Factors Limiting Maximal Horizontal Flight Speed. and Ways to Increase This Speed

It was established above that to increase the flight speed we must increase the angle у of deflection of the thrust force vector (see Figure 60) and the magnitude of the main rotor thrust force. The thrust force can be increased in two ways: by increasing the angular velocity CO; and by increasing the

main rotor pitch, since this leads to increase of the thrust coefficient c^

(see Figure 16).

Increase of the main rotor thrust force by increasing the rpm involves increase of the blade drag profile, and consequently increase of the fuel consumption per unit rotor thrust developed. This approach is not advisable. Moreover, increase of the thrust by increasing the rotor rpm is possible only up to a definite limit.

It is well-known that increase of the angular velocity increases the circumferential velocity u = U)R. In the forward flight regime the resultant

blade element velocity is W = u + V sin ip, i. e., at the azimuth 90° W = u + V.

With increase of the rpm there is an increase of the circumferential velocity and the flight velocity; consequently, the resulting velocity W will increase. Increase cf the resultant velocity of the flow past the blade elements is permissible only until the velocity reaches the critical value, i. e., until the appearance on the blade of a local flow velocity equal to the speed of sound (compression shocks develop, and blade shock stall manifests itself).

Thus, the first technique for increasing main rotor thrust by increasing the rpm is limited by the appearance of shock stall at the tip of the blade when the blade is located at the 90° azimuth.

Let us examine the second approach. Increase of main rotor thrust by increasing the pitch and the coefficient c^ involves increase of the blade element angles of attack. It is well-known that in the forward flight regime the blade element angles of attack vary in azimuth: the angles of attack are

smallest at the 90° azimuth, and they are largest at the 270° azimuth. The higher the flight speed, the larger the angles of attack of the blade tip elements at the 270° azimuth. Increase of the pitch leads to further increase /97 of the angles of attack. If the angles of attack at the tip elements approach the critical value, flow separation analogous to the separation from an airplane wing is formed at the end of the blade (Figure 64).

Moreover, with increase of the flight velocity the reverse flow zone expands; in accordance with the formula d = yR, the diameter of this zone increases with increase of the flight speed at constant rpm. Expansion of the stall separation and reverse flow zones leads to reduction of the main rotor thrust force and causes severe roughness. Flight cannot be continued under separated flow conditions, therefore, the flight speed can be increased only until the angles of attack of the blade tip elements become close to the critical value. This flight speed limitation is called the 270°-azimuth blade stall limitation. Thus, the maximal speed for the Mi-1 helicopter is limited to 170 km/hr, while the Mi-4 is limited to 175 km/hr.

If these limitations did not exist, the power available would permit a maximal speed at altitude of 205 km/hr for the Mi-1 and a speed of 225 km/hr at an altitude of 1500 m for the Mi-4.