Autogyro

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An autogyro (only an autogiro when produced by the Cierva Autogiro Company or one of its licensees (see below), sometimes called a gyroplane, gyrocopter, or rotaplane) is an aircraft supported in flight by a rotor driven solely by aerodynamic forces.

Though the autogyro resembles a helicopter, it is driven in flight by an engine-powered propeller similar to that of an airplane; the rotor turns due to autorotation to provide lift. Often mistakenly characterized as a hybrid between an airplane and helicopter, the autogyro is a distinct type of aircraft, invented in 1919, that made its first successful flight on 17 January 1923 at Cuatro Vientos Airfield in Madrid, Spain, predating the first successful helicopter by 13 years. All helicopters utilize rotor technology first developed for the autogiro; the helicopter owes its existence to the work conducted by Juan de la Cierva y Codorniu and his associates.

Image:Aurogyro-ELA-07-Casarrubios-Spain.jpg

1 Principle of operation
2 Flight Controls
3 General characteristics
4 Flight characteristics
5 History
6 Bensen's design
7 Records and Application
8 Kits
9 Warnings
10 See also
11 External links

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Principle of operation

An autogyro is characterized by a free spinning rotor that turns due to passage of air upwards through the rotor. Whilst in autorotation the rotorblade cannot stall, and control can be maintained through the rotor even at zero airspeed. Propulsion for forward flight is provided typically via a propellor, though jet thrust was employed on the Lockheed XH-51A compound helicopter flying in autogyro mode.

The spinning rotor blades generate lift through the phenomenon of autorotation in which the outer and inner spans of the blade generates drag in the plane of the rotor disk and the middle span of the blade generates thrust in the plane of the rotor disk. When the drag and thrust forces are in balance the rotor turns at a stable rpm. The vertical component of the total aerodynamic reaction is termed rotor thrust and sustains the autogyro in the air.

Pitch control of the autogyro is obtained by tilting the rotor fore and aft; roll control is obtained by tilting the rotor laterally. Tilt of the rotor may be effected by a tilting hub (Cierva), swashplate (Air and Space 18A), or servo-flaps (Kaman SAVER). Yaw control is provided by a rudder usually placed in the propellor slipstream to maximize control at low airspeed.

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Flight Controls

There are three primary flight controls: control stick, rudder pedals and throttle.

The control stick is termed cyclic and tilts the rotor in the desired direction to provide pitch and roll control. The rudder pedals provide yaw control, and the throttle controls engine power.

Secondary flight controls include rotor transmission clutch to drive the rotor prior to takeoff, and collective pitch to reduce blade pitch prior to driving the rotor. These secondary controls are fitted to the Air and Space 18A and McCulloch J-2 autogyros.

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General characteristics

Autogyros can take off and land in significantly smaller areas compared to airplanes, and depending on the model, can operate from helipads. When fitted with a jump start feature, an autogyro can takeoff from a standing start into forward flight, accelerate in ground effect, then commence a climb; hovering capability is not available however since the rotor is always declutched before the autogyro commences the takeoff procedure. If rotor collective pitch control is provided, an autogyro can execute a collective flare; otherwise landings are always made with a cyclic flare.

It is possible to land an autogyro in an area from which it cannot take off. An autogyro can easily execute a steep approach to a no-roll landing; however, the climb angle after takeoff is relatively shallow, similar to that of an airplane. Sufficient clear area must available after takeoff for the autogyro to turn and avoid obstacles during climb. This limitation, coupled with lack of hovering performance, is primarily responsible for autogyros being superceded by helicopters.

Certificated autogyros flown by trained and qualified pilots are notably safe. As intended by la Cierva, the rotor always turns regardless of the airspeed of the aircraft, though as airspeed decreases rotor rpm reduces to a minimum value at zero airspeed. Reduction of engine power increases the descent rate, though the autogyro remains fully stable and controllable. Directional control, provided by a rudder, can become nonexistent at low airspeed and low propeller thrust. For example, the Air and Space 18A gyroplane rudder rapidly loses effectiveness below 50mph airspeed when the engine is throttled.

