HOLES IN THE AIR 55 



The reason for the fall will be understood when it is recalled that, 

 for all ordinary velocities, wind pressure is very nearly proportional to 

 the square of the velocity of the wind with respect to the tiling against 

 which it is pressing. Hence, for a given inclination of the wings, the 

 lift on an aeroplane is approximately proportional to the square of the 

 velocity of the machine with reference, not to the ground, but to the air 

 in which it happens to be at the instant under consideration. If then 

 it glides, with propellers at rest, into air that is moving in the same 

 horizontal direction and with the same velocity it is in exactly the con- 

 dition it would be if dropped from the top of a monument in still 

 air. It must inevitably fall to ruin, unless indeed rare skill in balancing 

 or, possibly, mere chance should bring about a new glide after addi- 

 tional velocity had been acquired as the result of a cons'derable fall. 

 "Warping of wings, turning of ailerons, dipping and twisting of rudders, 

 and all the other devices of this nature would be utterly useless at first, 

 totallv without effect so lnnsf as wind and machine have the same 

 velocity, for, as already explained, there would be no pressure on them 

 in any position and consequently nothing that could be done with them 

 would at first have any effect on the behavior of the machine. How- 

 ever, as stated above, a skillful pilot may secure a new glide with a 

 properly constructed machine, and finally, if high enough, make a safe 

 landing. 



Of course, such an extreme case must be of rare occurrence, but cases 

 less extreme are met with frequently. On passing into a current 

 where the velocity of the wind is more nearly that of the aeroplane, and 

 in the same direction, more or less of the supporting force is instantly 

 lost, and a corresponding drop or dive inevitable. Ordinarily, however, 

 this is a matter of small consequence, for the new speed necessary to 

 support the machine is soon acquired, especially if the engine is in full 

 operation. Occasionally though the loss in support may be large and 

 occur but a short distance above the ground, and therefore be distinctly 

 dangerous. 



If the new wind layer is against and not with the machine an 

 increase instead of a decrease in the sustaining force is the result, and 

 but little occurs beyond a mere change in the horizontal speed of the 

 machine with reference to the ground, and a slowing up of its rate of 

 descent. 



"Wind sheets, within ordinary flying levels, are most frequent during 

 weather changes, especially as fine weather is giving way to stormy. 

 This then is a time to be on one's guard against the most dangerous 

 of -all " holes in the air." It is also well to avoid making great changes 

 in altitude since wind sheets, of whatever intensity, remain roughly 

 parallel to the surface of the earth, and the greater the change in alti- 

 tude the greater the risk of running into a treacherous " hole." iUso, 



