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gor Sikorsky began to work on he- R-4 was difficult, because develop- Ilicopter design in 1939, after con- ment of the VS-300 was continuing vincing (now United at the same time, and design deci- Technologies Corporation) to pro- sions had to be postponed until the vide a small amount of funding for results of VS-300 flight testing were development. United Air- available. craft supported this work, and also The prototype XR-4 made its first merged and Sikorsky to- flight on January 13, 1942, (Fig. 2) gether to reduce costs. (They were just as war fever was building in separated at the end of 1942.) The the United States following the helicopter, designated VS-300, made bombing of Pearl Harbor. After de- its first short flight on September 14, velopment flight tests by Sikorsky, 1939 (Fig. 1). After much trial and the helicopter made an epic five- error, this machine would become The Sikorsky day, 761-mile flight from Sikorsky’s the first practical single rotor heli- factory in Bridgeport, , copter and the prototype for most to Wright Field (now Wright Pat- flying today. R-4 Helicopter terson AFB) near Dayton, Ohio, for Army Air Corps evaluation in May The U.S. Army and the R-4 The Sikorsky R-4 was the first 1942. The success of the VS-300 led to production single rotor helicopter. Initial evaluations were suc- an order from the U.S. Army Air cessful, but somewhat strange to the Corps for a production version in Introduced to the military pilots who had never seen January 1941. This helicopter would U.S. Army Air Corps in 1941, an aircraft that could lift off verti- have two seats and serve as a trainer cally and fly sideways and back- and observation aircraft. It was des- it served as a rescue and wards. Eventually, service test ver- ignated as the VS-316 by Vought reconnaissance aircraft during sions (YR-4B) with a somewhat Sikorsky, and as the R-4 by the World War II. larger rotor and more powerful en- Army. Twice the weight of the VS- gines were ordered for operational 300, the R-4 would have a larger evaluation. main rotor, larger tail rotor, and Thomas H. Lawrence After successfully demonstrating more powerful engine, as well as Sikorsky Aircraft Co. their usefulness, a production order a two-place cockpit. Design of the Naugatuck, Connecticut for R-4B helicopters was awarded

Fig. 1 — The Sikorsky VS-300 was the first practical single rotor helicopter.

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tail rotor was almost eight feet in mented the R-4’s role as a simple and diameter, and it also had three lightweight training helicopter. tapered blades. • Fuselage: On the R-4, the fuselage • Engine: Power was sup- structure was made in two pieces, plied by a 190 hp Warner Super bolted together just aft of the engine Scarab air-cooled seven cylinder bay. The aft structure was almost en- radial engine. This was mounted tirely round steel tubing, but the for- in the normal airplane way, with ward part also included square-sec- the crankshaft horizontal, but the tion steel tubing and wooden engine faced “backwards,” with stringers. The aft framework was cov- the crankshaft pointing aft. This ered with doped fabric to reduce drag. drove a short driveshaft con- Zippers were sewn into the fabric at to Sikorsky Aircraft. These were dis- nected to the main gearbox. The various locations to allow access to tributed to the U.S. Army Air Corps, output shafts from this gearbox pro- components for inspection and main- U.S. Navy, and to the British Royal vided power to the main rotor, and tenance. Navy. The R-4s would prove their also to an aft driveshaft that powered The forward structure, containing mettle, making the first casualty evac- the tail rotor. the engine compartment and cockpit, uations in Burma in 1944. Rescues • Landing gear: The landing gear was covered with removable panels were originally made by landing the was of the “conventional” tail wheel made from thin sheets of dural or helicopter close to downed pilots, but type, but skids were provided on the magnesium alloy. As noted earlier, the it was soon realized that a hoist to haul front to prevent nose-overs during cockpit was extensively glazed with people up to the helicopter directly landing. The two-place cockpit was in Plexiglas windows, and doors were would be very valuable. This led to an the nose, immediately ahead of the en- provided on each side. effort by the U.S. Coast Guard to de- gine, and arranged so the pilot and In common with many aircraft of the sign and develop a rescue winch, and trainee sat side-by-side, with entrance 1930’s, the fuselage framework was today rescue hoists are a common fea- doors provided for each. The pilot sat constructed almost entirely of 4130 ture on many helicopters. in the right hand seat, opposite of the chrome-molybdenum steel thin wall Sikorsky set up the world’s first he- airplane standard, where the pilot was tubing. The tubing was readily avail- licopter production line, and a total of on the left side. able in various diameters and wall 131 R-4s in various configurations • Glass cockpit: Extensive glazing thicknesses, and techniques for calcu- were built. By the end of WWII, the R- was needed to provide the all around lating tube sizes and for welding them 4 was phased out of military service, visibility required by helicopters. In were well established. Another ad- as newer and more powerful heli- addition to the windshield, windows vantage was that changes could be copters from Sikorsky and other man- were placed in the doors, in the made easily by welding on new brack- ufacturers became available. cockpit overhead, and in the nose ets or reinforcing members as required. below the instrument panel. • Rotor hubs: Because weight is crit- R-4 helicopter design ical on all aircraft and especially on he- By modern standards, the R-4 has R-4 materials licopters, lightweight aluminum and a conventional appearance, but it was Because the helicopter itself was so magnesium alloys were applied wher- very unusual at a time when few he- unusual and unfamiliar, ever possible. The main rotor hub was licopters had been built, and those that had chosen to build the VS-300 of ma- made of steel, but the swashplate con- did exist all had multiple rotors in terials and processes common to air- trol assembly was made of magnesium coaxial, side-by-side, intermeshing, or craft design in the 1930’s, rather than alloy hubs with aluminum alloy plates tandem configurations. The multiple- risk new technologies that were riveted on. (Fig. 3) rotor designs had relatively small ro- emerging in the 1940’s. This had been • Gearbox: The main gearbox hous- tors, whereas the single main rotor of successful, so the R-4 was designed ing was a magnesium alloy, while the the R-4 was 38 feet in diameter and with the same idea, to reduce devel- shafts and gears were of alloy steel. had three tapered blades. The vertical opment time and cost. It also comple- (The fragility of the magnesium hous- ings was noted in the maintenance ertical lift aircraft had fascinated Igor Sikorsky since childhood, manual.) Similarly, the mechanical and he built his first helicopter in 1909 in the backyard of his flight control components consisted parents’ home in Kiev, Russia. Unfortunately, he could not get of flexible, corrosion-resistant steel it to lift off the ground. He realized that his meager resources cable and aluminum alloy pushrods, could not support all the development work that would be re- with pulleys and quadrants of alu- Vquired, so he turned his attention to the design of fixed wing planes. minum or magnesium alloys. Sikorsky would thereafter design a series of successful aircraft in Russia, • Main rotor blades: The blades including The Grand (designated as the S-21), the world’s first four-engine were primarily of wooden construc- airplane and the first with an enclosed cockpit and an enclosed cabin. After tion. The main rotor blade had a steel fleeing Russia because of the Bolshevik Revolution, Sikorsky immigrated to tube spar at the quarter chord location. the United States in 1919. He started an aircraft company in America in 1923 Ribs, built-up from plywood, were with few resources, and designed a long series of successful landplanes, sea- placed along the spar using brackets planes, and amphibians. to hold against the centrifugal force. The forward part of the airfoil was

