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Aviation History
1917
1917 - 0095.PDF
JANUARY 25, 1917. SOME PROBLEMS IN AEROPLANE CONSTRUCTION.* By CAPT. V. E. CLARK, CAPT. T. *N this paper we shall advance for discussion, with hopes ofsolution, some important problems connected with the construction of aeroplanes intended for military uses inthe United States. Many of these problems also apply to aeroplanes built for commercial and sporting purposes.Although the lessons on type development that are being learned in the European War are of immense value to us,many conditions that we must meet are peculiar to this country. • MILITARY FUNCTIONS OF AEROPLANES. .' We will first consider the various military functions(becoming more and more distinct), as we understand them at present. It must be borne in mind that other importantuses will, in all likelihood, develop. The aeroplane itself and its uses in war are so new that it is impossible to predict,with any degree of accuracy, the developments in even a few months. At present the aeroplane is being used in war forreconnaissance, fire control, rapid transportation of important officers or communications, demolition of valuable structuresby bombing, and to attack hostile aeroplanes in order to prevent them from performing these functions. IA. Strategical-reconnaissance machines.—For this work thefuel capacity should insure a flight of at least 500 miles without stop. The average speed during this flight shouldnot be less than 80 m.p.h. The military load consists of on<j pilot, one observer, a sketching outfit, a camera, a wirelessset and navigating instruments. The general rule is becoming more and more firmly established that no military aeroplaneshould be entirely defenceless against the attack of hostile aeroplanes. This and all other service types should carryone or more machine guns, and the general arrangement of the system should be such as to permit extensive fields offire in important directions. The useful load, that is, fuel plus the military load, and the speed range, determine thepower required. A power plant of about 200 h.p. would apparently satisfy most economically this problem, theprimary requirements of the power plant being reliability and fuel efficiency. Assuming this, the fuel will weighbetween 700 and 800 lbs. The military load will be almost 600 lbs. The complete aeroplane, fully loaded, will weighover 3.500 lbs. This aeroplane would also be adapted for long-distance transportation of important communicationsor officers. IB. Tactical-reconnaissance machines.—The fuel capacity ofthis type should insure a continuous flight of at least 250 miles at a speed of not less than 85 m.p.h. The militaryload should be about the same as that carried in the strategical-reconnaissance machine. A power plant of about125 h.p. is desired, the .primary requirement being reliability. The fuel will weigh about 225 lbs., the aeroplane, loaded,somewhat less than 2.400 lbs. 2. Field Artillery fire control.—The tactical-reconnaissancemachine can perhaps perform this duty, but it appears that the fire-control machine should be slower, and that one ofits primary requirements should be an extremely good field of vision. The engine should be of 125 h.p., or perhaps less. 3. Long-ranger bombers.—We here attack a more difficultproblem, owing to the heavy useful load with which we must climb from the starting field. There will probably be a widerange in sizes of machines intended for this duty. We will discuss what we might call an average type at the presenttime. The fuel capacity should permit going out at least 200 miles and returning safely, starting with a load of bombsweighing, say, 400 lbs. The machine should be capable of defending itself from hostile aircraft, so that it can operateindependently of escort. It appears that we need at least -'50 h.p., and that, depending upon the total useful load,300 h.p., or even 3150 h.p., would not be too great. If we assume 300 h.p., the fuel weight will be at least 900 lbs.and the total military load, including bombs, about the same. • This aeroplane will weigh, loaded, between 5,000 and 6,000 lbs.4. Pursuit machines.—The function of this type is to attack and drive off hostile aeroplanes of any of the threefirst-mentioned types, preventing them from accomplishing their purpose In fact, the employment of this type shouldafford a sort of offensive defence against hostile aircraft of all descriptions. While the types 1 A, I B, 2 and 3 are interestedprimarily in objects on the ground, the pursuit type is occupied solely with events in the air. This type is at presentdivided into the one- and two-place sub-classes :—• (a) The one-place machine carries fuel for 2 hours at fullspeed, about 130 m.p.h. The pilot is the only occupant. He controls the machine and operates the machine gun, A Paper read before the American Society of Automobile Engineers. F. DODD, and