Why A Rear Engine Installation

By Robert J Woods,
Chief Design Engineer, Bell Aircraft Corp

Part II of this article describes the problems met and overcome in the extended drive shaft engine installation of the Bell P-39

Note: the March, 1941, issue of Aviation magazine, which contains Part I of this article is not currently available. —JLM

In reviewing all the requirements (as set forth in Part I, Aviation, March 1941 p 36) it appeared that a new solution to the problem of engine arrangement was essential for single-engined military airplanes if they were to provide all of the desired characteristics. It was obvious for even first considerations that some of the equipment ordinarily installed in the nose of the fuselage of the airplane must be relocated in some other place in the airplane. The propeller, the tricycle landing gear nose wheel and the guns were obviously required in the nose under any circumstances and it appeared that the biggest gain could be made by moving the engine from the usual position in the nose to a position amidships of the fuselage. An extension drive shaft would be required, to carry the power from the engine to the propeller. This arrangement would permit the installation of the required equipment in a smooth clean bullet-shaped nose with a minimum of complication. It would permit satisfactory wheel base for the tricycle gear and would permit development of a fuselage shape with optimum streamlining. If the engine were located aft of the pilot the design would permit also maximum vision for the pilot, especially forward and down in the critical areas used in military operations. It would provide an arrangement wherein the bulk of the engine and its installed equipment would not necessarily govern the design of the airplane structure. Such an arrangement has most desirable advantages and facilities for improved installation.

There were new problems to solve in making this change. Preliminary investigations and engineering layouts made to check the arrangement indicated that satisfactory balance and a good installation of all the required equipment could be obtained. However, a question mark occurred over three problems; first, on a suitable type of structure to provide the necessary strength with lightness while affording adequate accessibility to the power plant and other installations; second, the problem of developing a suitable extension drive shaft to conduct the engine power from the remotely located engine in the center of the fuselage to the propeller in the nose of the airplane, and last, but not least, solution of the problem of vibration resonances occurring in the aircraft structure from impulses generated by the engine, propeller or drive shaft. These problems were all really one, "how to develop a satisfactory extension drive shaft installation."

The airplane preliminary design indicated that a shaft approximately 120 in long would be required to connect the engine and propeller. Since the airplane manufacturer was not equipped to furnish the engine and drive shaft equipment, a partnership type of development was necessary with the Allison Engineering Co, the engine manufacturer. Allison would build the engine and drive assembly and Bell Aircraft Corp would furnish the supporting structure, with both the power plant and aircraft projects being coordinated through the Materiel Division of the Air Corps.

Careful work in the preliminary design stages on both the power plant and airplane was required to insure that the basic design conditions, which were critical, were properly determined and were maintained fixed once they were established. Without the whole-hearted support and cooperation of all parties concerned the job could not have been done.

Extension shafts are not new in that they have been previously used in numerous airplanes in England, Holland, Germany and in the United States, but extension shafts have never until now been adopted and used on production aircraft of any type. Their use has been confined principally to experimental types of airplanes. Information available indicated that reasonably satisfactory operation had been obtained from the extension shaft installations, but no really satisfactory arrangement had ever received general use, with one exception. The Bell Aircraft Corp had had previous experience with an extension shaft arrangement on the Bell XFM-1 and YFM-1 Airacuda airplanes. In this instance the problem was to provide a 66-in extension of the propeller shaft of an Allison engine to drive the pusher propellers used on that airplane. The setup and development was comparatively simple in that a standard engine was used with an adapter flange mounted on the regular engine propeller spline connected to a relatively stiff, large diameter extension shaft. Design and construction of this shaft and drive arrangement by the Allison Co resulted in an installation which gave no major trouble of any kind during development and test and was the basis for the opinion that extension shaft installations were a satisfactory, practical mechanism that could be used on the new airplane without misgivings.

