angle of attack vs. pitch attitude

[brianjohn]

Could someone please try to explain the difference between angle of attack and pitch attitude??? This subject has made zero sense to me. I know I need to have a grasp of it as I'm moving into becoming a CFI. I cannot for the life of me figure out how an aircraft can stall at any airspeed, yet the P.O.H.'s give you stall speeds for specific configurations. So my question is in reference to an aircraft stalling.

 

[flywithastick]

Angle of attack (AOA) usually refers to the angle between the wing longitudinal axis (or effective chord line) and the air stream it is flying through.

Pitch attitude refers to the angle between the longitudinal aircraft axis and the Earth.

These can be widely different. For example, when a plane is diving it may have a -20° pitch attitude and a 0° AOA, a plane with quite a bit of power could be climbing at +20° pitch attitude and have a +5° AOA OR entering a loop with +90° pitch attitude and +10° AOA.

Wing sections stall at their respective critical AOA. Regardless of speed, pitch attitude or airspeed - right side up or upside down, a wing will stall at the critical AOA.

What makes AOA valuable is that it's a direct measure of "how close" a wing is to stalling. Airspeed is really an indirect measure of proximity to stall. The reason stall airspeed changes with configuration is pretty complex, but for various bank angles, density altitudes, loading, etc, the indicated stall airspeed will vary.

As for a plane stalling at a speed well above Vs, imagine a situation where the wing is forced into a high AOA condition. Whether by some "giant" grabs the tail and forces it down or the pilot's pulling back like heck, the wing may be forced beyond the critical AOA. Forcing the AOA to the critical angle like this will result in a stall - specifically, an accelerated stall (might break the plane too - be careful).

Back to the example of looping the plane, you have to be conscious of preventing the accelerated stall. Entering the loop or recovering on the backside, an unexpected accel stall can really wake you up or worse.

Hope his helps. anyone with a better explaination, jump right in.

 

[midlifeflyer]

Let's start with real simple, non-technical defintiions:

Pitch is how high or low the nose is relative to level. Level is roughly equivalent to being parallel to completely flat terrain below. (Following flywithastick's cue, I'm trying to be visual without pictures)

AoA is the angle between the wing chord and the relative wind. It's easy to get stuck thinking that the relative wind is opposite the direction of level travel — that it, too is parallel to that completely flat terrain below.

But it's not. Assume you're descending. The relative wind is likely to be opposite your direction of flight, but your direction of flight is not only forward, but downward.

So your AoA may well be 0, while your pitch is nose down.

You asked about stalling at any airspeed and the way stall speeds are referenced with respect to different configurations. That's exactly the point. It's those different configurations that make the wing reach its critical AoA at different speeds.

The phrase "stall at any airspeed" doesn't mean that an airplane with 40 KT clean stall speed at max gross weight in straight and level flight will, for no apparent reason suddenly decide to stall in at max gross weight in straight and level flight at 90.

What it means is that you have to avoid thinking "so long as I always stay above 45, the airplane won't stall". Because, put that airplane into a 60 degree bank and it may well stall at 90. (the extra load factor at 60 degrees bank mean that the critical AoA will be reached earlier).

 

[AV8OR]

Hey Mark,

Did yall ever get all the AA aircraft equiped with AOA indicators?

I was at Eagle for a short while and heard yall were working on that. One great idea I wish everyone would adopt. They're on every Part91 jet I've ever flown.

Regards

 

[AV8OR]

"Hey Mark,

Did yall ever get all the AA aircraft equiped with AOA indicators?

I was at Eagle for a short while and heard yall were working on that. One great idea I wish everyone would adopt. They're on every Part91 jet I've ever flown.

Regards"

Oops! After already posting, I went to your Website and realized that APA was Centenial Airport, not Allied Pilots Association. Sorry. Any AA pilots that can answer my question would be great.

 

[f18pugsley]

AOA is a key factor for aircraft that can change their gross weight significantly between takeoff and landing. For example, and FA-18 can take off at more than 50,000lbs and land at less than 32,000lbs. 'On speed' AOA between these weights will change by well over 40 KIAS. Even within the legal landing weights, landing speed will differ by almost 20 knots. That is why most tactical aircraft and many cargo aircraft set landing speed by AOA, not airspeed. In my very limited experience in the Herc, we do the same thing indirectly when we calculate landing speed based on weight. If we had an AOA guage, all we would need to do is cross check our landing configuration AOA with our airspeed like I could do in my previous platform.

The way I remember the correlation of AOA to gross weight is that when an airplane is heavier, it must go faster to get the equivalent amount of lift (but the angle of incidence of the wing stays the same.)

When trying to get the FAA to recognize the Hornet as a true multi aircraft (not C/L limited), it came down to the fact that Navy won't publish a min controllable airspeed with an airspeed instead of an AOA. They have correlated landing speed AOA to airspeeds, but Vmc and stall speeds are published by AOA only (since they vary so much with gross weight.)

Reference what midifeflyer said about AOA being relative airflow.

 

[midlifeflyer]

Oops! After already posting, I went

WHEW. For a moment there, I though it was me.

 

[Groucho]

AOA vs.Pitch

All the above posts are correct. It is sometimes hard to make this all make sense in an intutive way.

The only way to get a seat of the pants understanding of angle of attack vs pitch is to fly aerobatics.

I would recomend an aerobatics short course, if you can find one, to help understand how a wing really works.

 

[N8157B]

Suggested reading: Stick and Rudder.
Excellent book. Clears up alot of "mysteries". Check your local library.

 

[Rvrrat]

Beat me to it...

Suggested reading: Stick and Rudder.

Why an aircraft is also called an airplane...An inclined plane operated in the medium of air