# Do Alaskan bush pilots generate a Clmax of 5 with 90° flaps?

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?

• 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. – J Walters Aug 28 '19 at 15:09
• 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. – Fred Aug 28 '19 at 15:12
• 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. – 0xdd Aug 28 '19 at 15:39
• I'd say you are seeing oddball numbers because the effects of 90 deg flap are outside of javafoil's normal parameters. – John K Aug 28 '19 at 16:10

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.