3 Bensen's design

The Bensen Gyrocopter (TM), 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.

4 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 atop the vertical tube. The hinge lets the rotor tilt forward or backward. The hinge prevents the rotor from hitting the ground.

Since the rotor precesses like a gyroscope, exerting forward or rearward force causes the vehicle to roll left or right.

Later designs often substitute a more-complex between-legs control stick.

Another control is a simple set of rudder pedals that move the hinged back half of the vertical stabilizer. 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.

5 Flight characteristics

Autogyros are often regarded by fixed-wing aircraft pilots as "dangerously unstable", which is certainly true if one tries to fly a 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 the autogyro is pitch instability (pitch is the tilting up or down of the craft as viewed from the front or the back). Pitch instability is a problem because autogyros cannot handle negative-gee forces (positive-gee forces push people into their seats; negative-gee forces make people float out of them, such as driving down a steep hill at high speed in an automobile). Negative-gee forces "unload the rotor". 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-gees are introduced, rotor speed begins to decay and the gyroscopic forces stabilizing the plane are lost.

Negative-gees are usually 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. This is most likely at high engine throttle settings. If the pilot continues to fight the plane, the rotor (which is flexible) usually flops down and strikes 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 backpressure on the stick (to raise 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 lead to some unfortunate accidents and the autogyro's undeserved reputation for being "dangerous".

Another danger is "bunting over" or a Power Push-Over (PPO). A 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 translate some of his lift into forward motion. Too much tilt, and the autogyro's overall pitch will aim down towards the ground. When this happens, negative-gees occur, rotor speed drops too low to provide lift, and the autogyro tumbles end-over-end in a sommersault. It is impossible to regain control after a PPO.

Two factors lead to pitch instability: no or too small horizontal stabilizers (h-stabs) and high thrustline engine placement. A large h-stab, ideally in the prop wash (where the propeller blows on it) will reduce the tendency of a autogyro to over-pitch as a result of improper control input.

If the engine thrustline in a pusher-type 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, Benson-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 planes direction is controlled by rudder pedals.

6 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]

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).

7 Kits

Many autogyros are assembled from kits.

Kits with all parts, ready to assemble, are listed for US$19,550 as of 7/18/2002. 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.

8 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.

9 See also

• Fairey Rotodyne
• Carter Copter
• Wallis Autogyros

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