Effect of flight altitude and air temperature on helicopter flight characteristics
Answer 1. Increase of the flight altitude involves reduction of the air density. This leads to reduction of the thrust developed by the main rotor. Since the horizontal flight condition is the equality G = Y, with increase of the altitude the induced velocity must be increased; therefore, the induced power NL = GV^ will increase. The power required curve is shifted upward for low flight speeds.
At high flight speeds, the induced power increases only slightly, while the power required for motion decreases with increase of the altitude ; therefore, at high speeds the power required curve is shifted downard. As a result of this change of the power required and available, the minimal and maximal horizontal flight speeds increase up to the engine critical altitude. Above the critical altitude the maximal flight speed will decrease. Increase of the air temperature is equivalent to increase of the altitude.
Answer 2. With increase of the altitude the power required for helicopter horizontal flight increases. The power developed by the engine decreases. The power required curves are shifted upward and the power available curves are shifted downward. As a result of this displacement of the curves, the maximal speed decreases and the minimal increases; the speed range and the excess power decrease. Increase of the air temperature leads to reduction of the power required and available.