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Weight and Balance Accuracy

Prior to the first flight, all amateur built aircraft are weighed, the wheel distances are measured from a datum, and the CG and empty weight is calculated from the resulting wheel weights.

The Bearhawk uses the wing Leading Edge as a reference datum for these measurements, and Bob's plans provide the datum for leveling the aircraft.

Leveling the aircraft can be somewhat more difficult than it seems. Most builders use a spirit level, plumb bob, laser leveler, or clear hosing tube filled with water. However the range of measurements of the location arms suggests a large variation among builders. Given that all of the KITSET airframes are coming out of the same factory jig, the measurements should all be identical.

I've added a spreadsheet to simplify the weight and balance. I've used what should be an easily repeatable method of levelling the aircraft, and measured all the location arms using several different methods.

How to level the aircraft

Find the datum specified on the aircraft plans and stick a piece of masking tape to it (A).

Next, measure 510mm (20 inches) above the tape, and place second piece of tape (on the door) - (B).

Then find the point near the tail where the lower longerons meet the vertical tail post (C). Raise the tail until point (C) is level with point (B). Check that the aircraft is now level.

Then drop a plumb bob from the leading edge of the wing to the floor and make a mark. Next, drop the plumb bob from the center of the front gear main axle to the floow and make another mark. Confirm that the distance between these two marks is 65mm.

This measurement tends to be the most critical because it is located at maximum vertical distance under the leading edge. Most other measurements are largely horizontal, so even if not perfectly level there won't be much variation.

Hence, a small inaccuracy in the attitude of the aircraft can cause a sizeable error in the main wheel position, and the empty CG location. The resulting CG limits of 10.5 (forward limit) and 22.5 (aft limit) may then be out by up to several inches. It's for this reason that we need to be accurate in the initial measurements, followed by incrementally loading to the resulting forward and aft CG limits during flight testing to confirm the calculated CG limits.

The distance between the lines marked on the two pieces of tape below should be 510mm (20 inches) when the aircraft is level. You can actually see the red horizontal line from the laser level.

The photo below shows where the lower longerons meet the tail post.


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