Back Hiller HOE-1 / YH-32 Hornet

Hiller HOE-1 / YH-32 Hornet

Originally intended for the civil market with a price as little as $5,000, the Hiller Hornet was a tiny helicopter powered by ramjet engines mounted on the rotor tips. It was tested in 1956 as the HOE-1 or H-32 for the US Army for use as an artillery spotter or forward observation platform. For this role the Hornet lacked what we now call 'stealth', as it could usually be heard by the enemy and the glowing engines could be seen from a long distance. Hiller once claimed 'the Hornet's sound range compares favourably with that of a conventional-powered helicopter' but without an intercom system the pilots had to scream at each other to communicate. The museum owning the only remaining flyable example of the 17 Hornets built received complaints from neighbours a mile and a half away when they last flew it.

The high drag of the ramjets meant the blade angle had to be set to a very negative angle when power was cut, and this led to the Hornet plummeting at 15mps during auto rotation. Only a very skilled pilot could arrest this descent just before the ground.

Jim Winchester "The World's Worst Aircraft", 2005

Hiller HOE-1

Early in 1952, Hiller approached the Navy about authorizing construction of an evaluation quantity of Hornets. The proposal was accepted because it was felt that a revised Hornet might meet a Marine Corps requirement for an ultralight flying vehicle. By summer a fixed-price contract had been negotiated for delivery of five units, three to go to the Navy as the HOE-1. Impressed by the simplicity of the original HJ-1, the Army asked to take the remaining two under the designation YH-32. At the end of 1954, Army officials ordered a dozen more Hornets, and Hiller's own test and demonstration requirements brought the grand total of second-generation Hornets to twenty-five, including the three ULV gunships described below. This evaluation quantity of HOEs and YH-32s marked the first production of tip-jet-powered helicopters in history, and the first procurement of such vehicles by U.S. military services.

Coming only a year after the rift between the Navy and Hiller, the order for HOE-1s seemed to belie hard feelings on the part of Navy procurement officials. This small order for test and evaluation machines, however, was approved by the Office of Naval Research. When it came to the acquisition of operational aircraft, the Navy's grudge against Hiller was still in place.

Although the new military helicopter shared the name and designation of the original HJ-1 "Hornet", it was in fact an all-new machine with no commonality. Instead of wheels, it had lightweight skids. Gone too was the rudder worked by moving the collective lever side to side; the new HJ-1 had rudder pedals controlling a tiny single-blade tail rotor (these were reinstated for commonality with other military helicopters). While not needed to offset torque, the tail rotor - which furnished crisper yaw control than was provided by the earlier Hornet's rudder - helped during autorotation and permitted rapid sideways flight as required by increasingly stringent military control requirements.

The HOE-1/H-32 was a trim machine with perky lines that might just as easily have sprung from Walt Disney's drawing board as from the Hiller team's. In fact, it was designed by both Bob Anderson and the vice president of engineering, James B. Edwards, who had come to Hiller in January 1952 from Douglas Aircraft where he had been project engineer on the famous DC-4 and DC-6 airliners (Edwards would return to Douglas late in 1953 to fulfill the same function on the DC-7).

Unfortunately, as often happens with military involvement in a previously private venture, the HJ-1 suffered a growth in size and weight. A larger cockpit, fiberglass body and tail boom (the first structural use of fiberglass in aircraft construction), and other changes raised the empty weight from the original Hornet's 160kg to 240kg, and the gross weight climbed to 480kg. As a result, the service ceiling fell drastically from 3350m to 2100m. Even with 200 litres of fuel, the craft's range shrank from 65km to 45km, its maximum powered endurance at economy cruise being roughly thirty minutes.

The decreased performance frustrated the Hiller team, which felt that the HJ-1 had evolved away from the original concept of a truly simple helicopter. Worse still, the joint military program was worked over and over by the Army and Navy with so many extra-contractual changes that - including the dual certification programs described below - it eventually consumed more than a million dollars of Hiller resources.

The first of the HOE/H-32 series flew in September 1953, with deliveries to commence the following spring. The services did not receive them until the end of 1954, however, because these machines had been procured on a "certification specification" basis rather than on the standard "mil. spec." basis, and civil certification of the helicopter and its engine would take time, as both were new to the experience of the CAA.

The 8RJ2B ramjet engine was a refined and uprated version of the unit Hiller had developed in 1949. Now eight inches in diameter and weighing 5.7kg, it produced the equivalent of 45hp. Manufactured by Ryan in San Diego under license from Hiller, the 8RJ2B was made out of lnconel X, a high-nickel alloy also used in the Mach 6 North American X-15 research plane owing to its lightness and great resistance to heat. Because Inconel X corrodes on contact with lead - then commonly found in automotive gasoline, which was a likely fuel for this engine - the engine's interior was coated with a protective ceramic material.

