stalling speed

[flyguy]

Power doesn't affect stall speed. The airplane always stalls when it exceeds the critical angle of attack. You can exceed this at any power setting. Maybe you're confusing slow flight. " Any airspeed at which an increase in angle of attack, load factor or reduction in power will induce an immediate stall." When flying in slow flight, if you maintain constant back pressure and suddenly reduce power, you stall. Not directly because of power, but because with the reduction in power, the angle of attack will slightly increase with constant back pressure, thus exceeding CLMAX.

Slow flight and power also deal with the region of reversed command. Where you are on the backside of the power/drag curve and more power is required to go slower. This merely has to do with so much induced drag due to the high angle of attack that in order for the airplane to operate at such a slow airspeed and high angle of attack, you need more power.

To demonstrate that power doesn't affect stalling speed, take a 172 to up to about 4,000AGL, set the power to idle. Just pitch up until it stalls (power off stall). Let it recover then do a power off steep turn at 60' bank while trying to maintain altitude, you will accelerate the stall and probably stall somewhere in the ball park of 60kts. You can also initiate a descent at about 500'FPM power off, and abruptly pitch up (without exceeding airframe limitations) and you'll see an accelerated stall. Of course do this with an instructor.

That's what I was thinking.

 

[ananoman]

Power doesn't affect stall speed. The airplane always stalls when it exceeds the critical angle of attack. You can exceed this at any power setting. Maybe you're confusing slow flight. " Any airspeed at which an increase in angle of attack, load factor or reduction in power will induce an immediate stall." When flying in slow flight, if you maintain constant back pressure and suddenly reduce power, you stall. Not directly because of power, but because with the reduction in power, the angle of attack will slightly increase with constant back pressure, thus exceeding CLMAX.

Slow flight and power also deal with the region of reversed command. Where you are on the backside of the power/drag curve and more power is required to go slower. This merely has to do with so much induced drag due to the high angle of attack that in order for the airplane to operate at such a slow airspeed and high angle of attack, you need more power.

To demonstrate that power doesn't affect stalling speed, take a 172 to up to about 4,000AGL, set the power to idle. Just pitch up until it stalls (power off stall). Let it recover then do a power off steep turn at 60' bank while trying to maintain altitude, you will accelerate the stall and probably stall somewhere in the ball park of 60kts. You can also initiate a descent at about 500'FPM power off, and abruptly pitch up (without exceeding airframe limitations) and you'll see an accelerated stall. Of course do this with an instructor.

I don't see what an accelerated stall has to do with proving power doesn't affect the stall speed of an aircraft. If you are comparing apples to apples, you can make an airplane fly slower than its power off stall speed by using power to help support some of the weight of the aircraft.

You are correct that an airplane will always stall if Cl max is exceeded and that there are many variables that determine the airspeed where Cl will be reached. G loading is one of them and can cause an aircraft stall at any airspeed and power setting, but the simplest case to compare results is in an unaccelerated stall. You would not conclude that weight has no bearing on stall speed simply because you can pull enough G's to make a light aircraft stall at a higher speed than a lightly loaded one. So, pulling G's to make an aircraft with a high power setting stall at a higher speed than in a 1 G power off situation is not a fair comparison.

The next time you are out flying, try two identical stalls, with the only variable being power. Do a power off stall, maintaining altitude and note the speed at which the airplane stalls. For the next stall, gradually transition to slow flight, until it takes full power to fly straight and level. You should be able to maintain altitude at or below the normal power off stalling speed of the aircraft. The reason you are able to do this is because you are using power to support some of the weight of the aircraft. The wing will still stall at the same AoA, but the upward component of the thrust vector is now helping to lessen the weight that the wings must support. Cl max will now be reached at a lower airspeed. So you might want to rethink why reducing power during slow flight results in a stall.

For multi-engined prop airplanes things get even more interesting, as power will actually create lift due to the induced airflow over the wings. It is not an accident that the military uses 4 engined turbo-props when they want to haul a big load off a very short runway. At high power settings, the airflow induced by the propellers makes a tremendous amount of lift, allowing them to get airborne at fairly low airspeeds with a very short take off run.

 

[flyguy]

The next time you are out flying, try two identical stalls, with the only variable being power. Do a power off stall, maintaining altitude and note the speed at which the airplane stalls. For the next stall, gradually transition to slow flight, until it takes full power to fly straight and level. You should be able to maintain altitude at or below the normal power off stalling speed of the aircraft. The reason you are able to do this is because you are using power to support some of the weight of the aircraft. The wing will still stall at the same AoA, but the upward component of the thrust vector is now helping to lessen the weight that the wings must support. Cl max will now be reached at a lower airspeed. So you might want to rethink why reducing power during slow flight results in a stall.

For multi-engined prop airplanes things get even more interesting, as power will actually create lift due to the induced airflow over the wings. It is not an accident that the military uses 4 engined turbo-props when they want to haul a big load off a very short runway. At high power settings, the airflow induced by the propellers makes a tremendous amount of lift, allowing them to get airborne at fairly low airspeeds with a very short take off run.

What is happening in this example is you are using prop wash to generate lift. This is effectively lowering the AOA because much of the average relative wind is from the prop wash which is perpendicular to the longitudinal axis. The reason the indicated airspeed may appear to be lower than normal stalling speed is becasue the pitot tube is not measuring this wind from the propellor, but the actual speed of the airflow over the wings is the same.

 

[ananoman]

For the multi-engine aircraft, you are correct. Propwash over the wings is being used to generate lift. For the single, especially a high wing, this is much less of a factor. The reason you can fly slower with power in a single is mostly due to the vertical component of the thrust vector lessening the amount of lift the wings must create to support the aircraft.

Either way, power does effect stall speed. This is why you will never see a power on stalling speed in a POH or on any certification requirement. It is almost impossible to quantify.

 

[gtpilot]

For the multi-engine aircraft, you are correct. Propwash over the wings is being used to generate lift. For the single, especially a high wing, this is much less of a factor. The reason you can fly slower with power in a single is mostly due to the vertical component of the thrust vector lessening the amount of lift the wings must create to support the aircraft.

Either way, power does effect stall speed. This is why you will never see a power on stalling speed in a POH or on any certification requirement. It is almost impossible to quantify.

Ask Patty Wagstaff if power can affect stall speed....