May 112011
 

In 1947, at the request of the Bureau of Aeronautics (BuAer), Chance Vought Aircraft conducted a comprehensive design study of long range carrier based fighters. The purpose of the study was to determine the airplane performance, size and optimum arrangement with various available power plants, including turbojet, turboprop and reciprocating engines. The effect of reducing the radius requirement from the selected radius of 1,200 nautical miles was also determined as well as the effect of variation in cruising altitude. The results of the study were presented in CVA Report No. 7549, titled “Design Report — Long Range Carrier Based Escort Fighter Design Study.” This study showed that truly long range fighters could be designed for carrier operations at sizes no greater than contemporary carrier types such as the F7F and the AD-1.

Concurrent with this study, Chance Vought was also making very preliminary design studies for carrier based attack or strike airplanes. As the work progressed, the possibility of designing an airplane able to perform both missions was brought under consideration. The desirability of having an airplane which could perform more than one type of mission was obvious. However, there was some doubt whether a satisfactory airplane would result if it were designed initially for more than one type of mission. Experience and history had shown that in every case the airplane which had been designed to be an “all purpose airplane” had been found unsuitable for any purpose. The airplanes which had been able to satisfactorily fulfill more than one type of mission had been developed from a basic highly successful single purpose design by suitable modifications at a later date. Experience seemed to indicate that an airplane which was able to successfully perform a variety of missions had been first an inherently satisfactory design for the fulfillment of its basic mission. Such was the case of the F4U Corsair which was originally designed purely as a fighter, but which had been highly successful not only as a fighter, but as a VBF bomber and rocket attack airplane through subsequent continuing modification and development.

As the study work progressed, the desirability of supplying an advanced airplane to the fleet which would perform the various missions of the Corsair was kept in mind. It was felt that the fleet would probably have a real need for a “work-horse” airplane, if a thoroughly satisfactory one could be supplied. Vought’s desire in this connection was, of course, tempered by their knowledge of the facts mentioned in the previous paragraph, namely that no successful “multi-purpose” airplane had ever been designed as such originally. However, as Vought examined the situation more closely, it became apparent that the basic conditions were quite different from what they had been in the past and that it was possible to provide for two types of missions without significantly compromising the design of the airplane or adversely affecting its potential performance. This was possible in this instance where it had not been in the past, primarily because the fuel quantity required was much greater than was the case several years prior, due to the greater radii of action desired and the higher fuel consumption of more powerful modern engines. Since the outgoing fuel for the long range escort fighter mission was carried externally it could readily be replaced in part or in whole by bombs, with corresponding reduction in radius. Since the external fuel quantity required was so great, a substantial and adequate bomb load could be carried.

As the above general discussion indicates, Vought found it possible to present for consideration an airplane which performed both a long range fighter and a moderate range attack mission without significant compromise in size or performance over an airplane designed purely to perform the long range fighter mission. This airplane, which was designated by Chance Vought as the V-358, was a twin engine airplane powered by Pratt & Whitney PT-2 turboprop engines, as studies had shown that the PT-2 engine offered the optimum compromise between size and performance for the missions considered. The purpose of the proposal was not only to describe and discuss the airplane arrangement, but also to invite comment on the selection of basic characteristics which were subject to choice in arriving at the optimum airplane, as well as on the assumptions made and specification requirements used. Among the choices to be made were the basic radius of action and airplane size, the crew arrangement, and whether the airplane should be designed basically as a VF or as a VA airplane. The effects of the possible choices on the airplane characteristics such as size and performance are discussed below.

Power Plant

There were strict physical limitations imposed on the weight and size of carrier based aircraft by shipboard elevators, hangar deck clearance and general handling requirements aboard ship. Because of these limitations the optimum engine was one which not only provided good high speed performance but also had a minimum weight of engine plus fuel, giving the smallest airplane for a given radius of action. The long range escort fighter study showed quite conclusively that the turboprop engine offered the lowest power plant plus fuel weight for the radii of action under consideration and therefore offered an optimum compromise between the conflicting requirements of fuel economy, take-off and high speed. Other power plants, such as the reciprocating engine and the turbojet, had less suitable characteristics in this connection, either for the attack mission or for the basic long range fighter mission as reference to CVA Report No. 7549 showed. Although an airplane powered by two “hypothetical” turbojet engines in the long range fighter design study appeared to offer attractive performance, it must be emphasized that the power plant data used for this engine was extremely preliminary and undoubtedly quite optimistic within the time span under consideration, and therefore not comparable. The Pratt & Whitney PT-2 turboprop engine was selected as giving the optimum compromise between size and performance of the available turboprop engines that fit the power requirements of a dual purpose attack fighter. The major advantages of the PT-2 engine were its favorable ratio of jet thrust to shaft power, use of single rather than dual rotation propellers, better intake duct configuration than other turboprop engines considered, and the fact that the engine was optimal for a clean, light nacelle installation.

