 28, 2010 8:34:52 GMT -5
Hi Tom and thank you alot for paying interest to the issue that is annoying me. First of all I am using AFSD utility to monitore AoA , trim value, lift values, pithing moment values etc. I agree with you and I could verify that, AoA and trim vary as weight varies(ie as fuel burns), but suppose the case one flies the aircraft with zero fuel( illimited fuel checked in FS), that is weight constant.In this case and during flight, if one makes a change on cg location, normally, in the same manner trim and AoA change also.Why? Because, as stated by longitudinal stability equations, a change in cg position means a change in the value of the downward lift component of the tail responsible for the trimming pitching moment. In other words, the load supported by the lifting wing is not only the gross weight of the aircraft but : the gross weight of aircraft+down tail lift. Cl_alpha=(the gross weight of aircraft+down tail lift)/(q*Sw), so that for each trim value( ie down tail lift value), TBL 404 reads a value different. I think you agree with me, it is the elevator deflection that trims the aircraft and this means: an elevator trimming down lift force Cl_de*elevator deflection and an elevator trimming pitching moment Cm_de*elevator deflection. According to FS, the lift expression is: Cl= TBL404 value+Cl_flaps*flaps deflection+Cl_htail*incidence+Cl_de*elevator deflection+Clspoilers*deflection. And the pitching moment expression is: Cm=Cm0+TBL473+Cm_flaps*flapsdeflection+Cm_htail*incidence+Cm_de*elevator deflection+Cm_spoilers*deflection+Cm_gear+Cm_thrust.
Note that the trim moment expression called Cm_dt*trim tab deflection does not appear in the above equation and this means that it must remain hidden in the same way the force and moment developped by the pilot's wrist are. Because finally, the trim tab force and moment release what is normally supported by the pilot's wrist when aircraft not in trim and when AP not engaged. I suggest that you add an elevator deflection gauge besides the trim tab gauge.
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Post by Tom Goodrick on Dec 28, 2010 9:18:06 GMT -5
I do not have the AFSD utility. I use another utility that reads the parameters in the .air file. I had not seen those equations though I assumed equations like that were included in the sim.
So I see where you are concerned that you do not see the effect of trim in those equations. Well, the fact is that no one actually knows what the sim equations are. That has never been explained and never will be because the original sim designers did not explain everything to Microsoft when Microsoft took over the rights to the sim. I can tell you from my examinations that trim and elevator deflection seem to be additive though I have not seen a value for that total. I do have a gauge showing elevator deflection. I have not used it in a long time. I don;t recall seeing a trim effect. But now I will check.
I suspect there are a term or two that do not appear in your equation. But the flight performance seems reasonable on the whole. The down lift on the tail is generally considered negligible. Also, I am only reading AoA to one decimal. There could be a change below that level.
Is there any serious consequence to your trim/elevator discrepancy? I had thought the FS method was a good generalization that applies to a wide range of aircraft. Some RW aircraft do use tail incidence as a trim mechanism. Not all trimming depends on elevator deflection.
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Post by bkimoun on Dec 28, 2010 9:33:20 GMT -5
Hi again Tom, but just consider the case where the aircraft is flown manually with trim set to zero. In this case the pilot maintains a certain elevator deflection to have level flight and this situation lead to a lift component and a pitching moment component from elevator. How come then once the trim set to trimming value and hands off, there is no more a lift component from elevator deflection? A trim situation must then show an elevator lift component that loads the wing in the same weight the aircraft does.
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Post by Tom Goodrick on Dec 28, 2010 14:20:39 GMT -5
I just made a calculation using the Beech King Air 350 in level flight with an aft CG of 28.28%. Looking at the hstab offset, the wing apex offset and assuming the lift acts through the 25% mac point, the downforce at the tail comes out as 124 lb or 0.924% of the 13,445 lb weight. That does not seem significant to me.
