The Birth of Rotor Technology: The Development of the Helicopter During World War II (Part 2 - Final)

In the first part of our series, we examined the work of Igor Sikorsky, one of the pioneers of helicopter design. In this part, we continue where we left off, examining the contributions of other important designers.

Frank Nicholas Piasecki and the Tandem Rotor Design (PV-2, PV-3)

Although Sikorsky is credited with creating the United States' first successful helicopter, he did not work alone. Throughout the 1940s, other young and ingenious helicopter designers strove to realise their vision of vertical flight.

Frank Nicholas Piasecki, Arthur Middleton Young, Stanley Hiller, Jr., and Charles Kaman competed for recognition and ultimately produced successful helicopters.

On 11 April 1943, Philadelphia native Piasecki flew the second successful prototype helicopter in the United States. Born in Philadelphia in 1919, the son of a Polish immigrant tailor, Piasecki earned degrees in aeronautical and mechanical engineering from the University of Pennsylvania. After graduating, he worked as an engineer at both the Kellett Autogyro Company and the Platt-LePage Aircraft Company. Together with Harold Venzie, he founded P-V Engineering Forum in 1940. Piasecki set to work and designed the PV-2, a small single-seat helicopter he believed would revolutionise private transport. He built most of the first prototype in his garage and crashed on its maiden flight. Remarkably, Piasecki was the first person in the United States to obtain a pilot's licence to fly a helicopter before qualifying to fly conventional aeroplanes.

The cover photo shows the Piasecki PV-2, with its enclosed cockpit and burgundy-silver paint scheme, taking off in front of the Roxborough facility. The facility is more like a hangar.

Due to wartime shortages, Piasecki salvaged most of his parts from crashed aircraft and automobile scrap yards, but he equipped his first helicopter with a reliable and lightweight four-cylinder, 90-horsepower air-cooled Franklin engine mounted vertically beneath the rotor blades. The aircraft, which was entirely fabric-covered, including the pilot and fuel, weighed just over 1,000 pounds. Elliot Daland, a PV engineer who had also worked at Platt-LePage, added an innovative feature to the rotor system. While Daland followed the traditional wing structure by using steel tubing for the main spar and covering the wooden ribs with fabric, he added internally adjustable weights to dynamically balance each wing and adjust the centre of gravity, thereby reducing unwanted vibrations. The three-bladed rotor system was 25 feet in diameter, but Daland cleverly designed two of the blades to be foldable, reducing the PV-2's length to just 22 feet. When the blades were folded for storage, the helicopter was approximately 9 feet high and 8 feet wide, allowing it to fit in a garage or be towed behind a car. The pilot sat in a traditional seat in a rounded, glass-covered nose section. Piasecki's team installed conventional helicopter controls: a cyclic pitch control stick, a collective (lift) lever, and pedals controlling the single vertical tail rotor.

To highlight the versatility of the small PV-2, Piasecki performed an interesting stunt. He took off from the driveway of a private home in Falls Church, Virginia, flew a short distance, landed at a petrol station, and refuelled. He spent one ‘A’ ration stamp for three gallons of petrol. The astonished attendant filled the tank, cleaned the helicopter's windscreen, and watched as Piasecki took off towards a nearby golf course. He landed next to the first tee, retrieved his golf clubs from the small luggage compartment, and began playing a round of golf.

Flying the PV-2 rekindled Piasecki's interest in military transport aircraft, and he designed a much larger helicopter with stacked tandem rotors. The Navy Department, chastised by a Senate panel under President Harry S. Truman for its apparent indifference to the helicopter's obvious military potential, began searching for a new machine. In January 1944, the Navy awarded Piasecki a contract, and he began building a flying prototype of the PV-3. On 7 March 1945, in Morton, Pennsylvania, the PV-3 (XHRP/HRP-X) Dog House, the world's first successful tandem-rotor helicopter, took to the air for the first time. XHRP stood for Experimental Helicopter Transport; HRP-X indicated that it was a company prototype, not a US Navy helicopter. The later flying and officially designated Navy prototype, named Rescuer, fell under the XHRP-1 designation. Piasecki nicknamed the craft ‘Dog House’ because dogs were often used as test subjects in many situations.

