polar diagram in high altitude

Hi,

I wonder whether the lift over resistance coefficient diagram (aka polar
diagram or lilienthal diagram) of a given airplane changes much with
altitude.

The rationale behind the question is, that I read somewhere, that commercial
aircraft are flying in the so called "coffin corner", that is so high they
can't go faster because of compression and can't go slower because of stall.

Why should they do this, when they could well be a little lower and
comfortly fly at the point of optimal angle of access, well away from stall.

Do optimal point and stall point come closer together with increasing
height?

"dominik lenné" wrote in
:

Hi,

I wonder whether the lift over resistance coefficient diagram (aka
polar diagram or lilienthal diagram) of a given airplane changes much
with altitude.

The rationale behind the question is, that I read somewhere, that
commercial aircraft are flying in the so called "coffin corner", that
is so high they can't go faster because of compression and can't go
slower because of stall.

Why should they do this, when they could well be a little lower and
comfortly fly at the point of optimal angle of access, well away from
stall.

Do optimal point and stall point come closer together with increasing
height?





Are you sure you are refering to commercial aircraft ?

sure sounds as if you thinking of the U-2 with its VERY narrow
flight envelope at close to max.

On Feb 11, 5:50*pm, John Szalay john.szalayATatt.net wrote:
"dominik lenné" wrote :





Hi,

I wonder whether the lift over resistance coefficient diagram (aka
polar diagram or lilienthal diagram) of a given airplane changes much
with altitude.

The rationale behind the question is, that I read somewhere, that
commercial aircraft are flying in the so called "coffin corner", that
is so high they can't go faster because of compression and can't go
slower because of stall.

Why should they do this, when they could well be a little lower and
comfortly fly at the point of optimal angle of access, well away from
stall.

Do optimal point and stall point come closer together with increasing
height?

*Are you sure you are refering to commercial aircraft ?

sure sounds as if you thinking of the U-2 with its VERY narrow
flight envelope at close to max.- Hide quoted text -

- Show quoted text -

Glider pilots talk alot about polar curves. The peak is the best L/
D point. Lift is proportional to mass flow over the wings so thin
air (up high) and slow speed reduce lift. Drag decreases with thin
air, partially compensating. I'm not aware that any commerical flight
would be allowed to operate at a level where this makes a
difference. Plus, Class-A air space does not extend above FL600.

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