The Flying Platforms & Jeeps

[ds2knj]

The Flying Platforms & Jeeps

v1.1.0 / 01 sep 06 / greg goebel / public domain

* One of the more interesting aviation fads of the 1950s and the 1960s was the "flying platform" and the related "flying jeep". These were small rotorcraft, mostly based on ducted props. The flying platforms carried a single soldier and seemed attractive for scout missions, while the larger flying jeep appeared potentially useful for a number of different purposes.

While the flying platforms and jeeps had some advantages, they were not successful. The idea dies hard, however, with work more recently performed on unmanned aerial vehicles (UAVs) in a flying platform configuration, and on piloted flying platforms for civil and recreational use -- with most of these exercises also proving dead ends. This document outlines the history of the flying platforms and jeeps.

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[1] THE FLYING PLATFORMS
[2] THE FLYING JEEPS
[3] MODERN FLYING PLATFORMS
[4] FOOTNOTE: THE AVRO AVROCAR
[5] COMMENTS, SOURCES, & REVISION HISTORY

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[1] THE FLYING PLATFORMS

* The flying platforms grew out of research conduction by the US National Advisory Committee on Aeronautics (NACA, one of the precursors to the National Aeronautics & Space Administration or NASA) in the early 1950s on the feasibility of one-man flying platforms for combat use. The tests involved pilots "flying" tethered platforms, at first lifted by compressed air, and then by rotors.

The concepts investigated in the tests were based on thinking by NACA engineer Charles H. Zimmerman, who proposed that if the rotors of a helicopter were placed on the bottom of the aircraft, a pilot would be able to steer it just by shifting his or her weight, a concept Zimmerman called "kinesthetic control". It was hoped that kinesthetic control would allow a pilot to fly such platforms with little training. The tests demonstrated the technical validity of the concept. The NACA results were released to the public, resulting in flying platform prototypes from three companies: de Lackner, Bensen, and Hiller.

* In the mid-1950s, the de Lackner company privately developed a rotorcraft named the "DH-4 Helivector", later renamed the "HZ-1 Aerocycle", that looked something like a cross between a helicopter, outboard motor, and motorbike, which is effectively what it was.

The de Lackner machine consisted of a frame that supported the engine of a 30 kW (40 HP) Mercury outboard motor, with landing gear consisting of an arrangement of airbags on the ends of spars. The airbags were later replaced by metal skids. The engine drove a pair of 4.6 meter (15 foot) contra-rotating rotors directly beneath it, while the pilot stood vertically on a platform above the engine, protected from falling into the rotors by a safety harness, and hanging on motorcycle handlebars with it a twist-grip throttle. He used kinestetic control to fly the machine.

The Helivector / Aerocycle first flew in January 1955, and the Army ordered a dozen. De Lackner claimed the machines could fly at up to 105 KPH (65 MPH), carry up to 55 kilograms (120 pounds) payload besides the pilot, and fly for an hour. However, while the thing looked like it would have been a lot of fun to fly, it was also dangerous. Not only did the pilot stand above the whirling rotors, but the rotors were wide and close to the ground, making them a hazard on landings and takeoffs since they could easily kick up rocks and other debris.

In addition, although some sources claim that the Helivector / Aerocycle was easy to fly, others state that the test pilot insisted that novices could not pilot it safely. After two flight accidents in which the contra-rotating rotors flexed and collided, the project was abandoned before somebody got seriously hurt. At least one survives as a museum display.

* The Bensen "B-10 Propcopter" was no more successful. This ugly little machine consisted of a boxy frame with 1.2 meter (4 foot) props mounted vertically on the front and back of the frame. Each was powered by its own 54 kW (72 HP) McCulloch engine. The Propcopter flew in 1959 and apparently was tricky to handle. In any case, nothing came of it.

* The Hiller designs were better thought out, and attracted a good deal of public attention. Hiller developed their first flying platform on the basis of a contract awarded in late 1953 by the Office of Naval Research (ONR) for a one-man flying platform. The machine made its first flight in February 1955, and was named the "VZ-1 Pawnee".

The Pawnee featured a pair of contra-rotating rotors spinning inside a duct with a diameter of 1.5 meters (5 feet). Each rotor was driven by its own 30 kW (40 HP) two-stroke engine. The pilot stood above the duct, surrounded by a circular handrail and protected by a safety harness. He controlled the engines with a twist-grip throttle and leaned to guide the aircraft. The duct improved safety during takeoff and landing. The duct also provided additional lift, since there was a horizontal "lip" around its top edge that curved down into the duct. The airflow into the duct resulted in low air pressure above the lip, and the pressure difference between the top and bottom of the lip generated a net upward force, providing as much as 40% of the total lift of the aircraft.

