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Special Six-Page Report from RAF on The Stabilator to Download

Article by John Miller, SA Flyer magazine

There are few niche aviation pursuits that attract the contention found amongst gyrocopter enthusiasts. This is no recent development  the die was cast when Juan de Cierva licensed his unique rotary-wing invention, the Autogiro, back in the twenties and if we think "lawyer-mania" is a function of the late nineteenth century, think again. During the thirties, as technology accelerated following the Great War, patent offices hummed at the giant leaps in mechanical ingenuity. Cierva's aerial tractor autorotational device was just one of them. Apart from a short but stunning appearance during the fifties when Fairey Aviation built the only Rotodyne, and brief unfulfilled military interest, the Autogiro was to remain on the fringes of commercial aviation.

Cierva, amongst others, licensed his creation to a US gentleman by the name of Harold Pitcairn who was to drive the Autogiro's US development. Their patents not only enshrined early autorotational principles, but went a long way to determine current helicopter technology and it was only after Harold's death that Bell, Kaman, Piaseki and Hiller were obliged to pay royalties following the world's longest running patent action.

The Fairey Rotodyne was one of the technologies' last commercial adventures and since the British government ceased funding the project in 1961, autorotational flight has mostly become the preserve of recreational pilots and homebuilders. Nowadays, gyrocopters are almost exclusively pusher designs a configuration championed by the industry's celebrated designer, Igor Bensen. (Bensen wasn't the first. Lawrence Buhl flew the first pusher design in 1931 based on Cierva's unflown 1930 C.21 model).

Virtually all current pusher gyrocopters owe their basic design layout to Bensen and his original glider version. Bensen's early interest was formed whilst working as an engineer and pilot for General Electric, who had bought an army surplus Kellett XR-3 in 1945 to study its rotor hub technology. In 1951, Bensen joined the Kaman Helicopter company but by 1953 had started his own business marketing and selling, initially a gyro glider and then a powered version sold in kit form as the B-7 and B-8M, many of which are still around today powerd by 72hp, two-stroke McCulloch engines.

The first autogyros featured a fixed rotorhead, directional control being achieved by manipulating traditional ailerons, elevators and rudders. By the early thirties, Cierva was quietly developing his first "direct-control" rotor system by which control input was passed directly to the rotor plane to alter attitude. When Harold Pitcairn found this out he was disappointed that Cierva had not passed on information about his experiments, casting the seeds of distrust that were to lead to the eventual polarisation of the two companies. Nevertheless, direct control of the hub was to be universally adopted in all further gyrocopter designs.

During the sixties, Igor Bensen and protégé Ken Brock were to be the most successful Gyrocopter proponents and as this form of flying became more popular, other designs began to appear including the successful Rotary Air Force (RAF) 1000, designed by Canadian Dan Haseloh. Dan's interest was fanned by his uncle Bernard who was a gyrocopter enthusiast and designer. In 1987 Dan launched his single seater RAF 1000 followed two years later by the familiar two-seat RAF 2000.

The gyrocopter's apparent simplicity is deceptive, as are its basic principles of flight. Cierva's concept of continually autorotating blades, led the helicopter industry by some fifteen years. Gyro designers have historically attempted to usurp the role of the helicopter both by talking up the minimal runway requirements of their aircraft and by developing the "Jump-takeoff" concept where the main rotor is pre-rotated to well above its normal rpm and then suddenly "pitched" to provide a vertical capability. This takeoff method was developed by Cierva although the concept of collective control had been discovered by Argentinian, Marquis Raul de Pateras Pescara in 1924. It was to be used by a number of gyroplane designers including the Groen Brother's who are currently developing their commercial Hawk gyro. Collectively altering rotor pitch was used in jet-powered rotor form by Fairey for the Rotodyne and has been adopted as the primary control arrangement for all current helicopter designs.

One of the gyrocopter's greatest benefits, is its ability to operate safely in gusty wind conditions that would keep most ultralights on the ground. The reason for this is the inherent stability of its rotor disc and the disc's traditional resistance to turbulence. Upward and downward gusts have minimal effect on a gyro, which makes no use of a horizontal stabiliser. Late thinking has also determined that the engine's thrust line needs to be in line with the horizontal centre of gravity to minimize the achilles handling heel of gyrocopters, the dreaded Power Push Over (PPO). This phenomenon can follow a pilot induced oscillation (PIO) or an abrupt push of the control stick at the end of particularly energetic power climb. The best configuration to counter this tendency is the subject of heated debate amongst gyro pilots. However, one of the world's most famous authorities, Commander Ken Wallace, believes that, provided the gyro is well designed, they are quite safely operated without the need for a horizontal wing in any form.

