236 TAKING OFF OF AEROPLANES. 



(2) Aerodynamical ix'ciiliarities, owin^' to tlie teiuiity of 

 the almosplieie. 



Ill the former chiss the difficulties were not to be foreseen, 

 and much valuable information has ])een obtained as a result 

 of these flights. 



The latter class of difHcully, on the other hand, was 

 one that could have been ])redicted with certainly by anyono 

 who has had experience in the testing' of the performance 

 of aeroplanes under varying- conditions of the atmosphere. 

 Much trouble would have l>een saved if some fig-ures had 

 been worked out beforehand as to the maximum load per 

 horse power allowable under the unfavourable conditions of 

 low air density to be exj)ected in the hot and high tableland 

 of Central Africa. 



As Khodesia and the greater pait of the TTuion of South 

 Africa offer the disadvantageous conditions of high ground, 

 often at elevated temperature, it seems worth while to 

 consider in what way the ideas of aviation as obtained from 

 experience in Eiirope must be modified when applied to this 

 country. 



I. — The Effect of Temtekatuue and Altitude. 



As a result of a vast amount of investigation Avith the 

 greatest possible variety of machine during the late war, the 

 ])erformance of a machine in the air can be predicted with 

 a very fair amount of accuracy. The factors Avliich determine 

 tlie air speed on the level and the rate of climb are : — 



(1) The loading in i)ounds per horse power. 



(2) The wing-loading in pounds ]jer square fo(jt. 



(3) The density of the air. 



(4) The propeller efficiency, i.e., the proportion of the 



horse power Avhich is used in propelling the 

 machine. 



When these factors are taken into account, the perform- 

 ance of all machines, from the lightest scout to the heaviest 

 bomber, seem to fall into the same scheme. 



The propeller efficiency is the most uncertain term in the 

 work. It may reach about TO per cent., but the efficiency 

 depends on the air speed, and at the slow speeds of running 

 along the ground before taking off' it certainly will be 

 considerably lower than this figure. 



The most serious effect of a diminished density is in 

 lowering the horse power of the engine. At each stroke a. 

 smaller mass of air is drawn in. Though an " altitude 

 control " may correct the consequent over-richness of the 

 mixture, nothing but a motor comijressor or similar con- 

 trivance will obviate the reduction of energy supplied by each 

 stroke. 



