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THE primary purpose of this paper is to discuss some of the most pressing problems involved in choosing the propeller that is most suitable for use on a particular airplane. Propeller design is not dealt with, the discussion being limited to the selection of metal propellers of established design. Questions of noise, efficiency and diameter limitation are merely mentioned, and the emphasis is placed upon the choosing of propellers which will transmit the most engine power for the most needed condition of airplane performance; maximum and cruising speeds at altitude, or take-off and climb. Airplane performance enters only inasmuch as it is used to illustrate a case of power absorption. The proper choice of a propeller is becoming increasingly difficult to determine because of the current design trends of both airplanes and engines. Especially important is the fact that many of the supercharged engines now in use cannot be operated at full throttle below their critical altitudes.

After a brief consideration of airplane-engine practice in France, England and Germany, the author outlines the problems encountered in designing a twelve-cylinder aviation engine. He explains at some length the difficulties in determining the connection between propeller and engine and shows why valve-in-head location was chosen. Such features of engine design as the mounting of carbureter and exhaust pipes, methods of fuel and lubricant supply and details involved in selecting the lighting, starting and ignition equipment are considered.

The author gives a brief review of developments during the past year in the construction of aeroplanes, particularly as affected by the European War. He takes as an example the Renault twelve-cylinder engine, citing the respects in which the present differs from previous models. Such factors as the changes in cooling systems, method of drive, valve construction and starting devices are considered. The requirements of aeroplane engines, such as constant service, high speeds (of aeroplanes) and stream-line form of engines and radiators, are outlined. Propeller requirements are dealt with at length, curves being given by which the efficiency and diameter of the propeller can be obtained. In conclusion a number of different engine installations are illustrated and compared.

PROPER brake testing is stated to involve the measuring and recording of vehicle deceleration and rate of speed for every foot of individual wheel travel during the period from the initial application of the brakes up to the moment the car comes to a standstill. The brake synchrometer, designed to duplicate the conditions under which a vehicle is tested on the road, embodies the principle of traction between each tire of the vehicle, on one hand, and a rotor on the other hand. The kinetic enegery of the four testing rotors must be equal to the energy of a vehicle of a certain weight at a given testing speed. The author describes a brake synchrometer designed for testing vehicles of 3500-lb. weight, which machine, however, is adjustable to compensate for greater or lesser car weight, so as to include heavier or lighter vehicles in its test range.

Cavitation-erosion is a peculiar form of corrosion or pitting which occurs on the water side of Diesel engine cylinder liners, on hydraulic turbines, on centrifugal pumps and on high speed ship propellers causing extensive and costly damage. As a result it has been the subject of numerous investigations. Despite these studies its mechanism is still unknown although it has been established that the following factors affect the pitting rate of the metals: (a) Hardness, (b) Porosity and surface discontinuities, (c) Viscosity, temperature and molecular size of the contacting liquid, (d) Amplitude of vibration, and (e) the addition of corrosion inhibitors. Some recent experimental data which illustrates some of the above effects is included.

AGENERAL approach to the flutter problem is outlined, which can be used to investigate any mode of flutter of any structure provided the air-forces are known. The method can be used to investigate the possibility of flutter on such structures as airplane wings, tail surfaces, and bomb doors, aircraft propeller blades, vehicular bridges, buildings, and so on. With a well-known solution for the air-forces for two-dimensional flow over an airfoil with an aileron, equations have been derived which can be used to determine the flutter speed for the wing or tail surfaces of any conventional airplane. The use of still-air vibration tests in obtaining the structural friction of structures, and in checking a part of the flutter computations experimentally is indicated. Finally, to suggest the possibilities in the use of this method, a standardized procedure for its general application to aircraft flutter problems is outlined briefly.

STRESS corrosion cracking is a combination of stress and corrosive action that results in individual cracks of a brittle, intergranular nature. The author discusses such failures in compressor rotor blades made of a 12% Cr, type 403 stainless steel. A laboratory technique was worked out for producing similar failures at will. As a result of this study, it was recommended that compressor blades be stress relieved at 950 F. Since this has been done, no further cracking of the blades has been reported. Tests with three alloys in addition to the type 403 showed the former alloys to be superior to 403 in regard to stress corrosion cracking.

THE truck engine of the future will be capable of producing more horsepower and will operate at higher rpm, according to Mr. Michell. This means, he said, that new problems will arise in connection with the clutch, transmission, propeller shaft, and rear-axle design. The author discusses some of these problems as far as each of the above parts of the drive line is concerned.

CLOSED die forgings have long been preferred by engineers for dynamically loaded or highly stressed machine and vehicle parts for various reasons, such as: superior engineering properties, uniformity of quality, and low cost of producing the final finished geometry. A few years ago, crankshafts for road vehicle engines were among the heaviest closed die forgings being made in quantity production. Then crankcases and crankshafts for aircraft engines, components for landing gear, airframe members, and propeller parts progressively presented needs for larger and better closed die forgings. Mr. Dixon discusses here some of the resulting developments: hammers of larger capacity, improvements in heating and die blocks, and the like.

Gas-heated hollow propeller blades have been investigated both analytically and experimentally to determine heat requirements for the prevention of icing on the blade surfaces during flight. An analysis was made of a typical hollow propeller blade to determine the internal gas flow and temperature required to maintain the external surface temperature above 32 F. The basic wet-air equation is given and curves are presented to illustrate the blade heating and temperature distributions which were obtained for one set of conditions. Full-scale gas-heated propellers have also been investigated experimentally in an icing wind tunnel and typical results are presented. The rates of heating required to prevent icing are discussed and a modification of propeller blade interiors by using fins and partitions is shown by comparative experiments to permit large reductions in the required heat-source input.

AFTER citing the improvements achieved in the S-42 as compared with the S-40 type, the author states that many future improvements will be made in flying boats as a result of improved engines, propellers and airplane design. Some of the general problems of flying-boat design are compared with those of land air-transports. Although in the basic phases the problems are similar, the flying-boat designer diverges from the land air-transport-designer's viewpoint in several important features such as cruising range, speed, comfort, space per passenger, and size. Many advantages, particularly in seaworthiness, are found in connection with the construction of larger-sized ships, as the inherently easier take-off of the larger flying boat permits better shapes for seaworthiness. Sharper and more refined V-bottom hulls will be used.

In this paper, the author has first shown the increasing need, amounting in many instances to a necessity, for the adoption of the controllable pitch propeller on modern airplanes. Improvements in airplane design which bring about increased speed range have brought about a decrease in other performance items which can only be rectified by the use of the controllable pitch propeller. The improvement has been shown to be so great as to well pay for the added complication and expense involved. The several types of controllable pitch propeller developed in this country are described, the author going into considerable detail on the electric-driven type. The numerous test requirements are gone into in detail. In conclusion, the author notes greater progress in the development of the controllable pitch propeller in this country than elsewhere, perhaps necessitated by the accompanying advance in aerodynamic refinements here developed.