Stall Speed vs Altitude

[Skunkworks1011]

Can anyone help me understand these graphs?

I thought the Stall/CLmax would occur at a roughly constant KCAS. What could explain the kinks in the curves? The graphs are for a Gulfstream IV (blue) and a G450 (green). The have a virtually identical wing, except for fewer vortex generators on the G450.

Thanks!

I wasn't sure how to attach two files, so hopefully these links work:

http://files.engineering.com/getfil...&file=Screen_shot_2013-03-16_at_13.45.18_.png

http://files.engineering.com/getfil...&file=Screen_shot_2013-03-16_at_13.45.53_.png

 

[mohr]

Hi Skunkworks1011:
The pilot flies always KCAS (calibrated air speed, knots) information that gives the Airspeed indicator. From a certain pressure altitude, KCAS is practically constant. Obviously the TAS (true air speed) increases with the pressure altitude.
The shape of KCAS curves may result from low speed buffet boundaries at high pressure altitude.

 

[rb1957]

my thought was the steeply sloping lines (the middle segments) are constant mach number, and the slightly sloping lines (at the top) show slight calibration variation for essentially constant airspeed.

CAS is indeed what the pilot sees on the instrument, is close to EAS, whereas TAS has the density ratio factoring.

Quando Omni Flunkus Moritati

 

[Skunkworks1011]

Thanks for the replies.

If the middle segment was a constant Mach number, I believe the CAS would decrease with altitude.

Since we are dealing with low speeds, I wouldn't think compressibility needs to be considered.


Any other ideas?

 

[berkshire]

Maybe this will help,

http://en.wikipedia.org/wiki/Coffin_corner_(aviation)

B.E.

 

[berkshire]

For some reason the end of the url was chopped off
I will try below.
B.E.

 

[Skunkworks1011]

I am familiar with the concept of Coffin Corner, but I don't see how it is related to the shape of these curves.

Please explain.

Thx.

 

[dgapilot]

Don't know for sure, but could it be that the pressure laps rate is pretty constant up to about 3000 ft, then the rate changes and stays consistant again from there to about 23000, then the rate changes again from 23k on up. Just a thought.