The Pawnee handled very well in flight tests. The machine was then modified with longer landing gear legs to increase ground clearance, and eight vanes were mounted underneath the duct to improve flight control.

The US Army was interested in the Pawnee, and in November 1956 awarded a contract to Hiller for the construction of a larger version, which performed its first flight in 1958. It had three 30 kW (40 HP) engines, driving rotors in a 2.4 meter (8 foot) diameter duct. This more than doubled the rotor disk area, increasing payload and range while reducing noise and downwash.

The Army ordered a third Pawnee with a deep drum-like duct that improved lift performance and allowed the machine to carry heavier payloads. It also featured a circular landing skid instead of the multipoint wheeled landing gear of the two earlier Pawnees, and had a seat and conventional helicopter controls, since kinesthetic control became less effective as vehicle power and weight increased. This version first flew in 1959.

The Pawnee had its merits, but it was ultimately judged as too small, slow, and of limited usefulness and practicality for combat service. The Army abandoned the program in 1963, and two of the three Pawnees survive as museum displays.

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[2] THE FLYING JEEPS

* While the Army was pursuing the flying platforms, they were also investigating larger rotorcraft along similar lines, called the "flying jeeps". Some sources imply that they were intended mostly for hovercraft operation, with an ability to fly over obstacles or impassable terrain when necessary, while other sources indicate they were regarded as helicopter-like utility vehicles that operated normally as flying machines.

Whatever the case, the US Army Transportation Research Command began an investigation into the flying jeeps in 1956, leading to award of contracts for prototypes to Chrysler, Curtiss-Wright, and Piasecki in 1957.

* Chrysler developed two prototypes of their "VZ-6" flying jeep, delivering them to the Army in late 1958. The VZ-6 was a single-seat vehicle, shaped like a rectangular box, with two ducted rotors in the box front and back. There were rubber skirts around the bottom, and vanes underneath the rotors to provide airflow for forward motion.

The VZ-6 was powered by a single 373 kW (500 HP) piston engine and had a gross weight of 1,080 kilograms (2,380 pounds). Tethered flights performed in 1959 indicated that the VZ-6 was not very controllable, and was also badly underpowered. On the VZ-6's first untethered flight attempt, it flipped over. The pilot escaped serious injury, but the vehicle was badly damaged. The Army recognized the VZ-6 as a lost cause and got rid of both prototypes in 1960.

* The Curtiss-Wright entry was the "VZ-7", also known as the "Flying Truck", with two prototypes delivered to the Army in mid-1958. The VZ-7 was a simple metal truss with a pilot up front and four horizontal propellers at each of four corners. The props were all driven by a single 317 kW (425 SHP) Turbomeca Artouste turboshaft engine, mounted underneath the central beam. The props were originally ducted, but the ducts were removed after initial test flights. The aircraft was guided by differential pitch between the propellers, and a rudder in the turbine exhaust.

The VZ-7 was 5.2 meters long by 4.9 meters wide (17 by 16 feet) and had a maximum takeoff weight of 770 kilograms (1,700 pounds), with 250 kilograms (550 pounds) of that being payload. The VZ-7 handled well and was easy to fly, but it did not meet either altitude or speed requirements, and the prototypes were returned to Curtiss-Wright in mid-1960.

* The Piasecki flying jeep effort was the most successful of the three. The first of the class was the Piasecki "Model 59H AirGeep", which was given the Army designation "VZ-8P". The AirGeep was 7.9 meters long and 2.7 meters wide (26 feet by 9 feet), with three-bladed rotors in ducts in the front and the back. The pilot and passenger sat between the ducts. In the VZ-8P, the 2.4 meter (8 foot) diameter rotors were driven by a pair of 134 kW (180 HP) Lycoming piston engines, with the power linkages designed so that one engine could drive both rotors if the other engine failed.

The rotors spun in opposite directions to reduce torque effects. Control was provided by varying rotor pitch, as well as through vanes mounted in the downdraft. Forward motion was achieved by pitching the aircraft nose-down.

