Electric vertical takeoff and landing (eVTOL) technology has only become a reality in the past decade, but one battery-powered electric helicopter briefly made it off the ground more than 35 years ago.
A Brief S-52 History
Sikorsky’s model S-52 two-place helicopter was developed using company funds following the successful S-51/H-5. The S-52 was Sikorsky’s first helicopter with an offset flapping hinge and all-metal rotor blades, and was aimed at the military observation and medical evacuation role. The project engineer was Ralph Alex, one of the founders of the American Helicopter Society (AHS) and its first and 15th president.
First flight was on Feb. 12, 1947, with a 178-hp (132-kW) Franklin 6V4-165-B32F air-cooled, six-cylinder piston engine. The US Civil Aeronautics Administration (CAA) issued Type Certificate 1H2 to Sikorsky for the S-52 on Feb. 25, 1948. Further development of models S-52-1 through S-52-5 followed, with improvements to capacity and performance along the way.
By 1951, the S-52-2 model was flying with a 245-hp (182-kW) Franklin 6v6-245-B16F flat-six piston engine and an increase in capacity to four seats. The gross weight was 2,700 lb (1,225 kg). The S-52-3 first flew in August 1951 with the same engine as the S-52-2, but incorporated several modifications, most noticeably a downward-canted anhedral V-type horizontal stabilizer. The US Marine Corps took 78, designated as HO5S-1, starting in March 1952 and the US Coast Guard took eight, designated as HO5S-1G, starting in September 1952. The USMC’s HO5S-1 saw operational military service in Korea primarily as a medevac helicopter.
In 1959, Fred Clark began a long association with the S-52, beginning with aerial tours of Florida’s Cape Canaveral, then adding power line patrols and agricultural spraying operations. Prior to this, Clark had served in the US Air Force flying North American B-25s, Piasecki H-21s and Sikorsky H-19s until the late 1950s.
In 1964, Clark founded Orlando Helicopter Airways, Inc. in Sanford, Florida, to overhaul, modify and operate Sikorsky helicopters. Sikorsky agreed to sell Clark all the technical data for the S-52; for all practical purposes Clark’s company became responsible for the future of the S-52 helicopter. He had also acquired at least 17 military surplus S-52s, including five briefly registered in Canada.
Work at Orlando Helicopter Airways to convert an S-52 to electric power began around 1981, with ground testing starting in late 1983. The company expected battery-powered aircraft would be quieter and emissions free, have no infrared heat signature, and have an overall simple power system.
Orlando Helicopter’s mechanic Merrill Keller salvaged four 60-hp (45-kW) AC starter/generators from existing turbine engines and mounted them around a central transmission input shaft. The four starter/generators were linked by toothed belts. This 410-lb (185-kg) powerplant could produce 240–300 hp (180–225 kW) for 10 minutes using as many as fourteen 72-volt lead acid batteries weighing 600 lb (272 kg) that were installed against the firewall in the rear passenger seat area.
According to Helicopter International magazine Sept/Oct 1984, “Orlando expected to progress to a single 240 hp electric motor powered by a 100 lb lithium hydroxide battery pack. This should power the aircraft for up to two hours, with recharging carried out by simply replacing the lithium strips,” though this seems implausible.
By 1986, Orlando had received approximately $10,000 from NASA with the close involvement in the project of Dr. John Zuk at the Ames Research Center, along with Kaylor Energy Products of Menlo Park, California.
During ground runs and a brief hover of less than a minute at Sanford Airport, ex-Vietnamese Air Force pilot, Hue Dinh (a longtime Orlando Helicopter Airways employee), proved that the concept had potential for further development. This is believed to be the first passenger helicopter powered by electricity to get off the ground, even if the batteries did not (some previous VTOL observation aircraft hovered with an electric cable — the 1918 Austro-Hungarian PKZ-2 and the 1964 Canadian Westinghouse Servotech Periscopter — but they weren’t passenger helicopters).
During these tests, problems with motor synchronization were encountered and it was hoped the design would progress to a single Gould torpedo motor located directly under the rotor head. The proposed arrangement would use 100 lb (45 kg) of either silver zinc or lithium hydroxide batteries enabling a hoped-for 30 minutes of operation. This configuration was of much interest because it eliminated the main transmission, freewheel and clutch. Unfortunately, Dr. Zuk was unable to procure any additional funds from NASA and the project ended.
The S-52 airframe that was used for the electrification project has been forgotten to time, along with its identity (serial number and registration). All that is remembered is that the aircraft was painted yellow, as seen in the photo. Perhaps someone reading this can add to the story?
The S-52 Today
Fred Clark’s son, Brad Clark, founded Vertical Aviation Technologies, Inc. (VAT) in 1987 in Sanford, Florida, continuing development and support of conventionally powered S-52 helicopters. The younger Clark transformed the S-52 into a homebuilt, experimental-category kit known as the Hummingbird 260L available for purchase by home builders. The Hummingbird 260L had its origins in the S-52-3 helicopter, but with a significantly different appearance due to its Bell 206-style nose (which had been developed by Ralph Alex). It featured a Lycoming IVO-435-A1F derated 260-hp (195-kW) piston engine, among many other modifications.
The latest VAT Hummingbird kit is a further-developed 260L known as the Hummingbird 300L with a 300-hp (225-kW) fuel injected Lycoming IO-540 six-cylinder piston engine. Gross weight is 2,800 lb (1,270 kg). Hummingbird helicopters are sold by VAT in kit form for completion by owners in compliance with the Federal Aviation Administration’s (FAA) rule that 51% of the effort to assemble the aircraft must be by the owner.
After eight years of effort, Clark was finally able to acquire the S-52 type certificate from Sikorsky in October 2019. Clark is now responsible for all aspects of the S-52 helicopter, including its continuing airworthiness.
Clark’s latest S-52 related project is close to fruition. By summer 2021, he hopes to add the new S-52L model to the original 1H2 type certificate in the “primary category,” which is defined in FAA Advisory Circular AC 21-37 (June 14, 1994): “A primary category aircraft is of simple design and intended exclusively for pleasure and personal use… powered by a single, naturally aspirated engine… and a six-pound per square foot [29.3 kg/m³] main rotor disc loading limitation for rotorcraft. A primary category aircraft may have a maximum certificated weight of no more than 2,700 pounds [1,225 kg], a maximum seating capacity of four, and an unpressurized cabin. Although a primary category aircraft may be available for rental and flight instruction under certain conditions, the carrying of persons or property for hire is prohibited.”
The S-52L is not a rebuilt surplus S-52, nor a kit to be built by home builders. It is an entirely new-build, FAA-certified helicopter with a 300-hp (225-kW) Continental Motors Titan six-cylinder IO-540 engine derated to 245 hp (183 kW). Clark intends to build the S-52L under an FAA production certificate. Every part of the S-52L is produced by VAT in-house or by one of its approved vendors. The three-bladed, articulated-rotor helicopter includes hundreds of improvements to the original S-52 design, including compliance with FAR 27 occupant safety requirements and post-crash fuel fire standards. An electric powerplant is unfortunately not one of those improvements.
About the Author
Brent Wallace recently retired after a career in helicopter maintenance, management and regulatory oversight in Western Canada. In addition to his professional aviation interests, he has an interest in helicopter history. He has been a member of the Vertical Flight Society since 1984.
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