"Then shall the right-aiming thunderbolts go abroad: and from the clouds, as from the well-trained bow, shall they fly to the mark."
Wisdom of Solomon, Verse 21.
It is highly unlikely that King Solomon pictured six or seven tons of powered dynamite, piloted by a youth of the AAF or RAF, when these words were written more than 2,000 years ago. They are, however, peculiarly applicable, for the P-47 has more than justified the appellation, Thunderbolt a word synonymous with suddenness and terror.
The US Materiel Command knew what it wanted when plans were presented to Republic Aviation Corporation for a plane that would outclimb, outspeed, outdive, fly higher, and pack more lethal destruction than any single-engine fighter then in production. It knew that Alexander Kartveli, Republic's chief engineer, who designed the P-43, was enthusiastic about his dream ship, the XP-44, first interceptor to incorporate the 2,000-hp Pratt & Whitney radial engine. Not readily adaptable to quantity production for military purposes, that plane was redesigned and fostered the XP-47 in September, 1940. The P-47 gave Kartveli plenty of headaches it was no small task to design a plane of thin aluminum that could climb to 15,000 feet in six minutes, power it with a Pratt & Whitney Twin Wasp engine that could run a small factory, send it into high, thin air where men would die if they were not fed pure oxygen, and equip it with a battery of machine guns and ammunition that would make it a formidable opponent at 45,000 feet. To pack all this mightiness into one design required a big plane the biggest and heaviest single-engine fighter flying today, with a wing span of 40' 9-5/16", length of 35' 7", weight of over 13,000 pounds. When test pilots sent this goliath into power dives at 700 mph. and pulled out without mishap„ the Army knew it had something. Besides knowing that it would take men of extraordinary skill and ability to guide this flying destroyer, it was no less important that the P-47 have its own doctors and specialists, the ground crew attending to its particular service and maintenance problems.
If you were permitted to use a gigantic hacksaw and slice the top part of the fuselage from the Thunderbolt to look into its labyrinth of intricate machinery comprising more than a hundred pieces of equipment, and thirty-five cockpit instruments, you would probably give a low whistle, replace the top carefully, and go on about your business. But it's the business of every man in the ground crew to expertly inspect and check the inside and outside of this ship every time it lands or prepares for takeoff in a combat theater.
Because it would be impossible to note all of the P-47 service problems in a single magazine, only a cursory examination will be attempted here. It should, in its brevity, be enough to build genuine respect for the men who know and practice the routine contained in the Thunderbolt's 600-page technical order.
The P-47 is equipped with both geared and turbine superchargers to permit high altitude offensive operations where temperatures of 67° below zero are not uncommon, where the atmospheric pressure is less than one-seventh as great as the pressure at sea level, where most planes operate most efficiently. The turbosupercharger is a separate and remotely installed unit. The exhaust gases are directed from the exhaust collector ring of the engine to the large pipes on either side of the lower part of the fuselage. Forward of the fire wall, the pipe splits into a Y-shape duct, with one branch running along the lower fuselage the other leading outside the fuselage as a waste pipe. Equipped with a gate valve, the waste pipe is actuated by cranks and rods which link the valve to the supercharger regulator. The piping is housed in stainless steel shrouds which are insulated with asbestos to confine the heat from the exhaust. Gases impel the turbo and are expelled from the flight hood. Air is gathered in the scoop which is built into the lower half of the engine cowling, extends to the center of the fuselage before branching off. The upper branch directs cold air to the intercooler, where it is drawn into ducts on either side of the fuselage, expelled through the fuselage sides.
The lower duct is directed to the turbo, then impelled through ducts to the cooler, thence through the baggage compartment pipes. With so many parts, so much ducting, it is apparent why turbosuperchargers, weighing only a few hundred pounds by themselves, have added so much to American warplanes in the matter of weight and servicing problems. Fortunately, the General Electric turbo is a rugged unit, asking only normal care between overhauls of the plane. In the main, preflight and daily servicing consists of filling the supercharger lubricating tanks, using closed containers which exclude all dust and grit. After lubrication, supercharger rotor is checked for freedom of movement and the regulator is tested several times in off position.
