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Aviation History
1918
1918 - 0422.PDF
APRIL J8, 1918. U3 an engine of large diameter, and therefore probably of highhead resistance. These features are common of course to stationary radials and to rotaries. Of the stationary air-cooled Vee type we have the 70 h.p,Renault, an 8-cylinder engine, weighing about 360 lbs. and giving about 72 b.h.p. at 1,800 r.p.m. ; this engine is stillused on training machines. It is heavy, but reliable and long suffering. The 8-cylinder R.A.F. IA weighs about the sameas the Renault, and gives about 95 b.h.p. at 1,800 r.p.m. The 12-cylinder R.A.F. 4A weighs about 650 lbs., and givesabout 140 b.h.p. at 1,800 r.p.m. All these air-cooled Vee engines have the air screw mounted on an extension of thecam shaft, and that is to say the air screw revolutions are only one half engine revolutions; this I consider a disadvantagefor fast and light machines. Of vertical water-cooled types we have the 160 h.p. 6-cylinderBeardmore ; it weighs about 570 lbs., and gives about 170 b.h.p. at 1,400 r.p.m. •Of. the inverted vertical and the horizontal opposed typesthere are no models as yet in service, but there may be soon. •Both types possess great advantages from the aeroplanedesigner's point of view. These are:—very small bulk of engine above the air screw shaft to obstruct the view, lowcarburettor position (making for greater ease in employing a gravity petrol feed from the main fuel tanks), and the possi-bility of using short and simple pipes to carry the exhaust gases underneath the body of the aeroplane. Of Vee type water-cooled engines we have the " Falcon " "''.)! J 4 1 • if : .• • ; ;l • I '.1 • • r i R ii v., PuETo ROTATION. ADVANCE peRREv. SECTION PER REV , OUETOSPEED ALOHO AXIS. ""^""^ AC -'FACE'PITCH. AD'SRUE'P'TCM ^ ADVANCE PER KEY. REQUIRED rOW ZERO TMBU&-T. OB - FLIGHT PATH FOR SECTION, j -ANGLE OF ATTACK'FOR SECTION. . 9 -AN&UfcOf ATTACK| FOR ZERO LIFT. -ABOUT - TWQBL APED 'R I6HT HAND* AlR SCREW. FlG.2. CROSS SECTIONPP rCH ANCLE AT RADIUS Ri and "Eagle" Rolls-Royces, the 140 and 200 h.p. Hispano-Suizas, and several different powers of " Sunbeam-Coatalens." The Rolls-Royces, the Sunbeams and the 200 Hispano alldrive the air screw through a reduction gear; the 140 Hispano drives the air screw directly on the crankshaft. The petrol consumption of the best modern water-cooledengines is about .5 lb. per b.h.p. hour, and their oil consump- tion about x>4 lb. per b.h.p. hour. Their weight is about 2.8lbs. per b.h.p. without radiator and water, and about 3.4 lbs. per b.h.p. with these. These weights, however, are realisedonly in engines of over 150 b.h.p., say : smaller water-cooled engines are heavier per horse-power. Water-cooling is anaddition, and as such means additional liability to break- down and additional vulnerability, but it permits of simplemethods for variable and even cooling, and this is a necessity now. The weight per b.h.p. we have seen to be much the samefor all types, the air-cooled rotary being slightly the lightest; but the petrol consumption for the rotary is about 50 per cent,higher and the oil consumption about 200 per cent, higher than that of the water-cooled engine. It is hardly practicableat present to run an air-cooled engine at such a high com- pression as a water-cooled, so the power of the air-cooledengine falls off more rapidly at increasing heights than does that of the water-cooled. With the air-cooled rotary, it isuseless going to higher revolutions than about 1,300, trying to obtain greater power thereby, for any additional powerdeveloped in the engine by further increasing its revolutions will not be available at the air screw, it will be more than "accounted for by the additional power thereby absorbed by therotating cylinders, &c. Roughly then for'powers of below 150 b.h.p. we usuallyemploy an air-cooled rotary, for powers above this a water- cooled stationary. Now for the air-screw (Fig. 2). A well-designed air screwon a fast aeroplane is a very highly efficient source of thrust; it can transform, under good conditions, over 80 per cent, ofthe b.h.p. of the engine into " thrust horse-power." I make this initial statement, because many people seem to thinkotherwise, judging from the extraordinary forms of propulsion patented or otherwise advocated. 1 shall not define the technical terms connected withair screws, nor give reasons for my statements, but shall merely make a few general statements of their properties. First for efficiency. We shall define the efficiency of an air screw as the fractional value-work done by air screw divided by work given to air screw. If we denote the thrust in lbs. by T, and the " advance " (or speed along the line of this thrust) in feet per second by S, then the work done by the air screw is T C thrust horse-power. This value divided by the Soo *b.h.p. which the engine driving the air screw is developing, gives us the " efficiency " of the air screw. The " efficiency "quoted hereafter always means this only. If the air screw be stationary along its line of thrust, it is doing no work in thesense just defined, so its " efficiency " is zero, although it is giving a thrust. Directly it begins to " advance "it commences to do work and to possess therefore an efficiency.For any air screw therefore the efficiency varies according to the rate of its advance per revolution. The maximumefficiency usually occurs when the advance per revolution FlQ.3. •8 IO EFFECTIVE MCANPnCM CURVE OF AIR-SCREW EFFICIENCY ON ABA5E OF ADVANCE PER REV- ADVANCE PER REV EXPRESSED AS A FRACTION OF THE PITCH. is about .8 of the pitch. It is possible to calculate, or todetermine experimentally, the efficiency of an air screw at various rates of advance per revolution, and therefrom todraw a curve of efficiency on a base of advance per revolution (Fig. 3). The most efficient form of air screw as regardspitch, is one whose pitch is about equal to, or slightly greater than, its diameter. For the same blade form a twobladed air screw is somewhat more efficient than a four bladed. Assuming that only one factor varies at a time,thrust varies approximately as square of diameter, as square of pitch, and as square of revolutions ; while withinvery small limits thrust varies directly as face area of blades. When the question arises of the best air screw for any par-ticular aeroplane, we must know the b.h.p. given to the air screw shaft by the engine (on full throttle) at all rates ofrevolutions between its maximum power revolutions, and a rate of, say, two-thirds of this maximum. We have also toassume what will be approximately the maximum speed and the best climbing speed for the aeroplane. As no airscrew is equally efficient at all rates of advance per revolution, one and the same air screw cannot be the most efficient forhigh speed and for climb. Speaking generally, large diameter and fine pitch for climbing, smaller diameter and long pitchfor high speed. It is generally easier, therefore, to get air screw efficiencyat high speed than at climbing speed, for the pitch of the most efficient high-speed air screw will be greater in proportionto its diameter than will that of the most efficient climb- 420
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