Control surfaces behaviour while trimming

[JayPee]

I always presumed that changing the pitch of the trim tab causes aerodynamic forces to push the main surface out of its neutral position and thereby reducing required pressure on the controls.

 

The paragraph on manual reversion confirms this mechanism as it states that trim tabs are used to push the main surfaces in the required direction by making use of aerodynamic forces when in MAN REV mode.

 

However, if aerodynamic forces are the factor driving the main surfaces when adjustment of trim tabs is applied, how is it possible for trim to move the main surfaces when I'm idling on the ground and no aerodynamic forces are present? And why don't the trim tabs move in relation to the control surfaces? Trimming now looks more like a type of auto pilot continuously giving input to the main surface and not to the trim tab.

[Maverick-X]

I dont know how exactly the trim system of the a10 works... On commercial airliners the whole elevator is moved to reduce triminduced resistance.

 

I doubt that the elevator of the a10 is moveable, so I'd guess the elevon is pitched up or down regarding of your trim input. This would be realised by an hydraulic actuator. If this fails, trimming vía aerodynamic force for elevon-movement could be a backup?!

[Evil.Bonsai]

Need to watch more closely. You don't adjust the ailerons, you adjust the trim tabs at the backside of the ailerons. These are controlled electrically when you adjust the trim. Trim tab up moves the aileron down (aero forces pushing against the trim tab push it down, thus pushing the aileron down). The ailerons will just sit there until you move fast enough for air to push against the tab. Same thing applies to the elevators

 

edit: nm; I thought it was supposed to work this way, but just checked and the whole aileron moves, not the tab. I would've sworn I watched the tabs move previously.

Edited August 26, 2013 by Evil.Bonsai

[Flamin_Squirrel]

The normal method of trimming actuates motors that reposition the hydraulic actuators, moving entire elevator surface. When in manual reversion with no hydraulics this no longer works, so it has trim tabs too.

 

Detailed info for this kind of thing can be found in the A-10A manual (same airframe as the C). I don't have a link handy but it's not hard to find.

[JayPee]

OK. This is what I thought but I didn't want to bias the discussion.

 

So under normal circumstances, trimming an A-10 does not move the tabs, instead it moves the whole surface. However, when in MAN REV, trimming moves the tabs which in turn move the whole surface due to aerodynamic forces.

 

As a reply to Bonsai, when idling on the rwy my entire ailerons clearly move while the trim tabs don't when trimming for roll. I will check whether it is different in MAN REV.

[HugePanic]

Did you see these pages??

 

If I see it right, there is a difference from elevator to aileron.

 

In manual reversion, the elevator trimm tab is used for trimming only.

At the aileron, the trimm tab is used to control the aileron.

 

Would make sense, since you can apply more force pulling a stick than moving it sideways.

You also need "more" elevator control than aileron control at landing....

Edited August 26, 2013 by HugePanic

[JayPee]

I totally get it! Where'd you get those drawings from?

[HugePanic]

I totally get it! Where'd you get those drawings from?

 

search for 1988-TO-1A-10A-1-Flight-Manual-USAF

It was linked here sticky in the forum, but I don't know if the link is still active

[JayPee]

Roger!