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Autogyro

From Academic Kids

An autogyro (only an autogiro when made by Cierva (see below), sometimes called a gyroplane or Gyrocopter™) is an aircraft with an unpowered rotary wing, or rotor, that resembles a helicopter. It is powered by either an engine-powered propeller or a tow cable. The movement of air past the rotor causes the lift.

The Rehler Gyrocopter (autogyro)
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The Rehler Gyrocopter (autogyro)
Contents

General characteristics

Autogyros can take off and land in short fields compared to conventional fixed-wing aircraft. They can even land straight down in some cases. When they have a jump start feature, they can jump vertically and then start flying forward so avoiding a take off run (but this does not give them a hovering ability); but this feature adds weight, complexity and expense so it is not common. If they have a variable-pitch rotor, they can flare to a soft vertical landing, using excess momentum in the rotor to perform a soft landing; this is related to the way the jump start feature is implemented.

Autogyros are notably safe. If the engine should fail, the autogyro does not stall or spin. Instead, it begins to settle like a parachute. The pilot can usually maintain some directional control by slipping the rotor.

Autogyros are neither efficient nor fast. Fixed-wing aircraft are fast and use less fuel over the same distance, helicopters require much more power (and hence fuel) than a fixed wing aircraft for the same top speed.

They are typically more maneuverable than fixed-wing aircraft, but cannot hover as a helicopter can. When helicopters became practical, autogyros were neglected for nearly thirty years. Yet they were used extensively by major newspapers to move information from city roof top to roof top.

As the infrastructure for service, repair, training and building increases the number of gyrocopter users may increase. NASA is said to be exploring the use of these sporty flying machines to encourage personal air transportation for everyone.

There are three main types of autogyro: Early examples were tractor-type, meaning the engine and propeller were in the front of the aircraft, and pulled the plane forward. Such planes usually had small wings to provide better stability. Most autogyros today are pusher-type, meaning the engine and prop are mounted behind the pilot/passengers and push the plane forward. Latterly the Little Wing LW-5 (see [1] (http://www.littlewingautogyro.com/)) in the hands of Andy Keech (see [2] (http://records.fai.org/pilot.asp?from=rotorcraft&id=4246)) has taken numerous world records as well as exhibiting exemplary stability in flight.

The final type of autogyro has Vertical Take-Off or VTO capability. Aircraft (such as the Groen) with this feature have a rotor with adjustable blade pitch (like a helicopter's cyclic) and have the ability to use the engine to spin the rotor while on the ground. The rotor blades are turned flat so they produce no lift, and the engine is used to spin the rotor as fast as possible. When ready for takeoff, the engine is decoupled from the rotor and the blade pitch is set for maximum lift. The kinetic energy stored in the rotor lifts the plane a few feet off the ground, and the conventional propeller is used to give the plane horizontal airspeed before the (now unpowered) rotor speed decays too much to keep the gyroplane in the air.

History

Juan de la Cierva, a Spanish aeronaut, invented the first successful autogiro in 1923. His craft used a tractor-mounted forward propeller and engine, a rotor mounted on a mast, and a vertical stabilizer. His first three designs the C.1, C.2, and C.3, constructed by Parnall were unstable. His fourth design, the C.4, was successful.

The C-11 and some of his later designs had a power-coupling to the rotor, the so-called "jump" feature. The rotor would be sped up before the take-off roll. The coupling would be disengaged during the take-off as the airflow began to power the rotor. This allowed the craft to take off with almost no roll at all.

The C-19 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.

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 when Dr. Igor Bensen 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.

Since Bensen, a number of improved designs have been constructed. Two FAA-certified designs have been commercial failures, despite performing well.

Modern autogyros are quite frisky on the ground, and versions with brakes and tied rotors have been driven successfully in heavy automobile traffic.

Bensen's design

The Bensen Gyrocopter™, the protoype of many post WW2 gyroplanes, actually consists of three versions, the G-6, G-7 and G-8. All three were designed in both unpowered and powered forms.

The basic 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. All welds or soldered structural joints should be inspected.

The rotor is on the top of the vertical mast. The outlying fixed wheels are mounted on an axle (of tubing). The front-to-back keel (more tubing) mounts the forward wheel (which casters), seat, other tubes, engine and a vertical stabilizer. Some versions mount seaplane-style floats and successfully land and take off from water.