The AirGeep was first flown on 12 October 1958. Apparently it proved grossly underpowered and it was sent back to the shop, where the piston engines were replaced by a single 317 kW (425 HP) Turbomeca Artouste IIB turbine engine. The upgraded AirGeep flew in late June 1959. It weighed 1.1 tonnes (2,500 pounds) and could carry a payload of 550 kilograms (1,200 pounds), including the pilot.

The AirGeep was put through trials for both the Army and the Navy over the next few years. The engine was upgraded again to a Garrett / Airesearch 331-6 engine, which had a higher power-to-weight ratio. For Navy trials, which began in June 1961, the rotorcraft was fitted with floats, and redesignated the "PA-59 SeaGeep".

* Piasecki wanted to build a bigger and better AirGeep, and the Army Transportation Research Command obliged them by issuing a contract for what Piasecki called the "Model 59K" and what the Army called the "VZ-8P(B) AirGeep II", which made its first flight in the summer of 1962.

The AirGeep II was similar to the AirGeep, except that the aircraft was "bent" in the middle so that the rotors were tilted fore and aft, it seems to improve forward flight characteristics. The AirGeep II used twin 298 kW (400 SHP) Turbomeca Artouste IIC turboshaft engines, once again linked so that if one failed the other would drive both rotors. One engine could also be linked to the landing wheels to drive the machine on the ground. The increased power allowed a maximum take-off weight of 2.2 tonnes (4,800 pounds). The pilot and observer had "zero-zero" ejection seats, allowing safe escape if the machine was on the ground and standing still, and there were apparently seats for additional passengers.

* The flying platforms and flying jeeps had some merits. They were smaller than helicopters, and could operate in ground cover more successfully. However, helicopters could land more easily on rough terrain and had more convenient seating arrangements. Most critical, the flying platforms and jeeps had much smaller rotors, and so were fuel hogs. There were also apparently concerns about the practicality of the one-person flying platforms, since they provided relatively little capability in relationship to larger rotary-winged aircraft, while still presenting a good proportion of the same support headaches. Some sources also claim, very plausibly, that the flying platforms were unusually dangerous to fly in windy conditions.

In short, they didn't have sufficient advantages over helicopters to make them worth further development at the time. The idea didn't die out, however.

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[3] MODERN FLYING PLATFORMS

* There was actually a jet-powered flying platform. In the mid-1960s, engine manufacturer Williams Research developed a light turbofan engine, the "WR19", with a thrust of 1.91 kN (195 kgp / 430 lbf), which was actually used in a "flying belt" that could be strapped on somebody's back to allow flights of up to 20 minutes.

It was a cute toy but of no particular usefulness, and it was cancelled in 1969. Williams continued to tinker with the idea, coming up with a one-man flying platform powered by the WR19 or a derivative engine, known as the "WASP", which was later renamed the "X-Jet". This machine looked something like a flying trashcan on skids, and could carry a pilot directing the machine with two grip-type controls. It was eval‎uated in the 1980s and noises were made about a more capable successor, but apparently its endurance was too limited and, as was the case with most of the other one-person flying machines, it was hard to understand that it offered any utility proportional to its expense and complexity.

Details of the X-Jet program remain entirely obscure. One X-Jet is now on display at the USAF Museum in Ohio, while another is on display at the Seattle Museum of Flight. The WR19 and its descendants did prove to be useful powerplants for long-range cruise missiles.

* In the late 1980s, Sikorsky Aircraft flew a small doughnut-shaped unmanned aerial vehicle (UAV) named "Cypher" that was based on coaxial-rotor technology developed by the company in the early 1970s. The Cypher was clearly a flying platform in general concept, with a doughnut-shaped shroud to increase lift and provide safety in ground handling.

The first proof-of-concept Cypher was 1.75 meters (5.75 feet) in diameter and 55 centimeters (1.8 feet) tall, weighed 20 kilograms (43 pounds), and was first flown in the summer of 1988. This design was powered by a four-stroke 2.83 kW (3.8 HP) engine, and was so underpowered that it had to be mounted on a truck for forward-flight tests. These tests led to a true flight prototype Cypher that weighed 110 kilograms (240 pounds), had a diameter of 1.9 meters (6.2 feet) and was powered by a compact, 40 kW (53 HP) Wankel-rotary engine. After an initial free flight in 1993, the Cypher prototype was used in flight tests and demonstrations through most of the 1990s, ultimately leading to an improved design, the "Cypher II".

