4. Power

#### 4.1. Power required

The power required for flight is the second work that must be transmitted to the shaft of the rotor.

In general, for a helicopter in forward flight, the total power required at the rotor, P, can be expressed by the equation

$$P = P\_i + P\_o + P\_P + P\_y \tag{39}$$

where Pi is the induced power, P<sup>0</sup> is the profile power required to overcome viscous losses at the rotor, PP is the parasitic power required to overcome the drag of the helicopter, and Py is the climb (or descend) power required to increase (decrease) the gravitational potential of the helicopter [1].

Inductive power is consumed to produce lift equal to the weight of the helicopter. From the simple 1-D momentum theory the induced power of the rotor, Pi, can be approximated as

$$P\_i = k \cdot T \cdot \upsilon\_i \tag{40}$$

where k is the familiar empirical correction to account for a multitude of aerodynamic phenomena, mainly those resulting from tip losses and nonuniform inflow, and vi is induced velocity [1].

The profile power required to overcome the profile drag of the blades of the blades of the rotor is

$$P\_0 = Q \cdot \mathcal{Q} \tag{41}$$

where Q is the rotor torque, and Ω is the rotational frequency of the rotor.

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

$$P\_P = D \cdot V \tag{42}$$

The climb (or descend) power can be written as

$$P\_y = T\left(v\_i \pm V\_y\right) \tag{43}$$

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

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 some increase in parasitic and induced drag.

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

#### 4.2. Power available

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.

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

For rotating of the tail rotor about 8% is lost from the consumed power of the engine, for fan rotation about 5%, for friction about 7% in transmission, for auxiliary drive units about 1%, and for blowing parts of the helicopter about 2%.

This part of the power of the motor that is transmitted to the main rotor is called available power. It is defined as the difference between effective power and total loss.

Excess power—this is the difference between the available and the power required. The greater the excess power is, the greater the speed range is and the better the helicopter's maneuvering characteristics are (Figure 13).
