Getting my 2c in here because I can't over at eagle.ru. ( http://forums.eagle.ru/showthread.php?t=81992 )

Eagle-eyed user Fox One observes that elevator trim under manual revision in the DCS simulation is reversed from real life expected behavior.

Under MRFCS the elevator is controlled via mechanical linkage from the flight stick with electro-mechanical adjustment of tab bias for trim. First it is helpful to explain a small feature of the aircraft that is related but distinct from trim tab control to avoid a misunderstanding later. As aft stick pressure is applied the elevator tilts trailing edge up to apply nose up pitch control. Such elevator displacement causes increased air pressure on the elevator surface which produces a strong counter force. To reduce this force (in all modes) the trim tab actuator arm is geared to the movement of the elevator in a way to provide a fractional counter-counter-force. The result is that the qualitative behavior of additional stick pressure being required the more elevator angle remains but the intensity is lessened to spare the hydraulic system wear (PFCS) or the pilot's arm muscles (MRFCS.) This can be seen in PFCS or MRFCS simply by cycling the elevator and observing the change in tab angle relative to the elevator. This is not pilot-controlled trim but simply a design technique to reduce the neutralizing force on the control surface.

Trim is a change in artificial feel for the hydraulically-connect flight stick (PFCS) or electo-mechanically-driven bias to the tab actuator rods (MRFCS). In the case of MRFCS the total elevator trim tab movement is the sum of gearing to the elevator and trim motor bias. All this explanation is trying to say is: "When evaluating the effect of the cockpit trim input in MRFCS, leave the elevator in one place so as not to confuse what part of the tab movement was due to gearing and which part is pilot trim input."

Anyway.

In MRFCS, commanding NOSE UP trim the expected trim tabs movement is to angle downward, causing the neutral-force elevator position to be angled more upward which causes the aircraft's aerodynamically-neutral nose position to increase. The result being a net "nose up" trim input. It sounds complicated the way I write it but it's the normal method of elevator trim for aircraft with mechanical-linkage elevators. (Visual aid: http://avstop.com/ac/1-14.html )

And in DCS the elevator trim simply does the opposite. NOSE UP command causes the tab to elevate which increases downward pressure on the elevator. The elevator's new force-neutral position is angled down which is a net nose down trim result.

My compliments to Fox One for his patient but determined line of inquiry despite the several dismissive and off-topic responses. Poor guy had to get out an image editor and make educational material to get his point across. (Image: http://forums.eagle.ru/attachment.php?attachmentid=66288&d=1337595628 ) Personally I would not have been able to deliver the same patience.

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EDIT: I see it's been resolved. Very good

My experience with the DCS fixed wing planes (A-10C and P-51D) is that the simulation does not accurately model trim tab behavior. In A-10C, with the PFCS system engaged, changing trim actually moves the control surfaces (change trim while on the ground to observe).

If I recall correcetly, In MRFCS, the elevator is actually flown off of the trim tab, not the main actuator. The tab should move with control stick inputs as well and trim input should simply bias (e.g., offset) the elevator position for a given control stick input.

Will have to test this out...

What trim under PFCS does is mess with the artificial feel device only. Hydraulic controls have no natural feedback. Moving a hydraulic actuator requires no effort unless there is something artificial like a spring to provide resistance. If you've ever felt a FFB joystick with the power turned off you know this feeling. It does not care if it is flopped in one corner or the center or the other side.

So the A-10 has a spring and/or bob weight attached to the stick so that it feels better but the feel is 100% artificial for the benefit of the pilot. All the trim does (PFCS) is change where the center of that artificial feel is. PFCS doesn't use the trim tab for anything. The hydraulics laugh at the small air pressure at various elevator angles and will hold any angle forever without complaint.

In MRFCS the elevator is connected via cables and pulleys to the stick very much like a big Cessna 172. The trim tabs works like a big Cessna as well except instead of a manual linkage it's electric.

You're probably thinking of the ailerons under MRFCS. In MRFCS the ailerons are completely disconnected and are free hinging. If you flip the MRFCS switch on the ground you can see them just flop down under gravity. What controls where the ailerons fly happily in the breeze are the tabs and the tabs are mechanically connected to the stick. The change from the stick being connected to the hydraulic actuators to the mechanical tab linkages "tab shifters" is something that can take four seconds.

I don't know how the P-51D works. Does it have hydraulic primary controls?

I didn't realize that trim tabs weren't controlling control surface positions in hydraulic control systems, but, after your description, it certainly would seem odd to expect a little tab to counter the large hydraulic forces, as opposed to the low forces in a conventional "fly by cable" flight system. Thanks for the education!

Your explanation certainly describes the behavior seen in A-10C for a non-forcefeed back stick (i.e., adjust trim and the stick moves along with the control surface), although I think this occurs even before the hydraulic system is powered up. I don't know whether the P-51 has a hydraulic control system, but I see the same observation: stick and control surface moves with application of trim.

Not sure where you got that from but that isn't correct, trim has nothing to do with artificial feel on the ACFT. I think you're reading what the -1 says and interpreting it wrong, at least from my perspective. I'll post more tomorrow when I have more time.

Conflicting quotes from theA Dash-1:

#1 PITCH AND ROLL TRIM CONTROL SYSTEMS


#2A Pitch

#2B Roll

emphasis mine

The ROLL CONTROL SYSTEM SCHEMATIC is another source of study. Linkage path is mechanical stick-fuselage-wing-tab shifter-artificial feel device and then interfaces with the hydraulic servo-actuator and then finally the control surface.

Quote #1 suggests that if you hold the stick in a given position and actuate the trim switch, the only changes will be to the feel force. The positions of the linkage, stick, hydraulics, and surfaces will not change. Quote #2 suggests that (maybe?) trimming actually repositions the actuators even if the linkage/stick aren't allowed to move.

My money is on the #1 interpretation. I believe #2 is saying "Changes in artificial feel neutral position will result in actuator motion given that the stick is allowed to move with the changing forces." Otherwise I cannot reconcile the two.

Testing can be tricky in DCS with a spring-loaded joystick. Joystick input is pilot arm force and not direct manipulation. The A-10 stick can move when forces change even if the joystick is held stationary.

Aside: The genius of the artificial feel device being located outboard of the tab shifter and before the aileron is that it is only in effect when the tab shifters are normal. When the tab shifter shifts, the artificial feel device is not in the linkage path anymore. Stick forces would then be solely due to air loads on the aileron tabs. Such is not the case for the elevator and one feels both real and artificial loads combined in MRFCS. For the rudder there doesn't appear to be an artificial feel at all.

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Below a certain hydraulic pressure pitch and yaw control automatically revert to MRFCS mode, roll control however does not (different mechanism). This explains why that yaw and pitch control is possible even with a completely "off" jet while roll control is not possible.

Try trimming the P-51D on the ground with the engine off and see what moves. While in flight trim will alter the air loads which will make it difficult to hold the stick stationary.

Interesting aside discovered while testing. The pilot stick forces available to the player are not sufficient to place the stick through its full range of motion if the trim setting is not cooperative. For example a stick trimmed somewhat left may result in full right input not arriving at the right limit of stick travel. This would be possible in real life but would require more force than normal.