Guys I have a question about a biplane design I'm working on.
Both wings have a constant 15" chord. I'm planning on staggering the upper wing about 3 to 4 inches ahead of the bottom wing. My question is knowing this do I still need a 15" gap between the wings for efficient aerodynamics? Or can I move the gap closer say 12" or so?
Any help would be appreciated. I'd like to design a slow fliying biplane with 48" span wings. The airfoil is a thick flat bottomed Clark Y type.
Thanks for the input.
Tom

A gap greater than .7 of the chord is quite common on bipes.

0.7 is pretty much as close as I'd want to go. There's no definite rule and bipes with as little as a .5 separation have flown but with tight spacings comes mucho drag and steep glide angles. Depending on what you're doing this may be acceptable. But with tighter spacings there is also a loss of wing effectiveness to the point where a 1000 sq inch total may only act like it's a 600 to 700 sq inch. Keeping the spacing up around 1.0 to 1.3 minimizes these effects.

But it's tough if you are doing a fun fly type. But with those the extra drag will create a nice constant speed sort of effect and will help duct the air during knife edge. In short you can get away with pretty much anything when the model is super light and has a honkin' big engine.

This question was kicked around quite a bit a week or two ago - would suggest looking at this URL: http://www.rcuniverse.com/forum/Bipl...2019961/tm.htm

Rotaryphile,

I would like to add one or two thoughts. You stated you wanted a slow flying craft. The lighter the wing loading, then the less power and air speed that is needed to generate the needed lifting forces. I cut lightening holes in everything and I mean EVERYTHING.

If you want a good stable craft, build washout into the tips, raise the tip about 1/4!QUOT! or so on a 15!QUOT! chord beginning the bending of the wing at about a point 25 - 33% inward of the tip toward the root. The purpose here is to stall the root of the wing before the tip.

One last thought, set the top wing incidence at 1% greater angle than the lower wing. You can go even further if you like. The purpose is for the top wing to stall first, thus dropping the nose and regaining the needed air speed to maintain flight. This incidence differential is what makes biplanes such forgiving craft.

The above does not apply to aerobatic craft where the incidence is 0. However, I watched a home builder of a full scale aerobatic bipe, use a less efficient airfoil on the top wing to achieve the same forgiving characteristics.

Best regards,
THC

Hi Tommysta: Wind tunnel tests conducted back in the early 30s showed that an inch of stagger was roughly equal to a third of an inch of gap, in reducing induced drag caused by mutual wing interference. If you add three inches stagger, you can reduce the gap by about one inch, and keep induced drag the same. I like to keep the gap in my bipes about equal to or greater than the average chord, for taper wing bipes. Wind tunnel tests also showed that aerodynamic efficiency of bipe wings was maximized with a degree or so greater incidence on the upper wing, but controllability near stall was not improved, since forcing the upper wing to stall earlier also adds destabilizing pitch-up, as its drag increases much more rapidly than its lift decreases as it approaches stall.

Hi THC: I agree 100% with your quest for lightweight construction, particularly with bipes, which do not carry weight as well as monoplanes, due to their higher induced and parasite drag. I am not terribly big on lightening holes, though, preferring built up structures or thinner sheet fuselage sides with lots of stiffeners. I have found balsa plywood very good for weight reduction, and it withstands crashes extremely well. I make balsa ply by laminating three layers of balsa with epoxy adhesive, with the grain of the middle layer at 90 degrees to the outside layers. It is too easy to use excessive epoxy and end up heavy; I have found that spreading the epoxy very thin, and scraping as much off as possible with a wooden coffee stick produces a very strong laminate, provided that you use a lot of C-clamps and 3/4" ply to squeeze the balsa sheets together very firmly as the epoxy cures.

on very light model
No wash/twist wanted
No differences in AOA for top bottom wing needed
unless - you have some very weird setup with top wing way above effective thrust line.