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Suppose an aircraft is flying left wing low and hence rolls to the left.

If no other trimming options are available then a trim tab is needed (does it make any difference whether you try to pull the left wing up or push the right wing down?).

If you put the trim tab on the left flap, you would bend it down to create more lift.

However looking on line the solution seems to be to put it on the aileron and bend it up, which forces the aileron down to create more lift.

I suppose the aileron fitment is further from the centreline so will provide the required rotational moment for a smaller force, and so achieve the required objective for smallest drag losses.

Is this correct?
My thoughts ( probably wrong ) are that the tab is used to give a slight deflection of the control surface itself - giving you the 'correction 'needed .

However I'd want to correct the position of the actual surface as I (again thought) the use of trim tabs was to reduce the load needed to move the control surface - by virtue of pushing one aileron down or up the other will go the other way by the same ( ignoring differential ailerons for simplicity ) amount .

Logically - If you look to reduce lift on the right wing you avoid the adverse yaw created by adding lift to the left wing?

I'd be more concerned why its flying left wing down - I have seen incorrect rigging lead to some scary handing ( think old fashioned wing warping for control - but all the time & by accident :( ! ) and also repairs meaning one wing weighs more then the other …

I'd still ideally look for a adjustment in the rigging / controls to fix it rather than a tab ( rigging adjustment needing to still be within limits ) . ?
This is a trim tab you are talking about - job is to move a control surface, not do the lifting itself.
Ailerons are normally designed to change the angle of attack at the end of the wing, to give you additional lift (so aileron moves down), with minor deflection of the other one (up) to reduce adverse yaw by reducing its angle of attack and so reducing its lift. So if all else is equal, you would add a trim tab pointing up to force the aileron down to increase lift on that wing, lifting it up.
In saying that, more aircraft are fitted with a single fixed aileron tab, so it can work the other way too.
I assume the aircraft is flying in balance? Our Jodel was sensitive enough that if flying solo it would roll slightly left (so no instructor in the right seat). You could hold it with slight constant rudder pressure and still be in balance, so we added some tape to the rudder to act as a trim tab rather than to the aileron.
rogerb liked this
Charles, get someone to help you who truly knows what they are doing or you will be chasing your trim-tab tail!

Most aeroplanes will have a dedicated method of getting them in trim (with Robins it is by adjusting the flap rod actuators)
T67M liked this
The ailerons are connected, so whichever side you add the tab on, the final position of the 2 control surfaces will be the same.

As said by others, the tab moves the control surface, so tab down means aileron up means wing down.

There may be some small drag benefit by having tab down/aileron up, whichever side that means...
I've been plagued by this for years. My aeroplane banks to the left. Sometimes I adjust the rudder trim and fly crossed control, but other times I just put up with it and keep some pressure on the stick.

My aileron already has a fixed tab on it, and I did once adjust it so it constantly put some aileron in, but it was straightened at one of my annuals.

This is all if I'm flying solo...the aeroplane flies straight if I put someone like Charles in the passenger seat.
Our shareoplane rolls slightly left when solo, slightly right when dual. Once the aileron fixed trim tab was adjusted so that it flew perfectly hands off when dual, but this made it most unpleasant to fly solo, so it was bent back the other way. The rudder fixed trim tab is adjusted to fly feet-off at normal cruise speed, and doesn't vary much if at all with loading.

Both trim tabs are "nearly straight", but if they were significantly bent then I'd be asking the maintenance team to check the rigging and wing/tailplane incidence angles.