February/March Dakota Territory Air Museum P-47 Update

This month, the pilot’s seat, tailwheel uplock, cockpit enclosure, and ADI injection kit were major areas of restoration work for the fuselage. On the wings and empennage, doors for the gun and ammo bays saw attention. The inner landing gear doors were precisely fitted, and work on the elevators and flaps continued to show good progress.

Landing gear, gun, and ammunition bay doors were the main focus of the work completed on the wing this month.

Many details of systems and connections on the engine and accessories installation were completed this month.

One of the systems for the engine installation is the Anti-Detonation water Injection system (or ADI). A new old stock ADI kit, dated 9/5/44 on the box, is an exciting find.

The derichment valve decreased the amount of fuel to the engine under Water Injection. When the power is pushed into the water injection range the derichment valve will come into effect.

With the normal high power rich mixture, the engine could fail when water injection or ADI was used.

Chad and Chris worked hard on restoring the cockpit enclosure and test mounting to adjust the fit.

Curved wingtips like the P-47s are more difficult to restore than more squared-off tips like on a P-51.

The last items inside the cockpit are being completed. Warbirds flying in the environment of modern airspace almost always incorporate modern avionics as a safety concession. Often, these avionics are installed in a way that retains the original appearance of the cockpit. Original appearance combined with modern safety and technological advantages were the goals for the P-47.

On the lower left of the instrument panel, Aaron added a circuit breaker panel for modern avionics (it has six round holes and one keyhole-shaped hole). He designed it to mimic the oxygen control panel on the lower right side of the instrument panel.

The larger diameter line is for main and tail gear retraction hydraulics. The smaller tubes running near the bottom of the inspection panel hole are for the flap hydraulics.

As is the case with most new fighters and indeed any new aircraft design, there were some teething problems during the initial testing of the XP-47B. 

First, an explanation of why the first P-47 to fly was the XP-47B.  Customarily the “B” designation is for a model version of an aircraft built and flown after the “A” model.  In the case of Republic’s XP-47 series, the original XP-47 was a completely different design, a lightweight fighter powered by an 1150 hp Allison V-1710-39 liquid-cooled in-line engine. It was designed to carry only 2 .50 caliber guns while the P-47A was a version of this design without guns or military equipment intended to speed a prototype into testing. 

However, combat experience showed two guns to be entirely inadequate. So the original lightweight Allison-powered P-47 project was canceled. Remarkably, and contrary to common practice, the Army Air Corps issued a new contract that retained the P-47 designation for the next fighter project as well.

The new P-47 project was more closely based on past Republic fighters like the P-43. The availability of the new Pratt & Whitney R-2800 made great leaps in performance possible if it could be incorporated into an evolutionary design. This design was to be the XP-44. However, before a prototype was built, the Experimental Aircraft Division of the USAAC canceled the XP-44. It was at this time that the XP-47 lightweight fighter contract was also officially canceled. War experience information from the European theater showed that the USAAC needed a fighter with much greater range and firepower than the two experimental projects could ever offer.

A new set of requirements was drawn up and a contract was authorized to build a fighter to meet them. That fighter was to be designated the XP-47B. The new USAAC requirements were as follows:

1. The aircraft must attain at least 400 mph at 25,000 feet.
2. It must be equipped with at least six .50 caliber machine guns, with eight being preferred.
3. Armor plate must be fitted to protect the pilot.
4. Self-sealing fuel tanks must be fitted.
5. Fuel capacity was to be a minimum of 315 gallons.¹

The new fighter was ready to test fly on May 6, 1941. Republic’s chief test pilot Lowery Brabham lifted the big new fighter off Republic’s sod field. Brabham was immediately pleased with the takeoff and control performance. But one factor he wasn’t aware of was to shape the rest of his flight. A large amount of oil had accumulated on the exhaust manifold and stainless steel exhaust ducts leading back to the turbosupercharger during the long check out period before the test flight.

As soon as the waste gates closed and hot exhaust ran back through those pipes, the oil heated up and started turning to smoke. Smoke filled the cockpit and Brabham thought of getting out. He tried to get rid of the smoke by opening a small sliding panel in the cockpit enclosure but that only made things worse.

Fortunately, Brabham was an experienced test pilot with many hours of test flight experience that included dealing with in-flight problems. He knew losing the prototype of a fighter would set back the war effort by months. Not seeing any flames, Brabham made the decision to stay with the XP-47 and land at Mitchel field as had been planned all along due to the soggy grass field at Republic’s Farmingdale factory.

In the few minutes he had been in the air, Brabham was already convinced that the new P-47 had great potential. In fact some sources quote him as saying “I think we’ve hit the jackpot!” after the flight.

The first landing of the XP-47 went smoothly despite the cockpit smoke. Flaps, landing gear, and brakes all worked as designed. The big fighter met the AAC requirements with two exceptions.  It carried 298 gallons of internal fuel instead of the specified 315, and it weighed 900 pounds more than specified. But the AAC was willing to overlook these minor issues since the performance was unprecedented and met the specifications.

There were more issues to be solved as testing continued. The P-47 was flight tested at operational altitudes that hadn’t been customarily reached before, and the ignition was failing at these high altitudes. Oil pressure couldn’t be maintained because the oil was boiling in the low pressure, high altitude environment. Fortunately, pressurizing the entire ignition system and bleeding turbo pressure into the oil tank solved these two issues.2

^{1 Aviation Darwinism – The Republic P-47 Thunderbolt ( author unlisted), https://www.cradleofaviation.org/history/history/aircraft/p-47_thunderbolt_aviation_darwinism.html, downloaded 3-29/2022
2 Lowery ‘Brab’ Brabham,P-47 Pilots Association website, downloaded 3-2-30 2022, http://p47pilots.com/P47-Pilots.cfm?c=incP47BiographyHome.cfm&vm=BIO&pilotid=47&p=Lowery%20%27Brab%27%20Brabham}

Lowery Brabham’s flight test issues weren’t over with the first XP-47 flight. Another design problem related to turbosupercharger gases eventually caused the loss of the XP-47 in 1942. The tailwheel failed to retract and caught fire because of the very hot gases expelled by the turbosupercharger. Those gases exit directly in front of the extended tailwheel. That fire burned through the fabric on the elevators.

Brabham successfully bailed out, but the prototype XP-47 was lost. The elevators and all control surfaces were subsequently changed to all-metal construction.