Van's Air Force
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Wanted: W & B for comparison
- Thread starter daddyman
- Start date
planenutz
Well Known Member
RV-6 with an IO-360 and Sensenich metal FP prop.
Battery in the traditional location aft of firewall between pedals.
VFR machine but with dual GRT screens, COM, GPS/Com, XPONDER and steam ASI/ALT backups.
Total weight: 1116.6 LB at 70.18"
While overall slightly heavier than I wanted, the C of G position is absolutely perfect as it sits right on the fuel C of G line (70.00”). This means that C of G hardly changes with fuel burn and placement of baggage down the back is much less of a concern. As I'm sure you're aware, burning off fuel in flight typically moves C of G aft on most RV-6's and when you have too much baggage its easy to find yourself regretting it on approach as the elevators become over-sensitive.
I would guess that with a CSU up front your C of G will be slightly further forward than mine so placing the battery on the aft side of the firewall would work well for you. Access is easier on the forward side of the firewall of course. Remember that everything you put in the aircraft (fuel, bums & baggage) is aft of your empty weight C of G so the further forward you start with the better it will be when flying.
Battery in the traditional location aft of firewall between pedals.
VFR machine but with dual GRT screens, COM, GPS/Com, XPONDER and steam ASI/ALT backups.
Total weight: 1116.6 LB at 70.18"
While overall slightly heavier than I wanted, the C of G position is absolutely perfect as it sits right on the fuel C of G line (70.00”). This means that C of G hardly changes with fuel burn and placement of baggage down the back is much less of a concern. As I'm sure you're aware, burning off fuel in flight typically moves C of G aft on most RV-6's and when you have too much baggage its easy to find yourself regretting it on approach as the elevators become over-sensitive.
I would guess that with a CSU up front your C of G will be slightly further forward than mine so placing the battery on the aft side of the firewall would work well for you. Access is easier on the forward side of the firewall of course. Remember that everything you put in the aircraft (fuel, bums & baggage) is aft of your empty weight C of G so the further forward you start with the better it will be when flying.
Dan 57
Well Known Member
Not really correct Mike. Fuel is forward of the CG, and that is something to watch for, as whilst burning fuel your CG will wander towards the rear...
As for my ship, equipped as in my signature (and with an -8 tail), empty weight is 1068lbs at 70.74".
As an example, 100lbs of luggage, 2 occupants weighing 320lbs total, I'll hit the rear limit when the total fuel weight is below 130lbs, or about 21USG.
Both battery mounting have their pros/cons, but yes, CG would be a deciding factor for me too.
simpkinsona
Well Known Member
Mike’s answer is correct as long as the aircraft is only flown unmanned….
Scott Hersha
Well Known Member
My RV6 - O-360, carbon Sensenich light weight prop, battery on the forward side of the firewall, single 10” glass VFR panel.
Empty weight 1018.4 (unpainted, in the paint shop so that will change)..
Empty CG 70.36.
RV6’s tend to go tail heavy with luggage. Max load of 100 lbs is a lot. I generally bulk out before I’m anywhere close to 100 lbs in the baggage area.
Empty weight 1018.4 (unpainted, in the paint shop so that will change)..
Empty CG 70.36.
RV6’s tend to go tail heavy with luggage. Max load of 100 lbs is a lot. I generally bulk out before I’m anywhere close to 100 lbs in the baggage area.
planenutz
Well Known Member
Ok, I'll give you that technicality Dan, but my point was that if you can get your aircraft built so that the empty weight C of G is as close as possible to the fuel arm of 70.00" then you're making life real asy for yourself in future.
According to the RV-6 information I've always used, the fuel is located at an arm of 70.00". This is the arm used for weight and balance calculations and I assume this is the "average" of the total fuel distribution over the entire tank as calculated by Vans when the design was conceived.
If my aircraft has an empty weight C of G located at 70.18" then the addition and removal of fuel makes very little change to the aircraft C of G during flight. Yes, you are correct - it DOES make a difference, but only a very slight change. The weight of the total fuel consumption multiplied by 0.18.
How you would accurately determine your C of G at the point where you're installing the battery cradle into the aircraft I don't know, but with the heavier prop up front I would guess that on the aft side of teh firewall would be a good place for it.
According to the RV-6 information I've always used, the fuel is located at an arm of 70.00". This is the arm used for weight and balance calculations and I assume this is the "average" of the total fuel distribution over the entire tank as calculated by Vans when the design was conceived.
