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Post by sean on May 8, 2008 21:05:59 GMT 10
Go to "Polars / Different airfoils Vs the same Re". Choose your tip and root aerofoils in the box that comes up, choose your reynolds number (around 100,000 would be OK) then press "Draw". You need to look at the cl(alpha) graph... the stall is when the curve drops sharply down. You want your tip aerofoil to stall at a higher alpha (AOA) than the root. Remember if your wing is tapered the tip will be operating at a lower reynolds number which can influence the stall AOA. In the pic below, the aerofoil with the red line will stall before the one with the black line.
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Post by Pij on May 8, 2008 21:19:30 GMT 10
You want your tip aerofoil to stall at a lower alpha (AOA) than the root. Isn't that the other way around? I want the root profile (further forward) to stall before the tip (further back). I'd love to learn Profili or similar properly to check that. I need to learn more about the terms and symbols used (better study Model Aero again, borrowed it again today), and I need to import my images in a useful way.
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Post by sean on May 8, 2008 21:22:23 GMT 10
He he, yes, you're right! I got that back to front ;D. A good way to learn Profili is to just press buttons and see what gets spat out .
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Post by Pij on May 8, 2008 21:32:22 GMT 10
Yeah, but with the good Book in my other hand to see what those spit-outs actually mean!
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Post by felix on May 8, 2008 21:38:46 GMT 10
HEY PIJ, lol what i meant to say as sean pointed out was a HIGH aspect ratio wing.the round leading edge i meant as in the aerofoil -find an efficient semisymetrical with a nice rounded leading edge (keep a docile stall) at a 6% thinkness.also a 72inch tapering wingspan would be great! bigger they are the better they fly.also keep the sweep angle low is a must.PIJ you were spot on about the elevons mate,they are complex but well worth it. from what i've seen it has not been done before and offers alot of potential! (bet sean can see where i'm going here lol).could be built light as feather and thermal,durable for a good fast everyday sloper or in my interest loaded up and smoothed for speed. would also go for a single tail and a seperate elevator surface in the interest of effiecency.this would not be an easy build but who cares.....can always get a bee lol. rigidity would be something i think should be left to the builder,everyone builds their wing their own way anyway.a thermaller could do away with any spars and rely on tape an tip fins (that would be insanely light!) but in my case i would probably go for 3x 6mm cf spars,bi tape,spackle and brown paper/pva coveriing. there's my 2c
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Post by Pij on May 8, 2008 21:56:16 GMT 10
OK, that's a totally different creature (but I still had fun drawing the Fat one).
When I picture your 72" span, the 6% of not much still worries me. Even 3 carbon spars of 6mm each will give a lot of flex, I'm thinking. Maybe you need to start with something stiffer than EPP? Or as thevon suggested, use fibreglass and opposing surface elements.
Separate elevators makes sense with that much span. Single tail - what about a forward-projecting fuse too?
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Post by felix on May 8, 2008 22:26:44 GMT 10
hey pij i wouldn't worry too much about details like fuses and all that.....someone likely to want to build it will have enough experience and bad ideas to do there own tinkering.sort of goes back to the principle of trying to keep the cores as a platform to please as many principles as possible.besides it would be fun to see which route everyone goes and best of all would make it as cheap as possible.basically focus on making a good efficient set of wing cores only and let the builder do what they want. i know 6% would be very thin especially towards the tips even at a larger span and rigidity would take some thinking (half the fun) but there are plenty of ways to stiffen it.few ideas of mine would be lite ply ribs set into the foam to brace the spars together,diagonal crosshatch bracing on the wing surface (can't remember the name for it but was used to great effect on the british halifax and wellington ww2 bombers),glassing,carbon fibre ,brown paper etc etc. another thing i would consider is an EPP leading edge (say to 1/3 of the chord) and the rear half blue or white foam.or maybe all blue foam as it apparently takes glass and glues alot better.
all sounds rather complex but there is plenty of good all rounder designs but a no compromise wing is yet to be done (the F1 wing ;D).my main thought on this is without doing something unique and drastic you may just create another "nice flying wing". once again this is my 2c and others may desire something a little more normal and complete.
