Page 207 - Aeronautical Engineer Data Book

166 Aeronautical Engineer’s Data Book
Axis of
rotation
Lift Resultant
Angle of
attack
Blade
chord line
Tip-path plane
Relative Angle of
wind
pitch
Drag

Tip-path plane
Relative wind

Fuselage nose down
Axis of rotation
Fig. 10.6 Helicopter principles: lift and propulsion

rotor hub into the main drive shaft, so the
helicopter effectively hangs on this shaft.

10.3.3 Forward speed
The performance of standard helicopters is
constrained by fixed design features of the rotat­
ing rotor blades. In forward flight, the ‘retreat­
ing’ blade suffers reversed flow, causing it to lose
lift and stall when the forward speed of the
helicopter reaches a certain value. In addition
the tip speed of the advancing blades suffers
shock-stalls as the blades approach sonic veloc­
ity, again causing lift problems. This effectively
limits the practical forward speed of helicopters
to a maximum of about 310 km/h (192 mph).

10.3.4 Fuel consumption
Helicopters require a higher installed power
per unit of weight than fixed wing aircraft. A
large proportion of the power is needed simply