Biplane rigging

[Terry Rigden]

I'm just finishing off a small aerobatic biplane and I'm wondering how to set the decelage ( difference between the upper and lower wing incidence)

I want an aeroplane with a mild and predictable stall rather than one that flicks. The aerofoil is a 12% symetical section and the top wing is forward of the lower wing by about 20% of the chord and distance between the wings equals their chord

So should I set the top wing with a greater incidence so that it stalls first. Would this drop the nose as the forward wing looses lift first or would it raise the nose because of the extra drag?

Or would I be better with less ncidence on the top wing so the lower wing stalls first ?

Does the airflow ove rthe top (and forward wing) change the effective angle of attack of the lower wing ?

If I use decelage how much would be optimum ? I was thinking that about 0.5 of a degree but I have seen figures of up to 3 degrees used.

Any thoughts or experiences ?

Regards

Terry Rigden

[Sparky Paul]

The incidence setup with biplanes.. usuallly top more than bottom, to achieve a "safer" stall is a holdover from full-scale, and free flight.
With r/c, it's not needed.. the pilot provides the stability.
On my bipes, what ever decelage is there isn't because of planning.. :)

[Rodney]

My experience (some 6 Airomasters, 4 quarter scale bipes and a half dozen in between sizes) all flew best when the incidence of the upper wing was 1.5 degrees less than the lower wing. If you can, build your upper wing in a manner that lets you try different settings. Start with both equal or where the original designer specified then start decreasing the upper wing incidence a bit at a time if you do not like the flight characteristics. Of course, every change will cause you to also have to retrim the elevator neutral. You will find a point where the flight characteristics suit you best. Different designs will most probably require different settings.

[Terry Rigden]

Thanks for the advice guys

I only get one shot at this as the top wing will be fixed before the first flight and altering it won't be practical.

So what sort of flying charecteristics should I expect with negative, zero or positive declage ?

Regards

Terry Rigden

[tim hooper]

Won't having differing incidences just cause unnecessary drag in all situations?

tim :confused:

[HELModels]

I dont think so, here's why: The top wing rides in "ground effect" which reduces overall drag due to lift. At a high Cl, the induced drag of a biplane is less than a monoplane. parasite drag is higher on a positive stagger biplane than monoplane. To get the "ground effect" benefit And reduce parasite drag, you can increase the distance between top and bottom wing, or negatively stagger the wings.

[J.L. Frusha]

According to what I've read, Biplanes have higher drag than Monos of the same area. There is a compression between the wings that induces drag, plus there are twice as many wing vortices... Another source is the struts and bracing.

Won't stop me, though. I'm planning a 1/6th scale Hovey Whing-Ding II. An Ultralight from the 70's.

Mods: Either using the Princeton wing, or real ailerons and possibly flaps. Since wings are removable, will eventually have several sets...

NASA has proven that 'Scale' models perform near enough to the real-thing. I'd suggest using the design specs. AOI, for upper wing is different to soften stall characteristics. Parallel wings DO have better aerobatic capabilities, though...

[Salto]

At a high Cl, the induced drag of a biplane is less than a monoplane.
How can the induced drag of a biplane wing with much lower aspect ratio and 4 wing tips, be lower than a monoplane with the same wing area?

To get the "ground effect" benefit And reduce parasite drag,...
I don't think flying in ground effect will reduce parasite drag. As I understand it, flying in ground effect is like having an artificially higher aspect ratio, which reduces the induced drag.

Graham.

[HELModels]

There are certain regimes of flight in which the biplane is lower in drag than monoplane. I dont care how many vortices are whipping around, the top wing has less induced drag than bottom. Negatively staggering the wings and increasing the distance between top and bottom wing can reduce the interference drag.

[HELModels]

How can the induced drag of a biplane wing with much lower aspect ratio and 4 wing tips, be lower than a monoplane with the same wing area?


I don't think flying in ground effect will reduce parasite drag. As I understand it, flying in ground effect is like having an artificially higher aspect ratio, which reduces the induced drag.

Graham.

Shall I misquote you? No.

http://www.aa.nps.navy.mil/~jones/research/unsteady/panel_methods/anim2/

This link has lots of info done by legitimate researchers which validates some of the whacky ideas I seem to have regarding biplane interaction. Granted, it is a slightly different realm than aeromodels, but it is relevant.