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Aviation History
1910
1910 - 0241.PDF
MARCH 26, 1910. screw is merely a propeller situated in front of the machine so that it pulls instead of pushes. Most mono planes have tractor screws, and the pros and cons of the arrangement were discussed in our article on the Olympia Show last week. The first column of figures relating to screws in the table gives the diameters of the various propellers, and it IHE.DR.Aw [fLlGHT] and it is necessary to have a reasonable diameter in order to get sufficient disc area. The disc area of a propeller (13) is the area included in the circle described by the tips of the blades as they revolve. This disc area represents, approximately, the cross section of the slip stream ; in other words, it indicates the mass of air to which rearward acceleration is being imparted. If the mass is large the velocity necessarily imparted to it in order to obtain the thrust required will be small, whereas if the disc area or diameter of the propeller is small the will be observed that there is not a great deal of variation in these dimensions. The next column gives the pitch, but it has not always been possible to obiain this value, owing to the fact that it cannot be readily measured. The pitches given in this column are those represented by the angle of the blade. If, for instance, the pitch is given as 3 ft., it means that the machine would advance 3 ft. through the air per revolution of the propeller (12), if the propeller blade were mounted in solid guides, so that there was no possibility of slip. The air is not solid in this way, however; it is, in fact, only capable of offering an abutment for the thrust of a propeller by virtue of being accelerated backwards in the form of a rearwardly moving stream. This stream is called the "slip," and it represents a percentage (in the order of 30 per cent.) of the pitch, so that the machine does not advance through the air by the full amount of the angular pitch per revolution of the propeller. The Value of the Pitch Coefficient. The third column of figures relating to the screws in our table gives values of the pitch coefficients, which are obtained by dividing the pitch by the diameter. In most cases it will be observed that the pitch is in the order of one-half the diameter, but that in one or two cases the pitch is greater than the diameter; in other words, the pitch coefficient exceeds unity. The pitch coefficient is a very important factor in the efficiency of a propeller, and marine experience has taught designers of propellers for that purpose that a pitch coefficient in the order of unity should be obtained even with high-speed pro pellers (See FLIGHT, Vol. I, page 35c). Now it is possible in any propeller to have a pit h coefficient of unity if it is desired, but if those propellers that have a pitch coefficient less than unity in our table, were altered to have a pitch coefficient of unity, then they would either have to run at a slower rate of revolution speed or be made smaller in diameter. Those propellers that have a high pitch coefficient in our table are all indirectly driven from the engine through gear reduction mechanism. The natural high speed of a petrol engine does not, apparently, at the moment permit of the use of a pro peller of reasonable diameter with a high pitch coefficient, velocity imparted to the slip stream will be high. Now, whereas the power lost in the slip is only directly proportional to the mass of air in motion, it varies as the square of the velocity with which that mass is moving, consequently it is desirable to keep the velocity low by using a large diameter. The best proportion between mass and velocity is given, in the light of marine prac tice, when the pitch coefficient is unity, but in aeroplane construction the use of a direct driven propeller on a modern high-speed engine would cause the propeller diameter to be unreasonably small with such a co efficient, consequently designers have sacrificed the coefficient rather than the diameter. On monoplanes with tractor screws there is a very good reason for this, too, owing to the large amount of obstruction caused by the body of the machine. If the screw were much smaller in diameter than it is, practically its whole area would be screened in some cases. The pitch coefficient of a propeller is a factor that indicates the limiting brake angle. If the pitch coefficient of a propeller is unity, for instance, then the boss of the propeller will advance axially an amount equal to the diameter of the propeller while the tip of the blade moves through a path equal in length to the circumference. The ratio of these two movements is one to JT, or approximately one to three, which ratio represents the tangent of 180. This, therefore, would represent the angle made by the tip of the blade to the plane of revolution. Weight, Loading and Speed. The final series of figures contained in our table relate to weight, loading and speed. The weight given is the weight of the machine in flight and includes in every case an arbitrary allowance of 200 lbs. for the pilot and fuel. Dividing the weight by the area of the main planes, we have obtained the figures given in the column of loading. These figures represent the intensity of air pressure in lbs. per sq. ft. that must be obtained upon the main planes in order to maintain horizontal flight. It has been assumed that the whole ot the load is carried on the main planes, except in the case of the Roe triplane, where the tail is definitely constructed 239
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