Measurement of Maneuvering Loads in Flight

Abstract

D THE PAST 20 years, improvements in aircraft design have, to a large measure, been due to aerodynamic theory and wind-tunnel tests. There has been, however, one major weakness in both design tools and that has been in dealing with dynamic or nonuniform conditions. This is not because these two methods cannot handle dynamic conditions, for quite the reverse is the case. I t does mean that serious difficulties and complications are encountered. Spin tunnels have been successful in giving the general behavior of an airplane, but they have produced little quantitative data. Gust tunnels have encountered great difficulties in accurately measuring the dynamic loads and assimilating actual conditions. Design of large aircraft has been seriously impeded because of a lack of accurate knowledge of basic loads resulting from maneuvers. Many of the design loads for structural components of an airplane are governed by maneuvering loads. Because of this lack of information, the maneuvering loads required by the Civil Aeronautics Authority are based on an arbitrary normal force coefficient for the surface and the maneuvering speed, with the additional specification that the load need not be greater than that which is possible to obtain with a certain applied control force. Interest in dynamic loads has been greatly stimulated in recent years because of service failures, particularly of horizontal surfaces, which have occurred on several types of aircraft. Research work has been carried on by several organizations along both analytical and experimental lines. The analytical work has suffered for lack of sufficient flight data. Most calculations of maneuvering loads required a fundamental assumption of a certain rate of control surface movements. As this paper will show and as was brought out by Beman and Frost, maneuvering loads appear to be a complicated function of not only what the airplane and its control system can do but, in addition, what the pilot will or chooses to do. The experimental work has been handicapped because of the difficulties of making accurate measurements in flight and the absence of adequate instruments or methods. In general, the structural measurements made in flight in the past have been crude. Scratch gauges, deflection photographs, pressure distributions, and transit measurements of deflections have been made. The majority of measurements made during maneuvers have been with accelerometers. None of these methods has satisfactorily given the maneuvering load or stresses on the structure. With the advent of wire resistance strain gauges, the measurement of maneuvering loads in flight becomes possible and feasible. The technique of making measurements in flight has advanced a great deal in the past few years. It is the object of this paper to describe some of the work carried on along this line at Lockheed Aircraft Corporation. Throughout this paper the term ''maneuvering stress is taken to mean that increment in which is caused by, or entirely dependent upon, the maneuver being made (maneuvering = total — balancing stress).