The Navy and Liquid-Cooled Engines

Although the Navy effectively ended deployments of airplanes powered by a liquid-cooled engine in the late 1920s, that wasn’t the end of its interest and involvement in the technology. Its racers were powered by liquid-cooled engines up through 1930. The Navy funded development of the Wright H-2120 engine from 1933 to 1936 (click HERE). In 1938, the Navy’s Bureau of Aeronautics included the Allison-powered FL-1 Airabonita in its competition for a new high-performance fighter.*

 In 1940, the Navy began funding the development of Lycoming’s 2,200 hp H-2470 engine and in June 1941, BuAer contracted with Curtiss for the F14C to be powered by that engine. Pratt & Whitney’s even bigger H-3730 also received Navy funding for a time

At some point in World War II, the Navy acquired an Allison-powered early  P-51, 41-37426, from the Army Air Force.  It was one of a production lot of Mustang Mk 1As that were ordered by the RAF, some of which were taken instead by the Army Air Force. It was assigned BuNo 57987.

None of the post-1930 programs resulted in an operational aircraft—or engine for that matter—but the F14C (which flew with an air-cooled Wright R-3350) was only the penultimate effort. In a 21 March 1944 memorandum to BuAer, the Deputy Chief of Naval Operations (Air), Admiral J. S. McKain, asked for what proved to be the final evaluation of the technology:
1. It is requested that the Aviation experimental program be subjected to a study and review.
2. It is considered that experiences of the war and the present trend of design within the Navy may point to the necessity of reefing our sails and. departing on a new tack.
3. The weights and physical measurements of our newer airplanes are increasing rapidly with the result that we shall soon be forced into reduction of carrier complements. To obtain increased performance the natural and logical step has been an increase in power. However, the direct result of this has been larger engines presenting greater frontal areas with correspondingly larger fuselages to accommodate them.
4. Possibly we have reached the place in the development of the air-cooled engine where we should sever our many years of allegiance and look to a design of aircraft incorporating liquid cooled in-line power plants.
5. The size and weights of our projected designs is ample evidence of the fact that our engineers must concentrate on smaller and more compact aircraft with which to do the job at hand. If the liquid cooled in line engine represents the step necessary to achieve the results, we should then be spending some of our time and money on development of that article.

That direction proved controversial within BuAer, but it was not without supporters. The performance of the North American P-51 Mustang had impressed some. As a result, BuAer agreed to request proposals for a liquid-cooled engine powered fighter, as well as one powered by a combination of a liquid-cooled and jet engines. In July 1944, BuAer Military Requirements stated that the Navy’s next fighter should have a Vmax of at least 425 mph at sea level and 475 mph at a critical altitude of 25,000 feet. It would be able to climb to 30,000 ft. in five minutes. Its combat radius should be 300 miles in the escort version, 75 miles in the interceptor version. Either version should be able to remain in the air for six hours.

In August, Boeing, Grumman, Curtiss, Ryan, and McDonnell all informally declined an informal invitation to propose a liquid-cooled engine powered fighter, either because they sensed that the Navy wasn't serious about liquid-cooled engines or realized that the performance requirements were beyond challenging. In the event, only North American submitted a proposal, in October 1944; Vought responded with a design study that concluded “that other types of power plants were more suitable for future fighters.”

The North American proposal was based on its P-51H, the culmination of its effort to improve on the P-51D, focusing on a better rate of climb and roll. The Army had ordered the P-51H into production in April 1944. It was very similar in appearance to the P-51D but was slightly longer, with an elongated ventral cooling installation, and very different in detail.

The NAA-133 was to be powered by a Merlin V-1650-11 engine planned for the stillborn P-51L. The wing was redesigned to increase the wing area by 10 square feet (an increase in wing chord of about three inches since the wing span was not changed), substitute larger and slotted flaps, and incorporate wing folding. The ailerons were enlarged and hydraulically boosted. The landing gear was strengthened and an arresting hook and catapult hook added. The Mustang’s fuselage fuel tank was deleted and the wing fuel capacity was reduced to 150 gallons. The elimination of the fuselage tank allowed for a smaller horizontal tail while maintaining adequate longitudinal stability. The reduction in internal fuel capacity was to be offset by the addition of 50-gallon tip tanks when required. Here is the North American drawing of the P-51H with the NAA-133 changes shown in red:

(The NAA-133 fin and rudder are depicted with dashed lines although they are as depicted on the relatively crude North American three-view provided to BuAer.)
In November 1944, doubtless as part of this evaluation, a minimally modified P-51D was evaluated aboard Shangri-La by Bob Elder after shore-based trials.

As tested, rudder control power and the tail hook attachment were marginal. Elder stated that the rudder was on the stops at 82 mph in approach configuration and the hook structure strength limit was reached at 90 mph. As a result, he had to approach at 85 mph: “Fortunately the little lady exhibited marvelous speed control characteristics and even though operating at near minimum margins of directional and lateral controllability (limited by torque) wave-offs could be executed by judicious application of power.” He reported that visibility during approach was no problem: “Some of the radial engine fighters of that era, notably the F4U and F6F with cowl flaps open, had even more restricted forward visibility and I simply made a turning approach almost to touch-down as was the practice at the time.”

The November 1944 BuAer report on the North American proposal concluded that the NAA-133 failed to meet the desired performance by a wide margin, which wasn't a big surprise to BuAer's engineers, although it was superior to the F8F-1 and the F2G-1. The tip tank option was judged to be inferior to the use of conventional drop tanks. The engine-related “serious disadvantages… for carrier use” were:
(a) Increased vulnerability inherent in any liquid-cooled arrangement over air-cooled types.
(b) Increased maintenance because of addition of radiators and aftercoolers.
(c) The airplane as presented will possess the poor ditching characteristics of the P-51 series airplanes.
(d) Introduction of liquid cooled type will increase logistic problems, and necessitate an additional training program.

Fortunately, North American had also submitted a proposal, NAA-134, for a jet-powered fighter based on the P-51.

 The Navy ordered it as the FJ-1 as part of its initiative to enter the jet age.

The Air Force’s P-51H flew for the first time on 3 February 1945. However, only 554 were built before production was terminated in November 1945 as part of the draw down following the end of World War II and realization that all future fighters would be jet propelled.

For much, much more on the P-51H (although nothing on the NAA-133) see the excellent monograph by David McLaren published by Steve Ginter (http://www.ginterbooks.com/AIRFORCE/AF209.htm).

For more on the P-51D carrier trials including pictures, see Mustang: The Story of the P-51D Fighter by Robert W. Gruenhagen.

*For more on liquid-cooled versus air-cooled engines and the Bell Aircraft FL-1 Airabonita, see my FL-1 monograph also published by Steve (http://www.ginterbooks.com/NAVAL/NF81.htm).

Most of the above material is derived from the research of Ryan Crierie, who generously shares his hard-won information.

Modelers will find more information on the Seahorses HERE.