View Full Version : Effects of Power regarding stall.

27th Oct 2009, 13:12
G'day guys, I'm currently going through my FIR and came across this question I would know more about.

We teach in terms of the lift required is decreased since there is a vertical component of thrust acting as lift, thus a decrease in stall speed.

What bugs me is that I don't know what actually happens in terms of aerodynamics. Another instructor and I have speculated the following.

Assuming a single engine A/C power on stall no washout.
When the wing reaches critical angle what happens is that the wingtips are stalled. However, the wingroots are NOT because of the slipstream re-energizing the boundary layer.

If you further increase the AoA there becomes a point where the wingroots becomes stalled even with slipstream. Combine this with the already stalled wingtips tends to give you a fairly "violent" stall characteristic.

Any insights to this?


Pugilistic Animus
27th Oct 2009, 16:17
adding 'power' sorry OS :}more correctly thrust--- lowers the AoA in this case [aoa=the difference from where the airplane is going and where it looks like its's going] this is from a the 'flattening-out' of the flight path--as thrust contols altitude [basically]-you therefore require higher pitch attitudes for a stall to occur but the stall aoa does not change---this excercise is done to simulate an incorrect go around showing a stall ios not controlled by 'power' but by AOA:)


29th Oct 2009, 09:27

You can basically ignore the slipstream over the wingroot because a) the effect is minimal in most training aircraft and b) the effect of thrust on stall speed is the same when you have a pusher prop mounted behind the wing.

Thrust does not control the altitude or flatten anything.

The most simple way of explaining the situation is to consider the aircraft in level 1g flight.

As the aircraft slows, the angle of attack must be increased to keep lift constant - maintain level flight.

Since the wings are bolted to the fuselage, increasing the angle of attack in this case also increases the attitude of the aircraft i.e. the nose is moved progressively above the horizon in order to maintain horizontal flight with the reducing airspeed.

If we imagine the aircraft at the point of stall > speed = stall speed, AOA = stalling angle of attack. At this point, the "lift" and "weight" forces are balanced.

Now let's add some thrust (prop, jet, rocket - it matters little) along the longitudinal axis of the aircraft. This force can be resolved into two components - A force along the flight path (horizontal in this case) and the other at 90 degrees to this (vertical in this case).

Since the thrust was applied, the vertical forces are no longer balanced i.e. lift plus the vertical component of thrust now exceeds the weight.

To restore the balance without removing the thrust, lift must be reduced. This can be done by either;

a) Maintaining the same angle of attack but reducing the speed of the airflow (airspeed); or

b) Maintaining the same airflow (airspeed) but reducing the angle of attack.

Thus the effect of increased thrust is to enable either;

the same airspeed to be maintained but with a lower angle of attack (better stall margin) or

the aircraft to fly slower at the same angle of attack.

Thus the effect of adding power is to reduce the indicated airspeed at which the aircraft stalls.

At the stall with thrust applied, the attitude will be higher because this is necessary to acheive the stalling angle of attack.

Please note that I am talking about a true aerodynamic stall of the wings and not the case which may apply to some aircraft where the 1g stall speed is established during certification via the elevator being on the up stop for a certain period of time. In such cases, propeller wash can have a change in the effectiveness of the elevator and the resultant may not be as simple as given above.

Pugilistic Animus
31st Oct 2009, 18:20
You haven't had 'power/thrust required/available till you had it with me:8

although folks say that rant is like:\


Pugilistic Animus
31st Oct 2009, 18:57
but then the British may say 'what's he on about':}:}:}