This lecture is an attempt to give a brief survey of some facts observed during the German-Polish campaign, which are thought to be of interest to the aircraft industry. The authors wish to stress that they fully realise the scarcity of actual data on which any possible conclusions could be based. There is no doubt that a “Blitzkrieg” provides all of the experience to the winner, particularly if he enjoys an overwhelming superiority in the air and has secured a “total” victory, as was the case in this campaign. His vision was clear and far-reaching, his striking arm able to inflict blows, wherever required by tactical considerations.

The forces on an aerofoil have been measured at various angles of incidence for 0.5 < R < 2,000.

Below R = 2 the lift is found to be given approximately by :—

L, = (3/2) µlV sin 2x

and the drag at all angles of incidence tends to:

D = µlV.

An approximate theoretical estimate is also given of the effect of spacing, on the drag of a series of flat plates at R → o.

In seeking to analyse the stress distribution in a complex structure, our object is really twofold. In the first place, if the stress in each part is accurately known, the strength of each part separately can be assessed on the basis of tests on simple units and the necessity for testing the complete structure may be avoided. In the second place, if the stress distribution can be expressed as a function of the geometry of the structure, the effect of varying that geometry, particularly with the object of realising Holmes’ one-hoss-shay, can be examined theoretically and arbitrary-features of the design can be eliminated. If our knowledge were complete, any structure designed to a maximal or minimal condition would not permit any variation from one ideal form, the design would be uniquely determined by the stated requirements.

This paper is concerned mainly with that application of counter-rotating propellers referred to as “ dual-rotation ” propellers. This term is here defined to apply to a pair of oppositely-rotating propellers operating in close tandem, mounted on concentric shafts. Only the application to aeroplanes is discussed herein, for the application of these principles to helicopter design is considered another subject.

This paper presents a summary of the development and background of dual-rotation propellers and the benefits that can be derived from their use. Complete references are furnished for designers, no attempt being made to reproduce the somewhat involved theory and lengthy reference data.

A short history of the three applications of counter-rotating propellers is presented. The applications include multi-motored aeroplanes with oppositely-rotating propellers, aeroplanes with engines mounted in tandem (back to back) so that the propellers revolve in opposite directions, but primarily, aeroplanes with dual-rotation propeller installations.

Most present-day aeroplanes with maximum velocities of 350 m.p.h. and over have single-rotation propellers. At this velocity the single propeller is operating past the peak of its efficiency. The loss in efficiency increases rapidly with increasing forward velocity. Dual-rotation propellers offer an excellent means of maintaining peak propeller efficiencies up to aeroplane velocities of about 450 m.p.h. by absorbing the rotational energy losses in the propeller slipstream. At velocities above 450 m.p.h. up to the limiting velocity there will be only a small reduction from the peak in dual-rotation propeller efficiencies.

Sustained level flight aeroplane velocities appear to be limited to about 550 m.p.h. when propellers are used as the means of propulsion. The efficiency of a conventional single-rotation propeller will be so low, however, that this limiting velocity can be approached with such a propeller only at a prohibitive cost in engine horse-power. Any given velocity above 300 m.p.h. can be obtained with less horse-power if dual-rotation propellers are used to replace a single-rotation propeller with the same total number of blades. The percentage gain in performance will increase with increasing forward velocity when dual-rotation propellers are used.

It is possible that single-rotation propellers with more than four blades will never be commonly used. Instead, dual-rotation propellers with a total of four, six or perhaps even eight blades, will be used in the next decade on many high performance aeroplanes, especially those operating at high altitudes. The types of aeroplanes will include small single-seater racers and fighters and high performance, long range bombers and transports, particularly those with supercharged cabins.

The discussion brings forth the fact that there are few experimental data available in this country to allow a quantitative engineering evaluation to be made of the worth of dual-rotation propellers. Sufficient substantiated theory exists, however, to permit the general statements made above. An outline of the experimental tests that are required to check the theory is presented.

Following the discussion of tests made by the U.S. Air Corps on a pursuit type aeroplane equipped with a dual-rotation propeller system, comments are made on the relative advantages and disadvantages of dual and single-rotation propellers as they affect aeroplane design. Hub design problems associated with dual-rotation propellers are discussed.

Because of the increasing importance of dual-rotation propellers, additional pertinent references other than those used in this paper are presented in the Appendix.

The conditions in which the various parts of the ignition system of aero engines have to operate have during recent times become much more onerous than hitherto. Many of the developments which have occurred in modern engines have caused the emergence of new problems so far as the ignition system is concerned, and have made older problems which had been only partially solved, even more difficult. This paper presents a review of some of the more important factors which are involved, an account of how the problems which have arisen have been solved, and indications given of limitations which still exist in various directions.

The distribution of stress in an I-beam in torsion is investigated by a method of successive approximation, and the regions of initial yielding are determined for several values of the angle of twist. Formulas are given for elaborating the methods of Southwell and of Shortley and Weller in order to facilitate this determination.

The value of the stress-concentration factor in the fillets is found to agree well with results obtained by soap-film tests, and regions of plastic flow approximate closely those observed by MacGregor and Hrones by the use of the etching method.

The list of the Wilbur Wright lecturers is a long and distinguished one, and I feel it a great honour to have been asked to join this eminent company. I was invited just before Christmas, 1939, to address you to-night on the future of British civil aviation. I felt then that though only with great difficulty would it be possible to produce a lecture under war conditions, the invitation must be accepted, firstly because no greater compliment can be paid an aeronautical engineer, and secondly, to support the Council's policy of continuing the Wilbur Wright lectures, which were not interrupted by the last war, through this one.