" Experiment (2). Another tail was tried (TP2) ... A long
glide was also made with the elevator locked. During these flights
a marked improvement in the behaviour of the machine was obtained,
damped phugoids being described. ... It may certainly be said,
however, that with TPa and the other conditions of this experiment,
BE2 was proved to be capable of flying indefinitely with the elevator
locked in winds with gusts up to 30 m. per hour."
" Experiment (3). The same tail plane was fitted and the condi- tions were approximately the same except that the centre of gravity was considerably further back. . . . This very backward position of the centre of gravity, of course, made the aeroplane quite unstable, and increasing dives or rearings were performed almost as soon as the elevator was locked. ..."
" Experiment (4). The centre of gravity was brought forward and a considerable improvement was obtained. It was now found that even with the elevator free, damped phugoids were obtained. In the absence of gusts at the time, these phugoids were started by move- ments of the elevator control lever. When the machine had been forced to assume a sharp dive, the control lever was totally released and it was found that after two or three complete oscillations the amplitude became too small to be noticed. . . . The period of oscillations was found to be about 20 seconds."
" Apart from the practical utility of these experiments in develop- ing the particular aeroplane in question their wider significance un- derlies the fact that they agree with and confirm the model experi- ments on the full scale both as regards the characteristics of the tail planes and the interference of the main planes with them; and the two sets of experiments give data from which a tail can be designed for any aeroplane to give any degree of longitudinal stability required."
It appears from recent investigation of accidents that the type of instability described above is not avoided in all modern aircraft. The effect of the instability is serious and epidemic failures to control have been traced to this cause alone. 1 Some photographic records of longitudinal motion taken at a much later period will be found as re- productions in the Wilbur Wright lecture 2 for 1919. The actual time required for the testing of longitudinal stability is now so short that the production of records has been made an addition to the older established performance trials. Progress has been steady but rather slow and the influence of the tests is not yet evident in new design.
In the case of lateral stability the records of the early experiments at the Royal Aircraft Factory are of equal or greater interest with those on longitudinal motion:
" RE I rolling stability experiments, by Mr. Bush. . . . The wing flap controls were entirely abandoned and the aeroplane was flown 75 m. with two turns without their use. The rudder was used for steering or was kept straight to avoid complicating the investi- gation."
" The evolutions of the aeroplane bore out the theoretical expec- tations. Disturbance by a gust was followed by side-slip towards the low side, which brought the dihedral angle into effect, righting the machine. In both the above experiments the recovery from a roll seemed rather slow, and it was decided to double the amount by which the wings were bent up."
" The results of the above experiments were sufficiently satisfac- tory to warrant the abandonment of the warp and the use of wings with flaps for RE5 and other aeroplanes in course of design."
Rotative Stability. The rotative stability with the rudder in a fixed position was next examined. Up to this point the aeroplane had been usually steered on a straight course, which made recovery quicker. When the rudder is fixed, however, disturbance of level is followed by a turn towards the low side as well as a side-slip. If the directional stability is too great, the increased speed of the outer wing will counteract the restoring effect of the side-slip, and the aeroplane will continue to turn with increasing bank and angular velocity. The manoeuvre if not controlled ends in a spiral dive."
" Dec. 8 1913. In this experiment, the rudder was adjusted for straight flight and then held fast by the feet, friction of the heels against the floor making absence of movement certain. When all was ready the aeroplane was disturbed by the wing flaps, which were then returned to their normal position. The experiment is rather delicate, as any want of symmetry will cause the aeroplane to be stable when rolled in one direction and unstable in the other. It appeared, how- ever, that the aeroplane was just stable, righting herself slowly."
Complete stability test. The aeroplane was flown from Long Valley, Aldershot, to Froyle near Alton, and also from Froyle to Fleet, distances of 6 J and 8m., without the use of wing flaps or eleva- tor. The wing flaps were left free as usual and the elevator was locked. The flying was very comfortable, and the pilot considered that re- connaissance under these conditions would be considerably easier for a pilot alone."
