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Author Topic: FAQ: Elevator position, factors affecting trim, and trimspeeds.  (Read 6336 times)
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« on: August 19, 2009, 06:25:44 pm »

Since I keep finding myself posting these same sorts of responses I've decided that I'm going to put them here for safe keeping so next time I can just copy/paste!

Quote from: Zoring
It's not just the SE5a, and it is surely a bug, every single plane that i've flown (i haven't got the Nieuport 17)has its elevator tilted up by 10/15 degrees.

No, it's any airplane that has different elevator control surface travel in the aft direction than the forward direction.

The reason it isn't a "bug" per se is because the current control system we have was a choice made by the engineers to preserve the most accurate and realistic control system, but it does present issues for users who have non-FFB joysticks.

Understand that in most cases the real aircraft have greater travel of the control surfaces and flight control stick in the aft direction than the forward direction from aerodynamic center, while a spring centering PC joystick usually has equal travel forward and backwards, so in seeking to retain the real aircraft control ranges and limits as well as linearity of control responses in game, and directly proportional responses in game to the joystick inputs they have opted for the method that we have now.

The poster above with an FFB joystick will actually see that the joystick will tilt towards the monitor somewhat, but his spring centering joystick sits perfectly vertical. As a consequence of this, the FFB joystick leads to a more correct flying experience in RoF than a spring based stick does.

Also understand that there are sacrifices to be made for ALL users of spring based joysticks, and it does present an engineering dilemma trying to find the optimal solution for users of spring centered joysticks, while still trying to maintain real world aircraft responses.


Quote from: pierre
I was a (aerobatic) pilot in real life and if you had to fly a real a/c like this you would tire very quickly.

Please tell me what aircraft you flew that maintained trim throughout the speed range encountered during aerobatic flight? Or did you re-trim many times through each maneuver?

For competitive aerobatics I generally trim for approx. neutral trim by splitting the difference between the trim position required for upright flight and that required for inverted flight at a given speed. Of course with airpseeds ranging from 0 (or less than 0) up to Vne, there will be large control pressure changes, with no force being required only for an instant as you pass through the established trimspeed.

Quote from: pierre
I have never flown a WW1 a/c but I suspect one did not have to put up with flying with constant forward stick to stay level.

That's exactly what you would have to do in most of them. Any time you are flying faster than trimspeed you will need to apply forward stick pressure, and most of these designs have a trimspeed somewhat less than than cruise speed, so forward pressure at cruise speed is a given unless they have cockpit adjustable trim.


Quote from: [BFs]KaiserB_uk
I think you would [have to fly with the stick forward] - the SE5 operating manual as posted advises as such - however no pressure would be required to keep it there as aerodynamic forces would do that for you.

There would absolutely be muscle required to hold the stick forward. . .the aerodynamic forces will working against you, not with you. The greater the difference between the speed for which the aircraft is trimmed, and the speed at which you are flying, the greater the muscle required. This might be 10, 20, or 30 pounds in many cases.

The easiest way to "prove" an issue with this is to compare RoF to published trimspeed data. I've tested the aircraft for which I have this info and not surprisingly it does suggest that the RoF trimspeeds are too low in many of the designs, in the Pfalz this is actually below the normal climb speed, and nobody would ever trim an airplane like that! We also know that the DVII had a ground adjustable horizontal stabilizer (via spacers) and the Se5a had cockpit adjustable trim.


Quote from: 3DDevil
In modern aircraft, as others have pointed out, if you want to maintain the same altitude you use the trim wheel to trim the aircraft for neutral flight and the gain in throttle gives you extra speed.

Actually it's the opposite. Trim dictates what airspeed the aircraft will fly assuming no pilot inputs and throttle dictates what the airplane will do at that speed. Imagine an aircraft that has a trimspeed of 120 kmh, and suppose that 65% throttle gives you level flight. If you set power to 75%, the airplane will gently climb, and it will climb at 120 kmh. Set it to 100% and it will climb more steeply, but still at 120 kmh indicated on the airspeed. If you cut power to idle, it will descend, at, you guessed it: 120 kmh.

If you have cockpit adjustable trim, you must change the trim setting to increase or decrease the speed at which the aircraft will fly level, climb, or descend without force required on the elevator control. Nose down trim will increase the trimmed speed, nose up trim will decrease it. It is this way in which you change the speed of an aircraft, either with cockpit adjustable trim, or pilot applied inputs.

I know that is counter-intuitive at first, but that is how conventional aircraft operate. . .WWI or modern day. Though most modern day aircraft will allow the pilot to trim off the excessive control pressures to do this more easily and smoothly.

Quote from: 3DDevil
However I am POSITIVE that the UP ELEVATOR problem in the Fokker and others is a GAME PROBLEM/BUG and not what it was like in R/L. At neutral stick position I am sure the aircraft elevator should be neutral?!!