In the photo below, Frank Nicholas Piasecki, himself a helicopter pilot, conducted the first flight of the PV-3 (XHRP-X), the prototype of the first successful tandem-rotor helicopter, on 7 March 1945 in Morton Grove, Pennsylvania.

The photo above shows the HRP-1 in flight.

The helicopter's innovative twin-rotor design eliminated the need for a tail rotor, increasing efficiency and lift capacity. Known as the ‘Flying Banana’ due to its body shape, the PV-3 paved the way for military transport helicopters, leading to production models such as the HRP-1, and later inspiring the Piasecki H-21 and Boeing CH-47 Chinook.

Flying with a two-person crew in tandem seats, equipped with eight passenger seats and powered by a 600 horsepower Pratt & Whitney engine, the PV-3 was the largest helicopter flown at that time. Piasecki engineers mounted the single engine in the rear section of a lightweight steel-framed fuselage, and this engine powered a gearbox in the middle section via drive shafts; the gearbox in turn powered reduction gearboxes mounted beneath each of the three-bladed rotor systems. Using lessons learned from his days at Platt-LePage, Piasecki mounted the rear rotor hub in a higher position, thereby preventing the downwash from the front blades from interacting aerodynamically with the rear blades. Initially, the pilot seated in the front cockpit was protected by a Plexiglas windscreen, but the airframe itself was unclad. Later, the installation of a fabric covering greatly increased the aircraft's forward airspeed. The first prototype nearly suffered a disaster because the transmission, manufactured to automotive tolerances, could not meet the stringent demands of a helicopter's propulsion system.

When the gearboxes overheated during a test flight, Piasecki did not want Navy officials to learn of the problem. He sent the flight engineer to get ice and soda, which they poured over the transmission to cool it down. The HRP-1 also had two large vertical fins attached to its tail. These had not been present during the PV-3's initial test flights but were added to help the rotorcraft remain level during forward flight. In June 1946, the US Navy and Coast Guard ordered 20 XHRP-1 helicopters from Piasecki for search-and-rescue and service transport duties.

This model offered greater versatility and cargo capacity compared to previous helicopters. With the passenger seats removed, it could carry a one-tonne load or six standard stretchers. Its distinctive curved fuselage design inspired the PV-3 and other Piasecki models, which would later be nicknamed the ‘Flying Banana’.

The last of the completed HRP-1s was delivered in 1949. Twelve of these helicopters were given to the US Marine Corps for attack training, while three were given to the US Coast Guard in ‘HRP-1G’ configuration for search and rescue missions. Six HRP-1s were transferred to the civil aviation inventory after being withdrawn from military service.

The HRP-2 (PD-17), which followed the HRP-1, retained the side-by-side seating arrangement for the pilot and co-pilot and the improved cockpit visibility, while offering a structural evolution over the previous model with its all-metal, aerodynamic fuselage. The HRP-2, which made its first flight on 10 November 1949, entered service in 1950 and five were purchased by the Marine Corps.

In the early 1950s, the HRP series helicopters were removed from military inventory. Some models transitioned to civilian use, with at least one surviving to the present day as a museum exhibit. The PV-2 model was used by Piasecki for ceremonial flights until 1965, after which it was donated to the Smithsonian National Air and Space Museum in Chantilly, Virginia, where it is still on display.

Arthur Young and the Bell Model 30

Born into a wealthy Pennsylvania family, Arthur Young graduated from Princeton University in 1927 with a degree in mathematics. However, Young, who was more interested in philosophy than mathematics, was searching for a project to occupy his time when he discovered a book that sparked Anton Flettner's interest in helicopters. In 1928, Young set up a workshop on his family's property and began experimenting with various helicopter models. His first helicopter consisted of rubber bands, carved wooden propellers, and balsa wood strips, all purchased from a toy shop. The first model, with a 6-foot diameter, flew for about ten seconds before crashing. In the late 1920s and early 1930s, Young designed and redesigned several models, using everything from rubber bands to electric motors to power his experimental aircraft. He faced constant failure, and after the crash of a large model powered by a 20-horsepower outboard motor, he abandoned Flettner's fixed rotor blade ideas and continued his own aerodynamic calculations.

