Find Your Gyro’s CG Location:
The Double Hang Test
By Doug Riley, Gyro BFI
The position of the center of gravity (CG) of your gyro is critical to its flight stability. This is particularly true of the location of the CG above or below the thrustline of your propeller. The vertical position of the CG is sometimes referred to as the “vertical center of gravity” or VCG. This term, while handy, is a bit inaccurate: there’s only ONE CG (not a vertical kind and then some other kind). What we mean by this term is the number of inches that the CG is located above or below the thrustline. Knowing this number for your gyro can save your life.
The VCG can be calculated in the same way that CG location in the horizontal direction is calculated on other aircraft. It is easier to find the CG location by a fairly simple physical test, however. Here’s how to do it.
1. What you’ll need.
A. A camera on a tripod. You can use a digital camera, a Polaroid or conventional film, as long as you can make prints. If you have an assistant to operate the camera, good, because you’ll be in the gyro’s seat during the tests. Without an assistant, you’ll need a timer delay or long remote release for your camera, so you can climb aboard and snap a picture of yourself.
B. A hang-test setup. This is the same rig that you’d use for the familiar single hang-test. It consists of an overhead beam or tree branch high enough and strong enough to allow you to hoist you gyro with you aboard so that the lowest point on the gyro is clear of the ground. Typically, people use a “come-along” or block and tackle with at least a 3:1 ratio. A really neat way to do it is to use the bucket of a backhoe!
Pick a location and time when the air will be calm around the test setup. During the hang test, the gyro will weathervane into any breeze. In a pinch, you can stop the weathervaning with a stake in the ground against which a gyro wheel can rest loosely.
C. A hoisting sling. This may be the unfamiliar part. You must create a second hang point on your gyro other than the teeter bolt. The point should be at least a foot away from the teeter bolt and, in fact, the further away, the better. As you change the hang point, however, the gyro’s hang angle will change. The angle could get bizarre if, for example, you hang the gyro from its nose. To keep the angle reasonable, make a sling from heavy rope or metal cable, leading from a strong point on the nose to the top of the mast. Select a point along this sling for a second hang point, as shown in the photo.
CAUTION: The sling will impose some unusual bending loads on your frame. Watch the deflection of the mast and keel and stop if you see excessive strain. The sling should not be taut; the tighter it is, the more the bending load on the frame. It should have enough slack so that its peak (where you attach the hoist) is up above the blades.
D. A plumb line. The line must be about 8 feet long. Its string must be thick enough to show up in your photos.
2. Test Procedure.
The first test is a conventional hang test, without the rotor blades. Position your gyro under the hoist. Use a spare teeter bolt or a Grade 8 substitute bolt through your teeter towers for the hoisting. Set up the camera and tripod so that the gyro’s image fills the frame. Hang the plumb line from an overhead location so that it’s in the picture, between the camera and the gyro. Position it so that the line appears to extend down right through the hang point as the camera views it.
Fill the gyro’s fuel tank. Hoist the gyro high enough so it will clear the ground with you aboard. Then climb into the seat, get the machine to stop swaying, make sure the machine is broadside to the camera and snap a photo. Check your picture if you have a digital or Polaroid camera. Make certain that machine is not touching the ground anywhere and that it was not swaying when you took the shot. Take extra shots if in doubt.
Next, install the rotor blades and use a piece of rope passing from the middle of one blade to the middle of the other as a truss to pull the blades up into roughly their “coned” position (note that I didn’t do this when I took these pictures; the suggestion comes from other gyronauts and they are correct). Install your sling from the nose to the mast or rotor head. Double-check the sling’s strength and security, then hoist the gyro again. The gyro will hang tail-down this time. Make sure the sling does not press hard on the trailing edge of a blade (note the cloth cushion in the photo). With the plumb line still in the camera’s view and still lined up with the hang point, climb aboard and take another picture. Again make sure the gyro is broadside to the camera, is hanging free and is not swaying or twisting in the breeze.
Finally, here’s the easy part. Obtain prints of the gyro hanging in both positions. Stack a print of one position on top of a print of the other. Hold the stack over a strong light. Rotate one print until you’ve aligned the two images of the gyro so that they mate precisely. The point where the two images of the plumb lines cross is the CG. Stick a pin through both prints to mark the CG location.
The CG should be on or above the prop thrustline. The prop thrustline is a line extending forward from the center of the prop and square to the prop disk. If the CG is below the prop thrustline, you should move components of the gyro to raise the CG until the thrustline and the CG line up.
A horizontal stabilizer, specially set up, can compensate for small misalignments of CG with prop thrustline – misalignments on the order of 2 inches. That’s the topic of a separate article, however.