Tail Group Options

Most builders customize their projects to some extent and this is most obvious when it comes to the tail group. In this page I want to outline some principles and guidelines with respect to the Gyrobee tail group and then show some examples, , to illustrate the many tail-group options you might elect to pursue.

Vertical Stabilizer/Rudder

Because the vertical stab is partially immersed in the prop slip-stream in normal flight, rudder authority is rarely an issue in the normal flight envelope. If there is going to be a problem, it will typically occur when the engine quits and the vertical stab/rudder is exposed to the free-air slipstream, partially blocked by the structure of the gyro. This is a real problem for machines with a short-coupled tail boom and and enclosure. This is not an issue with the Gyrobee since the tal boom is relatively long and, without an enclosure, side area forward of the CG is limited. Any of the rudder configurations shown below provides plenty of rudder authority in a worste-case, engine-out situation.

One issue with any conventional rudder is the need to apply rudder to compensate for P-factor and torque. In the case of the Gyrobee with a Rotax engine, you will alsways be applying some left pedal in flight. An experienced pilot won't give it a thought, but it does complicate things slightly when you are first learning. It would be possible to modify the design to accept a Dominator-style "tall-tail" that would eliminate the requirement for differential pedal due to the fact that the vertical stab would span the entire propellor slip-stream.

Horizontal Stabilizer

An adequate horizontal stabilizer performs two functions. In normal flight in can act to significantly reduce the pilot's workload with respect to pitch stability. The second function is even more important and will be the focus of the remaining discussion. In the event that the rotor were to unload in flight, a gyro with its engine thrust-line above the center of mass (CoM or vertical center-of-gravity) will try to pitch forward and tumble. No amount of training and flight experience can prevent this tendency and the only effective remedy is a horizontal stabilizer with sufficient control power to resist the pitching moment. The larger the engine and the greater the offset between the thrust-line and the CoM, the greater the control power required to resist the pitching moment.

Magnitude of the Pitching Moment. The pitching moment, in foot pounds, is equal to the product of the vertical offset (in feet) times the peak engine thrust (in pounds). On a stock Gyrobee, the engine thrust-line will typically be 1-2 inches (0.083 to 0.166 feet) above the CoM. If we assume an engine thrust of 260 pounds (reasonable for a 447 in good shape and a bit low for a 503), the pitching moment can range from 21.7 (0.083 x 260) to 43.3 (0.166 x 260) ft/lbs.

Sizing and Positioning the Horizontal Stabilizer. Doug Riley quotes the following values with respect to the stabilizing force of a stab positioned in a 40 mph slipstream -

reasonable for a cruising ultralight gyro:

Flat Plate = 1.6 lbs/ft2

Airfoil Section = 3.2 lbs/ft2

Note that these values can be increased by a factor of four is the stab is placed in the propellor slip-stream with a nominal velocity of 80 mph. We will look at that important fact in just a bit.

Let's start with a simple analysis of the prototype Gyrobee, which uses the Brock KB-2 "stab". The fact is, placed where it is on a KB-2 (close-coupled in disturbed air) the stab cannot possibly be effective. However, the Gyrobee scenario is another matter. The KB-2 stab is not a flat plate - it has a modest airfoil section. Let's, for the sake of simplification, place it mid-way between a flat plate and an optimum airfoil section - 2.4 lbs/ft2. The control force exerted by the stab would thus be 2.4 times the area of the stab times the length of the moment are. The stab area is approximately 3 square feet and the moment arm is approximately five feet:

2.4 x 3 x 5 = 36 ft/lbs

The thrust-line offset on the prototype is under 1 inch, so I will assume the thrust-line vector to be about 20 ft/lbs, which, at 40 mph, is opposed by 36 ft/lbs generated by the horizontal stab. Basically, the effect of the modest thrust-line offset is covered with a reasonable margin. We have always been quite satisfied with the pitch stability of the prototype and the number crunching demonstrates why.

Now, if we move from our specific prototype to a more general model, we can see some areas of potential concern. If our thrust-line offset had been two inches (instead of under 1 inch), the pitching moment would increase to over 40 ft/lbs and the Brock stabilizer becomes marginal at best. Add the additional thrust of a 503 and the problem gets worse! There are two ways to increase the effectiveness of the stab:

Increase the stab area. Most of the Bees that have been built have bigger stabs and that cannot hurt.

Move the stab upward so it sits in the prop slip-stream. Many of the stabs shown below take advantage of this very significant boost in stab performace (see below).

The horizontal stab of Martin's Bumblebee is no larger, area-wise, than the Brock unit, and it is a flat plate. Normally we would expect no more than 24 ft./lbs. in a 40 mph slip-stream. However, Martin elevated the stab into the prop-wash, so, if we assume an 80 mph airflow, the stab would actually produce 24 x 4 or 96 ft/lbs! One of the nice things about positioning the stab in the airflow from the prop is that, as engine thrust increases, so does the stabilizing force generated by the stab!

The bottom-line is that virtually any of the tail-group options shown below will eliminate the possibility of PPO or "bunting" as long as you don't go higher than a 503 and build to keep the thrust-line offset within its nominal 1-2 inch range. Building your own tail group,

especially a design that places the horizontal stabb in the propellor slip-stream, can add considerable reserve stabilizing force that can cover a wider range of locations for the CoM and maximum engine thrust.

The prototype with the KB-2 stab. This arrangement is adequate given how our aircraft is set up, but the options shown below will provide much greater latitude.

Janos Dancso built a Watson-like tail for his machine. Plenty of stab area and its in the propwash!

Scott White used an original Holmann Bumblebee tail on his machine. The horizontal stab area is limited, but the stab is effective because it is mounted in the propellor slip-stream.

Johnny Skoch used a KB-2 vertical stab/rudder but built a bigger horizontal stab, placed as far back as it will go.

L.J. Norton used a Honeybee tail group. The horizontal stab is a flat plate but considerably larger than the one used on our prototype.

James Lee has a big stab positioned in the propellor slip-stream. This is probably overkill but James does report the machine is very stable. If you elect for large tail surfaces, you may want to look at ancillary bracing for the tail boom.

Curran Spottswood also incorporated a good-sized tail group with the horizontal stab places in the propellor slip-stream.