Most autogyros are neither efficient nor very fast (although Wing Commander Ken Wallis has achieved 120mph from 60bhp). Fixed-wing aircraft are faster and use less fuel over the same distance, helicopters generally require more power (and hence fuel) than a fixed wing aircraft (or autogyros) for the same top speed/load etc. It must be noted that autogyro development ceased prior to WW2 and with few exceptions have not benefitted from rotary wing advances applied to helicopters. When the latter became practical, autogyros became largely neglected. They were however used in the 1930s by major newspapers and the US Postal Service for mail service between the Camden, NJ airport and the top of the post office building in downtown Philadelphia, PA.

Autogyros can be of tractor configuration with the engine(s) and propeller(s) at the front of the fuselage (Cierva-type), or pusher configuration with the engine(s) and propeller(s) at the rear of the fuselage (Bensen-type). Early autogyros were fitted with fixed rotor hubs, small fixed-wings and airplane-type control surfaces. At the low airspeed at which autogyros can easily operate, the airplane-type control surfaces became ineffective and could readily lead to loss of control, particularly during landing. The direct control rotor hub, which could be tilted in any direction by the pilot, was first developed on the Cierva C.19 Mk.V and saw production on the Cierva C.30 series of 1934.

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 prior to a long taxi to bring the rotor up to speed sufficient for takeoff. The next innovation was a fully deflectable horizontal stabilizer that directed propeller slipstream into the rotor. Cierva license, Pitcairn-Cierva Autogiro Company of Willow Grove, PA, finally solved the problem with a light mechanical transmission driven by the engine.

The Groen Brothers Hawk 4 of the late 1992 is advertised as possessing Ultra-Short Take-Off and Landing (USTOL) capability, enabling the aircraft to take off and land within a very short distance (25 feet). This is merely a new name for performance autogyros have always possessed.

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Flight characteristics

Flight in any rotorcraft can be summed up as feeling similar to a cork bobbing on the sea. The rotor sweeps a large area and though it is very effective at damping out disturbances provides a somewhat nautical element to the flying qualities. Moving the cyclic stick tilts the entire rotor to a new position within no more than about one revolution and is thus a very sensitive control. There is nevertheless a small time lag between cyclic stick movement and aircraft response since the control system is essentially a mechanical relay system that only indirectly tilts the rotor: the cyclic control merely establishes the conditions for aerodynamic forces to reorient the rotor in the desired direction. Additionally, since the rotor is always turning at or above a minumum rpm, control sensitivity does not vary significantly with changes in airspeed.

Effectiveness of the rudder is dependent on airflow, and it rapidly loses authority as airspeed decreases; this can be partially offset by maintaining propellor thrust to generate the required airflow at low airspeeds.

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. Such autogyros are issued a Standard Airworthiness Certificate by the US Federal Aviation Administration. Bensen-type autogyros are invariably operated under a Special Airworthiness Certificate in the Experimental Category; there is thus no guarantee they will perform as claimed by their manufacturers. It is important to note that Bensen-type autogyros have established an extremely poor safety record due to significant stability and control deficiencies, exacerbated by common operation by unqualified pilots. NTSB accident records give a clear picture of the safety of autogyros with Standard Airworthiness Certificates compared to the hazards inherent in autogyros with Special Airworthiness Certificates.

