186 SYMPOSIUM ON AERONAUTICS. 



propellers is in accordance with these indications. There remains, 

 however, a margin of uncertainty regarding the influence due to the 

 neglected viscosity and also a query as to the amount of error which 

 would be introduced by using lower tip speeds for the model than 

 for the full-sized propellers. 



These two queries therefore stand out, representing two problems 

 which press for solution and which lie at the foundation of the 

 investigation of air propeller operation through the use of reduced 

 size models. 



We must therefore admit that the application of the law of 

 kinematic similitude, in the form commonly employed, to experi- 

 mental research on air propellers by means of reduced models, lacks 

 full authority in rational theory, and as a result the real justification 

 must come from experience. This means that the tests on models 

 and their interpretation in terms of full-sized propellers must rest 

 ultimately on carefully determined results given by the corespond- 

 ing full-sized propeller. This does not imply, however, that all 

 model measurements need to be checked by corresponding experi- 

 ments on full-sized propellers, for if so there would be no object in 

 the model experiments ; but rather, that a selected number of 

 experiments should be carried out, here and there over the field of 

 propeller forms and proportions, thus establishing the presumptive 

 degree of accuracy in model experimental work. With such margin 

 of error known, model experiments could be used freely, with suit- 

 able corrections if necessary, and the results would then have all the 

 accuracy which can attach to model experimental work corrected 

 by reference to direct experiment on full-sized forms. 



So much for the propeller itself. It must be remembered, how- 

 ever, that the propeller is but the connecting link between the prime 

 mover and the aeroplane, and that no matter how excellent the pro- 

 peller in itself, it must be adapted to the prime mover and to the 

 aeroplane in order to secure a harmonious and efficient combination, 

 or rather all three must be adapted each to the other, and it is in this 

 lack of adaptation that much of the trouble with and inefficiency of 

 the screw propeller in actual use arises. Thus no matter how effi- 

 cient the propeller itself at a suitable value of the slip, if it is too 

 small for the aeroplane, the slip will become excessive with corre- 