It is common for the vertical stabilizer to drag on the ground unless it is cut away. This is also why many frames have a small wheel mounted on the back end of the keel.

The rotor is not symmetric as in some helicopters. It has a true wing shape. Most light gyroplane rotors are made from aluminum, though aircraft-quality birch was specified in early Bensen designs, and wood/steel composite is still used in the world speed record holding Wallis.

Flight Controls

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

The Bensen pattern control stick drops down from a hinge that mounts the main rotor's bearing to the vertical tube. This is not the flap hinges on the rotor, but a separate, third hinge used to manage aircraft roll. The hinge lets the rotor tilt forward or backward. When the Bensen control stick is pressed forward or backward, the rotor precesses like a gyroscope causing the vehicle to roll left or right. The hinge has limits to prevent the rotor from hitting the ground when it is moving slowly.

Modern designs typically use a between-legs control stick instead, and the precession is handled by a mechanical linkage so that left and right stick motions are more intuitive than Bensen's simple design.

Another control is a simple set of rudder pedals that move the hinged back half of the vertical stabilizer, similar to a rudder on a fixed wing aircraft. This lets the pilot keep the craft lined up in the desired direction of motion. The stabilizer is mounted behind the pusher propeller, so one can steer the craft on the ground and during takeoff. Some builders use a pushrod between the rudder bar and stabilizer. Others use cables.

Some simple autogyros, including Bensen's G-6, do not use controllable-vertical stabilizers at all. They are fixed - this works for towed gyro gliders, but not for powered gyros.

The throttle and choke are usually levers mounted where convenient- often under the seat.

The rotor generates more lift on the leading side and less on the lagging side, and this causes the rotor to tilt backwards with forward airspeed (helicopters tilt their rotor in the opposite way as they use their rotor to drag the vehicle through the air, whereas a gyrocopter's blades are unpowered). This increases drag and has a lot to do with the relatively low top speed that Autogyros can reach.

Flight characteristics

Autogyros are often regarded by fixed-wing aircraft pilots as "dangerously unstable", which is certainly true if one tries to fly an autogyro using fixed-wing principles. Piloted properly, a autogyro is slightly safer than a fixed-wing aircraft because it cannot stall. A "stall" does not mean an engine-out event, it means a fixed wing aircraft is travelling too slowly for the wings to produce lift. Since the rotor of a autogyro is always spinning, it cannot stall. If forward airspeed becomes zero, the autogyro will slowly drift to the ground, rotor still spinning. A vertical landing in this manner will not critically damage most autogyros.

One weakness in certain types of autogyro is pitch instability (pitch is the tilting up or down of the craft as viewed from the front or the back). Pitch instability can be a problem because autogyros lose rotor control authority in negative-G forces (positive-G forces push people into their seats; negative-G forces make people float out of them, such as driving over a hump back bridge at high speed in an automobile). Negative-G forces "unload the rotor" and rotor control authority is lost. A flying autogyro hangs from the rotor much like an object hung from a string. As long as the plane is hanging from the rotor, stability is maintained. The instant zero or negative-Gs are introduced, rotor speed begins to decay and the forces stabilizing the plane are lost.

Negative-Gs can be caused by Pilot-Induced Oscillation, or PIO. PIO happens when a pilot adjusts his pitch too much too quickly, then makes a countering control input to bring the pitch back. The countering input often overcompensates, and the autogyro begins to buck like a bronco. You can see a similar effect when some learner-drivers are doing kangaroo-hops in a car with a stick shift and clutch. This is most likely at higher engine throttle settings. If the pilot continues to fight the plane, the rotor (which is flexible) can slow down due to the lack of positive G force, and can flop down and strike the spinning propeller, which destroys both and sends the autogyro into an uncontrolled fall. The way to avoid this during an incipient PIO is to apply gentle back pressure on the stick (to raise the nose in pitch) and cut engine power. Note that this is the exact opposite of what fixed-wing pilots are trained to do when in trouble, which has led to some unfortunate accidents and the autogyro's undeserved reputation for being "dangerous."

