Aileron reversal effects on swept-back wings in general and elevon reversal effects on tailless swept-back wings in particular are discussed on a non-mathematical basis, attention being confined to the orthodox flap type of control. The main purpose of the paper is to convey in the simplest terms possible a clear physical picture of the conditions producing loss of control power, emphasis being naturally laid upon the part played by structural wing distortion. Certain qualitative features relating to the two phenomena are also discussed. As a general introduction to the discussion on aileron reversal effects, the definition of “aileron power” in relation to the actual dynamic condition of rolling is described at some length. For elevon reversal effects on tailless aircraft the effect of wing flexibility on both “elevon power” and on trim in steady symmetric flight is considered.

With the descriptive treatment adopted the analysis is of necessity broad and general but is designed to appeal to those not too familiar with the subject. The results of certain calculations on a hypothetical wing, which may be of interest, are included. A mathematical analysis for the quantitative estimation of both aileron and elevon reversal effects is given in the Appendix.

Once the linearised lift problem for a supersonic wing has been solved and the disturbance velocity potential or the pressure is known at every point on the surface, the complete flow round the wing and the downwash behind the wing may be found by the following procedure.

A method is given for calculating approximately the changes in the roots of a stability secular equation caused by a change in any of the parameters involved. General formulas are given applicable to any quartic equation, and special formulae are also given applicable to the stability of an aeroplane: lateral stability in the text, and longitudinal stability in an appendix.

The method of using the formulae is illustrated by applying them to a particular calculation of the lateral stability of an aeroplane, and a check of the results is made by comparing the predicted approximate changes with those calculated by solution of the modified period equations. It is shown that the formulae are reliable, for this typical case, for any reasonable changes in any parameter other than nv.

If the changes in the derivatives are made equal to the probable error with which they can be measured, the formulae enable us to evaluate the probable errors of the roots. These are found to be considerable, and to arise mainly from uncertainties in yv, nv and nr: if these could be reduced to 0.03 in yv and 0.006 in the others, the uncertainties in the roots would be reduced to some ten per cent, of their values, except for a larger uncertainty in the root corresponding to the slow spiral motion.

Experience has shown that it is often necessary to retain many degrees of freedom in order to calculate critical nutter speeds reliably, but this entails much labour. Part I discusses the choice of a minimum set of freedoms and suggests that this should be based on the equation of energy and the use of the Lagrangian dynamical equation corresponding to any proposed additional freedom. The methods for conducting flutter calculations so as to minimise labour are treated in Part II.

It has been shown that since the performance of a turbo-jet engine can be expressed theoretically as a function of the two variables and , the performance of a turbo-jet aircraft can also be expressed as a function of two variables, and Two methods can then be used for the reduction of the performance to standard conditions. In the “non-dimensional” graphical method, flight test measurements of, for instance, are plotted against and , and the standard value of is read from these curves at the standard values of and . In this way, the relation between the parameters is established during the tests. The alternative method is an analytical one in which corrections are applied to, for example, the level speed for differences from standard in the independent variables. For this method the effect on the performance of changes in these variables must be predetermined.

In order to obtain full knowledge of the performance of an aircraft, one of the important quantities to be determined is the drag. The usual procedure is to measure the level speed of the aeroplane and to calculate the drag under these conditions from the engine thrust, as obtained from the engine makers’ specification. To do this, accurate thrust data are essential and in certain cases these may not be readily available.

In order to eliminate the possibility of systematic errors in the determination of engine thrust, a method has been developed whereby the drag at low Mach number can be measured directly,under such conditions that accurate knowledge of the engine performance is of minor importance. This method is satisfactory only on jetpropelled aircraft, since on conventional aircraft there are the added complications of propellers, slipstream, radiators and so on.

The method has been used to determine the drag of the de Havilland D.H. 100 Vampire and has given good results. The drag determined from earlier tests by the usual procedure is found to be in good agreement with that given by the new method.

In much of the recent work on the flow of an incompressible fluid past a swept-back symmetrical wing of small thickness at zero incidence it has been assumed that the disturbance introduced by the wing can be represented by means of a distribution of sources and sinks over the centre plane of the wing, the source strength being determined from the equation of the wing profile. It was found by Neumark that on the wing itself this procedure was liable to lead to serious errors. Neumark and Kuchemann studied certain special types of wings for which they were able to find the correct value of the pressure at the surface by evaluation of the pressure at points off the wing and continuity considerations. This process is rather cumbersome; it may therefore be convenient to regard the problem from a rather different point of view.