You are right that there is excessively nose up position of the elevator in some of the RoF aircraft, but only to the extent that the trimspeed of that aircraft is coded into the sim incorrectly (as this is actually the thing that should be driving the hands off position of the control surface). In real aircraft the elevator and the horizontal stabilizer rarely line up exactly when in flight. There are a lot of variables that effect the neutral position of the control surface such as airspeed, power setting, center of gravity, gross weight(as fuel burns CG and weight change) etc. In a speed optimized design rigged airplane, being flown at a specific speed, power, loading, CG, altitude etc the horizontal stabilizer may approximately line up with the elevator and yes it will be a little bit lower drag - but this is a pretty special case where an airplane will be built with this degree of precision as a real concern. In most cases, a pilot in steady state cruise flight will look back and see the elevator offset. I have some photos showing level cruise in various aircraft, none have a fully aligned horizontal stabilizer and elevator.

The bottom line is, don't worry about the visual representation of the control surface, as it will all work out in the air as long as the developers has set the aircraft up correctly with respect to trimspeeds. Having said that, I think that in general they do have issues with this in many of the aircraft in this sim.

Wow - that was far too long of a post for anyone to actually read now wasn't it?  :shock:


Quote from: [BFs]KaiserB_uk
So you're saying that, with your hands off the controls, the faster you fly the more the elevators deflect to put the nose up?

I know that's not what you're saying, do you see what I'm getting at?

Yes more speed = climb if above trimmed speed, but control surfaces will be neutral unless some input to the contrary is present (including trim tabs).

I might have missread what you posted, it sounded like you were saying that flying an aircraft in a condition which required forward stick inputs to maintain a constant flight path would not require muscle from the pilot (assuming non-adjustable trim). . .and that is incorrect, that’s all I was saying before.

With respect to the above quote, with hands off the controls there is no steady state condition of "faster" in a conventional airplane, as the aircraft accelerates above trimspeed it will very quickly pitch up to maintain that trimspeed, and it will oscillate around this speed until it reaches it, at least to the extent that it can do this without a loop or excessive speed resulting, airspeed will remain more or less constant

If you mean that mechanically the stick will center nearer to the forward stop in the Se5a then yes, that is correct (also true in most of these designs), but that is generally transparent to the pilot, and there is nothing magical about a control surface that is perfectly in trail with the fixed portion of the stabilizer either. What matters is what you must do to maintain the desired flight condition, and in aircraft without cockpit adjustable trim  at cruise speeds or higher that will almost always mean a constant forward pressure on the stick.


Quote from: 3DDevil
However I am POSITIVE that the UP ELEVATOR problem in the Fokker and others is a GAME PROBLEM/BUG and not what it was like in R/L. At neutral stick position I am sure the aircraft elevator should be neutral?!!

Here are a couple in flight photos that each show a steady state flight condition. All of these examples have the elevator trailing in the nose down direction.

In this photo of Citabrias we were at cruise speed in level flight. The lead aircraft had two people, I'm alone in the distant aircraft. Given the fact that we are at cruise speed, the expectation would be that nose down elevator would be required, and sure enough that's what the surfaces are trimmed for. The aircraft with two people on board has a slightly more aft center of gravity than the aircraft with only one on board, and actually has a slightly lesser deflection of the elevator as a result. If we were to slow well below cruise speed while maintaining altitude, these photos would show the elevator trailing in an upward direction.

This shot was taken in a steady state climb right after takeoff in an colorfully painted Fi-156 Storch replica. The aircraft was fully loaded towards the aft CG limit (we were flying across the country with gear filling the tail compartment). As you can see even in a climb the elevator is positioned in what would normally be thought to result in a nose down pitching moment. (Also note the position of the elevator trim tab)


Quote from: Jay_Oatway
I would love to know if people on the russian side of the forum have commented on anything about this.  Anyone here speak or read both Russ-Eng?

There are posts here about this from An.Petrovich. . .the summary of his explanation is pretty much to do with the fact that the real aircraft do not have uniform fore and aft travel of the stick from aerodynamic center.

This is a choice that they made to allow for linearity of control responses, the trouble is that a non force feedback joystick uses centering springs which hold the stick such that it does have uniform fore and aft travel, the result is that the position of the joystick commands a more nose up condition than the trimspeed would dictate.

I wonder what trimspeed tests would look like with a FFB joystick, and if they would better agree with known values. In either case, most users do not use FFB, and the trimspeed should be correct no matter which hardware people use provided that it's calibrated.


Quote from: An.Petrovich
Hallo ALL!
I've just read this thread.
This matter is regular, and I'm going to explain this accurately soon. BTW, thanks a lot for TX-EcoDragon who absolutely right posts about aerodynamic particularity of aeroplanes, and helps me much in this.

Quote from: 3DDevil
And An. Petrovich... I think I met you at the Monino Air Force Museum a few years ago... you look VERY familiar... and your English wasn't that bad at all!! Cheesy

Unfortunately, it was not me.
And I'm sorry for my bad English... I prefered to learn the aerodynamics and physics before. Smiley

. . . .