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Post by bkimoun on Dec 29, 2010 3:35:50 GMT -5
Good Morning Tom! Really a tricky issue the aircraft trimming as modeled by Microsoft! Tom, do you agree with me that the effect of elevator trim tab is no more than a substitution to the pilot's effort on the elevator yoke, so that the pilot can fly the aircraft hands off?( ie if we suppose an aircraft without a trimming tab, the pilot has to exert a constant and exhausting pressure on elevator yoke to fly the aircraft).So wether the aircraft is flown trimmed with elevator tab or through pilot effort on elevator yoke(without trim), for any condition of flight(climb, level or descent), the lift and moment due to elevator deflection must be the same. And using AFSD utility (in the case of not trimmed situation, ie trim=0), we can check that the elevator lift component is significant to be ignored.
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Post by Tom Goodrick on Dec 29, 2010 8:54:33 GMT -5
The trim in FS is sufficient to enable realistic control of flight of a wide range of aircraft. I fly everything from slow piston singles to multi-engine jets. I use the autopilot for the main portion of any flight after the takeoff and before beginning the approach. I have onlyt flown slow piston singles in real life so I cannot speak for the realism of more advanced aircraft. But from what I have read of pilot reports for more advanced aircraft, the process is similar to what I do in FS.
The way the trim works in FS is fine in my opinion. I use the digital trim indicator as described above. I always set a takeoff trim value appropriate to the aircraft. I put that info into a checklist for each aircraft. After lift off I transition to climb with manual trim adjustment. My only stick input is at rotation. From climb until the autopilot can be engaged in climb, I manage speed and climb rate with pitch trim and power adjustment.
The autopilot simply adjusts the trim to achieve the specified climb rate (given manual adjustment of power) until the specified altitude is reached. Then I adjust power for cruise. Following descent near the destination airport, I keep the autopilot on until vectored to the final approach. Then I switch to manual where I primarily adjust power and trim. It is good that the transition from autopilot to manual trim control is very smooth.
I am not convinced that the equations you presented show all the terms used in the sim. I don't know who developed AFSD or how thoroughly they disassembled the sim code. I know they got no help from Microsoft. I am not even sure Microsoft understands their own code. They certainly do not understand conventional aeronautical engineering or they would not have removed the wing incidence variable when they developed FS9.
But I don't think the downlift on the tail is a big deal, if, indeed, they do not account for it. It could be in the code that AFSD did not see.
There are serious simulations of particular aircraft used in flight schools. They cost several thousand $$ per aircraft. If you are so concerned about this, you should use those sims. I am sure they would model the exact trim tab geometry.
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Post by louross on Dec 29, 2010 12:30:50 GMT -5
Here're several comments-
Ran a few checks with the Twin Otter (the Bluegrass model which we worked on). Full T.O. weight:
1) 10* flaps- FS model, the rotate and lift off was about like the real one.
2) 20* flaps- FS model acted just like the T.O. with 10*, which is not real- the real a/c requires very little rotation and the lift off is much flatter.
3) No flaps- FS model very easy T.O., not much different than with 10*, which is completely innacurate- T.O. at max gross weight in the a/c requires definite exertion to rotate, a lot of muscle, and the a/c will continue rolling on the mains before lift off. In fact, if you actually forget to set the flaps, begin rotating, the nose won't lift- you keep pulling, still nothing, you may think- heavy forward CG and pull hard, then grab the wheel with both hands and pull with effort as the other pilot sets flaps and the a/c lifts of.
Also- I'm sure we all use procedures that we would not use in RW. For example the technique of adjusting pitch with trim. I don't have any idea what GA pilots do, maybe depends on their training. But if flying for a company- airline- you adjust pitch with the control column, and then trim out the pressure. In flying right seat in 727's and 732's, I never saw a Capt adjust pitch with trim. Never had a copilot do it with the commuters. In the jets, it was customary to hand fly below 10,000, auto above 10. If we were going to shoot a coupled approach, then from 10 thou to heading for intercept might be hand flown or with autopilot. Some ILS approaches were done with the Flight Director engaged, others not- depended on the mood of the pilot flying. lr.
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