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Aviation History
1921
1921 - 0074.PDF
anti-clockwise. Secured to the same spindle as the quadrant, and projecting downwards from it, is a lever (shown dotted) to which are attached the "aileron cables. It will now be seen that as the pistons and connecting rod travel to the right the quadrant and the lever behind it move also to the right, pulling down the port aileron and thus righting the machine. This, briefly, is the principle of the mechanism, and later we shall indicate detail differences between diagram and actual machine. The Original Feature Hitherto the stabiliser has not shown any very great origi- nality. All its components have been used or suggested before. We now come to the feature which separates the Aveline stabiliser from all others with which we are acquainted. Let us return to the diagram once more. If we suppose that the pilot of the machine does a flat left-"hand turn, it will be seen that centrifugal force would send the mercury up in the right-hand side of the disc, indicating that the machine is down by the starboard wing when, as a matter of fact, she is perfectly level, merely doing a flat left-hand turn. In other words, the mercury is acting precisely as would a pendulum. In order to counter this tendency to rise and fall with the action of centrifugal force, M. Aveline has had the brilliant idea of letting air pressure work against centrifugal force, thus keeping the mercury level. This has been done by the very simple means of fitting to each wing tip a Venturi tube. The Venturi tube on the port wing tip as connected to the right-hand end of the mercury and vice versa. In other words, the pipe leads from the tubes are crossed. Between the two points where the tubing enters the circular groove the latter is closed. Now the action of the Venturi tubes is to cause a FEBRUARY 3, 1921 Some Interesting Details The foregoing is a broad outline of the action of the Aveline stabiliser. It will have been observed, however, that so far nothing has been said regarding the method of returning the pistons of the working cylinder to zero when the machine begins to right itself. As shown in the diagram, the inclino- meter, or disc with the mercury, tilts with the machine. When, therefore, the machine begins to right itself there is nothing to prevent the pistons from travelling to the end of their stroke until the machine has " overshot the mark," and makes contact on the opposite side. This would give rise to oscillations, and to avoid these the actual arrangement of the apparatus is different from that shown in the diagram, ftistead of being mounted on the machine itself, the inclino- meter is mounted in front of the working cylinder, its axis carrying a quadrant pointing upwards and engaging with a smaller quadrant on the same pivot as that of the main quadrant. There is, therefore, a differential movement between the mercury disc and the main quadrant, so that when the quad- rant turns in an anti-clockwise direction the mercury disc turns in the opposite direction. The action is somewhat difficult to explain in words, but the two auxiliary quadrants are shown in dotted lines on the diagram, as is also the mercury disc in its proper position. The result of the differential movement is to shut off the air and so to stop the travel of the pistons. A similar arrangement is provided for the elevator control, but here, in addition to the differential movement of the disc and quadrant, the " setting " of the disc is also under the con- trol of the pilot. This is necessary in order to trim the machine. For any one setting the controls will keep the THE AVELINE STABILISER : On the left, a rear view of the casing over the working cylinder, showing the crank lever to which the control cables are attached. On the right, the relay tell-tale lamps and one of the control valves by means of which the pressure, and consequently the power of the control, is regulated. When the lever points forward the control is in gear, and turning the lever back throws the control out of gear. depression above the mercury. When, therefore, the machine turns to the left, the starboard Venturi tube is moving faster, causing a greater depression over the left branch of the mercury. The port Venturi tube, on the other hand, is moving slower, and consequently the depression is smaller on the right-hand side. Both these differences in pressures tend to prevent the mercury from being raised into the right-hand branch of the circular groove and there make contact and set in motion the mechanism which would pull the starboard aileron down. Put differently, the action of centrifugal force is counteracted by the aerodynamic forces brought into play by the Venturi tubes. The set for the elevator is similar in general principle, with, however, one important difference. Here only one Venturi tube is employed, the opening for the second one communi- cating with the atmosphere. A few minutes' consideration will make clear the reason for this arrangement. When the machine is travelling along at normal" flying speed there is a slight depression, depending upon the speed of the machine, above the corresponding end of the mercury. The effect of this is to raise the mercury into the forward end of its groove, and as this condition represents the zero line, or plane of reference of the elevator system, the two metal contacts are so placed as to be just above the ends of the mercury. In other words, the normal level of the mercury is at an angle with the horizontal. If the machine is flying along at, for instance, 60 m.p.h., and the engine stops, the speed will drop. Consequently the depression above-the forward end of the mercury will decrease, and the mercury will fall, rising at the other end until it makes contact at the lower end, and thus sets in motion the gear for pulling the elevator down until the speed, coupled with the downward slope of the machine, breaks contact and the machine proceeds in a glide. machine at one speed. If, therefore, it is desired to fly at a different speed, the pilot turns a handle in his cockpit, which —via a Bowden cable, worm and sector—alters the relative position of quadrant and disc. In other words, a new reference plane is provided, which forms the zero or normal datum line to which the controls return the machine every time it. has deviated from it through any cause. Adjustment of the Stabiliser _'. i:^ It has already been mentioned that the two air pumps keep up a pressure of in the neighbourhood of 50 lbs. per sq. in. in the air container. If this pressure were to be admitted to the working cylinder the controls would operate very brusquely and the machine would " hunt." In order to avoid this a small valve is provided in the pilot's cockpit by means of which the pressure of the air admitted to the working cylinder can be regulated to a nicety. There is no need to go into a detailed description of this valve, suffice it to mention that it regulates the air by means of a disc with holes of different sizes, ranging from 1 mm. to 4 mm. in diameter. On the barrel of this valve is a small lever which, by means of a cam and internal pistons, covers and/or uncovers the four tubes issuing from this small cylinder. When the lever is pointing forward the control is in operation, while by turning it back the control is out of gear. There are two of these levers, one for elevator and one for ailerons. Thus the pilot can throw either or both out of gear at will. For instance, he can control the elevators himself and have the stabiliser work the ailerons only. Or he can set the elevator adjustment to any angle of climb or to straight flight, and control'the ailerons himself. Finally, he can throw the entire mechanism out of gear by means of a single lever, when he has straight-through control as if the stabiliser were not fitted at all. 74
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