24 



NATURE 



[September 2, 191 5 



The example of Denmark has proved a potent influ- 

 ence in the measures which have been taken to develop 

 the agricultural resources and products of Ireland. 

 The export value of eggs, butter, beef, mutton, pork, 

 bacon, and hams from Ireland was in 1912 1 1,820,356/., 

 but in 1913 we received from Denmark alone butter, 

 eggs, and bacon to the value of nearly twenty-two 

 millions sterling. 



It cannot be said that this huge difference in value 

 is attributable to a better soil or climate, and, as a 

 matter of fact, it is entirely due to the better education 

 of the people, to a more scientific treatment of the 

 soil and of the animals concerned, and to a stronger 

 sense of the advantages of co-operative effort. It is 

 clear that given these conditions, Ireland, with its 

 much larger acreage and more productive soil, could 

 raise for export the greater part of this importation 

 of food products ; and as with agriculture, so with 

 the smaller factory and cottage industries. It is made 

 clear in Mr. Fletcher's address that there is abundant 

 scope for their establishment and development, of 

 which Belgium affords a striking and pregnant 

 example, which the people of Ireland would do well 

 to follow, and thus bring the small town and the 

 countryside into close and harmonious relation. 



J. H. Reynolds. 



AEROPLANE STABILITY. 



SOME phases of fore-and-aft or longitudinal equili- 

 brium in flying are discussed by Dr. Orville 

 Wright in a recent publication of the Smithsonian 

 Institution, entitled " Stability of Aeroplanes." Al- 

 though a beginner finds most difificulty in mastering 

 the lateral control, it is his lack of knowledge of 

 certain features of the fore-and-aft equilibrium which 

 leads to most of the serious accidents. 



In an ideal flying machine the centre of gravity 

 would lie in the line of the centre of resistance to 

 forward movement and also in the line of thrust, 

 but in practice this is not always feasible, since the 

 machine must be built to land safely as well as to 

 fly. In flying, a low centre of gravity — that is, one 

 below the centre of support, causes an oscillating 

 movement about the lateral axis like that of a pen- 

 dulum. On the other hand, a high centre of gravity 

 tends to cause the machine to roll over in landing, 

 and consequently a compromise is adopted. 



The two principal methods used in preserving fore- 

 and-aft equilibrium have been the shifting of weight 

 so as to keep the centre of gravity in line with the 

 changing centre of lift, and the utilisation of auxiliary 

 surfaces, known as elevators, to keep the centre of 

 pressure in line with a fixed centre of gravity. The 

 first method has been found impracticable on account 

 of the impossibility of shifting large weights quickly 

 enough, but the second is used in most of the modern 

 flying machines. 



Flying machines of the latter type should have their 

 auxiliary surfaces located in the front or rear as 

 far as possible from the main bearing planes, because 

 the greater the distance the greater is the leverage, 

 and consequently the smaller the amount of surface 

 required. No part of either the main surface or 

 auxiliary surface should be exposed on their upper 

 sides in a way to create a downward pressure for 

 maintaining equilibrium. 



The downward pressure of air is used to some 

 extent, however, on account of its adaptability, in pro- 

 ducing more or less inherent stable aeroplanes. Dr. 

 Wright describes an aeroplane in which equilibrium 

 js maintained by an arrangement of surfaces so placed 

 that when a current of air strikes one part of the 



NO. 2392, VOL. 96] 



machine, creating a pressure that would tend to dis- 

 turb the equilibrium, the same current striking 

 another part creates a balancing pressure in the oppo- 

 site direction. '"This compensating or correcting pres- 

 sure is secured without the mechanical movement of 

 any part of the machine. While this system will 

 control a machine to some extent, it depends so much 

 on variation in course and speed as to render it 

 inadequate to meet the demands of a practical flyinu; 

 machine. 



In order to secure greater dynamic efificiency and 

 greater manoeuvring ability, auxiliary surfaces 

 mechanically operative are used in present flying 

 machines instead of the practically fixed surfaces of 

 the inherently stable type, but they depend to a 

 greater extent upon the skill of the operator in keep- 

 ing equilibrium. If the operator were able to " feel " 

 exactly the angle at which his aeroplane meets the 

 air, at least 90 cent, of all aeroplane accidents would 

 be eliminated. Instruments for this purpose have 

 been produced, but they are n9t in general use. The 

 average flier does not realise how dangerous it is to 

 be ignorant of this angle, nor does he know when he 

 is "stalling." By "stalling" Dr. Wright means 

 coming to rest in the air, or nearly so. 



The danger from "stalling" comes when the 

 operator attempts to check the machine's downward 

 plunge by turning the main bearing surfaces to still 

 greater angles of incidence, instead of pointing the 

 machine downward, at a smaller angle of incidence, 

 so that the speed can be recovered more quickly. 

 Most of the serious accidents in flying occur, after 

 long glides from considerable heights with the power 

 reduced, when an attempt is made to bring the 

 machine to a more level course several hundred feet 

 in the air. The machine quickly loses its speed and 

 becomes " stalled." Those who have seen the novice 

 make a "pancake" landing have seen the beginning 

 of a case of "stalling" which might have been fatal 

 had it taken place at a height of 100 or 200 ft. in 

 the air. 



The greatest danger of flying comes from mis- 

 judging the angle of incidence. If a uniform angle 

 were maintained, there would be no difficulty in secur- 

 ing fore-and-aft equilibrium. Experiments made 

 during the past year or two have brought about a 

 considerable advance in the development of automatic 

 stability. A device described by Dr. WVight com- 

 prises a small horizontal wind-vane so mounted on the 

 machine as to ride edgewise to the wind when the 

 machine is flying at the desired angle of incidence. 

 In case tTie machine varies from the desired angle, 

 the air will strike the vane on either its upper or 

 lower side. The slightest movement of the vane in 

 either direction brings into action a powerful mechan- 

 ism for operating the controlling surfaces. If the 

 wind strikes the vane on the underside the elevator 

 is turned to cause the machine to point downward in 

 front until the normal angle is restored, and if the 

 air strikes the vane from above, an opposite action 

 upon the elevator is produced. The author maintains 

 that a machine so controlled is not liable to " stall- 

 ing." Another method for maintaining fore-and-aft 

 equilibrium utilises the force of gravity acting on a 

 pendulum or tube of mercury, and a second employs 

 the gyroscopic force of a rapidly revolving wheel. In 

 both these systems, however, the angle of the 

 machine is regulated with reference to the horizontal, 

 or some other determined plane, instead of the angle 

 of the impinging air. Other faults render the pendu- 

 lum and mercury tube useless in regulating fore-and- 

 aft equilibrium, although the pendulum is found to be 

 useful in regulating the lateral stability. 



