Vertical rewind: Tip-powered pioneer

The young, innovative, and talented engineer Stanley Hiller was one of the leading helicopter pioneers in the industry’s earliest days. Among his many achievements was the development and creation of the ramjet-powered Hiller HJ-1/YH-32 Hornet. The aircraft, publicly unveiled at the Hiller Helicopters factory in Palo Alto, California, in February 1951, took an innovative approach to the anti-torque problem. The two ramjets that powered the helicopter were mounted on the tips of the aircraft’s two main rotor blades. With no torque to counter due to the lack of a traditional engine and transmission system that would normally generate it, there was no need for a tail rotor.

The small Hiller Hornet (right) sits next to a Hiller Model 360 helicopter. Jeff Evans Collection Photo
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While Hiller was not the first to explore the idea of using jet-tip engines to power a helicopter, the Hornet was the first such aircraft designed as a practical, rather than experimental, model.

Eight years earlier, in the midst of the Second World War, Austrian engineer Friedrich von Doblhoff built and flew his WNF 342, which sent compressed air and fuel inside the rotor blades to combustion chambers on the blade tips for propulsion.

Later in the 1940s, McDonnell Aircraft designed and flew a lightweight experimental helicopter called the XH-20 “Little Henry” that had ramjet engines on the tips of the rotor blades; and the Marquardt Company flew a helicopter called the M-14 “Whirlajet,” which had two pulsejet engines on the end of the rotor blades.

Tip-jet contemporaries of the Hornet included Hughes Aircraft Company with its XH-17 (first flight 1952), American Helicopter Company with its XH-26 (1952), and Sud-Ouest in France with the S.O.1221 Djinn (1953).

The Hiller Hornet was powered by ramjets at the tip of the rotor blades. Two utility versions were manufactured, with both flying in the test program. Jeff Evans Collection Photo

Hiller had long been fascinated by the idea of finding new ways to perform vertical flight that eliminated the need for a tail rotor. He built and flew the first coaxial helicopter (the XH-44) in the U.S. in 1944, when he was just 19. Two years later, his company had developed and flown the first jet torque-compensating helicopter, the J-5. This used a fan blower, placed underneath the engine, which could be directed to counter torque.

After the civil certification of Hiller’s successful Model 360 three-place helicopter in 1948, Hiller challenged his engineers to come up with a design for a jet-powered helicopter for the civilian market. He wanted a two-seater that was simple, low-cost, ultralight, and easy to fly and maintain.

Picking the ramjet

First came a secret engine development program. Over two years, the team explored 17 different designs of jet engines, with more than 5,000 modifications incorporated to arrive at the final design. Hiller’s engineers evaluated turbojet, pulsejet, and ramjet engines, and tested them extensively on a whirl stand and a static test stand. The Hiller tip-mounted 11-pound (five-kilogram) ramjet engine showed the most promise and was selected for the aircraft. Ramjet powerplants have no moving parts, and the engines for the Hornet could be changed in only a few minutes with the simplest of tools. Each unit developed 31 lb. (14 kg) of thrust, which was the equivalent of 34 horsepower. It cost about $150, but was only expected to last for about 500 hours of use.

Hiller’s engineering team next looked at designing a proof of concept helicopter in order to flight test the new powerplant. The first of three experimental test ships were built at the company’s Palo Alto plant, with the first flight of the experimental XHJ-1 Hiller Hornet taking place during August 1950. That flight was piloted by Hiller chief test pilot Frank Peterson, but test pilot Bruce Jones did most of the experimental flight testing on the aircraft. Hiller was also checked out on his new helicopter.

Pilot Bruce Jones starts up the utility Hiller Hornet with Ed Bennet carefully watching. Note the flames coming out of the Ramjet engines. Jeff Evans Collection Photo

The company soon had three prototypes flying: the original streamlined fiberglass-enclosed two-seater, and two stripped-down utility versions. A fourth Hiller Hornet was manufactured, but it never flew.

