An autogyro (from Greek αὐτός and γύρος, "self-turning"), also known as a gyroplane or gyrocopter, is a type of rotorcraft that uses an unpowered rotor in free autorotation to develop lift. Forward thrust is provided independently, by an engine-driven propeller. While similar to a helicopter rotor in appearance, the autogyro's rotor must have air flowing across the rotor disc to generate rotation, and the air flows upwards through the rotor disc rather than down.
The autogyro was invented by Spanish engineer Juan de la Cierva in an attempt to create an aircraft that could fly safely at low speeds. He first flew one on 9 January 1923, at Cuatro Vientos Airfield in Madrid. Cierva's autogyro is considered the predecessor of the modern helicopter. The aircraft resembled the fixed-wing aircraft of the day, with a front-mounted engine and propeller.
The success of the autogyro garnered the interest of industrialists and under license from Cierva in the 1920s and 1930s, the Pitcairn & Kellett companies made further innovations. Late-model autogyros patterned after Etienne Dormoy's Buhl A-1 Autogyro and Igor Bensen's designs feature a rear-mounted engine and propeller in a pusher configuration.
The term Autogiro was a trademark of the Cierva Autogiro Company, and the term Gyrocopter was used by E. Burke Wilford who developed the Reiseler Kreiser feathering rotor equipped gyroplane in the first half of the twentieth century. The latter term was later adopted as a trademark by Bensen Aircraft.
- 1 Principle of operation
- 2 Flight controls
- 3 Pusher vs tractor configuration
- 4 History
- 5 Certification by national aviation authorities
- 6 World records
- 7 In popular culture
- 8 See also
- 9 References
- 10 External links
Principle of operation
An autogyro is characterized by a free-spinning rotor that turns because of the passage of air through the rotor from below. The downward component of the total aerodynamic reaction of the rotor gives lift to the vehicle, sustaining it in the air. A separate propeller provides forward thrust, and can be placed in a puller configuration, with the engine and propeller at the front of the fuselage, or in a pusher configuration, with the engine and propeller at the rear of the fuselage.
Whereas a helicopter works by forcing the rotor blades through the air, drawing air from above, the autogyro rotor blade generates lift in the same way as a glider's wing, by changing the angle of the air as the air moves upwards and backwards relative to the rotor blade. The free-spinning blades turn by autorotation; the rotor blades are angled so that they not only give lift, but the angle of the blades causes the lift to accelerate the blades' rotation rate, until the rotor turns at a stable speed with the drag and thrust forces in balance.
|on YouTube of Groen Hawk 4|
|on YouTube of Pitcairn PA-36 in 1941|
Because the craft must be moving forward with respect to the surrounding air in order to force air through the overhead rotor, autogyros are generally not capable of vertical takeoff (except in a strong headwind). A few types such as the Air & Space 18A have shown short takeoff or landing.
Pitch control is achieved by tilting the rotor fore and aft, roll control by tilting the rotor laterally. The tilt of the rotor can be effected by means of a tilting hub (Cierva), a swashplate (Air & Space 18A), or servo-flaps. A rudder provides yaw control. On pusher configuration autogyros, the rudder is typically placed in the propeller slipstream to maximize yaw control at low airspeed (but not always, as seen in the McCulloch J-2, with twin rudders placed outboard of the propeller arc).
There are three primary flight controls: control stick, rudder pedals, and throttle. Typically, the control stick is termed the cyclic and tilts the rotor in the desired direction to provide pitch and roll control (some autogyros do not tilt the rotor relative to the airframe, or only do so in one dimension, and have conventional control surfaces to vary the remaining degrees of freedom). The rudder pedals provide yaw control, and the throttle controls engine power.
Secondary flight controls include the rotor transmission clutch, also known as a pre-rotator, which when engaged drives the rotor to start it spinning before takeoff, and collective pitch to reduce blade pitch before driving the rotor. Collective pitch controls are not usually fitted to autogyros, but can be found on the Air & Space 18A, McCulloch J-2 and the Westermayer Tragschrauber; and are capable of near VTOL performance. Unlike a helicopter, autogyros without collective pitch or another jump start facility need a runway to take off; however, they are capable of landing with a very short or zero ground roll. Like helicopters, each autogyro has a specific height–velocity diagram for safest operation, although the dangerous area is usually smaller than for helicopters.