Comparing the experience and information available on other extension shaft installations without experience at Bell Aircraft on the shaft developed and furnished by Allison for use on the Airacuda, it became apparent two considerations were included by the Allison engineers in the solution of that extension propeller shaft problem that have not generally been considered in other designs. These were, first, extremely careful consideration of the torsional vibration characteristics of the engine and shaft system; second, careful detail design to permit angular flexibility in the drive and to eliminate eccentricities from occurring in the drive mechanism when angularity was present. It appeared that these two factors were the principal differences between the very successful Airacuda shaft installation and the generally satisfactory extension shafts used in other instances. On this basis it was believed that a satisfactory extension shaft turning at crank shaft speed, 120 in long could be developed for the new P-39 airplane.

In conference with Allison and from a study of preliminary shaft designs worked out by Mr R M Hazen, chief engineer of Allison, it was decided that satisfactory torsional vibration characteristics could be obtained and that if adequate flexibility could be provided in the drive shaft at each end and at the center and with concentric bearing supports at the flexible joints, a satisfactory extension drive shaft of extremely light weight and compact design could be produced. The reduction gear housing were to be removed from the engine and the shaft driven directly from the crank shaft at crank shaft speed. It was proposed that a reduction gear unit of 9:5 gear ratio would be located in the nose of the airplane provided with a hollow offset propeller shaft to permit the automatic shell-firing aircraft cannon to be fired through the center of the propeller hub. Drives for the machine gun synchronizers and a propeller governor were to be provided for on the independent reduction gear box. The solution looked very good and the Allison Engineering Co agreed to develop and furnish for installation in the new P-39 airplane a 1150-hp, type V-1710 Allison engine equipped with such an extension drive shaft arrangement. This power plant arrangement permitted a most compact and efficient installation and an ideal airplane arrangement.

The new airplane was laid out incorporating an ideal nose shape and a cleanliness of fuselage contour never achieved on an airplane of the P-39 type with the engine installed in the nose. The problem Bell Aircraft was faced with was to provide a sufficiently light and satisfactory structure. It was necessary to support the independent reduction gear box and propeller on the nose, the armament installation, the retractable tricycle gear, nose wheel, the pilot and his cockpit equipment and to connect together the airplane wings and tail besides adequately hooking up the power plant components. It was decided to build what constituted essentially the front half of the fuselage as a stressed skin, sheet metal platform incorporating two main longitudinal beams. These beams were continuous from their attachment to the propeller reduction gear box back through the gun compartment, under the pilot’s cockpit station and under the engine to connect to an oval monocoque rear fuselage assembly which carried the tail surfaces. A top deck over the beams constituted the floor of the cockpit and the gun compartments and the engine bearer rails were attached to the top of the beams where they passed under the engine. Transverse beams in this platform provided the wing carry-through from the two detachable outer wing panels. The tricycle gear nose wheel was attached to the two longitudinal main beams of the front fuselage section and retracted by folding aft between the beams. A very desirable arrangement for accessibility was possible in having the upper portion of the fuselage above the beam section constructed principally of removable cowling. This arrangement permitted the manufacture and installation of the complete pilot’s cabin as a unit assembly and made possible the use of large automobile type doors on each side. It also permitted unexcelled accessibility to the engine installation. Facility for such work was increased because the engine was so located that the wing could be used as a working platform by the mechanics. By simple disconnection of the 6 bolts in the extension drive shaft flange the engine could be removed by being hoisted directly from its support on the longitudinal main beams without disturbing the propeller or armament installations, an important time and trouble saver. An accurate and complete mockup was made and these conditions were checked and the opinion of all concerned was that the accessibility and conditions for simple and improved installation was in excess of anything available in comparable airplanes of the conventional arrangement.

