Multiple Wings Question.

[Wing Zero]

When a plane has another wing, how does it effect the craft over all? If I had a craft, which had two wings of the smae airfoil, shape, and size, what problems would I have? If I had three or four wings, what kinds of things would I encounter? Does a wing that sits behind another wing (inline) affected differently at different distances apart, or the same? How do you calculate CG, and other problems on such craft? Does it change depending on the type of wing and/or use? Okay... example and for sake of discussion, let say we have a Sailplane. The body is about 5' long, similar to a common ARF beginners plane. The first wing is a common Clark Y poly, with about five degrees of forward sweep (2m span) The second wing, same foil and shape would be at three degrees of forward sweep, at about 80% of the size of the first wing. This wing would be about a foot behind the first... what would the outcome be? There are other formats, but I thought this would be a good one to explore first. All up weight about 1 maybe 1.5 pounds more then a normal RE poly ship with a 2m span. Thanks. -Wing Zero

[JetPlaneFlyer]

Biplanes are generally less efficient than monoplanes because the airflow over one wing interferes with the other wing, reducing the overall efficiency... This is why you don’t see high performance biplane gliders.

Triplanes and quadroplanes suffer from the same issue, only more so.

Of course if extracting the last few % of efficiency is not your main objective then a biplane glider could be fun.

Steve

[Brandano]

Not to mention the fact that at each wing tip you have the usual induced drag vortexes. Without going into detail for the mechanism by which these are formed, the energy that goes into the vortex is taken away from the plane, and so it is a net loss. You can probably make an efficient biplane glider, by going for a tandem setup and getting rid of the tail surfaces, like Burt Rutan's Quickie. The long AR of all wings and loss of tail drag and tip vortexes would compensate for the increased wing vortexes. However the biggest problem is still making all that stressed structure light enough.

[ghoti]

Wings on the U2 have a very high aspect ratio and taper amost to a point, hence a lot of lifting surface with minimum drag from tip vortices. Make your wings infinently long and no vortices at all! Every control or lifting surface produces a vortex or two. How about using one wing with a fuselage at each end, tee-hee!

[Brandano]

Well, then you'd have two fuselages producing a vortex each. There's no way to escape it, as long as the upper and lower surfaces of a wing have different pressure (or speed, choose whatever lift theory you want) air going over them, you'll always have a vortex forming behind it in some way. The trick we use to fly is to put energy in an airflow and get some back as lift, but since we can't make a 100% efficient system we never get all of it back, and what's left is wasted by the air to make those pretty curls you see in condensation streaks.

[Tom Harper]

Wing Zero,

What is your goal?

If you are trying to gain efficiency by adding wings, it probably isn't going to happen (see Brandano above).

If the goal is to experiment with multiple surfaces try a semi-scale Gerhardt Cycleplane. The idea was to gain efficiency by using many, very high aspect ratio wings. If you do the calculations it will come out less efficient than a single wing of modest aspect ratio. But building and flying such a model is a lot more fun than doing the calculations. Although I doubt any model could approach Prof. Gerhardts light wing loading.

If your goal is an unusual biplane model that demonstrates very competitive flight characteristics, then base it on the work of John Dunne. His designs were ultra stable and very efficient. The upper wing section is thickened at the tips and washed out to the zero incidence angle. A scheme that is not used today.

At the size of a model, and with modern materials, you don't need all that bracing. Probably would not win any soaring contests, but would get a lot of attention at the flying field.

[BMatthews]

Placing one wing behind the other with no third stabilizer surface is called a tandem wing.

The method for finding the CG for any combination of horizontal surface variations is pretty straight forward and the all follow the same set of calculations.

It is possible to calculate a neutral point for any combination of forward and rear surface areas. Placing the C of G on that neutral point will produce a neutrally stable model in all cases (sort of). Our usual need for some positive stability to make flying chores easier demands that the C of G be placed slightly in front of this neutral point and the aircraft is then trimmed to provide an aerodynamic countering force.

As a conventional single wing and tail becomes larger in the tail the neutral stability point and thus the CG location moves back to the point that with a large enough stabilizer the CofG can be located at the wing's trailing edge and it'll still be stable. As the stabilizer becomes larger and is roughly the same size as the forward surface we call it a tandem wing. As the rear surface becomes even larger than the forward surface it becomes known as a canard. All though this transition in sizing the NP and thus the CG moves back based on the same basic calculations.

Online CG plug in calculator.....
http://www.geistware.com/rcmodeling/cg_super_calc.htm