Design Configurations

A study was made to determine the penalties in airplane size and performance that would result from providing for a second crew member. Two airplane arrangements were considered. The first was primarily a long range fighter airplane with a single place cockpit and only space provisions for a second man and his equipment in the aft fuselage, completely remote from the pilot. When a second crewman was needed, he and his equipment were carried as overload, causing an increase in the stalling speeds of the airplane. The second configuration considered was primarily a medium range attack airplane with a two place cockpit and weight provisions for the second man and his equipment included in normal useful load. With the constantly expanding tactical use of modern electronic equipment for navigation and search it became increasingly important to pick an airplane configuration that provided adequate crew facilities and flexibility of space provisions. At least having provisions for a two man crew appeared to be mandatory for some fighter and all strike missions in view of the possibility of escorting drone bombers on the former and the handling requirements of elaborate radar equipment on the latter. Vought studies had shown that this desired flexibility of arrangement could best be obtained with a twin engine airplane, as it offered greater fuselage space for crew and equipment and, most important, provided an unobstructed nose section for guns and radar scanners. Both the basic single place and the basic two place configurations were capable of performing the long range fighter and the medium range strike missions.

Design Summary

The V-358 design booklet summarized in this article compared the two basic arrangements discussed above. The combat problems used for the VF and the VA missions were presented, as were the basic characteristics of the PT-2D power plant. Performance curves showed both the VF and the VA airplane arrangements plotted against radius of action on both the VF and the VA combat problems. These curves showed an infinite number of airplanes for either design, making it possible to select a particular radius of action as a fighter for either configuration and thereby determine not only the size and performance of that airplane as a fighter but also the equivalent radius of action and performance as a bomber. Vought noted that the change in size with radius of action was not very great. This was due to the fact that the wing areas for these airplanes were all determined by meeting the landing stalling speed requirement, and fairly large increases in take-off fuel load did not have a significant effect on landing weight since only ¼ of the internal fuel was included. All the airplane illustrations presented were at the 1,200 nm fighter radius point. The image browser on page 1 shows three-view and inboard profile drawings of the single place VF and two place VA arrangements, as well as a size comparison of the VA version to the AD-1 and the F7F; it will be noted from this comparison that the V-358 airplanes were generally no greater dimensionally than either of these contemporary service types. Also shown is a flight view of the basic fighter version of the V-358 with wing tip drop tanks, as well as a flight view of the basic attack airplane with 2,000 lb bombs on the wing tip stations; the drop tanks and bombs were interchangeable from fighter to bomber. An alternate stores arrangement for the attack mission, with a 2,000 lb bomb on the centerline station and 1,000 lb bombs on the center section stations is also shown. Pictorial presentations of the two basic cockpit arrangements are shown, as are the advantages of nose wheel well access made possible with the two place cockpit. Typical emergency egress provisions are shown, as is the carrier spotting pattern. A summary of the physical characteristics of the chosen basic VF and VA airplanes is also included. The performance of these airplanes is summarized in some detail in the accompanying tables above.

As a matter of general interest in power plant selection the general summary chart was taken directly from the long range fighter design study report (CVA Report No. 7549). Vought noted that the airplanes on this summary chart were not fully comparable with the V-358 airplane studies herein. This is the result of more up-to-date data on permissible design maximum lift coefficients and tail volume coefficients than were available for the fighter design study. However, since the long range fighter design study was primarily a power plant comparison and the airplanes in the study were fully comparable one to another. This had no effect on the relative performances with the various power plants and various design radii of action shown.

Aerodynamic Characteristics

The basic airplane configuration and airfoil sections used for the V-358 were based on the attainment of as high a critical Mach number as possible from both a drag and control critical viewpoint, without sacrificing good low speed characteristics and a high maximum lift coefficient. Airfoil sections, such as the CVA-4 sections of 11% thickness, were considered as being best for obtaining good high and low speed characteristics.

In order to keep airplane size to a minimum, the greatest possible flap span was desirable so as to obtain a high maximum lift coefficient. The V-358 design studies were based on the use of only 15-20% semispan “feeler” ailerons for this reason. Lateral control was obtained with circular arc spoilers in addition to the feeler ailerons.

The most advanced propeller designs, incorporating wide blades, thin sections and other characteristics to give good propeller efficiency at high Mach numbers, were used for the V-358 design studies. Propeller efficiencies were estimated on the basis of the latest NACA wind tunnel information on high activity factor blades at high Mach numbers.

Rejection

BuAer did not award Vought a contract for the V-358, possibly because it saw more promise in the turboprop-powered Douglas A2D Skyshark, which was ordered in 1947, the same year as Vought submitted this proposal. Development of the Skyshark was hampered by severe problems with its Allison XT-40-A-2 turboprop power plant and gearbox; the program was eventually cancelled in 1954. The Navy had to rely on the less advanced but more dependable Douglas AD-1 Skyraider for the attack mission for many years to come, with turboprop aircraft playing a minor role in its inventory.

All images from NARA Archives II, College Park, MD, RG 72

Sources:

“Model V-358 Attack-Fighter Design Study,” in the files of the National Archives at College Park, MD, RG 72.

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