ACADEMIC AMERICAN ENCYCLOPEDIA

HELICOPTER

{hel'-ih-kahp'-tur}
 
The helicopter is a type of aircraft that obtains lift and
propulsion from one or more powered rotors, which are
PROPELLERS that rotate in a horizontal plane.  The helicopter
is capable of remaining motionless in hovering flight because
the powered rotors produce lift even at zero forward speed.
Horizontal flight is achieved by tilting the rotor forward with 
respect to the flight path to produce a propulsive force in
that direction.
 
HISTORY
 
Leonardo da Vinci is generally credited with sketching and
describing a helicopter in 1483.  But the helicopter, just as
the airplane, required a lightweight power source, which the
gasoline engine supplied.  The helicopter was perfected between 
1936 and 1941.  Although modest flights had been made prior to
this time, it was not until 1935 that a coaxial helicopter
constructed by Louis Breguet and Rene Dorand in France achieved 
flights of sustained duration.  With the perfection of the
single-rotor type by Igor SIKORSKY in the United States
(1939-41), the helicopter became a practical aircraft capable
of carrying a useful load and performing diverse commercial and 
military functions.

LIFT AND PROPULSION
 
The engine transmits its power to the rotor system to drive the 
lifting rotors.  This results in a torque reaction on the
fuselage of the helicopter, which will tend to cause the
fuselage to rotate in the opposite direction unless a
compensating device is provided.  The single-rotor type
achieves torque compensation by a small rotor (tail rotor)
operating in the vertical plane;  the tail rotor not only
prevents fuselage rotation but also provides rudder control.
Alternatively, the helicopter may use a configuration of two
equal-size lifting rotors, rotating in opposite directions.
The axes of rotation of these rotors may be coincident, with
one rotor on top of the other (coaxial configuration), nearly
coincident (synchropter), spaced fore and aft (tandem
configuration), or side by side.  The rotor blades of most
helicopters are hinged such that they move up and down freely
during translational (forward) flight to provide lift
compensation--just as in the AUTOGIRO.  The pilot's control
stick is connected to the rotor blades such that motion of the
control stick tilts the lifting rotor, providing translation in 
the direction of tilt.  The large-diameter lifting rotor
enables the craft to hover efficiently.  The maximum speed of a 
conventional helicopter is limited to about 400 km/h (250 mph)
because the lift depends on the relative velocity of the air
past the rotor blades, which is a combination of rotor
rotational velocity and the translational motion of the
helicopter.  Stalling of the blades occurs on the side of the
disk where the blades are moving backward (the retreating side) 
at high flight speeds due to the low relative velocity.
Typical helicopter cruise speeds range from 128 to 240 km/h (80 
to 150 mph), with the latter figure characteristic of recent
types.  Helicopters are made in various sizes to carry various
payloads.  In the event of a power failure, the helicopter
descent can be slowed and the landing cushioned if the pilot
windmills (autorotates) the rotor as in the autogiro.
 
APPLICATIONS
 
The utility of the helicopter derives from its unique ability
to execute vertical takeoffs and landings (see VTOL), to hover
in the air, and to fly slowly in all directions.  The
capability of VTOL and the related short takeoff and landing
(see STOL) have been incorporated into various types of
convertible aircraft, some of which use the helicopter
principle of flight.
 
Application of the helicopter in both military and commercial
spheres has been widespread.  In the United States there are
about 13,000 helicopters in use in the military, primarily
employed by the U.S.  Army, and about 8,000 are in commercial
use.
 
Commercial applications include traffic reporting,
transportation, fire fighting, logging, crop dusting, and (in
the construction industry) flying cranes.  One of the major
commercial uses of helicopters in recent years has been for
transportation of personnel, supplies, and equipment to and
from offshore oil platforms.  Helicopter transportation is used 
for short-range passenger transport, such as between nearby
cities or between the AIRPORT and central area of a city.
 
H. C. Curtiss, Jr.
Bibliography:  Carey, Keith, The Helicopter (1987);  Fay, John, 
The Helicopter (1987);  Gablehouse, Charles, Helicopters and
Autogiros, rev.  ed.  (1969);  Gessow, Alfred, and Myers,
Garry, The Aerodynamics of the Helicopter (1967);  Gregory, H.
F., The Helicopter (1976);  Keating, Bern, Chopper:  The
Illustrated Story of Helicopters in Action (1976);  Lambermont, 
Paul, and Pirie, Anthony, Helicopters and Autogiros of the
World (1970);  Taylor, Michael J., and John W., Helicopters of
the World (1978).

Copyright (c) 1992 Grolier Electronic Publishing, Inc.  All rights reserved.