EVENING DISCOURSES. 



Thursday, August 26. 



Sovie Requirevients of Modern Aircraft. 

 By Sir R. T. Glazebrook, K.C.B., F.R.S. 



The differences between the requirements of military and of commercial 

 flying are not merely due to the fact that the military flying machine has 

 developed into a formidable weapon, whilst commercial aircraft must be a means 

 of rapid transit and transport. 



The lecturer quoted from a paper, recently communicated by Squadron Leader 

 K. M. Hill, to characterise these differences : ' If commercial aeroplanes are to 

 compete successfully with other forms of transport, they must compete on grounds 

 of economy, speed, and reliability, but such achievement will not be of tJie 

 slightest value until a standard of safety nearer to that reached by railways 

 and shipping ie attained. The most pressing difficulties seem to be those of 

 flying to a place and landing when there is a mist down to the ground, of the 

 comparative unreliability of the light aero-engine, of the space which any 

 aeroplane requires to land in, and of the imperfect control of small aeroplanes 

 at low speeds and of large ones at any speeds.' 



The lift, the upward force on an aeroplane, is greatest for an approximately 

 rectangular wing when the long side is horizontal and at right angles to the 

 direction of flight, when the lift W = A', p i v^, where p is the air density, s the 

 area, v the speed of the machine, and kj the lift coefficient, depending both 

 upon the section of the wing and on the angle between the bottom chord of the 



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wing and the flight direction, i.e., the angle of attack. As the angle of attack 

 increases, and the wing is held more and more obliquely, the lift coefficient 

 increases until the angle is about 15 deg. ; at this, known as the stalling angle, k^ 

 suddenly drops, and the wing ceases to support the weight it has borne. The 

 actual values reached by ki before stalling differ appreciably ; 0'6 or 0"7 is about 



