Calling all physics majors,
I'm wrestling this big 27% scale plane on and off the balancer with the help of a friend and I'm a bit nose heavy. I stack 2.5 oz of weight at a distance near the tail and it balances. Now, instead of adding more weight to the plane I start shot-gunning some new servo and battery pack mounts rearward of the canopy thinking I'm going to balance. Trouble is, will I get lucky? It's been awhile since my physics classes in 79.... does anyone have a formula for mass versus moment. I should be able to put a calculator to this and get the radio position in the ball park without calling my friend over each time I balance. Yes, I've got a sweet digital scale.
Jack

I think it's a pretty straight forward relationship. If it takes 2.5 oz say 24 inches from the balance point to make the plane sit level, it will take 5 oz 12 inches from the balance point to make it sit level.

moment=weight X distance.

If you are talking about moving items (rx, battery etc) around then it's a bit more complex. I THINK you'd have to weigh whatever you're moving, find the difference in distance from the old location to the new location and the multiply to find the new moment.

In the above example 2.5 X 24=60 ounce-inches Lets say your battery pack is mounted 1.5 inches behind the balance point and weighs 8 ounces. It's moment is 12 ounce-inches. To move the battery pack to get it to balance...(60+12)/8=9 inches behind the balance point.

Oh, man am I ever getting a headache!

CP, just kidding, when you you mentioned oz/inches, it reminded me of torque settings on some robotic stuff at work. So, your saying we can just sum up the oz/in per component? That's not bad at all to understand. I immediately thought someone would bring up something about PI and circular momentum or vector analysis. Now there's a headache!!! Letz see... wheres dat calculator.
Jack

Down with lead!

Inches times ounces before and behind the cg is what it's all about. If you have a metric scale and metric tape, grams and centimeters are a little easier to calculate. If you move a 2 oz servo back 20 inches, you have 40 oz.-in. added to tail moment, which must be made up in the nose.

I put a screw-eye or eye bolt in a hardpoint which I build in to the cg of my model (top for high wing, bottom for low wing), and hang it from the ceiling. This enormously simplifies playing around with cg, both fore & aft and lateral balance. Lateral balance can often be improved by mounting servos, batteries, other components up against one side or t'other of the fuselage.

Rather than add tailweight ballast, I am inclined in serious cases to chop the nose. This is a scary prospect unless you can calculate with confidence how far back you need to cut. After you have all the straightforward component placement taken care of, just hang everything from a cord and place lead ballast on tail hinge line until the model is level. Weigh the ballast, measure the distance from cg to tail hingeline, multiply weight times distance and that's how much moment you need to take out of the nose. Weigh your engine/prop/spinner/nosegear/firewall/tank power module. The next tricky part is to find the power module's cg. You can lay it all out on a little balsa tray, tie string across the four corners and nudge the hanging point where the strings cross until the tray hangs level. Straight down from the string gives you the power module cg. Move that power module cg back enough distance so that power module weight times distance moved will equal the moment of the tail ballast. Cut with confidence, re-glue the firewall and fly a balanced plane.

Sometimes it is as easy to move the wing forward as to move the engine back. The cg moves with the wing.

I suppose if you wanted to be really weird you could calculate it in pound-centimeters....as long as you're consistent with the units of measurement!

Well, here's the numbers.
before changes, components to be moved plus lead weights all totall 101 oz-inches
After the components move rearward without the lead weights I get 152 oz-inches. Yikes! Sounds like a 50% change.....
If the 101 oz-inches balanced for a 5.5" CG, how can I figure the resultant CG?
Jack

I looked at the messages and I don't see what you are getting at. If you are looking at re-distributing weight to figure out where to place items then I think I see what you are doing. If you are trying to calculate the CG then you need to also take into account the plane in your torque calculations.

When you just move a component that weighs 3 ounces from say 2 inches behind the balance point to 4 inches behind the balance point, you are adding only (4 -2) X 3 = 6 ounce inches, not 12!

I have found in cases where I am moving a lot of components around that I need two fairly detailed diagrams, one for before, one for after, with every piece that moves detailed as to weight and distance from cg. I use + for nose moments, - for tail moments. When you get an initial trial balance with ballast at one end or the other, then all your changes in the + and - column need to come out to zero after you've made all your changes. This is more like accounting than physics.

These measurements and calculations need to be accurate, but not necessarily precise, since you can use your battery as a wild card, placing it after every other change has been made for final tuning. In fact, you might just leave the battery out of all preliminary calculations. If everything you change works perfectly, then just put the battery right at cg. If you're off, fix it with battery. If you're way off, use two batteries in redundant hookup. This also facilitates further balancing adjustments based on flight trimming.