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Aviation History
1912
1912 - 0065.PDF
Conducted by V. E, "If a model were designed for twin propellers with a pitch of 2 ft., but when bent they are found to have a pitch of 22 and 26 ft. say whtchof the two would have the most tower and make the model circle. Such is the pretty little problem sent us this week by a corre spondent. When we consider that the propeller is of bent wood and therefore not of uniform pitch, that it is moving axially through the air, and therefore that it is the dynamic and not the static thrust that is required, and that we must know the respective speeds of rotation of the propeller blades, and the deformation, if any, of the same while in flight, &c, &c, the problem certainly appears a somewhat unprofitable task from the theoretical point of view. Two of the chief factors in determining the thrust are the number of revolutions per minute and the pitch of the propeller, whose product appears in the equation, one becoming larger as the other grows less. Turning now to the more practical solution of the problem, we might take what is known as a " variable-pitch" propeller, fitted with a hub and adjusting screws, and setting the blades at the correct angles to give the stated pitch ; determine the actual thrust given in the two cases. This solution is useless, however, for more than one reason: one of which is that the type of propeller is different, as our correspondent specifies that it is to be of bent wood. One method remains—that of making two propellers of the same diameter and of 22 in. and 26 in. pitch respectively, and of testing them in actual flight, interchanging the propellers after the first experiment, in which case the model should, of course, circle in the opposite direction. Let us take a o-in. propeller, and find the difference between the tip-angles using the formula : Pitch = 2 ir r tan a. We find that in the case of a 22-in. pitch o = 37° 52', and in the case of the 26-in. pitch a = 400 33'. Difference is therefore 400 41'. It is not easy to measure accurately the tip-angle of a bent wood propeller of cambered surface, but in this case the angular difference should certainly be sufficient. As the diameter is increased this difference, of course, grows less. Will any reader who may have had practical experience with a case similar to the above kindly send results ? 44 Propcller-Pitch and Length oF Flight." In response to a request of ours, Mr. C. C. Allport, of the Conis- borough and District Aero Society, kindly sent us one of his propellers for inspection ; the propeller being one of those fitted to his 7-oz. models, which, as recently notified in FLIGHT, flew an estimated distance of 3,000 ft. On calculating out the theoretical distance, allowing 20 per cent, slip, we have 2,240 ft. The model is stated to have flown in a calm, and as Mr. Allport has pointed out since the model flew in an approximately elliptical course, even if started with wind, the latter part of its flight must have been accomplished against it. There is, therefore, in this case, considerable discrepancy between the two results. As it so happens, we have received this week ailetter from Mr. R. P. Grimmer, giving us certain details about the well-known Mann monoplane (see illustration). The chie/ details are: Length, 36 ins.; main plane, 20 ins. by 5 ins. ; propellers, 9 ins. diam. ; pitch, 34 ins. ; slip, approx., 20 per cent. ; total weight, just over 5 ozs. ; weight of rubber, about l£ ozs. ; number of turns obtained on each motor, about 1,000; propeller runs 600 revolutions per minute ; best flight in calm air, 750 yards. Referring to last-named fact, the pitch and 20 per cent, slip, 1,000 turns give theoretically a distance in entire agreement with results actually obtained. We shall be glad to hear from any of our readers results obtained (in calm air) with respect to theoretically possible distances, and lengths of flights actually obtained. To be thoroughly reliable these latter should be straight flights. To Determine the Pitch of a Propeller. Referring to the method given in the issue of December 16th we omitted to state that it was applicable only when the pitch was small. Supposing we have a 9-in. propeller with a tip angle say of 50°, first draw to any desired scale a line equal to 2 ir r, that is to the path traced out by the tip during one revolution, which in this case is Vx9, that is 28? in. Call this line AB. From A draw a line AC making an angle of 50° with AB, and from B draw a straight line Tertically upwards to meet AC in C. BL is the pitch. This is obviously the case ; take a full-size drawing. If we wrap the paper on which figure is drawn round a cylinder 9 m. in 65 JOHNSON, M.A. diameter, point C will come vertically above point A and will there fore represent the distance between two eonsective threads. If P = pitch we have by Trigonometry P = 2 «• r tan o ; a being the angle, in this case $0°. The latter is a very convenient method if one possesses a table of Natural Tangents. Applying these two methods to the above we find by the first method the pitch to be 33-5 inches and by second 33'66. If BC = BA then the pitch is evidently equal to 3f times the diameter. In this case 0 = 450 and tan a we know equals 1. Conversely a pitch of 2% times the diameter gives an angle at the tip of about 38°, and IJ times the diameter an angle of 25". Notes. We have received a long communication from Mr. R. P. Grimmer in which he takes exception to certain remarks made by us in the issue of December 23rd—re fashionable 4-oz. models. He contends that the larger models possess several important disabilities, one being that they are a danger to the general public. Mr. Grimmer states that he once had the doubtful pleasure of being hit by a heavy model in full flight, and that the impact was such as to cause him consider able damage (evidently one serious objection that can be urged against the rubber motor is that it is a silent one). He furthermore states that they are expensive to build and maintain, that a vast quantity of rubber is required, and that such models are therefore a severe strain on the purse of the owner unless he is wealthy, which many aeromodelists unfortunately are not. All of which is perfectly true and of which we were fully aware when we made our remarks. The fact remains, however, that there are plenty of places where they can be flown with perfect safety, just as there are many districts where they cannot. The general public is always impressed by size. We would rather not state some of the remarks that we have heard onlookers make with respect to small models, as we have no desire to hurt anyone's feelings. We know, personally,, of several instances in which gentlemen were induced to become aeromodelists after they had seen a large model in actual flight. Finally, it is possible to fit on to large models very light automatic recording instruments and automatic controls by which much valuable data may be gained, which is absolutely impossible of achievement with smaller models. If model aeroplaning is to be made a complete success we must cultivate and encourage not one side but every side. Another remark criticised by Mr. Grimmer is one made in the issue of January 6th, viz. : " No fin or rudder per se will make your model fly straight." He states that the models with which his name is associated (the Mann monoplane) when fitted with fins invariably fly in a straight line. It is rather a coincidence surely that in the same number of FLIGHT (Brighton and District Model Ae.C. notes) it is stated " Hervey several times did 300 yds. with ' Mann.' The machine will circle and fly out of the ground." Of course the machine may not have been fitted with a fin, or may have had it removed, which Mr. Grimmer states is always done when trying for duration, the fin being removed in order that the model may circle ; but it certainly looks as if Mr. Hervey was trying to fly straight. Of course the fault may have been Mr. Hervey's. We should scarcely have referred to the matter in this way had Mr. Grimmer not used the word " invariably," a very strong term The 100-secs. duration "Mann" monoplane. Brighton, September 16th, 1911.
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