PC680 vs PC925 in RV-8

[Airzen]

I will have rear mounted battery and it seems like PC-680 is far more popular in RV-8s than anything out there.

However, I am also reading posts where several RV-8 builders have had issues with CG being to far forward. Consequently, I am wondering if it makes sense to consider 10lbs heavier PC-925. It looks like it'll make CG go aft around 0.5 inches, plus added benefit of more capacity.

Has anyone considered PC925 in RV-8?

To give you an idea, I have IO-360 with constant speed Whirwind 200RV, raven inverted oil and glass instrument panel. ELT and Strobe Powersupply are mounted in rear fuselage.

Any opnions/suggestions?

[DanH]

Originally Posted by Airzen

Has anyone considered PC925 in RV-8?

Yes, I use one. Helps the solo CG (390 and Hartzell BA), and the extra capacity (cranking and endurance after alternator failure) is nice. Fits the stock battery tray when mounted on its side.

[Airzen]

Thanks Dan.

I hope you wont mind me asking a couple of more follow-up Qs.

1) Whats your CG with the configuration you mentioned?
2) Is it favorable for Aerobatics?
3) Any idea how heavier IO-390 is when compared to IO-360?

[chris mitchell]

Alternatively, add lightness at the front.

You already have a light prop. Use a gear-driven alternator and a lightweight flywheel/starter ring. The added benefit is that there is no alternator belt to break. I'll be doing just this in the RV-4 that's now lurking in my garage.

The PC 680 is 10 lbs lighter. A lightweight flywheel is only 2.95lbs. Standard flywheel weight??? but a lot more than 3lbs!

Chris

[Andy Hill]

To give you an idea, I have IO-360 with

Parallel (180HP) or Angle (200HP) valve variant? Makes a lot of difference

Whatever, I suspect the Whirlwind means Fwd Battery probably best bet, and certainly not a heavy rear Batt. Conversely, a 200HP with Hartzell likely needs all it can in the rear.

[Airzen]

I got

- IO-parallel valve (180 hp)
- Sky-Tec Light Weight Starter (7.8lbs)
- Plane-n-power altrenator (6.5 lbs)

PS Chris: Not sure I can shave more off the nose without going to automotive starter as you mention (and I am not confident myself to do those kinds of mods As for alternator, I think at 6.5 lbs its already pretty light.

PS Andy: you might be right, thats why I am seeking info.

[DanH]

Originally Posted by Airzen

Thanks Dan.

I hope you wont mind me asking a couple of more follow-up Qs.

1) Whats your CG with the configuration you mentioned?
2) Is it favorable for Aerobatics?
3) Any idea how heavier IO-390 is when compared to IO-360?

1. More to that question than just engine/prop choice.
2. Heavy pitch forces when solo. More pleasant to fly with a passenger, which was the primary mission goal.
3. They say 8 lbs.

[Saville]

Originally Posted by DanH

Yes, I use one. Helps the solo CG (390 and Hartzell BA), and the extra capacity (cranking and endurance after alternator failure) is nice. Fits the stock battery tray when mounted on its side.

I'm looking to install a PC-925I would like to install a PC-925 in my RV-8. But the only way it will fit in the standard aft compartment battery tray is if I set it like this:



Closing the door will short those lugs.

When you say "mounted on it's side" which direction did you have the lugs face?

The battery came with smaller bolts which fit the smaller diameter thread in the battery itself, and have no upper, wider diameter, lug.. That will keep the lugs from hitting the metal door.

In addition, I'd line the door with rubber just in case.

But I'm wondering if the smaller bolts, along with a lock washer, will stay tight?

thanks

[longline]

I installed a PC925 in a C-190 years ago. Those 8mm bolts were never, ever, an issue. They stayed tight until they met a wrench. There was more than just a little vibration involved with this aircraft, too.

This is a superior battery. I am using dual PC680 batteries in my RV8, which has yet to fly.

Naturally, the cable termination was different than standard, but we do not need to worry about that in experimental aviation.

[Saville]

Originally Posted by longline

I installed a PC925 in a C-190 years ago. Those 8mm bolts were never, ever, an issue. They stayed tight until they met a wrench. There was more than just a little vibration involved with this aircraft, too.

This is a superior battery. I am using dual PC680 batteries in my RV8, which has yet to fly.

Naturally, the cable termination was different than standard, but we do not need to worry about that in experimental aviation.

So you went with a cable end that had a smaller hole?

What was the order of stacking?

flat washer, cable, flat washer, lock washer?

Or cable, flat washer, lock washer?

thanks