Nigel R Posted October 25, 2021 Share Posted October 25, 2021 On 24/10/2021 at 00:09, David Ramsden said: , I've decided on two parallel carbon tube spars rather than spruce 'I' beams. with or without shear webs? Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted October 25, 2021 Share Posted October 25, 2021 I would urge you to fit shear webs. Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted October 25, 2021 Share Posted October 25, 2021 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. 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". 1 Quote Link to comment Share on other sites More sharing options...
Nigel R Posted October 25, 2021 Share Posted October 25, 2021 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 Quote Link to comment Share on other sites More sharing options...
David Ramsden Posted October 25, 2021 Author Share Posted October 25, 2021 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. 1 Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted October 25, 2021 Share Posted October 25, 2021 Sounds a good plan. Quote Link to comment Share on other sites More sharing options...
Jonathan M Posted October 25, 2021 Share Posted October 25, 2021 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. Quote Link to comment Share on other sites More sharing options...
Jonathan M Posted October 25, 2021 Share Posted October 25, 2021 52 minutes ago, Peter Jenkins said: Sounds a good plan. Agreed. Quote Link to comment Share on other sites More sharing options...
Shaun Walsh Posted October 25, 2021 Share Posted October 25, 2021 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. 1 Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted October 25, 2021 Share Posted October 25, 2021 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. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted October 26, 2021 Share Posted October 26, 2021 Yes, there is huge flex on airliner wings and they have to stay in one piece for a few airbourne hours here and there... 16 hours ago, Jonathan M said: Tapered rod is available Neat. Quote Link to comment Share on other sites More sharing options...
Jonathan M Posted October 26, 2021 Share Posted October 26, 2021 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. Quote Link to comment Share on other sites More sharing options...
Martin_K Posted October 26, 2021 Share Posted October 26, 2021 You can arrange things to make a stiff wing, i.e. reduce flex along the main spar. Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted October 26, 2021 Share Posted October 26, 2021 Geodetic? Quote Link to comment Share on other sites More sharing options...
Martin_K Posted October 26, 2021 Share Posted October 26, 2021 (edited) Geodetic sounds better than triangulated. Peter, in a previous post you mentioned torsional strength. I think this wing would break more easily by twisting than bending. Edited October 26, 2021 by Martin_K Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted October 26, 2021 Share Posted October 26, 2021 Only solution is a test specimen you can break! Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 27, 2021 Share Posted October 27, 2021 Like this? (actually no problem with the structural design but this was a destruction test which eventually established that faulty heat treatment of metal fittings had caused an in-flight failure of an identical glider in severe mountain turbulence) 1 Quote Link to comment Share on other sites More sharing options...
Peter Jenkins Posted October 27, 2021 Share Posted October 27, 2021 Exactly so! Until you have measured the forces the structure is subjected to in flight and then tested a similar structure to destruction you will not know if your structure is safe in flight and by what factor. Which is why our model structures are, generally, far stronger than the full size. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted October 27, 2021 Share Posted October 27, 2021 Bendy Boeing: https://www.youtube.com/watch?v=m5GD3E2onlk Quote Link to comment Share on other sites More sharing options...
robert chamberlain Posted January 17, 2022 Share Posted January 17, 2022 I have 100 feet of 7/16 surgical tubing plus 400 ft of string for a high start. Any ideas on how many pounds I could stretch the tubing for different weights of sail planes?-- Bob Quote Link to comment Share on other sites More sharing options...
Simon Chaddock Posted January 25, 2022 Share Posted January 25, 2022 By careful selection of a spar material I did achieve a pretty high aspect ratio with a foam wing. 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. The strength of the spar comes from the 1 mm hard balsa top and bottom 'flange' that lies flush with the wing skin. 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. 1 Quote Link to comment Share on other sites More sharing options...
Martin Hodgson Posted January 30, 2022 Share Posted January 30, 2022 I always felt the eagle spar was too small in diameter. A bigger, woven tube would be stiffer spanwise and torsionally stiffer, while not adding much, if any, weight. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.