Aileron Differential?

[fly boy3]

So am I Simon. Cheers FB3, and its because I am always financially challenged LoL.

Edited By fly boy3 on 06/12/2011 20:56:12

[Paul Adams]

Hi Martin, Just confirming what your saying, in both my test cases above (symmetrical wing trimmed for level flight in normal flight and symmetrical wing, model inverted trimmed for level flight inverted) that if I initiate a roll it does not matter which way round the differential is (more up than down, or more down than up) as the rolling forces will be the same and they will produce the same yaw?

 

We must be missing something as this does not compute. There must be something we are missing. The differential must be to do with the effect of the aileron on the particular aerofoil profile, dihedral, tailplane and fin surfaces and there relation to the main wings etc (aerobatic model). For a non aerobatic models we are probably looking at much greater differentials but we are not so concerned with the full 360 degree roll correction. 

Edited By Paul Adams on 06/12/2011 21:51:29

[Martin Harris - Moderator]

Hopefully, BEB will be along soon to explain this in his elegant fashion but let's try to think slightly differently.

 

When you want to turn right, which way to you move the stick?

 

When you want to turn right from inverted, which way to you move the stick?

 

I think you'd agree that the input is the same and therefore the adverse yaw produced by the force required to initiate the roll will need the same correction.

[Paul Adams]

Fully 100% agree, and that's my point. The wing does not know which way up it is (assumed trimmed for level flight in either test case). So the movement of the up going aileron must be the same in both cases and the movement of the down going aileron must be the same in both cases. When differential is dialed in this does not happen.

[Tim Mackey]

BEB is away for a few days, and may not have ready internet access .

[Simon Chaddock]

It appears an innocent question about differential ailerons has opened a debate on the aerodynamics of rolling as polarised as the "up wind/down wind turn" one!

And it appears other forums have the same problem!

 

So let me ask would a differential ailerons set up have the same yaw reducing effect when in sustained inverted flight?

 

The truth is out there - somewhere!

[BB]

Posted by Biggles' Elder Brother - Moderator on 05/12/2011 14:29:43:

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

Used 60/40 & 70/30 & 65/35 on my Wot 4. 60/40 worked best.

BB

[Olly P]

Up and Down relative to what?

 

I've deliberatly kept quiet.....

 

If you are trimmed for inverted flight and initiate movement in the roll axis by use of ailerons then the ail that moves up relative to the wing is doing exactly the same thing regards lift relative to the fuselage.

 

Therefore the effect on the yaw relative to the airframe is the same, it may look different from the ground, but in a 'pure' sense the reaction needed is exactly the same.

 

Olly

 

 

P.S. Frames of reference are not dirty words, crevice thats a dirty word

 

Olly Pearson ....Aero Eng department

[Martin Harris - Moderator]

That (I think) is the view that I generally subscribe to but Paul's logic makes sense in the case of a symmetrical 0/0 model. I have to admit that the more I think about this, the more I'm having trouble making much sense of it!

[ken anderson.]

on some/most of my 'sports models' that have been kapowed and repaired over the years...... had lots of twists,warps etc......and one i rember called the 'extra slim'..... that i decided to build the wing in my hand after laying the spars down with a couple of ribs attatched ... had the biggest twist ever..... i even got an award for the 'crepe' build at our AGM.. ! .. it was the best flyer ... and excellent at prop hanging....most ARTF .. modellers never give it a thought(differential)...and dont need to.....it would be interesting to hear from anyone in australia(them been upside down) allready .. ...(i'm sounding like myron b....)

 

ken anderson ne..1 .... upside down dept....

[Olly P]

Martin - the thing we are interested in is the yaw effect of the roll input.

 

This effect will differ on every setup on every model, depending on a multitude of things, and how much input there is. Indeed this is different on full size also. I know a fullsize glider I used to fly had a 2:1 ration on aileron deflection to prevent adverse yaw!

 

Some modelts will have sufficient vertical fin size/fuselage cross section etc to resist this induced roll.

 

Without getting into the mathematics of it (as I can't remember them!) the effect of the same input on any set up is the same if inverted or vertical, we just need to change our frames of reference to suit what we think of as default.

