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Flaps behaviour


Alfredson007
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Can anyone tell me the idea behind the flaps autotrim system?

 

if select take off flaps during approach, the nose dives a bit (as many low-wing aircraft do), when i select landing flaps nose rises quite a bit. I know that there is some sort of autotrim -event when i select the landing flaps, but rather than just making the most nose up trim requirement lesser, it adds the opposite reaction quite violently, why it trims so needlessly "over" (nose-down changes to nose up)

 

In case i was unclear: why doesn't it just trim a bit nose up but leaves something for me too? AFAIK the autotrim is supposed to relieve the sudden change in pitch.. ?

 

Thanks.

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Real Life - Lowering Full Flaps

  • When lowering the Flaps, the forces on the elevator change, changing the control force/position of the RL stick. IIRC an electric motor adjusts a trim tab to compensate and null out the change in trim.

DCS - Lowering Full Flaps (Takeoff -> Landing Position)

  • Using a FFB stick - the nose drops (and FFB centre position changes) and then rises as the flap trim tab compensates.
     
     
  • Using a regular spring centred joystick - the nose doesn't drop as the joystick centre position isn't affected by the change in control forces. Instead, as the flap compensation trim adjusts - the nose rises, necessitating a trim adjustment in the opposite direction.

Note: Here is the original thread - Flight model - different joystick, different behavior when full flaps

In case i was unclear: why doesn't it just trim a bit nose up but leaves something for me too?

TL; DR: It's because of how DCS models flight controls. AFAIK little if any trim adjustment is needed in real life.


Edited by Ramsay

i9 9900K @4.7GHz, 64GB DDR4, RTX2070 Super 8GB, 1TB NVMe, 6+4TB HD, 1+1TB SSD, MSFFB2 joystick, X52 Pro Throttle, TIR5, Win 10 Pro x64, 1920X1080

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I just kind of wish the "feel" would be more realistic in DCS with a regular stick, if in real life the pilot nearly do not have to do anything but go with the flow so to speak when applying ldg flaps..? if i understood correctly

There is this description of a nose up pitch change when deploying the flaps (US L-39ZA 1991 Flight Manual) -

 

attachment.php?attachmentid=168372&stc=1&d=1504151466

 

However... the left trim tab adjusts automatically when the flaps move from 25° to 44° position (page 1-47, page 62 in the pdf)

 

TRIM TABS

=======

...

Longitudinal trimming is provided by trim tabs fitted to the right and left elevators and operated by an electrical actuator which deflects both tabs simultaneously up or down.

 

It should be noted that the left trim tab automatically deflects downward when the wing flaps are moved from 25° to 44° positions.

...

 

Edit 2: Additional Sources

The L-39C Flight Manual T.O. 1T-L39C-1 (Aero-L-39-Flight-Manual.pdf) found on the Internet describes the characteristics a little differently, though it's hard to know how much is genuine as it was shown to be a fake document ("Data Basis: Estimated") in an Australian court case (Aero Vodochody performance charts have "Data Basis: Calculated").

 

TRIM TABS

The trim tabs provide aircraft trimming along the longitudinal and lateral axes.

 

Longitudinal trimming is provided by trim tabs fitted to the left and right elevators. The right trim tab is operated by electrical actuator which deflects the tab up or down. The

left trim tab is controlled by the flaps extension and will deflect automatically when the wing flaps are moved from to 44 degrees (landing) position, thus effectively introducing

a "nose up" trim for the flare. The right trim tab is controlled by the trim switch located on the control stick in either cockpits.

 

FLAPS

Flaps extension has no significant effect to longitudinal characteristics. Slight nose-up moment with extended flaps is easily trimable

L-39_Landing_flaps_causes_nose_up.thumb.JPG.be414597367afefd9f177a147b43b6a8.JPG


Edited by Ramsay
Correct inaccuracies

i9 9900K @4.7GHz, 64GB DDR4, RTX2070 Super 8GB, 1TB NVMe, 6+4TB HD, 1+1TB SSD, MSFFB2 joystick, X52 Pro Throttle, TIR5, Win 10 Pro x64, 1920X1080

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