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Battery Relocation to firewall
Posted: Mon May 24, 2021 8:15 pm
by matthewhinmann120cm
I’m new to the forum, and a new Cessna 120 owner as well! So here goes...
N120CM needs to go on a diet. The last W&B recorded has the plane at 1053 lbs E.W., of which I have verified by rolling it on scales. Since then, I removed the interior and old sound-proofing material. 14lbs worth.
The battery installed is a Concord RG-35A. 30lbs of overkill, so I would like to install something like an Odyssey SBS J-16 or 545.
My questions are, what type of battery have you all upgraded to (suggestions?), and has anyone relocated the battery to the firewall? My IA says I need an STC or field approval for all of this... The TCDS and IPC never actually state what type of battery, other than it needs to be 12v 25amps. So I thinking more of a minor alteration? Am I correct in my thinking?
I imagine that I could save a few more pounds by removing the starter wires and wire runs to the current battery by moving it all FWD to the firewall. Has anyone done this?
Any input would be awesome. Thanks
Re: Battery Relocation to firewall
Posted: Tue May 25, 2021 6:40 am
by 6643
I and my IA agree that the Odyssey battery is a minor modification. Refer to AC43-13.2b. Moving it to the engine compartment, maybe not. I wouldn't do that anyhow; moving the CG forward will slow you down!
It has been done, though. I may have a copy of a 337 somewhere.
Re: Battery Relocation to firewall
Posted: Fri May 28, 2021 4:49 pm
by matthewhinmann120cm
6643 wrote: ↑Tue May 25, 2021 6:40 am
It has been done, though. I may have a copy of a 337 somewhere.
Thanks for the information. If you have the copy of the firewall battery installation, that would be great!
Thanks for your time
Re: Battery Relocation to firewall
Posted: Fri May 28, 2021 6:09 pm
by 6643
I'll see if I can dig it up when I get back home.
Re: Battery Relocation to firewall
Posted: Sat May 29, 2021 10:13 pm
by 5099
I may be a party pooper but, don’t our birds already have a nose heavy issue? Look at all the folks using wheel extensions on the landing gear. Wouldn’t moving the battery forward add to that issue besides giving a nose heavy condition in flight?
You mentioned that your bird was overweight. This may be a dumb question but, is your plane painted? If so, how many coats of paint are on it? Just wondering.
Re: Battery Relocation to firewall
Posted: Sun May 30, 2021 1:24 pm
by a64pilot
You gain nothing by firewall mounting the battery, except the CG shift.
It’s definitely a major alteration and in my opinion the biggest hurdle would be structural. How are you going to reinforce the firewall to carry the weight and how do you substantiate it? My guess woud be by hiring a structural DER, and that’s likely way more money than it’s worth.
I think most of our airplanes are likely heavier than our W&B’s say, but they carry the weight well. The issue is engine performance, not flight handling qualities.
It really will slow you down a tiny bit and may make a nose over more likely.
On some aircraft, my Maule for instance it’s actual load capacity was determined by CG, the IO-540 would lift an astonishing amount of weight, but there was a point to where loaded heavy pushed it past aft CG limit and that made it dangerously unstable in pitch, on that aircraft ballasting to max fwd CG was good, but I suspect that a 120/140 is going to run out of HP well prior to aft CG being a limiting factor. mine does anyway.
The extra 12 lbs from a three blade prop put the Maule right at the max fwd allowable CG with two in the front seat at min fuel.
Re: Battery Relocation to firewall
Posted: Sun May 30, 2021 9:25 pm
by matthewhinmann120cm
5099 wrote: ↑Sat May 29, 2021 10:13 pm
I may be a party pooper but, don’t our birds already have a nose heavy issue? Look at all the folks using wheel extensions on the landing gear. Wouldn’t moving the battery forward add to that issue besides giving a nose heavy condition in flight?
You mentioned that your bird was overweight. This may be a dumb question but, is your plane painted? If so, how many coats of paint are on it? Just wondering.
I actually didn’t know the 120/140 tended to be nose heavy. That’s good to know.
I noticed it has a heavy layer of paint in a few places, and pretty much through out the entire plane. The decision is to strip it of its current old paint job and polish it. I think that will save us 8 - 10lbs...
Thanks for the reply!
Re: Battery Relocation to firewall
Posted: Sun May 30, 2021 9:57 pm
by matthewhinmann120cm
a64pilot wrote: ↑Sun May 30, 2021 1:24 pm
You gain nothing by firewall mounting the battery, except the CG shift.
It’s definitely a major alteration and in my opinion the biggest hurdle would be structural. How are you going to reinforce the firewall to carry the weight and how do you substantiate it? My guess woud be by hiring a structural DER, and that’s likely way more money than it’s worth.
I think most of our airplanes are likely heavier than our W&B’s say, but they carry the weight well. The issue is engine performance, not flight handling qualities.
It really will slow you down a tiny bit and may make a nose over more likely.
On some aircraft, my Maule for instance it’s actual load capacity was determined by CG, the IO-540 would lift an astonishing amount of weight, but there was a point to where loaded heavy pushed it past aft CG limit and that made it dangerously unstable in pitch, on that aircraft ballasting to max fwd CG was good, but I suspect that a 120/140 is going to run out of HP well prior to aft CG being a limiting factor. mine does anyway.
The extra 12 lbs from a three blade prop put the Maule right at the max fwd allowable CG with two in the front seat at min fuel.
That’s all good information, and stuff I didn’t think about. Thanks for that. Maybe I’ll just install the SBS-16J in the current location. I think that would give me the best results for weight reduction/C.G balance.
Re: Battery Relocation to firewall
Posted: Mon May 31, 2021 7:43 pm
by 5099
matthewhinmann120cm wrote: ↑Sun May 30, 2021 9:25 pm
5099 wrote: ↑Sat May 29, 2021 10:13 pm
I may be a party pooper but, don’t our birds already have a nose heavy issue? Look at all the folks using wheel extensions on the landing gear. Wouldn’t moving the battery forward add to that issue besides giving a nose heavy condition in flight?
You mentioned that your bird was overweight. This may be a dumb question but, is your plane painted? If so, how many coats of paint are on it? Just wondering.
I actually didn’t know the 120/140 tended to be nose heavy. That’s good to know.
I noticed it has a heavy layer of paint in a few places, and pretty much through out the entire plane. The decision is to strip it of its current old paint job and polish it. I think that will save us 8 - 10lbs...
Thanks for the reply!
I found 7 coats of paint on mine when I striped it.
Re: Battery Relocation to firewall
Posted: Tue Jun 08, 2021 12:09 pm
by meloosifah
I replaced my battery with an odyssey - removed the old battery box and installed the approved odyssey hold down bracket. I saved 1.5 lbs on the box and 7 on the battery. My IA wrote it as minor using the established PMA docs and a previous 337 for reference. I would not even consider mover the battery forward. I can’t imagine the effort would be worth the pound or two of wiring and the massive CG change. Currently I can load the plane with full fuel, two fatties and a big kid in the child seat and never get out of CG all the way to empty. I would be over gross but the plane would still be safe and controllable.