Two Cypher II prototypes were built for the US Marine Corps, which called the UAV "Dragon Warrior". The Cypher II was of similar size to its predecessor, but had a pusher propeller along with its rotor and could be fitted to a configuration with wings for long-range reconnaissance missions. In its winged configuration, the Cypher II had a range of over 185 kilometers (100 nautical miles) and a top speed of 230 KPH (125 knots). The program seemed to have then faded out, with no mention of it on the Sikorsky website. However, defense programs that disappear sometimes come back again, so we may not have seen the last of the Cypher.

* There are a number of other schemes for flying platforms floating around these days, but it is unclear how seriously to take some of them. Moller International has been working on its own small flying platforms since the mid-1980s. Moller has produced a series of small ducted fan UAVs, known as "Aerobots", using a single fan or eight ducted fans, powered by rotary engines. These aircraft have received little public attention, however, and it is difficult to know if they are actually in serious use or have remained test machines.

An Israeli company named Aero-Design & Development (AD&D) has worked on a flying platform named the "Hummingbird" that has a strong resemblance to the Hiller Pawnee flying platforms, intended as a recreational vehicle. Although AD&D flew their machine in 1997, the company seems to have faded out of sight.

An American company named Millennium Jet of Sunnyvale, California, has been working on a recreational flying platform aircraft with the comic-book name of "SoloTrek Exo-Skelitor Flying Vehicle (SoloTrek XFV)" that got a certain amount of public fanfare for a time. This was an extremely unusual design that looked like an exotic piece of exercise equipment with twin ducted fans bolted on at the top. The fans tilted together to provide forward propulsion, or individually to spin the aircraft around.

The pilot stood up in the frame and controlled the vehicle with two handgrips at the end of armrests. The right handgrip was a joystick for directional control and the left handgrip was the throttle. The pilot had flight instrumentation as well, provided by a display built into helmet goggles. Headphones in the pilot's helmet provided a buzz that got louder as the fuel ran out.

The first of two prototype SoloTrek XFVs performed its initial flight in 2002. It was powered by a 55 kilogram (110 pound) Hirth F30 two-stroke, four-cylinder engine with electronic ignition and an output of 90 kW (120 HP). The props were built from a nylon / carbon fiber composite and could take bird strikes without damage, except to the bird. The second prototype apparently featured a more powerful engine and other improvements. The first prototype is now in the Hiller Museum in San Carlos, California.

Millennium Jet envisioned a production machine as being powered by a turboshaft engine, such as the Williams International WTS-125, which provides 93 kW (125 HP); it would also feature an automatic parachute safety system. The production machine was to have a fully-loaded weight of 363 kilograms (800 pounds), a cruise speed of about 95 KPH (60 MPH), a maximum speed of about 130 KPH (80 MPH), and a fuel capacity of 38 liters (10 US gallons), giving it a range of about 240 kilometers (150 miles). Millennium Jet also publicised designs for a larger flying machine named the "DuoTrek", which would use engine and ducted-fan systems derived from the SoloTrek, but mated to a more conventional helicopter-style fuselage. However, the company also seems to have faded out of sight.

Another US company, PAM Group of Williamsburg, Virginia, has been working on a flying platform since 1989, and has flown a proof-of-concept demonstrator designated the "PAM 100B Individual Lifting Vehicle (ILV)".

The PAM 100B looks like an interesting amalgam of different early flying platform designs. It is built around a simple tubular frame with skids, about 3 meters (10 feet) across, fitted with twin two-cycle, four-cylinder Hirth F-30 piston engines with 145 kW (195 HP) each, with each driving a 2.8 meter (9 foot 8 inch) rotor, with the twin rotors arranged fore and aft. The platform can in principle be safely landed on a single engine.

The two Hirth engines also drive two 30 centimeter (2 foot) diameter thruster propellers that rotate in the horizontal plane to provide directional control. However, the main directional control is provided by the pilot, who stands in a frame on top of the platform and uses kinesthetic control to fly the machine.

The PAM 100B has an empty weight of about 300 kilograms (660 pounds), a maximum payload of about 200 kilograms (440 pounds), top speed of about 100 KPH (60 MPH), and a range of about 40 kilometers (25 miles). The company envisions it as useful for applications such as cattle ranching or low-volume crop spraying.

PAM Group is now selling the PAM 100B as a kit, designated the "PAM 200", with a cost in the range of $50,000 USD. They are also working on improvements, such as a turbine-powered "PAM 100T", and an airbag system. As the platform is intended for work at treetop heights, a parachute would be ineffectual, but an airbag would be a very useful safety measure should both engines fail. A UAV version, the "PAM 300", is in the works as well.