Following his own concepts, Young invented a semi-rigid rotor system. The main rotor blades were attached to the hub so that they could move up and down like a seesaw. He added something new to the seesaw rotor system: a stabiliser bar. There were weights at both ends of the bar, and it was directly connected to the rotor blades via cyclic control linkages. Young discovered that when the rotor became unstable in pitch or roll, the gyroscopic inertia of the bar effectively dampened rotor disturbances with cyclic pitch control inputs, stabilising the rotor system. Finding this impractical in his models, Young replaced the bar with a flywheel arrangement that could be remotely controlled during flight.

Determined to build a full-scale helicopter, Young attempted to attract aircraft manufacturers' interest with his concepts. No one showed interest until word of his experiments reached aviation entrepreneur Lawrence ‘Larry’ D. Bell. Bell, who had been fascinated with aeroplanes since childhood, began his career in the aircraft industry working for Glen Martin's Martin Aircraft Company. He then moved to Consolidated Aircraft as a design engineer and became general manager at the factory in Buffalo, New York. He left Consolidated Aircraft and founded his own company, Bell Aircraft, in the late 1930s. Government ‘Victory Bonds’ and lucrative contracts during the Second World War ensured Bell Aircraft's success. The company produced the P-39 Aircobra and P-63 King Cobra for the United States, and the P-400 for the Free French, British, and Soviet air forces. Towards the end of the war, Bell Aircraft also produced B-29s under contract at a new factory built near Marietta, Georgia.

Following reports that Young had successfully flown and landed his remote-controlled helicopter model in his barn workshop, Bell invited Young to demonstrate his model at the Bell factory. During a trip to Germany, Bell had witnessed several flights of the Fa-61 and was impressed by the helicopter concept. On 3 September 1941, Young manoeuvred his remote-controlled helicopter model around the crowded factory and then showed a film about his experiments. After the demonstration, Bell and Young agreed to build two full-scale helicopters; Bell would provide the financing, and Young would do the design.

On 24 November 1941, Young and his assistant Bartram ‘Bart’ Kelley arrived at the Buffalo factory to oversee the initial construction of the two prototypes specified in the contract between Bell and the inventor. However, the Japanese attack on Pearl Harbor and several misunderstandings slowed down the initial research. War-time production had monopolised Bell's interest, and his concern about what would happen if the helicopter engine failed led him to halt funding for Young's project. To allay Bell's concerns, Young placed an uncooked egg in his model and flew it up to the factory ceiling. He then cut the helicopter's power and completed a smooth automatic landing on the ground without breaking the egg. Convinced, Bell agreed to continue. Such trust developed between the two men that Young transferred his patents to Bell Aircraft. On 10 November 1941, Young filed a patent application for the stabiliser bar rotor system; final patent approval from the US Patent Office was granted on 11 September 1945.

Young decided that the only way to progress with his helicopter was to leave the complex aircraft manufacturing facility in Buffalo entirely. He found an empty car dealership in Gardenville, New Jersey, and persuaded Bell to provide a team of thirty-two engineers, machinists, sheet metal workers, and mechanics to build the Bell Model 30 helicopter, which went down in history as the Bell Ship 1. Young also persuaded Bell to allow him to develop the first prototype independently of company management. In June 1942, Young and his team moved into the new facility and began work on the helicopter. They scaled up Young's demonstration model sixfold to build Ship 1, which took approximately six months. Various construction materials were used in the construction of Ship 1. Plywood beams and steel tubes formed most of the skeleton fuselage, while riveted magnesium formed the tail cone. Young constructed the rotor blades from a laminate of pine and balsa wood, reinforced with a steel rod running along the leading edge of each blade. The open cockpit was partially protected by a fairing; everything else was exposed. A vertically mounted 165-horsepower Franklin engine drove both the 32-foot-diameter main rotor and the torque-compensating tail rotor via a geared transmission and drive shafts. The Gardenville factory copied Young's model transmission exactly because no one at the Bell factory knew how to design and build a transmission from scratch. Young also fitted the Ship 1 with a unique control system; there were no rudder pedals, all control functions were performed by hand.

