I understand the Alaskan bush pilots use 90° flaps in the Valdez STOL competition. I think this would stall the flaps. And if so, it wouldn't increase lift and reduce stall speed, but Javafoil says the opposite.

Perhaps this is more for lift/lower stall speed than drag. Since they land flat halfway down a long runway they don't need drag to slow down.

Just to do a test, I input a NACA 2415 foil in javafoil, 25% chord flap, 90 deg flaps deflection. And as expected, lots of drag, but Clmax jumps from 1.5 to 5 at about 0 deg AOA.

Is this realistic? If not, what is the real Clmax?

  • $\begingroup$ A STOL landing is typically best accomplished at high drag with power on. This is especially true of aircraft with a high power to weight ratio, as is typically the case in these competitions. The STOL landing technique will typically involve a higher than normal (e.g., greater than 3°) descent angle with a touchdown near, or even beyond, wing stall. Thus, the greater drag of greater flap extension likely aids in maximizing the short landing performance. $\endgroup$
    – J Walters
    Aug 28, 2019 at 15:09
  • $\begingroup$ They actually land pretty flat WITH POWER OFF in this competition as it is in halfway down a runway and not a clearing surrounded by trees. It's really a test of short landing roll, rather than a typical STOL landing with a short approach and short landing. $\endgroup$
    – Fred
    Aug 28, 2019 at 15:12
  • 5
    $\begingroup$ To be clear; while this behaviour is most common amongst Alaskan bush pilots, other bush pilot subspecies around North America have been observed to use 90* flap deflection as well. Biologists believe this is a form of territorial display. $\endgroup$
    – 0xdd
    Aug 28, 2019 at 15:39
  • 2
    $\begingroup$ I'd say you are seeing oddball numbers because the effects of 90 deg flap are outside of javafoil's normal parameters. $\endgroup$
    – John K
    Aug 28, 2019 at 16:10

1 Answer 1


The following graph is from Cahill, Summary of Section Data on Trailing-Edge High-Lift Devices. As you can see, for a plain flap, with gaps fully sealed, the $C_{L_{max}}$ maxes out at around 60deg.

enter image description here

When I played around with xfoil, things became unreasonable beyond 40deg. Probably some assumptions in the boundary-layer equations broke down. Since Javafoil utilizes the same computational approach as xfoil, I would expect the same behavior.

On the Alaskan Bush pilots, are we sure they are 90deg deflection, and not just some high deflection that looks almost vertical? With multi-element, slotted flaps, you can do better than plain flaps.


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