[.[. F. I.
the tunnel where the models are placed. The forces on the model are measured by a 3-component photo-recording balance. The test results included herein comprise measurements of the lift, drag, and pitching moments of six airfoils. The sections chosen for tests have thickness ratios of o.o6, o.o8, and o.io; three are based on the Clark Y profile and three on the R.A.F, 6 profile. The tests were made over a wide speed range and for several angles of attack, varying from that of zero lift to I2 ° , in order to investigate the effects of compressibility on the airfoil characteristics. Report No. 466, Aircraft Power-Plant Instruments, by Harcourt Sontag and W. G. Brombacher, 57 pages, tables, illustrations, 23 )< 29 cms. Washington, Superintendent of Documents, I933. Price fifteen cents. This report supersedes that on aircraft power-plant instruments, published in I92I as N. A. C. A. Technical Report No. I29, which is now, on the whole, obsolete, Aircraft power-plant instruments include tachometers, engine thermometers, pressure gages, fuel-quantity gages, fuel flow meters and indicators, and manifold pressure gages. The report includes a description of the commonly used types and some others, the underlying principle utilized in the design, and some design data. The inherent errors of the instruments, the methods of making laboratory tests, descriptions of the test apparatus, and data in considerable detail on the performance of commonly used instruments are presented. Standard instruments and, in cases where it appears to be of interest, those used as secondary standards are described. A bibliography of important articles is included. Report No. 467, The Experimental Determination of the Moments of Inertia of Airplanes, by Hartley A. Soule and Marvel P. Miller, I5 pages, illustrations, 23 × 29 cms. Washington, Superintendent of Documents, I933. Price five cents. The application of the pendulum method to the experimental determination of the moments of inertia of airplanes is discussed in this report. Particular reference is made to the effects of the air, in which the airplane is immersed, on the swinging tests and to the procedure by which these effects are taken into account. Consideration of the effects of the ambient air has shown that the virtual moment of inertia of the airplane about any given axis of oscillation must be regarded as made up of three distinct parts; namely, that of the structure, that of the air entrapped within the structure, and that of the apparent additional mass of external air influenced by the airplane's motion. As the true moment of inertia consists only of the moments of inertia of the structure and the entrapped air, the apparent additional moment of inertia due to the influence of the external air is determined and deducted from the virtual moment of inertia. The apparent additional moment of inertia is obtained by computations utilizing the results of experiments made to determine the additional-mass effect for plates of various aspect ratios. The procedure described in this report has been used for some time, and the data on several airplanes for which the moments of inertia have been found are included. The precision is believed to be within limits of -4- 2.5 percent, 4- 1.3 percent, and 4- o.8 percent for the X, 3(, and Z axes, respectively.