Hardly unique with its inward retracting main wheels, retractable tail wheel, the P-47 nonetheless demands special landing gear attention because of its size, loaded weight. In general inspections, wheels are first checked for distorted rim flanges or ribs, security of nuts, bolts and cotter pins. Then tires are checked for inflation pressures which are varied to assure proper landing and takeoff characteristics according to gross weight on each mission. Because of the tremendous ground pressure built up by the P-47, tire pressure must always be watched carefully to maintain the prescribed rolling radius as depleted fuel and military load exaggerates overinflated hazards when planes return from combat. For example, the tail wheel tire requires inflation pressure of 50 pounds when the gross weight is 12,000 pounds, 55 pounds when the loading is 13,000 pounds.
Using air compressing equipment developing pressures up to 800 pounds per square inch, or carefully handling an air cylinder of approximately 2,000 pounds, shock struts are inflated with air, or with carbon dioxide in emergencies. When the hydraulic fluid in the main landing Aerol falls below the level of the filler opening, the shock strut is deflated by depressing the air filler valves, removing the transfer valve plug, and transfer valve body. Fluid is replaced with careful attention to the maximum permissible torque of 400 to 600 inch-pounds. Fluid level in the tail wheel strut is then checked by backing off filler plug allowing air to escape until fizzing of fluid-air mixture ceases. As new fluid is added, several tests are made by alternately extending and collapsing the strut until fluid is level with the plug opening when strut is collapsed and plane in normal taxiing position. The main landing strut is never inflated beyond the dimension prescribed on the instruction plate because the strut is designed to operate from that position, is varied only according to changes in loading.
The flaps of the P-47's distinctive low wing are actuated by means of the flap-control switch and the pressure is equalized by means of an equalizer valve, which insures simultaneous raising and lowering of the flaps. Control surfaces and tabs are tested for warpage and failure, hinges are checked for excessive play, cables are inspected for excessive wear, proper routing, and attachment to control surfaces. The pulleys are carefully aligned on bracketed cable guards, and rudders and elevators are checked for evidence of buckled ribs or stiffeners. The screws attaching wing tips to wing panels are tested for tightness, and cowling, inspection doors, and covers are properly fastened and secured.
The presence of entrapped air in brakes and parking brake controls is detected by a soft, spongy feeling of the brake pedals. Travel of brake piston never exceeds ¾" when pushing hard on the brake pedal, with safe operation demanding sufficient slack in parking brake control cables to allow full application of brakes without locking them.
Because fuel is the lifeblood of any airplane and increasingly important as engines are boosted in power, self-sealing P-47 tanks are inspected for evidence of deterioration before each flight. At the same time, structures and fuel-cell fittings are checked for leaks, collapse of cells which always give the first visual evidence of tank failure. The main tank has a capacity of approximately 200 gallons and the auxiliary tank about one-half of that. A 200-gallon external tank can be installed on the P-47. After careful inspection and repair of all fuel lines and the hopper type oil tank, fuel cells are refilled just prior to flight with 100 octane fuel. Gasoline of less than 91 octane rating is never used because reduced-power operation demanded by low-grade fuel cuts severity of detonation sufficiently to allow flight for only a short time.
The propeller is a four-bladed Curtiss, constant-speed, multi-position propeller. The blade angles are controlled by means of a reversible electric motor. The electrical energy required for operating the motor is taken from the airplane power supply and passes through brushes mounted in a housing attached to the engine nose, and then to slip rings which are fixed to the rear of the propeller hub. From the slip rings, the electrical energy passes through connector leads in the hub to leads in the speed reducer and thence to the motor. The motor changes the blade angles through a two-stage, planetary-type speed reducer which drives a master bevel gear. This gear meshes with a bevel gear attached to the shank of each blade. Thus, depending upon the direction of rotation of the motor, the angle of the blades is increased or decreased.
With ignition off, the prop is inspected for bent or damaged blades, nicks, scratches and looseness while the spinner is cleaned with denatured alcohol then coated with castor oil. All propeller controls are then checked for lost motion.