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covered with thin plywood to provide a smooth aerodynamic surface and in- crease stiffness; while the aft part was covered with fabric to reduce weight and maintain the blade center-of- gravity forward of the quarter chord location. The trailing edges of the ribs were tied together by steel strapping to maintain the airfoil contour and re- sist edgewise bending caused by Cori- olis forces. • Tail rotor blades: The tail rotor blades were of simpler construction, being all wood. The spar was made of spruce, with alternating laminates of maple and mahogany filling out the airfoil. The whole tail rotor blade was covered with fabric, and a thin brass sheet was molded and fastened to the leading edge to protect against erosion.

Other helicopters Fig. 2 — Experimental test pilot Les Morris makes the R-4 first flight in January 1942. Sikorsky would design several other helicopters during WWII. The next was the R-5, which was twice the weight of the R-4 and had two pilots seated in tandem. It had compound- curvature Plexiglas windows and a more contemporary aluminum semi- monocoque airframe. The third design was designated as the R-6. This helicopter fitted the R-4 dynamic components (rotor, tail rotor, gearboxes) into a new, more stream- lined airframe powered by a more powerful Franklin engine. Nash Kelv- inator would produce 200 R-6s during WWII. This design minimized appli- cation of “strategic” materials, and had a plastic cockpit section and an all- magnesium alloy tail boom.

Design analysis While it would be easy to dismiss the R-4 as a primitive 1930’s tech- nology aircraft, in fact it represented a rather clever design, and the engi- neers showed great acumen in their Fig. 3 — The complexity of the rotor hub required use of many different materials. material choices. Steel tubing for the fuselage simplified the attachment of the pilots were the result of flight ex- configuration would come to domi- the load-producing elements, such as perience. In fact, many later heli- nate the world’s helicopters. And the the main rotor, tail rotor, and landing copters and vertical take-off aircraft R-4 was the first production helicopter gear. The blades were typical of wing built by fixed wing manufacturers of that type. design for the 1930’s, and allowed the failed to provide visibility as good as designers to focus on important para- the R-4. In general, the R-4 designers For more information: Tom Lawrence is meters such as diameter, rotor rpm, took full advantage of available ma- a senior engineer at Sikorsky Aircraft Cor- chord, and airfoil rather than on struc- terials, with the material selection poration in Stratford, Connecticut. He is a tural design. based on the specific requirements for member of the American Helicopter So- ciety and serves on the Board of Directors The tail rotor design showed each part. of the Igor Sikorsky Historical Archives. knowledge about the properties of dif- However, the single greatest feature He has been researching helicopter history ferent woods and how to best use was Igor Sikorsky’s faith in the bene- for over 20 years, and is currently writing them, skills that are now mostly lost. fits of the single rotor helicopter. Much a book on the history of Sikorsky heli- The expanse of windows provided to derided at the time, the single-rotor copters.

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