Designing the P-47 Thunderbolt

Sept. 1, 1982
In June 1940, a small group of men met in a room at Wright Field, near Dayton, Ohio. One can only guess at their mood. Since April, the Nazi blitzkrieg had poured into the mold of Western Europe like molten metal, and the Dunkirk evacuation in May had left only England, among the Allies, free of Hitler. The American “neutrality” of a year earlier was now a quaint memory as Washington began to transfer quantities of military equipment to the British.

Only one fact was more remarkable than the lightning swiftness of the war: the utter lack of military preparations in the US. This meeting at Wright Field was being held to change that.

Only hours before, on June 19, Gen. H.H. Arnold’s signature had endorsed the report of the Emmons Board (named for Maj. Gen. Delos C. Emmons, then commander of the GHQ Air Force), which had concluded earlier in the year that most, if not all, of the Army’s aircraft currently in service powered by the hoary Allison V-1710 would be inadequate in combat.

Among those present at the meeting were Lt. Col. Howard Z. Bogert and Maj. Franklin O. Carroll of the Air Materiel Division Experimental Engineering Section; Capt. Marshall “Mish” Roth, Project Officer; Julius Hulman of the Power Plant Laboratory; Paul B. Smith of the Technical Staff; and Maj. Lawrence “Bill” Craigie, Bogert’s assistant.

Two executives from Republic Aircraft were also present; C. Hart Miller, vice president of sales, and Alexander “Sasha” Kartveli, the vice president of engineering.

Defining Specifications

Kartveli, an immigrant from Russia, listened as the attendees began to describe the specifications that would be needed in fighter aircraft for the war that they tacitly agreed was coming. (Carl Spaatz and Ben Kelsey had already seen what the British thought was the minimum airplane necessary, and Bogert had properly been filled in.)

The fighter would require a ceiling of 40,000 feet, a maximum true airspeed of at least 400 mph at its optimum altitude, and no fewer than six machine guns, with room enough for two more and all of them mounted not in the nose but in the wings. The European practice of using heavy-caliber nose-mounted cannon often required that the propelling powder charge be reduced to keep the recoil forces within reason, so the 20-mm and heavier shells, while destructive when they hit, lacked the reach of machine gun slugs. And if enough machine guns could be massed on target, the force could be as great as that of a six-ton truck hitting a brick wall at fifty mph—enough to down a bomber.

As the specification took shape, Kartveli may well have mentally torn up the vellums for his XP-44 fighter. He knew the airplane being described in this room was something entirely different.

The guns and ammunition alone would weigh a ton or more, he reckoned. The airspeed and the operating altitudes would call for every bit of horsepower Pratt & Whitney could produce. But above all, he realized that this airplane would be far larger than any fighter then imagined.

Mish Roth later told of how Kartveli had sketched the outline of the fighter in a compartment on the train ride back to New York. Kartveli’s primary concern was wing loading, the single design point that seemed most critical to fighters. Based upon an estimated combat weight of 12,000 pounds, on the back of an envelope he drew a semi-elliptical wing with an area of 300 square feet. (Although neither Roth nor Kartveli knew it then, that’s the way the airplane would turn out.)

Republic had its AP-10 lightweight fighter, the Army’s XP-47/47A, on the boards, a comparatively microscopic airplane to be powered by the anemic Allison. In September 1940, that contract was altered to incorporate the specifications from the Wright Field meeting. On May 6, 1941, Republic test pilot Lowry L. Brabham flew the six-ton XP-47B prototype from Republic’s muddy grass to the concrete runway at Mitchel Field. Oil in the exhaust duct smoked the cockpit so badly that Brabham nearly parachuted. However, he managed to stay with the airplane long enough to get a feel for it and to proclaim upon landing, “We’ve hit the jackpot!”

Although it took Republic only eight months from contract to prototype, the pieces that formed the Thunderbolt (Hart Miller gets credit for the name) were, in fact, beginning to fall into place decades earlier—and that’s the real story of the P-47.

The Turbocharger

In every country outside the United States, the only way to supercharge an airplane’s powerplant had been a direct-drive or geared blower. In the 1890s, a University of California engineering graduated named Sanford B. Moss had posited the notion of turning a propeller using a nozzle and turbine buckets.

Throughout the early 1900s, Moss and others who attempted to design a turbine engine encountered stubborn hurdles in metallurgy and in compressor efficiency. Like today’s fusion-power researchers, Moss was unable to design a compressor that would leave him with net power from the turbine to do some work—the “break-even” point could not be reached.

Then Moss went to General Electric in Schenectady, N.Y., with an idea: The hot exhaust gas form a reciprocating engine could be put to some useful purpose driving a turbine; the turbine could be linked to a compressor and gas generator. Around the time of World War I, a GE turbocharger was affixed to a Liberty engine, and, soon afterward, the Army turbocharged its Curtiss D-12s and Allisons.