The PAM Group website also has a set of "future concept" images that show off ILVs featuring a true ducted flying-platform configuration. It seems like a long shot, particularly given the way the road to the flying platform is strewn with the skeletons of failures, but the company has managed to survive where others have failed, and it would be hard-hearted not to wish them luck.

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[4] FOOTNOTE: THE AVRO AVROCAR

* One of the most unusual flying machines ever built was the Avro "Avrocar", an attempt to build a true "flying saucer" based on ducted-fan principles. It is hard to categorize it as either a flying platform or jeep, but since it is hard to categorize at all, it might as well be included here.

The Avrocar was the brainchild of John Frost, an aeronautical engineer at the Malton-Toronto plant of Avro Canada, who conducted studies in the early 1950s on extremely unorthodox vertical-take-off aircraft, beginning with one design that looked like a shovel blade named the "Avro Ace" or "Project Y-1". and then moving on to saucer-shaped aircraft, originally designated "Project Y-2". Frost felt that such vehicles could offer vertical take-off capability as well as high-speed forward flight.

The US military became interested, leading to the award of a US Air Force contract in 1955 for experimental development that evolved through various designations such as "Project Silver Bug", "Weapons System 606A (WS-606A)", "Research Project 1194", and so on. The whole project was originally kept a deep secret since it was felt to be a major potential breakthrough, though in hindsight it is tempting to wonder if it was also out of fear of mockery.

After considering and eliminating a number of different configurations, the effort focused on development of a proof of concept vehicle named the "VZ-9-AV Avrocar" which was funded by both the US Army and US Air Force. It looked very much like an ordinary but greatly scaled-up modern "Frisbee" toy, 5.5 meters in diameter and 1.1 meters thick at the center (18 by 3.5 feet).

There was a fan 1.5 meters (5 feet) in diameter built into a duct in the center, and two little single-seat cockpits with individual clamshell bubble canopies, giving the vehicle the looks of a bug-eyed 1950s spaceship of some sort. The Avrocar was powered by three Continental J69 jet engines, which were arranged in the fuselage in a triangle around the central fan. The exhausts of the jet engines drove the fan and were also ducted around the around the edge of the Avrocar to provide lift at low speed. Engine thrust could be redirected to provide forward thrust, with the saucer-shaped airframe acting as a "lifting body".

Frost was extremely enthusiastic about the possibilities of his flying-saucer concept, envisioning an "Avrowagon" for family use, an "Avropelican" for naval use, a scaled-up transport version, and a Mach-2 fighter-interceptor version.

A wind-tunnel test model and a flying prototype were built. The test model was sent to the NASA Ames Center in California for wind-tunnel tests. First tethered flight of the flying prototype was at Malton on 29 September 1959, followed by the first untethered flight on 5 December 1959. Although Frost and his staff recognized that the Avrocar was inherently unstable and had incorporated an electromechanical stabilization system, it wasn't up to the job, and once the Avrocar picked itself up to above chest height and got out of ground effect, it bobbled around drunkenly.

An improved stabilization system was considered, but Avro was in chaos due to the cancellation of the "CF-105 Arrow" interceptor program. The chaos filtered down to the Avrocar program, and the US backers of the program lost interest. The program was axed in December 1961. Whether it would have ever flown right remains an open question. Various implausible rumors still circulate that the military actually did get the Avrocar or something like it to work, accounting for many UFO reports.

The flight prototype ended up on display at the US Army Transportation Museum. The wind-tunnel test model in the possession of the Smithsonian Air & Space Museum. It remains in storage at the museum's Garber Restoration & Storage Facility in Maryland. One wonders if they take it out for a spin every now and then, and abduct somebody just for fun. The truth is out there.

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[5] COMMENTS, SOURCES, & REVISION HISTORY

* Another interesting or at least humorous flying platform, the Bensen "B-12 Sky-Mat", was developed in the late 1960s. The Sky-Mat was basically nothing more than a rectangular metal frame assembly with a seat and controls at the center, surrounded by ten -- count them, ten -- 7.46 kW (10 HP) piston engines, each driving a two-blade rotor. It was one of the most dubious-looking flying contraptions ever made. It would be a good guess that keeping all ten of the engines working right would be a pain, and just looking at the thing suggests that its flight behavior was at least somewhat wobbly.

