One of the hottest topics in the aerodynamics thread of the Rotorcraft Conference for the past year has been an ongoing series of discussions (and often arguments) about the merits of horizontal stabilizers, center-line thrust, and having the horizontal center of gravity in front of the rotor thrust line. Greatly emboldened by Jean Fourcade's article on Longitudinal Stability of Gyroplanes (Rotorcraft, June-July 1999, p.14-19), they have concluded that the only really stable/safe machine is a high-profile gyro with a tall tail and centerline thrust. Any other configuration is judged to be inferior, unstable, and dangerous to varying degree.

On the surface, the Gyrobee doesn't look particularly appealing from a stability standpoint. Put bluntly, it looks like a typical high thrust-line machine. The result is that I now regularly get e-mails from prospective builders either asking about the stability of the machine or from others who say they won't build it because "the engine thrust-line is too high" or other similar nonsense. Let me make one thing quite clear - you can build or not build the Gyrobeefor any reasons that suite you, but anyone who is silly enough to think that I would spend over a decade flying an inherently unstable aircraft isn't connected with reality. I am certainly not an ace pilot or an adrenaline junky. I'm a gray-haired, over-weight college professor in his early 60's with a wife, three daughters, and two grandchildren. I'm also not in the business of selling you anything, so I haven't got an ax to grind or a reason to put down other aircraft!

In contrast to how the aircraft looks, the experience of all Gyrobee pilots was consistent - the aircraft was a pleasure to fly and very stable in pitch. In contrast, the guys into theoretical analysis said that couldn't be so. Just for the record, here is what I consider to be indicative of a stable flight envelope:
 

• Stick-fixed Flight. Trim the aircraft to a specific cruise speed and then anchor your hand against your thigh. You can move your thigh back and forth to keep the machine flying in whatever direction you want, but no stick movement in pitch! You can do that for as long as you want in a Gyrobee. In many other machines the aircraft will start to get active - or even divergent - in pitch, at which point you have to quit or the &^%*#$ machine may kill you. The Gyrobee will just keep flying!

• Behaviour in Thermals. The Gyrobee will balloon up in rising air and down in descending air, but there will be little or any change in pitch or trimmed airspeed, no matter how active the thermals. Other designs will show significant pitch changes which, if bad enough, may tempt you to chase the machine in pitch using the stick. This is a recipe for disaster!

Well, it turns out that both sides are correct - the Gyrobee is stable and there is nothing wrong with the current ideas about thrust-line and vertical C of G. As always, "the Devil's in the details...". Although the Gyrobee looks like most other pusher designs that trace their origin back to the classic Bensen heritage, the vertical C of G of a typical machine, built per the plans, turns out to be no more than about 1 inch (often less) below the engine thrust-line.  This, combined with only moderate thrust (447 or 503 engine) and an effective horizontal stab, makes the possibility of a power push-over a non-starter. The relatively high vertical C of G (relative to many other aircraft with similar geometry) only holds true if you build it per the plans and keep the weight down. Most things that you might do to add weight will also lower the vertical C of G, degrading your safety margin with respect to pitch stability. Keep the aircraft as close to stock as possible and you can stop worrying about pitch stability!

In an effort to provide the most honest assessment possible with respect to Gyrobee flight handing, I have compiled a set of pages (linked below) that discuss pilot reactions to the aircraft:

Don and I have been flying the prototype for eleven flying seasons. For what its worth, here is a summary of our perceptions.

Geoff is an experienced ultralight pilot who has also flown the SportCopter Lightning.

Doug is an experienced gyro pilot who has taken the time to record some detailed comments on how his machine handles.

Ron had the fun of doing the test-pilot chores for Barry Kropelin's Gyrobee. Here are his comments.

I will add more comments as they come in, but so far the opinions are unanimous with respect to the fine flying qualities of the aircraft.