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History

Juan de la Cierva, a Spanish engineer and aeronautical enthusiast, invented the first successful rotorcraft, which he named autogiro in 1923. His aim was to create an aircraft which would not stall, following the stall-induced crash of a three-engine bomber he designed for a Spanish military aeronautical competition. His craft used a tractor-mounted forward propeller and engine, a rotor mounted on a mast, and a horizontal and vertical stabilizer. His first three designs, C.1, C.2, and C.3, were unstable due to aerodynamic and structural deficiencies in their rotors. His fourth design, the C.4, fitted with flapping hinges to attach each rotor blade to the hub, made the first successful flight of a rotary-wing aircraft, piloted by Alejandro Gomez Spencer, on 17 January 1923. The C.4 was fitted with conventional airplane ailerons, elevators and rudder for control. During a later test flight, the engine failed shortly after takeoff and the aircraft descended slowly and steeply to a safe landing, validating la Cierva's efforts to produce an aircraft that could be flown safely at low airspeeds.

Image:Autogyro-Avro-620.jpg

This success eventually became well known and after further limited Autogiro development in Spain, la Cierva accepted an offer from Scottish industrialist James G. Weir to establish the Cierva Autogiro Company in England following a 20 October 1925 demonstration to the British Air Ministry at Farnborough. Test pilot for these flights was Frank T. Courtney. From this point on, Britain became the world center of rotary-wing aircraft development.

A crash due to blade root failure in February 1927 led to an improvement in rotor hub design. Adjacent the flapping hinge a drag hinge was incorporated to allow each blade to slightly oscillate horizontally and relieve inplane stresses generated as a byproduct of flapping motion. Development work on means to accelerate the rotor prior to takeoff was also undertaken. Efforts with the C.11 in Spain showed that development of a light and efficient mechanical rotor transmission was not a trivial undertaking and led to the adoption of the intermediate expedient of inclining the horizontal stabilizer to redirect the propeller slipstream into the rotor while on the ground. This feature was later introduced on the production C.19 series of 1929.

Further Autogiro development led to the Cierva C.8 L.IV which on 18 September 1928 made the first rotary-wing aircraft crossing of the English Channel followed by an extensive tour of Europe. US industrialist Harold F. Pitcairn had in 1925 visited la Cierva in Spain upon learning of the successful flights of the Autogiro; in 1928 he visited la Cierva in England after taking a C.8 L.IV test flight piloted by Arthur H.C.A. Rawson and being particularly impressed with the Autogiro's safe vertical descent capability, purchased a C.8 L.IV with a Wright Whirlwind engine. Arriving in the United States on 11 December 1928 accompanied by Rawson, this Autogiro was redesignated C.8W.

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The Cierva "Autodynamic" rotor used drag hinges with offset axes to perform this to good effect with great simplicity, but the Pitcairn collective pitch control advanced the "jump" ability.

The C-19 technology was licensed to a number of manufacturers, including Harold Pitcairn in the U.S. (in 1928) and Focke-Achgelis of Germany. In 1931 Amelia Earhart flew a Pitcairn PCA-2 to a then world altitude record of 18,415 feet (5613 m).

Image:Cierva-Duxford.JPG In World War II, Germany pioneered a very small gyroglider "rotor-kite", the Focke-Achgelis Fa 330 "Bachstelze" (Water-wagtail), towed by submarines to provide aerial surveillance. It's reported that German gyro pilots were often forgotten in the heat of battle when the submarine dived suddenly. The Japanese also developed the Kayaba Ka-1 Autogyro for reconnaissance, artillery-spotting, and anti-submarine uses.

The autogyro was resurrected post WW2 when Dr. Igor Bensen (a doctor of Divinity) saw a captured German U-Boat's gyroglider, and was fascinated by its characteristics. At work he was tasked with the analysis of the British "Rotachute" gyro glider designed by expatriate Austrian Raoul Hafner. This led him to adapt the design for his own purposes and eventually market the B-7.

Post WW2 autogyros, such as the Bensen B-8M gyrocopter, generally use a pusher configuration for simplicity and to increase visibility for the pilot. For greater simplicity, they generally lack both variable-pitch rotors and powered rotors. It must be noted that Bensen autogyros and its derivatives have established an abysmal safety record due to their deficient stability and control characteristics greatly worsened by use of a teetering rotor, and their marketing as a build it yourself and teach yourself how to fly it aircraft.

