The original single seat gyroplanes were a great step forward for personal aviation.  They provided exciting rotary wing flight at low cost and many were built in home garages with hardware store parts.  Wonderful as it was to build your own aircraft for a few hundred dollars, the downside was that some of the designs were inherently unstable.    

Unlike an airplane where the propeller is directly in front of the fuselage providing center line thrust, the gyroplanes had engines and propellers mounted above and behind the pilot meaning the thrust line was high and therefore constantly trying to push the nose of the aircraft down.  As builders began to put bigger engines with bigger propellers on their gyros for more power they naturally moved engines higher for propeller ground clearance, which made the problem worse accentuating the inherent instability. 

Finally some designers realized the problem.  The solution was to lower the engine in relation to the cabin resulting in center line thrust.   However, to do that they had to put long legs on the gyro so the propeller would still clear the ground.  The result was some very tall gyros that you needed to climb up into but at least they were much safer as the thrust line was more centered. 

Along came Two Seat Enclosed Gyroplanes
The first popular enclosed 2 place side by side gyro was the RAF.  Its sexy styling made it a big hit and many kits were sold and built.  Unfortunately it has a very poor safety record due to two factors.  It has a high thrust line and no horizontal stabilizing surface at its tail.  The high thrust line is trying to push the nose down and the lack of a horizontal tail surface makes it more susceptible to instability.  Experienced RAF pilots mange their machines well but too many new pilots end up as accident statistics.

If one looks at most every flying object from a lawn dart to an arrow shot from a bow to any airplane whether it be a Piper Cub or the Space Shuttle you will see they all have some horizontal tail surface for stability. TThe RAF factory does not provide horizontal stabilizers but fortunately nearly all their customers have added custom built horizontal tails to their machines thereby providing some stability.  RAF recently has come out with a horizontal stabilator on the rotor which helps trim the rotor but it does not address the inherent instability of the airframe.   

Stability Modifications
In an effort to stem the horrendous accident rate of the popular RAF, the design team at Groen Brothers Aviation took time away from the design of their Hawk 4 turbine gyro to address the problem. They came up with a stability modification kit for the RAF and sold it at a very reasonable cost.  There is no doubt their efforts have saved lives and they are to be congratulated.

Realizing they had a hit on their hands the folks at Groen Brothers then made further design modifications to the airframe and increased the cabin size resulting in the SparrowHawk.  To enhance stability they added a horizontal tail surface and also a large vertical tail.  In order to get the thrust line on center they lowered the engine in relation to the cabin.  This required putting long legs on the gyro so the propeller would clear the ground.  While a few pilots complained about the gangly look of the machine and the difficulty of climbing in and out, most were pleased with the machine's stability and maneuverability.  It was safe, stable and easy to fly making it a great gyroplane for training new pilots.  A new Sparrowhawk II is now offered to improve on some elements of the original Sparrowhawk.

Center Line Thrust
The solution GBA had applied to RAF’s instability problem was to apply center line thrust.  At this point everyone in the gyro industry in America was saying center line thrust was the answer to all the stability problems that plagued gyros. 

The Europeans
Across the Atlantic in Europe several designers took a different approach to solving the same problem.  Vittorio Magni in Italy and Raphael Celier in France tackled the problem with new ideas.  They both wanted stability but also efficient aerodynamics and good styling.  Raphael studied seaplanes which often use high mounted engines (it keeps them away from water spray) and also relied on his background of designing small drone aircraft for the French military. 

Rather than adapt from the RAF cabin shape which had been a marketing success, both European designers started with a clean sheet of paper.  Both also realized the key element was balancing everything on the entire aircraft not just putting the thrust line in the center of the airframe.  Now cabin shape became an integral aerodynamic element as well as a matter of styling.

Cabin Aerodynamics
Magni designed and produced a beautiful and stable 2 place partially open cockpit machine where the occupants sit in tandem, one behind the other, while Raphael designed the Xenon where the pilot and passenger sit side by side in a fully enclosed cabin.  Both machines have slightly high thrustlines but exhibit total stability in all areas of handling and both are beautifully styled.  The sleek front of the Xenon is reminiscent of a Citation Jet. 

Not only is the Xenon’s cabin contributing to stability, its shape is more aerodynamic providing less wind resistance so the machine goes faster on less horsepower.  In addition the cowl over the engine compartment provides smooth airflow to the propeller.  In pusher engine installations with no engine cowling the air flowing past the cabin gets disturbed when the cabin ends abruptly.  This creates vortexes and eddy currents just before the air gets to the propeller making the propeller less efficient.

The Xenon’s design allows the air to flow smoothly over the cabin and back to the propeller so the propeller is more efficient allowing less horsepower to give more speed and better climb performance.   Bringing things full circle, the more efficient airflow from the propeller means the tail and rudder surfaces can be smaller and lighter yet still provide the needed stabilizing force.  Smaller surfaces mean less drag and less drag means better performance.

ASTM Standards
In January 2007 the ASTM gyroplane committee came up with stability standards for gyroplanes which have been adapted by the Federal Aviation Administration.  The design standards specifically do not require center line thrust, what they require is stability and the standards recognize that there may be several ways to achieve that. 

Sparrowhawk, Magni and Xenon have all achieved aerodynamic stability
 which makes them safe and stable machines to fly. 

Handling
In airplanes one often gives up some stability for more aerobatic handling. This concept does not translate directly to gyroplanes, as the worst gyro is still so much more maneuverable than the most aerobatic airplane.  In gyros one does not really need to give up maneuverability to obtain stability.     

The Magni and Xenon have a more aerodynamic shape than some gyros which may make them handle better in gusty winds.  A pilot may have less work , more pleasure and higher performance flying a more aerodynamic aircraft but again all 3 are stable aircraft to fly and all 3 maneuver well.

For further information about the Xenon Design read

Gyroplane Design Secrets

The Truth about Engines

Last modified: 03/21/07        Copyright Future Flight 2007