I was just reading in your design area in the 'what are the advantages of a canard wing aircraft'.

You comment that a well designed biplane can have less induced drag than a monoplane of the same span. Now for the stupid question - Why have we not seen any biplane HLGs, or better yet Mosquitos?

I can see that the HLG class is large enough to limit the optimum weight for the average human arm to launch, but what about those little 'skeeters? Seems like with the severe span limit, you could use two of those little wings to grab more air (better mass flow).

From : Don Stackhouse

J.P., that's not a stupid question at all, it's a very good one! Unfortunately, there are some other factors that get in the way for any type of HLG. Biplanes have lower induced drag than monoplanes, but higher parasite drag. The flow fields from the two lifting surfaces fight with each other, causing something called "interference drag". In addition, there is extra parasite drag from the supporting structure required to keep those two wings in proper formation. More gap between the wings will improve the interference drag, but the extra structure required will increase the parasite drag. It's your basic "no-win situation".

At very high lift, such as in a thermal turn, the induced drag dominates. In this mode the biplane is in its own element. If all you had to do was thermal, we probably would see the biplane as the "arrangement of choice". Unfortunately for the biplane, there are very few types of aircraft that only have to operate at high lift. HLG's have a particularly wide range of lift coefficients where they must be efficient. They must have extremely low drag at zero lift for launch, at low lift for penetration, at moderate lift for cruise and search, and at high lift for thermalling. With the possible exception of F3B models, there are very few other classes of sailplanes with as demanding a performance envelope as HLG's. Because of the need for low weight to keep induced drag low despite the limited span, HLG's must do it without the help of elaborate flaps and other sophisticated forms of variable geometry, making their design even more difficult. Anything more than four servos and the weight is likely to be too high. Mosquito class models are even more sensitive in this regard. In my experience, proper design of a Mosquito class model is one of the most difficult and delicate balancing acts in the entire realm of model sailplanes. Because of the high speed requirements, where parasite drag dominates, biplanes usually don't make sense here.

There are some classes of models that do tend to be more of a single-operating-point design problem, where biplanes do make a lot of sense. Weight lift contests such as the SAE competition would be one possibility, and indoor R/C models is another. I'm currently working on an indoor model of my own, and it's a biplane.

Don Stackhouse
DJ Aerotech