I am considering building a revised version of a very nice .25 powered biplane I once had. But I would like to reduce the inter-wing gap to eliminate the cabane struts. On the stock version the wing gap is 1.04 times the wing chord. (For comparison the gap/chord ratio on a Sig Hog Bipe is 1.0) If I reduced the ratio 30% to 0.73 what would be the consequences? Will I ruin the model's flying qualities? Note that the wings on the stock version are staggered 25% of the chord.

.7 is about as low as gap should go.
Will you notice a difference?
Only if you can compare the previous plane to the new one.

If a smaller gap causes a loss of total lift would it make sense to perhaps add one bay to both wings?

??
If the wings are too close, and operate inefficiently, then adding span won't help...
If that's what you mean by "bay".
A gap between .7 and 1 is normally found to work well.

Extensive wind tunnel tests in the late 1920s and early 1930s pinned down the effect of biplane wing gap. Gap to span ratio is more useful in determining the effect of gap on induced drag, but I reduced some of the best figures to aspect ratio 6.0, then related induced drag to gap/chord ratio at this typical aspect ratio.

Reducing gap/chord ratio from 1.0 to 0.7 will reduce maximum lift coefficient by 4.9%, increase induced drag by 7.6%, and increase minimum wing profile drag by just under 1% for thin airfoils, but considerably more with a 15% thick symmetrical airfoil. It would probably be best to accept the slight weight penalty of keeping the gap about equal to the chord (a good all-round compromise), unless the airplane is intended for top level precision aerobatics and a lower drag center is absolutely necessary.

I have found by experiment that adding span while keeping gap constant greatly helps to reduce induced drag - high induced drag is a serious bipe bugbear, and increasing span helps almost as much as it does in a monoplane. Monoplane induced drag is proportional to the square of the span, so increasing span by a measly 10% will reduce induced drag by about 19% in a monoplane, and about 17% in a typical bipe. I would highly recommend increasing the span a bit, but be sure to increase the vertical tail area by at least the same proportion, since longer wings produce higher yawing moments, and thus force the vertical tail to work harder down near stall.

Actually, it would probably be helpful to increase the vertical tail area even more, since nearly every bipe I have flown could have benefited by increased vertical tail (fin and rudder) area. The Ultimate is an excellent example of a bipe with sufficient vertical tail area, although it could benefit a lot from less induced drag by using higher aspect ratio, tapered wings.

Frank Courtney, renowned WW2 era experimental test pilot said, "I have never flown an airplane that has enough vertical tail area."

Reference: Biplane Secrets, by Carl Risteen, from Air Age's "How To's", Vol. 2 - available through hobby shops and bookstores.

A related question has popped into my mind. If one uses a staggerwing configuration, how close should the wings be vertically then? I'm thinking that the LE of the top wing would be even with the TE of the bottom wing. I know I could make the vertical gap smaller, but by how much?

Thanks for that very detailed explanation.

Stagger reduces induced drag in biplanes, but its effect is roughly a third as much as that of gap. Increasing stagger by 0.3 of chord will reduced induced drag by about the same degree as increasing gap by 0.1 chord, within the normal range of stagger. Going to positive stagger of 50% of the mean chord will increase maximum lift by about 5%, according to old wind tunnel tests. The same amount of negative stagger, like on the Beech 17, will slightly improve lift/drag ratio, and make the stall more abrupt, compared to an unstaggered layout.

Unusually high stagger will tend to require increased horizontal tail area and elevator area to fight the tendency of the wing combination to resist pitch, much like a tandem layout such as the Rutan Quickie, which has no horizontal tail.

Most of my own bipe designs use a gap of about 1.25 times the mean chord. This would result in an impractically large gap at an aspect ratio of 6. I like to use tapered wings, with the gap at the root about equal to the chord, which is easy to accomodate and gives good aerobatic line-holding. Layout is generally similar to the little indoor bipe in website
http://ramicrolite.com

My previous post has a bad typo - induced drag is, of course, inversely proportional to the square of the span, not directly proportional.

Thanks so much for the explanation, Rotaryphile. I've been competing in the SAE Aero Design West competition for the past two years, and will do it again next year, and the rules for next year limit wing span to a maximum of five feet, so a biplane is definitally in order. This competition is in a way an exercise in increasing the lift to drag ratio, so every little bit will be helpful (such as staggering the wings).

No problem, Natoquick. If you need a lot of lift within a limited wingspan, a bipe can be a solution, and bipes can be built lighter than a monoplane with the same wing area. Being an inveterate design junkie, I would love to hear how your design is progressing.

Well, since it's during the summer, nothing has progressed so far. This is a collegiate design competition, with a lot of emphasis placed on the design part (written report, plans, oral presentation, and payload prediction). Then the teams meet up in April and June (East and West competitions, respectively) to prove their design. Similar to the AIAA Design/Build/Fly competition, only there are no special tasks: just lift a bunch of weight and get it back down on the ground in one piece. For more info:
[link=http://www.rcuniverse.com/forum/SAE_Aerodesign_pics_%26_discussion/m_1741764/tm.htm]Another Thread About the Competiton[/link]
[link=http://www.sae.org/students/aerodes.htm]The official website[/link]
[link=http://www.barney.gonzaga.edu/~mhabjan/]My Team's Website[/link]

I've got some ideas floating around in my head right now, but nothing too definite. I'm probably going to be Chief Aerodynamisist for next year's team, so I've been really thinking on the best way to decrease drag and yet still be able to lift a lot (which is tricky, since as lift increases, so does induced drag).

Hi Natoquick:

Here's an old NACA test report which investigated the effect of larger than usual stagger in bipes: http://naca.larc.nasa.gov/reports/19...i?page0002.gif

Bipes are, generally speaking, no champs in the lift/drag sweepstakes, but within certain design limitations, particularly within a given wingspan, a bipe could be interesting. Adding a second identical wing to a monoplane may not hurt its lift/drag ratio, assuming that parasite drag in not increased by much, and its lift will be increased by about 90%, permitting it to fly much slower. The thrust required to maintain flight will be higher, but the thrust horsepower will probably be lower, so that the airplane may be able to lift a given load while using less power.

I like to convert bipes to equivalent monoplanes for drag calculations, and a bipe of aspect ratio 8, with normal gap and stagger, will have induced drag about the same as a monoplane with the same wing area, with aspect ratio around 5.5 Two wings of a given design can be built considerably lighter than a single wing of double the area, due to the old square/cube law.

I find that it really pays to be very careful to minimize parasite drag on bipes, by use of sleekly airfoiled cabane and interplane struts. My own design taper-wing bipes have fully cowled engines, and are quite aerodynamically slippery overall, resulting in glide angles roughly the same as those of most monoplanes, and they can turn considerably tighter.

Interesting problem.

Thanks a ton, Rotaryphile. I'm really looking forward to the coming competition.