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Aviation History
1913
1913 - 0033.PDF
JANUARY 11, 1913. [/jlGHT THE COLLAPSE OF MONOPLANE WINGS. By GRIFFITH BREWER. ABOUT a year ago, M. Bleriot, with his characteristic courage, reported to the French Government that mono planes were liable to collapse in the air, not by the breaking of the stays under the wings, but by the breaking of the supports over the wings. Until then the upper stays were regarded as simply performing the office of holding up the wings when the machine was not in use, and it was not until M. Bleriot pointed out that these supports could be brought under flying strains, due to pressure being received on the upper surface when a machine is directed downward too suddenly, that makers of monoplanes recognized the necessity of strengthening the upper supports. In spite of this strengthening, however, accidents in which the wings break downwards continue to occur, and it therefore becomes vitally necessary either to abandon the use of monoplanes altogether, or to look more deeply in order to ascertain the cause of this type of structural collapse. The suggestion that I wish to put forward is, that a monoplane may be flying in a straight path, and suddenly, without any change in the angle of flight of the machine, the pressure may be caused to leave the lower surface and come on to the upper surface, and this with a momentum which imparts a sudden strain to the upper wing supports greater than the strain usually carried by the lower stays. The wings of monoplanes project like arms from the sides of the fuselage, and it is therefore obvious that it is easier to twist the outer ends of the wings than it would be to twist the shoulder portions which are attached to the fuselage. In fact, the greater the span, the more readily will the wing tips be twisted when subject to twisting strains. It is more than ten years since the Wright Brothers' experiments at Kitty Hawk, as described by Wilbur Wright in the Automotor Journal at that time, showed that the centre of pressure below a plane moving through the air at small flying angles, travelled backwards as the angle of incidence decreased and as the speed of the plane increased. What, therefore, is the effect of change of speed on the wings of a monoplane ? Without changing the path of flight of the machine, the speed of travel may increase, and this causes the centre of pressure to travel backwards thus tending to turn the wings over forwards, owing to the pressure below the front portion decreasing whilst the pressure increases below the rear portion. The effect of this change in the centre of pressure, is a progressive twist from the shoulders of the wings outwards, the left wing twisting like a right-hand corkscrew and the right wing twisting like a left-hand corkscrew. The amount of twist is of course very small and is probably not perceptible to the eye, but with continual changes in the speed of the machine this twisting effect is continually taking place. This twisting of the wings does not have the effect of changing the pressure from the lower surface to the upper surface until the critical angle is passed, and then the ends of the wings flip over, taking up the slack suddenly. Let us picture an example of what may take place. A monoplane is coming down from a height with the engine cut off. Before reaching the ground, the pilot restarts the engine and thus increases his speed of flight, causing the centre of pressure to travel backwards under the wings and tend to turn the machine downwards. The machine, however, aided by the gyroscopic action of the rotary engine, resists being diverted suddenly down wards, and the wings, therefore, bend or twist forward as the pressure decreases under the leading portion and increases under the rear portion. If the change of speed is such as to merely change the position of the centre of pressure without twisting the wings sufficiently to bring the pressure on the upper portions of the tips of the wings, then the use of the elevator corrects the flying angle. If, however, the wings are twisted to such an angle as to bring pressure on the upper portions of the wing tips sufficient to exceed the support below the wings, a quick downward angular movement of the ends of the wings takes place, and the upper stays then receive the strain of the slack being suddenly taken up, in addition to the pressure received from the downward inclination of the ends of the wings. Probably the whip over in the change of direction of the strain does the actual breaking, and the wings then presented to the air at a downward inclination are slammed down by the wind like suddenly released doors. If this theory is correct, it is not only when starting an engine that this danger occurs. On a windy day, a machine continually runs into whirls, which have the effect of altering the speed of the machine through the air, and also of altering the angle of incidence. The centre of pressure is thus continually moving backwards and forwards beneath the wings, which twist in pro portion to the length of their span and the slackness of the stays, and so they are continually approaching towards and retiring from the critical angle, where, if reached, they would whip over to a negative angle. Change in direction of air gusts, and speed of machine in flight must both have considerable bearing on this question, and when it is remembered that a change of less than 5° in the angle of the wing tips is often sufficient to reverse the direction of pressure on the wings, it will be recognised how close to the safety margin many monoplanes continually fly. The truss systems of the biplane structures make the wings equally strong in both directions, and so this collapse danger is entirely absent. In all cases a rotary engine would appear to increase the danger, because the wings in twisting find themselves opposed by a more unyielding base than would be the case with an engine having no gyroscopic effect. I should have liked to have followed up this diagnosis with a curative prescription, but it will perhaps be best to leave my readers to criticise the theory before carrying it any further. ® ® ® © N.P.L. Exhibit at Ghent. IN the British Section at the Ghent International Exhibition the National Physical Laboratory will hare an exhibit illustrating the experimental investigation into the science of flight carried on at the Laboratory. There will be large scale models of wind channels and also of the special tank which permits of photographs being taken of the actual currents and eddies which play about and round the wings and framework of an aeroplane. 33 c 2
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