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W&B issues?

Lufthans

Well Known Member
Hi guys

Question on W&B. Or rather- the balance aspect of it:

I am rebuilding my RV3B. I bought it in (somewhat) flying condition, and during the one flight that I did, elevator trim was pretty much neutral.

I have since taken the plane apart, all the way to tail and wings off, and I am working on a weight shedding program.

Heavy aluminum prop replaced by wooden Catto. 1950s Boat anchor starter and generator replaced by lightweight items. Two heavy magnetos and heavy carburettor replaced by EFI/ electronic ignition. Instrumentation shaved down to a bare minimum.

All in all, I expect to lose some 30 lbs from what was already a very lightweight (750 lbs) aircraft.

That’s all nice and good, however, pretty much all the weight that I am taking out comes from the nose. My engine is a narrow-deck O-320, which really only is a few lbs heavier than an O-235, so that doesn’t help much.



I AM going to add a second battery, which helps, however the plan was to make both of them EarthX units, with which I’ll end up below the weight of the single original battery.

I am 6’4” and 220 lbs, and for sake of legroom, I will be making an effort to sit as far back in the plane as possible.

Maybe the batteries should go on the firewall, and maybe I’d have to stick to Odysseys, but will that be enough??

Then the question: How sensitive is the -3 to CG? Am I messing things up here to the point where I need an extra heavy crush ring under the prop, or will I be ok? With the wings and tail off, I really can’t weigh until things are pretty much done.

Thoughts welcome!
 
Battery location is definitely used to keep within CG limits in an RV-3.

Do a (rough) W&B, then use that to determine battery location.

On the RV-3 I built (NA 13B Mazda rotary engine) I had the battery (lawn tractor) between my feet.

In my current RV-3B, with heavy engine mount, additional electric water pump, heavy tuned intake and otherwise overbuilt engine installation, the PC680 battery is far aft in the baggage compartment.

I know it doesn't really answer your question. But then you have to determine your main mission: aerobatics or cruise.

Finn
 
My -3 had a narrow deck 0-320, light starter, wood prop, Odessa battery between the pedals, 755# plus 185# of me and was a bit nose heavy. It flew much better with 15# in baggage and if I had not sold it, I planned to move the battery to the rear.
 
My -3 had a narrow deck 0-320, light starter, wood prop, Odessa battery between the pedals, 755# plus 185# of me and was a bit nose heavy. It flew much better with 15# in baggage and if I had not sold it, I planned to move the battery to the rear.

That sounds promising, since I was fearing it'd be too light on the nose. Thanks.
 
Battery location is definitely used to keep within CG limits in an RV-3.

Do a (rough) W&B, then use that to determine battery location.

On the RV-3 I built (NA 13B Mazda rotary engine) I had the battery (lawn tractor) between my feet.

In my current RV-3B, with heavy engine mount, additional electric water pump, heavy tuned intake and otherwise overbuilt engine installation, the PC680 battery is far aft in the baggage compartment.

I know it doesn't really answer your question. But then you have to determine your main mission: aerobatics or cruise.

Finn

Thanks Finn.

Interesting concept, with the 13B! Initially, I planned to put a Subaru EJ-25 on the nose, possibly even turbocharged for those 5000 fpm rocket climbs. I've got a (NA) EJ25 on my Jodel and I LOVE it. 100% reliable, zero cooling issues and smooooooooth. Unfortunately, my calculations showed it to become too heavy for this little airframe.

I've done some rough CG calculations, based on an somewhat unreliable W&B sheet that was in the POH that came with the plane. Bottom line is that the CG shift because of all my modifications will be around 1-1/4" towards the rear. If I don't load up the baggage area to the max, I'll remain within CG.

I think I'll have both batteries between my legs, where the single one is now, and if I end up with a too far aft CG after all, I'll move one of them to the void underneath the front of the crankcase, where the huge starter and generator used to be. That would shift CG forward by 5/8".

I'll get there.

Oh, and mission is formation flying mixed with quick dashes up to cloud altitude to play with cumulus clouds. For travelling, I'll take my Jodel.

So I can definitely live without the luggage capacity. And what's more, if I DO plan to travel with it, I have this RV8-like hatch that gives access to the space between instrument panel and firewall. I could always build a little luggage shelve there to carry the heavier stuff and balance out the rear luggage.

Thanks!
 
When I switched from a metal prop to a Catto 2 blade on My RV-3 I removed a Concord battery from the baggage compartment and put a Earth X between the rudder pedals. CG change was less than 1/2".
 
Weight and Balance

My RV-3 originally had an O-360 and CS on it, and the Concorde battery was behind the luggage compartment when I bought it. It now has an IO-320.
When I replaced the Sensinich prop with a Catto, I had to adjust big time.
I put a much lighter battery in it and moved it forward of the baggage compartment bulkhead, but its at the back so I can easily put things back there.
When I did the W/B, I found that I could not put the allowed #50 lbs of baggage back there at zero-fuel, I would be out. But I definitely don't want nose heavy so I placarded for #30lbs max baggage and that keeps me in limits in any fuel condition. I don't plan to ever fly it to zero fuel, but it makes me feel safer if I do get down to 5 gal (which I have).
N66GB is about #830 lbs, she was built in '84 so I have done all the spar mods and such. She flys straight, clean and fast. Whatever you do...don't be tail heavy.
Best
 
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