By August, the 8RJ2B had logged 559 hours in the air, 2104 hours on the whirlstand, and 1545 hours in free airstream static tests. Its early teething troubles had been completely solved, even the once nagging problem of flameouts. If flameouts should occur, the engine now reignited itself so quickly that the pilot never perceived the problem. Another selling point was the remarkable portability and simplicity of the 8RJ2B. An untrained person could change a Hiller Hornet's engine in just three minutes with nothing more than a screwdriver.

Ramjet testing on the ground included static duration runs, which were a sore point with residents of Belle Haven and East Palo Alto. One test in March 1953 lasted 200 hours, while another the following February ran for 150 hours. At night when all was otherwise still, moreover, an inversion would often form over the entire San Francisco Bay area, causing the bam-bam-bam of the whirling ramjets to skip over much of the local area and bounce squarely into the fashionable Atherton and Menio Park neighborhoods. Needless to say, it was not welcome. Nor could the company president himself escape it; the disagreeable noise often greeted Stan Hiller when he returned home to Atherton at night.

Harried residents telephoning the plant at Palo Alto to complain often found sympathetic listeners there, many of whom yielded to the temptation to suggest that the sound "must be coming from the Navy at Moffett Field." Fortunately, the problem was significantly reduced in the spring of 1954 by construction of a circular barrier 5.5m high and 12m in diameter, which was designed to muffle jet engine noise. Since the structure could not entirely encircle the facility without depriving it of sufficient air, however, the side facing the bay was left open. The necessity of building the barrier hinted at the seriousness of the tip-engine noise problem; in a world increasingly sensitive to aircraft noise, the issue was a significant obstacle to realizing Hiller Helicopters' vision of tip-turbine flying cranes.

During the last days of ramjet certification in 1954, several overspeed runs were conducted on the whirlstand. At peak rpm, when the tip jet was subjected to as much as 14000 Gs (boosting its "weight" to some 80300kg), supporting bolts sheared and the 8RJ2B shot away through protective walls, acoustical barriers, and property fences before coming to rest some 150m from the test site. The test stand also departed the area, flying off toward San Francisco Bay. It was later recovered from nearby mud flats. The only known structural failure of a Hiller tip-power rotor, this test far exceeded expectations and overwhelmingly satisfied the stringent certification requirements.

On October 28, 1954, the CAA awarded Hiller's 8RJ2B engine Type Certificate No. 280, officially approving it for commercial production and sale should Hiller so desire. Truly pioneering developments in power plant technology would soon overshadow this unit, which would be forgotten as Hiller moved beyond the ramjet in its tip-propulsion program (the subsequent power plant research lies beyond the scope of this book). As the first American-designed and manufactured jet engine to be approved for commercial sale, nevertheless, the Hiller 8RJ2B represents a milestone in U.S. aviation history.

Unfortunately, parallel certification for the "Hornet" helicopter was not granted. The reason was the cold drag problem associated with autorotation, described above for the earlier HJ-1. The new military Hornets descended steeply at almost 18m/s, but their massive rotors stored so much energy that landings were simple after one became accustomed - if one ever could - to the high sink rate. Hiller test pilot and marketing executive Robert Boughton found that whereas one would begin to flare 15 to 18m above the ground in a normal helicopter, the "Hornet" demanded that one begin pulling up on the collective a full two to three hundred feet off the ground. Enough inertia was stored in the tip engines to permit the pilot to touch down, rise into the air again, and land a second time.

A solution to the unacceptably high sink rate was proposed to the military during development of the H-32 and HOE. The company suggested that the flameholders in the mouths of the ramjets be modified to function like controllable shutters. During autorotation, they could simply rotate shut to close off air-flow through the engine and greatly reduce cold drag.

Flameouts had by now been all but eliminated. Even if one engine failed, the "Hornet" could be flown some distance to a safe landing, although it could not maintain altitude. An automatic low-fuel power reduction further enhanced safety by alerting the pilot to land while sufficient fuel remained to do so under power. Still, with range so limited and the possibility of fuel starvation so great, flameholder shutters were clearly a desirable feature.

The military declined because its strict acceptance standards made no provision for such devices. Without them, the Hornet's autorotation proved unacceptable to the CAA, which otherwise found the diminutive craft satisfactory. Civil certification was accordingly denied. Failure to achieve this production license removed any possibility of Hornets reaching the private market, although it is doubtful that Hiller would have chosen to market a civil version in any event.

