Weight and Balance today.
Below is a description of the process, which worked, but I wasn’t exactly pleased with the answer. As they say, numbers don’t lie.
- Set up the laser, and align it perpendicular to a long straight edge taped to the floor
- With the laser as a reference, put one scale under each of the main wheels
- Align the plane with the laser such that the beam crosses the center of each main wheel (chocks worked well here)
- Level the airplane with a scale under the tailwheel
- Extend a tape measure such that one end is aligned with the laser and the other is well past the center of the tailwheel
- Find the distance between the leading edge of the lower wing and the center of the main wheels using a plumb bob, ruler and laser beam (this is because the manufacturer references the CG as a distance back from leading edge of the lower wing)
- Move the laser along the long straight edge until the beam crosses the center of the tailwheel
- Find the distance from the center of the main wheels to the center of the tailwheel using the laser and the tape measure
- Power up the model such that the control surfaces are in their neutral position, and then record the weight of each wheel
- Add the weights together to find the total weight
- Calculate the CG from the main wheels using the formula: Weight of tailwheel multiplied by the distance between the main and tail wheels, divided by the total weight
- Subtract the distance from the main wheels to the leading edge of the lower wing and compare to the manufacturer’s suggested CG
- Reposition equipment as needed, reweigh the model and recalculate the CG
- Rinse and repeat as necessary
My first result showed the CG as too far forward, by almost 1 inch.
The second result was better for the CG, within a quarter of an inch.
Unfortunately, the second result was after I repositioned the batteries back in the tail, which I had hoped to avoid.
During the renovation, I had moved the engine back by 1/2” using shorter spacers, and rearranged all the electronics.
Good news is that it’s nice and tidy, but the bad news is the battery location.
I’m not sure if the weight is good or bad. Should a 100cc biplane weigh 30 lbs???
From the manufacturer’s manual, the weight should be between 26.5 and 27.5 (RTF).
