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The greater part of the experiments performed with the car at Saint-Cyr differed considerably from the results obtained by model tests, and apparently these differences followed no specific law. According to theory, and the results obtained by different laboratories, the performance of a full size wing should be better than with a model, but the Saint-Cyr tests showed that such was not always the case. The center of pressure movement differed in almost every case, and as a direct result, the pressure distribution of the large wings was materially different than with the model. The lift-drag ratio results varied, sometimes being better for the model than for the large wing. These differences can probably be explained as being due to variation in air currents, side winds, etc.

Model Tests

The whirling arm method of testing as used by Langley, Maxim-Vickers, and others, is a form of "towed testing," but is also open to serious objections. Unless the arm is very long, every part of the model surface will not move at the same velocity, the outer portions moving the faster. As the forces produced by an air stream vary as the square of the velocity, this may introduce a serious error. The fact that the body passes repeatedly over the same path introduces error, as the body after the first revolution is always working in disturbed air. The centrifugal force, and the currents set up by the arm itself all reduce the accuracy of the method.