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Aviation History
1910
1910 - 0087.PDF
at the side of one of the struts is to work the elevating plane* The peculiarities in this model are the planes which we fixed at the top of the main planes, which give the aeroplane a greater lift at the sides than at the centre, giving lateral stability ; also the planes fixed between the main planes are for stability, since one can decrease or increase the angle of incidence to increase or decrease the lift at the side you require. Our photos will clear the matter. Hoping continual success to FLIGHT. Kensington. MONTOYA BROTHERS (E. AND M.). PROPELLERS. [328] In connection with air propellers, there is one point that 1 do not remember to have seen, which must have a most important bearing on their efficiency. Air is a gas, and is therefore compressible. To follow the effect of compression, it is simpler in the first instance to take the case of a fixed fan, which is simply rotating about its own axis. To take a definite case. Maxim, in " Artificial Flight," pages 36 and 37, gives figures relating to an experiment. His propeller was 18 in. diameter and 24 m. pitch ; when run at about 2,500 revs, per min. the thrust was 14 lbs. This 14 lbs. thrust represents the reaction or resistance of a volume of air of area approximately equal to the disc area (1767 sq. ft.) to being set in motion. Neglecting for the moment the front of the propeller, and assuming that the back does all the work, the air immediately behind the area swept by the disc is compressed at an average pressure of 7 -o. lbs. per sq. ft. in excess of normal atmospheric pressure of 147 lbs. Now air, within wide limits, is la perfect gas, and providing the temperature remains constant, answers to Boyle's law, PV = const. As the pressure of the air behind the propeller is increased, its volume must be diminished. Now, no new air has been created by the propeller, consequently the increased density must be balanced by a rarefication of air in another place. There can be no doubt about the position of the rarefied air ; it must be in front of the propeller. But if the air is rarefied its pressure must be less than normal (147), from which it follows that the 14 lb. thrust represents the difference in pressure between that on the front and on the back sides of the disc area, or, in other words, that both sides of the propeller blades have to be taken into consideration. A definite weight of air is thrown out by the propeller of a density, fP, greater than "076 lb. per cubic ft. (which is density at 147 lbs. per sq. in. at 620 Fahr.), at a velocity, V2. If the temperature remains constant, then the same weight of air must be fed into the propeller, but its density, tf, is less than d2, therefore its velocity, V1, must be greater than V2. In a well-designed propeller, working at its most efficient speed, it is probable that both sides of the propeller-blades do an equal amount of work. At high pressures per unit area of blade, and consequent high velocity of air current, a propeller might be starved of air, when the pressures could only be maintained by a rise in temperature behind, and a drop in temperature in front of the propeller, and as the efficiency of the propeller depends upon the weight of air acted upon, any energy expended in altering the normal temperature of the air may be written off as lost so far as useful work is concerned. Between the two sides of the disc area there is an average difference of pressure of 7/9 lbs. per sq. ft. It is, I think, quite clear that the lotal difference of 14 lbs. represents the difference in pressure between the two sides of the projected blade area. Maxim's projected blade area is about one-fifth of the disc area, which gives us an average difference of 30-5 lbs. per sq. ft. of projected blade area. What is to prevent this air compressed by the blades from slipping back through the four-fifths of disc area unprotected by blades ? An example will be useful. On a well-designed engine the valve springs will be powerful enough to make the valve lifters or lappets follow the contour of the operating cams at any engine speed. If a valve-spring be taken out the valve will be lifted to its highest point practically the whole of the time, because after it has received one blow by the cam nose its weight is insufficient to overcome its momentum, and it has not had time to drop far before the cam nose comes round again. The same thing should apply to the air compressed by the pro peller blades. Other things being equal, the greater the pressures the greater will be the weight of air which slips back through the spaces between the blades. The weight of air which slips back represents so much lost energy, anil this can be diminished by decreasing the pressures ; but if we require the same total thrust we shall then require more blade area, but increased area obtained by increased width of blades does not make for efficiency, so that we must increase the length of blades, and consequently the diameter of the propeller. Now the thrust of a propeller depends upon the weight of air thrown out, and the weight depends upon the velocity—knowing the thrust of Maxim's propeller, we can calculate the velocity of the air current. Thrust in lbs. _ velocity in ft. per sec, x weight of air thrown out per sec. 32 Weight of air thrown out per sec. •» area of current in sq. ft. x weight per cu. ft. x velocity ; . •. Velocity = ,y/ ,73627 x'.o76 = approx. 58 ft. per sec. ; but on Maxim's propeller rJ-m- *P»tchinft. 60 = S3 ft. per sec. ; which gives a difference of 83 — 58 = 25ft. per sec, or 25 x 1767 x 076 = 3-35 lbs. of air per sec, which probably represents the amount of air which flowed back through the spaces between the blades. This air would lessen the rarefaction which was trying to exist in front of the propeller, so thai in this particular case the positive pressure on the back was much greater than negative pressure on the front of the blades. I am not attempting to teach ; I have set down my conclusions as they have come upon me, in the hope that someone who is farther ahead may spare a moment to assist those who are plodding along behind. Bristol. G. H. CHALLENGER. [329] I thank you for your reply in issue of January 1st to my question as to means of measuring thrust. As a case in point, I note that in Mr. Twining's book on model aeroplane making the thrust of the 8-inch propeller described is given as 1 '1875 ozs. at 2,100 r.p.m. This value to four places of decimals is far more precise than most people would require, and I wish I knew how it is reached. From your very full reply to my question, for which I am great!v obliged, I understand that the thrust is proportionate to the power of the screw to accelerate air from rest. This brings one up against the question of the number of blades. In ventilating fans, such as the " Blackman," the blades are numerous, so closely set as hi overlap, and Davidson's gyropter appears to have screws of this type. But nearly all aeroplanes have only two-bladed screws, only one or two going as far as four blades, and marine practice is similar. The Brothers E. and M. Montoya Model Biplane. 83
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