Pusher vs tractor configuration
Modern autogyros typically follow one of two basic configurations. The most common design is the pusher configuration, where the engine and propeller are located behind the pilot and rotor mast, such as in the Bensen "Gyrocopter". It was developed by Igor Bensen in the decades following World War II, and came into widespread use shortly afterward.
Less common today is the tractor configuration. In this version, the engine and propeller are located at the front of the aircraft, ahead of the pilot and rotor mast. This was the primary configuration in early autogyros, but became less common after the advent of the helicopter. It has enjoyed a revival since the mid-1970s.
Juan de la Cierva was a Spanish engineer and aeronautical enthusiast. In 1921, he participated in a design competition to develop a bomber for the Spanish military. De la Cierva designed a three-engined aircraft, but during an early test flight, the bomber stalled and crashed. De la Cierva was troubled by the stall phenomenon and vowed to develop an aircraft that could fly safely at low airspeeds. The result was the first successful rotorcraft, which he named Autogiro in 1923. De la Cierva's autogyro used an airplane fuselage with a forward-mounted propeller and engine, an un-powered rotor mounted on a mast, and a horizontal and vertical stabilizer. His aircraft became the predecessor of the modern helicopter.
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Juan de la Cierva invented the modern autogyro (autogiro in Spanish) in the early 1920s. His first three designs (C.1, C.2, and C.3) were unstable because of aerodynamic and structural deficiencies in their rotors. His fourth design, the C.4, made the first documented flight of an autogyro on 17 January 1923, piloted by Alejandro Gomez Spencer at Cuatro Vientos airfield in Madrid, Spain (9 January according to Cierva). De la Cierva had fitted the rotor of the C.4 with flapping hinges to attach each rotor blade to the hub. The flapping hinges allowed each rotor blade to flap, or move up and down, to compensate for dissymmetry of lift, the difference in lift produced between the right and left sides of the rotor as the autogyro moves forward. Three days later, the engine failed shortly after takeoff and the aircraft descended slowly and steeply to a safe landing, validating De la Cierva's efforts to produce an aircraft that could be flown safely at low airspeeds.
De la Cierva developed his C.6 model with the assistance of Spain's Military Aviation establishment, having expended all his funds on development and construction of the first five prototypes. The C.6 first flew in February 1925, piloted by Captain Joaquín Loriga, including a flight of 10.5 km (6.5 mi) from Cuatro Vientos airfield to Getafe airfield in about 8 minutes, a significant accomplishment for any rotorcraft of the time. Shortly after De la Cierva's success with the C.6, Cierva accepted an offer from Scottish industrialist James G. Weir to establish the Cierva Autogiro Company in England, following a demonstration of the C.6 before the British Air Ministry at RAE Farnborough, on 20 October 1925. Britain had become the world centre of autogyro development.
A crash in February 1926, caused by blade root failure, led to an improvement in rotor hub design. A drag hinge was added in conjunction with the flapping hinge to allow each blade to move fore and aft and relieve in-plane stresses, generated as a byproduct of the flapping motion. This development led to the Cierva C.8, which, on 18 September 1928, made the first rotorcraft crossing of the English Channel followed by a tour of Europe.
US industrialist Harold Frederick Pitcairn, on learning of the successful flights of the autogyro, visited De la Cierva in Spain. In 1928, he visited him again, in England, after taking a C.8 L.IV test flight piloted by Arthur H.C.A. Rawson. Being particularly impressed with the autogyro's safe vertical descent capability, Pitcairn purchased a C.8 L.IV with a Wright Whirlwind engine. Arriving in the United States on 11 December 1928 accompanied by Rawson, this autogyro was redesignated C.8W. Subsequently, production of autogyros was licensed to a number of manufacturers, including the Pitcairn Autogiro Company in the U.S. and Focke-Wulf of Germany.
In 1927, German engineer Engelbert Zaschka invented a combined helicopter and autogyro. The principal advantage of the Zaschka machine is in its ability to remain motionless in the air for any length of time and to descend in a vertical line, so that a landing may be accomplished on the flat roof of a large house. In appearance, the machine does not differ much from the ordinary monoplane, but the carrying wings revolve around the body.