The gyrocopter's natural stability is derived from its low centre of gravity with the airframe, engine and occupants weight being suspended from the lift generating rotors. The engine provides forward thrust and a simple vertical stabiliser, directional stability. Dan Haseloh, the RAF 2000's designer, believed a horizontal stabiliser wasn't necessary for his machine. However, the company, (Dan Haseloh was killed in June 1998 after a mid-air collision whilst flying for a promotional video), has recently developed a rotor-disc stabiliser mounted to the mast and hub and adjusted in pitch by the pilot's control input. Some homebuilders have also included a small stabiliser on the lower rudder frame. More of this later.

Like most gyrocopter designs, the RAF 2000 uses a single length-wise keel, upon which is mounted the undercarriage, cockpit, power plant, control systems and rudder. The gyrocopter is an exceedingly short device, measuring a mere 12 feet from the nose to the end of the rudder tip. An eight-foot (folding) vertical section carries the teetering blade rotor, control rods, 2.2 litre 130hp Subaru boxer engine and its flexible prerotator drive. The undercarriage is fixed to the keel. It is in essence, an elegantly simple structure and obviously robust, especially the masthead, which, just like a helicopter, carries the RAF 2000's entire weight once airborne.

As the gyro is a homebuilt, no two interior layouts are the same apart from the location of the trim adjusters, throttle and control sticks as well as the simple bicycle brake grip used to engage the pre-rotator. There are however some essential instruments, in particular the rotor rpm readout, typically presented in digital format. This is placed right in front of the left-seated pilot in command and always sits next to an equivalent engine tachometer. As the RAF 2000 is a two-seater, it is sensitive to weight location in both roll and pitch. Consequently a pair of prominent trim knobs occupy a central position on the floor between the occupants.

The egg-shaped cabin appears tiny for two people but is perfectly adequate. The RAF 2000 is almost unique amongst gyrocopters in having this feature and it goes a long way to provide an all-weather  or rather, an all temperature capability. The doors are quickly removed for those really warm days.

It is curious to see how some fringe aviation pursuits have aligned themselves with the flat-four Lancia derived Subaru engine. Amongst other things, it is an all aluminum unit and it's weight is eminently suitable for aircraft and as Rotary Air Force has discovered, ideal for their RAF 2000. The engine comes from Subaru's four-wheel-drive shopping and kid's runabout Legacy model the engine is called, in the trade, an EJ22 overhead cam. For the RAF 2000, it produces 130hp in normally aspirated form and boasts a 1500 hour TBO. Weight is around 100kgs. Those who may be nervous about adopting obscure automotive engines into aircraft should be reassured that Steve's Auto Clinic has a great deal of expertise on this type of application. Through their distributor, Soob Aviation in Port Elizabeth, overhauls can be done for under R20,000. The engine drives a 2:1 reduction belt to the four-blade composite propeller.

Flying the RAF 2000

There's something quite comical to stepping into a gyrocopter and taxiing out. On the ground, apart from the rotor blades, there's little to suggest that it actually flies. Indeed, normal procedure is to taxi out with the rotor static. To get to this stage had taken a leap in faith. My last and only gyrocopter experience was some years ago flying a European-manufactured tandem machine, a VPM. I was alarmed by the handbrake-type pre-rotator handle breaking off in my hand and the sudden departure of one of the drive belts during our pre-takeoff checks. When we eventually got airborne, the high vibration through the controls was enough to end any spark of enthusiasm that may otherwise have won over another sympathetic member of the flying media (I discovered much later that the vibration was due to ill-balanced blades).

It happened that Southern Africa's RAF distributor, Dave Armstrong was attending Oshkosh. As usual, he was accompanied by his family and was spending a great deal of time at the busy Rotary Air Force exhibition stand. Dave introduced me to CEO Don LeFleur a legendary figure amongst gyro pilots. Their enthusiasm wore me down and with a certain amount of misgiving, I volunteered to accompany demonstration pilot Duane Hunn on an RAF 2000 flight out of Fond du Lac airfield where they were giving rides.

Duane is RAF's Chief Flying Instructor and has some 5 000 RAF 2000 hours including 3 000 on his own 1994 machine. This is a lot of airborne time and it went a long way to settling any disquiet I felt before climbing aboard his bright yellow gyrocopter. I was about to be converted.

After a careful pre-flight, the Subaru boxer engine fired up easily and was allowed to warm up before Duane called for departure clearance. With a burst of power, (the throttle is mounted on a tube that stretches the entire width of the seat below your knees) we moved off towards the active runway holding point. Helicopter pilots have, for years, scoffed at the gyro's inability to takeoff vertically. However, this misses the point somewhat, especially as a Robinson R22 helicopter is about US$100k more expensive.

