My buddy has been flying one of the Hangar 9 Fokker D7's for the last week. Yesterday I noticed that the outside wing was dragging behind in turns and suggested some aileron differential. He said "Yeah, the instruction manual suggested that" and programmed his radio to the suggested values....more DOWN travel than up. That fixed the problem, it flew just as if he were coodinating rudder in the turns.

Now, that's the exact opposite of what I would have suggested. My understanding is that the rising wing makes more drag so you set up the aileron movement to give more up travel than down to take away some of the drag from the top wing. Am I missing something here?

For those not familiar, the D7 is a slow, draggy bipe with smallish ailerons at the tips of the top wing only. It floats along nicely but the Saito .80-something will just pull it through a loop and gives very limited vertical at this stage of its breakin.

You are right, more down then up is what differential should be. If the manual tells him otherwise and it now flies great so be it.

Why it fixed the problem is an example of how many, many things in aerodynamics are just flat out UNFATHOMABLE.

Aileron differential is usually done with cambered wings. It is done by rigging the down going aileron to move less, and the upgoing one to move more. That insures that the inside wing will induce more drag and the outside wing induces less drag.

You understood correctly why differential is needed. Some of the reasons anyway. And how differential is usually done.

Why did the D7 work better when it shouldn't have? In theory, theory and practice are equal. In practice they are often not.

I would also suggest using more rudder and learning to coordinate rudder and aileron in turns. WWI planes typically turn more with rudder than aileron, and usually in coordinated turns lead with rudder and add aileron, and you should see a much smoother turn with less adverse yaw.

And aileron differential is definitely more "up" than "down". It's indeed odd that setting it up the other way worked that well.

I would guess that drag is not the key component in this exceptional example. Lift must be playing a part... somehow.

David

Yes, it's quite mysterious. Without the 'reversed' differential the airplane noticably dragged the outboard wing.
One unusual feature of the ailerons is that they're wider at the tip than at their inboard end (barn-door type) and they have a broad balancing type horn at their outboard end (I've seen it on several WWI types). Maybe that has something to do with it?

Use the rudder for a coordinated turn like Ed says. The same reason you use it on full size stuff.

Guess your right as others had said. If using a two servo wing set up the amount of diifernetial can be simply programmed in with a JR tranny. I would suggest though, that you should also consider having to learn co-ordinated rudder/aileron turns.

The model is suffering from low power -(low speed ) which produces (of course) -higher angle of attack .
the proper differential can be established for only ONE speed range - and if the wings are rigged correctly -this will be more UP than Down aileron travel
On an aerobatic bipe 4 ailerons - the best rigging is wings and ailerons -0-0
All of the early aircraft suffered from the same problems - too heavy and underpowered -which equalled - high AOA to get enough lift. (and more resultant drag.)
The H9 model will essentially need similar rigging . Be sure to NOT rig one wing at a different angle than the other - for best results.

Yes the D VII is draggy and low powered (don't know about the relative wing incidences), and I understand and can do coordinated turns. My question is why the seemingly backward differential works.
My buddy flew the airplane all afternoon yesterday and it looks much more "coordinated" with the reverse differential. Just doesn't make sense to me!

Good chance the reason it works is because of what Dick has mentioned. Draggy and low powered really does cause the wing to need to create more lift. And it does it with AOA. And we really don't know for sure what shape the ailerons are in for effective work. But our models usually are not dragging about. So they're usually not going into the turns like this one is.

There is one thing we can assume. If it is working, it's doing it by either lessening drag over what was a higher drag situation on the outboard wing, or increasing drag over what was a low drag situation on the inboard, or both. Those are what caused the problem in the first place.

I just maidened a H9 Camel yesterday and used 20% aileron differential on my DX7.

Recommended throws are 1 inch up and down for high rates.

I noticed a really slow roll rate on low throw rates (3/4 inch) .

Could this be due to the differential applied?

I'm going to try the 3/4 throws without differential today and compare the turns as well as rolls.

root

ps I failed to mention that I also had an ail/rudd mix of 25% on a switch and it might have been on at the time of the roll. Could that have been the culprit as well??

roll rate will always be slow - it's part of the overall design.
fiddle with differential to eliminate OR accent a barrel roll appearance to the roll.
aerobatic ,contemporary bipes have extremely fast roll rates -but the whole setup/power to weight is far different.
I love the old Camels and that era models, are just plain, plane fun.

If you read what the old pilots wrote then you will find that to roll any of the biplanes up to the mid thirties needed both hands on the stick. To roll a 'Camel' the instructors recommended that the stick be moved full aft and full over at the same time applying full rudder in the direction of the roll. If you think about it, and try it on your tranny, you will see that this is a 'flick' maneuver. It is unlikely that a real Camel could actually do an aileron roll, certainly not against the engine torque. These old airplanes, and it includes all of them, not just specific aircraft, were rudder/elevator aircraft, and the models are the same. So, try using a bit of aileron mixed with your rudder, not a bit of rudder mixed with aileron. Forget rates, go for max possible mechanical movement on everything and use the proportional feature of you radio to fly the thing.
Evan.

Thanks Evan,

That was nicely put.

I will try the scale approach of more rudder less aileron in the turns and won't expect axial rolls.

I did not use the AIL / Rudd mix switch or the aileron differential today and didn't notice a difference in the roll rate or the basic flight characteristics.

I'm going to depend on the proportional features of my brain and thumbs to fly the thing. I may even learn to be a better pilot. That's one of the reasons why I bought this airplane in the first place.

How did you find out about the Camel instructors? The early planes in general? Can you point me to some sources?

Dunno Root, Much of what is known comes from the 'Aces High' book, the Camel instructor is non other than Capt. W E Johns, the 'Biggles' creator. Bits of the RFC pilots training manual can still be found, where you will learn how to do such esoteric things, as a 'correct loop', learn the 'proper landing method' etc., even some basic combat maneuvers. Otherwise read what modern aviators say about flying the reproductions, the Cole Palen (Rheinbeck) crowd should be able to help.
Evan.

Thanks again Evan.