Biplane difference incidence

[dusty IV]

I'm finishing up a .46 U Can Do bash that I'm converting to a biplane. I got the bones for free from a friend who crashed it. The damage was minor and I decided the basic layout would make a good bipe. er maybe.

It's had to be a stagger wing to get the CG location correct. I lowered the bottom wing but kept it in the same location front to back.

I'm not familiar with how to lay out the difference in incidence between the two wings. Got any idea if the top wing has more or less incidence compared to the bottom wing? How many degrees?

The top wing has a bit of sweep back.
I'm after maximum stability/ stall resistance.

[MarkusN]

With standard stagger (top wing in front of the bottom one) the top wing has less incidence. This is to compensate for the circulation of the bottom wing which is directed upwards at the location of the upper wing. (conversely circulation of the upper wing is directed downwards at the location of the lower wing.)

I only remember a rule of thumb of 0.5 to 1°, I don't know how to calculate it precisely off the top of my head.

For stall resistance you want the top wing to stall first, lest the plane fall into a deep stall when it loses lift on the rear one of the two staggered wings. this means that you probably won't lower the incidence of the top wing as much as would be optimal for induced drag.

[olmod]

If the upper wing is forward its known as forward stagger,here is a recomendation out of "Design for Aeromodellers"(old MAP publication) incidences of
lower wing =Y upper wing =X for forward stagger it recomends that X is from the look of the illistration is ~2deg more positive and goes on to explain how the lower wing then acts as a stabilising factor when Y aproaches stall.
hope that helps :) cheers.

[JetPlaneFlyer]

There you have it Dusty.. Two diametrically opposite answers to the problem, both with some reasonable sounding justification :rolleyes:

What about split the difference and make both wings the same incidence ;)

Steve

[vintage1]

Mmm. Tuning a biplane for minimum drag sounds a bit like putting low profile radials on a bicycle ;)

I'd set the top wing a degree and a half more positive, and let the drag issues get submerged in the strut work.

Err on the fowrad CG side if you add more wing area..it reduces the effectiveness of the tailplane.

[MarkusN]

Mmm. Tuning a biplane for minimum drag sounds a bit like putting low profile radials on a bicycle ;)

Point. I guess the idea is more to have both wings usable for a similar range of angles of attack (thus using near maximum lift of both wings), while still making sure that the forward staggered wing stalls first.

[Rodney]

I can not speak for a stagger wing but: in some 14 to 15 biplanes I have built and flown inclucing 7 Aeromasters, two quarter scale Lazy Aces, two 1/4 scale Moths, two Phaeton 90's and two Phaeton 40's, all flew best with the upper wing about minus 1.5 degrees with respect to the lower wings incidence. If you can, make your upper wing mounting in a manner that will allow you to experiment a bit as just a degree or two of change can make a huge change in the way the plane will handle. If you do that, come back and let us know what worked best for you.

[dusty IV]

I went through this guessing game with my smaller conventional Jitterbug type bipe. Gave up on theory and set the top forward wing with about 1.5* positive incidence compared to the bottom wing. This worked great and the plane doesn't have any stall that I can find.

The wing on my U Can Do has the top wing set behind the bottom wing. Like in a stagger wing Beach. So I'm thinking maybe negative incidence. Why you ask? I have no idea except if the wing is back a wards maybe the incidence would be to. I wonder what Beach did?

But on the other hand , drat I hate crashes.