QUESTION:
Why do planes fly at
very high altitudes between U.S. and Europe when there are no mountains in the
way?
ANSWER:
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) nand the true air speed as (IAS). 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.
While
reduced drag is the primary reason for high- altitude flight, there are a other
factors which benefit from flying at such levels.
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