Stall speed

[Donsa320]

Sbty One,

You mention slow airplane and low altitude. Are both speed and altitude factors? Whether together or apart, at what altitude and/or at what airspeed does this become a factor?

As a VERY general statement, the effects are minimal below 200kts IAS and 12,000ft altitude. Your old E6-B is inaccurate for TAS computations at any altitude at higher IAS speeds. You need a Jepp "CR" type computer or something similar which does allow compressibility error to be considered. So even in King Airs and Turbo-Commanders, etc, the E-6B will be in error for TAS computations. TAS will be less than an E-6B says it is!

~DC

 

[Stby One]

Midlifeflyer, the speed is the key factor here, however its closely linked with altitude. At low altitude you can fly 340 KIAS and still only show around mach .5. In most commercial jets you are then well below your critical mach number. but at 41000 340 KIAS would be around mach 1.2

In a jetfighter you probably have a MCRIT just below mach 1, while as a B737 maybe has a MCRIT just above mach .7 (dont quote me on that). MCRIT is defined as your free stream mach number at which you first will start to see supersonic airflow over some part of the aircraft, normally the upper sides of the wings since that is where the difference in speed of the free stream airflow and the airflow disturbed by the aircraft is the greatest.

When you get supersonic airflow over the wings a shockwave is created. It acts like a wall and either changes the laminar boundary layer to a turbulent one or simply destroys the boundary layer thereby reducing the coefficient of lift. If speed is increased in an aircraft not designed for high transonic to supersonic speeds, the boundary layer will eventually separate causing a high speed stall.

A C172 that cruises at around Mach .15 wont have any compressibility problems. But since its wing is designed for low speed flight it would get compressibility problems long before a commercial jet.

 

[Mojave9240]

Thank-You All!!

Great info thanx all!!

 

[midlifeflyer]

Stby One and DC, Thanks to both of you. Great explanations.

 

[uwochris]

I believe the formula to calculate the new stall TAS is:

CAS of the stall speed for your desired config/weight * Square root of(density of the air at altitude/ density of air at sea level).

The stuff in the bracket is referred to as the "density ratio." You have to find the specific ratios in a table/graph, which are often published in aerodynamic textbooks, such as "Flight Theory for Pilots" or "The Advanced Pilot's Manual."

As said above, the IAS does not change.

As a reference, the formula for calculating TAS = EAS (or CAS)/ square root of density ratio. As an approximation, you can simply estimate this by increasing IAS 2% for each 1000 ft of density altitude.

 

[Stby One]

Thanks uwochris,

I couldnt remember the exact formula but thats the one. Since the density ratio at 41000 is close to 25%, the TAS will be twice the EAS (or CAS).

 

[Singlecoil]

By the way, I found an awesome atmosphere calculator at:

http://www.aerospaceweb.org/design/scripts/atmos.shtml