Why do airplanes fly so high?
(Q&A by Eric Soder)
Drag on an aircraft is greatly reduced at altitude, so it can fly faster. In
essence, that is because there are fewer air molecules to get in the way. Engineers
express the way altitude affects speed by calculating two speeds for an aircraft:
the indicated air speed (IAS) and the true air speed (TAS). IAS is measured
by air molecules entering a forward-facing, open-ended intake--the air pressure
this creates moves the speedometer needle.
At sea level, IAS equals TAS. If you fly higher, the air is less dense. So,
to get the same number of molecules down the intake, you fly faster. Now TAS
is no longer the same as IAS, and at cruising level TAS is about twice IAS.
Lift and drag both depend on IAS, not TAS. The higher you go with the same speed
on your IAS speedometer, the faster you will actually go over the ground.
Engines for commercial airliners are also designed to operate better at altitude.
Optimum fuel performance is achieved at about 80 to 90 per cent of the engine's
maximum revolutions per minute (RPM). Air density decreases with altitude and,
at fixed RPM, so does thrust. Only at high altitude will the drag be low enough
to allow maximum engine efficiency. To push the aircraft along at the same speed
at a lower altitude, where the air is denser, the engines would have to operate
at a lower percentage of maximum RPM.
Source: http://www.newscientist.com