Small Models and their Airfoils.

[Pearse]

Hello.

I've been looking into messing about with "small" airplane models and have done some reading at the library, although mostly on aerodynamics in general. However, I recently came across this very informative page that's much more specific: http://jef.raskincenter.org/published/airfoil.html

I've been lurking about here a bit and thought I'd ask the more knowledgeable people here what they think of the foil suggested. One thing I'm not certain on is the angle of attack. I'm pretty keen to try it out so I guess I could just go through trial and error, but I'm sure there's a better way than that.

Also, on a slight tangent, what resources does everyone use for selecting airfoils? I've been searching the internet but there doesn't seem to be any kind of really large database that includes specs for foils, like Lift Coefficient etc. I would've thought any collection of foil designs would naturally include that kind of stuff with each design. Maybe I just haven't searched enough?

Hope I'm not asking too much here!

[Ollie]

See:
http://www.profili2.com/eng/default.htm
Profili2 includes the XFoil (virtual wind tunnel) applied to many, many airfoils, even your airfoil design.

[space_case]

The website does mention something rather important:

Somewhere between Re = 50,000 and Re = 100,000 conventional airfoils become superior (just where the changeover occurs for any particular design depends on many factors, too complex to discuss here).

I worked for a while on a program that dealt with 'small' aircraft and we found this line to be about 100,000. Problem is, the solver in xfoil does not like to converge for extremely thin airfoils in the viscous mode and you would need some other (Navier Stokes) type of CFD. However the viscous effects are crucial - and in fact dominate - in this regime.

So first calculate your Re# - then go from there. If you wind up on the lower side I can pass you a thin airfoil we used that worked in practice. I don't know if I have any hard numbers on it. I think the guy that chose it started with one of the 'thin' freeflight airfoils and then just took it down to a camber line.

[Pearse]

So first calculate your Re# - then go from there. If you wind up on the lower side I can pass you a thin airfoil we used that worked in practice. I don't know if I have any hard numbers on it. I think the guy that chose it started with one of the 'thin' freeflight airfoils and then just took it down to a camber line. I calculate an approximate Re# of 35,000, so your foil sounds quite interesting.

Thanks for the replies.

[space_case]

I calculate an approximate Re# of 35,000, so your foil sounds quite interesting.


I have it on another computer so I will drop it off later.

[davidfee]

Try something like this:

[space_case]

Yes you probably want to go with a thin free flight airfoil as above. The FF forum here would also be a good place to ask questions. It's best to use something tried and true instead of modeling it yourself unless you really have the tools and know how to do so. There are unsteady effects present and I don't think that aerodynamics in this regime are anywhere nearly as well solved as larger scale phenomena.

In the project I mentioned, we had reasons to use these thin, 'camberline' type of airfoils. The one attached was derived from a FF airfoil and worked OK in practice at ~Re=70,000 on a several different small aircraft. The coordinates seem to be a bit off in the traditional sense at the LE and TE.
-----------------------------------------------------------------------
0 0.0001
0.0100 0.0074
0.0200 0.0142
0.0300 0.0205
0.0400 0.0263
0.0500 0.0316
0.0600 0.0366
0.0700 0.0411
0.0800 0.0453
0.0900 0.0491
0.1000 0.0526
0.1100 0.0558
0.1200 0.0586
0.1300 0.0613
0.1400 0.0637
0.1500 0.0658
0.1600 0.0677
0.1700 0.0694
0.1800 0.0710
0.1900 0.0723
0.2000 0.0735
0.2100 0.0745
0.2200 0.0754
0.2300 0.0762
0.2400 0.0768
0.2500 0.0773
0.2600 0.0777
0.2700 0.0780
0.2800 0.0782
0.2900 0.0784
0.3000 0.0784
0.3100 0.0784
0.3200 0.0783
0.3300 0.0781
0.3400 0.0779
0.3500 0.0776
0.3600 0.0773
0.3700 0.0769
0.3800 0.0764
0.3900 0.0759
0.4000 0.0754
0.4100 0.0748
0.4200 0.0742
0.4300 0.0736
0.4400 0.0729
0.4500 0.0722
0.4600 0.0714
0.4700 0.0706
0.4800 0.0698
0.4900 0.0690
0.5000 0.0681
0.5100 0.0672
0.5200 0.0663
0.5300 0.0653
0.5400 0.0643
0.5500 0.0633
0.5600 0.0623
0.5700 0.0612
0.5800 0.0601
0.5900 0.0590
0.6000 0.0579
0.6100 0.0568
0.6200 0.0556
0.6300 0.0544
0.6400 0.0532
0.6500 0.0520
0.6600 0.0507
0.6700 0.0495
0.6800 0.0482
0.6900 0.0469
0.7000 0.0456
0.7100 0.0443
0.7200 0.0429
0.7300 0.0416
0.7400 0.0402
0.7500 0.0388
0.7600 0.0374
0.7700 0.0360
0.7800 0.0346
0.7900 0.0332
0.8000 0.0317
0.8100 0.0303
0.8200 0.0288
0.8300 0.0273
0.8400 0.0258
0.8500 0.0243
0.8600 0.0228
0.8700 0.0213
0.8800 0.0197
0.8900 0.0181
0.9000 0.0166
0.9100 0.0150
0.9200 0.0133
0.9300 0.0117
0.9400 0.0100
0.9500 0.0083
0.9600 0.0066
0.9700 0.0048
0.9800 0.0030
0.9900 0.0011
1.0000 -0.0008

[Pearse]

Thanks a lot guys, that's really helpful. :)