Stall Speed and Altitude

[uwochris]

Hey guys,

I have yet another question for you

I am just wondering what is the best (and easiest) way to explain to a new student why the stall speed (in IAS) remains relatively constant with altitude?

To any instructors here, how do you explain this, without really relying on the lift equation?

On a similar note...

The stall speed is related to a specific AOA- this AOA does not change with altitude.

Likewise, the speed for best range and endurance are also related to a specific AOA and do not change with altitude (only weight, load factor, etc).

The best rate of climb speed (Vy) has also been related to maintaining a specific AOA (where L/D ratio is optimized); however, unlike the others, this speed DOES change with altitude.

Why the difference?

Perhaps Vy is not in fact related to a specific AOA??

Thanks again.

 

[Mr. Cole]

Chris,

Well, you are correct in part, the IAS for the stall doesn't change with altitude, but the TAS does. I'm sure you understand the lift equation and with everything being equal stall speed changes with weight or G-loading, but the critical AOA is always the same. So this explains how we can find the stall speed at one weight/load if we know the stall speed at another weight/load, assuming everything else is the same, i.e. air density and the other values that go into the lift equation. Vs1 = Vso*sqrt(W1/W0), no biggie.

But as we climb the air density is not the same as at sea level, but is decreasing. So given the same weight and same critical AOA the stall speed should increase since you need to go faster to get as many molecules going over the wings. But for some reason the IAS at stall remains about the same. But this is where understanding the difference between TAS and IAS comes into play. The ASI is calibrated at sea level density and isn't recalibrated for different air densities at higher altitudes. This is unlike your altimeter than can be recalibrated. The airspeed is inversely proportional to the air density. As you climb, the true air density falls, but the value being used to compute airspeed is still sea level density, and is higher than reality. That's why TAS increases with altitude.

So in theory the stall speed is going up because the density is dropping, but your ASI doesn't take falling air density into account, so your IAS at stall remains the same as far as the ASI is concerned. If you're showing an IAS of 60 kts, then you must be traveling at a TAS that sends enough air molecules into the pitot tube for that indication. If there are enough air molecules entering the pitot tube to register 60 kts, then the plane doesn't really care what the TAS is. Likewise if you have enough air flowing across your wings to indicate the stall speed, the wings are seeing enough air to keep you flying. But the TAS will vary.

Dave

 

[jayme]

Vy changes with altitude because available power decreases as altitude increases.

 

[TDTURBO]

Vy changes with altitude because available power decreases as altitude increases.

Not with a turbo it don't.

 

[Crizz]

Not with a turbo it don't.

True avaliable SHP remains constant, but avaliable thrust decreases... the propeller is less efficient.