Quote:
Originally Posted by CobraBall
Aircraft fly high for two reasons. Less air with the same air to fuel ration burns less fuel. Higher altitude (colder) the higher the true airspeed.
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That just means that they have fewer HP available. A throttle does the same thing. Piston engines often have superchargers that would blow the heads off at low altitude, that are used to help burn more gas up higher, for a faster, but less efficient cruise speed. The altitude does help with reducing drag and avoiding weather.
Colder air is denser, which gives both more drag and more lift.