§ 44. General Characteristics of Horizontal Flight /88

Rectilinear flight of a helicopter with constant velocity in the horizontal plane is termed horizontal flight. This is the primary flight regime for the helicopter. Since the Earth is a sphere, flight at constant altitude takes place along a curvilinear trajectory. But the radius of curvature of the Earth’s surface is so large that the curvature of the Earth’s surface can be neglected in flight.

Only in flights on supersonic airplanes at a speed 2-3 times that of sound is it necessary to consider the Earth’s curvature. We shall use an example to demonstrate this. An airplane is flying horizontally at a speed of 1000 m/sec or 3600 km/hr. The airplane weighs 10,000 kgf. Let us find the centrifugal force which arises as a result of curvature of the Earth’s surface


г – mV Fc " R

where m is the airplane mass, kg;

V is the airplane speed, m/sec;

R is the radius of the Earth, equal to 6 370 000 m.





= 1570 N,


or Fc = 161 kgf.


We see from the example that the Earth’s curvature should be considered when flying at 1000 m/sec, since the airplane lift becomes 161 kgf less than its weight. But for the same flight weight and a velocity of 180 km/hr the centrifugal force is 0.4 kgf.

Therefore, in the following we shall consider the Earth’s surface to be



The following forces act on the helicopter in horizontal flight: weight /89

The horizontal flight conditions are expressed by the equalities

Y = G or G – Y = 0;

P =~X or P-X =0; par par

T = S or T – S = 0; t. r s t. r s

ГМ = 0. /L eg

The first condition ensures constant flight altitude, the second provides constant velocity, and the*third specifies linearity of flight in the horizontal plane.

The forces Y, P, Sg are the components of the main rotor thrust. Accordingly, the main rotor thrust in horizontal helicopter flight performs the functions of propulsive, side, and lifting forces.

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