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Aviation History
1909
1909 - 0014.PDF
4'' JANUARY Z, 1909. take flight without its launching gear, hence it is not legitimate to attribute its relative lightness to the superiority of its design.* Horse-power.—The next point of comparison is that of the horse-power employed as related to the weight and velocity, thus touching on the question of the relative efficiency of the two machines. The author has shownt that for equal perfection of design the resistance to flight of two machines of equal weight is approximately independent of the velocity of flight, consequently the horse-power will vary directly as ihe velocity of flight, and the Voisin machine is entitled to more power both on account of its greater weight and on account of its greater velocity ; in the absence of more exact information we may take the velocity of the Voisin machine as being 10 per cent, greater than that of the Wright, this is roughly in accordance with the figures already given. The declared b.h.p. of the motors is sometimes not very reliable ; it is customary to use the expression in a rather elastic manner. Let us make an estimate based on the cylinder dimensions and revolution speed of the two engines, assuming the same mean pressure for both. Employing the figures already given, and for the purpose of comparison assuming a mean pressure = 72 lbs. per sq. in., as appearing at the brake, we have, at the speeds coriesponding to the declared b.h.p. :—• bore. stroke. revs. b.h.p. Wright 4-25 ins. x 41ns. at 1200 ... 247 Voisin (Antoinette) ... 4-35 ins. x 4*15 ins. at IIOO ... 492 which agree remarkably well with the declared h.p. in both cases. It is still questionable whether the declared speeds of revolution are those actually employed in flight. The author believes that in both cases the speeds are, if anything, under stated, at least for the ordinary conditions of flight; they may, however, be taken in good faith, and we accept as, a fact that the b.h.p. supplied to the Voism machine is almost exactly double that fitted to the Wright. On the above basis the Wright machine is fitted with 1 b.h.p. for every 45 lbs. sustained, which rate would give the .Voisin machine 34 b.h.p., or, allowing for the difference in the speed of flight, 38-5 b.h.p. should be sufficient to place the machines on an equal footing. But the actual b.h.p. of the Voisin is 49"2, or an excess of about 28 per cent., and this excess must either be accounted for as a swplus of power, the measure of which is the rate that the machine can increase its altitude, or it represents a loss of efficiency in the propulsion or sustentation. Now there does not seem to be any substantial difference between the reserve lifting power of the two machines ; they both appear to have about 10 or at most 20 per cent, surplus power. Mr. Wright claims more, but the performance of his machine does not seem to bear out his claim.* We may consequently infer that the- loss of power in the Voisin machine is correctly represented by the fore- going figures We will now endeavour, with the data at our disposal, to ascer- tain the cause of the loss of efficiency in the Voisin machine. The flight velocity and the motor revolution speed (together with the ratio of the gear reduction in the case of the Wright machine), allow us to calculate the effective pitch of the propellers ; we already know their diameter and from the pitch diameter ratio we can form a close estimate of their efficiency, we shall then be able to form an estimate of any remaining difference in the efficiency of the two machines. Without going into the method by which the computation of the propeller efficiency is effected, it may be remarked that the method involves the assumption that in each case the designers haveapproxi- • It has been recently reported that the Wright machine has undergone alterationsby which it is enabled to rise from the ground by its own power, whether the machine has been fitted with permanent wheels, or whether it is mounted temporarily on a trolley which it leaves behind when it rises, the account does not say ; probably it is the latter. t "Aerial Flight " ; Vol. II , " Aerodynamics," Chap. VII. X The rate of increase of altitude of a machine having a reserve of 40 per cent, would be quite sensational. Thus at 1,000 metres per minute velocity, ihe power required for horizontal flight may be represented by a loss of altitude of about 130 metres per min., and an additional 40 per cent, would give an actual rate of ascent of over 50 metres per minute. The Wright machine does not, in the author's opinion, show so great a capacity of ascent. At_ the titn_e of the author's visit a passenger of about 60 kilogs. weight was being carried ; the machine should still have been able to rise at a rate of over 1 metre in two seconds. It is on record that on one occasion Mr. Wright took up with him a passenger weighing 100 kilogs. ; but on the other hand, on another occasion, he failed, after repeated attempts, to raise another passenger of approximately this weisht. It may consequently be inferred that an addition of 100 kilogs. to the 500 kilogs. normally carried—that is, an addition of about 20 per cent.