How to design a wing?

[denysy]

Hi,

I have a cheap electric plane (http://interactivetoy.com/presskit/texts/stealth.htm), that doesn't have airlons and turns/climbes by changing the speed of two engines. I want to use all the electric stuff from it to make a new plane. I got few pieces of epp foam for free and want to use them to make wings (two 34.5x6.3x0.9cm pieces, and one 29.5x16.2x1.4cm); I'm not sure what density they have, but I'd guess 1.3

Here's how much everything weights:

2 motors and props 11.8 X 2 = 23.6g
Receiver and all wiring 11.3g
Std 4-cell pack 31.5g

Each engine gives 25g of thrust.


So my question is how do I pick airfoils, how do I know how big the wings should be, how do I calculate the drag, speed etc?

Denys

[Ollie]

You begin by desiding what type of flight characteristics you want the plane to have. Then you try to quantify and prioritize those characteristics. This data then becomes a standard against which you can judge the virtue of the many design decisions to resolve conflicting objectives that are involved. You can start with the characteriatics of the model that the equipment came from. Calculate its performance and deside how you want to modify it for "improvement." The process goes something like this: analyze, modify, reanalyze, judge results and consider new modifications. With sufficient iterations, the process should converge on your objectives.

Nature evolves its designs by a slow process of random variations and natural selection. You can evolve your designs so very much faster by insiteful variations.

Equations analyze. Design decisions synthesize.

[denysy]

I am absolutely happy with flight characteristics of the first plane. The reason I I'm making this one is because I've crashed the first one many times and after numerours repairs it has became too heavy to perform the way I want it to. What I'm asking is how to chose what kind of airfoils to chose and how big the wing should be. Maybe there's some kind of airfoils-table that would give me different airfoils and tell me what lift and draugh they produce? At least that's what I'm looking for.

[Ollie]

Just copy the airfoil and wing area of the original plane to get the original performance.

If you use EPP for the wing you will have to thicken and mylar tape the wing for stiffness. If you thicken the wing airfoil you will increase both weight and drag, degrading performance.

I suggest that you copy the original wing in balsa wood instead of foam. Find an unrepaired part of the wing and cut out a section. Measure the weight and area of the section. Pick the thickness and density of balsa wood sheet to match the weight per area of the original undamaged wing. Soak the balsa in warm water and form the camber to the same camber as the original. Hold the shape while the balsa dries out to lock in the camber.

One of the reasons for shying away from the analytical approach is that the model's wing operates at a reynolds number where it is extremely difficult to find reliable drag data for airfoils. Another reason is that it is not necessary to "reinvent the wheel" to meet your design objectives.

If you want to design something using analytical tools, pick something larger and more powerful that operates at a size and speed where data is available. For reliable data to be available, the reynolds number should be above 60,000.
That means that the product of the wing chord in feet times the minimum airspeed in feet per second has to be equal to or greater than 60,000/6,380.

[denysy]

The first plane airframe is very complex. I can't reproduce it.

[MikeSt]

Originally posted by denysy
The first plane airframe is very complex. I can't reproduce it.

If you want to build something, you can buy the JSF plans for $15 from here

http://www.acesim.com/rc/jsf/jsf-guide.html

If you just want to replace your plane, the easiest solution is probably to just buy another one :)


Here is a nice little applet that lets you experiment with wng loading / flight speed

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

A drag estimator
http://www.gylesaero.com/_frames/f_dragcalc.shtml

Mike

[Ollie]

Flying wings are the most difficult of all to design. To learn how to do it see:
http://www.b2streamlines.com/winglinks.html
Pay particular attention to the next to last link, parts one through five.

For a practical solution, just buy a replacement.

[MikeSt]

Originally posted by Ollie
Flying wings are the most difficult of all to design.


Ya think ? <g>

I had a "brilliant" idea for "cheap design" ... or so I thought.

Cardboard !

So I "folded" up a little delta wing about 15" span, mounted some gear on it and went out to the field, full of excitement ;)

Total "design" and build time about 2 hrs <g>.

..About 2 hours later I gave up trying to get it to fly, by moving the battery pack back and forth. It would either "mush" / stall, nosedive, or flip straight up. "Reflex" ? .. twist ? ... Heck my "design" might have "accidentally" had some twist in it ... especially after 40-50 crashes :)

One of these days, I'll get motivated and actually build the JFS plane from the plans that I bought two years ago <g>.

I like the looks of the "Carbon Falcon Arf" ... but I'm not too excited about the ~$200.00 Cad price.

For now, I'll stick to "conventional" wing & tail designs, which are much more forgiving, and easy to at least get *flying*.

Mike

[AndyKunz]

There was a guy at the NEAT Fair this year who had an all-sheet B2 that flew really well. We talked to him for quite a bit, but alas, he didn't use plans.

Andy

[Ollie]

Here are some hints if you want to experiment with a balsa wing replacement. The airfoil should be close to a thin flat plate. One or two percent camber can be made to work but its high point should be well forward at about 15% or less of the chord from the leading edge. The tip panels should be washed out about 5 to 8 degrees. The airfoil for the center panels should be equipped with a reflexed elevator that is used to set the glide trim. The CG should be well forward to begin and moved aft gradually as the reflex of the elevator is reduced to achieve pitch trim. When the CG is too far aft the wing will loose pitch stability and the CG moved forward some to restore a little stability. The less stability required the more efficient the flight capabilities.

The lower the gross weight, the better the model will climb.