Cold War

The Hiller Hornet, Was Ramjet Powered

The Hiller YH-32 Hornet emerged in the early 1950s, a period marked by rapid advancements and experimentation in aviation technology.

Developed by Hiller Aircraft, a company renowned for its innovative approach to helicopter design, the YH-32 was conceptualized as a unique solution to the complexities and weight issues associated with traditional helicopters.

The central innovation of the Hornet was its utilization of ramjet engines mounted directly on the rotor tips to power the rotor blades. This novel approach effectively eliminated the need for a conventional engine and transmission system, significantly reducing the helicopter’s mechanical complexity and overall weight.


Stealth Capabilities

In 1956, the HOE-1, also known as the H-32, underwent testing by the US Army for potential use as an artillery spotter or forward observation platform. However, the Hornet faced significant challenges in this role due to its lack of what is now recognized as ‘stealth’ capabilities.

Innovative Propulsion: It was one of the first helicopters to use rotor-tip ramjets for propulsion.
Innovative Propulsion: It was one of the first helicopters to use rotor-tip ramjets for propulsion.

Its noise level was usually detectable by enemy forces, and the glowing engines were visible from a considerable distance. Hiller Aircraft had claimed that the Hornet’s noise level was comparable to that of conventional helicopters, yet the absence of an intercom system meant that pilots had to shout to communicate with each other.

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The only remaining flyable Hornet, now housed in a museum, once elicited complaints from residents living a mile and a half away during a test flight.

One technical challenge with the Hornet was the high drag caused by the ramjets. When power was cut, the blade angle had to be set extremely negatively, resulting in the aircraft descending rapidly at 49 feet per second during auto-rotation. Only highly skilled pilots could manage this rapid descent and safely land the helicopter.

The concept of powering a helicopter using tip jets was first pioneered during World War II by Friedrich von Doblhoff, an engineer in the German-controlled Wiener Neustadt Flugzeugwerke, a small aircraft factory in Vienna.

This innovative idea led to the construction and successful flight testing of two prototypes at the behest of the German Navy, though flight experience showed that the fuel consumption of the tip-jets was prohibitively high.

Hiller Hornet

Stanley Hiller Jr., a visionary in vertical flight, once imagined a future where personal helicopters replaced walking, envisioning a helicopter in every garage.

Legacy: While the Hiller Hornet was an innovative concept, it remained largely experimental and didn't enter widespread production or use due to its limitations.
Legacy: While the Hiller Hornet was an innovative concept, it remained largely experimental and didn’t enter widespread production or use due to its limitations.

This dream was captured in a February 1951 Popular Mechanics cover illustration, depicting a suburban neighborhood where residents flew personal helicopters, closely resembling Hiller’s own design, the Hiller Hornet, announced that same year.

The late 1940s and early 1950s represented a period of intense exploration in Vertical Takeoff and Landing (VTOL) technology.

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This era saw the development of innovative aircraft concepts, including turboprop fighters that launched vertically, “convertiplanes” that functioned as helicopters and airplanes, and various tilt-wing and tilt-rotor designs. Among the pioneers of this era was Stanley Hiller, the designer of the YH-32 Hornet, an experimental helicopter powered by rotor-tip ramjets.

Hiller’s fascination with helicopters began in 1941 at just sixteen after being inspired by Igor Sikorsky’s early helicopter flights. His initial focus was to eliminate the tail rotor from his designs, believing it unnecessarily diverted power from the lifting rotors.

Hiller Hornet Double Copter

Despite his young age, Hiller was no stranger to innovation. Six years earlier, he had built a toy car that he drove through Berkeley’s streets. By twelve, he developed the Hiller Comet, a 19-inch model racing car capable of speeds up to 107 mph.

Supported by his father, he formed Hiller Industries, producing these models using a novel die-casting technique for nonferrous metals, later utilized in World War II aircraft manufacturing.

Development Era: The Hiller Hornet was developed in the early 1950s.
Development Era: The Hiller Hornet was developed in the early 1950s.

In 1942, Hiller established the Hiller Aircraft Corporation, developing the XH-44 Hiller-Copter. This double rotor craft was his solution to the torque issue in helicopters. He patented a system of coaxial double rotors rotating in opposite directions to cancel out torque.