Thus, by the time Alexander Kartveli went shopping for a turbocharger for his AP-4/YP-43 fighter, Dr. Sanford Moss’s device was available, for sale, and listed in the General Electric catalog. A later NACA study estimated that the Pratt & Whitney R-2800 without a turbocharger was approximately twenty-eight percent efficient. With the turbocharger, the efficiency rose to thirty-three percent. The turbocharger remains one of the most significant contributions of American technology in that era.

The Engine

It was the early 1930s, and Pratt & Whitney was in deep trouble. After a brilliant lead under the direction of Fredrick Rentschler in the development of the Wasp and Hornet—lightweight, air-cooled radials that dominated both military and civil airframes—the company had mired in a proliferation of engine designs that seemed to be headed nowhere.

Archrival Wright had cleaned out its sock drawer in 1932 to concentrate on the development of the Cyclone and the new R-2600 and -3350. Ailing engineering vice president George Mead of Pratt placed his faith in his manager. Luke Hobbs, a steady hand who had signed on at McCook after World War I before coming to Pratt & Whitney.

Together, Mead and Hobbs began to thin the Pratt & Whitney garden until they could concentrate on just two viable designs: the R-1830 Twin Wasp and a 1,800-hp, 2,800-inch development of the twin-row R-2600, to be called the Double Wasp. At about the time they succeeded in setting this course, the Army began insisting that Pratt & Whitney initiate development of new liquid-cooled engines.

Hobbs, bewildered by this turn of events and determined to reverse it, set to work on a remarkable study which demonstrated conclusively that the supposed drag advantage of liquid-cooled engine installations was mythical once airplanes attained high enough airspeeds and the engines grew to sufficient horsepower. In fact, he drew a four-row radial with a spiraling cylinder arrangement that promised lower drag than the equivalent liquid-cooled engine (and, as it happened, he described perfectly the future four-row Wasp Major that powered the B-36).

It was an old argument, this liquid-vs.-air-cooling controversy. The Navy had satisfied itself on that score as early as 1926 when the Wasp first emerged. Around 1927, Carl Spaatz had flown a Wasp-powered Navy fighter, and came away impressed. His enthusiasm wasn’t enough to alter policy until the 1928 Los Angeles National Air Races, when Navy fighters flew not only fast, but inverted! At the race, the new Boeing XF4B delivered an average 172.6 mph, compared to the Army’s best 147 mph (in a D-12-powered Hawk).

That persuaded the Army to equip with the Wasp-powered Boeing, designed the P-12A, and, by 1930, the 1st Pursuit Squadron in 450-hp P-12As was flying combat formations at 30,000 feet. (See September ’79 issue cover painting by Keith Ferris and article on p. 120 by Jeff Ethell.) But in subsequent years, the Europeans at the Schneider Trophy Races had demonstrated that higher horsepower with then-current fuels seemed to drive engines toward liquid cooling, and the Allison gained its adherents in the US.

An Engine in Hand

By 1938, Pratt & Whitney was in such a precarious position that only a massive order from a besieged France kept it going. In that year, US expenditures for military aircraft totaled only $122 million. The French order got Pratt onto a war footing early, and, by May 1940, England added its orders for R-2800s, taking over the French orders as well after Hitler’s little jig at the railroad car in Compiègne.

With Mead’s health failing, it was now up to Rentschler to return from & Whitney organization out of its crisis. He sized up the situation, immediately agreed with Hobbs’s assessment of the Army program, and he used his personal relationship with Hap Arnold to sit down with him and talk some turkey.

Arnold arrived in Connecticut for the meeting with Rentschler in early 1940, shortly after the new Chance Vought F4U Cutlass had flown at 405 mph with a Double Wasp aboard. Arnold viewed first-hand the progress of the liquid-cooled projects, then witnessed a run-up of the new R-2800. Rentschler wasted no words: a 2,000-hp R-2800 was in hand; the liquid-cooled project would never be ready in time for the shooting. A good engine is better than an ideal engine when the ideal can’t be built in time, he argued. Further, Rentschler promised to abandon the liquid-cooled project and absorb the loss if Arnold would let him build a 3,000-hp radial instead.

Arnold never hesitated. His only question to Rentschler was, “Why didn’t you tell me all this sooner?” Pratt & Whitney went on to deliver about half the horsepower used in World War II, and the R-2800 Double Wasp engine became a classic, eventually reaching 3,400 hp.

Republic placed its first order for a Double Wasp for a second-generation version of its model AP-4, which had already been expanded once into the P-43; the new AAC designation: P-44. Now the Thunderbolt was only one step away.

The Airframe

In 1915, one of the Czar’s naval aviators lost his right leg. The pilot, Alexander P. de Seversky, promptly replaced the lost leg with a wooden one and got back into airplanes again. Shortly after de Seversky was sent to the US as part of a Russian mission, he asked to remain in the US and was assigned as a test pilot and engineer for the government.

In 1931, he formed his own company and began to build an amphibian, the SEV-3, in Earl D. Osborne’s Edo Aircraft works at College Point on Long Island. It had a semielliptical wing. But anyone who could afford an amphibian was already buying Grover Loening’s giants. In 1934, fellow ex-Russian Kartveli arrived with credentials that included time with Bleriot and Fokker. De Seversky and Kartveli mapped out a “modular” design strategy of one airframe matched to a variety of wings and engines.

