Weight and Balance - Main Gear Arm?

[RONSIM]

Just performed a new weighing on my -6A to re-do the weight and balance calculations. The datum for the (1992 vintage, tip up) -6A is 60" forward of the wing leading edge, which, on my airplane puts it right at the spinner tip (IO360, Hartzel CS). When I take the measurement from the datum to my main gear axle, I get right at 81.88 inches.

I ran across a W&B calculator for a -6A, on the web (or in the forum), which had the main gear arm at 84+ inches aft of datum. I am sure my measurements or correct, so where does this 84+ inches come from?

FYI -- My airplane - empty weight, no fuel, full oil, level, in flight configuration:

Nose Gear - 301lb
Left Main--- 442
Right Main--446

Total 1189 at 68.17 inches (Allowable range 68.7 to 76.8)

(As soon as I put fuel in it, CG moves into allowable range)

 

[WAM120RV]

Arms

Remember RV 3.4. and 6s are all built as one offs so it is unlikely that any two will have exactly the same measurements.

To help you ensure yours is correct,if you put the AC in the flight position then:

1. used a vertical level to drop down from each wing leading edge and made a mark on the ground.
2. Measured 60 inches forward of this mark and then made a line joining these two datum marks, establishing a datum line to measure from.
3 Using a vertical level and marked the position of the centre of each axle on the ground.The centre screw in each wheel pant is good for the mains.
4. Then measured from the datum line to each axle centre marked on the ground.

Then your measurements are good and these are what you should used for your calculations, not someone else's from the web which may be different to your Aeroplane.

 

[Toobuilder]

Remember that the datum is often "in space" rather than lining up with a particular location on the aircraft. My work airplane has its datum (F.S. 0.0) several feet in front of the nose, and my Rocket's spinner tip is 3.5 inches behind the zero datum. So by default, the "hard" datum on Vans aircraft becomes the LE of the wing. There's no problem using that as your baseline measurement as long as you remember to add the 60 inches (or whatever value is appropriate) to all subsequent measurements.

I do agree that having the gear a few inches off is well understandable between aircraft of this vintage. It drives home the point that W&B is a very aircraft specific process - taking measurements at face value off the Internet can lead you down the wrong path.

 

[ChiefPilot]

Was the aircraft level when you made the measurements? Was their weight on the wheels? Mine measured 25 and 1/16th inches behind the leading edge of the wing with the aircraft level and sitting on the main gear, for a moment arm of 85.0625.

 

[Mel]

Datum is set by the user! It's only a reference point.

I use the leading edge of the wing as my datum. The only reason for using a point forward of the airplane is to avoid using negative numbers.
My opinion is that if you can't do basic geometry, maybe you shouldn't be doing weight & balance calculations. But that's just me.

On my airplane, in flight position, the main wheels are right on the datum.
Guess what! Zeros make calculations easier.

 

[RONSIM]

Another builder

had published his measurements on the main gear arm and they were within a quarter inch, or so, of my measurements -- so, I think I am good. Airplane feels good, flys great, and keeps a smile on my face!

Happy Thanksgiving, to all!

Ron

 

[AlexPeterson]

I seem to recall that mine was different from the plans. Leveled the longeron using water in a long piece of clear tubing, weight on the gear, dropped strings from the leading edge and the axles. String and gravity doesn't lie...

 

[Mel]

I seem to recall that mine was different from the plans. Leveled the longeron using water in a long piece of clear tubing, weight on the gear, dropped strings from the leading edge and the axles. String and gravity doesn't lie...

As long as you have the string sufficiently ballasted.