Development of the autogyro continued in the search for a means to accelerate the rotor prior to takeoff (called prerotating). Rotor drives initially took the form of a rope wrapped around the rotor axle and then pulled by a team of men to accelerate the rotor – this was followed by a long taxi to bring the rotor up to speed sufficient for takeoff. The next innovation was flaps on the tail to redirect the propeller slipstream into the rotor while on the ground. This design was first tested on a C.19 in 1929. Efforts in 1930 had shown that development of a light and efficient mechanical transmission was not a trivial undertaking. But, in 1932, the Pitcairn-Cierva Autogiro Company of Willow Grove, Pennsylvania, United States, finally solved the problem with a transmission driven by the engine.
Buhl Aircraft Company produced its Buhl A-1, the first autogyro with propulsive rear motor, designed by Etienne Dormoy and meant for aerial observation (motor behind pilot and camera). It had its maiden flight on 15 December 1931.
De la Cierva's early autogyros were fitted with fixed rotor hubs, small fixed wings, and control surfaces like those of a fixed-wing aircraft. At low airspeeds, the control surfaces became ineffective and could readily lead to loss of control, particularly during landing. In response, Cierva developed a direct control rotor hub, which could be tilted in any direction by the pilot. De la Cierva's direct control was first developed on the Cierva C.19 Mk. V and saw production on the Cierva C.30 series of 1934. In March 1934 this type of autogyro became the first rotorcraft to take off and land on the deck of a ship, when a C.30 performed trials on board the Spanish navy seaplane tender Dédalo off Valencia.
When improvements in helicopters made them practical, autogyros became largely neglected. Also, they were susceptible to ground resonance. They were, however, used in the 1930s by major newspapers, and by the United States Postal Service for the mail service between the Camden, New Jersey airport and the top of the post office building in downtown Philadelphia, Pennsylvania.
World War II
The Imperial Japanese Army developed the Kayaba Ka-1 Autogyro for reconnaissance, artillery-spotting, and anti-submarine uses. The Ka-1 was based on the Kellett KD-1 first imported to Japan in 1938. The craft was initially developed for use as an observation platform and for artillery spotting duties. The Army liked the craft's short take-off span, and especially its low maintenance requirements. Production began in 1941, with the machines assigned to artillery units for spotting the fall of shells. These carried two crewmen: a pilot and a spotter.
Later, the Japanese Army commissioned two small aircraft carriers intended for coastal antisubmarine (ASW) duties. The spotter's position on the Ka-1 was modified to carry one small depth charge. Ka-1 ASW autogyros operated from shore bases as well as the two small carriers. They appear to have been responsible for at least one submarine sinking.
In Russia, the Kamov A7–3A autogyro was the first armed autogyro to see combat action. During the Winter War of 1939–40 an A-7 passed its combat christening in Finland, where it was used to coordinate artillery fire. Later in 1941 after the German invasion, Kamov A-7's performed artillery spotting in a battle near Elnei, but its low speed and a lack of fighter escort made the A-7 an easy target for the Germans. They then switched to night operations and in 1942 A-7 autogyros worked supporting military groups, performing harassment bombings and supplying guerrillas with cargo.
The autogyro was resurrected after World War II when Dr. Igor Bensen, a Russian immigrant in the US, saw a captured German U-Boat's Fa 330 gyroglider and was fascinated by its characteristics. At work, he was tasked with the analysis of the British military "Rotachute" gyro glider designed by expatriate Austrian Raoul Hafner. This led him to adapt the design for his own purposes and eventually market the Bensen B-7 in 1955. Bensen submitted an improved version, the Bensen B-8M, for testing to the United States Air Force, which designated it the X-25. The B-8M was designed to use surplus McCulloch engines used on flying unmanned target drones.
Ken Wallis developed a miniature autogyro craft, the Wallis autogyro, in England in the 1960s, and autogyros built similar to Wallis' design appeared for a number of years. Ken Wallis' designs have been used in various scenarios, including military training, police reconnaissance, and in a search for the Loch Ness Monster, as well as an appearance in the 1967 James Bond movie You Only Live Twice.
Three different autogyro designs have been certified by the Federal Aviation Administration for commercial production: the Umbaugh U-18/Air & Space 18A of 1965, the Avian 2-180 Gyroplane of 1967, and the McCulloch J-2 of 1972. All have been commercial failures, for various reasons.
