Aileron Differential?

[Steve Houghton 1]

Can someone explain why we use aileron differential? What are the benefits of having less down movement than up?

[Tim Mackey]

'cos the down going aileron produces more drag and can induce a yawing effect.

Im sure BEB will be along with a more detailed explanation - or correction if Im wrong .

[Biggles' Elder Brother - Moderator]

The down going aileron creates lift - more lift equals more drag. So if ailerons move equally then the wing on the down-going aileron side (the outside wing in a turn) will experience more drag which will tend to yaw the model so its nose points out of the turn. This is called "adverse yaw".

 

Adverse yaw doesn't just look ugly it brings other problems; it promotes side slip which increases overall drag. Also if the model has dihedral the effect of the side slip will be to try to roll the model out of the turn. All in all this is not desirable.

 

Two methods for dealing with it;

 

1. Live with it but must use some rudder in the direction of the turn to counteract it, effectively countering the adverse yaw with oppisite rudder induced yaw. Makes for nice flying problems if you like that sort of thing (I do) where you have to judge just the right amount of rudder to use - you can mix the rudder in with ailerons of course but I feel I'm cheating if I do that. (Just me - I'm not critising anyone else - if you like mixing rudder that's fine).

 

2. Alternatively, by letting the down going aileron have less travel you balance the drag between the two wings better. Up-going ailerons make some extra drag as well - but not nearly as much as down-going ones - becuase of the lift effect of a down-going aileron.

 

Hope that helps.

 

BEB

[Biggles' Elder Brother - Moderator]

Spot on explaination in very few words Tim!

 

BEB

[Steve Houghton 1]

Thanks BEB, I now understand. Is there some magical percentage of down to up which applies to most models or is it a case of suck it and see or rely on the kit manufacturers recomendations?

[Biggles' Elder Brother - Moderator]

No, the optimum value very much suck it and see. About 50% is a good place to start though.

 

BEB

[Steve W-O]

There is another reason, more important to me.

 

Flying slowly, say on approach, the down going aileron effectively increases the angle of attack, and can cause a tip stall. The same can happen in a high speed turn.

 

I use between 25% and 50% down compared to the up

[Tim Mackey]

Posted by Biggles' Elder Brother - Moderator on 05/12/2011 14:30:24:

Spot on explaination in very few words Tim!

 

BEB

Yippee

[Ultymate]

Most aerobatic pilots, myself included, would only use differential to correct any tendency for the roll to be a barrel roll as opposed to axial.

[PatMc]

Posted by Ultymate on 05/12/2011 18:12:24:
Most aerobatic pilots, myself included, would only use differential to correct any tendency for the roll to be a barrel roll as opposed to axial.

What does that do to a roll that you start & finish inverted ?

[Cyclicscooby]

So.. I presume, that on a flat sheet foamie wing, the initial drag effect would be less, as there is no proper foil shape, so either way the aileron would creat drag.. ?!

I appreciate the motors down thrust will create a pressure difference over the flat wing, but less than a foil.. ?!?!

So.. in short, the effect of 'down-side drag' will be less obvious.. ??

 

Personally, I have two flat-sheet planes, and both have NO-differential, and roll nice n straight.. Just curious really

 

 

Luv

Chrisie.. xx

[Ultymate]

Posted by PatMc on 05/12/2011 19:07:20:

Posted by Ultymate on 05/12/2011 18:12:24:
Most aerobatic pilots, myself included, would only use differential to correct any tendency for the roll to be a barrel roll as opposed to axial.

What does that do to a roll that you start & finish inverted ?

Whether a roll starts or finishes inverted or right way up makes no difference at all to a well trimmed aerobatic model.

[PatMc]

There's always more drag from the downgoing aileron if the movement is equal. The adverse yaw will be more pronounced with high aspect ratio wings than low aspect ratio because of the greater leverage involved (ailerons further out from the centre line). But differential also reduces aileron effectiveness in roll which for aerobatics might be a good reason not to use it but to correct any adverse yaw with rudder instead.

 

BTW the Wright brothers used coupled rudder with aileron (wing warping) from their No 3 glider onwards to counteract adverse yaw.

NOLOPKN - FGAD

 

And for JC's benefit - I don't know this nugget of information from personal experience - I read about it.

[PatMc]

Posted by Ultymate on 05/12/2011 19:25:00:

Posted by PatMc on 05/12/2011 19:07:20:

Posted by Ultymate on 05/12/2011 18:12:24:
Most aerobatic pilots, myself included, would only use differential to correct any tendency for the roll to be a barrel roll as opposed to axial.

