5. Ground effect

The parasite power, PP, is a power loss as a result of viscous shear effects and flow separation (pressure drag) on the fuselage, rotor hub, and so on. Because helicopter fuselages are much less aerodynamic than their fixed-wing counterparts (for the same weights), this source of drag

Py ¼ T vi � Vy

In addition, when calculating the power required of the helicopter, the required power of the tail rotor must also be calculated. The power required by the tail rotor typically varies between 3 and 5% of the main rotor power in normal flight, and up to 20% of the main rotor power at the extremes of the flight envelope [1]. It is calculated in a similar way to the main rotor power, with the thrust required being set equal to the value necessary to balance the main rotor torque reaction on the fuselage. The use of vertical tail surfaces to produce a side force in forward flight can help to reduce the power fraction required for the tail rotor, albeit at the expense of

Figure 13 shows the net power required for a given helicopter in straight-and-level flight.

The power needed to rotate the main rotor transmits to the main rotor from the engine through the transmission (Figure 13). But the main rotor cannot get all the power, which is developed from the engine, as part of it is spent for other purposes and does not go to the main rotor.

PP ¼ D � V (42)

(43)

can be very significant [1]. The parasite power can be written as

where Vy is the climb (or descend) velocity. In hover regime Vy = 0.

The climb (or descend) power can be written as

36 Flight Physics - Models, Techniques and Technologies

some increase in parasitic and induced drag.

Figure 13. Power required and power available in straight-andlevel flight.

4.2. Power available

When the helicopter flies near the earth's surface, the efficiency of the rotor system increases because of the interference of the airflow with the ground [2–4]. The rotor downwash is unable to escape as readily as it can when flying higher and creates a ground effect. When the rotor downwash reaches the surface, the induced flow downwash stops its vertical velocity, which reduces the induced flow at the rotor disk (Figure 14).

Figure 15. Influence of ground effect on the rotor drag.

Figure 15 shows the effects of this on the power required to hover. If the hover height in ground effect must be maintained, the aircraft can only be kept at this height by reducing the angle of attack (AoA) so that the total reaction produces a rotor lift exactly equal and opposite to weight. It shows that the angle of attack is slightly less, the amount of total rotor thrust is the same as the gross weight, the blade angle is smaller, the power required to overcome the reduced rotor drag (or torque) is less and the collective control lever is lower than when hovering out of ground effect. Therefore, there is better lift/drag ratio.

These conclusions are also true to flight in ground effect other than the hover, but the effect is smaller.
