Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Hot Wings, May 18, 2017.
Gee this thread just keeps splintering, now we are on to simple flying wings of all things!
VB, your third sketch could give the right mix, of simplicity and ease of construction
Vipul, get of this thread if you're here just to promote a company that has nothing to do with aviation!:hammer:
Thanks, but this design makes the pilot choose between his knees and ankles... or his lower spine... as the landing gear.
A quick check of the calendar (and a visit to the bathroom scale) verified that I was unfortunately not 25 years old and/or 180 pounds anymore... which resulted in the wheeled version called AiRecliner ( next step from "Air Chair").
For whatever the value of an opinion from a non-trained and uneducated mad sketcher may be - if you look online for the Fauvel 17% airfoil used by Debreyer, I think you'd be pretty darn close for the type of open ultralight you drew. Certainly in the "safe and sane" range.
Can someone inform me as to why Fleas and planks don't just use symmetrical airfoils? The Clmax is the same for the Fauvel 14% and the NACA 0012, the Cm/Calpha is a little better for the 0012, and the Cd/Calpha is virtually the same (slightly higher for the 0012 until high alpha where it is slightly lower), why not use a symmetrical airfoil?
Did you consider CL/alpha and Cl/Cd?
IIRC the cambered airfoils will develop the lift at a lower alpha than the symmetrical foils, and hence have a better drag at the same lift.
Thank you. Cl/Cd is significantly higher for the Fauvel pretty much across the board; I also found a Wortmann airfoil (fx049915-il) that has a negative Cm that is smaller in magnitude than -0.1 and has a L/D that is about 50% higher @ Alpha = 5° than the Fauvel - any thoughts on that? I mean, yes it has some moment, but the L/D is much higher (it does have some reflex).
Of course, even a symmetrical airfoil has to have some up elevon which pretty well simulates the reflex on a reflexed airfoil; it seems that that might bring the symmetrical airfoil a little closer if done right with more or less sealed gaps?
Exactly. Camber moves the drag bucket. A symmetrical airfoil has it's minimum profile drag at CL=zero. Great for control surfaces but not very useful for wings. Increasing the camber moves the point of minimum drag to a higher CL
Older flying wing article, note no mention of driveshaft issues.
... and in the meantime ....
OK, so I redrew it with a more appropriate airfoil. Nothing special about this idea, it's just a plank. But the airfoil I'm looking at is a Liebeck LA2573A; I didn't choose it because of the way it looks, I don't like the way any of these reflex airfoils look because they seem to be designed to only fly right side up. But while this airfoil has a Cl/Cd of about 80 at a Cl of 0.6, which is pretty much the same as the Fauvel, Marske, Hepperle, and Gottingen reflexed airfoils, it has a significantly higher Cl/Cd at a Cl of 1.0; about 127 compared to about 110 to 115 for the others. I think this might give a better climb rate to this low powered, low wing loading aircraft. Am I doing this right, or am I thinking wrongly again here?
Have you considered the pitching moment?
If you don't have reflex on the airfoil you are going to have to cruise with the elevons always up a bit, which will effect your drag. It is better to use a reflexed airfoil to begin with. Take a look at the Hepperle MH46. It is a reflexed airfoil with relatively low drag and positive pitching moment. As I understand it, this is essential for a plank.
Why this concern about flying right side up? Are you planing on doing aerobatics with your design?
Stay AWAY from the Liebeck flying wing airfoils. I can't remember which one is which, but the Minibat flying wing glider used the Liebeck wing with absolutely disastrous results.
Yes to the pitching moment; I've been using that as one of the main criteria for looking at these things.
It's an irrational prejudice I was harboring :gig:.
Well, I can see nothing appreciably different from the airfoil graphs so I would want to find out what the reason for that was before ruling a particular idea out, but, if it was the Liebeck airfoil that was at fault, then my next choice would be the Marske XM-1D,
And the Marske XM-1D:
AoA for best L/D of an airfoil and L/D of an airplane aren't the same thing because of the other drag components and induced drag. Generally an airplane has it's best L/D when the wing is at its AoA for minimum drag. Best rate of climb is at the airplane's speed for best L/D not the airfoil's. However the best angle of climb is close to the wing's CLmax called "peak section power factor". The speed for best rate of climb will get you to altitude faster but at a shallow angle. Best angle of climb is good for clearing obstacles but the altitude gained in a given period of time is lower because your airspeed is low. The rate of climb is mostly about available horsepower. Angle of climb is of course also dependent on available horsepower but also the lift to drag ratio of the wing (not the whole airplane) which is determined by: 1 span 2 span efficiency (AKA effective span) and 3 section power factor. In that order. Span efficiency of conventional airplanes with slab sided fuselages is usually around 0.7, less if you have an open cockpit (air leakage is bad). Hershey bar wings have lower span efficiency than tapered wings. Elliptical wings can have a little better efficiency but the stall tends to be abrupt and very sensitive to side-slip angle. Winglets can get you to span efficiency higher than 1. Peak section power factor (high L/D at high CL) is great in theory but since planks control pitch by reflexing the trailing edge that power factor that you saw in the polars isn't actually available.
Some of the Liebeck airfoils have a truly nasty stall. This software isn't reliable after the cl over alpha curve starts to round off so you can't realy get stall data for any airfoil from it. Thus the bad one may look as good or better than the old reliable ones.
I knew that! Actually, I did know that, but my reasoning was that all other things equal (same aircraft, same wing area, span, planform, etc), out of two very similar airfoils the one with a higher Cl/Cd over the same range would be superior. But, as you guys have been pointing out, other characteristics are equally important such as the stall behavior; the article from RCSoaringDigest about the MiniBat cited more than one cause of the bad handling (including poor CG location w/respect to the main gear causing difficult to anticipate control response at rotation speed), but the airfoil was one of them due to a couple of reasons - one due to a bulbous lower leading edge shape that caused drag at higher speeds - and the airfoil also had bad performance at Re < 3,000,000. The Liebeck airfoil used on the MiniBat had a very forward and very pronounced hump on the upper surface that looks suspect as the cause of the separation of flow at low Re. The Liebeck LA2573A shown in post #872 has a much more normal looking upper surface and the drooping LE is also gone and it has a nice large radius LE; I would hope that the code used by http://www.airfoiltools.com is more accurate than that used to design the MiniBat's airfoil.
Now, looking at the Cl v Cd chart for the LA2573A from airfoiltools, there is a pretty screwy looking curve for Re = 100,000. It's pretty much gone by Re = 200,000 and by 500,000 it looks almost the same as at Re= 1,000,000. It's really not much different - according to airfoiltools software, anyway - than the Marske...
Edit: Xfoil was used to generate the lift/drag polars on the airfoiltools website; I don't know anything about the efficacy of xfoil.
This can be true of any as yet unused airfoil that has only ever been evaluated by computer programs, so I am a little at a loss as to how to select an airfoil other than to go with one that has proven successful on another design previously. Which may be the thing to do since the XM-1D was apparently a nice flying sailplane.
-Watermanns ARROWBILE=one of the bests flying cars...
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