THE HANDLHY PAGE SLOT Its Construction and Principles
EVER since flying first became possible, no factor has engaged the attention of the aircraft designer
and the inventor, more than safety and from earliest days the biggest aim has been the production of a machine with a high degree of control. One has only to run through the records of aviation history to find evidence of this fact and to realize, that until comparatively recent years, the goal was far from being attained. In the early days not a few flying fatalities were due to structural failure and, in a measure, to ignorance of the fundamentals of aerodynamics on the part of both pilot and designer. The war, of course, speeded up the development of such to a phenomenal extent, so that by 1918 accidents due to breakages had been very much reduced and the skill of pilots in general raised to a much higher standard. But accidents still occurred and a big proportion were due to the pilot losing control of his machine through stalling. It was realized years ago, that if this loss of control at the period of stall could be
overcome, aviation could really become safe, but the problem which faced the designers was one of extraordinary difficulty and it was only after a period of prolonged and painstaking ‘research that ‘any hopeful results were obtained.
As an outcome of this work, the Handley Page slot or slotted wing has been evolved and there is no doubt that it is one of the outstanding aeronautical developments of recent years. Like most clever inventions, it is amazingly simple and those to whom its principles are explained inevitably remark on the extr,:me implici, y of the devic.. Before describing the slot and its action, it is necessary to go into the subject of the stall and. the incipient spin which generally follows the stall, how it occurs and why it occurs. When a machine is flying normally the air stream passes over the aerofoil or wing in a steady flow
and induces lift (approximately two-thirds being deived from the upper u ac : and o .ly one-t ird from the und,:r surface). When the speed of the machine is reduced to a certain point, the airflow i no longer steady over the wing and eddies occur over the rear area of the wing and along the trailing edge where the ailerons are situated. Of course, there is then no longer any lift and the wing sinks (in other words, the machine stalls), while any movement of the aileron is useless in recovering control and in attempting to raise the dropped wing, by moving his stick, the pilot actually increases the drag and reduces the lift with the result that the trouble is accentuated and the spin follows. The diagrams in figure 1 will explain more clearly what occurs.
Incidence and Airflow. In figure A the wing is shown in side view, moving from right to left, and in the normal position of flight with the airflow passing over it. In figure B the wing
is moving upward, and in C is moving both upward and forwards. The three figures A, B, C therefore, show that the angle at which the wing attacks the air when the wing is simultaneously moving forward and rising is less than when it is only moving forward. In other words the angle of incidence of an aerofoil moving both upward and forward is less than that of a wing merely moving forward. The lift of a wing increases with the angle of incidence up to a critical point when the lift is at the maximum’ afterwards it fails off and this critical angle is known as the stalling angle of the wing. The graph in figure D gives some indication of how the lift of the wing increases with the angle of incidence up to about 15 degrees and then diminishes. Applying this graph in an example of an aeroplane flying with an angle of incidence of 100 and with a wing lift (a), it
will be seen that an uneven movement of air will cause the wing tip (t2) to rise. The wing tip (ti) begins to fall and thus the aeroplane starts to roll in the direction of the arrows. Therefore the upward and forward movement of the wing (t2) will be of a smaller angle of incidence than before and the lift will be less than (a) in figure D. The downward and forward movement of wing (t1), on the other hand, will have a lift greater than (a), in other words the rising wing loses lift and the falling wing gains it and the machine will quickly return to its normal position. If we now consider the same machine being flown at an angle of incidence of 22° with a wing lift b (in figure D) we shall find that something very different occurs. The decrease in the angle of incidence of the wing tip t2 as it rises results in its lift becoming greater than B while the lift of the wing tip ti as it falls, diminishes. Therefore the machine does not tend to resume its normal position but falls over and spins.
Recovery from the Spin.
Recovery from a spin as we know, can only be secured by increasing the speed of the machine to such a point when the air streams over the wing are steady enough and of sufficient velocity to make the control surfaces again operative and this procedure can only be carried out by a dive and with considerable loss of height. The whole problem therefore centres round the matter of the retention of steady air flow over the surfaces of the wings and it is precisely this which the Handley Page slot secures. The ‘slot can best be described as a miniature aerofoil which is attached to the leading edge of the main wing by hinges so that it is enabled to move upwards and
away from the recess in which it rests under normal conditions. When in the ” out ” position there is a gap between the leading edge and the slot and it will be readily grasped that when the slot is in this position and the wing is stalled a stream of air is induced through the gap over the upper surface of the wing.
This air stream in passing through the gap is compressed to some extent and so in passing rearwards it smooths out the turbulence set up through the stall and the ailerons at once become effective and allow the pilot to keep the machine on an even keel laterally even though it be in stalled flight.
The whole action of the slot is quite automatic betatise as soon as the aircraft stalls the airflow over the ing at this large angle of incidence a: once draws the slot from its recess and its controlling action immediately comes into play. On normal flight being resumed, the slots are forced back into their recess, again through the air pressure. The fact that the slot is entirely self-operating obviously adds greatly to the value of the device for it relieves the pilot of any extra manipulation in an emergency, leaving him free to control the machine in a perfectly normal way.
The Handley Page Slot is being fitted to machines of all types and sizes in increasing numbers and no better proof of its utility in preventing machines from spinning is to be found than that it has not been used on high speed service craft because it prevented the pilots from throwing their machines into intentional spins, an evolution which is part of the R.A.F. flying routine. Latterly, however further developments have been made whereby it can be used on the fighter type, its action being brought into play at the will of the pilot.
G. G. O. M.
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