Hiller foresaw many applications for the Hornet, primarily as a flight school trainer, a commuter aircraft, and for pleasure flying. Potential military uses included as an air ambulance, artillery spotting, reconnaissance, and observation. Other potential industry uses were for aerial photography, publicity, courier service, and for executive transportation.

The simplicity of the new Hiller-Hornet was described in a 1951 special edition of the Hiller Copter-News. “Powered with two tip-mounted ramjet power plants developed by Hiller — engines which do not have a single moving part — the Hiller Hornet has only two hand controls and fewer instruments on the panel than the average automobile,” the magazine stated. “Contrary to most jet aircraft, the Hiller Hornet’s sound range compares favorably with that of the conventional-powered helicopter.”

Located on the aircraft’s small instrument panel were the tachometer, fuel flow indicator, air speed indicator, and the altimeter. Just below the altimeter was a starter button for igniting the ramjet engines. Hand controls included an overhead cyclic control stick, and a collective pitch and throttle just left of the pilot’s seat. This mechanism was also attached to the rudder installed at the tail of the helicopter. Its movement horizontally to the left or right controlled the direction of the flight. A collective pitch stick for vertical ascent and descent was so designed and located as to control the helicopter directionally through movement in a horizontal plane.

Three Hiller Hornets take flight at the Hiller Plant in California. Jeff Evans Collection Photo

The helicopter was reportedly very stable due to the proven Hiller Rotor-Matic paddles and the aircraft’s high-inertia rotor.

A helicopter for the masses

The small, utility two-place helicopter weighed 356 lb. (160 kg) empty, with a gross weight of about 900 lb. (410 kg), and length of about 12 feet, nine inches (3.9 meters). It was designed to sell for only $5,000, and was small enough to be parked in the average home’s garage. Hiller’s idea was that the Hornet would be a commercial helicopter for the masses, and he hoped that the price would drop even further when engineering and research expenses were written off over time.

The aircraft’s rotor diameter was 23 feet (seven meters), and the height to the top of the rotor was 7 feet, 10 inches (2.4 meters). Its normal cruising speed was 70 m.p.h. (110 km/h) with a top speed of 80 m.p.h. (130 km/h). Its range with two passengers was close to 50 miles (80 kilometers). The ceiling at full gross load was 12,000 feet (3,650 meters). The rate of climb was 1,100 feet (335 meters) per minute.

The helicopter had a tubular main structure, made from a combination of aluminum and steel. The two-bladed all-metal rotor system attached to a teetering type rotor hub. The auxiliary starter was located behind the pilot’s seat and could be hand cranked by using an electric power unit, or by a small gasoline engine. A 25-lb. (11-kg) luggage compartment was behind the seat. The fiberglass cabin enclosure could be dismantled in a few minutes, and underneath was a tricycle three-wheel undercarriage.

An issue of Popular Mechanics in 1951 shows the Hiller Hornet being used a commuter aircraft. Jeff Evans Collection Photo

The aircraft’s fuel tank held 37 US gallons (140 liters), and the Hornet was a bit of fuel guzzler, burning through about 50 US gallons (190 liters) per hour. The panel in the cabin featured a low-fuel warning gauge.

Autorotation on a Hornet kept the experienced pilot honest. The ramjets retarded main rotor windmilling, resulting in the helicopter descending at a staggering 3,000 feet (915 meters) per minute towards the ground. However, with careful flaring, one could still have a soft landing.

Getting going in the Hornet was quite simple. Start the auxiliary engine, depress the clutch, and once the speed of the main rotors had reached about 150 rpm, you pressed the starter button to ignite the fuel in the ramjets. When the rotor blades reached just over 500 rpm, the Hornet was ready for liftoff. No warmup was required.

