CG question

I'm hoping to hang the engine on my project in the next few weeks. I'm going to use a wood prop. I'm planning on mounting the battery forward of the firewall but am not locked into anything yet. The motor is a 320 B3B. I'm going to use a Vans or B&C alternator.( I can't believe how heavy the stock unit is!) My question is with the 4s tendency to be tail heavy its it necessary to replace the stock starter with a lightweight unit from B&C or will it benefit from a few more lbs forward.

Thanks
Skip
dues paid

Depends on the mission Skip

Skip,
You know my plane pretty well, and I have the O320H2AD, Sterba woody prop, lightweight starter, small 35amp alternator from Vans, battery in the tunnel per plans. I also have the 3.5 gal smoke system in the baggage compt aft of rear seat. My plane weighs under 950# and I almost always fly with 2 on board. If the tanks are full, and back seater is under 180 lbs, its not too bad. Wife and I can travel with a couple light bags in back, and its fine. If you plan on a 200# backseater and running tanks below half, it will be a pitchy not so pleasant experience. Im all for light weight, and limiting the AFT weight/load options. I recommend building light, and with the battery FWD of firewall (Im going to move mine there), you will be fine. Now come see us at 8A7 soon!

Data per the W&B sheet in my airplane...

My 4 is completely per the plans, wood prop, semi heavy home brew paint, manual trim, no interior other than seats, no autopilot, single EFIS, no steam gauges, PC-680 battery on the firewall, fire extinguisher.
Engine is stock O320-D1A, OEM Prestolite starter, Plane Power alternator, carb.
Without oil it weighs 948 pounds and with 7.0 quarts of oil it weighs 961 pounds, no fuel.

Per Vans the design CG range is 68.7? to 77.4? aft of datum
Calcs ?
Empty: 70.55? (7 qts. oil, no fuel)
Solo (me @ 170#): 72.01? (Oil & full fuel)
Pilot & wife (130#) + baggage; 15# fwd, 50# aft: 76.90? (Oil & full fuel)
Max passenger @ 205#, no baggage: 77.38? (Oil & full fuel)

I?m going to retain the Prestolite starter until it malfunctions, and I?m very happy with the PC-680 on the firewall. That puts weight farther forward and leaves the whole area between your legs for baggage, tent, ?in flight snacks?, whatever. Shorter heavy gauge wires to boot!

My -4 is about 1050 lbs empty with an IO-360, CS prop, and the battery between my feet. My CG is fairly far forward, but I still won't fly with anyone more than 200 lbs in the back seat. I would not spend any money on lightweight engine accessories. That being said, B&C makes great stuff. Save for a constant speed prop...

Good luck,

Dean

Four ward CG...

egp8111 said:

I'm hoping to hang the engine on my project in the next few weeks. I'm going to use a wood prop. I'm planning on mounting the battery forward of the firewall but am not locked into anything yet. The motor is a 320 B3B. I'm going to use a Vans or B&C alternator.( I can't believe how heavy the stock unit is!) My question is with the 4s tendency to be tail heavy its it necessary to replace the stock starter with a lightweight unit from B&C or will it benefit from a few more lbs forward.

Thanks
Skip
dues paid

Click to expand...

Skip,
My RV4 was nearly identical to yours.
I had no CG issues and limited myself after numerous experimentations to a Max of 200lbs aft of the roll bar. Lexus starter/GEO Metro alternator, Sterba prop (at first). I would adjust fuel load to match my mission. More gas equals more forward CG, less, better solo aerobatic and dogfight handling.
I found that 1/4 in both main tanks equaled a very light, maneuverable machine. 1/2 Tanks for flying (small) passengers, full for cross country.

After 10 years and 1000 hours I moved/replaced the battery to the firewall (best place IMHO) with an Odyssey PC625 and turned the tunnel into a glove box and installed rear passenger foot-wells. I was also running the Catto prototype prop, slightly heavier and better performing than the Sterba.
The CG after all that was improved, perfect at 1/2 fuel, my goal.
V/R
Smokey

Smokey, was your PC625 on the engine side of the firewall? If so, I see the battery operating temp range is from -40?C (-40?F) to 45?C (113?F). Did you ever survey the temperatures on the engine side?

Battery up!

CATPart said:

Smokey, was your PC625 on the engine side of the firewall? If so, I see the battery operating temp range is from -40°C (-40°F) to 45°C (113ºF). Did you ever survey the temperatures on the engine side?

Click to expand...

Cat,
Yes, engine side of firewall. I placed it in a Van's battery box lined with shiny home wall board insulation. It sits on top of the "shelf" formed by the rudder pedal wells.

Never had a temp problem with it or the RVX in the exact same spot. It was a win win with better starts (shorter wires) forward CG and a new storage area where the battery once lived.
V/R
Smokey