In the past few years a consensus has developed that contends that the single most significant factor relating to pitch stability in sport gyroplanes is the relationship of the engine thrust-line to the vertical position of the center of gravity. If the engine thrust-line is aligned with or below the vertical CG, the aircraft will be have in a stable fashion in all flight modes. In contrast, if the engine thrust-line is located above the vertical CG, the aircraft can tumble or bunt-over if the rotor is unloaded at the same time the engine is providing significant thrust.

In the September 1999 issue of Rotorcraft, Antonio Alvarez wrote an article, "Numerical Determination of the Center of Gravity in Rotorcraft", that described how to calculate the position of the CG, both horizontally and vertically. Antonio's article was very complete and thus the math was a little overwhelming to many pilots and builders.

When downloading the program, you can save it directly to the WINDOWS directory or copy it to that directory once you have it transferred to your computer. In a bit I will talk about how to use it, but let's start by looking at what data we have to measure on the gyro.

This is a much-simplified version of Figure 1 from Antonio's article. Our goal is to calculate the position of the CG in terms of its horizontal position ahead of the main gear wheels and its vertical displacement above the ground. To calculate these two coordinates, we need to measure the following with the gyro blocked up so the keel is level, half a tank of fuel, and the pilot in the seat:

• Wheel-base. The distance between the nose-wheel and main gear axles.
• The radius (1/2 the diameter) of the main gear wheels (R) and nose-wheel (r)
• The weight measured at the nose-wheel and each of the main gear wheels.
• Measure the vertical distance between the center of the prop hub and the ground

Dimensions can be either inches or meters, but the same units should be used for all linear measurements. Weight values can be in pounds or kilograms, as long as the same units are used for all calculations.

Once the measurements shown above have been taken, you need to block up the main gear wheels:

 You need to record the height of the blocks (they should be the same under each man gear wheel, and the new value for the weight measured at the nose-wheel. The block height should range between 10 and 20% of the measurement for the wheel-base.

The basic description above is appropriate for machines that rest on the nose wheel when loaded. The procedure for tail-draggers is just a bit different.

• Instead of the nose-wheel radius, record the tail wheel radius
• Block up the tail wheel so the machine is level and record the weights at the main gear wheel and the tail wheel.
• Record the height of the tail wheel block that was required to get the machine in a level attitude.
• Measure the distance from the prop hub to ground level
• Remove the tail wheel block and measure the new weight on the tail wheel

There are two ways to run the program - either directly from Windows or under DOS.

From Windows 95/98:

• Click on START
• Click on RUN
• Type CGCALC in the window
• Click RUN

The program is self-prompting and you can re-size the Window as desired.

To run the program from DOS:

• Click on START
• Click on PROGRAMS
• Click on MS DOS
• Type CGCALC and <ENTER>
• When you are done, type EXIT and <ENTER> to return to Windows.

If you are measuring a taildragger, enter the tail-wheel weight whenever a nose-wheel weight is requested. When asked to enter the main gear block height, enter the height to which you blocked the tail wheel.

The program will give you both the horizontal and vertical position of the CG. In the case of a tail-dragger, the horizontal CG is behind the main gear, not ahead, as the program will indicate. The horizontal CG is ahead of the main gear in the case of a machine with a nose-wheel.

The program will indicate the relationship of the engine thrust-line to the vertical CG. If this value is zero or negative (a minus sign precedes the value), all is well. If the value is positive (no minus sign!), the engine thrust-line is above the vertical CG, which is not so good, as the engine thrust could result in bunt-over with sufficient engine thrust and significant unloading of the main rotor. The bigger the difference, the greater the potential risk. In such a case, you need to make sure of the following:

• Use a horizontal stab. If it is large enough and on a long-enough moment-arm, it can significantly damp and bunt-over tendency if rotor loading is momentarily reduced.
• Avoid Gusty Conditions. Strong gusts and heavy thermal down-drafts can momentarily reduce rotor loading. Avoid such conditions if possible and, if you get caught, fly at the lowest possible throttle setting and get the aircraft back on the ground.
• Keep the rotor loaded. In essence, don't do stupid things like pushing the stick forward at the top of a zooming climb.