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Aviation History
1927
1927 - 0291.PDF
31 APRIL 28, 1927 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT have the same high efficiency, the aspect of commercial aviation should be considerably changed, and the day brought much nearer when aviation can pay for itself without Government sub- sidy. The subject is one of such importance that the present design appears to merit close inspection. In his nontechnical article Mr. Martin says: "It will amaze the thoughtful business man to find that more than half of the resistance of the present-day, or ' bird- cage,' type of aeroplane is of the useless variety." He then goes on to explain what this high drag or resistance means in loss of speed and carrying power, and in increase in fuel consumption. Abbreviating the title of his "" birdcage " type of machine to BC (one does not know whether a subtle piece of humour is meant to be conveyed by this choice of reference letters!), Mr. Martin com- pares the economy of the '• BC " machine with the P.M.3. Briefly, his argu- ment is as follows :— If the horse-power of the two machines is torn- pared, i.e., power required to drive the two machines *- 8-4."- 17009, 4-3-8' 76 P.M.3. 2-0" 110 120 ISO 1*0 ISO 160 170 180 ISO 200 SPtEO IN MILtS PER HOUR Gottinne^1 J P-M.3: Performance curves based upon the area 1 78^° el results- The mai° data used are: Wing1 ' °3 sq- ft.: engine power, 1.600 b.h.p. Total loaded weight, 17,000 lbs. THE MARTIN P.M.3 : General arrangement drawings. at the same speed, say of 110 ni.p.h.. the thrust of the " BC " machine, which Mr. Martin claims to have, at that speed, an L Dof 4 only, will require to be one-fourth of the total weight. At the same speed the L D of the P.M.3 is 20, and the thrust required is only one-twentieth of the total weight. If the paying loads are compared, it is found. Mr. Martin says, that the " BC " machine weighs, roughly, as much as it carries, i.e., if the machine weighs, without fuel. 10.000 lbs., it can carry 10.000 lbs., and its total loaded weight will be 20.000 lbs. With a lift-drag ratio of 4. this machine would require horse- power for ;~>.000 lbs. thrust at 110 m.p.h.. and a quantity of fuel in proportion to this power. The P.M. type, on the other hand, would only require power for 1,000-lbs. thrust at the same speed. If the P.M. machine required 200 lbs. of fuel per hour to drive it at this speed, the " BC "' 'plane would require 1.000 lbs. of fuel per hour, or, roughly, 7,000 lbs. on the New York-Chicago trip, leaving 3.000 lbs. for paying load. Assuming that the light weight of the P.M. machine was the same as that of the " BC." the more efficient machine would have a pay load on this trip, at the same speed, of 8.600 lbs. Mr. Martin points out that this supposition about weight would be wrong if the P.M.3 motors were proportioned for only 110 m.p.h.. because in that case the motors would weigh one-fourth of the weight of those required to drive the " BC" type. He claims that this difference in motor weight, together with the simpler construc- tion of the P.M. machine, would equal the fuel weight, so that the New York-Chicago passenger and fuel load for the P.M.3 at 110 m.p.h. would be 10,000 lbs. The P.M. machine has four times the required power, and, in fact, is claimed 262c
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