The first public account of the methods and principles of performance testing of aircraft is, it is believed, that presented to this Society in a paper read by Mr. H. T. Tizard in 1917. Since that paper appeared there have been few changes in testing methods—a fact which indicates how very well the requirements had been met by the proposals therein described. During the period of nearly ten years which has intervened, however, attention has been directed to the analysis of the data provided by the tests. At that early date, the designer was no doubt content to check his predicted performance by the information as to rates of climb and speeds made available by the work of the R.F.C. Testing Squadron; and if he found discrepancies, they were probably attributed to scale effect or to some other of those conveniently indefinite agencies which sometimes enable the theoretician who has been rash enough to predict a performance to evade the consequences.

In the past the usual method of dealing with the articulated rod has been to treat the problem graphically. Although this method may be sufficiently accurate for a first approximation the inherent difficulties often lead to serious errors after the first few stages. A graphical method is always sufficiently accurate to determine the travel of a piston operated by an auxiliary rod ; but, the double differentiation of the displacement curve by graphical means (to obtain the acceleration curve), is not only inaccurate but a long and tedious operation.

To remedy this defect a Fourier's series has been developed which can be differentiated at sight and which gives a remarkable accuracy.

The geometrical properties of the mechanism have been analysed and thus equations are formed giving the resultant radial and tangential components at the crank pin, the bending forces and tensions on the master rod, etc.