If my aircraft has an empty weight C of G located at 70.18" then the addition and removal of fuel makes very little change to the aircraft C of G during flight. Yes, you are correct - it DOES make a difference, but only a very slight change. The weight of the total fuel consumption multiplied by 0.18.
How you would accurately determine your C of G at the point where you're installing the battery cradle into the aircraft I don't know, but with the heavier prop up front I would guess that on the aft side of teh firewall would be a good place for it.
WingsOnWheels
Well Known Member
I would stick the battery on the firewall. It is unlikely you will run into a forward CG issue on the 6. Mine is a 6A as well (so heavier nose already) and I had to add a 20# crush plate to my Catto prop to get the CG forward.
With the extra 20#, it pushed my empty weight to 1086.
The real advantage is Maintenace. The battery down under the panel way forward is a pain in the rear to get to. On the firewall is nice and easy. The CG difference is minimal, since you are only looking at 4-5 inches shift in the battery CG.
With the extra 20#, it pushed my empty weight to 1086.
The real advantage is Maintenace. The battery down under the panel way forward is a pain in the rear to get to. On the firewall is nice and easy. The CG difference is minimal, since you are only looking at 4-5 inches shift in the battery CG.
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simpkinsona
Well Known Member
You’re not getting it. During flight your CG is several inches aft of the empty CG. This due to the occupants and any baggage. The CG will move aft as you remove weight at the 70 inch location. This discussion reminds me of the one where the FAA inspector wanted the empty CG within W&B flight limits.
-Andy
I was going to say the same thing. If you buy a lead acid battery its worth a thought.
If you are buying an EarthX or similar Lithium battery its not. EarthX weighs 3.9 pounds. That's equivalent to 20 feet of #2 wire not counting connectors and insulation. So if you are not careful the wire you need to run to the back is as heavy as the battery... .
Easier and lighter overall (you save the long heavy wire) to screw a pound of lead in the tail if you really need it.
Oliver
planenutz
Well Known Member
I'm pretty sure I have a very good handle on C of G.
My aircraft has an empty weight C of G arm of 70.18". The fuel arm is located at 70.00". If I put pilot, passenger and baggage into the aircraft the C of G will move aft, certainly, however from full of gas to empty in this configuration the C of G change is minimal at only +0.71" - lets call it three quarters of an inch. In this configuration I have not exceeded the aft C of G limit of 76.8" as I'm still sitting at 76.66".
However, if my EW C of G started at, say 72.00", then the maximum I could take off with is 54 lbs of baggage because in the same fuel configuration the C of G would be at 76.78" upon landing. This is of course all assuming we haven't exceeded the MAUW. Starting with a more rearward EW C of G means the C of G change is greater as the lever arm is greater.
It is true that a number of RV-6's with an O-320 and VFR panel have ended up with an EW C of G at or beyond the 72" datum and this immediately restricts your baggage carrying capacity. I read of one aircraft on this forum where the guy stated his EW C of G was at 74" and I cannot understand how that could be so far aft - it must be an error as it would be a terrible aircraft to fly and very limited if this is correct.
So going back to my original response - if the OP places his battery forward or aft of the firewall (never in the rear behind the cabin) then his EW C of G will hopefully be at or around the 70.00" fuel arm and the use of fuel from the tanks will have very little effect on his C of G change during flight.
Lets take the example that he places the battery forward of the firewall and that in the end his EW C of G ends up at 69.00 inches, forward of the fuel load at 70.00 inches. In the zero pilot configuration with full fuel at take-off and zero fuel at landing the C of G will actually move forward by 0.14 inches. If he adds the same two people and the same 100 lbs of baggage as me, the total maximum C of G change is just +0.64 inches. He could take another 38 lb of baggage more than I could in my aircraft and still not exceed the aft C of G limit on landing with zero fuel.
So again - it is in the best interests of the OP to place his battery at or about the firewall. It is certainly not in his interests to place it any further aft and certainly not aft of the cabin. The EW C of G does not have to end up being aft of the fuel datum. The further forward you can get it to begin with the less effect fuel consumption can have on any change that can occur in flight.
With the exception of the error I made that DanH picked me up on (I was a bit exuberant there) - "fuel, bums and baggage" - clearly I should have just said "bums and baggage" - I don't see anything incorrect in my statements.
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