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Aviation History
1934
1934 - 1414.PDF
26 SUPFLSMBMT TOPLIGHT APRIL 26. 1934 THE AIRCRAFT ENGINEER The condensers, of course, have to get rid of the " latent heat " of the steam and the quantity of steam generated is a big factor in the design. If the rise in temperature of water round the cylinder jacket is 20 deg. Fahr. the quantity of heat carried by 1 lb. of water is 20 B.T.TL, whereas the quantity of heat carried by 1 lb. of steam is 966 B.T.TT. It is a well- known fact that no steam is produced until the tem- perature of evaporation is reached, and this is controlled by the pressure. Whilst evaporation is taking place the temperature remains the same until all the water is evaporated. The heat supplied to the water after raising it to evaporative temperature until it is all converted to steam is termed the " Latent Heat " of steam. As the heat transferred from the surface of the wing condensers to the atmosphere must pass through the boundary layer, the condition of the layer becomes of great importance and at some point between the L.E. and T.E. the laminar layer becomes turbulent and is effected by the steadiness of the air advancing on the L.E. of the wing. Hence the importance of keeping the surface of the wing condensers as clean as possible. Having obtained the quantity of heat to be dissipated in terms of h.p. from the engine manufacturers, make Then C = '—- (1 • WkL* + 12 • 27k% 4- ] 7 • 54) (vad)-u ^^ Example:—An aeroplane with a wing section of 8 ft. chord, giving approximately a perimeter of 16.32 ft., is required for tropical summer conditions with a mean steam pressure of ^ lb./sq. in. within the wing con- denser. The total length of condenser is 40 ft. and cooling surface is of " standard properties," i.e., 25-1 per cent, of top surface and 15.2 per cent, bottom surface. What h.p. can be dissipated at an altitude of 9,000 ft aond an air speed of 150 ft./sec. with a /cL of 0.3P C=67'6X4Q(21-34)106 -692 X 16-32) + (1-022)-26 = 142-3 b.p. see Tables I and II R & M 1481 On climb there is an additional amount of heat to be dissipated, depending upon the rate at which the boiling point, corresponding to the pressure, is falling as the altitude increases. At 5,000 ft. the boiling point of water falls about 1.05 deg. C. in a 1,000 ft. of pres- sure altitude, so that with a steam cooling system a 1,000 ft. ascent has the same effect as a supply of 200 190 180 RO 160 150 KO 150 HO 110 I0&90 80 70260 50^40 30 20 10 0 10 20 30 40 5$ 60 70,? 80 30 100 110 120 130 140 150 160 170 BO f / f t /I KL- 125 / 7~ t< / / A s / / / r / /\ 1 "I 1+ 1\ // /K L-32 |/ \\ \\ / / I\ // L\ LOWER SURFACE ifA" I/O r,V "I— 30^- 4 OX 50^- 6 )% 70,Z- 8 90^- .g $2. 8 UJ <Dz 5 FRO M LE A uJ $ IOOA- 1002- V> \ -- —t \ -•« UPPER SURFACE \ 1 L \ ^ \ \ \ \KL-I25 \ \\ l \ \ \ \ •32 \ \ v \ N\ \ \ \ ft HALEY Fig. VI. : On left of datum line, cooling from lower surface compared with total cooling of standard surface at the same kL. On right of datum line, cooling from upper surface compared with total cooling of standard surface at same kL. a layout of the wing condenser using about 25 per cent. of the top surface and 15 per cent, of the bottom sur- face, making the area of the surface approximately 0.2015 dh, where d = total profile of aerofoil surface in feet and L = length of L.E. occupied in feet. These proportions will be governed largely by the design of the wing and wing tips, and must be tried out. To obtain the amount of cooling from the condenser let C = total cooling in h,.p., d = total profile of aerofoil section in ft., i- = length of L.E. occupied in feet, v = speed of oncoming air in feet per sec.; o- and p. are the density and viscosity of the air, expressed relative to I.C.A.N. ground level values, M = the mean tempera- ture difference in deg. C, 7fL = a function of the lift coefficient. sensible heat sufficient to raise the cooling water and its associated metal 1.05 deg. C. If We is the water equi- valent of the system which must be maintained at the boiling point corresponding to the pressure, and " R '; the rate of climb in feet per minute, then this addi- tional cooling is about 1-05 Well .X TZ—. horse-power.1,000 23-6 Two distributions of emissivity taken from results studied in R. & M. 1163 are illustrated in Fig. VI. one referring to a fcL value of 0.125 taken as typical for level flight and the other with a 7cL of 0.32 as typical of climb conditions. The abscissa; are distances measured from the L.E. over the profile towards the T.E. expressed as percentage of the profile distance 410 6
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