A careful analysis of this structure indicated that satisfactory light weight strength and rigidity could be obtained. There were, however, as has been mentioned, certain problems as yet unsolved. One of these was the fact that the airplane fuselage beam assembly was to all intents and purposes a part of the engine assembly since it was really a crank case extension and acted as a housing and support for the extension drive shaft and independent gear box assembly and propeller. It was, therefore, obvious that the transmission of power through the shaft would impose not only certain torque loads on the fuselage beam structures, but would also subject it to critical vibration as it occurs in many phases in a high power aircraft engine. It was also obvious that the engine would induce these torque and vibration conditions on the fuselage structure at all times and for all conditions of flight including when the airplane structure was loaded to design limits by the air loads of prescribed maneuvers. Therefore, many calculations were made and numerous physical tests were instigated to check the vibration resonance period around the vertical, lateral and rotational axes of freedom to insure that no resonance would occur in the airplane structure that would prevent satisfactory function of the engine shaft drive or unrestricted use of the airplane. How well this work was done is borne out in the final flight and ground tests of the airplane, where the only resonance that was found for any condition of operation was a local vibration in a sub frame at the cockpit doors that acted up in flight at 2,200 rpm engine speed. This small item was quickly modified to eliminate its case of St. Vitus dance.

The Allison Engineering Co converted their designs to metal as rapidly as possible, and within a few months after the beginning of this development work produced a complete engine shaft and gear box assembly test setup. A special support was built on the test stand at the Allison plant to support the extension shaft and propeller drive gears in a manner that would simulate the conditions of support as they occurred on the actual airplane. This supporting structure permitted the forward end of the extension shaft to bounce to the vibrations induced by the propeller and torque with a total vertical amplitude of about 3¼ in which would correspond to the maximum deflections that would occur in the airplane structure under the full design load. Very complete and exhaustive tests were run on the drive shaft at the Allison plant and during these tests the entire drive arrangement was run for considerable length of time at horsepower ratings well above the 1,150-hp normal rating established for this equipment, the peak being close to 1,400 hp. At one time during the engine tests the gear box at the forward end of the extension shaft was deflected 1.6 in out of line and held in this position for 20 hr of engine testing at full power. A more severe test for the flexible drive connections and operation of the shaft under a difficult condition could not have been devised. In some 300 hr of test stand running and approximately 110 hr of flight test of the engine drive shaft and gear box units only one minor difficulty occurred which required change.

Due to the circumstances it is believed that a case history of this minor difficulty should be told here. With the magnitude of this development project and the interest evidenced in it, all the testing was conducted under extremely high pressure. All of the engine tests were watched with an extremely critical eye by the engine manufacturer, the Army Air Corps, airplane manufacturer and aviation industry at large. The trouble which developed in the engine occurred in a quill drive shaft located in the accessory drive section of the engine. This quill drive shaft was a new mechanism that had not been used in any previous Allison engine and was part of the modification of the basic engine design to permit the use of the extension propeller drive shaft. Due to the unfortunate limitations of standardized technical nomenclature the engine accessory quill drive shaft was none the less a drive shaft and could be and was readily confused with the extension propeller drive shaft although no similarity of function or design existed. The quill shaft trouble was readily tracked down and determined to be a condition of flutter that occurred in the small spring quill accessory drive shaft. At idling engine speeds the flywheel effect of the rotating mass of the engine crank shaft system was inadequate to prevent the torsional resonance of the extension propeller drive shaft from deflecting through to the accessory drives. A simple dampener mechanism attached to the quill shaft eliminated the trouble but the damage had been done. The aircraft industry grapevine carried the news that the Allison extension drive shaft engine developed for the Bell P-39 was having shaft trouble, Three years later it is still not uncommon for representatives of Allison or Bell to be queried as to whether the extension shaft trouble was ever completely overcome. The true story of the shaft trouble is that the propeller extension shaft drive has never given a minute’s trouble or displayed any undesirable functional characteristics whatsoever and that the small quill accessory drive shaft in the engine has been properly fixed and is operating satisfactorily. The story of the shaft trouble is told here in the hope that it is no longer news.

As has been stated before, the development of the extension drive shaft installation was the major problem to be solved in the construction and development of the P-39 Bell Airacobra airplane. All of the ability, research and test facilities of the Allison Engineering Company and Bell Aircraft Corporation were concentrated on the solution of this one basic problem and the satisfactory results obtained from the installation must reflect to some extent the effort placed on this development work. The extension drive shaft installation is, however, not the only development and innovation incorporated in the P-39 airplane.

