268 TRANSACTIONS OF SECTION G. 



iiifinite, thus accounting for the unpleasant sensation and its limitation to a 

 very narrow zone near the plane of rotation. 



A systematic study of the phenomena involves an examination of the theo- 

 retical sound effects due to moving surfaces and moving sources, and certain 

 difficulties present themselves which were evidently anticipated by the late 

 Lord Rayleigh in his "Theory of Sound." So far as these difficulties are 

 due to physical considerations I find that they in great measure disappear, when 

 instead of working with the velocity potential we notice that it is on the con- 

 densation that the sound effects mainly depend. At this stage the mathe- 

 matical work becomes rather heavy, and confirms Messrs. Lynam and Webb's 

 use of Bessel's Functions in this connection. I have, however, given a simple 

 graphical construction for the vibration curve due t-o a revolving source, with 

 special reference to cases in which the velocity of revolution exceeds the velocity 

 of sound. The fluctuations producing the effect of sound are due partly to 

 the varying distance of the source from the observer, and partly to the varia- 

 tions in the interval between the time of emission and the time at which the 

 disturbance reaches the observer. An important practical application consists 

 in determining theoretically the law according to which the intensity of the 

 sound should diminish with the distance as well as its dependence on the rate 

 of revolution. So far as can be ascertained at present, the effect of a revolving 

 source is more like a doublet than a variable fixed source, the condensation due 

 to the former varying as the inverse square, and of the latter as the inverse 

 first power, of the distance. On this assumption the sound of the exhaust should 

 be heard further off than that of the airscrew, even when both are of the same 

 pitch. 



For arranging the Farnborough experiments thanks are due to Mr. McKinnon 

 Wood and Mr. Lynam. 



6. The Problem of Steep Landing and Short Run by Wind Tunnel 

 Investigation. By E. Eolleston West, D.S.O., B.A., 

 A.M.l'.C.E. 



With the advance of commercial aviation, the problem of lajiding in a small 

 area over obstacles is assuming great importance, both from considerations of 

 safety and expense. The two essentials to its solution are steep gliding angle 

 and elow landing speed. Till now these two desiderata have been mutually 

 contradictory. 



Mechanical landbrakes are inadequate, as an aeroplane when landing is largely 

 supported still on the wings. There remains aerodynamic methods. 



With the engine cut off, two air forces act on the machine, the Lift L and 

 Drag D. The ratio L/D measures both gliding angle and efliciency, hence, 

 unfortunately, the more efficiency the flatter the gliding angle and the larger 

 the aerodrome required. 



The three aerodynamic requirements for landing in a small aerodrome are a 



arge drag coefficient K^. a large lift coefficient Ky. and a small Ky/K^ ratio. 

 The second is important, since landing speed V varies as ~y^- and energy of 



machine to be dissipated varies as V^. 



A wing flown near its stalling angle complies with these three requirements, 

 but insufficiently. 



Experiments carried out in the wing tunnel of the Aircraft Manufacturing 

 Co., Hendon, showed that flaps along the wing greatly increased drag, but 

 reduced lift; hence landing speed was high and lateral control was also 

 impossible. 



A normal plane between wings also spoilt lift, and filling in undercarriage 

 struts gave inadequate drag. 



Experiments on a special aerofoil showed that dipping the trailing edge gave 

 high lift, but not very large drag ; while dipping leading edge gave very high 

 drag and a slight reduction in lift. In the case of dipping the trailing edge, the 

 aeroplane would have to fly vers', much nose down so as not to stall. In the 

 case of the dipping leading edge the opposite effect is obtained and the attitude 