What does that do to a roll that you start & finish inverted ?

Whether a roll starts or finishes inverted or right way up makes no difference at all to a well trimmed aerobatic model.

The differential is acting in reverse from inverted so if it makes no difference how is it doing anything

[Ultymate]

I think you're confusing yourself here between differential that's been programmed in to keep a rolls axial or to hopefully as a measure to prevent tipstall at low speed on finals etc.

[Paul Adams]

Am I correct in thinking that aileron differential is only used on non aerobatic models?

 

The reason I ask is consider a model undertaking a level roll (i.e. one where you add a bit of bottom elevator when inverted to keep the model flying level) when fitted with aileron differential.

 

I see it will go through three stages:-

 

Stage 1 - When you roll from level flight to knife edge the down going aileron ( i.e. the aileron nearest the ground) will go down less than the up going aileron.

 

Stage 2 - When going from Knife edge through inverted to knife edge the down going aileron (i.e. the aileron nearest the ground) will this time be greater than the up going aileron. Since bottom elevator is used to maintain level flight the wing during the inverted phase will still be producing lift (i.e. upward force on the model).

 

Stage 3 - Then from Knife edge back to level flight you are back to the correct way round.

 

I see how aileron differential will cancel out the Yaw effect during stage 1 and 3, but wont it make the yaw effect during stage 2 much greater?

 

 

 

 

 

 

 

 

 

 

 

 

[Paul Adams]

Ultymate - That explains it.

[PatMc]

Posted by Ultymate on 05/12/2011 19:56:12:
I think you're confusing yourself here between differential that's been programmed in to keep a rolls axial or to hopefully as a measure to prevent tipstall at low speed on finals etc.

 

No, no confusion. Differential is intended to counteract adverse yaw.

Not many people would start a roll at low speed on finals especially from inverted.

 

Posted by Paul Adams on 05/12/2011 20:29:23:.

...

I see how aileron differential will cancel out the Yaw effect during stage 1 and 3, but wont it make the yaw effect during stage 2 much greater?

 

 

That's the way I see it, Paul.

[Ultymate]

Paul some aerobatic models do need aileron differential plumbed in. The yaw effect is a result of design and if not corrected by differential leads to the roll being a barrel roll and not axial as is required when flying aerobatics. If you like the "designed in" yaw effect leads to what would be a normal axial roll about the aircraft's centre line becoming a "mild snap roll"/ barrel roll. Some high end models have figures for aileron differential listed in the build manual. A large scale Hangar 9 Extra, the one in my avatar is one such.

Edited By Ultymate on 05/12/2011 20:44:55

Edited By Ultymate on 05/12/2011 20:46:35

[PatMc]

Sorry Ultymate but that doesn't seem to make sense.

 

Can you explain why the adverse yaw isn't exacerbated during the inverted phase ?

[Ultymate]

A roll (axial) with a good aerobatic model only becomes a four stage affair when flying a "slow roll" where rudder inputs are needed in knife edge and possibly sight down elevator at the inverted stage, however a normally paced roll at most may need a breath of down elevator when inverted.

P.S. you will not be doing slow rolls with a Widthing 

Edited By Ultymate on 05/12/2011 20:59:22

[PatMc]

Sorry that doesn't explain anything. It will still have wrong sense correction for adverse yaw when inverted no matter how fast it's rolling.

You don't get axial rolls with a WT anyway.

[Ultymate]

Sorry but when flying at a "normal"/reasonable speed the aircraft doesn't know it's inverted. The terms up and down going aileron is relative to the airframe and not the sky being up and the earth being down. Hope this helps as I failed my GCSE in the explanatory sciences.

[Steve Houghton 1]

Heck, I seem to have started something. I fly slope soarer's and all I know is that my models don't have any differential dialled in and they roll axially very well. A guy turned up at the slope a week ago who did have differential and his model was barrel rolling all the time. Each time he adjusted the amount of down aileron, so there was more of it, the rolls became more axial. When I was taught to fly back in the 70's, I was taught to add a little rudder in the turns to smooth them out and turn tighter if I wanted to, so this is something I've always done, with no adverse affects, so that will be cancelling out any yaw I guess.

[PatMc]

If the aircraft doesn't know which way up it is it will still be aware which one is sticking out further into the airflow & yaw in that direction. So if it happens to be upright with differential ailerons the adverse yaw will be counteracted for reasons already explained by BEB. But when it's inverted the adverse yaw will be increased instead since it's the same aileron that's creating the most drag, which the aircraft will be aware of.

 

Bottom line is - for aerobatic aircraft don't use any differential but make any yaw corrections with rudder .