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Aviation History
1931
1931 - 0175.PDF
FLIGHT, FEBRUARY 20, 1931 -teart "• 'irst calculating the free air thrust horse power and"^h' by the slip factor. The result is the same, but an iis? y pnerati' ^ is saved in the routine work. Ch'ning 8 and 12. Nett efficiency = ^ I A_BCIT 1 K/Tf (13) Fo'- ;• particular airscrew and body combination "if 3-A-KB-S,A-BC 1 —— "J L is a constant. The thrust horse-power (T.H.P.) is obtained by multiplyingthe B.H.P. (P) by the nett efficiency. The whole of the cal- culations is for the purpose of obtaining the T.H.P. at various speeds. Full Throttle Characteristics By the above method the only curves necessary, excepttor those given (Figs. 1, 2, 3, 4 and 5), are those connecting the B.H.P. and the R.P.M. of the engine (this is supplied bythe engine makers) also the P/«3 and R.P.M. at the same altitude.These curves can be used for any aircraft using that par- ticular engine, and for any altitude by means of equation 6,where P/K3 always refers to ground level conditions. Where the power curve (as in the case of a superchargedengine) is available only at the rated altitude " x " ft. equation <i is replaced by P 1—EF—at x1 ' O, a — X »' aa \2TT-P-D5at x1 I/ 550 (14) Knowing the R.P.M., the aeroplane speed is obtained from . _ Jj-P, R.P.M. ~ T-467 ~ 60~~ 1111>h (IS) The efficiency and thrust horse power available on fullthrottle are then obtained by means of 13. Cruising R.P.M. and B.H.P. (Engine Throttled) At any given value of J j the value of I' n" is constant at thesame altitude. This means that the power absorbed by the airscrew isproportional to the cube of the rate of revolutions, the upper limit being placed on the revolutions by the maximum powerthe engine can supply. Starting with any value of Jx (say 0-4, 0-5 or 0-6) andcorresponding P/w3 from equation 14, the B.H.P. for any R.P.M. can be obtained by multiplying the numerical valueof Pjn3 by M3 = revs./sec3. A series of points can be obtained at various values of Jjand «, the corresponding aeroplane speeds being obtained by means of equation 15. At any given value of Jx the efficiency is constant so thatthe thrust horse power is calculated in the usual manner as for the full throttle case. In this way the constant R.P.M. curves for the long rangeAvian (FLIGHT, November 7, 1930, p. 1215, Fig. 3) wereobtained. The points where the T.H.P. at constant R.P.M. and the curve of T.H.P. required for level flight cross, give thecondition under which the aircraft is flying level. If the aircraft is flying at any other point along these curves,it merely signifies that height is being gained or lost. j 1 /1/ FIC5 1 •06 — —' PIC / / / 32 0-2 4-6 n PE "JI 06 07 08 09 '0 . MAXIMUM /^IDTH OF 61ADE % this c 20 0 T \\ \ FK \ \ 35 ^- A 6 8 l-O 1-2 i PE 4 '-6 "version from K<? Qr to P/M3 and by means of the"ing P/«3 with R.P.M. and H.P. at rated altitude, ;S'~p between R.P.M. and H.P. at rated altitude and by reading off the values of R.P.M. and H.P.,to P/«», and multiplying H.P. by the appro- ;ng to the altitude under consideration. Fig. 6 showsvariation of density andengine power with altitude. 169 72 000 20 000 16.000 6.000 t '400° tL " 12.000 1 10.000 D QC 2 8000 6000 4000 ZOOO 0 \ \ Fli I 1 cr • REtATIVf /SFMS/fr Pe • f/VG/fff POHW? FAC7O/t \X 5 6 \ \ \ w \ 5-6 7 8 -0 10
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