—represents approximately the limit of the capacity of the machine. Beyond this Mr. Wright, has admitted (at least to the author) that his gliding angle is about 7 degs. ; this at agiois weight of i,ino lbs. pives- 140 lbs. thrust required, and at 58 ft. per sec. the thrust h.p. becomes i4'5- Now Mr. Wright also agre- s 24 b h p. as the power of his motor, which if 40 per. cent in excess of his requirements gives 17'! b.h.p. as ordinarily utilised, or the total efficiency of gear and screw-propeller would be 85 per cent.—a manifest absurdity. If Mr. Wright's statement may be taken to mean that the thrust h.p. required is about 15 to 16 h.p., ar>d that hi-; reserve of power is 40 per .cent, to include that lost in propulsion, then the whole matter is clear. It is possible that the author misunderstood Mr. Wright's meaning. mately determined the form of best efficiency under the restricted - conditions of the pitch-diameter ratio adopted. That this assump- tion may not always be correct is obvious, but that it is somewhere near the truth in the two cases under discussion the author has been- able to satisfy himself. The method beyond this consists of a simple and elementary application of the principles laid down in the author's "Aero- dynamics," chap. IX. Firstly, to determine the effective pitch. This in the case of the- Voisin machine is given by the distance travelled, divided by the number of revolutions in the same time. In the Wright machine the result has to be multiplied by the gear-ratio. In the case of the Voisin machine, we have 66-18-3 = 3-6 ft. as the effective propeller pitch ; in the Wright machine we have 5 " _ Q-6 ft Or the 20 x 10 diameter in terms of effective pitch in the two cases is, Voisin 2'l ; and Wright "S8. The efficiencies found by the author as appropriate to these pitch- ratios are respectively : Voisin, -54 ; Wright, '68, or deducting in, the latter case 5 per cent, on account of the chain drive (certainly not a too great allowance for the power consumed by a chain running at about 16 ft. per sec), we have the efficiency of propulsion: Voisin, -54 ; Wright, -63. In the table that follows, col. I gives the foot-lbs. given out by the respective motors per revolution on the basis already employed, Wright Voisin Ft.-lbs . p e revolution . 70S i,55o 2 Ft. pe r revolulion . 29 3 Efficiency . •63 •54 4 £ m J 155 23O 5 Weight . lbs. 1,300 1,720 6 Tan . •12 •135 7 j 7° 7° 40' i.e., 72 lbs. per sq. in. mean pressure. Col. 2 gives the feet traversed by the machine per motor revolution. Col. 3 gives the efficiency of propulsion as above. Col. 4 gives the thrust in lbs. calculated from the three preceding columns. Col. 5 gives the weights of the machines augmented by an amount that would absorb the whole thrust in horizontal flight, that is the maximum weight that can be sustained in flight. Col. 6 gives the resulting value of tan y ; and col. 7 gives the equivalent in degrees. .• - ,. lo be concluded.) . T •• ' : •.'•"- ® : © ® 0 • '."..•' Aeronautical Patents. Applied tor in 1007. ' Pubtished December -^st, 1908. 26,884. M. S. BJELOVUCIC. Propellers. ' • .: Applied for in 1008. ' Published'December 31st, 1908. ' : 9,069. L. BLEKIOT. Balancing and steering apparatus. 31,363. MOTORLUFTSCHIFF-STUDIENGES. Rudders for airships. i' 3 6 (/.8 3 b 3 6 12 Months, Post Free .. 2 6 5 0 10 0 FLIGHT. Telegraphic address : Truditur, London. Telephone : 1828 Gerrard. SUBSCRIPTION RATES. .'". : FLIGHT will be forwarded, post free, to any part of the world at the . following rates: — UNITED KINGDOM. . ABROAD. 3 Months, Post Free ... 6 „ 12 Cheques and Post Office Orders should be made payable to the Proprietors, of FLIGHT, 44, St. Martin's Lane, W.C., and crossed London and County Batik ; otherwise no responsibility wilt be accepted. . Should any difficulty be experienced in procuring FLIGHT from local newsvendors, intending readers can obtain each issue direct from the Publishing Office, by forwarding remittance as above. NOTICE.—Advertisement instructions should reach the office, 44, St. Martin's Lane, W.C., by first post, Thursday. The latest time for receiving small alterations for Advertise- ments is 12 noon, Thursday. No alterations can be made after that hour. 16
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