Biplane wing positions

[MooseFlyR]

I've tried looking around and I haven't found good enough expalantions. So, I thought I'd ask the experts...

Case in point: Small biplane with 16" WS and thin wings. 0 incidence in lower, slight positive in upper. Wingloading numbers look fine. Very little stagger. Both wings same size. Being small, there is not huge spacing between. Triplane in similar setup too. Flies ok on DD and fast, but struggles on geared with what should be plenty of thrust. Fast is what it was designed for, but some are trying to do the other with minimal success.

Some incidence should help lift, but about the stagger positioning. Does not enough interfere with lift and/or create drag? Any other suggestions to help geared slower speed flight envelope? (while keeping the same basic wing size / thickness)

I know the small thin wings are more for speed, but it just seems the flight envelope could be a bit wider with a little tweaking. Thanks for any suggestions or explanations.

[Ollie]

There is interference between the wings of a biplane. The closer the wings are together, the more drag and the less lift. A spacing of 1.5 chord lengths between the wings will reduce their efficiency by roughly 20%. A spacing of 2 chord lengths will cut the penalty in half.

If you want to widen the speed range on the high end, reduce drag. If you want to lower the stalling speed use flaps or reduce wing loading.

[MooseFlyR]

Thanks Ollie. That's a start. I don't have one, but I'm trying to help out and learn at the same time. The planes are built and flying so not much can change except for wing positions and angles and such. Great little speed demons, and asking to tame them down might be a bit much, but folks are trying and I'm sure would appreciate any help.

Can anyone please explain the stagger relationship beyond it effectively lengthens the chord? Possible effects of too much or too little?

Thanks

[Ollie]

With enough stagger the biplane morphs into a tandem wing configuration and the horizontal tail can be despensed with. The French "Flying Flea" and Burt Rutan's Quickie are examples. The Flea has positive stagger and the Quickie has negative stagger.

The design gets tricky because the aft wing is flying in the downwash of the fore wing. Down wash extends several chord lengths above and below a wing. The fore wing reduces the efficiency of the aft wing because the fore wing slows the horizontal velocity of the wake when the down wash is produced.

The down wash behind a 747 is so powerful and lasts so long that it isn't safe to land behind one for a couple of minutes after it passes.

[MooseFlyR]

Thanks again Ollie. Yeah, I've waited to take off behind a big bird before. I'm starting to think that the spacing of maybe 1/2 chord length at best isn't helping any. BTW adding some incidence and some "flap" in the ailerons helped a little.

[Ollie]

It may help to restate some fundamentals. An airfoil produces lift and drag by virtue of its shape and angle of attack (angle to the line of flight). When a control surface or flap is deflected, it changes both the effective mean line camber and angle of incidence (angle to an arbitrary reference line).

If the ailerons are drooped it changes the shape of the airfoil increasing its maximum lift coefficient and incidence. This in turn changes the pitch trim, increasing the trimmed pitch attitude and lowering the airspeed just like a little up elevator trim. The increase in maximum lift coefficient lowers the stalling speed too. Depending on the actual angles and location of the hinge line involved, the stall margin may be increased or decreased.