What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

PC680 vs PC925 in RV-8

Airzen

Well Known Member
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?
 
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.
 
Thanks

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?
 
Alternatively, add lightness at the front. :D

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
 
To give you an idea, I have IO-360 with
Parallel (180HP) or Angle (200HP) valve variant? Makes a lot of difference :eek:

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.
 
details

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.
 
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.
 
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
 
Saville

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.
 
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
 
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.

The Sky-Tec is essentially an automotive starter.

One way to drop a little more weight is to go with P-mags. They are 1.5 lbs lighter than a standard mag. Install two and you drop another 3 lbs.
 
I replaced a Concorde RG-25XC with a PC-925 in my RV-6. I needed the extra weight on the nose to help out my CG. I had to trim some metal off the battery box cover to give some clearance near the positive terminal, and fabricate new spacers inside the cover to properly secure the different outer dimensions of the PC-925. With the Skytec starter, this battery cranks the O-320 engine super fast.
 
Although you said you 'will have' a rear mounted battery, indulge me for a moment...
I have an RV-8, which I mounted the battery in the rear. It offset my Mazda turbo rotary well, and my CG was just right.
I flew the rotary for 16 hours, it was powerful, it never failed, but it wasn't for me.
I installed a parallel valve IO-360, and used a Skytec lightweight starter, a Superior front induction sump, Airflow performance fuel injection and my own direct induction chin scoop. I installed a CATTO 3 blade prop. The engine is conical mount and I use one P-mag. I use a Denso mini alternator with the small diameter belt pulley type flywheel.
In short, I did everything I could to reduce weight.
So I moved the PC-680 battery to the firewall. The CG is still aft of the previous CG with the rotary engine, and I removed 3.2 pounds of battery cable!
Best of all, I don't have to fish around in the back to access the battery. I prefer having it on the firewall for ease of access.
A side note: Many of use build our planes, then do the weight & balance and hope for the best. If your EAA chapter has a set of scales, you should weigh & balance the aircraft at some progress milestones, like with wing & engine on, and do a few calculations. Then decide where to put your battery, strobe power supply and ELT?
 
Another data point for your consideration:

Angle valve
Pmags
Whirlwind CS
Grove gear
PC680 in rear

The last thing in the world I want is more weight aft. As it is there is literally no way to load my plane and get to the front of the CG envelope. Overload the front baggage with 75 lbs and leave the rear seat and baggage empty and I'm still not there but I can go off the back end quite easily. My thought was the "heavy" angle valve needed the weight in the rear. The Grove gear, light prop, pmags and crank without counterweights pretty much negated that I guess.

I've never felt I needed more battery, despite 10:1 Pistons.
 
Weight is the enemy. You may want to save even more with one of the Lithium batteries. I saved over 12 lb with a Shorai and havn't looked back.

Bevan
 
Weight is the enemy. You may want to save even more with one of the Lithium batteries. I saved over 12 lb with a Shorai and havn't looked back.

Bevan

The weight savings appeals to me....but the failure mode does not. Especially with my battery inside with me. I know the new ones have electronics in them to prevent meltdown fires but one thing keeps coming to mind. My Dynon D6 was working well until the electronics failed. My Pmag was also working quite well until its electronics failed. Both of those pieces have a lot more expensive electronics in them than the new batteries...but neither of them burst into flames when the electrons got confused.

I'm going to let some of the braver in here life test that tech for a while longer before I jump on the wagon. Probably quite a while longer. Still, I agree with you.....weight is the enemy so I am tempted.
 
I'm not sure about the normal location for the battery on the RV8 but I would think the logical place for a very light battery is forward of the firewall. Shorter cables also equals less weight. My firewall is well sealed from the cabin. The failure mode of LiFePo batteries is rather benign compared to the earlier flavours of lithium batteries as I recall. Do your own research.

I had trouble getting fuel to the engine one day cause the lines had been pumped dry for W & B. Forgot to prime them. My hangar neighbor thought I could taxi away on the battery power alone. :eek:

Bevan

My opinions only. Do your own research.
 
Back
Top