Three FAA-certified designs, Umbaugh U-18/Air and Space 18A of 1965, Avian 2-180 of 1967, and McCulloch J-2 of 1972 have for various reasons been commercial failures.

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Bensen's design

The Bensen Gyrocopter was adapted directly from the Hafner Rotochute and Focke-Achgelis Fa 330A-1 "Bachstelze" autogiros of WW2. Bensen's adaptation, termed Gyrocopter, was available in three versions, B-6, B-7 and B-8. All three were designed in both unpowered and powered forms.

The basic Bensen Gyrocopter design is a simple frame of square aluminum 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.

Power is supplied by a variety of engines, though rarely one certificated for use in aircraft. McCulloch drone engines, Rotax, and other designs have been used in Bensen-type designs.

The rotor is atop the vertical mast. Outlying mainwheels are mounted on an axle. A front-to-back keel mounts a steerable nosewheel, seat, other tubes, engine, a vertical stabilizer, and commonly a small fixed tailwheel. Some versions mount seaplane-style floats for water operations.

Many light gyroplane rotors are made from aluminum, though GRP-based composite blades (Sport Copter, Averso, Revolution, RAF eg) and GRP-skinned blades are increasing in number. Aircraft-quality birch was specified in early Bensen designs, and a wood/steel composite is still used in the world speed record holding Wallis.

The rotor system of all Bensen-type autogyros is of two-blade teetering design. This single feature is responsible for the majority of accidents in this type of autogyro due its lack of tolerance for mishandling. A teetering rotor does not directly control the fuselage attitude but merely reorients the thrust vector which then causes the fuselage to swing into alignment beneath it. If a low G condition occurs, rotor thrust decreases and causes degradation of control. A certificated rotorcraft fitted with a teetering rotor is required by airworthiness standards to maintain a loading of at least 0.5G. If the rotor is powered as in a helicopter, rotor RPM is maintained even though control authority decays; in the case of an autogyro, rotor RPM and control degrade simultaneously and prompts the usually "self-trained" pilot to overcontrol and precipitate contact between the rotor and the rudder.

All autogiros produced by the Cierva Autogiro Company and its licensees were fitted with articulated rotors controlled about a tilting hub. This design has significantly higher tolerance to mishandling due to offset flapping hinges which generate a control moment even under low G conditions and provides control of the rotor. Overcontrol of this rotor can still result in contact with part of the fuselage however. It must be noted that unlike the majority of Bensen-type autogyros, Cierva Autogiros were invariably flown by trained and qualified pilots, which produced a safety record not exceeded in general aviation until 1972.

Bensen-type designs commonly also have an unstable relationship between propeller thrustline, aircraft center-of-gravity, and rotor drag. If the propeller thrustline passes above the aircraft center-of-gravity and rotor drag decreases suddenly, the Gyrocopter goes out of balance and pitches down rapidly. This has the additional effect of unloading the rotor. This condition is unrecoverable and has caused many fatalities.

The thrust line of autogiros produced by the Cierva Autogiro Company and its licensees passed through the aircraft center-of-gravity, thus eliminating any pitching moment due to reduction of rotor drag.

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Records and Application

As of 2002, Wing Commander Ken Wallis, an enthusiast who has built several gyroplanes, holds or has held most of the type's record performances. These include the speed record of 111.7mph (186km/h), and the straight-line distance record of 543.27 miles (905km). The record picture is continually changing, and on 16 November 2002, Ken Wallis increased the speed record to 207.7 km/h - and simultaneously set another world record as the oldest pilot to set a world record! See: [1]

Ken Wallis also built and flew one of the most famous autogyros - "Little Nellie" - in the James Bond movie "You Only Live Twice".

Hours flown

Autogyros are often used to herd range animals. An autogyro 'cowboy' holds the world record for total hours in the air each week.

The Bensen design has also been used by hobbyists, sight-seers and scientists (for game counting).