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Post by felix on May 8, 2008 22:28:49 GMT 10
oh and btw that little fatty could also be something else lol,sorta looks like a weasel with some sexy curves....elliptical wings/highly tapered tips tend to be an aerodynamic death trap though.
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Post by felix on May 9, 2008 20:47:14 GMT 10
hey pij spent a time today scribbling on a note pad today during lunch to get a potential subject.... started by reverse engineering and starting with given attributes that should be added: that the chord shouldn't below 8" in any area (general reynolds # rule of thumb),minimum depth of root shouldn't be less than 20mm,high aspect ratio,minimum sweep etc etc. came up with this: span 72" root chord 12" @ 6% equals 18mm thickness (may be a better option to make at 7% =21mm thick and taper down towards the tip?) tip chord 8" @ 6% equals 12mm thickness sweep at 12 degrees (measured from root chord to leading edge) area equals 576" no dihedral
the aerofoil section is something i wouldn't have a clue about? i have my own twisted unproven theories here but will leave that alone.if i were to do it i would draw it free hand so someone with a little more experience here would be handy lol.
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Post by Pij on May 9, 2008 20:57:25 GMT 10
If you want to base it on known successful concepts, maybe use proven plank airfoils too?
That's a lot of wing.
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Post by sean on May 9, 2008 21:18:00 GMT 10
Hey Felix. Sounds great! Nice and original too! For the aerofoil 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 . Not for some time at least . The current catwalk aerofoil for RC flying wings is the PW51. Standard it's 8.5% thick. Probably not the most original choice but you couldn't go wrong with it, it's known for being fast on heavy planks but people are using it on light planes with success too. Thinning to 6% would be interesting but with EPP construction you'd be hard-pressed to get a stiff wing (in my experience stiffer wings are generally faster and nicer to fly), especially with such a long span. To get the aerofoil templates, use Profili. When thinning the aerofoil also use Profili - it must be thinned around the camber line, not just squashed vertically. Profili will do all that for you. There are of course many other aerofoils you can use, but they must be specifically designed for small flying wings otherwise performance and handling will suffer (they must have a pitching moment of zero or at least very close to zero). When you run an aerofoil analysis in Profili, there's a graph showing the pitching moment coefficient (it's the graph labelled cM(alpha)). Also RCGroups is a good place to search for this info. If you're getting your cores CNC cut then all you need to do is tell the fellow doing it the wing's dimensions, what foil to use at the root and tip, the amount of washout you want, and bob's your uncle!
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Post by felix on May 9, 2008 21:28:01 GMT 10
thanks sean,i'm with you on the stiffness side of things.i know stiffness in this design could be an issue and take some more out of the box thinking......give me a set of cores and i'll figure it out in no time ;D
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Post by Pij on May 9, 2008 22:11:02 GMT 10
I have an idea about the stiffening. First you mark out where you want your spar, top and bottom. It may be a curve to add torsional strength, just make sure you mark the line the same on top and bottom. You route a 4mm groove along those lines/curves, top and bottom. Along that path, you put a mark every 120mm or so, for braces to be inserted. Then, you have trapezoidal pieces (maybe balsa, maybe an old credit card, whatever, liteply sounds good) which are cut to match the thickness at each bracing location, minus 8mm. You stick them through the wing, at the marked spots. They should just reach the depth of the routed groove, top and bottom ie the braces fit between the grooves without protruding into the grooves. Then you glue 4mm fibreglass rod into the grooves, making sure it is glued well onto the trapezoidal braces. Then you have the rigidity and low weight of a braced beam. Almost as strong as an I-beam, but much lighter. And this one can be made to a curve, which is tricky with an I-beam. edited: I-beam, not H-beam.
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Post by felix on May 9, 2008 22:14:16 GMT 10
now your thinking pij lol,that would work well.all would be needed is multiple spars IMO and a way to brace the rear spars to the front to conteract twisting forces (maybe some small vertical triangular "fins" out towards the tip on the top surface)......actually someone who may be a great assistance here would be thevon (andrew).in his own words he is obsessed with spars and stiffness.
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Post by Pij on May 9, 2008 23:00:07 GMT 10
If your 2 spars were laid out something like this, I don't think they'd need to join to lend strength to one another.
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