For normal flight the description of lateral stability given in these abstracts still represents the position. The experiment is still delicate and it may be doubted whether any aeroplane has an appreciable degree of lateral stability. The early work on stability cleared the way to a large extent; the temptation to
'Accidents Investigation. Advisory Committee for Aeronautics, Jan. 1919. R and M No. 617, also R and M No. 629, Dec. 1918.
2 Supplement to Aeronautical Jour., July 1919.
complex design for safety was removed and dangerous instability rarely exists so long as a pilot is alert. The introduction of aerobatics and the training of pilots to loop, spin, roll, etc., at the same time as it inspired confidence in the ability to control an aeroplane also led to conditions far removed from those of normal straight flight. It was then found that the stability of aircraft under extreme conditions has great importance, particularly when the angle of maximum lift has been reached or exceeded.
A very large proportion of accidents arises from engine failure whilst near the ground. In holding up the nose of the aeroplane whilst attempting to turn back into an aerodrome, the pilot not infrequently stalls the craft and violent lateral instability results. Recovery from the effects of this instability is rare and much study has been made of the phenomenon.
There is now little doubt as to the cause of this instability but the methods of removing it are far less clear. The same cause which produces instability removes the effectiveness of the controls; it is probable that high -lift wings have charac- teristics antagonistic to those of stability and further investiga- tion of the subject is required before satisfactory design for speeds less than that of stalling can be reached.
More recent papers on various aspects of stability will be found in the reports of the societies and bodies 3 dealing with aeronautics; there are no striking developments but much solid work has been done by a few workers in the subject. There are difficulties in the nature of variation of nomenclature which make the comparison of work laborious and in an attempt to deal with this aspect of the problem of stability the Royal Aeronautical Society, acting as a sub-committee of the British Engineering Standards Association, has drawn up and recom- mended the use of a particular set of symbols and axes of refer- ence. Still in its infancy as regards application, the subject merits greater attention. It is scarcely likely that the degree of stabil- ity still undefined thought suitable for military use will be that correct for civil uses. Extreme manoeuvrability is con- sidered to be essential in the first and safety in the second. Whilst not wholly incompatible it is clear that a degree of stabil- ity can be introduced without discomfort in a straight and un- eventful flying which is disliked for the purposes of aerial fighting. (L. Bw.)
IV. MATERIALS AND METHODS or MANUFACTURE
The aircraft pioneers, being their own designers, builders and financiers, used the simplest design, manufacture and assembly, and the cheaper materials.
Between 1912 and 1914 came a striving for efficiency; fixed charges were relatively high, and research costs were extremely great for the small output of the day; this conduced to the quest for the best materials and made costly machining to reduce weight and establish types permissible. In the World War the aerodynamic advances made in this way were used, but as bulk production set in before schemes and tools for bulk production existed, aeroplanes had to be made regardless of cost until the tools were evolved.
Standardization of materials, of sizes and of parts and com- ponents, notably bolts, nuts, bracing connexions, piping con- nexions, etc., common to most types of aircraft, had previously to 1914 been started, but was extended in 1915 to cover tubes, bracings, methods of jointing, length of bracings, wheels and axles, airscrew bosses, etc. Also some of the larger components, wings, elevators, rudders, and ailerons, which could be utilized on more than one type, were standardized. Master and work- shop gauges were made and distributed to ensure interchange- ability. Continuous records of tests led to the selection of the most suitable materials, and to standard specifications. These have been continuously evolved up to the present day, and their dissemination has spread far and wide much acquired knowledge.
3 Reports of the Advisory Committee for Aeronautics to date, Jour, of the Royal Aeronautical Society.
Reports of the National Advisory Committee for Aeronautics, United States of America.
" Applied Aerodynamics," L. Bairstow.
" Aeronautics: A Class Text," E. B. Wilson.
" Aeronautics in Theory and Experiment," Cowley and Levy.