Speed

The CarterCopter fixed wing/autogyro hybrid has been unofficially flown in tests at speeds above 170 mph. The claimed theoretical top speed for this general design is in excess of 450 mph.

In the late 1950s, the (15 tonne) Fairey Rotodyne, another hybrid was capable of 213 mph.

Andy Keech made a TransContinental flight from Kitty Hawk, N.C. to San Diego, Ca. in October 2003 and set 3 World Records. The 3 records are for 'speed over a recognised course', and are verified by tower personnel or by Official Observers of the U.S. National Aeronautic Association. In March 2005 he set another World Speed Record, ratified by the FAI:

Sub-class : E-3a (Autogyros : take-off weight less than 500 kg)

Category : General

Group 1 : piston engine

Speed over a closed circuit (500km) without payload : 139.67 km/h,

Date of flight: 20 March 2005

Pilot: Andrew C. KEECH (USA)

Course/place: North Little Rock, AR (USA)

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Kits

Many autogyros are assembled from kits.

Kits with all parts, ready to assemble, are listed for US$19,550 as of 18th July 2002. This is extremely inexpensive for an aircraft. This includes an engine, the major expense. It can be reduced. Some people are clever at scrounging materials. However, scrounging increases one's construction time and program risk. Buying both the engine and rotor hub is recommended by most vendors.

Some people who actually completed an autogyro have said that it took them about a year, working in their spare time. Careful estimates place most build times at 100 to 200 hours.

Kit vendors often say that since it has relatively few parts, hobbyists can assemble it more rapidly and correctly than most fixed-wing kit aircraft. Kit vendors recommend working on it every day for an hour or two.

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Warnings

Most vendors recommend that a new pilot have at least ten hours of instruction by a rated instructor in small fixed-wing aircraft, followed by at least two hours of instruction in a dual-place autogyro with an experienced instructor. An autogyro is more similar to a fixed-wing aircraft than to a helicopter. One must be able to land safely and reliably before attempting to fly any aircraft alone.

Autogyros are relatively safe, but not foolproof. There were 19 fatal autogyro accidents reported to the FAA between 1996 and 2001. Autogyros are aircraft. Do not neglect safety precautions: training, instrumentation, flight rules, preflight checklists, and periodic inspections and maintenance. As discussed, the autogyro is not a helicopter/fixed wing hybrid craft and the particular handling characteristics and limitations must be thoroughly learned so the aircraft can be operated safely. Previous experience in the other types of aircraft may even prove to be less of a benefit, since some procedures may have to be "unlearned", before being qualified to fly the gyroplane.

As mentioned, there is a slight delay between control input and aircraft response - a characteristic of inertia in the spinning rotor blades. Inexperienced pilots may be inclined to repeat or overemphasise a control input owing to a perceived lack of response. The resulting response may then be excessive and the pilot may attempt to compensate with opposing inputs, again with excessive control motion. These inputs can quickly put the aircraft into an increasing cycle of responses which may exceed the safe flying limits. This phenomena is termed "Pilot Induced Oscillation" (PIO), and has lead to loss of control crashes and fatalities. Pilot Induced Oscillation is readily corrected in a certificated autogyro operated by a trained pilot; in a Bensen-type autogyro no amount of training may be sufficient to avoid catastrophe.

In the United States private, recreational, and commercial pilot licenses with rotorcraft category and gyroplane class rating are issued, or the rating is added to an existing license for other aircraft; holders of sport pilot licenses can also qualify to fly autogyros. Requirements include completing required training times, passing written exams, and successfully doing oral and practical tests. Sport pilot license in-flight tests can be conducted in single-seat aircraft, but a "single place only" limitation is placed on the certificate in such cases.

"Learning to fly the rotor" is a vital ingredient for safe flight in an autogyro - models and rotary kites can help the learning process, and towed gyro-gliders and boom-trainers are ideal tools for this as well as being cheap to build and fly.

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