Helicopter lifting capacity is limited in large part by the weight of components such as tail rotor assemblies, clutches, transmissions, and cooling blowers. A helicopter that can do away with some, or all, of these items can increase its useful load by a significant margin. The larger the helicopter, the more these parts begin to impinge on the useful load of the aircraft. Modern helicopters have been able to reduce some components necessary to piston engines through the use of gas turbines. In 1948, turbine engines of a size suitable for the small airframe of a helicopter had not been proven reliable to the degree that a manufacturer would be willing to attempt production of such an aircraft. Helicopter pioneer Stanley Hiller, and a number of enterprising competitors employed an alternative solution by using rotor tip mounted ramjets to power a small compact helicopter that demonstrated the technologies he hoped to use in the design of giant aerial cranes. Hiller's design became the first production helicopter to be powered by ramjets, and entered limited military service with both the Army and Navy as a technology demonstrator.
Stanley Hiller established himself as a leading contender in the nascent helicopter market of the 1940s with the development of his Model 360, which saw military service through the Korean War as the UH-12/H-23. With the financial well being of his company established, Hiller sought out new niches for his lightweight designs, since Bell, Piasecki, and Sikorsky were dominating the market for utility and transport helicopters. Hiller had begun experimenting with tip-jet technology in 1945, as a means of simplifying the increasingly complex and heavy mechanisms necessary for vertical flight. Tip-jet technology originated in World War Two with Baron Friedrich von Doblhoff, an Austrian who developed and flew several practical models. Doblhoff's "cold-cycle" tip-jet rotors used high-pressure air from a compressor that was ducted through the rotor blades to their tips to power the blades around at a sufficient rpm to generate lift. Hiller's experimentation initially revealed that a "hot-cycle" system, which used exhaust gases and an afterburner type arrangement at the exhaust ducts, created greater efficiency and thrust. However, it did not take Hiller long to recognize that the greatest weight savings, and propulsive efficiency could be gained, by mounting the engines directly onto the tips of the rotor blades.
Hiller began to develop his vision for giant flying cranes, utilizing tip-mounted ramjets or jet turbines, by developing a simple version that could be sold as an affordable aircraft for the masses that was small, lightweight, easy to fly and maintain, transportable, and above all else, cheap. In 1948, Hiller began work on the HJ-1 Hornet single-seat sport helicopter. The most difficult challenge was developing the ramjet engines, which had seen very little practical use, but were far easier to engineer than jet turbines. The ramjet uses a shaped pipe in a high-speed airflow and the combustion of large quantities of fuel to create a significant pressure differential that produces thrust. The ramjet accomplishes the same effect as a turbine engine but without any moving parts, which results in a considerably lighter and more reliable structure. The disadvantages are that a high-speed airflow must already be established before the ramjet can begin to operate, and the fuel consumption goes well beyond that of a turbine engine. Hiller used the well-established pulse jet design from the German V-1 flying bomb as a starting point, but quickly discarded it in favor of a pure ramjet. A year later, Hiller perfected a ramjet engine that weighed only 5 kg (11 lb.) and put out 14 kg (31 lb.) of thrust when the rotor tip was moving at a maximum operational speed 207 m/sec (680 ft/sec) at 550 rpm. Since the two bladed rotor of the Hornet had one ramjet mounted on each tip a total of approximately 27 kg (60 lb.) of thrust was produced. This does not seem like much, but the only mass that the engines had to move was their own weight in addition to the small, lightweight rotor blades, for which this thrust was more than adequate. Since the rotor was freewheeling, there wasn't any torque that required a tail rotor. The high speed airflow required to start the ramjets was achieved through the use of a small 50 horsepower motor that spun the rotor up to 150 rpm at which point the airflow was sufficient to sustain the operation of the ramjets. Ignition of the fuel, which could be gasoline, kerosene, or fuel oil, was accomplished through the use of a device, similar to a glow plug, referred to as a "flame-holder," which insured re-ignition if the ramjet flamed out.