 

I will dig into the logic mentioned above when i'm not being paid to think about other things i.e. not at work!

[Martin Harris - Moderator]

Olly - that seems very much along the lines and logic of my thinking as I've been trying to argue but the symmetrical section points raised by Paul are nagging at me...

 

What glider was that by the way? I've flown a wide variety from vintage to flapped glass and not come across one that didn't need considerable rudder input in turns to balance the adverse yaw. Perhaps advances have been made recently but I understood that the reason there was insufficient differential or other solutions as used in the power world, such as Frise ailerons was that it would lead to excessive drag and reduced performance.

Edited By Martin Harris on 07/12/2011 11:30:13

[Ultymate]

Martin a lot of aerobatic gliders now run fully symmetrical wing sections these days, I have two in my hangar and neither require rudder in the turns.

[Martin Harris - Moderator]

Full size or model, Ulty?  Whichever, is there differential?

Edited By Martin Harris on 07/12/2011 11:56:12

[Ultymate]

Model Martin, I spend far too much on modelling but could not stretch to fullsize. Neither of the two gliders in question have or require differential. One's a four servo wing (Vector 3) which I use both ailerons and flaps as ailerons with about 50%flaps to ailerons. The other's a 2 servo wing Wasabi no differential but loads of movement on high rates and gets very close to 3D flight. One thing I've found on the Vector is that if you increase the aileron travel too much the roll rate actually slows down, in other words there's a sweet spot which if over exceeded slows down the aeros. I suppose with gliders more so than with a power model with propwash the fact is that everything you hang out in the breeze can become an airbrake and not necessarily produce the expected result. Like many things in life there are compromises and trade offs to be made.

[Martin Harris - Moderator]

Posted by Ultymate on 07/12/2011 12:12:12:
One thing I've found on the Vector is that if you increase the aileron travel too much the roll rate actually slows down, in other words there's a sweet spot which if over exceeded slows down the aeros.

 

I think it's stalling the wing with the downgoing aileron due to an excessive AoA from the resulting chord line.

 

Regarding the lack of differential, I suspect things are very different with a high speed bank and yank operation and (I'd guess) more twinkle rolls than slow ones? 

Edited By Martin Harris on 07/12/2011 12:28:46

[Ultymate]

No definitely don't fly the Vector "yank and bank" style, the Wasabi yes sometimes but most of my aeros whether powered or glider tend to be large slow and classical. I do play with some 3D but it only forms a small part of my flying.

[Biggles' Elder Brother - Moderator]

Guys I am in Germany at the moment - with somewhat flaky internet connection and little time!

 

I will post a reply to questions when I return at the weekend!

 

BEB

[Ultymate]

A final thought I've had, if you guys that are as you say are exploring "slow flight but not 3D" (not entirely sure what you mean) and are inputting up elevator and rudder you could well be inducing tip stall scenarios as said inputs are basically spin inducing.

[Andy Gates]

Ult

 

Very true, but with aileron differential set up, the down going aileron with its smaller deflection does not cause the wing to tip stall while the up aileron with its larger deflection has caused the required bank to take effect. This is a reduced effect and not a cancelling of the tip stall.

 

Flying close to the stall with coordinated aileron and rudder in this way is great fun and does tweak the thumb reflexes - or so I have found.

 

What I was meaning by the "not 3D" comment was that the wing is still flying and that the plane is not hanging in the air on the propeller thrust alone.

[Ultymate]

Andy I don't deny that such flying is exciting but not to be advocated for everyone, in helecopter parlance it's amost akin to flying in "deadman's curve". I would not be advocating it for the inexperienced particularly at low level or on finals etc .

[Ultymate]

Aaaahhhhhh!!!!

Edited By Ultymate on 07/12/2011 16:01:10

[Ultymate]

Aaaahhhhhh!!!!!

Edited By Ultymate on 07/12/2011 16:01:40

[PatMc]

I'm down with a bad attack of shingles right now so will dip out of this but may add a few thoughts at a later date.

[Ultymate]

Posted by Ultymate on 07/12/2011 15:40:22:

Triplicate post   AAAAAhhhhh this "B" forum

Edited By Ultymate on 07/12/2011 15:59:53