North American ETF-51D; The “Sea Horse”

September 14, 2023

When it comes to carrier fighter aircraft of World War Two, there is one very notable attribute that they generally share; Air cooled radial engines.

This type of powerplant was preferred because it was considered far more reliable, especially for naval combat. After all, damage to the cooling system of any aircraft fitted with a liquid-cooled engine would rapidly mean the loss of the aircraft, probably long before it could get close to its home carrier. Radial’s were also, broadly speaking, simpler in maintenance than comparable inline engines; again an important factor for carrier air groups where keeping aircraft flying was done in far more cramped and complex conditions than on an airfield.

It is notable that the only exceptions to the use of radial engines in carrier fighters were by Germany and Italy, who built basically conversions of existing fighters which never flew off a flight deck and therefore don’t really count, and the British, who built the Fairey Fulmar, Firefly and the Seafire. This was largely due to the fact that the British didn’t get a decent enough radial engine into service until towards the end of the war, and they basically went all in on the Rolls Royce Merlin and Griffon.

As a result, the best carrier fighter the British produced that saw combat in that role during World War Two was the Seafire, which if you’ve read my article on you will know was a variable quantity.

So, the epic carrier actions that today get most of the attention were flown by aircraft like the Mitsubishi Zero and its great rivals the F4 Wildcat, F6 Hellcat and the F4U Corsair.

But the US Navy, despite officially stating that they would not be buying any inline powered aircraft from as early as 1921, still kept an eye on aircraft with those powerplants and occasionally toyed with the idea of bringing them into service. One example of this is the 1938 Bell Airabonita, very broadly a navalised P-39 that competed to be the US Navy’s new fighter.

Though the aircraft wasn’t selected for service, losing out to the Vought Corsair, the Navy did continue to display interest in liquid-cooled aircraft, actually investing in Lycoming’s XH-2470 H-block 24-cylinder engine with a view to fitting it on the Curtiss XF14C naval interceptor. And even before that project ultimately fizzled out, the Navy was still closely watching other developments in the field.

They received one of the early Mustang Mk.I’s that the US government had retained from the Royal Air Force’s order for the type for testing, which they used for assessing the then new fighter. This represented the start of the US Navy’s interest in the aircraft, which only became stronger as the capability of the Mustang began to truly be fulfilled with the introduction of the P-51 with the Packard-built Merlin engine.

And that in turn led to what is commonly known of as “Project Sea Horse”; an attempt to build a carrier Mustang.

The history of this project is somewhat confused, including the name as Project Sea Horse seems to have been an informal nickname given to concept by those involved with it.

There is also some confusion as to the intentions for this scheme. It is often stated that the project was intended to provide long-range fighter escorts for the upcoming bombing campaign that the USAAF was actively preparing for against the Japanese home islands. Neither land-based nor carrier-based aircraft had the range to escort the B-29s on their long-missions, which really got into their swing from late November 1944 when the Twentieth Air Force began operations from the recently captured Marianas Islands.

But that doesn’t seem to have been the principle reason for the US Navy’s interest in the carrier-capable P-51, though no doubt it could have served that role if such an aircraft was ever fielded. Instead, the Navy initially requested that their Bureau of Aeronautics investigate more fully liquid-cooled powerplants in March 1944.

The Navy had already issued orders in late 1943 for both new jet-powered and mixed jet-and-prop aircraft for assessment, which ultimately led to the McDonnell FH Phantom and the Ryan FR Fireball respectively. But there were still concerns that these new aircraft would not perform, as well as a recognition that though radial’s were getting more powerful, they were also getting considerably bigger, which in turn meant bigger airframes and a consequent impact on the limited numbers of aircraft that could be carried in the cramped confines of a carrier. So, in July 1944 feelers were put out for proposals for a liquid-cooled inline carrier aircraft that should have superior speed and range to the current fighters conducting operations.

Now to be fair, basically all of the aircraft companies recognized that the US Navy wasn’t serious about the idea, and simple declined to take part, leaving North American the only builder that displayed an interest. They proposed the NAA-133.

This was a navalised version of the lightened P-51L, which was an even more formidable version of the P-51H “super prop” that was in production and intended to become the new standard USAAF Mustang. The NAA-133 would have had a Merlin V-1650-11 engine that produced 2,270hp with water injection, wingtip tanks, enlarged folding wings, bigger flaps and arrestor gear.

But North American and the Navy also wanted to see how the type would handle the rigours of carrier landings and so a single P-51D, number 44-14017, was modified for testing. Designated as the ETF-51D, the aircraft acquired a tail hook, catapult linkage and reinforced undercarriage.

This began making shore-based test flights in September 1944 so that the pilot, Lt. Bob Elder, could get a proper feel for the aircraft, especially in the tricky landing. The P-51’s wing was notably not designed for low-speed maneuvering, and the aircraft had a stall speed of 82mph. As the arrestor gear was only reliable if the aircraft was not surpassing 90mph when attempting to land, that’s a pretty tight window.

However, after conducting 150 flights Elder was confident in his ability to land the naval Mustang on a carrier, and on the 15th November 1944 he did just that on the USS Shangri-la.

Elder conducted several landings and take-offs, coming in at a precise 85mph with each landing. Despite the concerns about the suitability of the P-51 for carrier landings, Elder reported that the aircraft was extremely responsive and that the visibility from the P-51s cockpit for carrier landing was better than purpose built radial aircraft such as the Hellcat and Corsair because of the much smaller frontal cross section.

But this was in the hands of a tremendously skilled and experienced test pilot, and the ETF-51D certainly had to be very carefully handled and it was recognized that the type wasn’t suitable for deployment. Indeed, after conducting 25 flights Elder himself concluded that:

‘Although I had “premiered” many US Navy aircraft carrier landings, no such experience had been as interesting as with the Mustang.’

Which I am sure is test pilot speak for “probable death trap for the average Joe”.

Plus the aircraft was essentially redundant anyway, either as an escort for the B-29 raids or as a high speed long-range fighter for the carrier fleet. In early 1945 the island of Iwo Jima fell to the advancing US forces, allowing USAAF P-51s to provide escort to the bombing campaign, while the US Navy was soon to field even more formidable fighters equipped with radials while actively working towards deploying jets with even better performance on their carriers.

And North American weren’t to bothered either as they had received an order for a new jet design evolved from the P-51 for the Navy – the FJ-1 Fury. This would go on to form the basis for a whole new family of fighter and attack aircraft, most famous of which is the F-86 Sabre.

So that is the story of the P-51 Sea Horse, a fascinating little footnote in the story of one of the most famous fighter aircraft to ever fly.



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