I made a small electric biplane recently. All from 0.505 blue insulating styrofoam. The Pitts Special was the outline inspiration. The span is 30", constant chord of 5", approx area of 295 sq in. The plane weighs 16 oz with rudder, elevator, ailerons, ESC. I only put ailerons on the top wing. Intended to put them on the bottom wing but the constant removal of the bottom wing led me to put them on the top wing, which is permanently attachec. After flying the plane I plan to put ailerons on both the top & bottom. The roll rate wasnt good. The blue foam is reinforced with 2" wide bi-directional fiberglass tape & covered with Econocote. Really light & strong

Now, the point of the post. I made the wings with different airfoils. The top wing is barely semi-symmetric, a raised-LE flat-bottom might be the way to label it. The degree of curvature is about 25% that of the top surface. The top wing has Pitts looking sweep and about .75" dihedral on each tip.

The lower wing is almost fully symmetric. No sweep, no dihedral.

My feeling was the lift characteristics were linear mathematically and the characteristics of both airfoils would be available to some degree. (They would add up, so to speak, much like the properties of oil in a multi-grade variety ... or wave phenomena, etc.) I wanted a slightly positively stable plane that wouldnt croak upside down.

The CG seemed happy about mid-way on the upper wing, which was staggered 50%. Decalage (sp?) was eyeballed to be 0* lower, -1* upper (very close to 0.5, enough to see it was slightly lower AA than the lower wing.

As for flying ... it barely flew on a 3:1 280 speed, 8 cells. It soared on a slope really well with no power. With a 4:1 speed 300, 8 cel, 8x6 prop it really flew well. When the speed dropped, the tip stalls were meaty. It was good that the recovery was pretty fast...

Anyway, I lost all power about 50' up and shattered the fuselage. (read homemade battery packs & worn out connector leads :O ) It was not covered at all. I plan to replace it with a 1" wide, near profile fuselage.

Does the idea of two different airfoil profiles make any sense? Before I rebuild the fuse, I want to know if I should also make another wing. If I do so, I will have both very close to full symmetry.

Derek

------------ My feeling was the lift characteristics were linear mathematically and the characteristics of both airfoils would be available to some degree. (They would add up, so to speak, much like the properties of oil in a multi-grade variety ... or wave phenomena, etc.) I wanted a slightly positively stable plane that wouldnt croak upside down. ---------------

The stability of the airplane is a function of the CG location relative to the Neutral Point of the airplane and doesn't really have any thing to do with the airfoils. All you get with the mix is that at a certain negative angle of attack the top wing stops creating negative lift slightly before the top wing. There is not much to gain from not having both fully symmetrical in a model. Sometimes the mix isn't necessarily a beneficial thing to do.

In full size man and cargo carrying biplanes like the Beech Staggerbi that never do more than a loop and roll once in a long while then an airfoil optimized for upright cruising would be a better choice.

---------------- As for flying ... it barely flew on a 3:1 280 speed, 8 cells. It soared on a slope really well with no power. With a 4:1 speed 300, 8 cel, 8x6 prop it really flew well. When the speed dropped, the tip stalls were meaty. It was good that the recovery was pretty fast... -------------

It is pretty heavy for a speed 280. Fast recoveries are always welcome!

The little bipe described, is a classic case of " do very small, light models really respond to the rules for man carrying sized craft?" question.
my own findings are that:
A, you really can't make it light enough to simulate the flying abilities of the big un.
B, The very best one can do is to make it as light as possible and disregard the airfoil-
Rather choose a foil based on the strongest -light"streamlined board " one can make.
as for relative AOA of upper and lower panels - keep em the same -Al of my own attempts with small (as described size) stuff like this is that the only things that mattered were wingloading and CG.
You can't overpower it . it is impossible.
However- if you figure out how to do that - let me know .

It's really too small a plane to worry about finessing the aerodynamics.
This is my long-suffering Goldberg Mirage with the wing on upside-down AND backwards!
Other than having to reverse the rudder to have any control at all due to the inverted dihedral, the plane flew. Not well, but it did fly.
With the wing mounted right side up but still backwards, it flew well... for a Mirage.
All you need on these small airplanes is a curve to change the direction of the air flowing, front-to-back.
Flat plates do it with alpha..

The first Pitts Special had a top symetrical wing and a flat bottom lower wing. I think that was the right way around.

Yes -that's one way to do it -
There are other setups -full scale-which were extremely good - for example-- various permutations of the Bucker Jungmann also the Jungmeister - --and they are not the same plane -
Airfoils are great examples of "it worked so it must be right" -
Problem is - if the wing loading is low enough - the airfoil means less and less -
bipes with 0-0 setups and absolutely equal foils work just great - with low loadings and high power setups.
The main compromise now is setting the plane up so that the pilot does not pull 10 g's before he knows it!
In some new monoplanes it is worse - I saw pics o Jurgis Karis' (sp),latest effort -- it looks like a huge radial with vectoring surfaces - -incredible example of with enough power anything will fly.
I would bet that takeofk is possible in 5 seconds from hitting the throttle

Dick, do you have a site for the airplane you mentioned or some locating material. It sounds interesting.

Paul's data points are really interesting. It takes more courage than I could muster. Of course it could mean that the original airfoil was the dog of the century!

Finesse in airfoils pays off when you are designing for full scale long distance load carrying machines. There a few counts of drag saved converts directly into dollars saved. When we were doing the F-15 wing we spent a lot of time trying to get a compromise that lowered the drag and hopefully still hung on in the high load factor transonic region where buffet onset is a real problem. Even then when we went into flight test we tufted the upper surface to the wing to see where the air was going. It was interesting stuff.

Finesse in little light airplanes sometimes is wasted because we just can't evaluate it from the ground. You can measure roll and loop rates but beyond that every feedback from the airplane is through the eyes. Not the most precise range finders around although they arn't too bad. You can get a feel for landing speed that isn't too bad but anything else is really tough to quantify.

The original (full size) Pitts had an arrangement that made sure the upper wing would always stall before the lower wing. The designer received a U.S. patent on this idea. It made sure that during a stall the airplane would pitch forward, because the lower wing was behind the cg, and it was still flying. The lower (and rearmost) wing had a thicker and more stall resistant cord than the upper wing.
Puff

The new aerobat by Karis (sp) - is -strangely enough -- in the new March issue of RC Modeler - I don't remember the page number -
He was a top Sukhoi pilot and this is his "shot " at an even hotter setup --
The Pitts setup to provide a predictable stall- -wasn't that back in the late 40's? Betty Skeltons Stinker? (early 50's)
current aerobats have a rather (ahem) more aggressive performance arrangement.