244 . TAKIXG OFl-' Ol' AEllOPLAXES. 



extivme case of 0,000 feet at 100° F., the rate of climb is 

 only 102 feet per minute. It is also clear that the superior 

 performance of the " Yoortrekker, " Avhich finished the flight 

 to Capetown, is not due, as was supposed by some, to 

 superiority of the machine or its eng-ine, but merely to the 

 fact that its loading- was more suitably tempered to the 

 disadvantag'eous conditions of altitude and temperature. 



Y. EfIECT OE lluillDITY. 



It is worth while considering- the eliect of Avater yapour 

 in the air on the performance of the machine. For a given 

 pressiu-e the amount of dry air in a given volume diminishes 

 with increase of humidity, and the effect, unimportant at Ioav 

 temperatures, might become important at high temperatures. 

 For example, saturation at 100° F. will decrease the relative 

 density of the dry air from 0-740 at 0,000 feet to 0089, :i 

 change of TO per cent. The factor of reduction of engine 

 poAver is thereby reduced from 0'78() to 0-(i77, a reduction of 

 8 per cent. The relati\'e density of the dry air is AA'liat must 

 be used in the engine factor, since the vapour drawn in at 

 each stroke is passi\'e as far as combustion is concerned. 

 On the other hand, the aerodynamic effect of humidity is 

 negligible, the square root of the density being veiy little 

 altered by tli^ presence of Avater A^apour. In Fig. 1 the 

 alteration of engine power factor for presence of saturated 

 water A-apotir is shoAvn. 



It appears as if even the extreme case of saturation at 

 100° F. Avould only produce al)otit 8 per cent, increase in the 

 run to take off. 



It does not appear therefore that (apart from the effect 

 on the fabric and the extra Aveight due to deposition) humidity 

 should have mtich detrimental effect on the performance of 

 the aeroplane. 



VI . — CojS'clusion . 



It is clear from the above considerations thai the vIcaas 

 as to possible loading per horse poAver gained from experience 

 during the war, more particularly from bombing machines, 

 must be modified when applied to flight in Central and South 

 Africa. One of three courses is open: Either (1) otir aero- 

 dromes mtist be made larger (say tAvice as big as under 

 Etiropean conditions) ; or (2) the loading of the machines in 

 pounds per horse poAver must be considerably reduced ; oi- 

 (3) some effectiA'e way of OA'ercoming the loss of horse poAver 

 Avith diminished density mttst be employed. 



With regard to (2), long- non-stop flights musi give place 

 to shorter flights of three or four hundred miles or less. 



