First Analysis Of The Thunderbolt

by Peter Masefield
Technical Editor, The Aeroplane, London, England

With top performance at 30,000 feet, the Republic P-47 outguns and outclimbs the enemy's formidable Focke-Wulf FW-190 fighter.

April, 1943, marked the appearance of the eagerly awaited Republic Thunderbolt fighter in the battle-skies of Europe. Flown by — among others — the famous Eagle Squadrons, now with the Eighth Air Force Fighter Command under Maj Gen Frank Hunter, the Thunderbolt did not take long to open its account. Four of the formidable Focke-Wulf FW-190 fighters fell to the eight .50-caliber machine guns of the Thunderbolts over France within a few days of "going on operations."

As must be expected in offensive operations far into enemy territory, not all the Thunderbolts returned. As a result correspondents are privileged to describe such of their features as will be known to the enemy.

To anyone familiar with the trend of British and German fighter design since the war began, the Thunderbolt is of particular interest. It is the largest, and one of the fastest, single-engined fighters in the world. It is certainly the fastest airplane ever to have a radial engine. It is turbosupercharged and it has heavy armament. A close examination of the Thunderbolts at their operational stations reveals many points of exceptional interest — some of which must remain restricted data.

The version of the Thunderbolt in service at present is the Republic P-47 It differs a good deal from the prototype. The first thing one notices about it on the ground is its enormous size. At the nose a tall man finds that he can just peer into the air intake at the bottom of the oval cowl which encloses the 2,000-hp 18-cylinder engine. As one pilot alighted after a flight, an RAF squadron leader hung about expectantly. "Is there anything you're waiting for?" he was asked. "Yes" he said, "they told me she was flying fully loaded. I've seen the pilot get out — but where are all the troops`?"

Seriously — how does the P-47 rate as a single-seater fighter? The answer is: — very highly indeed — within its specialist class. All fighters today are specialists and the Thunderbolt is no exception. You cannot have a fighter — or indeed any airplane — which will be supreme in every category. Any attempt to satisfy all of them leads to "a Jack of all trades and a master of none" — the bad old "general purpose" theory which proved so expensive in World War I and in the early days of World War II.

Thus it would not be fair to the Thunderbolt to compare it with, for example, the Spitfire, as a dog-fighter. Obviously it was not designed as such. But as a destroyer of high-flying enemy bombers between 25,000 and 35,000 feet the P-47 is supreme. At those heights it is very fast indeed — attaining practically 420 mph on the level. Furthermore it can outdive any airplane in existence today and its battery of eight .50 machine guns, all mounted in the wings, are deadly in action.

Naturally such a large and heavy airplane — with a power loading of 7 lb/hp and a span loading of 8.4 lb/sq ft — must pay for its level flight performance in rate of climb. The P-47 is not a fast-climber as fighters go today and thus uses a fair amount of fuel to attain its operating height. But set it at 30,000 ft and the world is at your feet — in more senses than one!

For all its size the Thunderbolt is pleasant to fly. The cockpit is large and comfortable and although visibility might be better — a fault common to the Spitfire also — there is at least the consolation of remarkably complete armor plate protection all round and especially behind.

Of all the controls the ailerons stand out for their brilliance — a great tribute to the Republic designers. They are both powerful and light and, in consequence, the rate of roll of the Thunderbolt compares well with that of any other fighter flying today — an important point. The Thunderbolt becomes airborne with full load at about 100 mph.after a run of about 200 yards with flaps slightly down. When coming in to land the approach is made at about 120 mph and the machine sits down at just over 100 mph. Once on the ground it remains firmly put and, because of the wide track landing gear, is easy to control there.

Thus, to summarize the fighting qualities of this remarkable machine:—

Undoubtedly, in a balanced air force the correct tactical role for the Thunderbolt would be as the top layer of anti-bomber defenses. Flying at 30,000 ft. or more, squadrons of Thunderbolts can dive down to destroy fast bombers or their fighter escorts, flying at lower levels, and can use their speed to get away once their ammunition is exhausted.

In offensive operations the Thunderbolts should be valuable as high cover for day bomber formation, diving on the enemy fighters which may climb up to intercept. In these operations the P-47s already have done well.

The specialist job to be avoided is dogfights at medium altitudes with such swift climbers as the FW-190 and the Zero.

So much for the tactical angle. Now for the technical — insofar as it may be described. The history of the Thunderbolt runs remarkably parallel to that of the Typhoon which is in much the same general category. In fact many of the problems met with in developing the Typhoon have appeared in exactly the same form in the Thunderbolt.

Design work began in 1937 — work on the Typhoon began in March of that year. Both machines were intended for 2,000-hp engines: In June, 1938, the designs for the Thunderbolt were submitted to the authorities of the US Army Air Corps.

At that time the air-cooled motor was not looked upon with favor for the new generation of fighters and some delay ensued (American and British technical departments obviously being very much alike). Finally a contract was given, the number P-47 allocated and the Republic Aircraft Corporation began production at Farmingdale, NY in 1941. Alexander Kartveli, vice president in charge of engineering, was responsible for the general and detail design.

Some bottlenecks occurred in the early stages of production. Engines and four-bladed propellers were in short supply. But today the P-47 is in large-scale production, not only at Republic but by Curtiss as well.

When the Thunderbolt first reached the European battlefront it soon was found that it was likely to be confused with the FW-190. For this reason, the front part of the cowling of all those in service is painted white. The cowling neatly encloses the big 18-cylinder engine which has a diameter of 52 inches and a swept volume of 2,804 cu in (45.9 liters). This engine is restricted to 100-octane fuel and gives 2,000 hp for takeoff.

