load distribution along wing spars

[modelbuilder]

Hi folks I am a builder of model sailplanes so all that follows must be taken in context. My current project is an 18ft span hi-performance electric motor launched radio controlled sailplane. The total weight will be no more than ten pounds in flying trim and the maximum "G" load that I want to stress for is 5 G. The spar must therefore be capable of supporting 50 lbs at the centre which I have already designed, tested and am happy with. My question is this:

1) How far from the centre of the fuselage can I start to reduce the strength of the spar, and by how much? If I was hanging weights on the (inverted) spar to simulate positive wing loads, and the 50 lb limit is in force, what would the weight be at say 1 ft, 2 ft, 3 ft, 4ft,etc. In other words, how is the weight distributed along the span?

2) Is there a rule of thumb about this that will enable me to easily adapt, for instance, if I find the the spar is too strong (and heavy) my next project would use lighter construction and so on?

Thanks Guys

 

[der8110]

Weight should be distributed along the span in proportion to the area of the wing. If a rectangular planform, then a linear taper. If tapered, in proportion to the taper.

Your exercise here is basically a cantilever beam in bending, one going left and one going right. Your plan to test by inverting the aircraft and holding the fuselage fixed is correct. Hanging indivudual weights is fine, the more the merrier. Don't want to have the individual weights applying concentrated point loads. Your one per foot (9 per side, 18 total) is probably the least to try. So if a rectangular wing, then all weights are equal.

As far as rules of thumb - there are all sorts of good references out in the internet, heck, even Wikipedia. The property of your spar that relates to its strength is called Moment of Inertia. So do a few word searches, and just follow the links.

 

[40818]

For a homebuild, then the numbers aint the driving force, your sailplane probably isnt going to get any where near 5g. So if your interested in optimisation, then i would think more along 3g, and save some weight.
Normally your post would be redflagged as we try to keep no students and homebuilds in our traffic, but i must admit a sympathy for homebuilds, as i feel they embody a nature of engineering whereby failure is just another hurdle to pass over.
the answers your looking for are simple mechanics/statics, as you dont want to overcomplicate it with actulaties.

The answers out there just stop, think, google, and calculate.
good luck.

 

[39minuteman]

You could also check this post, as some general discussion regarding wing load could be found there

http://www.eng-tips.com/viewthread.cfm?qid=143930&page=1

But as 40818 said: Keep it simple.

Good luck

 

[modelbuilder]

Thank you all for your valued input. I shall indeed go fishing on the internet now that I know what to look for!

 

[rb1957]

along the lines of der8110, a uniform load is simple to work with and only slightly conservative (it overloads the outbd wing, moving the CP outbd increasing the moment slightly).

personally, i'm surprised that you can be happy with your root section if you don't know the root bending moment.

to learn more google "spanwise lift distribution", "shrenk" (not shrek !)

 

[thruthefence]

Sometimes "homebuilders" grow up to be engineers.

ie "Scaled Composites"