Fuselage download

The slipstream from the rotor exerts a downward force on the helicopter fuselage, which is in addition to any vertical drag associated with axial flight. This means that in the hover the main rotor must generate sufficient thrust to support not only the weight of the helicopter but balance this download. The rotor wake contracts from the diameter of the rotor to its ultimate wake size in about a quarter of rotor radius

Подпись: POWER COEFFICIENT (CP= P/pAVT )
Подпись: Performance Theory 17

THRUST COEFFICIENT FUNCTION (CT15 = (T/pAVT2)15)

 

Fig. 2.4 Typical hover performance results.

 

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[2.6]. Even so it is often assumed that the fuselage is immersed in the ultimate wake and receives the full effect of the downwash. Simple estimates of download use a projected area and assume a drag coefficient of 0.3 [2.6]. It is often convenient to express the vertical drag as a percentage of gross weight (GW) and typical values range between 1% and 4% of GW.

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