I'm replacing a Concorde RG25 X/C battery with an EarthX900. The weight delta is 18.6 lb, and with the battery located several inches behind the rear bulkhead, the CG change won't be small enough to ignore. I'd like to generate new W&B paperwork by calculation rather than re-weighing. The process to do so is straightforward, but I can't seem to find information showing the ARM at the center of the battery tray. Clearly a lot of other RV-10 owners have switched from AGM to LiFePO4 before me. I'm hoping that those who have done so have the ARM info. I have tried contacting tech support at the Mothership during telephone hours, but thus far haven't gotten through.
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Calculating a new Weight & Balance after a battery change
- Thread starter tom_AZ
- Start date
You could use a tape measure to find out how far the battery is from a known datum point. In the RV-7 the datum is 70 inches in front of the wing leading edge. So I would measure how far something is in relation to the wing leading edge and then convert this to the moment arm.
That arm is not published. Will need to use a different one and add to it based upon your measurements. Can be a challange as many published points like pilot and baggage are not clear as to where the exact point is physically. I assume the baggage point is the mid point of the baggage floor, but hav no idea if that is accurate. Probably easiest to use the wing leading edge as a reference, as that is the base point for all W&B points. Beginning point is based upon a set distance fwd of LE.
sansoneservices
Well Known Member
Yeppers, tape measure to the leading edge, add 70 for arm
per my wt and balance Excel spread sheet, moment arm for the aft baggage area bulkhead is 189" from the standard Vans' reference point. Measure from there to the battery center, add to189.
(For a complete wt and bal spreadsheet you need to account for baggage location, so spreadsheet has fore/aft baggage area limits. So 189" is measured to the base of the forward side of the bulkhead.)
(For a complete wt and bal spreadsheet you need to account for baggage location, so spreadsheet has fore/aft baggage area limits. So 189" is measured to the base of the forward side of the bulkhead.)
Yes. 70 is for an RV-7, he asked about a -10.
sansoneservices
Well Known Member
Thanks for the correction. Hopefully someone conducting a w&b will look up the exact # rather than blindly taking a number off a social forum
Pet Peeve!
I fail to understand why everyone is so concerned about where the datum is. It is anywhere you want it to be. It is only a reference!
I personally always use the leading edge of the wing. Everything aft of that is a positive number and everything forward is a negative number.
If you can't deal with negative numbers maybe you shouldn't be doing Weight & Balance!
I fail to understand why everyone is so concerned about where the datum is. It is anywhere you want it to be. It is only a reference!
I personally always use the leading edge of the wing. Everything aft of that is a positive number and everything forward is a negative number.
If you can't deal with negative numbers maybe you shouldn't be doing Weight & Balance!
Mel, you and I agree 99% of the time. But not on this. Yes, I agree that it is meaningless to quote stations without saying where the reference point is. But using something other than what Vans suggests only leads to confusion on threads like this.
As to your last comment: I used to do primary instruction (stopped due to liability concerns). You would be amazed at how much confusion there can be with (a few) students when some arms are negative. I think Vans anticipated this when recommending a reference point in front of everything.
Correct!
Datum line
- What it is: An imaginary vertical line or plane used as the zero point for all measurements.
- Location: It is established by the manufacturer, often located at the firewall, but can be different for each aircraft model.
Drop a plum bob leading edge of left wing. Measure said location to the middle of the battery. Add 99.44 inches, done. This is using the Vans published datum.
Norman CYYJ
Well Known Member
When measuring a distance from whatever you use as a datum line make sure you are measuring in a straight line perpendicular to that datum line or else all of your measurements will be wrong. Leveling the aircraft makes it easier.
Because vans lists all critical arm locations that you need and they are from the datum that van sets. However, if you pick an arbitrary datum, you have to do a whole bunch of math to get usable arm lengths, like pilot, fuel, etc. to match the arbitrary datum number. I fail to see the benefits to using anything other than what Vans provided. It is especially helpful when people are posting w&b info here and the points are not all different because every builder chose a different datum.
For example, bob was kind enough to give the 189” location to the OP. That number would be meaningless if bob used an arbitrary datum.
