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Re: [Phys-L] Aeroplanes and air temperature



Are you asking what the top speed is of a plane on an infinitely long flat
pavement? Surely reducing air density and hence drag (with all flaps
stowed) will help, right?

It seems to me that reducing the air density makes it easier to reach a
desired speed, but the reduced lift means you need a higher speed. From
that point of view, it seems like a tradeoff to me.

On Wed, Jun 21, 2017 at 2:22 PM, Bill Norwood via Phys-l <
phys-l@mail.phys-l.org> wrote:

Carl,
- At what temperature, safety margin aside, do you think that none of the
planes could fly regardless of runway length?
Bill

On Wed, Jun 21, 2017 at 2:16 PM, Carl Mungan <mungan@usna.edu> wrote:

I'm not so sure. The density is presumably proportional to pressure
divided
by absolute temperature.

Absolute temperature at 119 F divided by that at 72 F is a bit less than
109%. A 9% effect isn't negligible but isn't going to require a runway
twice as long or something. Speed squared down the runway is proportional
to distance along the runway approximately.

The other thing I'm wondering about is the air pressure. We're changing
both the number of molecules per square area (perhaps not important since
the change doesn't reach all the way up to the top of the atmosphere but
only near the surface) and the speed of the molecules (and hence their
impact speed and frequency). I suppose to zeroth order P is independent
of
T for an open sample of gas but it isn't obvious to me excluding
large-scale convective effects.

On Wed, Jun 21, 2017 at 1:55 PM, Bill Norwood via Phys-l <
phys-l@mail.phys-l.org> wrote:

Antti,
- I find the explanation satisfactory.
- Thanks for sharing and asking.
Bill Norwood, U of MD at College Park

On Wed, Jun 21, 2017 at 12:50 PM, antti.j.savinainen via Phys-l <
phys-l@mail.phys-l.org> wrote:

Hi,
there has been recent news on aeroplanes which cannot fly because the
air
temperature is too high. Here is one explanation why this is so:
https://www.wired.com/story/phoenix-flights-canceled-heat/
How do you find the explanation?
Regards,
Antti Savinainen, Finland
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--
Carl E. Mungan, Professor of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/
_______________________________________________
Forum for Physics Educators
Phys-l@mail.phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@mail.phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l




--
Carl E. Mungan, Professor of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/