By Brent Wallace and Kenneth I. Swartz Vertiflite, Nov/Dec 2021
(Note: This is an updated and revised version of what was published online in June 2021)
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 1961, Fred Clark began a long association with the S-52 in Florida. Prior to this, Clark had served in the US Air Force (USAF) between 1956 and 1959, flying B-25s and Sikorsky H-19s, but primarily Piasecki H-21s with the 20th Helicopter Squadron in Tennessee, including temporary deployments to Greenland to support construction of the distant early warning (DEW) Line radar site construction and to Nevada to monitor above-ground nuclear weapons tests. Clark moved to Florida after his discharge from the USAF in 1959 and established Trans-American Helicopter, Inc., which purchased the assets of Helicopters International / New England Helicopter Service, and pioneered citrus grove spraying in the Lake Wales area using Bell 47s.
In July 1961, Clark traded a Bell 47D (Serial no. 61, now on display at the Canadian Museum of History in Ottawa) for three S-52-3 helicopters owned by Genair, Ltd., in Canada. He soon put them to work with his Rotor Wings Inc. unit on powerline patrols for Florida Power Corp., spraying irrigation canals and operating sightseeing flights.
“The S-52 was faster, more comfortable, more spacious and had more range than a Bell 47 and you could trim it so you could fly hands off on long powerline patrol flights,” recalled Clark in a recent interview.
So, Clark established a subsidiary called Bee Line Helicopter that used S-52s to shuttle passengers between NASA’s then-Cape Canaveral Launch and Operations Center and the Orlando airport. Bee Line flew many famous passengers, including German-American rocket pioneer Wernher von Braun, and television broadcasters Jules Bergman (ABC) and Walter Cronkite (CBS).
“One of our regular jobs after every rocket launch was to rush the news film to the airline terminals at McCoy [now Orlando International Airport] so that it could be flown to New York City in time for the ABC, NBC and CBS evening news,” Clark recalled.
In 1964, Clark founded Orlando Helicopter Airways, Inc., (OHA) at Orlando’s Herndon Airport (now Orlando Executive Airport) 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 and acquired at least 17 military surplus S-52s from various sources.
In 1981, Clark was busy operating and modifying Sikorsky H-19s and H-34s at Sanford Airport for utility use and from piston engines to turbine power when he connected with Dr. John Zuk at NASA’s Ames Research Center who was interested in developing a proof of concept electric helicopter for research purposes. Clark can’t recall exactly how he was introduced to Zuk, but the NASA scientist had access to some discretionary funds he could use to launch the project. OHA expected battery-powered aircraft would be quieter and emissions free, have no infrared heat signature, and have a simple overall power system.
OHA needed its airworthy S-52s for its powerline patrol contracts, so Clark selected an aircraft that had been in storage for years; it had been damaged in 1966, when a tornado destroyed the company’s heliport beside Highway A1A in Cape Canaveral. This S-52 (FAA registration N8002E) had served with the US Marine Corps, flown executives for a sugar company and had been used for many other tasks.
A copy of the maintenance logbook, still in Clark’s possession, says that in November 1982, “Airframe Bureau No. 125517 (serial 52010) removed from storage and modified for experimental helicopter operations… Roy Kaylor of Kaylor Industries designed the control system. Merrell Keller of Orlando Helicopter Airways designed and manufactured the electric motor installation. All testing conducted at OHA hangar at Sanford, Florida.”
Keller salvaged four 60-hp (45-kW) alternating current (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.
Clark recalls that Zuk introduced him to Roy Kaylor, from Menlo Park, California, to design the electronic control system for the helicopter to maintain the right rpm during flight. Kaylor had founded Kaylor Energy Products, Inc., during the 1970s energy crisis to sell plans and parts for a “kit car” conversion of the Volkswagen Beetle to electric power, which he marketed through classified advertisements in magazines such as Popular Mechanics. The kit included an electric motor, lead acid batteries and an optional hybrid-electric motor and fiberglass sports car body.
The starting procedure for the helicopter was quite simple. The pilot would turn on a master switch to arm and engage the motors. He would then toggle a second switch into the “Start” position to begin turning the motors and run them up to 2,000 rpm. Once stabilized, the start switch was moved to the “Run” position to take the motors to 3,200 rpm for takeoff and flight, recalled Clark.
The motors would maintain rpm without further adjustments, but there was a switch on the collective to make fine rpm adjustments as needed.
Clark says the project moved quickly and it took about six months to convert the S-52. According to the maintenance log, the helicopter had 829 hours total time when ground testing began at Sanford in 1983, with two hours logged each month in March, April and May, and three hours in June.
After a break for the summer, ground runs resumed, along with taxi tests in September (four hours), October (four hours) and November (six hours). During ground runs and a brief hover of less than a minute in November, Clark and long-time Orlando Helicopter Airways employee (and ex-Vietnamese Air Force pilot) Hue Dinh proved that the concept had potential for further development.
This is believed to be the first passenger-carrying vertical takeoff and landing (VTOL) aircraft powered by electricity to get off the ground with batteries onboard (some previous VTOL observation aircraft hovered with an electric cable — such as the 1918 Austro-Hungarian PKZ-2 and the 1964 Canadian Westinghouse Servotech Periscopter drone).
During the 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 have used 600 lb (270 kg) of silver zinc batteries to enable a hoped-for 30 minutes of operation. This configuration was of great interest because it eliminated the main transmission, freewheel and clutch.
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 now seems implausible.
Clark said OHA received $50,000 towards the cost of the project from NASA. However, OHA was never able to secure a larger torpedo motor from the US Navy for further tests and development of the lithium hydroxide batteries stalled, so the helicopter was returned to storage in June 1984.
In later years, Clark corresponded with pilots who had flown this specific S-52 — originally one of the first HO5S-1 examples delivered to Observation Squadron VMO-6, which performed the majority of the helicopter-borne medical evacuations for wounded Marines in Korea, evacuating over 5,000 seriously wounded personnel.
In the early 2000s, the helicopter was restored in Florida to its original Korean War configuration and donated in 2006 by the Nancy and Fred Clark family to the Smithsonian Institution’s National Air and Space Museum for display at the Steven F. Udvar-Hazy Center at Dulles International Airport, in Chantilly, Virginia.
Meanwhile, Clark’s son, Brad Clark, founded Vertical Aviation Technologies, Inc., (VAT) in 1987 in Sanford, Florida, to continue OHA’s work modifying and supporting S-52 helicopters. The younger Clark subsequently transformed the S-52 into a homebuilt, experimental-category kit known as the Hummingbird 260L, now available for purchase by home builders. VAT acquired the S-52 Type Certificate in October 2019.
The history of OHA, VAT and the Hummingbird will be described in future articles in Vertiflite.
About the Authors
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.
Ken Swartz runs the agency Aeromedia Communications in Toronto, Canada. He specializes in aerospace market analysis and corporate communications. He’s worked in the regional airline, commercial helicopter and commercial aircraft manufacturing industries for 25+ years and has reported on vertical flight since 1978. In 2010, he received the Helicopter Association International’s “Communicator of the Year” award.
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