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Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Hephaestus, Jan 1, 2020.
1/3 scale can probably teach you a lot of things, but it may not be good for airfoil evaluation. Chances are, an airfoil in 2D will do better in full scale. Also, separation on any rounded surface may happen at a different location. Depending on just what the shapes are, flow at higher Reynolds numbers may separate later. At the tip, unless it's fairly sharp, at a higher Reynolds number the flow may wrap around the edge some and make the wing act shorter than it is. To some extent, the round tips may be doing that already. I seem to recall, though, that the edge of the tip on the Lazy Bee is relatively sharp.
I seem to recall that this Reynolds number issue is one reason that the Facetmobile, and, of course, the preceding scale models, had a relatively sharp leading edge.
I could spend forever trying to find the perfect airfoil - at the end of the day the real worry is is coefficient of lift, and it's pitching moment, and to some extent stall behaviour.
Right now this one seems good enough, tamed the higher speed squirrely pitch. This isn't a world record aircraft - just needs to get off the ground and return to it safely. I'm going with an Orion idea - doesn't need massive optimization or more days thrown at it. Chances are I could throw a 2412 at it and get similar performance.
I wasn't suggesting more analysis, but rather that I doubt X-plane is very accurate in this case, unless they've gone to a lot of trouble specifically for low aspect ratios.
I think you may be right about the 2412, though it seems to me that a smaller pitching moment might be good. I suspect that stall behaviour of the wing will be significantly different than what the 2d airfoil data shows.
You might take a look at NACA 23112, it's quite popular in Mignet types because of the low pitching moment.
LAR are less prone to tip stalls, so the NACA 5 digit could be a good choice.
There we go, now I've got it overlayed nice enough that it makes sense... ClarkY, 2412, 23012, nlf0115 comparison.
Are you going to build the wing to laminar accuracy and finish? No? Then trip the flow at 20% and recalculate.
I'm thinkin' the airfoil is not real critical for this Lazy Bee. Something flat on the bottom and with a decent leading edge radius is gonna be good enough for what this is. Steal the airfoil form the Airbike/MiniMax, or the Kolb, or even the Cub.
Now none of those calculations actually model laminar flow if I remember. Xplane xfoil etc stick to generally non laminar flow.
At a Re of 2.5million (Vs) you don't need perfection and the airfoil still works as a standard non laminar airfoil. Note the CM vs alpha curve - just a bit more stable, but following very similar profile.
MomC would probably be the better choice for a 103 - but Canada doesn't have silly max speed rules so
Xfoil does model laminar flow. If you are using xflr5 to run it, it is easy to trip the boundary layer at x% of chord.
With the huge chord, a lowish Cm seems like a good idea. Shouldn't be too critical otherwise. I'm not usually a big fan of the NACA 5 digit series, but I think they'd be good choice here.
I agree, 231XX or 241XX in your choice of thickness would be a good place to start.
Not sure, don't see the 23112/24112 as better. I know I didn't like the 23112 on the model. Might be they're a bit too low on cm.
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