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Aviation History
1917
1917 - 0096.PDF
fore, all other things being equal, less power will be requiredto overcome the total resistance. Arrangements with two propellers.—Four different systemsfor two-propeller installation have been suggested :— 1. Two engines, one on each side, mounted out on thewings. The fundamental weakness of this system is that these great masses, removed so far from the centre of gravityof the aeroplane, produce great moments of inertia, and con- sequently slow periods of oscillation. The machine is " logy "and probably not satisfactory for any but " hydro " purposes,' in which case a " snappy " machine is impossible at best. 2. Two engines mounted in the central body betweenpilot and observer, each driving its own propeller through bevel gears and shafts, or by other method, the two systemsbeing independent. 3. Oi^ large engine, mounted in the central body, drivingboth propellers, one propeller at each side. 4. Two engines, mounted in the central body, with asystem of clutches connected with the transmission system in such a manner that either engine, or both engines, candrive both propellers, it being possible for the pilot to shift during" flight. The last system presents many advantages over the others,but it is entirely possible that excessive weight and com- plexity will render it impracticable. The system, as a wuole,must be reliable. In the design of any system of transmission for the two-propeller arrangement the engineer must bear in mind that the structure of the wings supporting the propeller andtransmission is very light and rather flexible, usually vibrating during flight. The information at hand indicates that, to date, nosuccessful aeroplane of the two-propeller type has been developed, but it is urged that the possible advantages aresuch as to warrant great effort on the part of engineers toward this improvement. Methods of reducing vibration.—The problem of reducingvibration of the aeroplane in flight, initiated by the engine, is a serious one. It is difficult to realise, without actualexperience, the viciousness of this vibration, especially when the engine is of the eight-cylinder type, even though it isrunning normally. After one experiences this vibration, it is easy to understand why ignition systems, gasoline-supplyjoints, water-cooling systems, delicate instruments, and even wire terminals and structural joints of the aeroplaneitself, deteriorate so rapidly. The vibration throughout the aeroplane can, of course, be reduced by better design of theengine mounting, but we cannot hope to eliminate it entirely in this manner, if the engine itself is not of the proper design. Co fhurt tee Cut vtf (Fou, Qi Mai Co, 1k i I • £> ,»k htdnts > •n^rt- Tortff to Ceibttreltrs Fig. 1.—Gasoline supply system (suction pump) formilitary seaplane. We must not, in this connection, get the idea that the engineis always operating at the same speed during flight. We can, for instance, if flying at extremely high speed, turn thecrankshaft over at, say, 2,000 r.p.m. ; whereas, if our sole object is to remain in the air without losing altitude, as whenspotting for artillery fire, we can use a crankshaft speed of, say, not more than 1,200 r.p.m. The vibration at anyspeed should not be excessive. Starting motor for engines.—The development of light JANUARY 25, 1917. starters is a matter of immediate importance. For instance,a seaplane equipped with two engines, one out on each wing, would be utterly useless without reliable starters. It seemsquite probable that electric starters will be preferable, if the weight can be reduced sufficiently, and if the danger ofspilling electrolyte be eliminated. It appears that any engine of over 140 h.p. requires a starter. Reliable provisionfor starting the engine in extremely cold weather is necessary. Gasoline supply system.—To date none of our pilots isanxious to fly cross country with any except gravity feed. The gasoline-supply system, Figs. 1 and 2, required by theUnited States Army for twin-engine seaplanes, is as follows :— The flow of fuel shall be from the main supply tank incentral body to the gravity service tank located at the centre of the upper wing ; from gravity service tank by gravity,along the lower wing panels, to the small headers at the carburettors of the two engines, and from the small headersin each case to the carburettor. These tanks shall have fuel capacities sufficient for operation at full-rated power, asfollows :—Main supply tank, 4 hrs. 35 mins.; gravity service tank, 25 mins.; each header to carburettor, 1 min. Thedesign and material of the gasoline supply system throughout shall be such as to obtain extreme lightness as far as con-sistent with strength and resistance to corrosion. Main gasoline supply tank in central body.—This shall bedivided by one vertical longitudinal bulkhead and one vertical transverse bulkhead into four gasoline-tight compartments.Proper swash-baffle plates shall be installed. The tank shall be o^ sturdy construction throughout. The main tank shallBe of such shape as to properly fit the central body. It shall be securely fastened in the structure of the central bod 'in such a way as to be undisturbed by any possible motion of the aeroplane. The structure shall be such that the tankwill withstand an internal pressure of at least 7 lbs. per square inch without leakage of gasoline. The design shall be suchthat there will be no ill effects from drumhead vibration. Suitable means shall be provided for quickly and convenientlyfilling and for completely draining all four compartments. Each filling hole shall have a suitable screen filter, 100 meshto the inch. Plugs or caps for filling holes shall be airtight and provision shall be made for " safetying " them positivelyin place. Suitable gaskets shall be used. Provision for reducing to a minimum the rate of leakage due to bulletholes by lining the inside of the tank with a special material, is highly desirable. Suitable gasoline-supply gauge shall beinstalled. There shall be leads from the bottoms of the four compartments to the upper gravity service tank. Supply of gasoline from main to gravity service tank.—Thisshall be by two methods :—First : Air-fan driven pump, so Fig. 2.—Gasoline supply system (air pressure) formilitary seaplane. designed as to maintain proper air pressure in or suctionfrom the main tank system, and to operate satisfactorily during flight. An alternative and better method will beto instal two such fans, each fan maintaining pressure in any two of the four compartments of the main tank. Whenany one or two of the four compartments of the main tank leaks (because of bullet hole or through other cause), anarrangement by which pressure or suction can be maintained through the leads from the tight compartments is highly 96
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