They succeeded in converting the SEV-3 to a trainer for the Army. This otherwise obscure BT-8 got the company into production in quantity and led to the 1935 competition for a fighter. A succession of postponements kept putting off the date for a decision by the Army until 1936, when, by a seeming miracle, de Seversky won with an airplane that became the P-35. And in the P-35 are the lines that would eventually be stretched into the massive P-47, but only after a confused and pressured Army managed to get its peacetime procurement process hopelessly tangled.

By 1938, de Seversky himself was in trouble with his financial backers due to colossal mismanagement and failures on the P-35 program. By 1939, he was gone for good, off to write the stirring book Victory Through Air Power. The Seversky Aircraft Corp. in October, with Kartveli staying on as vice president of engineering.

The wing that bore the breed from P-35 through P-47 and all Kartveli’s designs in between was based on the S-3 airfoil, a conventional, non-laminar-flow shape mated to a longchord, semielliptical planform. It was thought that this wing came close to an idealized lift distribution—an ellipse—in the spanwise. Among its blessings, it had ample room for the guns it would one day carry.

Kartveli’s first turbosupercharger, was installed in the Ap-4/YP-43, and he established its location—in the tailcone—as well as the extensive ductworks for exhaust gases and intake air then. The turbocharger and its ducts were laid out first, and the airplane designed around them. The turbocharger was the size of a small washing machine, and the ducts capacious enough for a man to crawl through them.

Intake air traveled a total of forty-three feet from the nose scoop to the turbocharger and back to the injection carburetor; a branch of this duct provided intercooler air, which was dumped overboard through two exists in the side of the fuselage. The energizing exhaust gases traveled nineteen feet back to the turbine side, and a cleverly arranged boundary layer of cold air protected the stainless steel from the intense heat at the exit shroud. (The XP-47B prototype was lost when the tail-wheel was inadvertently left in this heated stream long enough to catch fire and consume the fabric tail surfaces.)

The R-2800 needed four propeller blades to absorb its horsepower, and, even then, Kartveli had to build the main gear with a “shrink” mechanism that would shorten it by nine inches at retraction to leave enough room for the outboard guns.

The Debugging Process

The NACA helped considerably with the debugging process, running a drag clean-up program on the P-41 that was carried over intact to the -47. High-speed dive tests confirmed a serious aileron snatch problem, and an NACA Frise aileron modification devised by Gough and Gilruth at Langley solved the problem.

An epidemic of tail failures that arose in 1941 continued through the long, hot summer of 1942 at Langley before tests revealed that loads at high speeds could balloon the fabric coverings; all-metal tails resulted. Earlier compressibility studies after the XP-38 inflight disintegrations revealed the problem had resulted in an electric dive-recovery flap that was added to the P-47 as well. (Still, cadets were taught to be patient in their dive recoveries, to wait until reaching the dense air at lower altitudes.) Velocity-G-force recorders revealed how enormous were the aeroelastic problems at these speeds, and the wing was beefed up to compensate. And the NACA “paddle” propeller blades added 400 fpm to the climb—which it needed.

A Reputation for Ruggedness

When the Germans’ elegant FW-190 began to threaten the P-47’s edge in 1942, Pratt & Whitney added the water-alcohol injection that brought the R-2800 the final mile in power output on new high-octane fuel. Two of its Missouri engineers, Arthur Smith and Donald Hersey, had figured that out in 1938 and patented the idea.

The Thunderbolt got pressed-paper wing tanks, a bubble canopy, rocket tubes, and bombs on the wings and belly, but, throughout the war, changes to it were minimal. It was produced in greater numbers than the P-51, which took over long-range bomber escort and converted the “Jug” into a low-level attack airplane whose rockets and Browning .50s busted tanks and railroads with ease. No other righter ever achieved its reputation for ruggedness.

Martin Graham, a Pratt & Whitney representative in Europe immediately after the tide turned in the Ardennes campaign, told in the most stirring words of the way the P-47 helped to win the war:

“I was in the Ardennes a few days [after the battle turned]. We came to a thick woods where von Runstedt had hidden a munitions dump. He couldn’t have picked a better spot so far as air reconnaissance went. That had been a really heavy woods before we blew it all to hell.

“Even blown up, though, right through the center of that woods, you could see by the shattered trees and the torn branches where the P-47s had gone through. You’d have to see it to believe it. Those crazy kids flying the P-47s couldn’t see what was hidden from above. So they went right into the forest to find out. They cut a path right through the top of that woods. … one minute, you think of an airplane as a fragile thing, and then you see something like that.”

George C. Larson is Technical Editor of Business and Commercial Aviation magazine. His book, Fly on Instruments, won a 1981 award for excellence from the Aviation/Space Writers Association. He is a commercial, multiengine, and instrument-rated pilot. He was a member of the editorial staff of Flying magazine for six years. His most recent article for AIR FORCE magazine was “Computer-Generated Images for Simulations: The Cost of Technology,” February 1982.