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EFFECT OF STING STIFFNESS ON MEASURED DERIVATIVES FOR A MODEL DESCRIBING PITCH OSCILLATIONS OF LOW REDUCED FREQUENCY,
LOCKHEED MISSILES AND SPACE CO INC PALO ALTO CA
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Experimental pitch damping derivatives are usually obtained from wind tunnel tests by using either forced or free oscillation techniques. In both cases the model is as a rule sting mounted. The effect of sting stiffness on measured derivatives for a model oscillating in pitch is analyzed. The method of forced oscillations is used in the analysis but the conclusions are valid also for the free oscillation technique. It is shown that, for a representative re-entry body model, a sting with a natural frequency at least 4 times higher than the model pitch frequency is needed if the effect of sting plunging is to be kept below 5. The relations derived make it possible to estimate the sting plunging effect by use of static data and the requirements on sting stiffness may, therefore, be reduced somewhat. It is essential, however, that the sting natural frequency is appreciably larger than the model pitch frequency if reliable data are to be obtained. Such a sting will give too low conservative damping derivatives for a statically stable model if the plunging effect is neglected. Author
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