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Aviation History
1917
1917 - 0143.PDF
FEBRUARY 8, 1917. effective pitch of the propeller. Suppose the chord XY °l v 1 section at any radius V, makes an angle a with the helix line, Fig. 9. Angle a is called the angle of attack of the sectwn. As (V -;- N) changes owing to a variation m either V or N, or in both, the blade section will have a varying angle of attack, an increase in (V -*• N) decreasing of Attach Fig. 9.—Showing pitch angle of propeller. the angle of attack and vice versa. The efficiency of such an . element is expressed by --•••'" ••••?•• •- JauA ' ~ tan (A + G) where G is the gliding angle, which is a function of the angle of attack and varies with the type of section employed 3 t 1 t i / z J / . J / 1 ,y / J y r• \ i < - \ \ I!5 DISTANCE «D«NCEOPEB RtvOlUTION rff Fig. 10.—Relation of propeller efficient and effective pitch. With the usual section used in propeller design G is a minimum when the angle of attack is about 40. It would, therefore, be advantageous from the viewpoint of efficiency of the section to keep the angle of attack at 40 throughout the speed range of the aeroplane. This can be accomplished by means of a flexible blade whose pitch angles could be changed a varying amount from the tip of the blade to the 70 "0 OO to t>t> •0 la 0 / / / / / / / 1 • 000 n <*f VOIL'TIONS PER Fig. 11. -Assumed brake horse-power of engine drivingvariable-pitch propeller. root or hub section. Such a blade is out of the question in the light of present-day practice. A good approximation to such a blade could be more simply had by rotating the blade about its axis perpendicular to the shaft. With the usual type of section employed the approximation is good, as the value of G does not change greatly for a degree or so on either side of the best angle of attack. A mean valui" for the angle of attack could, therefore, be found giving practically the same efficiency as though all the sections were at the best angle of attack. Fig. 10 shows curves in which efficiency of a propeller is plotted against (V -4- N). The full line gives the efficiency for a fixed blade, the dotted line the efficiency of the same blade were the angle of attack kept at approximately 40. It is assumed that the fixed- blade propeller was designed for a maximum efficiency at a value of «(V •*- N) of about 6 ft. PROPELLER STRESSES. In connection with the subject of propellers, it may be of interest to give a brief review of the variation of stress that occurs in a propeller blade under an assumed condition of 12.—Points of maximum propeller stress. flight. The blades of a propeller are subject to the following stresses when an aeroplane is in any but a straight-line flight:— ; . 1. Shear due to aerodynamical forces. 'rrlfi 2. Torsion due to the distance between the centre of gravity, of the blade section and the point of application of the resultant of the air reactions. 3. Tension due to centrifugal force. 4. Steady bending due to aerodynamic forces ; torque and thrust imposing a distributed load on the blade, the hub being the fixed point of support. 5. Reverse bending due to gyroscopic forces, which occur only when the aeroplane has rotation about an axis, as in making a turn or pulling out of a dive. As a matter of fact, an aeroplane is continually turning to some extent if the flight be in disturbed air. Each of these forces produces a maximum stress of tension and compression in different parts of the blade, hence the resultant fibre stress at any point will be equal to the algebraic sum of the individual stresses at that point. It is sufficient to calculate the stress at the RADIUS IN FEET Fig. 13.—Fibre stress in propeller blade at point A (see Fig. 12). points a, b, c (Fig. 12) along the blade, as these points will be those of maximum stress. The shear in any case is small and can be neglected in design. The torsion is also small. In good designs, when the thrust is great, the point of application of the air reactions is but little removed from the
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