A short history:
Before I’ve developed the “Auto Level Flight” mode, all aeroplanes pitched up much right after you started a mission, If you started a mission in the flight. So, I was need to set the elevator (and the control stick in the cockpit, of course) in the balanced position for the level flight. And this position is around the neutral of elevator, it’s true. Moreover, this position was holding (freezing) while you take your joystick the first time.
Now, there is no necessary to preset this initial position of control stick and hold it, because now the aeroplane always starts in the flight with “Auto Level Flight” mode ON. So, I’ll eliminate this atavism in the next patch, I think.

BTW, you are interested: why when you have your joystick in neutral position, the airplane’s elevator is not? I’ll answer it soon, please wait a bit.

Neoqb Lead Engeneer


3DDevil, with a little more sleep I read the part of your post which Petrovich quoted and see that you might have been saying the same thing I was in my response to you, that you would have to combine trimming with throttle changes to increase speed, I read it as trim for level attitude, then use throttle to accelerate (the written difference is subtle - but it's there). This trim=speed/throttle=climb/cruise/descend concept is a common point of confusion with simmers and even many pilots.

Quote from: 3DDevil
Correct me if I am wrong, but to me your photo of the colourful plane on climbout, looks as if the ELEVATOR would be acting in a nose down pitching moment

That's exaclty my point. The position of the control surface is only one factor of the many that dictate what the aircraft is doing at a given point. As I said in my post before that one:

"In real aircraft the elevator and the horizontal stabilizer rarely line up exactly when in flight. There are a lot of variables that effect the neutral position of the control surface such as airspeed, power setting, center of gravity, gross weight(as fuel burns CG and weight change) etc"

Next time you are in a small aircraft, have a passenger slide their seat all the way aft and then watch the nose rise. The elevator will need to move further towards a "nose down position" to maintain cruise. . .if you are the one flying, you'll feel it even better.

The Fi-156 Storch is a tailheavy design in the first place, and the day this photo was taken I had a passenger in the rear seat (who took the pic) and the tail of the aircraft loaded with gear so we were also near the aft center of gravity limit. As CG shifts aft, the airplane tends to pitch up, as CG shifts forward, it will tend to pitch down. What you see here is simply showing how that all comes together.

Quote from: 3DDevil
. . .and the trim tab is way up in the nose up trim position, in effect fighting each other??... why would the trim tab be like this on takeoff?

The trim tab is "up" however it is not in the "nose up position. The way a trim tab enacts change to the elevator position is analogous to how the elevator enacts change to the airplane. In other words, when the trim tab moves up, it drives the trailing edge of the elevator surface down in the same way that the elevator moves up, driving the tail of the airplane down.

So trim tabs will pretty much always "fight" the position of the elevator - though this is functionally inherent to the design.  

Quote from: 3DDevil
.... You are obviously an expirienced pilot, but I too have flown aircraft and this doesnt seem correct to me? ...Are you sure that photo wasnt taken at the moment of pitch down to level out??

I was leaving Rolla MO trying to beat the thunderstorms to Kansas, and on my way to the West coast. . .we were full throttle, climbing at best rate of climb speed (47kt in a Storch!) and didn't level off until reaching cruise altitude a few thousand feet higher.

Quote from: 3DDevil
I still cannot understand how an aircraft can have, in your photo's case, down elevator position and be level or even climbing?

This topic is really too big for a forum post, but if you consider the way that a wing generates lift, and how forces balance in a steady state condition, and factor in what I've previously posted, you should come to see how it all works - and that the elevator position is not a fool-proof predictor of what the aircraft is actually doing.

Here's a related link:[/quote]

Quote from: Jay_Oatway
I know, I read those An P. posts a while back. But your reply stated it alot more clearly I must say! 1:  The elevator tab of any plane in game I have ever seen should not look like this when you are waiting to take off no matter what it does in flight.

You are right that in the real world a parked airplane with conventional controls will have the elevators deflected down due to gravity (unless a control lock is installed). Once again, the fact that they don't do that in the sim is because we have spring based joysticks which return to center even when there is no airflow over the control surfaces. Generally this is nothing worth complaining about as it does not affect gameplay in anyway. Though, I must confess that when I take screenshots of a parked airplane I'll often push the stick forward for a little more authentic look!

As for point 2, yes, at the absolute minimum it's a must that RoF correctly model trimspeeds for the aircraft. Some curve control would be great. . .though responses should not adjustable to be faster than that which is possible, only detuned for those that want it.
And given that we are talking about the Se5a which has a cockpit adjustable trim system in the real world, this plane in particular needs some work done.

« Last Edit: August 21, 2009, 09:05:27 am by TX-EcoDragon » Logged


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« Reply #1 on: September 06, 2009, 11:55:38 pm »


The FAQ repository for trim/elevator/trimspeeds Smiley

Great descriptions by the way - short and to the point.



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