The Gyrobee
and
Center-Line-Thrust

I have added this page as a result of a discussion I had with Dick DeGraw at Mentone in 2004. Dick, one of the most respected gyroplane designers around, made a passing remark that he would like to see me convert the Gyrobee to a center-line-thrust (CLT) configuration. My response was that a number of Bees have had their vertical-center-of-mass (CoM_v = to what most people call the vertical center-of-gravity) measured, using a variety of techniques, and the engine thrust-line on a stock Gyrobee (full fuel, and a 200+ pound pilot) is typically just 1-2 inches above the CoM_v. In effect, for all practical purposes the Gyrobee already was a CLT machine!

Dick was surprised and said I should make that more obvious in talking about the design. His surprise is understandable, since the aircraft looks like a "typical" pusher/Bensen derivative, many of which are notorious for having a thrust-line that is significantly higher than the CoM_v ! When stability discussions, driven by C. Beatty and J. Fourcade, began to look at alignment of the thrust line and CoM_v  as a key element in gyroplane stability, I found myself arguing that other factors had to be equally significant. I did so for two reasons - the fact that I assumed the Bee had a high thrust-line and the fact that the aircraft has gotten consistent rave reviews in terms of stability and handling  by everyone who has flown it. Either the Gyrobee didn't fly as well as everyone thought it did, which I knew to be nonsense, or there was something wrong with the theorizing with respect to stability! The resolution of the apparent conflict turned out to be simple - the Gyrobee did exhibit excellent stability and there was nothing wrong with the stability theories. The problem was the assumption that the Gyrobee obviously had a high thrust-line!

When actually measured, there was no more than a 1-2 inch-offset between the thrust-line and CoM_v , well within the range of a modest horizontal stab in terms of damping! Lets look at why this is so:

The problem was, nobody (no matter what they say now) was paying much attention to CoM_v when the geometry of the Gyrobee, based on Holmann's Bumblebee) was being finalized in the late 80s. Designers and builders were making decisions based on other criteria. Sometimes this led to machines with marginal to dangerous stability characteristics. In the case of the Gyrobee, the dice rolled in our favor and the decisions contributed to enhanced stability:

• Airframe Weight Distribution. In order to meet the weight requirements of Part 103, the basic airframe was configured to be strong but absolutely as light as possible. This is particularly evident in the wheels and landing gear, with the result that minimizing weight in the lower part of the airframe has the effect of raising the CoM_v.

• Fuel. The fuel tank is elevated above the keel tube, thus raising the CoM_v.

• Tall Mast. A tall mast was necessary to provide for a larger and more efficient prop. It was also thought at the time (in error!), that a tall mast would enhance so-called "pendulum stability". It doesn't, but it does place the blades and hub bar (which may represent 15-20% of the total aircraft mass) higher above the rest of the structure, thus raising the CoM_v. The fact that we were using relatively heavy blades (Rotordynes) also helped!

• Engine Thrust. Power is king in the world of sport gyroplanes and it is not uncommon to see a single-seater sporting an engine with 400-600 pounds of thrust. We did not have the luxury of a big engine if we were to meet Part 103 weight and speed requirements and settled on a Rotax 447 with a modest ~250 pounds of static thrust. We made the engine work by keeping airframe disc loading much lower than conventional machines (~1 psf) by keeping the weight down and using a 25-foot rotor disk. In short, we had enough power to fly well but not so much extra power as to constitute a hazard!

• Engine and Re-drive Mounting. As folks began to use Rotax engines on gyros, they were often mounted inverted, with the re-drive directed upward to achieve enough prop clearance. This is bad as the engine represents at least 15% of the total aircraft mass and inverting it lowers the CoM_v while directing the redrive up raises the thrust-line. Not a good combination! We mounted the engine upright simply because that is the way such engines were designed to run (you will never see an inverted test stand in the Rotax factory in Austria!). This raises the CoM_v while directing the re-drive down lowers the thrust-line. All-in-all, a good combination.

• An Effective Horizontal Stab. I will talk more about stabs shortly, but we chose the stab from a KB2. This stab is not very large, but it does have a modest airfoil section, which makes it better than a flat plate. In the KB2 the "stab" functioned as a rock guard under the prop. We put a long tail-boom on the prototype so the stab could be moved toward the rear. It was not in the propwash, but it was big enough, along with the airfoil section to do the job of dampening any tendency toward PPO (see later discussion on stab size and location).

The result was that all these decisions, often made without reference to stability, combined to produce a machine where, in the prototype, the thrust line was about 1 inch above the CoM_v. All Gyrobees so far have fallen in the 1-2 inch offset range. If we use 2 inches - the maximum typical offset, at a thrust of about 300 pounds (assuming the use of a 503 - the biggest engine specified), the over-turning moment is 300/(2/12) or 50 foot-pounds. This is well within the stabilizing range of any of the horizontal stabs now in use (see below) and the result is an aircraft that exhibits the same stability as a well designed fixed-wing ultralight.

The task of any Gyrobee builder is not to screw up the basic design and degrade its stability. Here a list of things you don't want to do:
 

• Adding extra weight to the airframe - treat the machine as if you were shooting for Part 103, even if you intend to register it as Experimental. Extra weight can only degrade flight performance and, if it is added below the existing CoM_v  it can degrade stability.

• Alter the orientation of the engine and re-drive - these are important parameters that you mess with at your peril.

• Use a bigger engine - a Rotax 503 or its equivalent is the largest engine I recommend for the Bee and, in most cases, a 447 is just fine. If you want a more powerful engine, find another gyro to put it on.

• Blade weight - the installation of very light blades, unless compensated by the addition of a prerotator, will lower the CoM_v. Given blades currently available, this should be a non issue if you have an adequate horizontal stabilizer.

HORIZONTAL STABILIZER PARAMETERS

If you haven't strayed from the plans, your maximum thrust-line offset should be 1-2 inches. Note that if you weight in well under 200 pounds, the thrust-line may be right on or even below the CoM_v. In any case, the horizontal stabilizer the the final element of your positive stability prescription.

The critical elements for an ideal horizontal stab include:

• Adequate area and moment arm

• An airfoil section

• Placement in the propwash

• Use a 2-3 degree negative incidence (trailing edge higher than the leading edge)

All of these issues are interrelated. For example, the stab on the prototype Bee was fairly small (see above) and was placed below the propwash, but it was on a somewhat longer moment air. We always considered it to be adequate, but, in fact it was marginal. With heavy blades, like Rotordynes, it was solid as can be. However, if you further lowered the CG by using very light blades (like Dragon Wings), it didn't feel right. The key is to have a large enough stab to cover any reasonable range of CoM_v due to different blades, loading, etc.

The Watson tail, included in the documentation, meets any reasonable stability requirement. It has a total horizontal stab area of 6 square feet (a pair of 1.5 x 2 foot panels), a symmetrical airfoil section, and is placed in the propwash. Actually, it is probably slight overkill BUT, if you are going to err one way of the other, having too much stab area is the side you want to be on.

If your stab is going to be mounted below the airflow from the prop, make it somewhat bigger. Do the same if you are using a "flat plate" section instead of a real airfoil. If "bigger" isn't practical, then extend the tail boom a foot or so to get a longer moment arm.

The Completions  page will give you lots of ideas for how to approach the tail group, including the horizontal stab.

Ralph E. Taggart (taggart@msu.edu)