In “Practical Aerodynamics,” Major B. Baden–Powell quotes the following; formulae for the pressure of the air on a moving plane when expressed as a function of the angle of attack α, taking the pressure as unity when α=90°.

I. Duchemin

II. Rayleigh (hydrodynamical)

IIa. Gerlach

III. Renard

IV. Soreau

V. Eiffel, 1907,

VI. Another formula frequently quoted is

and a more complicated formula, depending on the aspect ratio of the plane, has been proposed by Soreau.

Consider any form of aircraft provided with a small rudder plane placed behind its centre of gravity and at a certain distance l from it, which we may speak of as the length of the tail. When this tail is subjected to a given wind pressure, the moment of this pressure about the centre of gravity, tending to turn the machine round, is proportional to l. From this point of view a long tail is preferable to a short one.

On the other hand, the pressure of the air on the tail plane depends on the normal velocity of the wind relative to it. Hence the normal velocity of the plane itself is limited in magnitude, and the angular velocity with which the machine turns about its centre of gravity must therefore be less when l is large than when lis small; that is, the long–tailed machine turns more slowly than the one with a short tail, and this is a disadvantage.

Some time ago when leaving my laboratory I happened to notice a large number of cheels gliding overhead that seemed to me to show something unusual in their mode of flight. I guessed that they were after locusts, a flight of these insects having passed over Agra three days previously. On reaching my house I found, with the help of my binocular, that my conjecture was correct. The following are the notes that I took at the time:—

IIth August, 1915, 3.0 p.m. A few small thin clouds. Air rather hazy at a height. Wind S, occasionally moving small branches.

A large number of cheels were gliding in a way that seemed to me unusual. They were at heights between 200 and 400 metres. They were generally slow–flex–gliding and did so in all directions and with frequently changing course. Two were seen rapidly losing height, feet foremost. Occasionally they dived or at least glided steeply downwards at speed.