The first single-seat HJ-1, referred to as the utility model, consisted of an open steel and aluminum tube frame that used a rudder for directional control. This design first flew in 1950. The next HJ-1 enclosed the pilot and passenger sitting side-by-side in a cabin made of fiberglass, which was one of the first applications of this new material in the aviation industry. Hiller used this aircraft to begin his marketing campaign for the design while the aircraft was undergoing certification tests with the Civil Aeronautics Authority (CAA). The third HJ-1, which was the version intended for mass production at a price of $5,000, contained a number of changes. On the first two aircraft, the collective could be moved side-to-side to control the rudder in place of pedals. The new design used rudder pedals to control a small tail rotor. The tail rotor was an asymmetric design that consisted of a single blade balanced by a counterweight, and was driven by the starter motor. The cockpit was also significantly enlarged to accommodate two passengers. These changes increased the weight from 356 lb. to 510 lb., while decreasing the service ceiling to less than 7,000'. Stability of the design was excellent because of the incorporation of Hiller's patented Rotor-Matic servo control system, which allowed the aircraft to maintain a hover with hands off the cyclic. The Hornet series was one of the first helicopter designs to use all-metal rotor blades, which had not been used before that time because previously poor metallurgy often caused structural failure with the rapid flexing that rotor blades experience in flight.
The Korean War began before the new HJ-1 configuration could be successfully marketed. Hiller's limited production capacity was quickly taken up by military orders for his UH-12/H-23, and the HJ-1 program was halted. In 1952, Hiller talked the Navy into purchasing three HJ-1s under the HOE-1 designation, as a possible answer to a Marine Corps request for a flying ultralight vehicle. The Army, which had fallen well behind the Marine Corps in the use of helicopters, was not about to be left out of another project, and placed an order for two HJ-1s under the YH-32 designation. One improvement was retrofitted to the aircraft after they were ordered by the military. The tail boom was extended and two downward canted horizontal stabilizers were installed to improve stability in forward flight. The first HOE-1/YH-32s were not ready for delivery until late 1954, because Hiller was certificating the aircraft to Civil Aviation Authority standards as opposed to military specifications. The Army saw the aircraft as a possible low-cost replacement for its jeep in certain liaison, wire laying, and observation roles, and ordered an additional dozen for extensive field trails, after receiving its first two.
The HOE-1 proved to have a number of operational problems that precluded its deployment in the field. The small helicopter could not be safely approached while the rotors were turning since they hung close to the ground. The lightweight construction of the helicopter meant that it could easily be blown off balance in moderate wind conditions. The flames coming out of the ramjets produced an incredibly bright white halo when the HOE-1 was flown at night. This was a considerable disadvantage in the military environment, and the effect prompted a number of UFO sightings when operated in the vicinity of populated areas. The noise generated by the ramjets was also quite considerable, and did not endear the United Helicopter's Palo Alto, California facility to its neighbors. The most serious limitation was the enormous fuel burn of the ramjets, which consumed fuel at approximately ten times the rate of a piston engine providing the same power output. The fuel capacity of the HOE-1/YH-32 was only 136 kg (300 lb.) This had to supply a full power fuel burn of nearly 272 (600 lb.) per hour, which led to a maximum endurance of just over thirty minutes including start-up and shut down. The never exceed airspeed of the helicopter was mere 62 knots, which resulted in a maximum range of less than 30 nautical miles. Hiller appreciated the limitations of the aircraft, but believed the design concept would prove itself once reliable turbine engines could be found to replace of ramjets. However, the centrifugal fuel pumps required for turbines, which would be subjected to several thousand times the force of gravity, needed considerable reengineering before they could reliably be used for a tip-jet design.
The HOE-1/YH-32's most worrisome problem was its flight characteristics during autorotation in the event of engine failure. The ramjets created considerable drag on the rotors when they were not running, which necessitated a rapid descent to keep the rotors windmilling at a sufficient rpm to prevent an unrecoverable rotor stall. The rate of descent required during autorotation was nearly 3,000 fpm, which was almost twice that of most other helicopters. Additionally, because the ramjets increased the rotational inertia of the blades to a high degree relative to the weight of the aircraft, the flare into landing out of the autorotation had to be initiated much earlier and more gradually than was done in other helicopters. Flaring out of autorotation usually occurred below 100' in most other types, but this aircraft required a flare to begin between 200' and 300' above ground.
