wingstrut

Gentlemen, I have a 1/3 scale Sopwith pup and I just set up the ailerons to where the ailerons move 2.75 inches above the wing and 1.00 inch below.
Before I fly the aircraft can anyone tell me if this will work or do I need more aileron deflection below the wing.
Do I need any aileron movement below the wing at all.

I think this is how the ailerons work.
The aileron moving above the wing creates drag causing the wing to go down, the aileron moving below the wing causes lift, helping the plane on it's axial roll.
It also creates some drag, that is why some aircraft use aileron differential.

Is there a formula for this, I can't seem to find one.
Any help would be appreciated, Thank you, Sincerely, Ron

rmh

Formula ?
not really -
you are on the right track tho-
on these big ol PUP models the speed you fly em has a lot to do with how they perform rolling /turning . Also everything happens in slow motion
At your typical cruising speed add aileron -and watch what happens -
IF it banks and gois straight or slightly turns in the direction of the roll - good enough.
The real thing took rudder with aileron to do the turn
IF it rolled a bit yet appeared to crab the other way - you would need more differential.
also - if you have not already done it
make up a couple of 2ft long sticks with a notch on one end - then measure relative leading edge to leading heights and relative trailing edge etc..
ideally the wings will be flying at the same angles and especially no differences -side to side.
There is a lot of guesswork about how models bipes should be setup
try to keep everything about the wings as equal as possible -that is do not set one wing with a different flying angle than the other wing

rmh

BMatthews

At slower flying speeds the down traveling aileron adds lift and MUCH more drag than the up travel aileron. So differential is used to try to equalize the side to side drag and reduce any adverse yaw.

Your thinking on how ailerons work is a bit off. The ailerons alter the camber and angle of the area of the wings that have the ailerons attached. Reflexing the ailerons up initially reduces drag in that area as it unloads that area of the wing by both reducing the camber and at the same time reducing the angle of attack for that portion. In effect it makes the wing act like it has a lot of washout twist. It's only if the angle of the aileron becomes quite extreme that it produces a rise in drag at the point where the lower side becomes stalled and pulls a large separation bubble in its wake.

On the other side the down traveling aileron both increases the angle of attack and increases the camber of the airfoil over that area. It acts as if the wing is twisted to produce a washin effect. But both of these effects produce an increase in drag that is associated with the increase in lift. And if flying slowly the deflection pushes the lift of that portion of the wing up into the region where a small increase in lift produces a large increase in drag.

In an extreme case if you're flying near the stall speed and use a mitt full of aileron you can push the down aileron to raise the angle of attack and camber to the point where that wing stalls and you get the classic wing drop or snap roll and dive towards the ground. Glider flyers and folks with models like your Pup quickly learn to use small amounts of aileron when flying slowly and be patient as the model sloooooooowwwwly rolls into and out of the turns.

But all this changes if you let the speed build a little. At a reasonable cruise speed there's no need at all for differential as the wings are operating at a much lower lift coefficient where changes in the Cl result in little or no change in the Coefficient of Drag or Cd. It's only when flying slowly that the differential is nice to have as it makes rolling into the turns look smoother thanks to the reduction in adverse yaw.

But because these effects are related to the Cl and thus to the flying speed there's simply no formula other than "rules of thumb" about how much differential to use.

Case in point. A buddy of mine built an absolutely incredible SPAD 13 in balsa and tissue that was rib for rib and stringer for stringer a copy of the original. To keep the model in character he planned on flying it very slowly and near to the stall much of the time. He played around with the ailerons to the point where there was about 30 degrees up travel and less than 5 degrees down. Rather extreme differential. You know what? It STILL had a lot of adverse yaw when flying it in the manner he wanted. It was because the up aileron wing reduced its drag while the other side kept what it had. THAT was enough to produce adverse yaw. He kept asking me what it needed to "fix it" and I kept telling him to fly a trifle faster and that he was trying to fly it aggressively too close to the stall speed. He resisted at first but finally tried it and found that it only needed to fly a little faster to make all the adverse yaw go away. The model still look incredible in the air but now it isn't dragging its tail around every turn.

So hopefully you see now that differential is a bit of a "Band Aid" for adverse yaw in slower flying models. Some can help but if you insist on flying slowly enough even 100% differential won't cure your problem. It also shows that you can't do a calculation since anything you determine is only the right amount over a rather small speed range.

Top_Gunn

My Feedback: (6)

As nobody else has mentioned this, I will. The reason airplanes have rudders is that the pilot can use them in turns to prevent adverse yaw.

rmh

in some cases - that works
Some aircraft us rudder ONLY for turning - the use of ailerons not required nor is banking needed
The rules change as the airframes evolve.

Top_Gunn

My Feedback: (6)

Yes, if you're talking about weird planes usually made of slabs of foam. The OP has a WWI biplane. Like most planes, it turns by banking and the rudder is used to prevent adverse yaw. "Nearly all cases" would be a lot less misleading than "some cases."

bjr_93tz

There's no real formula to it other than the trial and error method. A fixed amount of differential (and an aileron to rudder mix) will reduce your workload uder a given set of conditions but learning to use the rudder thumb will help you out when a bit more (or less) rudder is needed.

