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Old Dec 29, 2003, 10:27 AM
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Airfoil NACA 23014 ?

I am designing a scale model of a Cessna Bobcat twin trainer of the 40's. The fullsize has a NACA 23014 to 23012 airfoil, which I would prefer to stay with for my model . Is this any good for a 1:8 scale model ? (light/medium loaded, 64" span)

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Old Dec 29, 2003, 10:30 AM
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Get yourself a copy of Profili-2 and check out the performance of the full-size section against low reynolds model alternatives.
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Old Dec 29, 2003, 10:32 AM
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Got to http://www.profili2.com/ $10 and you are away!
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Old Dec 29, 2003, 10:59 AM
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The ESA (Evans Simitar Airfoil) is based on the NACA23012 but with enough trailing edge reflex to give it a positive pitching moment. The ESA was tested by Dr. Selig , etal. and the results reporteed in Summary of Low-Speed Airfoil Data, Vol. 3.

I expect that the NACA 23012 will have a high maximum lift coefficient, fair inverted performance, a low but negative pitching moment coefficient and a very abrupt stall with a serious stall hysterisis that delays stall recovery. A tapered wing planform will require wash out to avoid tip stall in the landing approach. The landing approach should be flown with a generous tip stall margin of airspeed.
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Old Dec 30, 2003, 05:53 AM
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Thanks Ollie, Anton.

Does not sound like my kind of airfoil. Here are some more stats: Aspect ratio approx. 2, taper is 66%. Original had 2 deg. washout, which I copied. I am not particulary interested in inverted performance (I don't expect the original to be either, being a twin trainer), but I like slow approaches. I am aiming at 50 grams/dm2 loading. Still would like to stick to the original, but how about the good old 2412 as an alternative?

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Old Dec 30, 2003, 08:55 AM
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Max z wrote:
Here are some more stats: Aspect ratio approx. 2, taper is 66%.

AR looks more like about 5.5:1

2412 looks a lot better, 23012 pitching moment changes erratically from negative to positive with angle of attack, polars look ragged. 2412 stays mainly negative and loads up the tail nicely, much nicer polars.

NACA 23012 12% - Re = 100000
Alfa Cl Cd Cl/Cd Cm - Not good!
-6.5 -0.6135 0.0405 -15.1481 -0.0116
-6.0 -0.5743 0.0344 -16.6948 -0.0084
-5.5 -0.5330 0.0300 -17.7667 -0.0051
-5.0 -0.4912 0.0272 -18.0588 -0.0022
-4.5 -0.4472 0.0248 -18.0323 0.0004
-4.0 -0.4037 0.0220 -18.3500 0.0025
-3.5 -0.3621 0.0206 -17.5777 0.0051
-3.0 -0.3235 0.0189 -17.1164 0.0075
-2.5 -0.2847 0.0176 -16.1761 0.0095
-2.0 -0.2491 0.0149 -16.7181 0.0157
-1.5 -0.0918 0.0166 -5.5301 0.0011
-1.0 0.1125 0.0165 6.8182 -0.0266
-0.5 0.2283 0.0154 14.8247 -0.0389
0.0 0.2895 0.0147 19.6939 -0.0395
0.5 0.3325 0.0142 23.4155 -0.0359
1.0 0.3730 0.0141 26.4539 -0.0321
1.5 0.4148 0.0142 29.2113 -0.0288
2.0 0.4567 0.0146 31.2808 -0.0257
2.5 0.4989 0.0153 32.6078 -0.0229
3.0 0.5416 0.0161 33.6398 -0.0205
3.5 0.5854 0.0169 34.6391 -0.0183
4.0 0.6297 0.0179 35.1788 -0.0163
4.5 0.6749 0.0189 35.7090 -0.0144
5.0 0.7206 0.0200 36.0300 -0.0127
5.5 0.7665 0.0212 36.1557 -0.0110
6.0 0.8120 0.0224 36.2500 -0.0093
6.5 0.8573 0.0238 36.0210 -0.0077
7.0 0.9024 0.0254 35.5276 -0.0061
7.5 0.9476 0.0270 35.0963 -0.0046
8.0 0.9933 0.0288 34.4896 -0.0032
8.5 1.0391 0.0304 34.1809 -0.0020
9.0 1.0789 0.0320 33.7156 0.0000
9.5 1.1172 0.0339 32.9558 0.0022
10.0 1.1540 0.0358 32.2346 0.0044
10.5 1.1887 0.0376 31.6144 0.0067
11.0 1.2202 0.0393 31.0483 0.0094
11.5 1.2478 0.0412 30.2864 0.0124
12.0 1.2719 0.0432 29.4421 0.0155
12.5 1.2943 0.0454 28.5088 0.0186
13.0 1.2993 0.0483 26.9006 0.0231
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Old Dec 30, 2003, 10:06 AM
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Quote:
AR looks more like about 5.5:1
Offcourse. I only measured the span of the bit of wing sticking out from one side of the fuselage, duh.... !
Measured it again, is actually closer to 4.5:1 . I think I will change to 2414 / 2412, thanks Anton. Or I will change to an airfoil I have used with great success in the past, the HL80 (there are some more digits behind that, but I forgot, anyway, I saved it in CAD).

Max
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Old Dec 30, 2003, 11:03 AM
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An S shaped bend in the Cm vs alpha is not unusual at a reynolds number of 100,000. Many useful airfoils have such a characteristic because of changes in a laminar seperation bubble with angle of attack. It just means that there needs to be a slightly more forward CG so that stability is maintaimed over the whole range of useful angles of attack.
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Old Dec 30, 2003, 11:56 AM
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Ollie, Agreed S shape seems common but this one at lower re seemed to cross the zero point several times. As you say just an indicator of bubble shape and I suppose movement.

The polars just looked a lot better for the 2412 down to low re. I was thinking that this cm character might contribute to tip stalling on a slow flying 23012 wing. Does the laminar separation bubble behaviour at low re effect stall speed?


This sort of thread makes me realise it's what we see manifest itself if the CG is in a critical position, the models behaviour follows the changes, like planks with the wrong airfoil, wild ride!

I am just going to post another thread/question on wash-out in the modelling science section.

Tony.
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