The basic Bensen Gyrocopter design is a simple frame of square aluminium or galvanized steel tubing, reinforced with triangles of lighter tubing. It is arranged so that the stress falls on the tubes, or special fittings, not the bolts. A front-to-back keel mounts a steerable nosewheel, seat, engine, and a vertical stabilizer. Outlying mainwheels are mounted on an axle. Some versions may mount seaplane-style floats for water operations.
Bensen-type autogyros use a pusher configuration for simplicity and to increase visibility for the pilot. Power can be supplied by a variety of engines. McCulloch drone engines, Rotax marine engines, Subaru automobile engines, and other designs have been used in Bensen-type designs.
The rotor is mounted atop the vertical mast. The rotor system of all Bensen-type autogyros is of a two-blade teetering design. There are some disadvantages associated with this rotor design, but the simplicity of the rotor design lends itself to ease of assembly and maintenance and is one of the reasons for its popularity. Aircraft-quality birch was specified in early Bensen designs, and a wood/steel composite is used in the world speed record holding Wallis design. Gyroplane rotor blades are made from other materials such as aluminium and GRP-based composite.
Bensen's success triggered a number of other designs, some of them fatally flawed with an offset between the centre of gravity and thrust line, risking a Power Push-Over (PPO or bunt-over) causing death to the pilot and giving gyroplanes in general a poor reputation – in contrast to Cierva's original intention and early statistics. Most new autogyros are now safe from PPO.
21st-century development and use
In 2002, a Groen Brothers Aviation's Hawk 4 provided perimeter patrol for the Winter Olympics and Paralympics in Salt Lake City, Utah. The aircraft completed 67 missions and accumulated 75 hours of maintenance-free flight time during its 90-day operational contract.
Worldwide, over 1,000 autogyros are used by authorities for military and law enforcement. The first US police authorities to evaluate an autogyro were the Tomball, Texas, police, on a $40,000 grant from the United States Department of Justice together with city funds, costing much less than a helicopter to buy ($75,000) and operate ($50/hour). Although it is able to land in 40-knot crosswinds, a minor accident happened when the rotor was not kept under control in a wind gust.
Since 2009, several projects in Kurdistan, Iraq have been realized. In 2010, the first autogyro was handed over to the Kurdish Minister of Interiors, Mr. Karim Sinjari. The project for the interior ministry was to train pilots to control and monitor the approach and takeoff paths of the airports in Erbil, Sulaymaniyah, and Dohuk to prevent terrorist encroachments. The gyroplane pilots also form the backbone of the pilot crew of the Kurdish police, who are trained to pilot on Eurocopter EC 120 B helicopters.
In an 18-month period from 2009 to 2010, the German pilot couple Melanie and Andreas Stützfor undertook the first world tour by autogyro, in which they flew several different gyroplane types in Europe, southern Africa, Australia, New Zealand, the United States, and South America. The adventure was documented in the book "WELTFLUG – The Gyroplane Dream" and in the film "Weltflug.tv – The Gyrocopter World Tour".
Some autogyros, such as the Rotorsport MT03, MTO Sport (open tandem), & Calidus (enclosed tandem), and the Magni Gyro M16C (open tandem) & M24 (enclosed side by side) have type approval by the United Kingdom Civil Aviation Authority (CAA) under British Civil Airworthiness Requirements CAP643 Section T. Others operate under a permit to fly issued by the Popular Flying Association similar to the US experimental aircraft certification. However, the CAA's assertion that autogyros have a poor safety record means that a permit to fly will be granted only to existing types of autogyro. All new types of autogyro must be submitted for full type approval under CAP643 Section T. Beginning in 2014, the CAA allows gyro flight over congested areas.
In 2005, the CAA issued a mandatory permit directive (MPD) which restricted operations for single-seat autogyros, and were subsequently integrated into CAP643 Issue 3 published on 12 August 2005. The restrictions are concerned with the offset between the centre of gravity and thrust line, and apply to all aircraft unless evidence is presented to the CAA that the CG/Thrust Line offset is less than 2 inches (5 cm) in either direction. The restrictions are summarised as follows:
- Aircraft with a cockpit/nacelle may be operated only by pilots with more than 50 hours solo flight experience following the issue of their licence.
- Open-frame aircraft are restricted to a minimum speed of 30 mph (26 knots), except in the flare.
