Alpha Systems Angle of Attack video

[carusoam]

Rip,

Very interesting...

Restating...

Clean stall happens at a higher A/S AND higher AOA than dirty stall.

That is a challenge...

Did I get that right?

Best regards,

-a-

[MooneyCFII]

Rip,

Very interesting...

Restating...

Clean stall happens at a higher A/S AND higher AOA than dirty stall.

That is a challenge...

Did I get that right?

Best regards,

-a-

Well, close. First, the question is, "what are we measuring?" Start by drawing a line from the leading edge of the airfoil to the trailing edge of the airfoil. This is the chord line. AoA is the angle that the relative wind makes to the chord line of the wing. Now extend the flaps. You have changed the location of the trailing edge of the wing and hence the chord line. Now it is possible that the wing still stalls at the same AoA but since the chord line has changed, critical AoA will likely occur at a different angle relative to the longitudinal axis of the aircraft. Our perception is that the wing stalls at a "lower" AoA when, in fact, the wing still stalls at the same (or very similar) AoA. This is why we need our AoA indicators to be calibrated for flap deflection. 

 

I have heard a couple of AoA manufacturers say, "well, if you calibrate it clean you will have a "buffer" when you put the flaps down." I'm sorry, I don't want an unknown buffer. I want to know where approach reference AoA is and I want to know where critical AoA is REGARDLESS of where my flaps are set. (Oh, and I really want to know zero-lift AoA too but ... I'm abnormal. :-) 

[MooneyCFII]

On a related note, Garmin has just weighed in with their AoA indicator. It is a differential-pressure unit but, lo and behold, it is a three port device that senses dynamic (pitot) pressure too, making it a true AoA instrument ... for one flap position. <sigh> Nope, no flap position sensing or calibration. Poop.

 

But, hey, we are getting closer!

 

(I prefer differential pressure instruments to moving vane type because there are no moving parts to break. But we still need to solve the problem of calibration for flap position.)