This is the story of the Skytrain, whose fame has spread to the four corners of the globe; not by virtue of a high-powered publicity campaign, but by word of mouth and deed, passed on by the millions of men and women who have needed her, seen her and been glad. In the words of the Hon Winston Churchill, as he spoke of the RAF before the House of Commons four years ago this month, "Never in the field of human conflict, was so much owed by so many to so few." Only a few thousand of these ships have come off the production lines, but they are on active duty with the AAF Air Transport Command, the Air Service Command, and the various Troop Carrier Commands as well as being attached to tactical units. The Naval Air Transport Service, the Marine Combat Air Transport units, and many other organizations operate Skytrains as standard or auxiliary equipment. These operations are being carried on in all parts of the world; from Capetown to Cairo, from Chungking to Corpus Christi.
The Douglas C-47 Skytrain is correctly named "workhorse," in that this type aircraft has performed more diversified tasks than probably any other plane in military service. At various times and in various theaters, she has seen duty as a personnel and cargo transport, hospital plane, glider tow-plane, airborne infantry, artillery, engineer and paratroop transport, and on some occasions, even a makeshift, low-level bomber! There are records of her use as a mobile headquarters, an observation and reconnaissance ship, fighter and bomber decoy, mobile repair shop, flying supply room, and also an aerial post exchange! Truly, the C-47 has a well-earned name; holder of a thousand unknown records, savior of a million unknown lives the workhorse of the AAF.
Much has been written about the various types of Douglas cargo planes, but there has been quite some confusion as to exactly what aircraft are equivalent to what designations. As an introductory note, it might be well to go over the various types in an effort to distinguish the C-47 Skytrain from the rest of the Douglas models. The first twin-engined low-wing transport used by the AAF was the C-33, which was a modification of the standard Douglas DC-2 first used by the airlines. This was replaced by the C-39, which was a hybrid ship, using a C-33 (DC-2) fuselage, with DC-3 empennage, wings and more powerful engines. The C-53s were commercial DC-3s taken over from the airlines at the beginning of the war, which were stripped of seats and furnishings and converted into personnel/cargo transports by the airlines and the various AAF depots. A companion to the C-53 was the C-48, which was the same ship, but with the original furnishings still installed, used as a personnel transport. Actually, it was a commercial DC-3 with a coat of OD paint and accommodations for another crew member. The C-49, which has had a dozen or more modifications, was the first ship built by Douglas from the DC-3 prototype as a cargo transport. While the C-33, C-39 and C-53 had moderately strong flooring, they were not able to withstand the rigors of continually transporting heavy, concentrated loads. The C-49 which has had a strongly reinforced floor and an astro-hatch for the navigator, was built to carry heavy loads of cargo or military equipment. Strange as it may seem, the C-48, C-49 and C-53 are all known as Skytroopers by the AAF, as R4Ds by the Navy. Even the British call them all by the same name, the Dakota.
The Douglas C-47 Skytrain was the first Douglas plane designed and built as a cargo transport. Her distinguishing feature is the freight door on the port side of the fuselage, plus the fact that there is no after baggage compartment and therefore, no exterior access door. The C-47 and C-47A are equivalent to the Navy's R4-D1 and R4-D5, and the British Dakota I and III. These ships are built with strongly reinforced floors and bulkheads and can accommodate passengers, litters or cargo with only a few minutes time spent in modification. The only discernible difference between the two models is the fact C-47 aircraft have a single circuit, 12-V, grounded electrical system, while the C-47As have a 24-V system except where instrument deflections may occur. This difference necessitated a change in battery connections and generator types; the C-47 is equipped with two 88-Ah batteries connected in parallel and a 14-V, 50-A generator installed in each engine, while the C-47A has the batteries connected in series with 28-V, 50-A generators on the engines.
C-47 series aircraft are low-winged, twin-engined monoplanes of all metal construction. Their overall span is 95', overall length 64' 5½", and overall height 17'. Accommodations are provided for a three-man crew: pilot, co-pilot and radio operator. Folding benches installed along each side of the main cargo compartment provide accommodations for 28 passengers. In lieu of the passengers, 18 type M-60 litters and a medical crew of three may be carried, or the ship may be loaded with 5100 lb of cargo.
The wing is of the full-cantilever type and is of monocoque, stressed-skin construction. It is made up of a center section of constant chord, nacelles attached, and two tapered outer panels, each consisting of a main section, a detachable trailing edge section at the inboard end and a detachable tip. The ailerons are fabric-covered metal frames, with a controllable trim tab on the starboard side. The hydraulic wing flaps are of the split-trailing-edge type of all-metal construction.
The horizontal stabilizers and fin are of all-metal multi-cell construction attached in fixed alignment to the fuselage. The rudder and elevators are of metal frame, fabric-covered construction and are aerodynamically and statically balanced. The rudder and each elevator are equipped with a controllable trim tab.
The fuselage is semi-monocoque in design and is divided into six compartments: pilots' compartment, port and starboard cargo and baggage compartments, radio operator's compartment, main cargo compartment and lavatory. The landing gear consists of an independent unit mounted under each engine nacelle. They are hydraulically operated and are so arranged that they retract into the nacelles, leaving only the bottom of the wheel projecting. Each wheel is mounted on two oleo struts of the air-oil type for shock-absorption. Hydraulically operated Bendix 14×3 dual brakes controlled by toe pressure on the rudder pedals are installed on each wheel, The tailwheel is non-retractable and is of the 360°-swivel type, which can be locked in a fore-and-aft position for take-offs and landings.
