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Where's my CG going to finish up ?

P

Pittsartist

Well Known Member
A quick question.

I'm building an RV6 and as it evolves I'm getting a little concerned about finishing up with a rearward CG.

Here's the spec

RV6 / QB / Slider all to plans

0-320 B4B. B&C starter, B&C lightweight (8a) alternator, Bendix F.I., Bendix Mags. This is a conical "flatback" motor.

Oydessy PC680 battery, Traditional Flight instruments, Dynon EMS. Ray Allan Electric elevator trim

Composite Prop (probably the Sensenich ground adjustable one.)

Later type cowls

Not much filler, fairly lightweight paint job

Whats the prediction on empty CG ?. Should I be worried / be trying to move things forward / backward ?. Are there any quick tests (EG interim w & b calcs) that can give me a clue ?.

Any help appreciated !
 
Your airplane sounds similar to mine. You'll likely have a relatively aft CG. Aft CG concerns require me to land with about 50 pounds of fuel remaining with 370 lbs of people aboard and 60 pounds of baggage in the baggage area. With no baggage, I can't come near the aft limit.

I'd move as many things forward as possible.
 
Thanks Kyle, just the sort of comparisons I'm looking for.

the purchase of a propeller is starting to loom large - a metal one is looking increasingly attractive
 
Pittsartist said:
Thanks Kyle, just the sort of comparisons I'm looking for.

the purchase of a propeller is starting to loom large - a metal one is looking increasingly attractive
Click to expand...

It is all about the set of compromises you like best. A heavier crush plate or a steel flywheel ring would make a difference.
 
My -6 has the same aft CG tendency, but it's been mitigated by having a Landoll inertia ring on the flywheel, a heavy Concorde full-size RG25XC sealed battery immediately behind the firewall inside the cabin on the floor, a lighter weight tailwheel (1 lb on the tailwheel stinger equals 15 lbs on the nose, or so I've read somewhere here on VAF) all these things help out with the CG.

If you use a heavy flywheel inertia ring, you'll love the Sensenich GA prop. It's much smoother than the metal fixed pitch prop, and doesn't have that ugly 2600 rpm restriction that steals valuable horsepower by not being able to run the 320 engine at full power. A crush plate is not applicable to the Sensenich GA prop, only a bolt-on flywheel ring can be used with it. Mine is "turbine smooth". Pics here: http://www.vansairforce.com/community/showthread.php?p=805264#post805264

When the day comes that I have to replace my Concorde battery (23.5 lbs), I'll go with the big PC-925 battery (26 lbs) mounted in the same place. My empty weight is currently 1016 lbs, so gaining 2.5 lbs more isn't going to hurt much, plus it'll be up near the front where it'll help move the CG up a bit.
 
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Pittartist

I am not a definitive expert on any of this ..... but I will say that it sounds like you will be tending towards an aft CG. I believe that the 0-320 uses a longer engine mount. I suggest that you confirm this and confirm that you have the longer one for starters.

I also suggest that it is easy to build a quick spreadsheet that will calculate weight changes and their effect on CG. I have one but do not have access to it right now. i think that with using one, you will find Neal's comment about 1 pound on the tail equaling 15 on the nose to be way off. I suspect it is closer to 3:1. Also consider that it is very hard to lose 1 pound on the tail, but it is pretty easy to add weight on the nose.

Even though you may be in the acceptable limits, I found that in my "tending toward aft CG" RV-6, that with two aboard and low fuel, that control response in pitch became more responsive than I desired. There is a greater tendency to bounce at touchdown. You will be limited in aft baggage capacity.

I have since switched my composite RV-200 prop for a metal Hartzell BA with a commensurate 1+ inch forward movement of CG. The airplane is much happier. Pitch sensitivity is gone, landings are less likely to bounce and I have full baggage capacity.

I suggest that you look at it as closely as possible and that the heavier aluminum prop or a weighted ring may be the way to go.
 
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CG issue

I would be glad to send you my excel W&B sheet, which makes it easy to test where to put things. Quickly tells you where the cg will go when adding weight at any station.
Let me know if you want it.
Jim
 
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