ON AERONAUTICAL PROBLEMS. A417 
The ‘Boundary Layer ’. 
39. The reason why these regions of frictional effect exist is, that 
although air offers little resistance to the sliding of different layers over 
one another, it has a strong objection to sliding over the surface of the 
aerofoil. In fact, as Whetham showed thirty years ago, fluids refuse 
entirely to slide over solid surfaces at all. So the air which is in contact 
with the aerofoil is stationary, whilst at a very small distance away it is 
moving very fast ; and hence, within the immediate neighbourhood of the 
boundary there is rapid sliding, and air friction has important effects. 
We must not bemoan this fact, hard though it makes our theoretical 
problem ; for if air could slip over the surface of the aerofoil, we should 
find little resistance to motion, it is true, but we should equally find no 
‘lift’. 
What happens close to the surface of the aerofoil? That is, really, the 
ultimate problem of Aerodynamics. Somehow or other, in a film of air 
whose thickness is measured in thousandths of an inch, are generated those 
forces which produce the lifting power of an aeroplane. Osborne Reynolds’ 
theory of lubrication shows us that there is nothing impossible in this 
notion ; but our problem is immensely harder than his. Look, for example, 
at what friction does to modify the flow past a circular cylinder (Fig. 5) : 
these are two photographs kindly lent to me by Dr. Stanton. 
By the kindness of my friend Professor Karmdn, I am able to show 
some more slides (Fig. 6) in illustration of this phenomenon of ‘ eddy 
motion’. In his experiments, a circular cylinder was moved through 
water with continually increasing velocity, and the motion of the fluid was 
rendered visible by means of powder. The successive stages in the 
formation and ‘ break-away’ of the eddies are clearly revealed. 
The School of Prandtl.—Conclusion. 
40. Professor Prandtl has shown, in brilliant fashion, how much 
can be done with the theory of frictionless fluids towards “’xplaining 
the ‘lift’ of an aeroplane, provided that we are content to take 
these boundary effects for granted. It is he who now leads, with Professor 
Karman, the new attack,—the attempt to elucidate, by mathematical 
investigation, thé microstructure of the ‘ boundary layer’. To some this 
may seem a purely academic inquiry,—even as Sorby’s researches into the 
microstructure of steel were deemed to be academic sixty years ago; but 
remembering how completely the microscope has come to dominate 
metallurgy, I think they would be well advised not to be too dogmatic ! 
To me it seems the most fundamental and the most important problem 
in Aeronautics to-day. I wish that I had time to say more about it; 
but perhaps I have said enough to suggest that, both for its intrinsic interest 
and for its difficulty, it deserves study by minds as{numerous and as 
inventive as those which are probing the structure of the atom. 
1925 EE 
