×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

#### Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
To Your Posts
• Keyword Search
• One-Click Access To Your
Favorite Forums
• Automated Signatures
On Your Posts
• Best Of All, It's Free!
• Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

# Stall Speed vs Altitude

 Forum Search FAQs Links MVPs

## Stall Speed vs Altitude

(OP)
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/getfile.aspx?folder=9...
http://files.engineering.com/getfile.aspx?folder=0...

### RE: Stall Speed vs Altitude

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.

### RE: Stall Speed vs Altitude

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

### RE: Stall Speed vs Altitude

(OP)
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?

### RE: Stall Speed vs Altitude

(OP)
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.

### RE: Stall Speed vs Altitude

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.

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

### Reply To This Thread

#### Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!

Already a member? Close this window and log in.