THE HELICOPTER
In a true helicopter in normal flight, the upward thrust of the revolving blades must be equal to the weight; forward motion is produced by inclining the effective axis of the rotor forward which normally entails tilting the nose of the helicopter down. The actual means by which this is achieved is rather complicated and involves altering the cyclic variations in rotor blade incidence. Owing to the reaction of the torque of the lifting blades the whole aircraft will try to rotate in the opposite direction, this resulting in a tendency to yaw which corresponds to the rolling tendency due to the propeller on a fixed-wing aircraft. The yawing tendency can be counteracted either by an auxiliary propeller or by a jet reaction system at the tail; this can also be used to provide directional control and to save the necessity of having a rudder.
The development of successful helicopters has involved the solution of many other practical problems. The blade going into the wing gets more lift, and drag, than the blade going down the wind. This happens whenever the machine moves in any direction, forwards, backwards or sideways; but it become even more serious problem when moving at high speeds because the tip portions of the blade going into the wind meet compressibility problems before the aircraft itself is moving anywhere near the speed of sound. The helicopter has other failings too, vibration is apt to be excessive, as is the noise when compared with aircraft of similar power.
Attempts to solve these and other problems have resulted in wings that must not only rotate, but also have variable incidence, hinges to give variable dihedral (resulting in a kind of flapping motion of the blades), drag hinges to allow the blades to bend backwards, and tilting axes of rotation. Then there are auxiliary propellers at the tail, or jets, some have had jets to at the wing tips to rotate the blade, some have had more than one set of rotating wings (like contra-rotating propellers), and there have been many other ingenious devices, all helping to some extent to solve the problems, but all contributing to the complication and weight of an already complicated type of aircraft.
One development of considerable interest in the rotating-wing type of aircraft – all of which come under the general description of rotorcraft – was the Fairey Rotodyne, another trade name, although it did not prove a commercial success, this experimental was both gyroplane and helicopter, and to some extent a conventional aero plane too. For take-off and landing it was a helicopter, the wings being rotated by jets at the wing-tips, up to forward speeds. It was a combination of conventional aero plane, twin engine turboprop, and helicopter.
Modern types of helicopter can do extraordinary things; not only can they take off and land vertically, they can hover at will, fly sideways or backwards or forwards, land in confined spaces including the heart of cities, drop and pick up the mail, passengers and even parts of buildings, and they have proved in valuable in rescue operations, both over land and sea – including the retrieving of space – craft and their crews. All these things they can do better than conventional aircraft – but still they cannot fly at high speeds.
There has not yet been a real break- through as regards maximum speed, but its weight lifting capacity, one of its great assets, has increased even more rapidly than that of its fixed-wing counterpart. Moreover at the present time more attention being given to the solution of the other problems, including that of maximum speed.
Recent developments have included the use of so-called ‘rigid’ rotors which means dispensing with both the drag and flapping hinges, but making the blade themselves flexible enough to bend upwards and backwards under the lift and drag forces – not exactly rigidity! This has reduced complication to some extent, and has resulted in lighter rotor heads, of titanium, and lighter blades, of reinforced plastics. Another development has been the shrouded tail rotor, enclosed in the fin, reducing danger and giving less drag, the fin itself also having an unsymmetrical camber as to counteract some of the torque in the forward flight.
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