MadSci Network: Physics
Query:

Re: Why do airplanes fly so high?

Date: Thu Apr 21 09:07:51 2005
Posted By: Madhu Siddalingaiah, Physicist, author, consultant
Area of science: Physics
ID: 1112231595.Ph
Message:

Hi Tom,

Good question.

Your assumption is essentially correct.
Air density decreases at higher altitudes.
This allows planes to fly faster while using less fuel.

You are correct that there is less air, so there are limits to how high a
plane can fly with a given wing area. In general, high altitude flight
requires large wing area or high speed, both of which deliver greater lift.
Very light, slow flying experimental aircraft such as the NASA pathfinder
can fly as high as 71,000 feet at only 25 miles per hour! You can read more
about it here:
 http://www.nasa.gov/centers/dryden/news/FactSheets/FS-034-DFRC.html

On the opposite end, the SR-71 Blackbird has relatively small wings, but
flies very fast at altitudes above 60,000 feet. The SR-71 replaced the
older U2 which flew much slower, but had long wings like a glider.

Of course, there comes a point where there is so little air that no lift is
generated and no conventional plane can fly. The space shuttle is specially
designed to handle these conditions.

The additional benefit to flying high is the ability to fly above the
weather, which makes the ride more comfortable for passengers.



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