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Spar options for high aspect ratio balsa wings


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It is worth remembering that the bending load on a cantilever wing is a maximum at the root and reduces to virtually zero at the tip. For the best weight to strength the construction of the wing should reflect that.

For an open structure wing that relies on the spar(s) for bending strength it means the spar's resistance to bending can gradually decrease from root to tip.

As the wing is loaded over is entire area the actual bending load does not decrease at a constant rate but by a 'squared' function.

BendUni.jpg.3aa21f7808467da07cebd25e20473e32.jpg

A conventional construction balsa spar (top and bottom constant sections with a shear web) can thus be considered to be either weak at the root or unnecessarily heavy everywhere else, even if the wing itself is tapered.
It follows that the top and bottom spar sections themselves should be tapered to virtually nothing at the tip. The required torsional rigidity is likely to be the limiting factor.

 

My take on Colin Chapmans mantra "For performance add lightness" is "Add material where it is needed and take it out where it isn't".    

      

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tapered spars are quite easy (easy-ish?) to make from spruce or hard balsa, webs can be thicker at root and thinner in the middle and omitted at the tip, extra part-span spars can be added to root areas

 

not all of which is so easy to achieve with pre-formed carbon rods

 

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Hi Nigel, Peter, and Simon.

Thanks for the advice on spar tapering and shear webbing. When I said "two parallel carbon tube spars" I wasn't thinking of 'one above the other', I was thinking of 'one behind the other' with the forward one being the main spar glued to the back of the 'D box' sheeting and the rear one being half the diameter running through the unsheeted part of the wing. These would line up with (and accept) the rods protruding from the outer wing sections (that plug in to the center section). I'm confident this will be much stronger than the previous single spruce spar but I obviously can't taper them and there will be a weight penalty.

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2 hours ago, Nigel R said:

tapered spars are quite easy (easy-ish?) to make from spruce or hard balsa, webs can be thicker at root and thinner in the middle and omitted at the tip, extra part-span spars can be added to root areas

 

not all of which is so easy to achieve with pre-formed carbon rods

 

 

Tapered rod is available - as per the Eli 2mRES middle-section spar, the whole wing of which illustrates your post perfectly.

 

1044154380_Screenshot_20211025-1506493.thumb.png.abd7b573984eea031586d713e6909e1f.png

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A fellow club member and myself were flying RES gliders earlier in the year. Two different designs, he was flying an RES Eagle and I was flying a Circle Dancer.

The Eagle uses carbon fibre for the main spar and leading edge with a balsa trailing edge, it has no leading edge sheeting. The Circle Dancer main spar comprises over 20 components, the core is a thick, light, balsa section, vertical grain, with slots and holes for the ribs, it is also faced with very light glass cloth or similar and is very fragile, it has lite ply additions in the centre and at the ends where the wing joiners go. When the ribs are in place, spruce spars go on the top and bottom of the balsa section and the top leading edge section is sheeted but not the bottom. This is the same for the outer wing panels using smaller sections. The leading edge is balsa too.

He commented that he had watched how the wings behaved going up on the bungee, the Eagle wing flexes visibly on the way up, the Circle Dancer wing doesn't flex at all.

Two different construction methods giving noticeably different characteristics. 

I did wonder what the flexing would eventually do to the balsa/carbon fibre joints in the long term.

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Shaun, I think the answer to your last point is likely to be "wait and see".  Just because the wing flexes doesn't mean it's more likely to fail.  OTOH.....

 

I remember seeing the first full size glider with a carbon fibre main spar.  It was a Kestrel that had its fibreglass spar replaced.  The difference in wing bending between the two types was quite noticeable.  I'm not sure if the stiffer wing made the aircraft easier to fly or if there was no difference.

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15 hours ago, Shaun Walsh said:

I did wonder what the flexing would eventually do to the balsa/carbon fibre joints in the long term.

Interesting question. The flex itself is intrinsic to the combination of spar, LE and TE stiffnesses.  There are 22 ribs/riblets in each wing of the RES Eagle.  Each joint will have a degree of elasticity and resilience which, while not great at the individual level, amounts to quite a lot along the whole wing.  There will of course be a critical point beyond which any given joint is fatigued to the point of failure - but this would require repeated excessive force beyond those imposed by normal launches off a bungee of 6mm diameter.

 

27779098_Screenshot2021-10-26at09_30_27.png.4822b44fc5325e4b7698ac27cd5f5fd1.png

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  • 2 months later...
  • 2 weeks later...

By careful selection of a spar material I did achieve a pretty high aspect ratio with a foam wing.

CompleteA.JPG.dd6733d1f3ed54b54e657dc5fc056c5c.JPG

The wing was built up with ribs and skin all using 3 mm thick foam sheet. In those days real Depron.

A root test piece shows the internal structure of what is in effect a box spar but all made of 3 mm foam sheet.  

1pcBuild.JPG.e8400edb98c39fe911d9c9d96a0c9ab9.JPG

The strength of the spar comes from the 1 mm hard balsa top and bottom 'flange' that lies flush with the wing skin.

 1mmSpar.thumb.JPG.285c5900654d37aeb8404d723ff5ec2b.JPG

The 1 mm balsa flange is also fully backed by a 2mm foam sheet that itself is supported by the ribs and the shear webs to ensure the thin flange can carry the compression forces.

In the final wing the spar although broad at the root was tapered to a point a short distance from the tip. The final 5% of the span relying entirely on the strength of the foam.

To save weight the 2.1 m wing was built in one piece. It just fitted in my car.

It did bend in flight but then so does a full size Libelle wing. I tested the wing to a load that approximated to pulling 4g. I simply did not want to know what its ultimate strength was!

I have used the past tense because I no longer have it. When 'edge on' to the line of sight the visible area was tiny. I let it get a bit too far away, lost sight of it and never saw it again. 😟 

One day I might try again.

 

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