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The XH-44, notable for its secrecy during development, was a significant achievement, being the first coaxial helicopter, the first to fly west of the Mississippi, and the first with rigid, all-metal rotors.


Hiller’s innovation was recognized by shipbuilder Henry J. Kaiser, leading to financial support and the formation of the Hiller-Copter Division within Kaiser Cargo Company. In 1945, Hiller formed United Helicopters Inc., focusing on creating user-friendly, affordable rotary-wing aircraft.

At just nineteen, Hiller was already an accomplished entrepreneur, and his ingenuity extended to developing the Hiller 360.

Fuel System Complexity: The Hornet had a complex fuel system due to the need to transfer fuel to the rotating rotor blades.

Following a crash in his XH-44 model, he shifted to the more conventional tail-rotor design. The Hiller 360, a milestone in helicopter design, was widely sold for commercial use, exceeding sales of other manufacturers in the early years.

During the Korean War, Hiller’s workforce expanded significantly, with over 2,000 of the 360/UH-12 series produced over two decades. These helicopters were used globally and continuously improved, with the Army’s H-23D and the 12E model being notable examples of advancements.

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Hiller’s exploration of VTOL technology didn’t stop with conventional helicopters. He worked on the Flying Platform and the XROE-I Rotorcycle, a one-man collapsible helicopter for the Navy.

His company also contributed to the development of the X-18 tilt-wing “propelloplane” and the XC-142A transport VTOL prototype, a collaboration with other aerospace companies.

Priced at under $5,000

In 1951, Stanley Hiller introduced the HJ-1 Hornet, an innovative helicopter design powered by rotor-tip ramjets, aligning with his vision for a simplified, affordable, and easy-to-fly aircraft. Standing 7 feet tall and equipped with a 23-foot rotor, this compact helicopter could potentially fit into some garages.

Test Batch: A test batch of twelve YH-32 prototypes was ordered by the Army.
Test Batch: A test batch of twelve YH-32 prototypes was ordered by the Army.

Priced at under $5,000, it was competitively priced against sports cars of the time, and Hiller envisioned the Hornet as the ‘Volkswagen of helicopters’.

However, the onset of the Korean War put these plans on hold. In 1952, the Army ordered a test batch of twelve slightly larger prototypes, the YH-32, featuring a 7ft 8in frame and a tail rotor, which were delivered in 1956.

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The Navy also built a prototype but ultimately did not accept it. The Museum of Flight’s military YH-32 Hornet, discovered in a garage in poor condition, sparked curiosity about its origins. Stanley Hiller purchased and restored this aircraft, later donating it to the Museum.

The Hiller company’s innovative approach aimed to significantly reduce the weight of a normal helicopter by using rotor-tip powerplants.

Flying Crane

As explained by project engineer Harvey Holm in a 1953 helicopter design seminar, this design would negate the need for antitorque systems, complex engine installations, and large power transmission systems.

Military Interest: The U.S. Army expressed interest in the Hornet for use as an artillery spotter and observation platform.
Military Interest: The U.S. Army expressed interest in the Hornet for use as an artillery spotter and observation platform.

Stanley Hiller shared with his biographer, Jay Spenser, a former curator at the Museum of Flight, that the YH-32 Hornet served as a proof of concept. Its primary purpose was to demonstrate the feasibility of rotor-tip power for a potential large-scale “flying crane.”

The production and field testing of the dozen Army prototypes were meant to validate this rotor-tip power concept. In a larger ‘flying crane’ model, the weight savings from the powertrain would directly enhance payload capacity. The U.S. armed forces showed keen interest in such technology, considering its potential benefits.

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Howard Hughes, renowned for his ambitious aircraft designs, had previously built a flying test bed, the XH-17, featuring a 130ft main rotor diameter powered by compressed air funneled to tip nozzles from jet engines mounted on the platform.

This model, which flew in 1952, had the potential to be the first helicopter to carry a load equal to its own weight, with a maximum loaded weight of 105,000 pounds. Hiller’s ongoing development of ramjets for the YH-32 promised greater efficiency at the high speeds achieved by the tips of such lengthy rotors.


Hiller’s development of rotor-tip powerplants led to a significant achievement: the creation of the first ramjet engines to be certified by the CAA, and notably, the first American-designed and built jet engines of any kind to receive such endorsement.

