Stalls

[mocaman]

Ok I am getting ready for my CFI checkride and I am trying to explain stalls In a manner that is easy to understand. I know that an airplane will stall when the critical AOA is exceeded. But lets say I am cruising around in a 150 and pull the power....I keep pitching up and up and up and finally the plane stalls. Why is that pitch alot less than if I had full power in and then the plane stalled? I know that with full throttle there is excess power available, but I cant seem to bring everything together. I thought critical AOA on most light planes is about 18 degrees pitch up. I know that with idle power in the 150 the AOA is not even close to 18 degrees in the stall. Your ideas and thoughts are appreciated. Thanks!

 

[avbug]

Mocaman,

You're confusing angle of attack with pitch attitude.

How do you know what the angle of attack is on the airplane? Do you have an angle of attack indicator?

When in a climb with full power, how much of that thrust compensates for vertical component of lift? Think about it carefully.

When in a climb with full power, how much does the induced airflow over the wing alter the stall characteristics in the propeller slipstream? Think about it.

What is the angle of attack of the wing when under the influence of the propeller slipstream, vs. when it is not? Think about it.

 

[Amish RakeFight]

During power-on stalls, the plane stalls at a higher pitch and a lower airspeed due to the downwash from the propeller which provides some additional lift. There is also something called the lift of thrust which assists in a higher pitch attitude during the stall. More power allows for the pitch to seem highe, although the stalling AOA always remains the same.

18 degrees for the wings AOA - not pitch attitude.

Don't mix pitch attitude with AOA. Exceeding the critical AOA can occur with any pitch attitude.

 

[Doc Holiday]

Don't confuse or assimilate AOA to the pitch attitude of the airplane. It's true that the PITCH ATTITUDE will not be 18 degrees when the aircraft stalls. Remember that AOA is defined as the angle between the chord line of the wing and the RELATIVE WIND. The relative wind is not opposite and parallel to the pitch attitude of the aircraft. For one, there needs to be some angle of attack for a wing to produce lift. Another thing to take into account is the angle of incidence, the angle between the chord line of the wing and the longitudinal axis of the airplane. The angle of incidence by definition does not allow the angle of attack to match the pitch attitude of the airplane.

Remember that an airplane can be stalled at any airspeed in any pitch attitude. It is all about the relative wind.

 

[VW Pilot]

Simply put....you still have the propeller spining and producing thrust....but at that point, it is not enough to keep the flow over the wings smoothly. but enough to keep it going in a forward motion so some flow is still there requiring an even steeper pitch........without power, you are simply using the plane's kinetic energy to rotate until the speed bleeds off and the flow burbles over the wings.....or try keeping the plane level in flight without power....same thing. it is going to stall. with power it just acts as of someone is somewhat nudging the plane alone until the angle of attack is sufficient for a stall condition....my two cents....maybe someone else has a better explanation...

Ok I am getting ready for my CFI checkride and I am trying to explain stalls In a manner that is easy to understand. I know that an airplane will stall when the critical AOA is exceeded. But lets say I am cruising around in a 150 and pull the power....I keep pitching up and up and up and finally the plane stalls. Why is that pitch alot less than if I had full power in and then the plane stalled? I know that with full throttle there is excess power available, but I cant seem to bring everything together. I thought critical AOA on most light planes is about 18 degrees pitch up. I know that with idle power in the 150 the AOA is not even close to 18 degrees in the stall. Your ideas and thoughts are appreciated. Thanks!

 

[SPilot]

Don't confuse pitch angle and AOA. There is no exception: the wing will stall when AOA (16-20 degrees) is exceeded, regardless of pitch attitude.

Excess power relates to how much power is necessary to stay at altitude, not how much power is necessary to avoid stalling.

Reason pitch attitude is different for power-on and power-off, is that in a propeller aircraft you have accelerated airflow over the lifting surface, so with more throttle, you need to fly a higher pitch attitude to dissipate enough airspeed so that the airplane will reach reasonable stall speed (assuming no accelerated/G-force stall). Another way to think of it is that with power-on stall, you have excess power, so the airplane will have a climbing flight path, and you have to add that angle to the angle you need to stall the airplane.

As you dissipate airspeed, you eventually lose lift and begin falling down. So even if the pitch is about level in a power-off stall, your angle of attack is still beyond critical when the nose drops.

 

[nosehair]

I know that with idle power in the 150 the AOA is not even close to 18 degrees in the stall.

What everyone says about confusing pitch with AoA is true, and as an aspiring new instructor, take a lesson:

You should not have that confusion because your instructor should have explained the difference, but...it seems that your stall technique has given you that illusion. You should bring the nose up to about a 20 degree pitch attitude, and hold it, during stall practice. If you are practicing stalls without holding a constant pitch attitude, you are missing a very important element of stall training. Holding a constant pitch attitude as the airspeed slows to stall, and keeping a constant heading and wings level is how the controls feel during the landing. Not holding the nose up in a constant attitude is what teaches students to land flat. They intiate a flare and allow the nose to drop a bit and land flat. They don't have the "feel" of continuing the back pressure-holding-the-nose-up during and after the touchdown.

Carry the message.