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Weighing the plane

Before the inspection, it’s required to weigh the plane to, well, know how much it weighs, so you know how much stuff it can carry.  Additionally, it’s important to know where the center of gravity is, because after adding people, baggage, and fuel, the resulting center of gravity must be within a certain range for the airplane to be controllable.

First step is to jack up the plane so the scales can go under the wheels.  Dave Parsons (RV-7 builder) gave me his homebuilt wing jacks, which are basically hydraulic  jacks on a wooden stand.  The jack points are at the wing tiedowns.

After getting the jacks precisely positioned and the wheels chocked, I started lifting, alternating between the left and the right sides.  The nosewheel stays on the ground for now.  Here’s a picture taken just after I got the wheels high enough to slide the scales under.

With the scales under the main wheels, I lowered the jacks.  It’s also important that the plane is level when it’s weighed (and when it’s sitting on the ground, it’s not level).  Miraculously, with the main wheels on these scales and the nose wheel on the floor, the plane is exactly level.  However, the nosewheel also needs to be weighed, and with a 3rd scale under it, the plane would no longer be level.  My plan then was to note the weight on the main wheels with the nose on the ground, then put the nose gear on a scale and add/subtract from the nose weight based on how much the weights on the main wheels changes.

Here’s the plane sitting level.  Not sure whether it’s clear in the picture, but in person it definitely looks different.  The tail is much higher.

To lift the nose, I needed to pull down the tail.  To do this, I rigged something up with a furniture dolly, sand bags, and a tiedown strap.

Tail goes down, nose goes up.

For the nose, all I had was a bathroom scale.  It’s the electronic kind that you have to “tap” on to activate, turns off after ~10 seconds of inactivity, and “locks” when it has a stable weight on it.  This meant I had to tap on the scale, then run around to the tail to lower the nosewheel.  I also had to try to lower the nosewheel as slowly as possible so it didn’t slam down and break the scale.  It worked okay.

After that, I raised the wheel again, removed the scale, and put the nosewheel back on the floor.  The other important part of this process is measuring where the wheels are in relation to the airplane.  Due to manufacturing inconsistencies in the gear legs and to a lesser extent other parts, there are slight variances from plane to plane.  I used a plum bob to mark on the floor where the leading edge of the wings were, then measured how far each wheel was from that.

Strangely, when I put the nosewheel back on the floor, the left main wheel weight was the same, but the right increased by 4 lbs.  This could have been due to side load on the wheels–not sure.  I decided to take the average of the two measurements.  Also interesting, the second weighing had the left and right main gear weights exactly the same.  Here’s a picture of both scales showing 616.

The last step was to jack the plane back up, remove the scales, and lower it again.  Here are the results:

  • Left main: 616 lbs, 123.99 inches aft of datum
  • Right main: 614 lbs, 123.84 inches aft of datum
  • Nose: 376, 49.94 inches aft of datum
  • Total weight: 1606 lbs
  • CG: 106.6 inches aft of datum

Note the plane was weighed with everything on, except the wheel and leg fairings, and of course without paint.  These items will add weight, but still think we should end up with a plane that’s not too heavy.  Note we also have a full interior installed.

This is very close to what Van’s shows in their sample data, which is reassuring.  Thanks to Joe Hull for letting me borrow his scales.

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