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Aviation History
1910
1910 - 0366.PDF
I/yog Comparisons of Blade Form by Test. When we come to testing an aerial screw in still air it is very necessary to determine in the first instance what sort of information is available and in what way it should be interpreted. A propeller designed for use on an aeroplane in flight may or may not be a good fan, and is more than likely to prove an indifferent helicopter. It is a simple matter to show, mathematically, that the greatest thrust with the least horse-power is obtained from an aerial screw of infinitely large diameter and infinitely small pitch, conditions which do not satisfy those obtaining with the propeller in flight. If the results of a static thrust test of a propeller are expressed as they would be for a helicopter, that is to say, by the ratio of the thrust to the power expended in the shaft, the information is meaningless if taken by itself as an indication of the properties of the same propeller in flight. On the other hand, it is possible that two propellers of the same diameter and pitch but of different blade form might possibly be thus compared with some utility. We do not know if this is so, but we certainly think it is worth while trying to find out. Makers of propellers should, we think, consider it to their advantage and to the progress of their business to have the question settled one way or the other. The plant at the South western Polytechnic is available for their purpose, and it is in the hands of those who have the necessary scientific training to work accurately, and, that most desirable quality of investigators, an absence of preconceived ideas. Static Thrust Test Data. Among the data that can be determined by the static thrust test are the following :— A propeller revolving in still air on a stationary machine sets in motion a slip stream that should have a velocity equivalent to the product of the pitch multiplied by the revolutions, but it is possible that a direct measurement of the velocity of the stream itself, by means of an anemometer or other suitable instrument, would disclose a discrepancy between the two values. An anemometer has, as a matter of fact, been erected on the apparatus under discussion for this very purpose. Suppose, for example, that the measured velocity of the slip stream is less than the value represented by a pitch x revolutions. This would show that for some cause or other the blades were failing to obtain a proper grip of the air, and comparisons of different blade forms and different numbers of blades might quite possibly lead to very interesting and important information on this matter. The "Slip." While dealing with this particular point it may be as well to draw attention to the exact use of the term "slip," lest it be applied in an incorrect way to the above-mentioned difference that may exist between the measured velocity of the stream and the velocity represented by the pitch revolutions. " Slip " is a very natural term to apply to the above - mentioned phenomenon, but it does not have the same meaning in con nection therewith as in connection with the propeller in flight, and we suggest that it should be differentiated by using the term "fan-slip " as a term of reference to the slip of a screwused as a fan. It will be observed that the slip-stream plus the fan-slip are necessarily numerically equal to the pitch-speed (pitch x revs.) in the case of a propeller used as a fan. In the case of a propeller in flight, the pitch-speed is represented by the speed of flight plus the slip-stream plus the fan-slip. Ordinarily it is assumed that the fan- slip «5 zero, and that the difference between the pitch-speed and the flight-speed accurately represents the slip-stream. If there is any fan-slip, however, the slip-stream will be less than the amount thus calculated by a quantity equal to the fan-slip, and since the thrust is proportionate to the effective slip, the thrust will have an actual value correspondingly less than the apparent figure as deduced when fan-slip is supposed to be zero. Vet another factor that must be borne in mind is the effect of the wake that follows the machine in flight. If the propeller operates on this wake, which is a stream flowing in the direction of flight, the real slip stream will have a lower rearward velocity relative to still air than the apparent value indicated above, but the effective value for producing thrust will not be affected thereby. Now it is very important that estimates of thrust and speed should be reasonably accurate, and it is essential to know whether fan-slip should or should not be considered as a factor. If the fan-slip is found to be considerable as the result of making a static thrust test, then we see no reason to suppose that it should be negligible in the case of a propeller in flight, although it does not necessarily follow that the actual values are the same in both cases. In this connection it occurs to us as being distinctly important to have a means of directly measuring the slip stream in whirling table tests, and we commend this thought to the National Physical Laboratory in the MAY 14, 191°- hope they will devise a method of obtaining such data. Working on these lines, and combining the results thus obtained with in formation collected from trials of full sized aeroplanes (especially the gliding angles of such machines), would be the means of estab lishing a reliable working basis for calculating the thrust of propellers, and it would incidentally throw a considerable amount of light on the limitations of the application to this subject of the Newtonian method (see "Flight Manual," Note I). *- •n u r | § EFF K s i ----'' ^,~-' __CUK"E #-.^ -* ^Li£^ A ^, <s * SCALE. Of SLIP PER. CENT • 1 1 1 ) / s s . / • V i 5 a 0 at V) 9, -a m 20 25 3S Chart for two-bladed propeller of stated blade form having a pitch ratio (pitch -5- diameter) = i'2 T K=-- 2 -2; T = thrust (lbs.), V = flight speed ft./sec, D = propeller diameter. Experimentally plotted from observations of thrust efficiency throughout a range of flight speeds that are expressed in terms of slip, the slip being expressed as a percentage of the pitch speed (pitch x revolutions). Example showing the use of the chart: Thrust required, 80 lbs. at a speed of 41 m.p.h. (60 ft./sec), with a propeller 6 ft. diameter (pitch ratio I "2 ; p = 7 "2 ft.). T 80 K ~ V2D2 3600 x 36' 80,000 K, = (K X I.OOO) = J = 'OIK. 1 ' 130,000 J From the chart (KI = '615) indicates that the above conditions of workir.g will give a propeller efficiency of 68 per cent. The indicated slip being 34 per cent, of pit implies that the flight speed (V) = 56 per cent, of pn . •. 60 ft./sec. = tjb pn. .'. pn = —. = 107 ft./sec.; and since/ = 7-2 ft., .•. n = — = 14-9 rev./sec. = 890 revs, per minute. Coefficient of Area. Another point that could be investigated in connection with the static thrust test of a propeller is whether the product of the measured slip and the disc area of the propeller correctly accounts for the measured thrust in accordance with the fundamental equation:— g . .- . where T = thrust, m = mass, / = acceleration, p = density, 364
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