How worthwhile is aileron differential?

[Alistair Scolley]

Aileron differential is used to correct adverse yaw during a turn. When a plane is turning the wing on the outside of the turn is moving faster than the wing on the inside so it creates more drag. In a left turn, for example, the right-hand wing creates more drag so the plane yaws to the right and with many aircraft, this looks ugly. To correct this, we get the up-going aileron (on the inside of the turn) to move more than the down-going one. This creates more drag and helps the plane round the turn.

That, more or less, is the explanation I’ve seen but I’ve always wondered about it. To turn a model to the left from straight and level, we apply left aileron and the plane starts to roll. When it is at the desired angle of bank, we neutralise the ailerons to stop it rolling further. For the rest of the turn we hold the nose up with elevator but the ailerons are at neutral so differential aileron movement won’t have any effect. The only time the ailerons are off-centre is at the start and end of the turn, not during the turn itself so we’ll still get adverse yaw.

Is this correct or am I missing something?

Alistair

[cymaz]

I'm sure you're correct. Passed a certain point side slip occurs, I think differential is to help avoid this...I know someone will put us right.

[Frank Skilbeck]

Alistair, you are correct once banked over the ailerons are neutralised and hence there is no additional drag from the down going aileron, but it's on instigating the turn that the down going aileron creates more drag than the up going aileron and causes the plane to yaw out of the turn. So what happens you roll the plane and the adverse yaw causes a nose up attitude in the turn, which could be countered by using some rudder. But at low speed the increased drag of the down going aileron with the adverse yaw causing the upgoing wing to slow down could cause the upgoing wing to stall. Note if the ailerons move the same in either direction, the down going aileron will create more drag, hence you are not adding more up going up going aileron to increase the drag of the up going aileron, but reducing the down going aileron to reduce the drag of the down going aileron.

On a general sports model aileron differential doesn't make a lot of difference (but may help to keep rolls more axial), but on a largish glider the adverse yaw effect is quite noticeable, using aileron differential with a some more rudder makes entry into turns a lot neater.

[Brian Cooper]

I wouldn't get too hung up with the theory of it all. . . Aileron differential is used to get a model rolling axially (right down the centre-line of the aeroplane).

There is no hard-and-fast rule about how much diff to use -- it is a matter of trial and error -- but getting it right makes for a much better flying machine.

B.C.

[Chuck Plains]

I need all the help I can get, so yes, I think it's worthwhile. But even more so as I've yet to maiden my Zephyr flying wing. Apparently these really benefit from aileron diff.

Edited By Chuck Plains on 15/02/2015 17:43:17

[bert baker]

Posted by Frank Skilbeck on 15/02/2015 17:32:48:

Alistair, you are correct once banked over the ailerons are neutralized and hence there is no additional drag from the down going aileron,

hmmm, Is it a flat wing only question, so were not fighting dihedral as well

[Dave Bran]

Have only ever used differential on:

1. Dihedral wing with Aileron to reduce/eliminate adverse roll.

2. Flying wing with elevons to get axial roll

[Chris Barlow]

I have experienced adverse yaw whilst glider training in Venture gliders many years ago and it was always compensated for with rudder, which you can still do with RC.

Differential should be dialed in until adverse yaw is eliminated at normal speed, unless you use rudder to initiate the turns and balance the yaw/sideslip during.

On a wing/delta you can also use more aileron differential to act similar to rudder to turn the wing in the direction of bank to help is around the turns.

[bert baker]

I always think of it a being a bit like a motorcyclist putting a knee down.

If set up the wrong way round it makes for some horrible flying characteristics.

You tell the plane to turn right and it shoves it's nose up and tries to slide the tail into a left turn and then tries to tuck the right wing under as well.

If you are a happy to fly on rudder and elevator, why not set the ailerons up slightly wrong so you can feel the difference, and then go back to rudder-elevator if you find it that bad.

It is worth taking the time to set differential up

[eflightray]

I would also add, the lack of differential aileron is probably the main reason for many of the, 'down wind turn, fall out the sky' problem.

Some flier reduce speed too much on the down wind landing leg, start the bank, and the adverse yaw drag slows the plane even more. Crunch.

Also I have quite a few planes where I will hold a little aileron in a turn. I don't normally - 'apply left aileron and the plane starts to roll. When it is at the desired angle of bank, we neutralise the ailerons to stop it rolling further'

I dislike 'bank and yank' turns.

[J D 8]

My Major Mannock built in the 80's [not by me] has one central servo driving barn door ailerons on the lower wing only,ailerons move the same amount each way. If you try and turn only with aileron it initially just leans over and wants to keep going straight or even go the wrong way depending on speed,elevator only seems to bite once aileron is taken off. The first time I flew it was a real lesson.