One thing some glider guys find is differential can cause pitch changes when the ailerons are used but it's probably not noticable on a WWI biplane?

I use differential to keep the point rolls straight in a vertical upline when the wing isn't generating any lift and where "up" and 'down" doesn't exist so balancing drag isn't the whole story behind why differential can be required in some designs.

There's no harm in getting differential a bit wrong so feel confident to keep making adjustments until you like the results...

wingstrut

Thank you gentlemen, I should have mentioned that the WW-2 guys were telling me to put differential in the ailerons, I actually turn my pup with rudder and counter act the drop of the wing with opposite ailerons which makes for some pretty nice turns.
I also have been flying this plane for quite a while, I was making sure that I didn't mess up by using aileron differential and not be able to control the aircraft.
My rolls are scary, as soon as the pup gets on it's back it has a tendency to stay there, I have to use rudder to get it through the roll.
I normally don't like to roll this aircraft, but you know somebody always hollers out, won't that thing roll? I should just tell them the real one didn't.
I'm not trying to get away with not using the rudder in turns, because you can't turn one of these things without rudder, not well anyway.
I was also concerned that by using this method that I might create less control at slow speeds, you know those gusty days, landing a kite.
I set the bottom wing at zero with the stab and engine also is set at zero and my top wing is set at 3/4 degree negative incidence, I know this may not seem correct with a lot of guys but this aircraft flies beautiful.
I have never used differential before but with these new radios it is so easy to experiment.
Thank you all for your comments, I might try a little differential but I will just keep on flying like I always have elevator, rudder, ailerons, throttle that way my left hand won't feel like it got left out.

Ron

Waco brother #216

rmh

In some cases -

wingstrut

RMH, My pup goes a little toward the turn when I apply aileron, which is correct, then I apply a little rudder then a little opposite aileron to keep the aircraft from rolling over from the rudder input.

Ron

bjr_93tz

A barrel roll would look really nice with this type of plane. You sort of pull the nose up about 30deg before applying aileron and hold in some up elevator while you do the roll and the thing rolls in a corkscrew flightpath.

Or you could roll about 30 deg to the opposite direction first, then apply elevator and aileron at the same time to achieve the corkscrew flightpath.

Big key here is that with a "proper" barrel roll the wings are always positive G loaded so if you were a passenger you won't spill your drink and your plane never really knows it's upside down.

A normal straight roll with a pup would just look forced and unnatural.


At 1:28, the pilot explains his 707 roll was "a 1G maneuver" meaning the wing never unloaded from a positive G state..

http://www.youtube.com/watch?v=-KNbKFMBsQE

wingstrut

Thanks, I will have to try that one at 8000 ft. Ha!Ha!

rmh

Sounds about right
a little rearward shift of CG MAY be in order but BUT if you fly slowly most of the time - this can get dicey.
Our Bucker Jungmann (30%) can tolerate an aft CG but this design is very different from the PUP.
The Jungmann was designed to do aerobatics The PUP was done when everyone was struggling to produce a plane which simply would fly and shoot better than the enemy stuff .

Rob2160

Hi Ron,

I will add my 2 cents worth.....

I am a full size instructor and the subject of differential ailerons is part of the theory covered in briefings during the intial flying lessons..

I'll do my best to summarise....

A wing generates lift by deflecting air downwards..

The "curvature": of the wing is called "Camber"

The wing can change its angle relative to the airflow - this is Angle of Attack..

In General - more angle of attack / more camber equals more lift.

The penalty for producing this lift is Drag - it is called "Induced Drag" because it is induced by the wing as it generates lift..

If you increase angle of attack, you get more lift, but you also get more drag.

Imagine viewing the wing from the side.. with ailerons neutral both sides of the aircraft have the same camber and therefore lift and drag on both wings are equal - the plane should fly in balance..

Now imagine you apply Left aileron - the aileron on the left wing moves UP - the aileron on the right wing moves DOWN...

This changes the "Camber" of the wing... increasing camber on the right wing.. lowering camber on the left wing... similarly the effective angle of attack increases on the right wing and reduces on the left wing...

What happens?... the right wing develops more lift and rises, the left wing develops less lift and falls.. and the aircraft rolls left.. all good so far...

However.. the right wing now also has MORE drag and the left wing has LESS drag.. so if viewed from the top.. more drag on the right wing will cause the aircraft to YAW right... so as you roll left, it will Yaw right and vice versa..

This is called "ADVERSEYAW"

There are several design features that can reduce this..

1. Differential ailerons
2. Frise Ailerons

1. Differential ailerons - as you know, the upgoing aileron moves UP more than the down going aileron moves DOWN.. the theory behind this is that the Drag increase on the downgoing aileron is reduced slightly - reducing the overall adverse yaw.

2. Frise ailerons.. in this design.. when an aileron moves UP, part of it deflects into the under surface of the wing to increase the overall drag on the side with the up going aileron.. this drag matches the increased drag on the down going aileron and therefore the two drags balance each other and there is less or no adverse yaw..

