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Rh thin for structural reasons and the Royal Aircraft in the early days of 1913 designed the RAF6 wing on the basis of these experiments for the development of the aeroplane BE2. At a later stage, as engines of greater power were produced, further experiments led to improve- ment of wings at small angles of incidence and RAF6 was replaced by RAFlS (May 1915). It was found that the advantages of the latter at high speeds were appreciable in spite of the increase of wing area necessary to maintain a reasonably low landing speed.

Many attempts have been made to introduce new wing forms and those showing value on preliminary test have been investigated. It has invariably been found that guesses have been inferior to the results of systematic investigation. In order to facilitate comparison all results of wing tests are-^in Great Britain reduced to a stand- ard form. Different expressions are common in France and America but neither of the latter is international in the sense of being non- dimensional. In accordance with principles of dynamical similarity, the measured forces, lift and drag, have been divided by air density, wing area and sq. of speed in order to deduce lift and drag coeffi- cients. A centre-of-pressure coefficient is obtained by expressing the position of the centre of pressure by the ratio of its distance from the leading edge to the chord of the wing. The results are usually shown in curves as well as tables and, if uniformity in scale be adopted for the curves, comparison of wings is greatly facilitated, since superposition immediately indicates the relative advantages and disadvantages.

It is clear to most workers in the subject that the angle of inci- dence of a wing is a convenient but arbitrary variable. A more use- ful relation than lift to angle of incidence and drag to angle of inci- dence is that of drag to lift, and it is very common to find in the records of the aerodynamics laboratories the value of drag coefficient plotted on a base of lift coefficient. The idea was in effect used by Eiffel in 1910 in a system of polar diagrams. When comparing wings for a given duty a still further variation is sometimes made; the area of a wing depends on the specified landing speed and on the maximum lift coefficient. Only when both these quantities are included can the criterion be of greatest value. If it be presumed that the condi- tion of prescribed landing speed is to apply to an aeroplane with different wings it can be shown that at other speeds the lift coeffi- cients of the respective wings will be proportional to the maximum lift coefficients. Hence a curve of drag coefficient on the ratio of lift coefficient to maximum lift coefficient has direct uses.

Further elaborations have been used, one of which, due to the Royal Aircraft Factory in 191 1, 1 is equivalent to the plotting of horse-power on a basis of speed. A new point is thus brought into prominence for it is seen that the choice of wing form to meet given requirements is affected by the resistance of the rest of the aero- plane. Brief notes on the character of this additional resistance will be made at this point.

The aeroplane as a whole is made up from various parts: wings for support; body for holding the engine, pilot, load and control organs, and the undercarriage for leaving the ground and alighting. The same organs are required by float seaplanes and amphibians but in the boat-type seaplanes the body and alighting gear are combined into one structure. The wings themselves are usually supported by struts and wiring which add to the resistance and there is a dis- position to test and fit wings which are designed to be strong enough to support the weight of an aeroplane without external bracing wires. It is desirable here to emphasize the fact that the result may not be an effective reduction of resistance owing to the less advan- tageous types of wing section which must be used and to the greater mechanical difficulties of construction.

The resistances of the body and undercarriage are easily appre- ciated ; both vary very closely as to the square root of the speea and are scarcely changed by alteration of the angle of incidence of the aeroplane. At high speeds the added resistance is roughly equal to that of the wings whilst for the most efficient flight the proportion is more nearly I to 2, the wings having the greater resistance. There is a loss due to the engine which is not quite so evident as that due to the body. If water-cooling be adopted, the engine may be totally enclosed and so have no direct effect on the air flow, but in order to maintain the cooling, radiators in the wind are required. It does not matter whether the engine be air-cooled or water-cooled, a certain minimum resistance to motion must be incurred to provide the cool- ing. Experiments have indicated a relationship between the heat dissipated from a hot surface and the skin friction given by the mo- tion of a fluid over that surface, and the best known radiator is the honeycomb type. Disturbance of the air by a cooling surface which is such that the motion is violently eddying involves a higher resis- tance for a given dissipation of heat.

