2
$\begingroup$

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?

$\endgroup$
  • $\begingroup$ 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 '19 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 '19 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 '19 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 '19 at 16:10
2
$\begingroup$

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.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.