A really good Wiki article here that describes it better than my attempt above..

http://en.wikipedia.org/wiki/Adverse_yaw

In a real aircraft, the pilot also has rudder to balance any adverse yaw, and this is achieved by applying a little rudder in the same direction as the aileron deflection..

A yaw dampener in more sophisticated aircraft does this automatically..

So to get back to your original question.. try setting up your plane so that the downgoing aileron has 30% less down movement than the up going aileron... EG if 3 inches up.. set 2 inches down..

This will be a good starting point..

Now a feature that can really help is to add an aileron / rudder mix..

Set it up so that you have about 25% rudder movement in the same direction as ailerons..

I set all my scale aircraft this way and the difference is noticable.. they turn much nicer and look more scale (because the radio is automatically doing what a real pilot would do with his rudder pedals anyway)

Of course you can adjust these settings after a some of trial and error to see what works best in your model.. and put the mix on a switch so you can turn it off for aerobatics..

Hope that helps..

Rob2160

We had a saying at our flying school..

Watch him spin, watch him burn..

He held off bank in a descending turn....

What you described above is a big NO NO in real flying.. If you turn with rudder - EG left rudder and hold off right aileron you are actually setting the aircraft up for a nasty flick / spin if you get too slow...

The left rudder is producing left yaw.. holding off with right aileron is giving you more angle of attack on the left wing.. and more drag on the left wing...

if you approach a stall in this configuration, the left wing is likely to stall first and drop, likely resulting in a spin..

We used to teach this (at a safe altitude) and the results were sometimes a rapid spin entry that caught students off guard...

Try the differential ailerons and rudder mix.. that is a more accurate and realistic way of flying..

Rob2160

Yes, a Barrel Roll should always be a positive G manouver.. the absolute master of this was Bob Hoover..

A great and timeless video of this exceptionally talented pilot...

[youtube]http://www.youtube.com/watch?v=pMWxuKcD6vE[/youtube]

rgburrill

Finally someone (Rob2160) addressed the amount of differential the OP was setting up.  That is way too much.  I have always set the down to about 65 percent of the up - the OP is more like 36 percent.

rgburrill

BTW Rob, while Hoover was certainly great it was Tex Johnston who did that one in the 707.  And to me that is the greatest barrel roll ever.

Rob2160

Yes no doubt about it being impressive.. Would have been incredible to see.

wingstrut

I need to clarify something, When I say that I apply a little opposite aileron I mean a little, less than a 1/4 inch deflection which is nothing in this aircraft.
It helps to stabilize the plane to make a much smoother controlled turn.
I get a lot of positive comments on how my turns make the aircraft look like the full size and not a model.
If I gave it a lot of left aileron as in a flat turn and opposite rudder than I would have a stall.

I posted this question because I wasn't familiar with Differential ailerons, after reading all the great explanations now I know that I don't want it.
I love flying these ww1 planes and I will never mix controls in the radio, I can fly the plane better than the radio mix can.


Thank you all for the great explanations and for taking your valuable time in doing so..........Ron

Rob2160

I understand completely and you are very welcome Ron,

Cheers.

Rob

wingstrut

Thanks to you Rob, that was quite a lesson in aileron and rudder function.

Ron

rmh

Your comment about corrective aileron tells me you have it set close to perfect.
In model flying we don't have instrument panel - we watch and correct as required
When setting up models for aerobatics - we also found a setting such as you have is -"about right

IF you held the initial turn -the setting would result in a tightening spiral-
the very slight reverse aileron held against the rudder applied - kept the model in a predictable turn-
going further into MODEL flying
The right setup for predictable flying is still the best
When we fine tuned designs for predictable very slow rolls - the amount of dihedral would always be such that the application of rudder in final section of the roll- would SLOW the roll rather than increase it
much easier to control
Apparantly some feel full scale practice always applies to our models - it doesn't always apply but in this case it is about the same.
some models turn with no bank or very little - depends on what designs one flies -if you poo poo all types you just limit your knowledge.

wingstrut

RMH,
I think I lucked into it, I had a couple fellows fly it who also have the same aircraft and they said that it doesn't fly anything like theirs, it is more smooth at the controls.
But is not as responsive in the ailerons as their pups.
I am swinging a larger propeller because I have a larger engine, I fly at less than half throttle.
My wing incidence is also different than theirs, that is probably the main difference.
The aircraft balances perfect not nose heavy at all.
Models are just smaller versions of the full size, but flying characteristics do not scale down equally.
I'm not changing anything, the pup flies fantastic, I usually get three 18 minute flights on it every time I fly and then I don't want to land.

You have it correct, that is why I have to hold a little opposite aileron to keep the spiral from increasing and to help keep the airplane in the same altitude and attitude.

If I wasn't trying to fly the plane scale as possible then I would just bank it over and pull on the elevator, but it's more fun trying to make it look like the full size .........actually it's not to far off from being the full size..Ha!Ha!

Aren't these airplanes fun!!!
Thanks for the info RMH

Ron