THE LIGHT TOUCH

Performance is certainly the first consideration when judging the relative merits of sailplanes. Performance is readily measured and an excellent literature is available due to the work of Bikle, Zacher, Johnson, and others. The acid test of competition frequently confirms the results of engineering tests and sailplanes with superior performance are rapidly recognized and therefore thrive and improve the breed. Handling qualities (or stability and control or flying characteristics) are less definite. Requirements can be examined from two view points. The first considerations are "What is required for flight safety? Is a glider airworthy?" These are the concern of requirements having legal weight such as the FARs. One would expect that characteristics affecting flight safety would have Cooper-Harper ratings of 3.5 or better. The other viewpoint is "What is required for elegance, what are the qualities that make flight more enjoyable, and why is one glider more pleasant to fly than another?" Favorable answers to these questions are probably associated with Cooper-Harper ratings of 1.5 or better. With respect to safety, the author is of the opinion that the OSTIV requirements are adequate. The "return to trim" requirement may be too severe. The addition of a maximum value for lost motion would be appropriate, say, no more than 5% of full stick travel. 0therwise, the 0STIV list is necessary and sufficient to describe airworthy longitudinal characteristics. It is tempting to write a list of criteria for elegance. The author believes the ideal system would totally lack friction, lost motion, compliance and mass. All of the forces would be linear and light. The stick force gradient would be more than 1 kg/g but less than 3 kg/g. Since a very low stability Gradient would provide adequate signaling of speed changes with such a high quality mechanical system, the static margin could range between 5 and 10% MAC. 0f course, the zero values suggested of the ideal system are not practical. However, a realistic system probably could be built with no more 0.1 kg friction, 1 mm lost motion and 0.3% full travel per kg compliance. The mass of the system should be as low as possible consistent with structural safety. (The compliance and mass criteria tend to be in opposition, a very rigid system is apt to be heavy.)