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I need help picking an airfoil. I'm an artist and have limited knowledge of such things.

It's for a game with an aircraft similar to an Edge 540 / Extra 300. Unfortunately I have a limited selection of airfoils though and need some suggestions for both wing and tail airfoils. (Don't have the ClCdCm for the real Edge)

I do have access to many (not all) of the airfoils listed on this site though. https://aerodynamics.lr.tudelft.nl/cgi-bin/afCDb

Any guidance would be appreciated!

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  • $\begingroup$ since it is for a game and not for real life application, what difference does it make? $\endgroup$ – Federico Feb 24 '16 at 9:23
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    $\begingroup$ I understand that depending on the game they might be relevant or simply cosmetic. Is it a detailed flight simulator? Are you going to perform CFD simulations in the background? Or is it simply something that is there to give a paint of realism but has no effect on the mechanics and physics of the game? $\endgroup$ – Federico Feb 24 '16 at 9:34
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    $\begingroup$ Note that the Cl,Cd,Cm for an airfoil are quite different from those of a wing. And those of the whole aeroplane are yet another difference. $\endgroup$ – DeltaLima Feb 24 '16 at 11:06
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    $\begingroup$ I am using precalculated ClCdCm then simply assign to these variables the values that best fit a realistic behaviour, the selection of the airfoil has no importance to you. $\endgroup$ – Federico Feb 24 '16 at 11:12
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    $\begingroup$ Something symmetrical. NACA 0012 might work. $\endgroup$ – Fred Larson Feb 24 '16 at 17:42
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For the Extra 300 which has symmetrical airfoils:

enter image description here
Extra 300, image by Ampbomber (source: rcgroups.net)

From the service manual of the Extra 300S:

  • Wing: from MA 15S (root) to MA 12S (tip). This change in profile allows for good performance both at low and high airspeeds.
  • Tail, for the 3 surfaces: - Wortmann FX 71-L-150/30

Wing airfoils characteristics seems not available online. On X-Plane.org forum (How X-Plane Uses Reynolds Number), it is suggested to replace the airfoils by NACA 0015 and 0012.

For the pitching moment coefficient Cm:

  • Cm is null for a symmetrical airfoil (no camber).

Manufacturer manuals:

Similar project:

  • On SourceForge: Building an Aerobatics Aircraft for JSBSim: The Su-26.

As an additional information, a study was conducted to evaluate the lift coefficient of the wing (or maybe with the tail) and the lift / drag curve, at a reduced scale:

enter image description here Experimentally determined drag polar and lift curve for the Extra 300 (Figure 7)

However:

  • The scale change introduces a different Reynolds number, unknown adjustments must be made to extrapolate to full scale.
  • The curves are incomplete (alpha max undetermined).
  • The L/D ratio close to 2 is very low compared to the actual performances.
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There are a few things to consider for a good aerobatic airfoil. The transition between attached and separated airflow has a big influence for spins or snap rolls, and a good aerobatic airfoil needs only a small change in AoA (angle of attack) to fully transition between the two. To achieve this, it helps to shape the forward part similar to an ellipse (a lemniscatic function, to be precise), and to use a straight line for the last 70% to 80% of chord. The goal is to have a Stratford pressure rise on those 70% to 80% which gives the maximum possible pressure gradient without separation. Increase AoA just a little, and you have fully separated flow over three quarters of the wing chord.

When all works as planned, you can get the plane out of a spin by applying rudder at, say, 45° before the fuselage points in the desired direction and get out of the spin within 10° of that direction. Same goes for snap rolls, they get really snappy. It is a real pleasure to fly such planes, but you need some discipline. There are no aerodynamic warnings before you stall, and stalling comes immediately. On the other side, with the limited lift loss of such an airfoil and the powerful engines of aerobatic airplanes, stalling is no big issue.

The next thing to consider is relative thickness. Modern aerobatic aircraft are rated ±10g (note that FAR/JAR only requires ±6g in the aerobatic category), so a thick root airfoil helps to keep the structural mass down. On the other hand, maximum lift is possible with a thickness around 12%, so the compromise is to taper the wing from 15% at the root to 12% at the tips. The airfoil numbers given by mins refer to the relative thickness as percent of chord.

None of the airfoils in your collection (which looks awfully like an older version of Michael S. Selig's UUIC database) will fit well. Go ahead and design one with XFOIL and use the calculated polar.

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  • $\begingroup$ I hear ya but for time reasons as well as general understanding, using airfoils I already have access to is ideal. $\endgroup$ – Gullie667 Feb 25 '16 at 7:43
  • $\begingroup$ @Gullie667: Then for the wing pick a symmetrical airfoil where the location of the maximum thickness is close to the leading edge. Bonus if the rear two thirds are composed of straight lines. Definitely use the Wortmann FX 71-L-120 or FX 71-L-150 for the tail surfaces (L = Leitwerk) and maybe the FX 71-120 for the wing. $\endgroup$ – Peter Kämpf Feb 28 '16 at 12:36

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