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Aviation History
1953
1953 - 1411.PDF
FLIGHT, 23 October 1953 565 AUTOMATIC FLAP-CONTROL Details of a Suggested Electronic Servo-system EVOLUTION has resulted in the installation of an incredible number of "essentials" contributing towards making aircraft proof against accidents. Since, how ever, many of them are merely tools at the disposal of the pilot, rather than automatically operated safeguards, they actually expand the field of human fallibility and provide further sources of pilot error. In the author's view, a higher degree of automaticity is required to assist the pilot in his ever-increasing task. Such automaticity would have its greatest value during the critical landing and take-off phases. Trailing-edge flaps or other high-lift devices are at least partially extended before the aircraft begins to lose height on the final approach, and will be fully extended by the end of the approach. During this stage the pilot must be able to vary his rate of descent to correct for errors in judgment and the effect of wind; and the ease with which such correction can be made will depend to some extent on the maxi mum gliding angle at the approach speed. This in turn depends chiefly on the drag of the aircraft, and one of the uses of the flap is to provide extra drag for this purpose. Furthermore, to enable the pilot to make accurate approaches it is necessary to provide him with a good field of view forward and downwards. The view can always be improved by reducing the incidence used on the approach, and the trailing-edge flap forms a convenient method of achieving this end. A means of rapidly changing the lift coefficient without altering incidence can provide the pilot with a useful control for making rapid adjustments to his approach. Thus, the changes in lift coefficient may be obtained by rapidly altering the deflection of the trailing-edge flap. Trailing-edge Flap.—The use of trailing-edge flap to increase the maximum lift coefficient of the basic wing is now almost universal. The split flap is probably the simplest type, and it increases the lift coefficient by effectively increasing the wing camber without affecting the angle of attack. Maximum lift increase is usually achieved with the flap deflection of 70 deg or 80 deg. For other types of flap, the increase occurs somewhat earlier. Trailing-edge flap reduces the landing distance by reducing the speed at touch-down and thus reducing the energy which must be dissipated by the brakes and tyres. We may conclude, then, that use of such flaps has a very marked effect on me approach and flying characteristics of an aircraft. Change in Trim.—The pitching moment due to the use of flap is always a nose-down moment and is directiy proportionate to the increment in lift coefficient. These moments have a large influence on the change in trim. This change results from two opposing effects: (a) the change in pitching moments on the wing as the flaps are deflected, which gives a nose-down moment; (b) die increase in downwash over the tailplane, which gives a nose-up moment. To counteract these effects it is necessary to have a trimmer device positioned on the elevator which changes the elevator angle to trim out the pitching moments on the wing. This in turn is dependent on die tailplane volume. The bigger die tail the smaller is the elevator angle required to give a particular pitching moment. This immediately introduces complication in design, by reason of the fact that tail volume is determined by stability requirements. Ground Effect.—In aircraft with moderate tail-volume, trouble has sometimes developed during take-off and landing, by reason of insufficient elevator power. This is caused by the "ground effect," which alters the trim of die aircraft in the following way: (a) by altering the lift in the tailplane at a given incidence; (b) by altering the downwash over the tailplane. The change in the lift may be quite considerable in certain types of aircraft with low-set tailplanes. Automatic Flap.—Reflection for a moment on all these variable factors, which can so gready influence the landing and take-off manoeuvres of an aircraft, suggests that we must concentrate on a device which gives us the maximum amount of efficiency at these critical periods. The distance required by an aircraft to accelerate from rest to climbing speed is, we are told, proportional to the square of the climbing speed, or inversely proportional to the maximum lift coefficient. The drag during this stage is not important. The distance covered during the climb to a height of 50 ft is chiefly dependent on the difference between the thrust and the drag. If die maximum lift coefficient of an aircraft in take-off manoeuvres is steadily increased the drag coefficient will THIS article is a condensed version of a paper by Mr. W. J. Gatehouse, who feels that there would be considerable advantage in relieving pilots of the necessity for flap-operation during landing and take-off; he proposes, and describes, an electronic system in which application of flap would be automatically dictated by air-speed. He has clearly made a painstaking study of the problems involved and there appears to be no reason why his system should not work. We think, however, he will find quite a number of pilots to be chary of having such a vital operation taken out of their hands. A suggested scheme of installation (not to scale) for the principal com ponents of the automatic-flap system. also be increased, partly because of the profile-drag of the flap but mainly because of an increase in induced drag. To keep mis drag figure to a minimum we want a device which automatically gives progressive efficiency of the flap surface during the varying loads and accelerated speed of die aircraft—in other words, a proportional reduction of die drag figure with increasing speed; such an operation is beyond the capability of the human being and may be said to extend into the realms of the "mechanical brain." Similar considerations apply in the landing operation. The flap is partially extended before the aircraft begins to lose height and fully extended by the end of the approach, and during this stage the pilot must be able to vary his rate of descent to correct for error in judgment and the effect of wind gradients. The main purpose of the flap in this manoeuvre is to provide extra drag necessary for corrective action by the pilot. The automatic flap can increase efficiency in this respect, because the incidence of the approach will be determined by the stalling incidence and the margin in speed above stalling speed required for safety. An automatic flap, functioning and biased to operate at around a specified landing speed, could increase or decrease drag or lift, so giving constant approach conditions. Automatic flap could com pensate for change in aircraft trim, although the use of airbrakes or spoilers may have to be considered to increase drag under such conditions. Electronic Automatic Control of Flap Mechanism.—Fully automatic control of flap is the next consideration. Flap angle can be expressed as proportional to the air-speed of the aircraft— less spaed, more flap, and vice versa. By making use of this proportionality an electronic servo system functioning as a flexible yet positive coupling between an air-speed transmitter motor and a reversible electric motor could be used. In the arrangement envisaged (see diagram overleaf), a transmitter motor (1) (which could be positioned in the nose of the aircraft) operates, with the aid of a pitot head, proportionately to the air-speed of the aircraft. The pitot head communicates with a sylphon capsule which is coupled mechanically to the rotor of the transmitter motor. The rotor is energized by an external source of alternating current, and this current creates a magnetic field around the rotor, which in turn induces a current in the three stator windings of the transmitter motor. The current thus induced in each of the stator windings will be proportional to, and be a representation of, the transmitter motor's rotor-angle. The stator windings of both motors are connected in parallel and thus the same current flows in the stator windings of, both, thereby producing a similar magnetic field in the receiver motor (2). Now, whenever the rotors of the two motors are not in align ment widi each other, a voltage will be induced in the rotor
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