NASA-TP-1435

Abstract

INTRODUCTION <br>Experimental data for scale-model aircraft are used to define the aerodynamic characteristics of full-scale aircraft, verify theoretically predicted aerodynamic characteristics, and provide data where theory is deficient. To apply the data to a full-scale aircraft or its components with maximum validity, certain similitude conditions must be met. The similitude of the geometric configurations is a fundamental requirement, as is the similitude of the angles of attack. Reynolds number and Froude number, as well as Mach number in the case of compressible flow conditions, are pertinent parameters for steady-state (static) or dynamic test conditions. A number of other similitude parameters may be important, depending on the test objectives and aircraft elasticity. <br>In general, any one experimental technique will not satisfy all the similitude requirements for correlation of wind-tunnel data with free-flight data or for correlation of free-flight data obtained from models of different scale. Most tests are designed for certain similitude conditions at the expense of other parameters. For example, an elastic, rigidly mounted wind-tunnel model tested at full-scale Mach number and dynamic pressure through an angle of attack range does not properly account for the effects of mass on elastic deformation except at one angle of attack at a steady level-flight condition. Inertial aerolastic effects that occur in maneuvering flight must be accounted for theoretically. A comparison of the aerodynamic characteristics of one free-flying model with those of a model of different scale or a full-scale aircraft at the same Mach number may not be appropriate if Froude number similitude requirements are not met. A difference in Froude number could result in dissimilar angles of attack. <br>Although there are many references of limited scope in the literature on similitude, a comprehensive report is needed to clarify and summarize the many techniques for wind-tunnel and free-flight model testing with regard to similitude requirements, test objectives, and comparison of model and full-scale results. The fulfillment of this need is particularly appropriate in that remotely controlled, subscale, powered and unpowered models of advanced aircraft are currently being used to investigate stability, control, and handling qualities at routine as well as high-risk flight conditions. One of the prime factors necessary to determine the limitations of data obtained from a model is the degree to which the similitude requirements have been met. <br>This report provides a comprehensive review of the similitude requirements for the most general test conditions , from low-speed incompressible flow conditions to high-speed supersonic conditions. The fluid is considered to be a continuum that obeys the perfect gas laws for a fixed value of the adiabatic gas constant. The similitude requirements are considered in relation to the scaling requirements, test technique , test conditions, and test objectives. Limitations in test techniques are indicated, with emphasis on the free-flying model. <br>Scaling procedures are illustrated for free-flying models in incompressible and compressible flow. For incompressible flow, the kinematic properties are preserved by using velocities scaled from Froude number similitude requirements (Froude scaling). For compressible flow, the compressibility effects are preserved by using velocities scaled from Mach number similitude requirements (Mach scaling). In addition , summary tables and nomographs are presented to facilitate a rapid assessment of the scaling requirements for free-flying models and of the extent to which the requirements are satisfied for both Froude and Mach number similitude. <br>Although this report covers parameters encountered in dynamic model tests, it does not include discussions of other similarity effects that may be important in individual cases , such as the scaling of a viscous damper in the control system of a model with free control surfaces or, a more remote example, the scaling of physical parameters for an icing test. To prepare for such situations, the experimenter should refer to books on dimensional analysis, such as references 1 and 2 .