Stall Speed Options

[flyingsfuture]

An airplane will stall at a higher true airspeed as altitude increases, but will it stall at the same indicated airspeed regardless of altitude? what factors cause an airplane to stall at a different indicated airspeed?

[glnflwrs]

Indicated and True airspeed are almost the same in that to convert from IAS to TAS you go from IAS to Calibrated Airspeed, making up for the position and installation of the pressure points, then from CAS to True Airspeed which makes up for the altitude and temperature difference.

For all practical purposes an increase in the stall speed, shown as TAS, will show an equivalent increase in the Indicated airspeed. The two are different only in the temperature and altitude deviation.

I hope that Ranger or AirRabbit can give a much better explanation.

[talldude]

The following change an airplanes stall speed:
Gross weight
CG location
Load factor
Configuration....like flap settings
Airfoil contamination
Power setting

I hope thats all of them

[AName]

Ok, think of it this way. CAS is the speed at which the airfoil thinks it is going in regards to ram air pressure. CAS stall speed stays the same with any altitude because, regardless of the different OAT/Static Pressure, the airfoil itself is getting the same amount of air molecules over a given spot at a given time. As you climb, air gets less dense, so it takes a stronger relative wind to get the same ram air pressure as it would at a lower altitude with more dense air. This makes the TAS change in regards to CAS. So while TAS is the true speed at which the airfoil moves through the air, the difference in pressure will make performance different assuming a constant TAS and different OAT/Pressure. You with me so far?

Lets say at 2,000 feet, we have 200 air molecules per cubic inch (I know this isn't even remotely close, but it's just an example). If we move through the air at 100 inches per second (this would be our TAS) (again, not accurate, but just an example), a 100 square inch surface of the leading edge of the airfoil will be exposed to 20,000 air molecules per second (this would be our CAS). Now, lets say we climb up to 10,000 feet and we are still moving through the air again at 100 inches per second (again, our TAS). Well this time, our imaginary pressure at this altitude is 150 air molecules per cubic inch. Now, the same spot on the airfoil as show above will only hit 15,000 air molecules per second (CAS, which, as you can see, is less then our CAS at a lower altitude). Now, there isn't as much ram pressure, so we need to speed up the airfoil (relative windwise) to get the same amount of air molecules per second with the less dense air. If we increase our speed to 133.33 inches per second (TAS), the same given spot on that airfoil will get 20,000 air molecules per second (CAS) again.

Don't take that example practically, it's just the general idea.

[flyingsfuture]

Ok, so calibrated and indicated stall speeds will not vary with altitude, but true airspeed will. Will the indicated and calibrated stall speeds increase or decrease if the airplane is loaded too far aft or forward or will they always indicate the same stalling speed? What about if excessive load factor is placed on the plane?

[Piltdown Man]

Stall speed varies by the Square of the Load Factor. eg Pull 2g (60 degree banked turn), stall speed increases by a factor of square root of 2 = 1.41

Further forward the CofG, the higher the stall speed.

PM

[flyingsfuture]

Indicated or true?

[glnflwrs]

Both.

[flyingsfuture]

Okay, let me make sure ive got it. a plane stalled at 2000 feet will stall at the same indicated airspeed as it would at 10000, but it will stall at a higher true airspeed. A plane with a load factor greater or less than one will stall at a higher indicated and true airspeed. I am also assuming that moving the cg to beyond either its forward or aft limit will affect BOTH ias and tas stall speeds?

[Aspiring Boeing ...]

Glnflwrs, you know ur stuff!

[AName]

Okay, let me make sure ive got it. a plane stalled at 2000 feet will stall at the same indicated airspeed as it would at 10000, but it will stall at a higher true airspeed.

Yep

A plane with a load factor greater or less than one will stall at a higher indicated and true airspeed.

Yep (with a greater load factor at least). At any given altitude, an increase in IAS will cause an increase in TAS, so if the factor effects the IAS Vs, you know it effects the TAS speed.

I am also assuming that moving the cg to beyond either its forward or aft limit will affect BOTH ias and tas stall speeds?

Yep. Weight is a big factor in stall speed for IAS (which directly effects CAS and TAS). When you move the CG forward, the plane weights more so the stall speed goes up. The reverse is true for an aft CG as the plane weighs less. This is because with an aft CG, the horizontal stabilizer doesn't need as much force to keep the nose up as it would with a forward CG.

[glnflwrs]

AName is the pro here.

[jshsedgwick]

Ok, think of it this way. CAS is the speed at which the airfoil thinks it is going in regards to ram air pressure. CAS stall speed stays the same with any altitude because, regardless of the different OAT/Static Pressure, the airfoil itself is getting the same amount of air molecules over a given spot at a given time. As you climb, air gets less dense, so it takes a stronger relative wind to get the same ram air pressure as it would at a lower altitude with more dense air. This makes the TAS change in regards to CAS. So while TAS is the true speed at which the airfoil moves through the air, the difference in pressure will make performance different assuming a constant TAS and different OAT/Pressure. You with me so far?

Lets say at 2,000 feet, we have 200 air molecules per cubic inch (I know this isn't even remotely close, but it's just an example). If we move through the air at 100 inches per second (this would be our TAS) (again, not accurate, but just an example), a 100 square inch surface of the leading edge of the airfoil will be exposed to 20,000 air molecules per second (this would be our CAS). Now, lets say we climb up to 10,000 feet and we are still moving through the air again at 100 inches per second (again, our TAS). Well this time, our imaginary pressure at this altitude is 150 air molecules per cubic inch. Now, the same spot on the airfoil as show above will only hit 15,000 air molecules per second (CAS, which, as you can see, is less then our CAS at a lower altitude). Now, there isn't as much ram pressure, so we need to speed up the airfoil (relative windwise) to get the same amount of air molecules per second with the less dense air. If we increase our speed to 133.33 inches per second (TAS), the same given spot on that airfoil will get 20,000 air molecules per second (CAS) again.

Don't take that example practically, it's just the general idea.

so in laymans terms the thinner the air the higher the stall speed?

[AName]

AName is the pro here.

Nah, it's all just one big lucky guess

so in laymans terms the thinner the air the higher the stall speed?

In regards to True airspeed, yes. Indicated/Calibrated stays the same regardless of air pressure.

[Piltdown Man]

True or Indicated? IAS remains the same (ish) but True has to increase. In laymans's terms, the wing needs to move molecules of air around it to work. To produce X amount of lift is has to move Y amount of molecules per second, but the amount of molecules in any given lump of air reduces as you get higher, therefore you have to fly faster to get the same number of molecules per second to produce the same amount of lift. Clear as mud?

PM

This post has been edited by Piltdown Man: Apr 28 2007, 04:34 PM

[AName]

True or Indicated? IAS remains the same (ish) but True has to increase.

Uhh, that's what I said.

Edit: Nevermind, I think that was directed to jshsedgwick's question. Carry on.