Originally Posted by
propjunkie
Been looking over thread and want to make sure everyone is on the same page.
Indicated stall speed remains absolutely 100% constant regardless of altitude
The higher we fly our indicated airspeed will represent a higher and higher true airspeed due to the decrease in air density.
Indicated airspeed is measured by molecules being packed into a pitot tube well if we are at a higher altitude the air density is lower (less molecules to enter our pitot tube) so in order to obtain the same indicated airspeed at a higher altitude we must fly at a faster TAS.
In my example lets say we are flying at our stall speed, we are at our critical angle of attack, and are in unaccelerated level flight. If we increase our altitude and all factors (weight, CG, bank angle, ect.) remain constant the decrease in air density will cause our lift to decrease. We compensate for this loss of lift by either increasing AOA, or velocity (TAS) because in my example we are at our critical angle of attack we can only increase or speed, therefore an increase in altitude will increase the true airspeed the airplane stalls.
Even though the true airspeed the airplane stalls increases the indicated speed remains constant due to lower air density at higher altitudes. as said earlier "The higher we fly our indicated airspeed will represent a higher and higher true airspeed due to the decrease in air density." It is an absolute fact that indicated stall speed is the same at sea level as it is at FL600.
Ive found this discussion very interesting. Making me think about things I haven't thought about since getting an unused aero engineering degree 27 years ago.
Your post is however incorrect. As a former U-2 pilot I can tell you tell you it does change and have been well above FL 600.
The original post didn't define the type of aircraft or the regime of flight the were referencing. For all practical purposes, in an approach phase application, IAS at stall is a constant with variations for density being minimal. If you want to talk about high altitude/performance aero then you bring in other variables.
Bottom line, the atmosphere density combined with the approach speeds to any runway I can think of make your statement true in that application.
Lee