MODERN AVIATION THE PRINCIPLES OF HELICOPTER FLIGHT

Press, Volume XCI, Issue 27669, 27 May 1955, Page 12

MODERN AVIATION THE PRINCIPLES OF HELICOPTER FLIGHT

The principles of flight by which a fixed-wing aeroplane M* s remains there, and is controlled there, are known to most pedons. But since the helicopter from H.M-A.S. Sydney has made frequent appearances over Christchurch, the curiosity of many persons has been aroused about toe principles of helicopter ,? a ," ticularly, do many findl it difficult to appreciate how the whirling rotor of the helicopter provides both lift and forward flight, and many wonder what function is performed by the smaller rotor at the tail of the m A i good description of helicopters and the principles of helicopter flight » included in D. M. Desoutters All About Aircraft” (published by Fabei and Faber) from which many of the facts in this article are taken. There are many different types of helicopter, but in this article reference will be chiefly to the "classical type' perfected by Igor Sikorsky. The Bristol Sycamore carried in H.M.A.b. Sydney is of this type. This has a three-bladed main rotor, driven by an air-cooled engine mounted m the fuselage below the rotor-head. The rotor revolves around an approximately vertical axis. But one effect of this drive is a reaction on the part of the engine, which attempts to turn in the opposite sense to the rotor. This is known as rotor torque reaction, and has no noticeable effect on the ground, where the machine is prevented from rotating by the friction of the tyres; but once the machine is in the air the tendency is to set the fuselage spinning round. To prevent this happening, an antitorque rotor is mounted in the tail. This takes the form of an ordinary variable-pitch airscrew, driven by an extension shaft from the main engine. This airscrew provides the means of control in yaw; that is, it corresponds to the rudder of an ordinary aeroplane. If the machine is to be kept on a constant heading, then the airscrew pitch is set so that the thrust it develops at the tail will exactly balance the torque reactions from the rotor drive. If the tail rotor pitch is lessened, its thrust will no longer balance the torque; this allows the tail to swing over to the right (reacting to the swing of the main rotor) and makes the whole machine turn to the left. The reverse effect is achieved by increasing the pitch of the tail rotor so that its thrust overcomes the torque reaction Control of Tail Rotor The pilot’s control over the pitch of the tail rotor takes the form of a pair of “rudder pedals,” more properly called “tail rotor pitch control pedals." A point to be noticed about this yawing control is that it can be used whenever the helicopter is in flight: unlike the rudder of an aeroplane it does not need any forward motion to make it effective. Those who have seen the Sydney’s helicopter manoeuvring will no doubt have seen how the tail rotor control can be used to spin the helicopter first this way and then that, beneath its rotor in hovering flight. The lift of the main rotor is controlled by varying the pitch of the blades. Each rotor blade acts as a wing and is subject to the aerodynamic laws governing lift. Thus, it would be possible to vary the lift by varying the speed of rotation, but this would be a slow process, with an inconvenient time lag while the rotor was gaining or losing speed. In practice, it is found that the best method is to keep the speed more or less constant and .to yary the lift coefficient by adjusting the pitch, or angle of attack, of the blades.

This is done by the “collective pitch control lever.” To take off, the pilot first has to get his rotor running at the correct number of revolutions a minute, perhaps about 250. He then raises the collective pitch lever so as to increase the pitch of all three blades simultaneously. When the lift developed by the rotating wings is greater than the weight of the whole aircraft, it will begin to rise. From a position of hovering flight, where the lift exactly equals the weight, the pilot can raise or lower his machine by increasing or decreasing the pitch of the blades, and at the same time he can turn the fuselage right or left with the aid of the tail rotor pitch control pedals. A helicopter of the type we are

considering has no airscrew to draw it along in forward flight. This is a task the main rotor has to undertake, in addition to providing the necessary lift. To get a forward component of thrust in this way means that the main rotor has to be tilted forward; this accounts for the typical nose down attitude of helicopters of this type in forward flight. The total lift is now at an angle to the vertical and must be sufficient to provide the two components of vertical lift and forward thrust. The control used by the pilot to tilt the rotor so that its thrust line will be out of the vertical is the “cyclic pitch control.” The cyclic pitch control column occupies the same position in the pilot s cabin as the control column of an ordinary aeroplane, and it moves in a similar manner. Unlike the collective pitch control, it does not alter the pitch of all blades simultaneously. It changes the pitch of each blade as it passes through the rearmost part of the cycle. Equally, it decreases the pitch of each blade as it passes through the forward part of the cycle. Thus, if one thinks of the complete revolving rotor system as a single disc, the rear part of the disc rises as the front part drops. Pilot’s Three Controls

In a similar way the cyclic pitch control can be used to direct the lift of the main rotor to the rear of vertical, or to either side. Thus, with three controls, tail rotor, collective pitch and cycle pitch, the pilot can cause hi B machine to spin round on its own axis, to move up or down, and to move backwards, forwards, or sideways. Helicopter blades have four movements in flight—they rotate around the central axis, flap up and down, oscillate from side to side through about 10 degrees, and alter pitch. The vertical oscillation, or “flapping” counteracts the aircraft’s tendency to roll over (blades advancing have greater lift than those retreating from the aircraft’s direction of movement and at speed the aircraft, without the flapping devices, would roll over). Horizontal oscillation is given to the blades to compensate for the vast difference in drag between the advancing blade which has to force its way through the airstream and the retreating blade which has the force of the air behind it. There are, of course, many special problems with rotating-wing machines that are not met with in fixed-wing aeroplanes. For instance, the maximum speed possible for rotating-wing machines is limited by the speed at which it is possible to turn the main r . Some figures taken from the cr I the Bristol Sycamore serve to trate the sort of problems that have to be allowed for in the development of rotating-wing machines. The Sycamore has a rotor diameter of 48.5 feet, which gives a circumference of about 152 feet. At its maximum forward speed of 130 miles an hour, the Sycamore’s rotor turns at 287 revolutions a minute, so that the tip speed is 287 x 152 feet a minute, or 43,500 feet a minute. This is 725 feet a second or about 500 miles an hour. Thus, the advancing blade of the Sycamore, in these conditions, has a total air speed of 630 miles an hour. Less than oneeighth of a second later this same blade is in the retreating position, with an air speed of 370 miles an hour. The difference between the two speeds is, of course, twice the forward speed or the whole machine—26o miles an hour.

The helicopter has come a long way since it was invented some 450 years ago by Leonardo da Vinci. No-one can doubt that it will go very much further. Experiments with helicopters at present include work with many variations of rotor arrangement: the fitting of big turbo-jet engines, the use of fixed wings, which do nothing in vertical or hovering flight, hut which in forward flight can provide , up to one-third cf the total lift. This latter type of machine falls in the c-ass called “compound helicopters.” A proposed compound helicopter is under development by Fairey Aviation for replacement of the Douglas DC-3, for long so familiar in the skies over Christchurch. Whether this particular development Is successful or not, noone can dpubt that the rotating?wing on the Sycamore from H.M.A.S. Sydney is merely a pioneer of many that Christchurch people of this and future generations will see.