How can the maximal flight speed of the helicopter be increased?
In order to avoid blade tip stall at the 270° azimuth with increase of the forward speed, it is necessary to reduce the main rotor pitch. But this leads to reduction of the thrust and creation of a deficiency of the lift force (-AY) and of the propulsive force (-ДР) (Figure 65). In order to avoid these phenomena, use is made of an auxiliary wing, which creates the additional lift force ДУ.
The additional propulsive force AP necessary to increase the speed is /98 created either by increasing the engine thrust or by the thrust of additional propellers. Helicopters with an additional wing and additional propellers are called compound helicopters. In these helicopters there is an unloading of the main rotors such that they can operate with lower incidence angles, and blade stall occurs at higher speeds. Helicopters of this type include the
Mi-6 and Kamov’s prop-wing design with side-by-side rotors. The speed of the compound helicopters reaches 400-450 km/hr. However, they also have drawbacks : increased structural weight
and increased parasite drag as a result of the slipstream from the main rotor flowing over the wing.
This situation is particularly characteristic for the vertical flight regimes with low horizontal velocities.