An unconventional feature of the cowling is its oval shape. The reason for this is that the large air intake is situated in the bottom of the cowl under the cylinders with the oil coolers placed one on each side of it. The trunk from the air intake runs back along the bottom of the fuselage inside its contour flanked by the exhaust. Both converge at the turbosupercharger in the bottom of the fuselage midway between wings and tail. The turbo compresses the air and it then passes through intercoolers higher in the fuselage before being returned to the Stromberg injector carburetor and thence into the cylinders. In all, the air travels some 43 feet in its passage from nose to turbo and back to the engine. In the form of exhaust gases it then travels another 19 feet before being discharged from the vanes of the turbo. Certainly every pound of air is made to do its job in the Thunderbolt.

The deep air trunks along the bottom of the fuselage account for the low mid-wing form and the general dumpy appearance of the machine. Without this deep breast the Thunderbolt's lines would show fine form, accentuated by the rather long nose. Cooling might have been expected to be a problem with an air-cooled engine of high power operating at great altitudes. However, there seems to have been no trouble in this respect despite the fact that the cooling gills run only in a semi-circle over the top half of the cowl.

Probably the excellent cooling maintained at all heights and at all forward speeds is partly because of the propeller root "cuffs" which promote a good air flow from the Curtiss electric four-bladed constant-speed propeller. These root cuffs do, to some degree, the job for which the enemy employs a 12-bladed fan on the BMW 801. The Curtiss propeller has a pitch range of 30° and takes care of a very wide range of speeds.

The cockpit is situated two-thirds of the way back along the chord of the wing which measures just over nine feet at the root. It is the largest cockpit I have ever seen on a single-seater — and the most neatly arranged. The automobile-type door which distinguished the prototype is now replaced by the more conventional sliding hood. The modification added several miles an hour to the speed, presumably through the elimination of air leaks. In front is the standard flat, bullet-proof windscreen, more than two inches thick. A novel feature is that in front of this unaerodynamic military necessity is a Vee-shaped panel of thin Plexiglas, which thus preserves the air flow.

A normal control stick is standard with a trigger for firing the guns. On top is a button to release either the extra fuel tank, which can be slung externally to give a ferrying range of 1,000 miles, or a bomb. No details have been given of the size of missile which can be carried but one would not be surprised to see a 1,000-pounder in place.

The wings have straight taper on the leading edge almost to the tips and an elliptical trailing edge. Dihedral of about 4° begins at the root. An interesting point is that there are practically no wing fillets, but the right angle wing-fuselage intersection appears to give excellent results.

Both flaps and ailerons are provided with efficient slots and are of comparatively low aspect ratio.

Four Browning .50-caliber machine guns are installed in each wing outboard of the propeller disc. The guns are staggered to give clearance for the breeches and the protruding muzzles of the inboard barrels are well faired. The guns are cocked and fired electrically and each is operated individually so that, should part of the electrical gear fail, only single guns would be affected by the failure.

A small movie camera is installed behind a Plexiglas window in the leading edge of the starboard wing root. This camera can be switched on to record the result of combats, photographing the enemy airplane as the guns send their bullets into it. The developed films afford important confirmation of results and useful data for identification of the targets.

The pressure head of the air speed indicator extends some way forward of the port wing tip to minimize position error.

The wide track landing gear retracts inward toward the wing root and is completely enclosed behind well-fitting doors when it is up. A curious point is that the port leg invariably goes up first. Retraction is remarkably swift. There is no doubt that the distance of 15' 6" between the wheels makes the Thunderbolt exceptionally steady on the ground. Further, the forward position of the landing gear obviates any danger of nosing over. In consequence a deep tread is possible on the tires — inflated to 45 psi — and steering on the ground by means of the brakes is simplified.

No details of the internal structure may be given as yet, except to say that the self-sealing fuel tanks of unmentionable — but high — capacity are standard. Many interesting features remain to be described later on when the enemy cannot benefit from a survey of them.

In the Thunderbolt, the US Army Air Forces have a fast, robust and hard-hitting fighter. The radial engine has not prevented its attaining high speed at altitude and should be of particular value in tropical climates where liquid-cooled engines often provide many headaches. At present the output of the Thunderbolt engine is in the neighborhood of 0.71 hp/cubic inch of swept volume, and the Brake Mean Effective Pressure about 220 psi. These are relatively low figures and encourages the hope that there is a good deal more power yet to be had out of it. Already in the fastest class, the Thunderbolt with another 300 or 400 hp should be quite startling on top speed and the rate of climb very much improved.

And so, with the Air War working up to its climax, we can say of the enemy, with Will Shakespeare:
    "Be ready, gods; with all your Thunderbolts
    Dash him to pieces!"

Republic P-47 Thunderbolt
(one 2,000 hp, 18-cylinder, two-row
radial engine with turbosupercharger.)
Span——  40'9-5/16"
Length——  35'5-3/16"
Height——  12'4-11/16"
Track——  15'6"
Wing area——  305 sq ft
Aspect ratio——5.46
Propeller diameter——12'3"
Loaded——14,000 lb
Wing——45.8 lb/sq ft
Power——7.0 lb/hp
Maximum speed——over 400 mph
at 30,000 ft
Service ceiling——40,000 ft
Strikes per second——100
Punch per projectile——13,000 ft-lb
Firepower——2,368 muzzle hp

This article was originally published in the August, 1943, issue of Flying including Industrial Aviation magazine, vol 33, no 2, pp 47-48, 188, 190.
The original article includes a photo of a P-47 in flight and one of the production floor.
Photos credited to Republic Aviation Corp.