at a near constant RPM and thus a constant 
true airspeed and cannot operate at this most 
eficient condition over a wide range of altitude 
and gross weight as the fixed-wing airplane. 
The airplane is able to maintain an efficient 
angle of attack at various altitudes, and gross 
weights by flying at various airspeeds but the 
helicopter will operate with a near constant 
rotor velocity and vary blade angle to contend 
with variations in altitude and gross weight.