O. E. STRAHLMANN. or guns, of which there can be from one to four. He usuallyaims the gun, in action, by " pointing " his aeroplane. All characteristics are sacrificed to reasonable limits in order toobtain rapid climbing ability, high speed, rapid climbing ability at high speed and the greatest possible dodging1ability, or " handiness." In the engine, reliability must lx- sacrificed to a great extent to obtain low weight per horse-power, in order that the necessary attributes of the aeroplane can be obtained. Between 90 and 130 h.p. is desired. Atpresent by far the greatest percentage of engines in this type of machine are of the rotary air-cooled type. . (b) The two-place machine carries fuel for 3 hour! at fullspeed, about no m.p.h. Space is provided for two men, the pilot and the gun operator. This is, of course, somewhatlarger and less agile than the one-place machine, and, it is believed, is rapidly losing its popularity in favour of thesmaller type. The power required is from 110 to 160 h.p. 5. Oversea reconnaissance.— (a) The long-range machine ofthis type must carry fuel for 6 hours at not less than 75 m.p.h. Two men, wireless-transmitting sot and navigating instru-ments are carried. The 300 h.p. plant used on the bomber should answer for this typesatisfactorily, the greatest require-ments being reliability and fuel efficiency. (b) The machine used for short-range reconnaissance andcoast artillery fire control must carry fuel for 3 to 4 hours at speed of not less than 75 m.p.h. Two men, navigatinginstruments, wireless and other signalling apparatus will In- required. The 200 h.p. engine used in the land strategical-reconnaissance machine should answer. SOME PROBLEMS IN CONSTRUCTION.Tt is important that engineers work out the mechanical details of a great many problems in construction, amongwhich are the two-propeller system, the reduction of vibration, the development of light engine starters, gasoline supplysystems, devices required for safe landing and improvements in wing and propeller desig-H. The two-propeller system.—When an all-round field of fireis necessary, the best arrangement is to carry the two or., three operators and th* main supply of gasoline in a centralbody and to drive the machine by two propellers, one at each side of this central body. By such an arrangementmachine guns can be fired forward, in attack, and to the rear, in retreat, with extensive fields of fire in Iroth directions,above and below, to right and to left. This attribute is always desirable, and, in some types, as for instance in thebombers and reconnaissance machines, is essential. These propellers can be either tractor screws or " pushers." Theleft-hand propeller should turn clockwise and the right-haud propeller counter clockwise. This symmetrical arrangementis a great advantage, in that it permits, equalised torque and gyroscopic effects when turning in different directions.In addition, it makes for safety, because the downward velocity imparted to the inboard parts of the two slip-streamsthat strike the horizontal tail surfaces produces an inherent 'tendency'toward nose heaviness without power and towardtail heaviness with power. We can, therefore, design so that the line of thrust is considerably above the centre of gravity,compensating for this, and obtaining another convenient feature. A fourth great advantage of such a system is tin-fact that great power can be transmitted with good propeller efficiency without demanding excessive diameter and retainingsatisfactory structural safety factors. It is highly desirable that the line of thrust of the propeller be kept belowthc centreof gravity of the aeroplane, unless the two-propeller arrange- ment, as described above, be used ; a propeller of largediameter, with sufficient clearance, necessitates a high landing gear with its many great disadvantages. It appearsextremely difficult to build a propeller of wood, of satis- factory strength (if the speed of revolution be high), givinggood efficiency, to transmit more than ifx) h.p. Peculiarly- stringent climatic conditions making for rapid deteriorationhave increased this difficulty. In fact, the tendency to reduce cylinder diameter and increase crankshaft revolutionspeed is already necessitating a gear between crankshaft and propeller-shaft in order to keep the propeller speedbelow 1,300 r.p.tn., which is considered desirable. A fifth advantage of the two-propeller arrangement is that thetotal resistance of the air to progress through it of the com- plete aeroplane while flying under power will l>e diminishedowing to the fact that less total projected area of bodies will lie in the propeller slip-streams. The velocity of the airstriking objects lying in the slip-stream is. say, 20 per cent, higher than the velocity of air not in the slip-stream. Theresistance varies about as the square of the velocity. There- 95
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