Like other inventors, Young encountered both mechanical and human obstacles to progress. Ship 1 initially sat on a landing gear consisting of four 12-foot spider legs welded to a tubular body. The legs, with shock absorbers approximately 4 feet off the ground, were designed to prevent the helicopter from accidentally tipping over during hover tests. Due to the length of the legs, Ship 1 did not fit through the garage door, and every time the helicopter was taken out for testing, the mechanics had to remove and reattach the landing gear.

On 18 December 1942, Young's team rolled the craft out of the garage for its first test flight, but wintry weather delayed the helicopter's flight: it was too cold for the engine to start, but a more powerful battery powered the motor and on 29 December the helicopter made its first tethered hover flight. Bell did not have a designated test pilot, so Young took the pilot's seat and conducted the first test flight himself. Until then, he had only flown remote-controlled models.

In the photograph below, Model 30 Ship 1 is being prepared for its test flight. In June 1943, Ship 1 became the third helicopter to fly safely in the United States (Photo: Niagara Aviation Museum).

In the photograph above, Floyd Carlson is using Bell's Model 30, Ship 2, during a demonstration held at the 65th Regiment Ordnance Depot in Buffalo, New York, in May 1944 (Photo: Niagara Aviation and Space Museum)

In late January 1943, an unnecessary accident caused serious damage to Ship 1. Bell Aircraft's chief test pilot (and later chief engineer) Robert Stanley arrived at Gardenville and demanded to fly the helicopter. No one dared to oppose the company's chief test pilot, so he sat in the pilot's seat and started the engine. Stanley, like many aeroplane pilots, underestimated helicopters and neglected to fasten his seatbelt. When Stanley lifted Ship 1 off the ground, he rapidly operated the collective control like a pump handle. The helicopter reacted violently, and he was thrown from his seat, striking the spinning rotor. Fortunately for Stanley, the steel stabiliser bar missed him, but the wooden blades struck him, breaking his arm. The impact threw him several feet and he landed in a heap. In the subsequent crash of the pilotless helicopter, both the rotor blades and the tail boom were so badly damaged that they had to be replaced, delaying the test programme by several weeks. Ashamed of his carelessness, Stanley called Bell to inform him that he had delayed the helicopter project.

Bell sent Floyd Carlson to New Jersey as the designated Model 30 test pilot and to help coordinate the project. The Bell team rebuilt Ship 1 and began a series of tethered flights to iron out some of the craft's characteristics. On 26 June 1943, the helicopter finally made its first untethered flight.

As flight testing progressed, Young's engineers and pilots discovered a series of unforeseen problems. Air speeds above 30 miles per hour caused excessive vibrations that made the helicopter nearly uncontrollable. Engineers discovered that the vibrations originated from weak rotor blades, and a stiffening yoke eliminated the vibrations. Excessive wear on the transmission gears also delayed flight testing. Each one- or two-hour flight required the complete disassembly of the transmission and rotors to determine where the wear was occurring. As the engineering team overcame one problem after another, Ship 1 exceeded 70 miles per hour in forward flight.

While test pilots put Ship 1 through flight testing, mechanics and engineers built Ship 2, which was radically different from its predecessor. In line with Bell's requests, Ship 2 had two side-by-side seats so that Bell could ride in the helicopter. Ship 2 had an enclosed cockpit, whereas Ship 1 had an open cockpit design and only a small windscreen. Ship 1's propulsion system did not have a clutch, which meant that someone had to manually turn the rotor when the engine was started. Ship 2 included a clutch in the power transmission line to facilitate engine start-up. Ship 1's controls were radically different from any previous or existing helicopter control system. Young's initial arrangement combined cyclic control and collective and anti-torque control, operated by the pilot's right hand, with the throttle and a ‘pump handle’ operated by the left hand. There were no controls operated by the pilot's feet. Because Floyd Carlson insisted on foot pedals to control the anti-torque rotor, Ship 2 and subsequent Bell helicopters were equipped with conventional pilot controls.

