Chapter VI
Experimental Evidence and Verification of the Phugoid Theory
§ 64. Preliminary. The Importance of the Experimental Verification.—The experimental verification of the conclusions of the preceding chapters is evidently a matter of very great moment, for the results obtained from theoretical considerations, if experimentally established, form a definite proof, not only of the aerodonetic portion of the work, but also of the aerodynamic fabric in which the phugoid theory has its foundation.
The checks that have so far been applied to the theory are of various kinds. In the first place there is the general qualitative resemblance of the curves of flight, both as determined and illustrated by other workers and by the author himself, to the curves as plotted from the phugoid equation. Secondly, there is the direct measurement of the phase length, phase time, and velocity relationship as a check on the results given in § 36. Thirdly, there is the experimental determination of the condition of neutral stability for the phugoid of small amplitude, as a check on the investigation given in Chapter V., and the demonstration of the conditions of flight path stability and instability by means of model experiments. Lastly, there is the post-mortem examination of birds as throwing some light on the extent of the regard paid by nature to the requirements of theory; in this category may also be included the examination of gliding machines and models, made empirically in the absence of any knowledge of the equation of stability.
It will be shown in the present chapter that the theory is able to withstand every test that has so far been applied.
It is important to point out that although the model experiments employed for the purpose of demonstration are on a small
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