Parking your car in a crowded area may leave you sometimes with a clammy brow and unvoiced epithets. When you realize that a P-47 combines the weight of five 6-cylinder automobiles, it is obvious that parking and moving the plane on the ground requires a proportionate increase in ingenuity and skill. Generally, five men are required to tow the P-47 with one man in the cockpit to operate the brakes, one or more men at the tail, one at each wing tip. With the cockpit steering control in fixed position, the plane can be safely jockeyed between other planes, hangars or obstacles with a tow rope measuring 3½ times the P-47 tread width, at least one inch in diameter. In mooring the plane, wheel chocks are first placed fore and aft of both main wheels, parking brakes and controls are locked and the ship is lashed to the ground by means of ropes attached to parking lugs inset in the fuselage skin. The tail is then lashed to the ground with ropes. Obviously, conditions of terrain affect towing procedures. For safety and for general efficiency, tow ropes must be 3/8" in diameter and at least 6' long when moving the ship over soft ground. Using hoist facilities capable of lifting 14,000 pounds, a special sling is attached by clevis pins to lugs on the engine mount and headrest after rear top engine cowling is removed, and canopy pushed all the way back. Whenever the tail is hoisted to flight position, 150-pound weights are applied to each end of the bar through the lift tube.
Icing conditions occur when flying through rain or clouds when the air temperature is approximately freezing or colder. At very cold temperatures or at high altitudes (above 20,000 ft) icing is rarely encountered.
Special precautions are taken prior to takeoff and after landing due to temperature inversions when the ground air may be anywhere from 59° to 86° colder than at altitude. The winterization equipment includes an oil-tank immersion heater installed in the bottom of the oil tank and a special duct can be installed which will allow hot air to enter the induction system from the exhaust shroud. Air, preheated by the exhaust manifold, is conducted through a canopy defroster tube and a special valve operates the windshield defroster at the same time.
Portable ground heaters are used to heat the engine and cockpit before the first flight of the day. The unit weighs approximately 183 pounds and is easily handled by one man.
In the winter, the engine is 2½ times as hard to crank as in the summer, while the battery has less than one-half the energy. The battery should be removed and placed in a warm room, maintained, if possible, at a temperature of 68°. Or where storage space is lacking, a portable generator may be used to build up the battery and assist in starting. In servicing the single 24V battery located on the firewall, hydrometer readings are taken on at least two cells of the battery with the following readings commonly used as standard: Low (discharged) 1.200 and below; Medium (partially discharged) 1.250; High (fully charged) 1.275 to 1.300. Fluid is then returned to the cell from which it was withdrawn, distilled water is added when necessary. If any cell reading is below 1.240, the battery is turned in for test and recharge. Because electrolyte in batteries will freeze if exposed to low temperatures when partially or fully discharged, no water is added when the temperature is below freezing, unless the airplane is to be operated within a few hours.
When temperatures fall below 5° F, heat, when available, should be applied to that portion of the grounded airplane where instruments are located, if the airplane is to remain on "alert." Heat must also be applied to armament equipment to assure its immediate operability.
In parking the airplane on snow or ice, if possible, provide a layer of fabric, grass, straw, green boughs or other insulating material under the wheels to prevent their freezing onto the surface. Lack of such precautions frequently tears large chunks of rubber from the tires when the airplane is again moved.
Airplane protective covers, consisting of form-fitting covers or paulins with ropes provided for securing the paulins to the airplane will be used on the wings, tail surfaces, cockpit enclosure, fuselage and engine. Propeller blade covers will be installed. It is recommended that when the airplanes are kept outside in very cold conditions, a cover be placed over the main air scoop. This will prevent snow or ice from forming on the carburetor. Brakes should be off on parked airplanes to prevent locking by ice through condensation.
So when you see a P-47 take off, assured of the pilot's protection by the armor plate which will withstand the German .312, and Jap .303 (7.7mm) caliber guns, you know that his own eight .50 calibers will "give 'em hell." You know, too, that the ground crew has done expertly the job for which they've been trained, as the unsung heroes who make the "right-aiming Thunderbolts fly to the mark."
Previous Air Tech analyses of aircraft have included schematic diagrams and structural descriptions of fuel and oil systems the most important units in modern aircraft. However, after ten months in combat, the Republic P-47 has not been captured in recognizable shape by the enemy. Fuel and oil systems have, therefore, been omitted from this P-47 analysis in the interest of military security. These details will appear in Air Tech as soon as their revelation becomes valueless to our common enemy.
This design analysis article was originally published in the December, 1943, issue of Air Tech magazine, vol 3, no 6, pp 21-35.
The original article includes 7 photos and some 37 drawings and diagrams, including 2 three-view drawings.
Photos credited to Republic Aviation Corp and US Army Air Forces.