An aircraft CG range is the designer's planned relation of mass to lift. The aircraft designer sets a datum plane typically forward of the nose of the spinner - that makes for convenient positive number arithmetic. He then measures the distance aft of the datum of the empty CG of the prototype, setting safe foreward and rearward CG limits by testing that prototype. No matter how closely the plans are followed, EAB airplanes are going to vary from the prototype. If the build varies in the location of lifting surfaces it has to affect CG range. That's why it makes sense to double check measurements.

Builders agree that for the Rutan-derived canards the important reference plane is the wing root leading edge ("WRLE"), since it is a fixed point as regards the wings and thus the center of lift. The plans assign that point FS 113.9 - conveniently it falls at the outmost corner of the strakes and can be measured wings off. Given the WRLE, what are the stations for the leading edge of the canard, our other lifting surface? And what are the stations for the main and nose gear (reference points of the scales), and the several stations at which weight can be added or removed (ballast, pilot & copilot, fuel and rear passengers)? Time to do some measuring.

First step, find someone smarter - in this case Marc Zeitlin and Joel Ventura. I distilled their work into a 3-step process. First, set the WRLE and define the location of the landing gear relative to it. Second, set the aircraft on scales and set the empty CG. Third, measure the actual arms for the stations of interest. Here's the step by step.

1. On a clean floor, level the plane. The first several steps involve measuring distances and can be done wings off.

2. Drop a plumb line from WRLE to the floor and mark the spot (right & left). The plans define this as 113.9 inches, and I used it as the reference point for all other stations.

3. Drop a plumb line from the center of the main gear axle, mark the spot and measure the distance forward of the WRLE (for me, 4.1 inches for both left and right axles, for a main gear fs of 109.8). The plans call for 110, and the template draft POH calls for 109.5 +/- .5. The result is between plans and the POH and within tolerance. The 0.2" bias will very slightly increase the moment arm of the engine relative to the gear, so I might rotate 1-2 KIAS faster than otherwise. The discrepancy has no effect in the air. The landing attitude might be a degree or two nose higher and 0-1 knot slower. I might need to be just a little bit more careful about tip back of the aircraft while moving around the hangar.

4. Drop a plumb line from the center of the axle of the fully extended nose wheel, mark the spot and measure the distance forward of the main gear on the center line. Plans say FS16, which is 97.9 forward of the WRLE reference line. Mine measured at 96.14 - I will fully extend the gear and remeasure.

5. Drop a plumb line from the leading edge of the canard at the side of the fuselage, mark the spot (left and right). The plans say 18.7 but the manufacturer provided draft POH calls for this to be at 18.6, +/- .54. This is 95.3 forward of the WRLE reference. Mine measured at 81.4 - I absolutely need to remeasure!

6. By now there is a lot of tape on the floor! Snap chalk lines and make the measurements. Measure twice. At the end is determining if the build is in conformity with plans, and if not, that the variance is known and not a flight hazard.

7. Now it is time for the scales. My EAA chapter has a set of vehicle scales that are not certified but accurate within a half pound. The airplane should be ready to fly - all standard equipment and normal oil. I hold that you need to have installed all standard equipment - the ELT, POH, et al. Add 5 gallons of fuel and drain all usable. (With aft fuel sumps, the higher risk is level flight - drain with the aircraft level.)

8. The next step sets the arm of the various stations where weight will come and go. The following sequence is keyed to standard definitions of empty weight and to keeping the plane safely on the scales. For each step, add a _known_ weight at the given station. Write it all down - each measurement includes each of the 3 scales + the weight added.
- Empty weight and CG. This may require 50 lbs on the nose wheel to avoid a tip back.
- Add 100+ lbs at the pilot station and remeasure.
- Move that 100+ lbs to the copilot station and remeasure.
- Move that 100+ lbs to the right rear station and remeasure.
- Move that 100+ lbs to the left rear station and remeasure.
- Add max weight (24 lbs) at the most forward ballast station and remeasure.
- Add 50 lbs at the standard ballast station (for me, forward of the nose wheel housing) and remeasure.
- Move that 100+ lbs to the right strake, centered on the fuel cap, and remeasure.
- Move that 100+ lbs to the left strake, centered on the fuel cap, and remeasure.

9. Do some high school arithmetic and determine the station arm for each of the points.

10. Develop a cg worksheet or modify the Zeitlin template. ***Test the worksheet*** before leaving the scales - does the calculated CG match the actual? If so, load the same into Foreflight.

Going forward, look at the placard above the start button. T/O = WT + CG + DA.