What was it like to fly the Hiller HOE-1/YH-32? Start-up was easy if not instantaneous, since a small electric motor first had to spin the rotor up to 150 rpm, or a third its operating speed. Fuel pressure was then built up, fuel flow valves were opened, and the starter button was depressed to fire a spark plug in each ramjet. Ninety seconds later, the Hornet's rotor reached 450-500 rpm and away it flew. Pilots who had the chance to try it out reported generally pleasant and forgiving characteristics.

J.P.Spencer "Whirlybirds: A History of the U.S. Helicopter Pioneers", 1998

Another of Stanley Hiller's innovative helicopter designs the commercial Model HJ-1 Hornet was intended primarily for the civilian commuter market and first flew in 1950. The two-place craft was of exceedingly simple construction, consisting mainly of a reinforced steel tube framework overlaid with a skin of fiberglass and plastic laminate. The Hornet was powered by two Hiller 8RJ2B ramjets, one fixed to the end of each main rotor blade, with an auxiliary one horsepower gasoline engine being used to spin the rotor blades up to the 50 rpm required prior to ignition of the ramjets.

The Army felt that the Hornet might have some military value as a light and easily maintained battlefield observation platform, and in 1952 ordered two examples for service test and evaluation. These YH-32s (serials 53-4663 and -4664), which were delivered in mid-1953, differed from the commercial Model HJ-1 only in having two small tailplanes attached to the aft end of the tailboom. A follow-on order for twelve additional examples was placed in 1955 in order to permit a more thorough evaluation of the type's military potential. These twelve aircraft (55-4963 through -4974) were all delivered by late 1956, but within a year the Army determined that the YH-32 was unsuitable and all fourteen Hornets were subsequently withdrawn from service.

S.Harding "U.S.Army Aircraft since 1947", 1990

Technical data for HOE-1

Engine: 2 x 8RJ2B ramjets, 17kg of thrust each, main rotor diameter: 7.0m, fuselage length: 3.45m, height: 2.44m, take-off weight: 487kg, empty weight: 239kg, max speed: 129km/h, cruising speed: 111km/h, service ceiling: 3500m, range: 450km

Johann Wegmann-FACH Chile, e-mail, 19.08.2017

Dear Gentleman, motorglider Stemme SB10, with rotax of 90 kg,can be improved by Subsonic Ramjet Hiller!!I made an aproach for 1000 newtons x 2 under every wing, and booster in the tail,De Laval CD knozzles for reaching ramjets start taking off.After many years research with pulsejets, DCR Ramjet and Scramjet are in my files,high stagnation temp. for scramjet makes me study reversible stirling engine criogenics cooler with expansion valves,helium or glycol, many options, xcramjet X43A, only concentrate hgh spots of 10000K somewhere,if you can helpme, I handle FEA to simulate,Hypersonc is the future,Best Regards Johann wegmann,White Eagle,Cobra Squadron,Chile.

Vince, e-mail, 31.07.2015

There used to be an HJ-1 (the one with the big ruder) in the hangar at the San Jose State Aeronautics Dept. I wonder if it's still there?

Marc de Piolenc, e-mail, 11.07.2013

Seems to me that the original machine had enormous potential as the basis for a very lightweight compound helicopter - one that would take off and land vertically under tip ramjet power, but cruise as an autogyro with much lower fuel consumption. Doing that would required making ramjets that were conformal with the rotor blade to make their drag acceptable in cruise, but this seems feasible.

polo, e-mail, 17.06.2011

can i have buy plans for this.

jerry, e-mail, 14.05.2009

This model served as the basis for the toy G.I. Joe helicopter I had as a kid in the early '60s (when all G.I. Joe accessories were based on some kind of real-life equipment). I remember thinking it was kind of silly looking, but I guess such a small, self-contained design was necessary to accomodate a foot high doll, in correct scale, within acceptable size and durability limitations for a toy. Something like a Bell-47 would have been too large and too complex (with it's strutwork tail) for such purposes

Rolando BATRES, e-mail, 30.10.2007

can i have buy plans for this.

Do you have any comments concerning this aircraft ?

Name   E-mail


The advantage of a rotor-tip propulsion syste is that there is no torque, which is created in coventional helicopters by the rotor's motion working in opposition to that of the engine and its attachment to the structure.

On a warm day the Hornet could barely hover over a metre above the ground with two people and a full fuel load. Development of turbine engines allowed small helicopters to carry useful loads.

The Hornet's engine became the first ramjet to be certified for civil use in the USA.

The Hornet was able to run on a wide variety of fuels, but the fuel tank held only enough for 25 minutes flying or 65km range.

The rotor needed to be spun to 50 rpm by a small motor before the ramjets would start. Normal operating speed was 550 rpm.

The no-torque ramjet system eliminated the need for a conventional tail rotor, making the Hornet very mechanically simple.

All the World's Rotorcraft

Virtual Aircraft Museum