Limits of Helicopter Altitude
Country: United States
Date: May 2007
Assuming aspiration was not a factor, why can a
helicopter not fly higher and higher until it reaches space? I
understand the concept of 11 km/s escape velocity for an object
without propulsion as it needs to have the same kinetic energy as
the potential energy it is counteracting by moving away from the
source of gravitational pull, but i do not see why an object with
propulsion cannot be 'slowly' flown into space.
just gives the maximum height, but not why.
In the case of a helicopter, the down-force the rotors develop depends on the
density of air. The higher the altitude, the less dense the air, and the
less force the rotors can provide. Therefore, at a certain altitude, the
rotors will no longer provide enough force to counteract the weight of the
The earth's pull on the helicopter declines as well, but not as fast as the
density of the atmosphere.
Lift is produced by reaction with the air. The more dense the air, the
greater the reaction. As the air becomes less dense, the amount of
reaction (lift, that is) becomes less. At some point in a climb up
into the atmosphere the lift produced by the thinning air would be only
enough to maintain the altitude... not enough to climb. This is termed
the absolute ceiling for an aircraft.
A helicopter's altitude limit is the result of the atmosphere
becoming thinner as altitude increases. A helicopter gets it
lift by rotating its blades, which are much like wings that
travel though the air and generate lift. Just as an airplane
cannot climb into space because it reaches an altitude where
its wings cannot generate sufficient lift as the air gets
thinner at high altitude, a helicopter, with its "rotary
wings" (its rotating blades) also reaches a point where the
its blades cannot generate enough lift in the thin air to
take it any higher. Clearly a helicopter's blades could not
generate any lift at all in the vacuum of space since there
is no air at all, but even at a much lower altitude, the air
becomes thin enough that the amount of lift the blades
generate is reduced so much that this places a limit on the
helicopter's ability to climb any higher.
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Update: June 2012