* Since the flying platforms and jeeps are a very obscure topic, it is unsurprising that sources tend to differ greatly on details. For example, the numbers of Hiller Pawnees built seems to vary from three to six. My best estimate is that only three were built, since there were three different configurations, and given the highly experimental nature of the program there doesn't seem to have been much motivation to build duplicate prototypes.

One of the Pawnees also appears to have had the designation "VZ-1E", but it is uncertain which one. Similarly, some sources seem to hint that the de Lackner Aerocycle and the Helivector were different rotorcraft, but no details or photographs support this notion. Finally, the specific reasons given for abandoning these programs vary wildly from source to source.

Given how inconclusive the efforts to build flying platforms and jeeps turned out to be, it is surprising in hindsight how much public interest there was in them in the early 1960s. I vaguely recollect that Monogram made models of them, with even more vague memories of other toys, including flying ones powered by RC model aircraft engines, and clearly recollect them in action cartoons and comic books. Decades later they were largely forgotten, though interestingly a few years ago I received a technical data book on a Japanese animation series called GUNDAM from Japanese business acquaintances in which flying platforms, called "Wappas", were well represented.

* I initially wrote this document as something of a "knockoff" in 1997, on the basis that it would be a short writeup on a mildly amusing subject. As it turned out, the subject proved to be something of a annoyance, since as noted much of the information on these flying machines is sketchy or contradictory. I ended up doing many revisions and getting a lot of nitpicky criticisms from readers, as well as materials from various people with proposals for flying-platform-type machines. I got the feeling that I'd strayed into something like perpetual-motion-machine territory.

It quieted down after the v1.0.7 revision in late 2001. I don't think it's because I'd got it all straight by any means, I think it's just because I'd canvassed most of the easily available information on the subject and nobody else had much more to add. I suspect that there are parts of the story that nobody will ever get straight.

* Sources for this document include:

• US ARMY AIRCRAFT SINCE 1947 by S. Harding, 1990. I actually did not have access to this book, but substantial portions of it were duplicated on a Russia-based aviation site and proved useful in development of the v1.4 version of this document.
• VTOL MILITARY RESEARCH AIRCRAFT by Mike Rogers, Haynes Publishing, 1989. The v1.0 release of this document was heavily based on notes from this book.
• "Sikorsky Tests Proof-of-Concept RPV That Uses Coaxial Rotor Technology" by Stanley W. Kandebo, AVIATION WEEK, 21 May 1990, 24.
• "A Cypher That Adds Up" by Richard DeMeis, AEROSPACE AMERICA, January 1991, 40.
• "Spying Saucer" by Glenn Zorpette, SCIENTIFIC AMERICAN, June 1997, 40.
• "Hummingbird Flying Platform Hovers Near Completion" by Edward D. Flinn, AEROSPACE AMERICA, March 1998, 23:25.
• "Twin Rotors Propel Personal Aircraft" MACHINE DESIGN, 19 August 1999, 31:32.
• "First Steps Taken In Quest To Prove Backpack VTOL Aircraft" by Michael Mecham, AVIATION WEEK, 3 September 2001, 101:102.
• "Industry Collaboration Explores Turbine-Powered Flying Platform" by Stanley W. Kandebo, AVIATION WEEK, 24 September 2001, 57:58.

Bill Zuk, author of the book AVROCAR: CANADA'S FLYING SAUCER, corrected a number of details on the subject for the v1.0.5 version of this document. Those interested in more information on the Avrocar may be interested in this book, though as I have not read it myself I can say nothing more about it. Moller International's website also provided sketchy details on their Aerobots.

* Revision history:

   v1.0   / 01 aug 97 / gvg
   v1.1   / 01 jul 98 / gvg / Polished, added notes on Aerobot & Hummingbird.
   v1.2   / 01 sep 99 / gvg / Tweaked, added note on SoloTrek XFV.
   v1.3   / 01 oct 00 / gvg / Typos, added comments on Dragon Warrior.
   v1.4   / 01 jun 01 / gvg / Polished, added data on VZ-6 and Avrocar.
   v1.0.5 / 01 sep 01 / gvg / Corrected data on Avrocar as per Bill Zuk.
   v1.0.6 / 01 oct 01 / gvg / Updated information on Millennium Jet.
   v1.0.7 / 01 dec 01 / gvg / Added material on PAM 100B ILV.
   v1.0.8 / 01 dec 03 / gvg / Minor cosmetic update, added stuff on WASP.
   v1.0.9 / 01 oct 04 / gvg / Minor cosmetic update.
   v1.1.0 / 01 sep 06 / gvg / Minor cosmetic update.