Another danger is "bunting over" or a Power Push-Over (PPO). An autogyro's vertical airspeed (climb or sink rate) is directly coupled to airspeed. Increase forward airspeed, increase rate of climb. In order to maintain level flight at high engine throttle settings, the pilot must tilt the rotor forward to prevent climbing and maintain level flight. The rotor thus becomes more nearly horizontal, and the control stick becomes more sensitive.

Too much forward stick, and the autogyro's rotor can aim down towards the ground. When this happens, negative-gees occur, rotor speed drops too low to provide lift, and a high-thrustline autogyro is then pitched forward by the propeller thrust and tumbles end-over-end in a somersault. It is virtually impossible to regain control after a full PPO.

Two factors can lead to pitch instability: no or too small horizontal stabilizers (h-stabs) on too short a tail and high thrustline propeller placement which destabilises the force diagram. A large h-stab, ideally in the prop wash (where the propeller blows on it) will reduce the tendency of an autogyro to bunt over as a result of improper control input by damping the control response.

If the propeller thrustline in an autogyro is high -- meaning the axis of propeller power is above the center of gravity for the aircraft -- the autogyro tends to pitch forward under sudden power application (see PPOs above, as for why this is Bad). (Unfortunately, Bensen-type autogyros have a notably high thrustline.) If the thrustline is low, the autogyro tends to pitch up under sudden power application, which is harmless. It's difficult to have a low thrustline without a really tall autogyro (such as a "Dominator" style) however, so most autogyro designs simply try to get the thrustline as low as possible though still being slightly above the center of gravity.

In spite of these dangers, most autogyros are designed to reduce them. Also, the majority of autogyro pilot training involves avoidance of PIO and PPOs.

Autogyro rotors usually feature a teeter-hinge in the middle. Picture a autogyro or helicopter from above, rotor spinning clockwise. If the aircraft is flying forward, the rotor tips on the left are traveling faster than the aircraft, while those on the right are actually going backwards relative to the craft. If the rotor blades were fixed, this would produce uneven lift -- more lift on the left side, since those blades are traveling faster. The teeter hinge on each blade lets it "flap" up and down. As the blade swings on the left, the increased speed makes it flap up with a greater angle of attack to the relative wind. This increases drag and reduces lift. As it swings to the right, it's now going slower, relative to forward speed. This reduced drag lets it flap down and get a better bite into the air, increasing lift.

Pitch is controlled by a conventional joystick coupled to the rotor. Pulling back on the stick tilts the rotor back, increasing lift and decreasing forward airspeed. Pushing forward on the stick decreases lift and increases airspeed, as long as it is not pushed much beyond horizontal (see PPO above). The plane's direction is controlled by rudder pedals.

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 16/11/2002 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: [3] (http://records.fai.org/pilot.asp?from=rotorcraft&id=335)

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

Gyrocopters are often used to herd range animals. A gyrocopter 'cowboy' holds the worlds 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).

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

Sub-class : E-3a (Autogyros : take-off weight less than 500 kg) Category : General Group 1 : piston engine

Speed over a recognised course : 16.45 km/h

Date of flight: 12/10/2003 Pilot: Andrew C. KEECH (USA) Course/place: Kitty Hawk, NC (USA) - San Diego, CA (USA)

Rotorcraft: Little Wing LW-5 Sub-class : E-3a (Autogyros : take-off weight less than 500 kg) Category : General Group 1 : piston engine

Speed over a recognised course : 31.89 km/h

Date of flight: 22/10/2003 Pilot: Andrew C. KEECH (USA) Course/place: San Diego, CA (USA) - Kitty Hawk, NC (USA)

Rotorcraft: Little Wing LW-5 Sub-class : E-3a (Autogyros : take-off weight less than 500 kg) Category : General Group 1 : piston engine

Speed over a recognised course, round trip : 16.42 km/h

Date of flight: 22/10/2003 Pilot: Andrew C. KEECH (USA) Course/place: Kitty Hawk, NC (USA) - San Diego, CA (USA) and return

Rotorcraft: Little Wing LW-5

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.

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. 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.

See also


Lists of Aircraft | Aircraft manufacturers | Aircraft engines | Aircraft engine manufacturers

Airports | Airlines | Air forces | Aircraft weapons | Missiles | Timeline of aviation

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