Other new innovations, one of which was submerged cooling, were also incorporated in the Bell P-39 airplane. "Submerged cooling" is a technical term that describes an arrangement wherein holes are provided at suitable points in a basic airplane component, such as the wing surface, through which the air required for cooling the power plant is taken in and discharged. Internal ducts are provided to conduct the cooling air from the entrance hole to the exit hole and the radiators or other heat dissipation equipment are located in these internal ducts. Since the fuselage shape itself had been made to a true streamline form of optimum aerodynamic efficiency it was believed undesirable to destroy this condition by the addition of numerous scoops and ducts and external appendages such as were normally used to house the radiators and conduct cooling air to and from them. A large amount of basic research work had been done by the National Advisory Committee for Aeronautics regarding possibilities and desirable arrangements for submerged cooling and for proper design of internal ducts in the airplane wings. An early phase of this arrangement had been used very successfully on the Bell Airacuda and it was indicated that the large amount of cooling air required could be obtained with minimum effect on the aerodynamic characteristics of the P-39 airplane by taking cooling air in approximately at the stagnation point of the leading edge of the wing and expanding the duct internally to the Prestone and oil radiators and by providing a converging duct aft of the radiator leading to a discharge located at or near the trailing edge of the wing. From tests made on models and from design data available from NACA this arrangement for cooling was included in the airplane and is no small contributing factor to the superior performance of the P-39.

Another of the innovations in the airplane was the inclusion of the 37-mm automatic aircraft cannon of the type developed by the US Army Ordnance Department suitable for tiring 37-mm explosive projectiles at 2,000 ft per sec muzzle velocity at a high rate of fire. This gun, weighing close to 200 lb, had been developed for the use of our Army as an antiaircraft defense gun, and was at one time, not too long ago, considered too large and powerful to ever be carried by small fighting military aircraft. On the Airacobra the arrangement and equipment and facilities for mounting and carrying guns permitted the design of a proper placement and support and location of the gun and feed mechanism so as to permit its installation. The carrying of this powerful weapon, precedent for which has been only established in the Bell Airacuda, a large, twin engine multi-place fighter which carries two of these guns, is an important factor in providing increased firepower. Installation was also made in the isolated forward gun compartment of the P-39 fuselage to install two .50-caliber and two .30-caliber aircraft machine guns in a very compact, accessible and satisfactory manner. These five guns grouped closely on the pilot’s line of sight at the centerline of the airplane provide a concentrated mass of fire power that is believed to be the greatest carried by any single place, single engine military airplane in the world. The arrangement of the airplane and the gun installation is such that a 20-mm cannon of extremely high power firing explosive bullets may be substituted for the 37-mm cannon where it is desired to have higher muzzle velocity and a more rapid rate of fire with somewhat smaller size explosive projectiles. Provisions have also been made in the airplane to install four .30-caliber machine guns in addition to the other armament in the wing panels outside the propeller disc, to provide an increase volume of area fire in addition to the concentrated fire power located in the fuselage.

The placement of the pilot ahead of the engine in his little "office" located near the nose provides some innovations and visibility not heretofore realized on single engine tractor military aircraft. The vision provided for the pilot is excellent in all directions. So-called fighting vision for the pilot in the area forward and down is a very important consideration. Vision in this area is provided on the P-39 equal to that obtained in most twin engine airplanes and is a very desirable feature. There are numerous other innovations in detail design of the airplane which are beyond the scope of this short discussion; the use of magnesium alloys for certain parts of installation and construction permits a saving in weight and reduction in size of the airplane to a minimum, the completely tested and developed tricycle gear added much to the safety for operation and utility for the airplane, certain aerodynamic developments in the design of the wings, wing flaps and surface control system, make the Bell P-39 Airacobra a completely modern and distinctly advanced military airplane with great speed, exceptional facility for maximum utility and extremely powerful fire power. It is indeed a development in which all who have done their part should feel proud.

This article was originally published in the April, 1941, issue of Aviation magazine, vol 40, no 4, pp 46-47, 142, 144.
The original article includes a photo of the P-39 in flight, a photo of a fuselage in static load test, and a cutaway diagram of the airplane.