Warbird Digest has just received the January, 2021 report from Chuck Cravens concerning the restoration of the Dakota Territory Air Museum’s P-47D Thunderbolt 42-27609 at AirCorps Aviation in Bemidji, Minnesota. We thought our readers would be very interested to see how the project has progressed since our last article on this important project. So without further ado, here it goes!
This month work on the secondary cowl and turbosupercharger connections began. The modified oxygen bottle brackets were installed. Progress on the wings and cockpit systems continued. This restoration has been a long process, but every little step brings us closer to the test flight of the only Republic-built Razorback under restoration.
The secondary cowl covers the accessory section forward of the firewall. Several secondary cowl skin panels were fitted this month.
Both are part of a system that increases voltage to power radio systems; vacuum tubes in 40’s-era electronics require higher voltages than their modern, solid-state counterparts.
The restoration team has made many of the initial connections to electrical terminals on various switch boxes and control boxes. This month Aaron started putting the connected wires into bundled, wiring harnesses.
The wings gained some skin panels, extrusions around the gear openings, and inspection panels this month.
Several of these panels already have their covers installed – like the ones on the yellow zinc chromated reinforcing plate on the image above. The line of rectangular openings in the skin angling across the upper right of the photo are for inspection access covers for the flaps and ailerons.
42-27609 has a non standard location for the oxygen bottles, necessitated by the field installation of the Christmas tree fuel tank.
The Complex Turbosupercharger System
The P-47 had the ability to perform well at high altitudes, whether as an escort fighter or in air to air combat, but it could also operate at low altitudes in a fighter bomber role with great success. This operational flexibility is attributable to the Thunderbolt’s Pratt & Whitney R-2800 Double Wasp, twin-row, 18-cylinder radial engine coupled with its complicated turbosupercharger system. The turbosupercharger system made the P-47 Thunderbolt a complex fighter for the time period, perhaps the most complex single-engine fighter in the American arsenal.
It may be helpful to think of an aircraft turbosupercharger (#17 in diagram) as a big air pump. It takes in cool intake air (light blue in the diagram), and pressurizes it. That air, now hot from the heat of pressurization (dark blue in the diagram) is pumped through an intercooler (#11) to bring the temperature down, because cooler air is denser. Passing through the intercooler, the hot air is cooled and continues on to the carburetor. When it arrives, it is mixed with fuel, and the mechanical supercharger pushes it into the cylinders of the R-2800 via the intake manifold and intake valves.
The ideal air/fuel mixture for a normally aspirated gasoline internal combustion engine is around 14.7 parts air to 1 part fuel by mass. When an engine is supercharged, it can produce maximum power at a somewhat richer ratio of roughly 12.5-13 to 1. It is critical to always have sufficient air coming into the engine to maintain the correct ratio. As an airplane climbs, the air becomes thinner, and as a result, weighs less per unit of volume. The fuel volume also expands somewhat, albeit to a far lesser degree. Therefore, at higher altitudes a larger volume of air must be pushed through the engine in order to maintain the proper fuel/air ratio. This increased air volume requirement explains the need for either a turbocharger, or the second stage of a two-stage supercharger, at altitudes above 15,000 feet or so.
What Drives the Two Superchargers?
To operate, superchargers require power from the engine in some fashion; most being directly spun via a mechanical connection to the engine. The P-47’s R-2800 has a single-stage, single-speed, centrifugal type supercharger connected to the back of the engine, but it also has a turbosupercharger. While a mechanical driven supercharger needs to use some of the power produced by the engine, the extra power it allows more than makes up for the energy drain from its operation. An exhaust-driven turbosupercharger system also drains some engine power due to the resultant back pressure, but again, the gains far outweigh losses incurred in driving the turbo.
The turbosupercharger is spun by exhaust gasses (red in the diagram) ducted to a “bucket wheel” in the lower half of the C-23 General Electric turbosupercharger. In turn, that bucket wheel turns the impeller. The impeller takes in cool air in the center and, by centrifugal force, spins it outwards, pressurizing and pushing it out to the intercooler through ducting.
The Design Elements That Make a P-47 Look Like a Milk Jug
At some point in its history, the P-47 gained the nickname “Jug”. The origins of this sobriquet are frequently disputed/otherwise explained, but one of the more popular origin stories involves the name coming from the resemblance between the Thunderbolt’s deep-bellied fuselage and the shape of an American, forties-era milk jug.
The P-47’s milk bottle shape was largely dictated by the turbosupercharger system in the type’s fuselage. The oval cross-section fuselage looks bulky because the belly curves from the engine back to the turbosupercharger, but this design was necessary to allow room for the large R-2800 and the ductwork, intercooler, and turbocharger hardware enclosed within the fuselage.
Even the engine cowl opening’s unique, ovoid shape relates directly to the turbocharger, as the main air duct intake opening is located in the lower cowl; the cowl had to be designed around it. That’s why the P-47 cowl isn’t circular like those for many other R-2800 powered aircraft.
And that’s all for this month. We wish to thank AirCorps Aviation, Chuck Cravens for making this report possible! We look forwards to bringing more restoration reports on progress with this rare machine in the coming months. Be safe, and be well