Fortunately, Ship 2 emerged quickly, as an accident in September 1943 nearly destroyed Ship 1. Carlson flew Ship 1 to Gardenville Airport to perform Ship 1's first automatic landings. He completed the first two without incident, but at a very high forward speed. On the third attempt, Carlson sharply banked the helicopter to rapidly reduce forward speed, bringing the helicopter into a nose-up attitude just before touching down. The tail wheel, mounted quite far back on the helicopter's tail boom, suddenly touched down before the main wheels, tilting the tail boom towards the main rotor. Ship 1 then rolled over, and the helicopter pilot reported in his logbook that it ‘destroyed itself’. Fortunately, Carlson escaped unharmed. After examining the wreckage, the team members decided that Craft 1 could be rebuilt. However, all subsequent tests would have to be completed with Craft 2. The delays postponed Larry Bell's first flight in a helicopter until late 1943 or early 1944.

The Gardenville team quickly completed Ship 2 and continued the test programme. Bell invited a number of high-ranking military and government officials, including top researchers from the National Advisory Committee for Aeronautics (NACA), the precursor to NASA, to see the helicopter.

Although Bell Aircraft attempted to conduct the helicopter programme in secrecy, all activities at both the Buffalo, New York and Gardenville, New Jersey facilities heightened public curiosity about the strange flying machine. Eventually, Bell decided to publicise the helicopter programme and invited local newspaper reporters to witness a demonstration flight. In May 1944, Carlson flew Ship 2 indoors at a Civil Air Patrol meeting at the 65th Regiment Armoury in Buffalo, New York. Carlson's demonstration was the first indoor flight of any aircraft type in the United States. Despite the glare of spotlights on his face and the dust kicked up by the rotor's downward flow nearly blinding him, Carlson successfully completed several manoeuvres inside the armoury. The highlight of the demonstration was Carlson holding the helicopter in mid-air with one wheel resting on Arthur Young's outstretched hand.

On 4 July 1944, the modified and rebuilt original Model 30 performed in front of more than 4,000 spectators at a defence workers' rally at Buffalo Civic Stadium. After Bell's helicopter was unveiled to the public, the aircraft carried out various rescue and relief missions. On 5 January 1945, Bell Aircraft test pilot Jack Woolams parachuted from an early model of the Bell P-59, the first US jet fighter aircraft. He injured his shoulder but reached a farmhouse near Lockport, New York, after travelling over a mile through snowdrifts. With snowdrifts blocking all roads to the area, Carlson picked up Dr Thomas C. Marriott in Ship 2 and landed him at the isolated farmhouse in about five minutes. The doctor immediately treated Woolams' wounds and frostbitten feet, saving him from amputation. On the night of 14 March 1945, Carlson, again in Ship 2, rescued two fishermen who had been trapped on an ice floe on Lake Erie for twenty-one hours. When the Coast Guard requested assistance, Carlson conducted some impromptu tests. He needed to be certain that someone could safely climb into a helicopter suspended in mid-air. No one at Bell was sure what would happen when someone grabbed the helicopter. With the help of Crew Chief Harry Finagan, Carlson confirmed that he could control the helicopter while suspended in mid-air as someone climbed in. Carlson flew about five miles to the lake and rescued the fishermen one by one. The rescued men were annoyed when Carlson wouldn't let them take the large chain of fish they had caught with them.

In 1945, the Gardenville team made a decision that would affect the helicopter industry for years to come. A decision far more important than they realised at the time. Despite the agreement between Bell and Young stipulating that Bell Aircraft would only pay for two helicopters, they decided to build a third Model 30. The team had developed so much technology and collected so much data from the previous helicopters that they felt it was their duty to build a third helicopter, incorporating all the lessons learned from Ship 1 and Ship 2. By attaching two cinema cameras to the previous helicopters, the team had recorded a significant amount of performance data that could be studied repeatedly. As a result of this decision, in January 1945, work quietly began on the Model 30, Ship 3, which was as different from Ship 2 as Ship 2 was from Ship 1.

