features Vertical Rewind: Unconventional origins

All legends have an origin story, and this is the MD 500’s.
Avatar By Don Porter | July 29, 2020

Estimated reading time 10 minutes, 11 seconds.

Igor Sikorsky pioneered rotorcraft development, Frank Piasecki perfected tandem rotor helicopters, Arthur Young made Bell’s machines legendary — and Howard Hughes developed the Model 500 and its predecessor, the OH-6A. Or did he?

Executives sit in a Hughes 500 while a U.S. Army OH-6A hovers in the background. Don Porter Photo

In fact, helicopters didn’t interest Hughes, except as a way to make money. Instead, Malcolm Harned, a 41-year-old vice president at the Aircraft Division of Hughes Tool Company (HTC-AD), masterminded the project that would result in the OH-6A in Culver City, California. Harned, who joined the company in 1959, worked closely with two other engineers, Herb Lund and Harvey Nay. During 1961, the trio could be seen examining sketches scattered across drawing boards in the company’s Building 2. The facility had previously been the engineering nerve center for the Spruce Goose flying boat.

An aeronautical engineering graduate from Caltech, Harned had researched ramjets and hot cycle rotor systems before working on the Model 269A helicopter at Hughes. Chief engineer Nay headed light helicopter development there and worked closely with project engineer Lund. Assisted by a close-knit team of consultants, they transferred much of the technology from the 269A into a light observation helicopter (LOH) for the U.S. Army, designated the OH-6A. Hughes called it the Model 369A.

The design details of today’s MD 500 product line bear little resemblance to the 369A of more than a half century ago. But the helicopter’s overall design is similar. Consider the drive system, non-boosted controls, cooling system, static rotor mast, lightweight airframe, and simple rotor systems. A 1965 engineering report prepared by the Department of Mechanical Engineering at Stanford University reveals how the unique design emerged. The report was based on interviews with the three engineers at the heart of the 369A program.

Less is more

Harned focused on three principal objectives in designing the Model 369A: low drag, minimum weight, and reduced maintenance. That is, less of everything except the aircraft’s spirited performance that would later distinguish it in the Vietnam War.

I joined HTC-AD as a technical representative in 1968, and witnessed the result of those engineering efforts, as OH-6As were being manufactured in volume at the time.

A Hughes illustration showing the elements of the PH-6A’s design that would enhance crash safety. Hughes Image

“When we went into this competition, it seemed obvious to me that we would have to come up with a helicopter that was a whole lot better than that of any of our competitors,” Harned told the Stanford University report’s authors. “We felt that if our ship was only a little bit better, we would still lose the production contract, with our inexperience given as a cause of failure.”

The 369A design philosophy centered on minimizing airframe weight. “Only in this way can we get the type of performance wanted,” said Harned. The helicopter was designed to lift a payload equal to 110 percent of its empty weight.

Lund was tasked with conceptualizing different configurations for the aircraft, beginning with pencil-and-paper sketches. “It is usually much cheaper to catch a bad design feature in the sketching or layout phase than in the later phase of models and mockups,” he said in the report. Based on one of the sketches, a one-sixth-scale model was built and subjected to testing in a wind tunnel. All went well.

“We started with a cabin cross section very much like our 269A,” Lund said about his initial sketches. “To approximate more closely a tear-drop shape, we faired the sides into the top and bottom, thereby creating an oval cross section.”

Development work for the two-place, bubble-cabin 269 had begun in 1955, with a first flight on Oct. 2, 1956. Its fully articulated three-blade rotor was the same as that fitted to the McCulloch MC-4 tandem rotor helicopter. HTC-AD had bought the patent for the rotor from its inventor, Drago Jovanovich.

“From my very earliest sketches throughout preliminary design, I concentrated on locating the cargo compartment almost directly below the rotor head,” Lund said. “We felt the location of the cargo compartment directly under the rotor mast was one of the most important early objectives. [It] allows for indiscriminate loading without fear of dangerously shifting the center of gravity.”

Lund reiterated Harned’s obsession to cut weight and simplify the helicopter. “[He] was constantly questioning suggested designs in attempting to eliminate unnecessary weight or complexity,” he said. “He did not want us to add any extra shafting with its associated bearings and brackets.”

A Hughes company illustration showing a side view of the aircraft’s design. Hughes Image

The teardrop shape of the 369A constituted one of the first helicopters where drag reduction was emphasized. “There has been a belief that a helicopter’s aerodynamic drag level could never be brought down to that of fixed-wing aircraft,” Harned said. “We have made just the opposite assumption. We did a great deal of complicated analytic design just to determine the minimum drag for the fuselage alone.

“When we set a drag level of four square feet as our goal for this helicopter, many people thought we were being ridiculous. [They] believed that such a low level was unattainable.”

By comparison, the drag of the 269A equaled approximately 13 square feet of frontal area.

Light but strong

Harned concentrated on developing a strong, lightweight structure, with a truss that had a deep beam and a torque box forming the foundation. In Vietnam, hundreds of crewmen survived crashes that would have otherwise been fatal — owing to the frangible airframe and A-frame truss.

Almost all fuselage skins were fabricated from 0.016-inch thick aluminum, while heavier skin (0.032) was used for the tail boom. All excess metal was removed. Structural webs and beams were milled down to 0.040-inch-high protrusions.
The thin metal structure served as “crush zones,” absorbing much of the impact during crashes.