- All aircraft are restricted to a Vne (maximum airspeed) of 70 mph (61 knots)
- Flight is not permitted when surface winds exceed 17 mph (15 knots) or if the gust spread exceeds 12 mph (10 knots)
- Flight is not permitted in moderate, severe or extreme turbulence and airspeed must be reduced to 63 mph (55 knots) if turbulence is encountered mid-flight.
These restrictions do not apply to autogyros with type approval under CAA CAP643 Section T, which are subject to the operating limits specified in the type approval.
United States certification
A certificated autogyro must meet mandated stability and control criteria; in the United States these are set forth in Federal Aviation Regulations Part 27: Airworthiness Standards: Normal Category Rotorcraft. The U.S. Federal Aviation Administration issues a Standard Airworthiness Certificate to qualified autogyros. Amateur-built or kit-built aircraft are operated under a Special Airworthiness Certificate in the Experimental category. Per FAR 1.1, the FAA uses the term "gyroplane" for all autogyros, regardless of the type of Airworthiness Certificate.
Wing Commander Ken Wallis (UK) held most of the autogyro world records during his autogyro flying career. These include a time-to-climb, a speed record of 189 km/h (111.7 mph), and the straight-line distance record of 869.23 km (540.11 mi). On 16 November 2002, at 89 years of age, Wallis increased the speed record to 207.7 km/h (129.1 mph) – and simultaneously set another world record as the oldest pilot to set a world record.
Until 2019, the autogyro was one of the last remaining types of aircraft which had not yet circumnavigated the globe. The 2004 Expedition Global Eagle was the first attempt to do so using an autogyro. The expedition set the record for the longest flight over water by an autogyro during the segment from Muscat, Oman, to Karachi. The attempt was finally abandoned because of bad weather after having covered 7,500 miles (12,100 km).
As of 2014[update], Andrew Keech (USA) holds several records. He made a transcontinental flight in his self-built Little Wing Autogyro "Woodstock" from Kitty Hawk, North Carolina, to San Diego, California, in October 2003 and set three world records for speed over a recognized course. The three records were verified by tower personnel or by official observers of the United States' National Aeronautic Association (NAA). On 9 February 2006 he broke two of his world records and set a record for distance, ratified by the Fédération Aéronautique Internationale (FAI): Speed over a closed circuit of 500 km (311 mi) without payload: 168.29 km/h (104.57 mph), speed over a closed circuit of 1,000 km (621 mi) without payload: 165.07 km/h (102.57 mph), and distance over a closed circuit without landing: 1,019.09 km (633.23 mi).
On 7 November 2015, the Italian astrophysicist and pilot Donatella Ricci took off with a MagniGyro M16 from the Caposile aerodrome in Venice, aiming to set a new altitude world record. She reached an altitude of 8,138.46 m (26,701 ft), breaking the women's world altitude record held for 84 years by Amelia Earhart. The following day, she increased the altitude by a further 261 m, reaching 8,399 m (27,556 ft), setting the new altitude world record with an autogyro. She improved by 350 m (+4.3%) the preceding record established by Andrew Keech in 2004.
|2002||Ken Wallis (UK)||Speed over a 3 km course||207.7 km/h||Wallis Type WA-121/Mc (G-BAHH)||Oldest pilot to set record|
|1998||Ken Wallis (UK)||Time to climb 3000m||7:20 min||Wallis Type WA-121/Mc (G-BAHH)|
|2015||Donatella Ricci (ITA)||Altitude||8399 m||Magni M16 – Rotax 914 engine|
|2015||Paul A Salmon (USA)||Distance without landing||1653.0 km||Magni M22-Missing Link II (N322MG)||10 November 2015|
Norman Surplus, from Larne in Northern Ireland, became the second person to attempt a world circumnavigation by Gyroplane/Autogyro type aircraft on 22 March 2010, flying a Rotorsport UK MT-03 Autogyro, registered G-YROX. Surplus was unable to get permission to enter Russian airspace from Japan, but he established nine world autogyro records on his flight between Northern Ireland and Japan between 2010–11. FAI world records] for Autogyro flight. G-YROX was delayed (by the Russian impasse) in Japan for over three years before being shipped across the Pacific to the state of Oregon, USA. From 1 June 2015, Surplus flew from McMinnville, Oregon, across the continental USA, through northern Canada/Greenland and in late July/August made the first (and to date only) crossing of the North Atlantic by Autogyro Aircraft to land back in Larne, Northern Ireland on 11 August 2015. He established a further 10 FAI World Records during this phase of the circumnavigation flight.