The engineers retained the two-person enclosed cockpit design. Lacking funding for a third helicopter, the Gardenville team gathered materials from other projects to build Ship 3. On 20 April 1945, the newly designed helicopter took to the air for the first time. When Bell management learned of Ship 3's success, they approved the new helicopter as a flying test bed to provide additional data to Bell's engineering department, which had been tasked with designing the company's first marketable helicopter. Model 30 was renamed Ship 3, Model 47, and production was moved to Bell's Wheatfield facility near Niagara Falls in June 1945.

The improved performance of Ship 3 greatly impressed both Bell's test pilots and passengers. The helicopter's superior automatic landing capabilities made engine-less landings almost routine, and Bell pilots quickly demonstrated these to all passengers. Ship 3 quickly proved itself to be not just an advanced test bed, but also a helicopter that could be commercially marketed. With only a few relatively minor changes to the configuration, the Gemi 3 became the first successful commercial helicopter. The Gemi 3 fit perfectly into Bell's plans. In September 1943, Bell formally informed the Bell Aircraft board of directors that the company had a successfully flying prototype helicopter and that the company planned to enter the post-war helicopter market.

Stanley Hiller, Jr. and the Coaxial Rotor Design (XH-44)

Another young, independent designer, Stanley Hiller, Jr., started his first business at the age of sixteen by producing petrol-powered model cars. When the United States entered the Second World War, Hiller converted his factory to produce window frames for C-47 cargo planes. This profitable venture, with his father's help, provided the initial funding for the helicopter project. Hiller's father, himself a renowned aviation pioneer, encouraged the young man and taught him to fly. In 1937, Hiller saw footage of the Fa-61 in flight and decided to build his own helicopter. In 1942, while still a young man, Hiller founded the Hiller Aircraft Company and assembled a small group of engineers to design and build his first helicopter.

In the photo below, an H-13 (the military designation for Model 47) is being tested at Bell's facility in Fort Worth, Texas. (Photo Bettmann/Corbis)

The photograph above shows the Focke-Achgelis Fa 61 (also known as the Focke-Wulf Fw 61). Designed and built solely for testing purposes, the Focke-Wulf Fw 61 is considered the world's first practically usable helicopter. EADS photograph.

Hiller decided on a coaxial rotor system. Frenchmen Louis Breguet and René Dorand had proven this concept in their Breguet-Dorand gyroplanes in 1935, and despite its complexity, Hiller believed it was the best system for his helicopter. Hiller also thought that the external struts on the Fa-61 added too much weight and could not compete with Sikorsky's design. Furthermore, he wanted to build a helicopter adaptable for civilian passenger use, and the tail rotor was dangerous. In December 1942, his company began building a 13-foot-4-inch-long, steel-framed, fabric-covered fuselage.

As with other inventors, wartime shortages delayed Hiller's progress. His men gathered parts from every possible source and manufactured what they couldn't find. Hiller couldn't purchase a reliable power source on the open market, so the 18-year-old convinced Grover Loening, the War Production Board's chief aircraft consultant, to provide a Franklin engine, reduced to 65 horsepower from its original 90 horsepower. Hiller later upgraded the engine to a 125-horsepower Lycoming engine. Workers installed the Franklin engine in 1943 and began ground tests. When they first started the engine in their small workshop, the engine explosion and rotor wind blew out all the skylights. After cleaning up, flight tests began a few days later.

The photograph below shows the Hiller XH-44 during a test flight. Due to the photograph being black and white, the helicopter may appear to be painted white. However, in the original design, the helicopter was painted a highly distinctive yellow colour.

The photograph above shows Charles H. Kaman flying the Kaman K-125 NX60377 in 1947 (Kaman archive)

Like most other helicopter designers, Hiller tethered the XH-44 to the ground for its first flights: ‘X’ for experimental, “H” for Hiller, and ‘44’ for the year it flew. He had no experience flying rotorcraft and was concerned about safety, but the precautions he took did not prevent accidents. During the first flight from the family garage driveway, the tether cables were incorrectly adjusted and the 1,244-pound helicopter tipped over, causing minor damage to the handmade machine. Hiller moved subsequent flight tests to the football stadium at the University of California, Berkeley. On 4 July 1944, Hiller flew the bright yellow machine untethered for the first time. On 30 August, he flew his helicopter in a public demonstration in San Francisco. Hiller's success so impressed Henry Kaiser, the wealthy builder of Liberty ships, that Kaiser provided sufficient funding for Hiller to redesign his rotor system.

