Want Best Technique for Accurate Weight & Balance
 

When it comes to technique in performing Weight and Balance on my RV-7A, I have a few questions, while I wait for my scales to arrive....

A) Is it acceptable practice to take the arm measurements to the nose wheel and each main wheel prior to putting the airplane onto the scales?

I feel I can get more accurate distance measurements off of the scales with the aircraft in a natural stance on the floor but that creates a new question....

B) The aircraft has to be perfectly level while on the scales but normally sits slightly nose high in the natural stance on the floor. Will the datum to wheel measurements change significantly with the airplane sitting perfectly level?

I will have each wheel on its own scale. The scales are 3" high so I thought I'd construct looooong wooden ramps to roll the airplane onto the scales as opposed to jacking it up and letting it down on the scale. I feel rolling it onto the scale is better to prevent side loading of the scale. I also feel that the arm measurements taken prior to rolling on the scale will remain fairly static if I can roll the airplane as opposed to jacking it up.

My plan is to roll it onto the scales and check for level. If it is out of level, roll airplane off of scales, add shims under appropriate scales and repeat until I can achieve a level stance.

Is my thinking okay on this? Any way to simplify? Are there better techniques or more steps that I should consider?

You can fine-tune the leveling just by adding or releasing air from the tire(s).

For the measuring only bit you can level the plane with the nose wheel removed.

I found this, and a minor tire pressure adjustment in the mains, got my -6A level. With no scale platforms in the way it was much easier to drop plumb bobs and use squares to translate the axle positions to marks on the floor.

For the final weighing (not yet done) I can now use my wing jacks and lift the plane and just use blocks under the main wheels to remove the approx. 5 degree aft down stance of the -6A.

I did find the construction manual dimensions to be a bit off.

http://www.vansairforce.com/communit...d.php?t=142975

The datum is defined and correct only while the plane is exactly level.

For the 5 degree longeron slope I mentioned in the previous post the wing leading edge plumb bob mark on the floor would have an error of around 2 inches.

Thanks Bob and Gil! Good advice!

Typically you have to raise the mains about 2 inches to level a 7A model. Assuming the NLG stays in the same position, I'm not a math major but I guarantee you that raising the mains 2" does not move the wing LE (or datum) anywhere near 2".

I think it does...:)

Not 2 inches on the floor, but an effective 2 inches.

If the fuselage is sloped, then the real LE datum would be where a 5 degree sloped plumb bob would fall - if gravity could be defied....:)

It would be a major error in the datum line measurement.

Excuse my ignorance but what it the world is "an effective 2 inches" ??
Is this some kind of virtual measurement you are talking about?

We use plumb bobs to make marks on the floor and then take our ARM measurements from there, I'm not aware of any other method to measure ARMS.

From the FAA W&B hand book the method is:

Determining the Center of Gravity
When the aircraft is in its level flight attitude, drop a
plumb line from the datum and make a mark on the hangar
floor below the tip of the bob. Draw a chalk line through
this point parallel to the longitudinal axis of the aircraft.
Then draw lateral lines between the actual weighting
points for the main wheels, and make a mark along the
longitudinal line at the weighing point for the nose wheel
or the tail wheel. These lines and marks on the floor allow
you to make accurate measurements between the datum
and the weighting points to determine their arms.

This is the way I have done it. Only reason Plumb Bobs are in my box.

i

Walt, that is the correct way.

If you check my post I was trying to explain the errors that would occur if the plane is NOT leveled fore and aft.

The approx. 5 degree slope of a -6A would give a very significant error if it was not leveled. The plump Bob mark from a non-leveled LE would be about 2 inches off.