W&B questions

[rockwoodrv9]

I am re-doing my weight and balance and having trouble remembering what I did the first time! Mostly it is that my measurements are not the same as shown in section 14-8 of the manual.

The manual shows 91.88” from the main wheel aft of the Datum.
I get 91.0 on the Right and 90.8 on the left.
The nose wheel shows 34.5” and my nose wheel is 33.0

I have a James cowl but still use the standard motor – gear mount. I don’t remember things being an inch off the first time. Any ideas? Are there other 9A’s that are different than the drawing on 14-8?

[bruceh]

All you need is the moment arm lengths of each gear, and the weight on each gear. Your measurements will override what Van's estimates as the lengths from the datum. This will give you your airplanes total moment, CG and total weight. As long as the CG falls within Van's range, you are within limits.
You need to weigh the airplane with full oil, and empty the usable fuel.

Once you have the airplane's total moment, you can do the calculations for all of the various flight conditions - ie, gross weight, baggage, passengers, etc.

[rockwoodrv9]

Thanks Bruce.
I was concerned because my numbers were different and I couldnt understand why there are different than the manual.

[rmarshall234]

I have a 1956 bonanza and a few years ago I really took my time and did a W&B measurement - including dropping plumb bobs to establish all the stations - and it didn’t match the book numbers either. Those old Bonanzas were hand built too.

[BobTurner]

Being off by 1.5” is a lot. And everything (all three wheels) is off in the same direction. Triple check that you have properly located the datum point, measuring from the leading edge of the wing. If that’s off then Vans’ cg charts will be off too.

[rockwoodrv9]

Originally Posted by BobTurner

Being off by 1.5” is a lot. And everything (all three wheels) is off in the same direction. Triple check that you have properly located the datum point, measuring from the leading edge of the wing. If that’s off then Vans’ cg charts will be off too.

As I think this through, I did not level the plane before I took the measurements. I am not sure if that will make a difference but I will check that tomorrow. I cant see it being a 1.5" difference but maybe.

[Kyle Boatright]

Originally Posted by rockwoodrv9

As I think this through, I did not level the plane before I took the measurements. I am not sure if that will make a difference but I will check that tomorrow. I cant see it being a 1.5" difference but maybe.

It’ll make a big difference. A local RV-14 guy was very disappointed in his cg/baggage allowance until he remeasured with the plane level. It matched the factory data after that.

[bruceh]

The FAA aviation maintenance general textbook has more information than you will ever need about weight and balance in chapter 6.