|
Post by felix on May 11, 2008 12:02:38 GMT 10
yeah that spar arrangement is great,especially as you mentioned top and bottom in a sort I beam arrangment.extra stiffness could be had by a leading edge and drag spar combined with a few liteply ribs.not to mention the amount of strength the covering adds. wieght may add up but due to it's potential effeciency i doubt that will bother it one bit (my bee is now probably 750g + and has been flown in 8knots at SC). this wings potential if built light (single spar,minimum covering,micro gear etc etc) for thermalling/light breezes would be something else too.
|
|
|
Post by Pij on May 11, 2008 17:51:42 GMT 10
Is it "legal" to have a leading edge element? I thought EPP was kept for the leading inch or two to avoid collateral damage. What is a drag spar? Do you really think ribs would be a valuable addition? I'm beginning to get interested in this long wing idea of yours. I don't much like the flat-bottom-ness of the PW51 foil. I like this one: www.aerodesign.de/english/profile/profile_s.htm#sipkill I reckon go with a tried and true one from a trusted designer - I don't think many of us are going to be able to do a better job than some German fellow who's devoted his life to aerodynamics . A little off topic maybe,but does anyone have an opinion on these servos? The specs amaze me. www.hitecrcd.com/servos/show?name=HS-125MG
|
|
|
Post by sean on May 11, 2008 19:17:02 GMT 10
The shape of the underside isn't a good indicator of the aerofoil's characteristics - more important is the amount of camber. Profili says the Sipkill has 1.68% camber while the PW51 has 1.42%, meaning the PW51 should in theory have better inverted performance than the Sipkill.
For better aerobatics you could even reduce the PW51's camber a tad more, but that would harm its low lift performance.
|
|
|
Post by felix on May 11, 2008 19:23:03 GMT 10
pij, the leading edge spar is a tricky subject all depending on what flying the builder will do.yes a foam LE will be more resistant to damage -what i would think about doing is have a 6mm CF spar 10-20mm back from the leading edge (good compromise between total strength and survivability) a drag spar is basically a spar that runs along the TE of the wing,this i would safely say is necessary to stop the elevons binding in the TE grooves when the wing flexes. ribs are not necessary and best left to the builder too.done properly and bonded well to the spars 4 or so ribs per side would do well to resist twisting of the wing by allowing more of the force to be distributed and dispersed evenly throughout more of the total wing.done wrong the wing would not survive it's 1st high G turn lol. as for the aerofoil i am not really the one to offer theorised advise.what i do know though that in low reynolds numbers wings the aerofoil curve drawn around the edge of your shoe is at the worst only 10% less efficient than anything in a book and that's before taking in the cr*ppy surface of a foam wing. basic practice though would say to keep the thickness % very low,keep it semi symetrical and keep the leading edge reasonably round.the pitching moment i am not up to scratch on but guess it has got to with the centre of pressure range of movement (may have to study this a bit ).my guess is the apex of the aerofoil on a tailess design is further rearward than a convential (sean?) one thing though pij what exactly are you hoping to accomplish with this wing? this design would be a good thermaller,aerobat and ds'r but generally ruggedness and stability would be a long way from the duck or bee.
|
|
|
Post by Pij on May 11, 2008 20:20:00 GMT 10
Well, as you know, I wanted a rugged all rounder. But as we have explored aspects of your idea, I can picture it in a light (but not lightest) configuration - I see a good floater, but with some speed, 'batics and survivability.
|
|
|
Post by ezza on May 11, 2008 20:20:46 GMT 10
Stay away from them!! Little gear inside almost immediately develops slop, then strips. Maybe OK with something light weight. Do a search on RCG, plenty about them on there. Eric
|
|
|
Post by felix on May 11, 2008 20:47:15 GMT 10
pij a wing like this would be a very quick,low drag wing and be quite a handful if built like a typical sparred sloper (fairly heavy) but with if built light would make a great light wind/thermal flyer due to it's large area and efficient design.also it would take a fair bit of work/thought to build stiff.not sure if you built it to normal slope specs it would be a good all rounder like the bees,ducks and weasels. maybe a more convential design may be better suited?
......heay sean in your opinion what would be the best aerofoil for this (at 6% thickness)
|
|
|
Post by sean on May 11, 2008 22:49:54 GMT 10
Hmm, I'm not very experienced at building floaters but my suggestion would be a highish aspect ratio to improve the L/D and a low wing loading. If you want to fly it mostly in light air then you could get away with a flexy wing to save weight. I think 6% would be too thin from a structural point of view, especially if you use a higher aspect ratio than is typical on a combat style wing. It's true that for low reynolds numbers (ie small and slow) thinner aerofoils are the best but you'd need to compromise to build a sufficiently strong wing from EPP. An aerofoil with a reasonable amount of camber (around 2%) would help a lot on a floater but aerobatics and inverted performance would suffer. You could probably get away with a low camber aerofoil to improve aerobatics. It's all a compromise really. I reckon an aspect ratio of around 7 give or take (I think most combat wings would be about 5 but just a guess), a foil about 8% thick and a span somewhere between 1.2 and 1.5M would create a nice flying wing with better light air perfomance than a typical combat wing - but it wouldn't be as durable and might not be as aerobatic. It would probably be fairly quick in bigger lift too. If I were designing it for myself I'd probably tend toward 1.2M span to keep it fairly nimble and durable. More span and a higher aspect ratio would make a more efficient wing but with EPP construction it might become a bit weak and impractical. I think most flying wings suffer from a lack of yaw stability so reasonably sized fins would make it track straight and true. It also has to be said that any gains in light air efficiency over a standard lightly built Duck would probably be quite low, but it would be fun to see .
|
|
|
Post by felix on May 13, 2008 17:57:35 GMT 10
sean,say the pw51 aerofoil you mentioned at 6% would you consider that to be a fast,efficient aerofoil? has it been used on faster wings? reason i ask is i am still hooked on these dimensions for a high performance/speed wing. the rigidity would be a concern but i reckon,looking some more at the dimensions,with a bit of unorthodox engineering it could be built stiff,light and cheapish.look at most epp wings (epp is pretty hopeless on it's own) that are just bi taped and colour taped for instance,amazingly rigid for what they are.add a few straight CF spars and they become rock solid.now i reckon with i bit more thought with spar designs (more complex!) and whatever this design could be made stiff enough even for DS.it'll never make a SC combatter,plenty of better designs for that,but would make a very fast and fairly durable wing.dimensions are based alot more towards performance than ease of building and inherent strength compared to others but hey,gotta shove the envelope to be different lol seriously considering doing this one myself.......for now i'm off to sand the cr*p out of the spackled zero so it can be brown papered tomorrow.
|
|