How strong must a wing be?

[tumad]

Hello,

I've just finished my Extra 300 s, scratch built with my own plans, and was wondering how strong the wing must be to prevent it from snapping in half during a loop say.
Wingspan 1050mm
Flying weight 1600g
Electric.

Where must I support the wing, to measure this strength. Must the wing be mounted in the plane, or must I measure this with the wing out of the plane. Should the wing be able to support twice the weight of the plane, and if yes, measured where?

Thanks for your help.

P.s. The plane has flown already, with perfect results, but just to make sure...

[Dick Huang]

tumad,
Design it to withstand 12 to 15 g's positive.
Dick Huang;)

[DaveSawers]

Which means it should be capable of supporting 12 to 15 times the weight of the model. Faster, pylon like models need to be even stronger, perhaps up to 40 times.

How do you test it? Very hard to do meaningfully and accurately. Experience helps a lot!

[Bill Glover]

If the plane has already flown successfully then I wouldn't worry about it too much!

Otherwise you should probably suspend the whole model inverted, then load the wings up with weight (bags of sand or lead shot) evenly spread along the spar/CG line ... up to a total of 12 times the AUW (or whatever). I'm sure there's a more scientific way but for practical purposes that's close enough, and I'm sure it's what they used to do full-size. With an aerobat like an Extra you'd need to repeat the process with the plane upright to make sure you weren't going to fold the wings in an outside loop.

[Ollie]

Here is an analytical approach to answering your question:

1. Find the maximum coefficient of lift of the wing's airfoil.

2. Pick a redline speed that you don't think you can or will exceed.

3. Calculate the maximum lift that the wing could generate at the redline speed if a gust or control input suddenly drove the wing to maximum angle of attack before stall.

4. Assume the lift calculated in step 3 is distributed uniformly over the wing span. (This is a very conservative assumption that introduces a generous safety factor while simplifying the calculations.)

5 Integrate the lift distribution to find the sheer distribution.

6. Integrate the sheer distribution to find the bending moment distribution.

7. Calculate the sheer strength distribution and bending strength distribution of the spar structure to see if it meets the requirements of steps 5 and 6.


If you wish to pursue this approach "Ollie's Design Service" will be glad to help you through it in this open forum. How's that for an offer? The rest of the forum participants can check the work and comment on the proceedure.

[Sparky Paul]

Dive it!
Pull up REAL hard!
If it breaks, build #2 stronger. :D
Otherwise work with Ollie. It WILL be an education! :)

[MikeSt]

Ollie,

Not my plane, but I'm interested in the calcs.

Originally posted by Ollie


1. Find the maximum coefficient of lift of the wing's airfoil.



"1" (just a guess)



2. Pick a redline speed that you don't think you can or will exceed.


60 mph


3. Calculate the maximum lift that the wing could generate at the redline speed if a gust or control input suddenly drove the wing to maximum angle of attack before stall.


L = 1/2 rho * V^2 * S * CL

0.001185 * 7747 * 1.8 * 1

= ~ 16 lbs


5 Integrate the lift distribution to find the sheer distribution.

6. Integrate the sheer distribution to find the bending moment distribution.

7. Calculate the sheer strength distribution and bending strength distribution of the spar structure to see if it meets the requirements of steps 5 and 6.



*Now* you've lost me ;)

[Ollie]

Mike,

You left out step 4. The 16 pounds of lift must be distributed across the wing span. What is the wing span in your example?

We will also need the height of the spar, the material and crossection of the spar caps, the material and thickness of the shearweb.

[MikeSt]

Originally posted by Ollie
Mike,

You left out step 4. The 16 pounds of lift must be distributed across the wing span. What is the wing span in your example?


I was using the wingspan given above ~41.4 inches. Took a guess at a ~6in chord.


We will also need the height of the spar, the material and crossection of the spar caps, the material and thickness of the shearweb. [/B]


Spar height = .5 inch ??

Sparcaps, shearweb. I honestly have no idea what those are.

I'm not building a wing at the moment. But I'm curious about about the calcs.


Mike

[Ollie]

Step 4. OK, taking the span as 41.4 inches and the lift as 16 pounds, the uniform load distribution is 16/41.4= 0.386 pounds per inch.

Step 5. The integral of the load distribution is the shear distribution which is a triangle with zero shear at the tip and a shear of 0.386x20.7=8 pounds at the wing centerline.

Step 6. The bending moment distribution is a parabola with zero at the wing tip and 1/2 X 20.7 X8 = 82.8 inch pounds at the wing center line.

Step 7. Since the spar height is 0.5 inch the maximum compression load in the top spar cap is 82.8/0.5 = 166 pounds and the maximum tension in the bottom sparcap is also 166 pounds. If the top spar cap is spruce with a compression strength of 5,610 pounds per square inch of crossection, then the required crossectional area is 166/5610=0.0295 square inches at the wing center. The closest standard size spar cap is 1/8X1/4=.03125 square inches.

Eleven pound per cubic foot density balsa has a shear strength of about 300 pounds per square inch. The required thickness of a balsa shear web is 8/300=0.027inches. The neares standard size stock is 1/32 thick, vertical grain balsa shear webs.

[DaveSawers]

Which is all very well, but don't forget the safety factor you'll need to account for weak spots in the wood, more violent manoevres than you intended, heavy landings, etc. etc.

[Ollie]

Dave,

I was just working with Mike's numbers to illustrate the calculations.

If I were designing an aerobatic model, I would calculate the terminal velocity in a dive and use that for the red line speed. The result would probably be to design to a maximum wing load in the range of 20 to 40 G's. That should be adequate for everything short of a crash into a hard surface.