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Name: Elliot
Status: Other
Grade: Other
Location: N/A
Country: United States
Date: May 2007


Question:
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. Your page:

http://www.newton.dep.anl.gov/askasci/gen01/gen01200.htm

just gives the maximum height, but not why.



Replies:
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 helicopter.

The earth's pull on the helicopter declines as well, but not as fast as the density of the atmosphere.

Burr Zimmerman


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.

Larry Krengel


Hi Elliot,

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.

Regards,

Bob Wilson.



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