Magneto Arm, weight and balance revision

[Mike S]

We're installing new magnetos with 3.7 lbs per mag weight savings and I'd like to revise my weight and balance calculations. What is the arm for the mags? I suppose I could measure back from the propeller, as that arm is listed in the TCDS, or from the leading edge. Just wondering if anyone would like to share.
Thanks,
Mike

[3Dreaming]

Re: Magneto Arm, weight and balance revision

If I was doing it, unless you know the current W&B is accurate I would weigh the airplane to get a correct W&B. But I have access to scales.

[Mike S]

Re: Magneto Arm, weight and balance revision

If I was doing it, unless you know the current W&B is accurate I would weigh the airplane to get a correct W&B. But I have access to scales.

,
I agree, weighing it would be the best. But the current Weight and Balance was done in 2015 and I have no reason to believe it is not accurate.

[Edwin Otha]

Re: Magneto Arm, weight and balance revision

Most times the weight is reduced from the dry weight of the engine. Lots of stuff can be added and removed from the engines for applications.
EO

[woodmw]

Re: Magneto Arm, weight and balance revision

Mike,

Don't know much about the different brands of mags. Can you say the before and after brands you're swapping to gain 7+ lbs? (edit: of useful load)

[Guest]

Re: Magneto Arm, weight and balance revision

We're installing new magnetos with 3.7 lbs per mag weight savings and I'd like to revise my weight and balance calculations. What is the arm for the mags? I suppose I could measure back from the propeller, as that arm is listed in the TCDS, or from the leading edge. Just wondering if anyone would like to share.
Thanks,
Mike

Hi Mike,

Couple of thoughts, item 26/ 402 on TCDS is cabin air box and it's at -22" that gets you in the ball park and gives a sanity check as the firewall and the heat box are easy to measure to.

I like your idea about measuring back from the prop datum. Remember that the prop datum is going to be the distance from the leading edge to the prop center of gravity.

That's probably at the middle of the prop's thickness and ditto for the magneto so you want to measure to the middle the the mag (approx.) in the fore and aft direction.

Have you looked at the Continental engine TCDS to see if they list anything there?

Dave R

[Guest]

Re: Magneto Arm, weight and balance revision

Mike,

Don't know much about the different brands of mags. Can you say the before and after brands you're swapping to gain 7+ lbs?

He said he was loosing 7 lbs. Mike.

Dave R

[woodmw]

Re: Magneto Arm, weight and balance revision

My bad. Didn't proof read before clicking.(or after posting for that matter) Meant 'gain 7+ lbs useful load'. Anyway, just curious as to which mag brands would be so different in weight.

[Guest]

Re: Magneto Arm, weight and balance revision

I bought a light weight started for my champ to save 5 lbs spent about $600 to do it!

I got a light weight dry cell battery and saved about 4 lbs for about $200.

All together that's about $800 for 9 lbs. Golly LOL what's wrong with me.

Dave R

[PA1195]

Re: Magneto Arm, weight and balance revision

The Continental overhaul manual available online shows prints of various dimensions. I didn't note one for the CG of the magnetos but various components and parts/mating surfaces of the engines are described. To get from that to the datum would take something like the engine CG which is available, prop flange on the crankshaft, or engine mounts. A leveled fuselage and line from the datum with a dropped plumb should work also.

Gary

[Mike S]

Re: Magneto Arm, weight and balance revision

Can you say the before and after brands you're swapping to gain 7+ lbs? (edit: of useful load)

Right here in black and white, and now my bank account is red all over.