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On an aerobatic aircraft, such as an Sbach342, what type of airfoil does it have and what angle of incidence does it have in relation to the fuselage?

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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 nearly straight line for the last 70% of chord. The goal is to have a Stratford pressure rise on those 70% which gives the maximum possible pressure gradient without separation. Increase AoA just a little, and you have fully separated flow over those 70%.

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.

A major reason for mounting the wing at 0° incidence is that you need little elevator correction in a roll. Rolling happens about the longitudinal axis of inertia, and if the wing is aligned with this axis, it has 0° AoA at 90° and 270° of the roll and at 180° the inverse of the AoA it had in level flight. With a symmetrical airfoil, lift is always correct, and if the aircraft flies at zero stability, you need not to move the elevator.

Mistimed elevator movement in rolls leads to immediate changes in flight path direction (happens really easily, and is easy to observe from the ground), and it is very hard to pitch the whole airplane just at the right time so it has no lift (read: side force) in knife edge flight, but the right lift in inverted flight. Just for flying inverted, many regular airfoils are just fine, but it is in the transitions where the symmetric airfoil has a clear advantage. A fast roll with a cambered airfoil will result in a corkscrew-shaped flight path because you cannot pitch quickly, let alone precisely, enough.

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  • $\begingroup$ Thanks for explaining the relation between the rigging of the wing, the longitudinal axis of inertia and the symmetry of the airfoil. I never thought of it that it way, but it really makes sense. Great answer! $\endgroup$ – DeltaLima Jul 4 '14 at 21:49
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    $\begingroup$ I did my first roll in a Grob Twin Astir, a glider. There, you need first to pitch up, then stop the pitching motion. Then you can initiate the roll, but if you don't pitch down during the knife edge portion, the plane will change direction. Inverted, you need to keep the nose up, and so on. After my first roll, I was almost 90° off course. That makes you realize how important the elevator is in an unsymmetrical configuration. $\endgroup$ – Peter Kämpf Jul 5 '14 at 4:54
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Most of these aircraft, like the Extra 300, will have symmetrical airfoils at 0 angle of incidence. As DeltaLima pointed out, the angle of incidence is the angle of the chord line of the airfoil versus the longitudinal axis of the aircraft. Angle of attack is the angle of the airflow versus the chord line of the airfoil, which of course changes with flight conditions.

A symmetrical airfoil allows the wing to perform just as well upside down as right side up. An angle of incidence of 0 degrees helps the plane as a whole to perform in a similar way.

You will probably not find many specifics about the airfoil design, as these are specialized aircraft and airfoil design is generally a proprietary subject. However, you can look at symmetric airfoils in places like the UIUC Airfoil Database.

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  • $\begingroup$ You are quite right: Its specification doesn't tell any details about what is asked for :-( $\endgroup$ – menjaraz Jul 1 '14 at 17:45

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