The C-47 series aircraft are powered, for the most part, by Pratt & Whitney R-1830-92 Twin Wasp, air-cooled radial engines. They are equipped with PD-12H Stromberg injection carburetors and-have a compression ration of 6.7:1, an impeller gear ratio of 7.2:1 and a propeller reduction ratio of 16:9. Each engine is rated 1050 hp at 2250 rpm for sea level operation up to 7500 feet. They develop 1200 hp at 2700 rpm for takeoff using 100-octane fuel. Propellers are Hamilton Standard Model 23E50s of the three-bladed dural type that are both hydromatic and quick-feathering.
The fuel system consists of four fuel tanks within the center wing section, two tank selector valves, two cross-feed valves, two wobble pumps, two fuel strainers and an engine-driven fuel pump in each engine and a primer in each nacelle. The two forward main tanks have a capacity of 210 gallons each and the two aft auxiliary tanks hold 201 gallons each making a total fuel capacity of 822 US gal, or 684 Imperial (British) gal. Each engine is supplied by a separate fuel system, but the two are connected in such a way that the cross-feed permits both engines to be supplied by either port or starboard tanks and either fuel pump in case of an emergency. Pressure gauge readings are taken from a point adjacent to the pressure warning switch installed at the carburetor in the pressure line. The normal operating fuel pressure is 14 to 16 psi.
The oil system is also independent for each engine, and is carried in two tanks, one in each nacelle, and each having a capacity of 29 gal. Dilution equipment for winter starting is included and manually operated shutters controlled from the pilots' compartment are provided on each oil cooler for temperature regulation. The landing gear, brakes, flaps and automatic pilot are all operated hydraulically through two engine-driven pumps, one on each engine. The system is of a pressure-accumulator type normally operating at a pressure of between 800 and 875 psi. The Sperry A-3 Autopilot or the Jack & Heintz A-3A Autopilot are standard equipment, providing control of rudder, ailerons and elevators. The flight controls are of the conventional wheel and column type for lateral and longitudinal control and rudder pedals for directional control, and are in a dual, side-by-side arrangement for pilot and co-pilot.
The communications equipment is located in the radio-man's compartment on the starboard side of the fuselage between the starboard cargo bin and the forward wall of the main cargo cabin. The radio equipment installed in the Skytrain includes a command set, a liaison set, radio compass, marker beacon receiver and an interphone system. Some of the later models are also equipped with the SCS-51 instrument landing equipment which necessitates the installation of two more receivers for localizer and glide path reception. Other models are now being equipped with this apparatus at air depots and modifications centers as it will not be long before it becomes standard equipment.
Some models are equipped with a steam heating and ventilating system and others have hot-air installations. The former incorporates a boiler in the starboard engine exhaust collector ring which converts water into steam by the heat of the exhaust gases. The steam is passed into a radiator where it heats air brought in from the outside. In the latter system, outside air is introduced through scoops on the outboard sides of the nacelles, and is heated while passing through exchangers attached to the exhaust tailpipes. The hot air is conducted to the fuselage through insulated ducts, and is distributed to. the compartments in such a manner that the starboard engine heats the pilots' compartment and the port engine heats the rest of the ship and operates the defrosting system. Ice-eliminating equipment includes a propeller anti-icing system, an alcohol-type windshield anti-icer and rubber deicer shoes on the leading edge of each wing, the stabilizer and the fin.
A carbon dioxide powerplant fire extinguisher is standard equipment with outlets in each engine nacelle, but portable extinguishers are also carried. The oxygen system for the crew consists of a supply bottle with a pressure regulator and a high pressure gauge in the starboard cargo compartment, another on the instrument panel and outlets adjacent to crew positions for high altitude operations.
The main cargo cabin is equipped with a snatch-block, an idler pulley and tie-down fittings to facilitate cargo-handling. Provisions are made for carrying any one of the following cargo loads: four transport cradles with R-1830 engines or four Curtiss V-1570 engines; three cradles with Allison B-1710 engines; 18 litters, airborne task force equipment including a ¼-ton truck and any one of three types of field guns, and fittings are provided beneath the fuselage for transporting two three-bladed propellers.
Two A-2 and one B-3 life rafts may be stowed in port and starboard cargo compartments as a further safety measure for over-water flights. The parachute pack mechanism consists of six parachute pack containers, attaching fittings, release mechanism and release controls. The racks are located underneath the fuselage, and the release mechanism is provided by a type B-7 bomb shackle in each rack. The chutes may be released in series or in a salvo either manually or electrically by the jumpmaster or the pilot. These chutes contain the equipment and provisions for the normal complement of two squads of paratroopers.
Due to the fact that many modifications of the C-47 series have not been compromised, many of the operational performance figures have been classified as restricted information. However, it is known that the maximum speed is well over 200 mph, and the normal cruising speed is approximately 185 mph with a full load. The gross weight is 29,000 lb, but the maximum allowable takeoff weight is 26,000 lb. The operational ceiling is 30,000 ft with a tactical radius of 750 mi. As this article goes to press, we are advised that the Skytrain is now being equipped with Pratt & Whitney R-2000-92 Twin Wasps which gives the ship a power loading of 12 lb/hp. With this powerplant installation, the wing loading is now 25.3 lb/sq ft, the empty weight is 16,970 lb and the useful load is 8600 lb. This information, when evaluated, allows a new maximum cargo load of 6000 lb, an increase of 900 lb over the previously released figure.
This design analysis article was originally published in the September, 1944, issue of Air Tech magazine, vol 5, no 3, pp 15-22, 57.
The original article includes 4 photos and 6 drawings/diagrams.
Photos and drawings credited to Douglas Aircraft Co.