As a tech rep in Vietnam, I witnessed how static masts saved lives in crashes, while “Brand X” helicopters designed with dynamic masts could result in fatalities. If the OH-6A transmission suffered a seizure, the main rotor drive shaft would shear, and the rotor would then rotate freely and facilitate a safe autorotation.

“The rotor shaft passing into the transmission carries no axial load,” Lund remarked. “Our transmission takes none of the lift. Since [it] did not have to serve as a structural member, we decided to make it with walls thin enough to serve as efficient heat exchanging surfaces, thereby eliminating the extra radiator normally used to cool transmission oil.”

An illustration of the helicopter’s main transmission. If the OH-6A transmission suffered a seizure, the main rotor drive shaft sheared allowing the rotor to rotate freely, facilitating a safe autorotation. Hughes Image

Cooling the main transmission, engine oil, and engine compartment called for ingenuity.

“We wanted an engine oil cooling system with minimal mechanical or hydraulic complexity,” Harned said. “Existing helicopters usually have a radiator and a separate shaft for a fan. This requires an associated system of brackets, bearings, and driving gears or belts and pulleys.” He stressed that by increasing the parts count, mechanical failures could be expected and flight readiness would suffer.

“The aerodynamicist involved with fan selection kept wanting to use a higher speed fan, which was always more complex than a fan assembly mounted directly on the drive shaft,” he said. “The aerodynamicist and I continued to differ in opinion until I finally made the decision to mount our fan directly on the 6,000 rpm drive shaft.” A Volkswagen engine cooling fan was used for testing, and the production fan was patterned after it.

The 252-shaft horsepower Allison T63-A-5A engine and its drive shaft were mounted at a 47-degree angle. Only two gear meshes were required for the main transmission in contrast to approximately two-dozen meshes in transmissions having planetary gearing. A double spiral bevel gear reduced engine rpm from 6,000 to 2,000 for the tail rotor and 473 for the main rotor in cruise flight.

An aluminum tube weighing six pounds (2.7 kilograms) served as a tail rotor drive shaft. It ran directly from the main transmission to the tail rotor gearbox. The shaft had no bearings, universal joints, or extra gearboxes. Simple flex couplings took care of any misalignment. “The system used in our 269A is simple, lightweight, and inexpensive,” Harned said of why it was selected.

Performance enhancers

To save additional weight and ensure that the helicopter could attain maximum speed, the main rotor was fully articulated, its blades free to lead, lag, feather, and flap. The blades were repurposed from the 269A, but using four rather than three. An article in Aviation Week on Feb. 3, 1964, describes the system’s simplicity: “The blades are free to pivot up and down on a ball and socket pivot point with the straps providing the means of retention. A Teflon bearing requiring no lubrication is used to provide freedom for lead and lag.” Grease fittings common to older helicopters became passé.

Straps eliminated the need for flapping hinges. Each blade was supported at the end of a pack consisting of 15 laminated stainless steel straps. If any six of the 0.090-inch-thick straps fractured, the remaining nine straps were designed to carry the entire load imposed by the blades. A continuous load path reached across the hub to an opposing blade. The loads were not transferred through the hub, saving weight.

The main rotor hub. The use of straps eliminated the need for flapping hinges. Hughes Image

“Strap retention systems had been attempted before, but none had proven entirely acceptable,” said Nay. “When Mr. Harned made the decision to develop a strap retention system for the LOH, there was much popular sentiment for the simultaneous development of a conventional rotor system in the event the strap method failed.”

It took Nay nine months of design and testing to perfect the pack, involving “some of the most difficult design problems associated with this project,” he recalled.

The parts reduction and slimmed-down hub caused Harned to remark that it “probably made our LOH a hundred pounds lighter than a conventional helicopter.”

The tight “feel” of the controls was popular, and was owed to the helicopter’s non-boosted control of cyclic and collective pitch. There were no cables to stretch or bind. A hydraulic system or electronic stability augmentation weren’t needed. A large horizontal stabilizer, positioned at an angle, combined with plenty of control power developed by the main rotor, provided excellent controllability and inherent stability.

The Model 369A made its maiden flight on Feb. 27, 1963, followed by delivery of five prototypes to the U.S. Army for testing. The first production delivery was in August 1966. On June 30, 1964, the 369A earned a type certificate from the Federal Aviation Administration, paving the way for the commercial Model 500. However, due to assembly line bottlenecks for the OH-6A, the first 500s weren’t delivered until the summer of 1969.

Upon leaving Hughes in 1967, Harned joined Lear Jet Industries as its executive vice president, moving on to become president of Cessna Aircraft Co. in 1975. Nay joined Gates Learjet Corp. as vice president, later working for McCulloch Aircraft Corp. and retiring as vice president at Piper Aircraft Corporation. Lund remained at HTC-AD, renamed Hughes Helicopters in 1972, and became involved in developing a variety of rotorcraft.

None of the trio’s later projects offered anywhere near the satisfaction they experienced creating the 369 series, whose airworthiness became a legend. As with the erstwhile Douglas DC-3, today’s MD 500 joins a short list of aircraft that have proven irreplaceable.

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