Weight & Balance AFT CG Concerns

[lutorm]

I know this is an old thread, but now in 2024 there are two more avenues to mitigating the aft CG: removing the elevator counterweight per Sling notification #9 and swapping out the 915 for a 916.

The counterweight is 1.22kg at arm 5755mm. Removing this moves the cg forward by about 14mm for an empty aircraft and 7mm at MTOW.

The 916 weighs 75.44kg vs 74.23kg for the 915. The engine CG station is essentially the same (377mm instead of 376mm) so this increased weight also helps move the CG forward.

With the external alternator, no counterweight, and a 916, the rearward CG at max weight is significantly reduced. Unless you have a very heavy couple up front you'll still run out of CG before you run out of max weight in the rear seats, but using the N113EB example above you can have 150kg up front and 141kg in the rear seats (no luggage) before you have a CG problem. With the counterweight and a 915, CG starts running off at 134kg in the rear seats.

The drawback is that you start needing ballast in the rear if you fly by yourself. With just a 75kg pilot and full tanks, you need 8kg in the luggage compartment to keep the CG from going out the front limit. To me, this seems preferable since it doesn't impact the utility of the aircraft in the same way as being unable to use maximum weight does. It's mostly an inconvenience.

I wasn't going to remove the counterweight but this calculation makes me think I should reconsider.

[TSiDreamer]

I wonder if you could shave additional ounces/grams by not only removing just the counterweights, but the lever arm structure, as well. Once you remove the weights, the purpose of the lever arm goes away. On the other hand, retaining the lever arm mass may be necessary for a safe flutter margin.

Mike

[Magoomba]

Aft CG was a real concern of mine. For starters, my wife are both relatively small people (<300 lbs, combined), I have the parachute and the air conditioner system designed by CAB/Goodrich Aviation that installs in the rear cargo area (approx. 35-40 lbs). I added the extra alternator, both for the added power for the a/c, and the extra forward weight. CAB designed a placeholder for lead weights in front of the passenger side rudder pedals to help offset the rear bias of the parachute and A/C. With 25 lbs of lead in the holder, I'd really have to overweight the back seats and cargo area to find myself aft of CG, and not until getting low on fuel.

The most-common loading configuration I'll have is wife and me up front with A/C unit in the rear (in summer). By my calcs, we can carry just shy of 200 lbs of luggage and gear with the rear seats folded down and stay in CG envelope from, from full tanks to empty.

Another common load will be my son and me up front, with our wives in the rear. We'd have room for some light baggage and stay in envelope from full to empty tanks with the A/C in the back.

My real-world loading calcs only get challenging when trying to fly with heavy passengers. I end up getting over max gross around the same points as aft CG rears its head.