When an airplane manufacturer decides to produce a new type airplane, he has a definite goal in mind. The new aircraft may have to meet performance requirements of the Air Corps, the Bureau of Aeronautics, Civil Aeronautics Administration, an airline, or his own organization's design section. In any event, the new plane is being built for a special task, in addition to being an improvement over all previous aircraft of its class.
To achieve this objective, it is necessary that each part of the aircraft be as efficient and serviceable as is possible when taking into consideration such factors as weight and space. Inasmuch as the principal parts of the aircraft are considered, in addition to the aircraft structure itself, to be the engines and propellers, it is necessary that their characteristics and efficiencies under various operating conditions be considered during the early stages of design.
This article is primarily concerned with the correlation of propeller design with airplane design only.
In formulating the new aircraft design it is first necessary that the airplane manufacturer consider the aircraft requirements specified by his customer. This information includes facts on the type of aircraft (land plane, long range transport, pursuit, etc), the order of importance of its performance in takeoff, climbing, cruising or high speed, plus many other details pertaining to performance and cost.
Coordinating with his customer, the airplane manufacturer considers the power plant to be used and makes his selection, again bearing in mind the all-important items of performance and cost.
The next major item to be considered is the propeller, although in some cases it is necessary to select the propeller and engine simultaneously, since the efficiency and performance of each depends largely on the other. Quite frequently a current production propeller satisfies the airplane performance requirements. However, it is not unusual to find that a new model or even type of propeller is necessary. If the latter is necessary the design is made, usually in close cooperation with the engine and airplane manufacturers, to meet specified performance requirements.
The following data are therefore presented to the propeller manufacturer by the airplane designer so that he may aid in selecting the propeller most efficient for the aircraft that is to be constructed:
With this information available it is possible for the propeller manufacturer to submit to the aircraft designer an aerodynamic propeller performance study showing the relative efficiencies of certain propeller models most satisfactorily meeting the design data requirements. This study gives the propeller diameter, solidity ratios, effective helical tip speed, and thrust values for specified and recommended gear ratios. The values of the foregoing, along with calculated propeller efficiencies, are based upon engine ratings at their respective predetermined operating altitudes. The total weight of each propeller also is listed.
Only such propellers are included in the final study that indicate, in one or more conditions, efficiencies that will give the required airplane performance.
In many cases additional factors must be considered, such as availability, cost, blade material, etc. The availability problem at present is one of major importance due to defense priorities. The cost is usually dependent on the type of propeller selected, material used, and whether the selected propeller is a model in current production. Blade material has a definite bearing on propeller selection inasmuch as hollow steel blades usually provide a weight advantage and are more resistant to stone and cinder nicks. This particular characteristic of steel blades is of great advantage on certain types of installations and operations. On some installations the propeller is so located as to receive stones and similar objects thrown by the landing gear wheels or nose wheel. This can be of major importance, particularly if the airplane is to be operated from unimproved flying fields. A typical design of a Curtiss electric propeller with hollow steel blades is shown above.
From the results of the aerodynamic propeller performance study submitted by the propeller engineers, and bearing in mind other factors listed above, the aircraft designer proceeds to recalculate his airplane performance using the most suitable propeller efficiencies. As a result, he makes the final decision as to which model propeller will be specified.
If the propeller selected is a new design it must successfully pass a rigid test-stand endurance run, usually combined with an electric whirl calibration, to be approved by the CAA, Air Corps or Bureau of Aeronautics. Even though it is a current production model, or a new design installed on a certain model engine for the first time, it must meet vibratory stress tolerances for approved use, usually checked during trial flights on the "X" model or first airplane at initial installation.
In the case of the Lockheed P-38, it was found best that a three-bladed hollow steel propeller be used and that the left propeller should rotate counter-clockwise while the right propeller rotates clockwise.
On the Martin B-26 medium bomber, both propellers are four-bladed hollow steel types and rotate clockwise. Due to the power output of these engines and the limited propeller diameter, it was advantageous to utilize the higher solidity ratio of the four-bladed propeller.
Now that the propeller itself has been selected it is necessary to determine the type of propeller control system most suitable to the aircraft under design. The Curtiss electric propeller installation provides great flexibility in this respect, inasmuch as any one of the three general control systems is readily adaptable to various types of installations. A remarkably small amount of wiring and control items is required for the proportional governor system, which accounts for its wide usage in single- and twin-engine airplanes.
The automatic synchronizer control system has certain definite advantages and is in more general use on multi-engine airplanes.
Another Curtiss electric propeller control system which is sometimes used is the remote control governor type. This system permits governor rpm selection by operation of an electric switch rather than by a governor lever control.
Any one of the propeller control systems can incorporate normal rate full-feathering, fast rate full-feathering (which requires a voltage booster) or no feathering feature at all. The feathering feature is usually provided in all installations except the single-engine type. To determine the specifications of the exact control items, it is essential that the engine governor drive ratio, its direction of rotation, and voltage of the electrical supply system be known.
The selection of a specific control system is determined by considering which is the most applicable and efficient for the weight and cost involved. As in the case of some multi-engine airplanes the synchronizer system may be preferred over the proportional governor, or remote control governor systems, due to the automatic synchronization feature. Since each installation presents its particular problems a complete control study is necessary.
On some airplanes reverse pitch is often desired to improve surface maneuverability. In the case of the larger type airplanes it is usually desirable to provide dual control panels for the pilot and flight engineer. Such variations from standard control systems are not unusual and can normally be incorporated into the control specifications.
Recent trends in new airplane development have been toward the use of ground test rigs which duplicate power plant installation details and permit testing in advance of installation on the airplane. Preliminary propeller blade vibration tests and control tests can be conducted as part of the test program on the ground rig.
It can now be assumed that the basic propeller and its controls have been designed and fabricated concurrently with the airplane and its engines, all of which are now ready for the initial propeller installation.
Aerodynamic spinners are almost invariably specified for liquid-cooled engines, inasmuch as the combination of the spinner and cowl increases the high speed efficiency by reducing the profile drag. The use of spinners is becoming more general for air-cooled engines, particularly on the high speed type airplanes, where improved engine cooling results.
Blade shank cuffs, which recently have been developed, are specified for the most part on air-cooled engines in that their effect has been to increase the flow of air through the cowl at low speed, which aids in engine cooling under ground and climb conditions. The advisability of using cuffs is usually determined on trial flights of the initial installation.
On most installations it is desirable to conduct functional test flights to check general operation, high pitch set- tings. feather angles, etc. After such tests have been made and the requirements for acceptance have been met, it can then be said the propeller design has been completed.
The foregoing is a general discussion meant to outline the high points in coordinating propeller design and installation with airplane design. There are many additional details, more or less minor, which necessarily have been omitted, but it is hoped that the importance of considering the modern propeller in the early stages, and throughout the airplane design, has clearly been defined.
This article was originally published in the April, 1942, issue of Aviation magazine, vol 41, no 4, pp 70-71, 73, 202.
The original article includes the drawing above, three schematic diagrams of Curtiss control systems, and three photos: P-38, B-26 and an assembly photo of a three-blade propeller.
This HTML version has been edited to eliminate references to figures in the original.
Photos and drawings are not credited.