Hiller introduced not only the first successful coaxial rotor helicopter in the United States, but also the first successful all-metal rigid rotor blades. The 25-foot-diameter metal blades would not become excessively tapered under aerodynamic stresses, minimising the risk of collision during flight and thus making them safer than wooden blades. The control system consisted of a cyclic control stick, a collective stick, and directional pedals. Unlike yaw control in a single-rotor helicopter, which includes a torque-compensating tail rotor, the XH-44 pedals feathered the main rotor blades in opposite directions to provide directional control. Hiller slightly modified the rotor system to allow the blades to act like a seesaw in flight, similar to the design Young introduced with Bell Helicopter.

Hiller designed another improved coaxial helicopter but was unsuccessful. However, his company gained renown after the Second World War for an innovative control system for single-rotor helicopters. Contrary to aviation industry forecasters, thousands of pilots did not leave the military to create a boom in personal transport aircraft. By the end of 1945, Hiller shifted his focus to commercial and military helicopters. His groundbreaking invention came with the Rotormatic main rotor design. Hiller connected cyclic pitch controls to a set of small auxiliary vanes positioned at a 90-degree angle to the main rotor blades. These auxiliary vanes increased the helicopter's stabilisation, particularly during hovering, dampening pitch and roll deviations.

Charles Kaman and the Synchropter

In December 1945, with an investment of $2,000, Charles Kaman founded the Kaman Aircraft Corporation and began design work on a synchropter with two coaxial rotor blades.

Kaman followed Flettner's design principles but developed the concept further with the invention of servo-flap controlled rotor blades. Kaman organised his company to enter the post-war growing helicopter market and, with his creative genius, pioneered many firsts and broke numerous records with his rotorcraft.

Conclusion

The Fa-61 and VS-300 were the first practical helicopters in the sense that they could perform manoeuvres we now take for granted. Vertical take-off and landing, hovering, and forward, backward and sideways flight paved the way for production helicopters capable of carrying payloads, thus enabling countless military missions and civilian tasks to be performed.

Although limited in lift capacity, durability, and airspeed, a few helicopters in combat service, particularly in rescue and reconnaissance missions, demonstrated the versatility of the new machines. Twenty years after the VS-300's maiden flight, Sikorsky Aircraft General Manager Lee S. Johnson summed up its contribution: ‘Before Igor Sikorsky flew the VS-300, there was no helicopter industry; after he flew it, there was.’ These pioneering efforts laid the foundations for modern aviation after the Second World War and ensured that helicopters now occupy an indispensable place.

Every study on helicopters deserves in-depth analysis and comprehensive narration. However, in digital media, I am either forced to narrow the narrative or get stuck in long-winded series of articles. This dilemma sometimes becomes one of the most painful impasses in my writing process. Here we have reached the end of part two and our series of articles. I hope to see you again with a new article on a new topic.

If you haven't read the first part yet, I've left the link below.

The Birth of Rotor Technology: The Development of the Helicopter in World War II (Part 1)

https://strasam.org/savunma/havacilik-ve-uzay-sanayii/rotor-teknolojisinin-dogusu-ii-dunya-savasinda-helikopterin-gelisim-sureci-bolum-1-3835

References

https://www.lockheedmartin.com/en-us/news/features/2023/sikorsky-celebrates-100-years-of-innovation.html

https://www.thisdayinaviation.com/18-august-1943/s48-2/

https://forum.il2sturmovik.com/topic/76590-sikorsky-xr-4/

https://www.aviastar.org/helicopters_eng/piasecki_pv-2.php

https://www.airvectors.net/avpiaski.html

https://www.historynet.com/the-gardenville-project/

https://www.defensemedianetwork.com/stories/nazi-helicopters-henrich-fockes-fa-61/

https://www.aviastar.org/helicopters_eng/hiller_xh-44.php

https://www.heli-archive.ch/en/helicopters/in-depth-articles/kaman-k-1200-k-max

McGowen, Stanley. Helicopters: An Illustrated History of Their Impact. 2005