Close up of a ramjet
Ramjets are inefficient at subsonic speeds

Ramjets, characterized by their lack of moving parts and simplicity, can be quite compact. They operate by drawing in air due to their forward motion, with a flameholder at the front, equipped with a fuel nozzle and ignition line, igniting the fuel-air mix.

A combustion chamber follows, where the hot exhaust gases are expelled, completing the engine’s structure.

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Ramjets become efficient at very high speeds, and with rotor-tip speeds nearing the speed of sound, these small engines were more suitable than any other available options at the time.

Throughout the late 1940s, Hiller had been refining the ramjet, overcoming challenges in manufacturing engine parts from the heat-resistant but difficult-to-form nickel alloy, Inconel X.

The company succeeded in developing a ramjet about the size of a medium-sized watermelon, weighing 12.71 pounds, producing 40 pounds of static thrust (or 45 equivalent horsepower), and easily removable with just a screwdriver.This innovation represented a model of simplicity and functional design.

Splitter Tree

During two years of testing, it was found that the conventional rudder used on the flying test stand (Hiller Model XIIJ-1) provided less effective yaw control compared to a small tail rotor, particularly in crosswinds and at low rotor speeds during landing. The military’s need for controlled lateral movement led to the decision to equip military Hornets with tail rotors.

Starting the rotors, which required a small gasoline engine, took about five minutes to reach 50 rpm, as ramjets are incapable of generating thrust while stationary.

Tail Rotor Addition: Although initially designed without a tail rotor, later versions included one for better control.
Tail Rotor Addition: Although initially designed without a tail rotor, later versions included one for better control.

While this might have posed challenges in combat scenarios, it conveniently allowed for extra time during morning routines, such as enjoying a last cup of coffee.

Explaining the fuel system to colleagues in 1953, Holm highlighted its complexity, especially in transferring fuel to the rotating blades en route to the ramjets.

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A rotary seal at the rotor’s base and a “splitter tee” at the head facilitated fuel movement along the blades, where centrifugal force played a role. However, this design meant that fuel pressure varied with rpm, ranging from 25-40 psi from the fuel pump to as high as 2,500 psi at maximum speed.

Quick acceleration or deceleration was challenging due to the fuel quantity in the lines and the high pressures, a factor critical for pilots who rely on immediate throttle response.

Hiller Hornet Incredibly High Fuel Consumption

The innovative power concept of the Hiller Hornet was remarkably advanced for its time, yet it’s somewhat surprising that the most challenging aspect of its manufacturing program turned out to be the simple fiberglass tail boom.

In the early 1950s, fiberglass was a relatively new material for structural use. Hiller experimented with various shapes, chemical impregnation methods, and lay-up molding processes. By 1953, they believed they had mastered these challenges. However, the Hiller Hornet’s potential was ultimately limited by two significant factors.

Rotor Acceleration: The rotors were initially accelerated using a small gasoline engine as the ramjets couldn't develop thrust while stationary.
Rotor Acceleration: The rotors were initially accelerated using a small gasoline engine as the ramjets couldn’t develop thrust while stationary.

Firstly, the high fuel consumption of the ramjets posed a major drawback for a lightweight helicopter. The military test Hornets, for instance, carried fifty gallons of fuel which only allowed for twenty minutes of flight time.

To achieve practical range, they would have required much larger fuel tanks. Secondly, the helicopters were incredibly noisy, which was a significant drawback.

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Hiller Hornet 360

Though the Hiller company gained prominence with the Model 360 and its production during the Korean War, it was this very conflict that put an end to Hiller’s vision of widespread personal helicopter use.

Not every infantryman had the skills to be a good pilot, and experienced pilots feared that these flying machines could become dangerous in the hands of amateurs. Additionally, the potential for air traffic complications was a significant concern.

These unique, egg-shaped helicopters, while captivating in their design, ultimately became just a footnote in America’s post-World War II leadership in helicopter technology.

The powerplant and transmission issues with helicopters haven’t been entirely resolved, but they have been significantly reduced with the advent of turboshaft engines. These engines offer power-to-weight ratios three times greater than the piston engines used in early Sikorsky models.

Since the debut of the Hiller Hornet, advancements in powerplant technology have largely mitigated the drawbacks associated with conventional helicopters.