In the twenties, getting the rotors to wind up on early autogyros meant taxiing energetically around the airfield before sufficient rpm had built up to allow a takeoff. Later, this task was accomplished by winding a rope around the main shaft and pulling it much like a spinning top. Nowadays, such Heath Robinson solutions are thankfully a thing of the past and the RAF 2000's blades benefit from a flexible pre-rotator shaft, engaged by gripping and pulling on a clutch lever attached to the left stick. Duane gave me this job as I was sitting in the left seat. With the run-up complete and lined up on the runway, the lever is held until a magic 140rpm appears on the rotor tachometer. The throttle is then advanced fully and the RAF leaps forward.

It is important to grasp that when manipulating the stick, the gyro pilot is not maneuvering the body of the aircraft but the rotor disc. Consequently, special attention needs to be paid to it during all phases of ground operation as well as flight. It is vital that the rotors do not contact the ground easy to do if the stick is not held forward whilst taxiing. At about 100rpm, the rotors begin to support their own weight and forward pressure on the stick can be relaxed. The most important skill to be acquired when training is a feel for power, airframe and rotor disc harmony. The rotor slows down if the disc is presented to the airflow at too high an angle on takeoff, requiring more power to maintain rotor rpm. Conversely, too low an angle and the disc will also slow. If this sounds confusing, then it is. Duane explained that the power needs to be "feathered in" to achieve a smooth acceleration and best transition to takeoff, at the same time keeping the nose wheel "light" on the ground.

Done correctly and the RAF becomes airborne very smoothly with little perceptible change in airframe attitude. Duane further describes the process as not unlike that of getting a speedboat on the "plane" he says the gyro needs to "plane out" of the ground run to about 55mph with two on board before it gets on the step and flies away. We headed east, low level over the flat but picturesque Wisconsin countryside, taking in the extensive swamp areas that reminded me of the Okavango Delta. It was a very pleasing experience with a magnificent view in all directions one of the great attractions of gyrocoptering. Better still, was the almost complete absence of stick-shake and comfortable ride, even with the doors removed. I could get used to this!

Duane's RAF is fitted with the latest advance in gyro aerodynamics a "Rotor Stabilator" that he himself has developed. I was able to fly a standard RAF 2000 without this accessory and the difference enabled me to fly Duane's gyro, free of any PIO tendencies. RAF claim this device is set to revolutionize gyro handling. The stabilator is hinged at the mast and is controlled by an electrical trim pushrod attached to an extension of the torque tube. The stabilator stabilizes changes in rotor rpm, compensates for turbulent conditions and helps maintain a constant airspeed. It also dramatically reduces any PIO tendency. It appears to work very well and once testing is complete, it will be offered as standard on new machines.

Duane reckons a helicopter pilot will adjust to the gyrocopter quicker than a fixed wing pilot. However, for each, the handling experience is new. There isn't a great deal of "feel" through the stick although the RAF 2000 is very happy being trimmed "hands-off" for extended periods. The biggest challenge is to remember you are flying the rotor rather than the airframe and it's easy to make stick inputs in response to tiny changes in the attitude of the airframe. It is these airframe responses that lead to PIO and they need to be minimized to achieve a smooth path through the air. Despite my own helicopter time, I reckoned I needed a good couple of hours to get used to this input. I found I could happily control Duane's RAF immediately, the Rotor Stabilator, damping out the small pitch changes found on a standard model. Turning is also easy although gyro students are taught the effects of this maneuver on rotor speed. It requires no noticeable rudder input for a coordinated turn. The rotor speed rises during a turn a result of simple lift and drag vector principles, well understood by helicopter pilots. Upon returning to the straight and level, a gyro will start a gentle climb until the rotor rpm stabilizes again.

We eventually returned to Fond du Lac and I returned the controls to Duane. He talked me through the approach and landing phase, explaining that the gyro needs to be powered onto the runway. Interestingly, we held our height until over the threshold, gently reducing the power and flying the rotor disc to a 45-degree approach angle at some 60-65mph.

With an almost imperceptible back pressure on the stick, the RAF 2000 plopped ever so gently onto the runway, coming to a stop in a few meters. If I were in a Robbie, I would then lower collective and heave a sigh of relief that I was safely back on the ground. The gyro pilot needs to continue monitoring the rotor rpm and fly the disc as it is still effectively "loaded", making sure that the stick is moved forward as the rpm dissipates to avoid an embarrassing and expensive contact with the ground.


My ride in the RAF was a great aviating experience and I would be encouraged to do it again. I can understand the gyro's appeal and why passion and opinion run high within this activity. An RAF 2000 is not an expensive method of becoming airborne and kits sell for about half the price of current new ultralights.

Southern Africa RAF Distributor, Botswana based Dave Armstrong, left, Don LaFluer, Rotary Air Force CEO and Garth Armstrong, right. Seen here at Airventure 2003.

This article is republished with the permission of SA Flyer magazine


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 Last modified: 01/22/2016