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Aviation History
1915
1915 - 0046.PDF
"PROPELLER ALTHOUGH the efficiency of modern airscrews has reached a very high figure—as a result of the care and skill which have been exercised in their design and manufacture, and in the selection of suitable grades of material—there is but little doubt that a still better performance could be obtained were it possible to either entirely or partly eliminate what is colloquially termed "propeller flutter." This phenomenon may be seen on viewing an airscrew in motion from the side—the blade appears to, and does actually, vibrate in a plane containing the axis of the shaft to which it is fitted, and is effective in reducing the efficiency because the aerofoils forming the blades do not attack the air at a constant angle. There are several factors, in any power plant installed upon an aeroplane, that contribute to this. Firstly, no engine is entirely free from unbalanced forces and couples, although some will run very smoothly under load; hence, when it is mounted upon such an elastic framework as that of an aeroplane, the integral parts of the whole structure will be caused to vibrate. The longer and the more flexible the member is, the greater will be the magnitude of its vibration ; which is also increased if the damping forces are small, or if the impulses causing vibration synchronise with the actual period or multiples of the natural period of vibration. There is a tendency, therefore, for the airscrew, which is rigidly attached to the crankshaft of the engine, to vibrate in a plane approximately at right angles to the direction of its motion through the air, during rotation. A second cause of " propeller flutter " is due to the medium in which the airscrew works. Air is not a homogeneous substance, it varies in density according to the elevation and its location; but this would be of no importance whatever as regards the matter we are now discussing, provided that any change in the direction and the velocity of its motion were effected gradually. In practice, however, both the direction and velocity of the wind relative to the airscrew may vary over wide limits ; and since the normal mode of motion of any body is one of uniform velocity, the airscrew encounters a varying resistance and exerts a varying thrust. Now, when a blade is transmitting power, it is deformed slightly by the pressures acting upon the two surfaces—a variation in these pressures due to the cause we have just mentioned must permit the blade to resume its former shape or to be still more deflected, and if the gusts or other disturbing forces occur with some regularity, it necessarily follows that a blade will be caused to vibrate (again) in a plane approximately at right angles to its direction of motion. A similar effect will also be produced should the air screw be placed in close proximity to fixed parts of the structure of the aeroplane, as the flow of air over the surfaces of the blades will be subject to periodic interrup tion as the blades or parts thereof pass by them ; while the effect of a side wind upon the pressures acting upon the blade, although too complicated to examine in detail here, must also partake of a similar character. Propeller flutter is also caused by lack of uniformity of torque. Assuming that resistance to the rotation of the airscrew is approximately constant at any instant— and any fluctuation in resistance will only tend to com plicate matters and render the speed of rotation more irregular—the ideal motor would be one that would produce a continuous and uniform torque. Any excess or deficiency in the energy delivered to the propeller, above or below the average required for propulsion, must FLUTTER." tend to accelerate or decelerate the moving parts. The inertia of these parts—pistons, connecting rods, crank shaft, flywheel and airscrew of fixed engines, and the cylinders and crankcase also, but not the crankshaft, in rotary engines—resists the change of velocity and tends to keep the angular speed of the airscrew constant, but is only capable of so doing within certain limits and the speed must, therefore, vary to some extent. Hence, as the velocity of the blades is increased or decreased, there is a variation in the thrust delivered by the blade ; and, consequently, the same action as was indicated above as due to variation in the air velocity—the deformation or the partial straightening of the blade—must also here become evident: with this difference, however, that the pressure variation must in this case occur continually and have a high frequency. Lastly, structural deformation is sometimes responsible for the presence of this phenomenon, and it is more particularly in evidence when the airscrew is direct coupled to the crankshaft, without the interposition of an intermediate shaft, of an engine in which the crank shaft is under, rather than over, the usual standard of rigidity; or in which the end bearing is abnormally short. Under the explosion pressure in the cylinder, the crank shaft is bent to a slight extent and tends to tilt the end of the shaft outside the crankcase. This action is resisted in part by the bearings, but if these are short, or if the end-plate of the crankchamber is not sufficiently stiff, the major part of the movement takes place, and being trans mitted to the propeller, the blade tips move towards the front or to the rear, according to the position in which they happen to lie at the moment of" explosion. Hence, again, there is the same ultimate effect pro duced as that due to variations in the air velocity, in fact, the real origin of the vibration of the blade is the variation in the relative speed of the air over the blade. It will be clear that where the airscrews are driven by chains or by some other form of gearing, this particular action does not take place, and this may partly account for the excellent results obtained with the very crude propellers so driven on some of the earlier machines. Both of the last two effects to which attention has been drawn are, it will be observed, due to periodic forces; and these, if applied with sufficient frequency and at certain definite intervals of time, may be productive of marked results upon efficiency—altogether out of proportion to that which might have been anticipated from the small forces to which they are due. Their elimination is, therefore, especially desirable, and by use of (i) a large number of cylinders, (2) a flywheel, or its equivalent, of large capacity, and (3) an exceptionally rigid crankshaft and end bearing, much can be done in this direction, while the employment of a short intermediate shaft and affording a substantial support to the crankcase, should assist in diminishing " flutter " due to crankshaft deflection. The effects of gusts and other wind changes of a similar character cannot be prevented from arising, but these are negligible, except that due to the influence of a side wind upon the airflow over the airscrew, in comparison with the other factors mentioned. ® ® ® ® An 1870 Relic for Sale. MRS. REY, a member of the Royal Aero Club, wishes to sell, on behalf of the owners, who are in straitened circumstances on account of the War, a letter and a copy of 'Le Ballon Paste, which were sent by balloon from Paris during its siege in 1870.
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