After a nine-year wait (since 2010), permission to fly U.K. registered gyroplanes through the Russian Federation was finally approved, and on 22 April 2019, Surplus and G-YROX continued eastwards from Larne, Northern Ireland, to cross Northern Europe and rendezvous with fellow gyroplane pilot James Ketchell (U.K.) piloting Magni M16 Gyroplane G-KTCH. Flying in loose formation the two aircraft made the first Trans-Russia flight by gyroplane together to reach the Bering Sea. To cross the Bering Strait, the two aircraft took off from Provedinia Bay, Russia on 7 June 2019 and landed at Nome, Alaska on 6 June having also made the first gyroplane crossing of the international date line. After crossing Alaska and western Canada, on 28 June 2019, Surplus piloting G-YROX, became the first person to circumnavigate the world in a gyroplane upon returning to the Evergreen Aviation and Space Museum, McMinnville, Oregon, USA.
The first physical circumnavigation of the globe by gyroplane had taken 4 years and 28 days to complete, primarily due to the very lengthy delays in gaining the necessary permission to fly across Russian Federation Airspace. Over the nine years it had taken Surplus to complete the task, G-YROX flew 27,000 Nautical Miles through 32 countries.
In popular culture
Since their invention, autogyros have appeared in many works of fiction. Appearances where the autogyro features as a significant part of the story include:
Little Nellie, the autogyro featured in the 1967 James Bond film You Only Live Twice, was a Ken Wallis WA-116 design and was piloted by Wallis in its film scenes. In the film it was shipped by Q in four suitcases and assembled before use.
In Mad Max 2 one of the characters flies a homebuilt gyrocopter with a Volkswagen engine. He is named Gyro Captain after his machine.
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- "[permanent dead link]" Donatella Ricci, record di volo con l'"autogiro" 8 November 2015
- "FAI Record ID #17745 – Autogyro, distance without landing Archived 24 November 2015 at the Wayback Machine" Fédération Aéronautique Internationale (FAI). Retrieved: 22 November 2015.
- Norman Surplus (GYROX) list on FAI: 1 Archived 12 September 2014 at the Wayback Machine 2 Archived 12 September 2014 at the Wayback Machine 3 Archived 12 September 2014 at the Wayback Machine 4 Archived 12 September 2014 at the Wayback Machine 5 Archived 12 September 2014 at the Wayback Machine 6 Archived 12 September 2014 at the Wayback Machine 7 Archived 12 September 2014 at the Wayback Machine 8 Archived 12 September 2014 at the Wayback Machine 9 Archived 12 September 2014 at the Wayback Machine
- The KELLETT K-3 AUTOGIRO NC12691 Page of the Davis-Monthan Airfield Register Website Archived 15 June 2013 at the Wayback Machine
- "Doc piloting his gyro". Art Nocturne. Archived from the original on 1 October 2007. Retrieved 7 October 2007.
- "Tom Strong and his Phantom Autogyro". America's Best Comics. Archived from the original on 13 October 2007. Retrieved 7 October 2007.
- "Trivia: You Only Live Twice". Internet Movie Database. Retrieved 7 October 2007.
- "K.H. Wallis". Internet Movie Database. Retrieved 7 October 2007.
- "Hero – Gyrocopter". www.dota2.com. Retrieved 19 March 2019.
- Rotorcraft Flying Handbook, FAA Manual H-8083-21, Chapters 15–22. Washington, DC: Flight Standards Service, Federal Aviation Administration, U.S. Dept. of Transportation, 2001. ISBN 1-56027-404-2.
- Andreas G. Stuetz (2013). Weltflug: The Gyroplane Dream. Createspace Independent Pub. ISBN 978-1-4937-6094-7.
|Wikimedia Commons has media related to Autogyros.|
- "Development of the Autogiro : A Technical Perspective" : J. Gordon Leishman: Hofstra University, New York, 2003.
- Jeff Lewis' in-depth history of the Autogyro
- Popular Rotorcraft Association (United States)
- "Will Autogiro Banish Present Plane?". Popular Science. March 1931. p. 28.
- "Feathering of Blades Increase Gyro's Speed". Popular Mechanics. April 1932. p. 538.