CG considerations when adding an upper wing?

This could be interesting. Adding weight and drag in search of lower stall speed seems counter-intuitive, but perhaps you'll prove that initial reaction wrong ;-)

Cheers - and good luck

/daytripper

On Thu, 16 Aug 2007 21:30:55 -0400, daytripper wrote in :

Agreed.

But look in this newsletter for a reprint of Jerry Neuberger's article on calculating MAC & CG range for biplanes:

Let us know how it all turns out!

Marty

Yup Doubling wing area for a modest increase in weight will lower the stall speed..IIRC by about root two, so *to* 71% approximately.. Say

You will need to push the CG a bit more forward as the extra wing area will not have extra tail area to compensate..if you add an upper wing staggered forwards, it will need to move even further forwards. On a sopwith camel, which has a tiddly tail, and foward stagger, it ends up a little behind the LE of the lower wing for example.

Thoughts along the lines of 'surely it doesn't need to be *that* far forward?' are rapidly overcome on the first (and last) flight ;-)

If in doubt mock up a scale profile chuck glider out of balsa or depron sheet. Apart from being loads of fun, it sorts out the CG issue very quickly and at MUCH less final expense..

| Yup Doubling wing area for a modest increase in weight will lower the | stall speed..IIRC by about root two, so *to* 71% approximately.. Say | *by* ~25% if the extra weight is factored in.

Your reasoning is sound, but the problem with biplanes is that they're less efficient than monoplanes, as the airflows for the two wings interfere with each other which makes both less effective. So the new stall speed is likely to be faster than (old stall speed)*(1/sqrt(2)) even if the weight remains the same. But it's probably a good first guess.

| Thoughts along the lines of 'surely it doesn't need to be *that* far | forward?' are rapidly overcome on the first (and last) flight ;-)

If the plane has a landing gear, put the CoG way forward and try to fly. If it's way too far forward, it won't even take off -- which generally also means it won't crash, unless you ground loop it or hit the fence at the end or something :)

His reasoning is very sound. At stall speeds there is very, very little interference between the two wings if any at all. Interference only happens at faster speeds. Most of the interference takes the form of the large tip vortex generated by a biplane vs a mono wing. This tip vortex is why biplanes make such popular crop dusters. The big tip vortex smashes the spray directly down onto the crop canopy and cuts drift problems.

Even at speed there is little interference between the two wings with respect to lift generation. The reason is because of the greater lift available a biplane flys at a given speed at a lower angle of incidence then a mono plane with greater wing loading per square foot of wing. The lower incidence means less air from below the leading edge is deflected upwards in front of the wing. So the deflection caused by the upper wing does not have to so great as to interfer with the deflection of the lower wing shorting either wing of air flow. In fact if the upper wing is a bit in front of the lower wing the two induced air flows help each other. Just like a main sail and jib on a sail boat. The two together work better then the sum of the individuals.

But the tip vortex does cause a major increase in drag. In a model this is not all bad. It means that the extra drag will help slow the plane during landing. This is good as the two wings make the thing a real floater and without this drag it could be hard to get slow enough to get it on the ground. Models are typically so over powered that this extra drag is inconsequential during flight.