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Aviation History
1913
1913 - 0948.PDF
I/UGHT does not depend upon the amount of the angular dis placement, but upon the suddenness of the acceleration or precessional movement. The movement itself may be very slight in amount, but if it is sufficiently rapid it will bring a large couple into play. In the absence of actual figures, we can only rely upon the experiences of pilots, and up to the present time we have not heard complaints in this direction. If any pilots who have flown much in windy weather have experienced sudden twistings of their machines in the horizontal plane, accom panying sudden pitchings in the vertical plane, and vice versa, we should be very pleased to hear from them, and have their own accounts of the effects of such disturb ances on the control. There is, however, an aspect of the subject dealt with in Mr. Cloud's letter to which we attach sufficient import ance to commend the matter to the attention of aeroplane constructors. It is perfectly obvious that the vibration effect of a two-bladed propeller is a variable quantity during each revolution. Regarding the couple introduced by pitching, for example, the effect is a maximum if the pitching originates while the propeller is vertical, and is zero if the propeller is at that moment horizontal. Expressed in another way, we may say that if the machine oscillates about its wing spar in flight, then the two-bladed propeller will maintain a perpetual vibration owing to the difference in magnitude of the vibration couple. This vibration is of course quite distinct from the gyroscopic couple itself, being due only to the variation of the moment of the mass about the axis of forced pre cession. We believe that this aspect of the case has always been quite well known and appreciated by constructors and others, but we have no knowledge of any precise experimental evidence showing whether or no it is a serious factor in practice. It is commonly supposed, we believe, to be of no particular consequence; at any rate, the two-bladed propeller has never been seriously criticised on these grounds, so far as we are aware. For our own part, we are by no means anxious to raise false estimates of systems that are serving their purpose well, but we must confess to having been much impressed by a very simple experiment that was made for us by Mr, Cloud at the Westinghouse works. He had a little electric motor balanced on a trunnion so that it could be oscillated in any direction. On the motor spindle could SEPTEMBER 6, 1913. be placed metal bars to represent two, three, and four- bladed propellers. The difference in the vibration caused by the two and the four-bladed propeller, in favour of the four-bladed propeller, when the axis of the motor was oscillated, was so remarkable that we would certainly recommend those interested to look into the matter for themselves. We do not say more than this because we cannot offer any conclusive evidence that the experiment in any way represented the conditions of flight to scale. We merely record the fact that the difference in the effect—which could be felt by the hand when oscillating the motor, and was therefore to that extent all the more definitely im pressed on the mind—between the two and the four-bladed propeller was very much greater than we had previously imagined, and it may be that in actual fact the advantage of the four-bladed propeller under this head is in practical flight superior to what is generally supposed. The vibration due to the two-bladed rotating member in the experiment was exceptionally harsh and un pleasant, the whole frame being subjected to thuds of distinct violence and suddenness. With the three-bladed propeller, the vibration was still noticeable, but its violence was appreciably diminished. It had, as one might say mathematically, taken unto itself the charac teristic of a curve. With the four-bladed rotating member, the vibration was, by comparison, so slight as to be negligible.' It will be understood, as we have already remarked, that the above considerations do not relate to the gyroscopic couple as a whole. In fact the couple as such was greatest in the case of the four-bladed member, as it had the greatest mass, the same diameter, and was rotating at the same speed. Reverting for a moment to the subject of twin pro pellers rotating in opposite directions, when there are two twin-bladed propellers they should become vertical together and horizontal together. This is an important condition if they are to neutralise their individual gyro scopic effects. We mention the matter here because there is an Italian horizontal engine very ingeniously designed to drive a propeller at each end. The centres being rather close together, space for the rotation of the twin screws is obtained by placing one horizontal while the other is vertical. Thus, as long as the tip of one pro peller clears the boss of the other, they can rotate without running foul of each other. ® ® $ $ THE CORKSCREW TWIST. IT is difficult from the accounts that have been published of the extraordinary experiment made by M. Pegoud on a Bleriot monoplane in France on Monday to really grasp exactly what it was that the pilot did. The salient point in the descriptions is that for a distance of about 400 yards his machine was upside down and still under control. It is definitely stated also that it was not a case of looping the loop. We imagine, however, that the exhibition was in principle related to a performance of this order : that is to say, we suppose that during the inverted flight of the machine the wing pressure was operating against centrifugal force. What we suppose took place was this. The pilot made a dive of exceptional steepness, and having acquired a velocity considerably above his normal flight speed, he proceeded to flatten out. It has been demonstrated by Lanchester and others that a model thus launched can successfully loop the loop, that is to say, the momentum of its descent is used to carry it round the rising half of the circle, and beyond, while the wing pressure counteracts centrifugal force and so steers the machine of its circular path. If, at the bottom of the dive, when having flattened out, the upward curve of the loop is about to be commenced, the control is so used as to roll the machine into an inverted position, what would otherwise have been the top half of the circle completing the loop will now become a curve proceeding away from the origin of the loop; that is to say, instead of the two semi-circles forming one circle they will be drawn consecutively in the same direction, the first one convex to the earth, the second one concave. To call this second curve a semi-circle is, of course, incorrect, because while the machine is turning over, it will steer off to one side as if it were performing' an 974
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