The Army and Navy did not employ the HOE-1/YH-32s operationally, but did utilize them in some interesting tests. The effective employment of helicopters into forward combat zones during the Korean War sparked some interest in arming helicopters to provide fire support. Previously the U.S. Air Force had jealously guarded its air support role, and had prevented the Army from pursuing the development of armed helicopters. As the Korean War was ending, the restrictions placed on arming Army aircraft by the National Security Act of 1947 and the subsequent Key-West accords were easing. In 1955, the Army issued its first ever contract for an armed helicopter when it negotiated with United Helicopters to produce two YH-32A ULVs (Ultralight Vehicles). These were bare-bones YH-32s that had their cockpit fairings removed, and the tail rotor replaced by a twin tail boom arrangement that mounted small rudders. The ULVs were nicknamed the "Sally Rand," a well-known burlesque star, because of their stripped down configuration. These three-seat variants were used in 1957 at Fort Rucker, Alabama to test the employment of a wide variety of weapons, including missiles, rockets, and recoilless rifles. The blast of the recoilless rifles had necessitated the change from the tail rotor to rudders. These tests were successful, and proved the viability of the helicopter as a stable weapons platform. However, the performance limitations of the YH-32A did not allow the aircraft to progress beyond its status as a technology demonstrator, and the ULV project was the type's swan song.
Hiller's skycrane concept never progressed beyond the design stage. Hughes Aircraft constructed a large turbine powered lifting helicopter, the XH-17, which utilized Doblhoff's principles, but was too inefficient to warrant further development. In the early 1960s, the Army began a competition for the development of a heavy lift helicopter. Hiller responded to the challenge, but the competition was canceled before the design could progress off the drawing board. Hiller later approached NASA to develop a concept for a giant ramjet helicopter in the 1,000,000 lb. class to capture spent Saturn V stages as they parachuted to earth. NASA eventually decided that a reusable space shuttle was a more effective way of overcoming the economic burden of throwaway rockets than trying to catch spent rockets in mid-air with a giant helicopter. Ironically, by this time, Hiller had finally succeeded in overcoming the engineering difficulties in designing reliable tip-jet turbine engines, which would have made such projects feasible. He would not get another chance to develop the technology, before he left his company during the controversial loss of the Army's Light Observation Helicopter (LOH) contract to Hughes aircraft. The remnants of his company began a project to replace the ramjets on one of the old HOE-1s with turbojets, but the company closed its doors before this project was completed.
Several Hornets and HOE-1/YH-32As survive in the hands of museums and collectors. The aircraft in the collection of the National Air and Space Museum is the second HOE-1 to be completed. The aircraft was accepted by the Navy on December 27, 1955 and completed a mere 28.5 hours of flight testing through March 1957 at the U.S. Naval Air Test Center at Patuxent River Naval Air Station in Maryland. Most of this aircraft's flights were short hops under twenty minutes in length. Apparently, some of the HOE-1/YH-32As that have fallen into collector's hands may have managed a few further flights with their new owners, though the type was never certificated for civil operations, because of the difficult autorotation technique required. However, the 8RJ2b ramjet was actually approved for civil use during Hiller's attempts to get the HJ-1 certificated. The 1950s saw a number of attempts by other manufacturers to employ ramjets or other tip-jet propulsion to helicopters. Many of these designs flew successfully, but most were ultimately abandoned because of the same limitations encountered by the Hiller Hornet.
The fuel consumption, and noise created by ramjets, and other tip-jet designs, have precluded their use in more recent helicopter designs in spite of their considerable advantages. The goal of the affordable personal helicopter continues to be an elusive goal. The Hiller Hornet and its developments came close to delivering on their promise for simple, safe, cheap, and easy to fly aircraft, but they could not give the appearance of being anything more than an interesting experiment.
Rotor Diameter:7.01 m (23 ft)
Length:4.54 m (14 ft 11 in)
Height:2.39 m (7 ft 10 in)
Weight:Empty, 257 kg (567 lb.)
Gross, 490 kg (1,080 lb.)
Engines:2 x Hiller 8RJ2b ramjets, rated at 39 lb. thrust each
References and Further Reading:
Polmar, Norman. Military Helicopters of the World: Military Rotary-Wing Aircraft Since
1917. Annapolis, Maryland: Naval Institute Press, 1981.
Spencer, Jay P. Whirlybirds: A History of the U.S. Helicopter Pioneers. Seattle:
University of Washington Press, 1998.
Spencer, Jay P. Vertical Challenge: The Hiller Aircraft Story. Seattle: University of
Washington Press, 1992.
HOE-1 curatorial file, Aeronautics Division, National Air and Space Museum