Civil Aeronautics Authority (CAA) commercial certification was underway by early 1951, with commercial marketing of the Hornet planned for the spring of 1951. However, due to the war in Korea, and because of expanded military orders for Hiller’s Model 360 helicopters, this had to be adjusted. All production of Hiller’s helicopters was focused on the military.

A U.S. Navy Hiller Hornet at the Classic Rotors Museum in Ramona, California. Jeff Evans Collection Photo

With commercial certification on hold, Hiller gauged the military’s interest in the Hornet. The U.S. Army and U.S. Navy ordered an evaluation quantity of the two-place upgraded Hornet in June 1952. The improved updated HJ-1 Hornet was designated the H-32 for the Army, and HOE-1 for the Navy.

The H-32 became the Army’s first operational ramjet helicopter, and the HOE-1 was the Navy’s first tip-powered jet helicopter. Five HJ-1 Hornets were manufactured: two for the Army, and three for the Navy.

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Hiller engineer’s felt that the improved Hornet would be a good proving ground for the principles of tip-powered ramjet helicopters. Evaluations by the U.S. military would also prove the claims of lower cost and maintenance for this type of helicopter.

A military design

The new Hiller H-32 differed in many ways to the original tip-powered HJ-1 rotorcraft manufactured in 1951. The new improved version was on skids instead of wheels. The cockpit cabin was slightly larger, with a new fiberglass tailboom, plus a small one-piece tail rotor in the back. Inside were standard cyclic sticks and pedals to control the small tail rotor. The military had requested the changes to make the aircraft’s controls common with other helicopters in its fleet. The small tail rotor helped to improve yaw control in the performance of the H-32.

A Hiller YH-32A called “Sally Rand” on the tarmac at the Hiller Plant. Note the two litters on the helicopter. Three of these helicopters were manufactured. Jeff Evans Collection Photo

The empty weight of the H-32 increased to 530 lb. (240 kg), with the gross weight now 1,080 lb. (490 kg). The service ceiling was reduced to 6,900 feet (2,110 meters). The range dropped to only 28 miles (45 kilometers), while the endurance was reduced to about 30 minutes. Hiller was not impressed with the reduced performance caused by the military’s changes.

The ramjet engine was also updated with a larger diameter, and new weight of 12.7 lb. (5.7 kg). Called the 8RJ2B, the ramjet engine produced 45 horsepower. The CAA approved the new engine in October 1954 for commercial production and sale.

Hiller delivered the first Hornets to the U.S. military in late 1954, when service tests and evaluations began. The Army followed up with another order for 12 H-32s, for a total of 14. Hiller’s production run of the military tip-powered Hornets was the first of its kind in North America.

In 1955, Hiller received a contract for an armed helicopter called the YH-32A ULV (ultra-light vehicle). Three YH-32A helicopters were manufactured and evaluated at the Fort Rucker Army facility in Alabama during 1957. The helicopter used the Hiller Hornet rotor and systems with a twin tail at the back. Tests were carried out using missiles, rockets, and a recoilless cannon. The helicopter armed concept program was successful, but the military placed no further orders.

A U.S. Navy HOE-1 Hiller Hornet takes off. Note the small tail rotor installed at the rear of the tailboom, following the military’s request for one. Jeff Evans Collection Photo

Over time, the military decided that the aircraft’s short range, high fuel consumption, its potential for fuel starvation, and concerns with its autorotation capabilities would not meet the acceptance standards for both the Army and Navy. Within a year, the military withdrew the H-32/HOE-1 helicopters from service. Hiller decided to give up on marketing the Hornet commercially after the CAA did not approve civil certification.

All told, Hiller manufactured 25 Hornets, and about half have survived, on display in museums across the U.S. and in private collections.

Even with the end of the Hornet program, Hiller had not given up on the promise of tip-jet propulsion. He was already thinking of using the technology on a larger scale, dreaming of a future when large heavy-lift flying cranes, and passenger carrying sky buses would fly using tip-powered jets.

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