However lead with rudder and it turns fine,like many aircraft of the early years of flight rudder is the primary control to start the turn, in fact as Bert says it fly's fine rudder-elevator.

[Former Member]

[This posting has been removed]

[bert baker]

Also try feeding in a little bit of rudder, it will pay dividends later with big old vintage planes.

Don't over do it but do get used to feeding in some

[eflightray]

Posted by Andrew Ray on 15/02/2015 18:57:25:

Posted by eflightray neath on 15/02/2015 18:46:16:

I would also add, the lack of differential aileron is probably the main reason for many of the, 'down wind turn, fall out the sky' problem.

Some flier reduce speed too much on the down wind landing leg, start the bank, and the adverse yaw drag slows the plane even more. Crunch.

Nothing to do with anything other than flying too slow. By the way the aeroplane doesn't know it's flying downwind as the only speed reference it has is airspeed. The ground is only important when the aeroplane is transitioning to or from flight.

So you disagree that adding aileron could slow a model, and that adverse yaw can cause additional drag ?

And yes, I do understand about wind and a models air/ground speed, I have been flying models long enough, (around 60 years), to have grasped that.

[Former Member]

[This posting has been removed]

[bert baker]

Farnborough pass anyone.

Yep I agree with above jump in a full size, preferably a Tiger moth ACDC my favorite and you will understand the above, try a cross wind day just for some extra fun

Edited By bert baker on 15/02/2015 19:50:37

[bert baker]

Posted by john davies 8 on 15/02/2015 18:54:53:

My Major Mannock built in the 80's [not by me] has one central servo driving barn door ailerons on the lower wing only,ailerons move the same amount each way. If you try and turn only with aileron it initially just leans

Wow I had the same thing with my Telemaster 66", back in the late eighties

LMAO oh such fun

[bert baker]

Differential is achievable with one Central servo, there are many old books out there that shows how to set it up.

Of course these days things have gone modern, I still have a tin full of the elavon servo horn mixers.probabbly still worth using on a narrow fuz

Edited By bert baker on 15/02/2015 20:01:04

[Martin Harris - Moderator]

Posted by Andrew Ray on 15/02/2015 19:37:56:Hence full-size aircraft don't have an issue with the downwind turn.

Edited By Andrew Ray on 15/02/2015 19:40:17

An exception here, which certainly goes some way to proving the above, is full sized gliders thermalling (scratching) at very low altitude in reasonable windspeeds where the ground speed perception plays a significant part in stall/spin accidents. Just like a model flyer misreading his "airspeed" a 20 knot windspeed at 50 knots airspeed gives an apparent difference of 40 knots (30 knots upwind and 70 knots downwind) and where attitude is taught as the primary airspeed reference (ASI is used to cross-check) it is all too easy to subconsciously slow the glider downwind with the scenery whistling past the canopy.

[Tim Hooper]

I use differential on my scale-type models, which generally use a Clarke Y or semi-symmetrical section. No probs.

Mind you, I've got a couple of fun-fly aerobats that need to fly as well inverted as they do the right way up, and it seems to me that normal aileron differential will be counter productive when the model is inverted, so these models are set up to have equal up and down movement.

tim

[Former Member]

[This posting has been removed]

[Alistair Scolley]

Many thanks to everyone for their replies.

If I understand them correctly, adverse yaw is worst when initiating a turn and aileron differential is most effective then, so is worth having. I won’t get into the issue of the downwind turn, when planes seem to stall when we think they shouldn’t. I’ve always thought that aircraft know the laws of aerodynamics far better than we do, so we should react to what the plane is doing, not to what we think it should do.

Alistair

[bert baker]

For years and years I was guilty of only using rudder on take off, I was cured of this problem when I flew my first Precedent Stampe.

I now find myself using the rudder stick even if it is a elevator-aileron only plane.

Steve Carr did a really nice basic lesson flight, explaining the stick movements whilst flying one of his large planes.

[J D 8]

Andrews post has brought back memory's of gliding in the 70;s We had an instructor who if you flew into some lift on final's would inevitably call "I have control" and try to catch it which he did most of the time, but one time with myself onboard we scraped over the boundary at slow airspeed and just stopped flying dropping down almost vertical some fifteen feet with one hell of a thump.[soft ground helped some] With another pupil he caught a road sign near the boundary ripping the tail off !

Tough old bird that T21 it was repaired and flew on for many years.

[SIMON CRAGG]

Interesting topic.

On smaller (60" or so) models, it does not seem to make a huge amount of difference in my experience.

However, on larger wingspan models eg: Cub, PT19, (with a shorter moment arm), I have found that 20% more "up" than down, plus coupled rudder transforms the model into a much more benign, smooth flyer. Worth experimenting, as with modern gear it is straightforward to "dial in" the offsets etc,