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Aviation History
1941
1941 - 2042.PDF
136 FLIGHT SEPTEMBER 4TH, 1941. ROCKET PROPULSION —II tion " /=;—20 would not be excessive, in addition to the ordinary brake effort. This would probably give a total equal to gravity, i.e., -32.2 or thereabouts; the higher the better fqr the purpose intended. 2 t-secs 3 V/OZO 240 360 Referring to Fig. 4a, the uniform acceleration due to therecoil of the rocket is taken= —20ft. /sec. per sec. as above. This, being negative, is measured downwards from thedatum line Ox starting from the ordinate at t 0. Then inFig. 4b the corresponding velocity graph is given ; the velocity, prior to the point of time t0, is taken F, = 120ft./sec, and it falls at the rate of 20 ft./sec. per sec, becoming zero at the point of time t2 after a lapse of sixseconds. If the value of V had been zero at the point O, the velocity at tz would have been retrograde ( = tf), butthe ordinate OF, represents the integration constant. The velocity-with which we are concerned is V^ — tf, whichmeans that the ground is the datum to which velocity is referred. In Fig. 4c the distance travelled during the timeinterval <„, t.,, is denoted by S, and the graph is derived by integration of the velocity graph in Fig. 4b. That isto say, from point to point the ordinate in Fig. 4c is deter- mined by the area shown shaded in Fig. 4b, being eithercalculated or measured by planimeter. Thus values of S from instant to instant are plotted in Fig. 4c, this beingthe method of graphic integration, and the distance 360ft. (scale on left hand) is the distance run before the plane comes to rest; this scale is repeated on the right of thefigure with the zero at the end of the run. In the example ' plotted the assumption has been made that the deceleration(negative acceleration) is constant, and the values of i 2 V,and S are calculated at once from the law of the falling body, thus: Initial velocity taken = 120ft./sec. ; / (const.)= -20.Hence time taken £2 = 120/20 = 6 seconds, when V is zero. Then S=Vt 12 = 120x6/2 = 360 feet. The reader will havelittle difficulty in tracing this out by the aid of the dia- gram, although the changes in the zero position are a littledisconcerting. Perhaps it is easiest to think of the con- ditions as inverted, and the process as being one ofacceleration from the point of rest as in the right-hand scale in Fig. 4c The Figs. 4a, 4b, and 4c represent the hypotheticalcase of an aircraft brought to rest by the action of the rocket alone, a constant applied force. But, under actualconditions, this would be a means to be reserved for emergency, and would be additional to the usual wheelbrakes and windage and rolling resistance ; so that, based on the data assumed, the total stopping effort would belittle if anything short of that due to gravity, as though climbing a gradient having a slope of 45 deg. Research Needed There would be many practical difficulties to be over-come in connection with the applications of the rocket to aircraft, these centring mostly round the rocket itself andthe rocket composition. The writer regards it as by no means certain that the figure for the kinetic energy in,the efflux per lb. of rocket composition, i.e., 566,000 ft. /lb., does not overstate the case ; and in this there is an openfield for experiment and research. Indeed, until this is settled and a figure based on actual experience has beenestablished, the way is blocked to further progress. It would seem likely also that the fire risk, especially inthe last application suggested, might prove an obstacle. It is an open question whether there is any seriousfuture for the rocket in its application to aircraft, and one on which the writer would not like to express artopinion one way or the other. For the general purposes of propulsion it would seem that the rocket is too severelyhandicapped by palpable limitations. The chief of these are: (1) That the range of flight is paltry in comparisonwith other methods. (2) That the thrust is not under the direct control of the pilot and cannot be made so. (3) The•extent of the "take-off boost" necessary is very much greater than in an aircraft driven by engine and propeller. But for the purpose of giving a boost to a propeller-dnven machine the rocket may well have a future. Reference has been made to the fact that the expressionV /v as the efficiency for an accelerated condition, founded on the work done being proportional to time, and theenergy utilised as proportional to S (distance travelled), takes no account of the cumulative effect of loss of weight(mass), and that these expressions, therefore, are not exact. But where, as is always the case in practice, V is small com-pared to v, the resulting error is negligible, and the com- plication introduced in the treatment is great. AIRCRAFT IDENTIFICATION CHARTS ENORMOUS interest is being taken in the identification ofaircraft, be it of friend or foer by young and old, male and female, and in or out of uniform. Often it is a hobby as wellas a duty, and prowess in recognition is thus a matter for pride. The informative identification charts produced by Flight areextensively used by the Services and in Government circles, nearly 500,000 copies having been sold to the armed forcesand the public. The following types are available from Iuffe and Sons Ltd., Dorset House, Stamford Street, London,S.E.I. 0British Aircraft.—22^'m. by I4|in., printed on card for hang- ing, TS. net, by post is. 6d. Pocket transparent chart—two celluloid cards eyeletted forease of examination, is. 6d., post free. American Aircraft used by R.A.F. and Fleet Air Arm.—22in.by i4^in., printed on card for hanging, is. 3d. net, by post is. gd. German Aircraft.—22^in. by i4jin., printed on card forhanging, is. 3d. net, by post is. gd. Pocket transparent chart—two celluloid cards eyeletted forease of examination, is. 6d., post free. Troop carriers, 12Jin. by ioin., printed on card, folded totpocket, 6d. post free. Italian Aircraft.—25m. by I7£in., printed on art paper, od.net, by post 7^.
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