NA TURE 



[October 7, 1922 



without the aid of a motor. All the three names 

 mentioned above are really unsuitable. The term 

 gliding is reminiscent of descent in an aeroplane, 

 while the real interest of recent events has been in 

 the fact that pilots were able to stay in the air very 

 long without the help of a motor, and in fact per- 

 formed climbing feats. The term soaring is less un- 

 suitable, but it suggests climbing as the essential 

 thing, whereas, in reality, horizontal flight in a glider 

 is just as different from aeroplane flight as climbing 

 in a glider. Finally, sailing is quite inappropriate 

 as a description of the flight in question. Perhaps 

 the term wind-flight is a really suitable name for 

 flying without a motor, as distinguished from engine- 

 flight in an aeroplane. 



The wind is indeed the main instrument of motor- 

 less flight. Whether birds and other natural flyers 

 do or do not derive energy from the air in some mysteri- 

 ous manner of which we have, as yet, no knowledge is 

 a question that does not arise in the present connexion. 

 The successes achieved have been the outcome of care- 

 ful study of design and of movements in the air. In 

 construction the gliders used look like aeroplanes 

 without engines, and the determining factors in the 

 flights were the various types of winds that blew while 

 the machines were in the air. 



It is clear that in a quiescent atmosphere the net 

 result of any motion through the air in a motorless 

 machine must be a diminution in the total energy, 

 i.e. in the sum of the kinetic and potential energies. 

 It follows that in the absence of wind, real flight, 

 namely, flight in which the machine maintains its level 

 for some considerable time, or rises still higher above 

 the ground, is not possible without a source of energy 

 like an engine. It is the presence of wind that puts 

 in the hands of the pilot a source of energy, which 

 can be used to neutralise the loss of energy involved 

 in motion through the atmospheric resisting medium. 



Although it should be obvious that the wind must 

 be upwards or unsteady in order to supply this energy, 

 it is necessary to say a few words about the case of a 

 steady horizontal wind, since it has been claimed that 

 " once the airman has left the ground he gets his 

 energy from the wind, which may be level and steady." 

 This is not correct, as can be proved quite simply. 

 If we write down the equations of motion of a glider 

 through the air under the action of gravity, we get 

 three types of terms : 



(1) Accelerations in terms of the motion of the 

 glider relative to the earth ; 



(2) Gravity components ; 



(3) Forces and couples due to air resistance, these 

 being functions of the motion of the glider relative 

 to the air. 



It is useful to write the first terms, the accelerations, 

 with reference to the motion of the glider relative to 

 the air. When this is done for a steady wind, the 

 resulting equations are exactly of the same form as 

 if there were no wind at all, since the moving " air 

 axes " move uniformly as seen from the " earth 

 axes." This means that when there is a steady wind, 

 we get the actual motion of the glider as seen from 

 the earth, by adding the velocity of the wind to the 

 motion of the glider in still air ; in other words, to 

 an observer travelling with the wind, the motion of 



NO. 2762, VOL. I IO] 



the glider would not reveal any effects that can be 

 attributed to the steady wind. 



In a horizontal steady wind, therefore, real flight is 

 no more possible without an engine than in absolutely 

 windless air. Any argument that leads to a contrary 

 conclusion must have a fallacy somewhere, if we are 

 to have any confidence in the principles upon which 

 all our mechanics are based. It is true that a steady 

 horizontal wind can be used as an aid in gliding. 

 Thus, by pointing his machine into the wind the pilot 

 can get off the ground with less initial speed than in 

 still air. Further, when the machine is already in 

 the air the pilot can, by pointing it with the wind, 

 increase the horizontal distance travelled before 

 reaching the ground again. But a steady horizontal 

 wind cannot make the machine stay at the same level 

 in the air for any length of time, or climb. For these 

 purposes the wind must be upwards or variable. 



If the wind is steady, but has an upward component, 

 it helps in the attainment of real flight, which we can 

 call wind-flight. Thus, if a glider is so constructed 

 that in still air it performs a straight line glide with 

 speed U at gliding angle 6 below the horizontal, then 

 a steady wind of speed U, blowing at an angle 6 above 

 the horizontal, will keep the glider suspended in the 

 air indefinitely, if it points into the wind. And, more 

 generally, if the steady wind has speed U' at an angle 

 0' above the horizontal, where U' sin #' = U sin d, 

 then the machine will fly horizontally with speed 

 U cos - U' cos 6' relative to the earth, if it is given 

 this speed initially against the wind. If U' sin 6' is 

 greater than U sin 6, so that the vertical component 

 of the wind is greater than the rate of vertical fall of 

 the glider in still air, then the glider will climb with 

 horizontal speed U cos 9 - U' cos 6' and upward vertical 

 speed U' sin 0' - U sin 6. 



These results are simple and obvious. Given a 

 steady wind with sufficient upward vertical component, 

 a glider can perform real flights and make evolutions 

 similar to those of ordinary aeroplane flight. 



It is not necessary, however, to postulate steady 

 upward wind. If the wind is variable, and this is, 

 of course, usually the case, energy can be derived 

 from the wind, even if it is horizontal, or downwards. 

 This can be seen by a little analysis based on the 

 ordinary equations of motion of the glider. Thus, 

 suppose that the wind is in a straight line, but of vary- 

 ing speed. If we write the accelerations in these 

 equations in terms of the motion relative to the air, 

 we readily find that the motion of the glider relative 

 to the air is the same as if the air were at rest, and a 

 force per unit mass were given to the glider, in a direc- 

 tion opposite to that of the wind and proportional to 

 the acceleration of the wind. If the wind rises steadily 

 from zero to U' in time t, the motion of the glider is 

 found by taking the air to be at rest and assuming 

 that on each unit mass there acts, in addition to the 

 weight, a force Vjgt in a direction opposite to the 

 wind. 



If, then, the machine is pointed into the rising wind, 

 and the wind varies quickly enough, flying becomes 

 possible. If the wind is being retarded, similar pro- 

 pulsive effect is obtained by pointing the machine 

 with the wind. It follows that in a fairly sudden gust, 

 which can be taken to consist of a quickly increasing 



