W&B Arms

[Utah-Jay]

During a good chin wag in the hanger today we got to talking about W&B and the arms for the different inputs for the calculations

I am building a Companion, but the 4 Place should have the same arm for the fuel tanks, front seats and tailwheel

Also, when measuring lets say a pilot seat, at what part of the seat do you determine the actual arm distance? At what position is the seat measured, rear of the seat bottom? Middle of the seat bottom? Fuel tank, middle of the tank?

From memory the leading edge of the wing is the datum for W&B, is this correct?

Thanks in advance

[Utah-Jay]

Question answered thanks to Nev as he sent me a spreadsheet

[Chewie]

Sit in the front seat and reverse engineer the arm based on the scales.

[rodsmith]

When I weight my plane, I plan on doing just that for front and back seats, front and rear of baggage area etc,

[rodsmith]

During a good chin wag in the hanger today we got to talking about W&B and the arms for the different inputs for the calculations

I am building a Companion, but the 4 Place should have the same arm for the fuel tanks, front seats and tailwheel

Also, when measuring lets say a pilot seat, at what part of the seat do you determine the actual arm distance? At what position is the seat measured, rear of the seat bottom? Middle of the seat bottom? Fuel tank, middle of the tank?

From memory the leading edge of the wing is the datum for W&B, is this correct?

Thanks in advance

From previous discussion on this topic, be careful to accurately level the plane and get a good measurement between the wing leading edge and main wheel axles. An error on that measurement has a big impact on your calculated weight and balance.

[K Fostvedt]

Hello, I've just finished my 2 place patrol and I'm having lots of fun crunching the numbers for my certification paperwork... I'd really like to hear from anyone with a 2 place Patrol, 180hp and a Whirlwind prop. Specifically, I'm looking for the weight and balance moment arms I should be using and also the recommended performance numbers for first flight and testing. Thanks in advance, Kim.

[Nev]

Here's a LINK to the downloadable W&B spreadsheet, with instructions.

Also a LINK here to one method to accurately level the aircraft for weighing and measuring station arms.

Edit to add :
The factory kitsets of each model should be identical and therefore the arms of each station should be identical. There may be a small variance for the front seat positions which of course are adjustable, but also very close to the empty CG.

Anecdotally, there does seem to be a sometimes significant variation introduced during weighing the aircraft by not having it level - something I experienced myself (which throws the forward and aft limits out significantly, and can also be seen throughout the forum with changes to empty CG position). This is what led to finding a repeatable method to level our aircraft, and therefore keep the front axle arm exactly the same - as rod smith mentions above.

[whee]

Nev has done a fantastic job walking through this stuff and sharing it with us. We are all better off for it.

You should take it a step further and determine your own CG limits during flight testing. Doing so will help keep you safe even if you didn’t get the arms exactly right or didn’t get the plane leveled correctly. Yes, your CG range should come out the same as shown in the plans but just assuming you got it right is not a safe way to operate.

[Nev]

Thanks Whee !