Maximum Level Speed
The maximum speed attainable in level flight is likely to be limited by the envelope of retreating-blade stall and advancing-blade drag rise (Section 7.7). If and when power limited, it is defined by the intersection of the curves of shaft power required and shaft power available, (C) in Figure 7.5. In the diagram the power available has been assumed to be greater than that required for hover (out of ground effect) and, typically, to be nearly constant with speed, gaining a little at high speed from the effect of ram pressure in the engine intakes.
Approaching maximum speed, the power requirement curve is rising rapidly owing to the V3 variation of parasite power. For a rough approximation one might suppose the sum of the other components, induced drag, profile drag and miscellaneous additional drag, to be constant and equal to, say, half the total. Then at maximum speed, writing PPARA for the parasite power, we have:
Pav = 2Ppara = pVMax/ (7.14)
For a helicopter having 1000 kW available power, with a flat plate drag area of 1 m2, the top speed at sea-level density would by this formula be 93.4 m/s (181 knots).
Increasing density altitude reduces the power available and may either increase or decrease the power required. Generally the reduction of available power dominates and VMAX decreases. Increasing weight increases the power required (through the induced power Pj) without changing the power available, so again VMAX is reduced.