The placing of the radiator in the wind near the aeroplane may have important secondary effects. The body is made to approach a streamline form as closely as possible in order to reduce its resistance and the approach to the best results is found to depend greatly on the choice of shape. The magnitude of the possible effects of shape on resistance is most clearly shown by experiments on airship forms. The resistance of an airship envelope is only from I % to 2 % of that of a disc which would cover the section at the maximum diameter. It is true that the aeroplane body is far removed from this condition

1 See Flight, Jan. 13 1912, " An Aeroplane Study," M. O'Gorman.

but it is still sufficiently fine to have its resistance increased by an unsuitable disposition of radiator. There is little systematic knowl- edge as to the best arrangement, and the problem of engine- cooling and body form remains one of engineering difficulty and uncertainty.

Performance of Aeroplanes. Rapid development also costly was facilitated by the construction and test of numerous aeroplanes for 'war purposes. Not until 1917 did the measure- ment of engine power and aeroplane performance in Britain reach the stage of generality and accuracy necessary for the purposes of estimate and prediction. Other countries entered the field at still later dates and it will be seen that aviation is still in early infancy. Progress is now less rapid, the main aerodynamic features having been brought to a state at which the work of all the better designers produces nearly the same result. So true is this statement tjiat curves can be drawn relating engine power to speed of flight, rate of climb and total weight curves which show what a designer can attain but rarely exceed. The greatest changes in 1917-21 were in the power plant and here limits are becoming clearly discernible. The changes in the weight of the aeroplane structure due to more advantageous use of material were also small, and in all direc- tions new advance can only be won by assiduous study. The period of striking progress is over and has given place to one in which greater training and knowledge are required than in the past. This is particularly true in matters relating to the reliabil- ity and safety of aircraft.

Stability. The idea of stability as applied to motion is very old and standard methods of dealing with mechanical problems were gradually developed by the mathematicians of the last century. Laplace applied his knowledge to an examination of the stability of the solar system, i.e. he accepted the theory of gravitation as accounting for observations and made an exten- sion to see whether the motion was permanent or in a state of change. The ideas of stability are quite different from those of performance and at the present day it is safe to say are not understood by designers with the degree of intimacy which leads to incorporation in design. It is true that some rough generaliza- tions exist and are acted upon; by placing the centre of gravity of an aeroplane very far forward longitudinal stability is en- sured whilst a rearward position tends to instability and danger. Similarly, the fin's dihedral angle on the wing is known to affect lateral stability. Present-day (1921) aeroplanes border on neutral stability for the conditions of straight forward flight and this has come about by trial and error, corrected by the likes and dislikes of a pilot during aerial fighting. So long as the pilot be alert and the aeroplane of moderate size, say less than 6,000 Ib. weight, it is possible to control the craft in the air in the condition in which it leaves the works. The few attempts to make very large aeroplanes, 20,000 to 50,000 Ib. in weight, have led to early disaster owing to the inability to approach, on such scale, the necessary degree of refinement of control and stability. Alternatively it may be said that the attempt to develop large aircraft has overstepped the reasonable limits of caution and has placed on the pilot a strain which he is physically incapable of withstanding.

Even in the smaller craft there are many which in normal flight require the unremitting attention of the pilot and which if left to themselves for a minute would be in a dangerous and probably uncontrollable condition of flight. This is not a neces- sary state for an aeroplane and there is no insuperable difficulty, given training, in ensuring, without an appeal to trial in the air, that an aeroplane will fly itself for long periods. The opinion has been expressed that aircraft of the present day would be of commercial value were the obviously removable defects dealt with. Reliability of the engine installation is probably the most urgent need, but following that comes the application of the known theories of stability.

Broadly speaking the quality called stability is readily de- fined. An. aeroplane is taken into the air and a given state of motion produced by the pilot and maintained for some time. This operation does not involve stability but requires adequate control. When flying steadily suppose that the pilot ceases