Critical angle of attack
 

Hello guys,

I just could not find anything in POH nor on the web about critical angle of attack of Cessna 172, 152, PA 28, 34. I know the speeds but what I want to know is the angle at which these types stall in clean configuration and with full flaps.

Thank you for replies:)

Do you really mean angle of attack, or do you mean something else?

In what way will a pilot measure the angle of attack of his aircraft's wing?

E.g. on AIRBUS

Pitch - FPV

I really do mean the angle of attack at which those airplanes' wings stall. I know that it is somewhere around 15-20 degrees but as each airfoil has one specific critical angle of attack for given config. I was wondering what the angle was.

Well 172s and 28s etc have similar aerofoils so I'd assume its
17-18* AoA relative to the chord line of the wing irrespective
of flap. Anyway those types do not come with AoA meters as
standard equipment, so the question's a bit redundant.

cessna 172 is using a naca 2412, ok without flaps, it stalls behind AoA=22deg
http://upload.wikimedia.org/wikipedi...pha-Polare.png

The Incomplete Guide to Airfoil Usage

Cessna 152: root NACA 2412 tip NACA 0012
Cessna 172 Cutlass: root NACA 2412 tip NACA 2412 mod
Piper PA-28 Cherokee: NACA 65-415
Piper PA-34 Seneca: NACA 65-415

NACA report 824
gives
2412 Clmax (1.5-1.6) at about 16 degrees
64-415 Clmax (1.4-1.6) at 14-16 degrees
for a smooth, unflapped aerofoil, not accounting for 3D effects.

That looks fairly typical.

glider pilots use "side window pitch strings" for AoA measurement

a wool fathom and sticky tape outside the side window, and two edding-lines inside
http://www.dg-flugzeugbau.de/uploads...aden-4.jpg.jpg

http://www.pprune.org/data:image/jpe...6m+5QuQqcv/9k=
http://www.pprune.org/data:image/jpe...6m+5QuQqcv/9k=

The Cessna 170A & B had some washout in the wing construction, i.e. reduced incidence (and thus AOA) in the outboard wing. This makes the outboard wing stall last, improving aileron response through the stall. (Various sources quote the amount of washout between one and three degrees).

I expect the derivative 172 has similar washout. Does the PA-28 and its derivatives?

Washout has been a common practice in rigging prewar aircraft as well; it makes the airplane more "honest", albeit with a small potential performance penalty.

Of course, this raises the question "What AOA do you measure to define stall AOA? Root or tip?"

Indeed. But that means the answer "it stalls when the wool passes this line" is much more useful than "it stalls at 16 degrees"!

I know that we are not interested in AoA once flying. I was just looking for this answer because this was asked at one of the technical interviews another ppruner attended.

I really thank you for all the replies, it helped a lot!

Agreed, and some jet pilots have more sophisticated AoA indicators to look at. But the aircraft mentioned don't. Propellor wash would upset a piece of wool stuck to the windows.

The OP mentioned having tried to find the information in the POH for the required types. Critical AOA isn't mentioned in those documents because it's impractical for a pilot to measure the wing's AoA.

As we all know, that's why recognition of an impending stall is so important. :)

Well, the C172 does have an AoA indicator. Unfortunately, the resolution of the indication is rather low... stall horn on or stall horn off. But an AoA indicator it is. :8

Noox, what interests you more than alpha?

Speed is what interests me the most. Once I do Vs1g stall I know at which speed I will stall the airplane. As I do not have alpha vane or any other AoA indicator (for example the eyebrows) speed, low speed buffet and stall warning is the only indication of imminent stall.
But now I was just interested at what AoA it stalls specifically on 172, Seneca etc.