Please help a mathematical dunce

bencmaster · Post by **bencmaster** » Sun Apr 06, 2008 6:56 pm

I find C of G calculations tough at the best of times as I am a bear with a very small numbers brain.

So If I want to remove an obscelescent part from my Storch (just behind the prop)..no not the engine..

and aslo bring the battery 18" forward, then I know it should mean less ballast on the tail (currently a big lump of metal).

But what's the simple way to calculate the new figures without a full blown weighing session?

Help much appreciated

Ben

Mark A · Post by **Mark A** » Sun Apr 06, 2008 9:30 pm

Quite easy really.

First work out the total moment for the aircraft. i.e empty weight multiplied by empty CG moment arm.
Work out the moment for the part you are removing i.e. Its weight multiplied by its position re. CG datum. Remember to use the right sign convention if moments are fwd or aft of the datum.
Subtract the moment of the parts removed from the total moment to get the new moment.
Subtract the weight of items removed from the aircraft basic weight to get the new basic weight.
Divide the new moment by the new weight to get the new empty CG position.

Of course if you are adding parts you do the same sums but adding instead of subtracting.

A couple of minutes in Excel should have it sorted.

bencmaster · Post by **bencmaster** » Mon Apr 07, 2008 8:25 am

Has anyone a copy of an Excel C of G equation?

Rob Swain · Post by **Rob Swain** » Mon Apr 07, 2008 11:37 am

Mark A wrote:A couple of minutes in Excel should have it sorted.

You're not an accountant, by any chance?

Redwing · Post by **Redwing** » Mon Apr 07, 2008 2:34 pm

You need to know where the "datum" is defined on your plane, that the c of g position is referenced to. This is so you can measure the distance from there to where you are adding or removing weight.

Then, if you want to calculate how much weight you can remove from the weight at the tail, and keep the same c of g as you have at present.

Measure distance from datum to tail weight in inches= Dtail

For moving the battery forward 18 inches, you can remove
battery weight times 18 divided by Dtail
from the tail weight.

For removing the object from behind the prop,

Measure Distance from datum to object= Dob
Measure weight of object= Wob

You can remove Wob times Dob divided by Dtail from the Tail weight.

That's my understanding of what you would need to do to keep you c of g in the same place, while reducing your tail wheel weight, but you might want to get a qualified second opinion, or post your figures, to double check!

Steve Brown · Post by **Steve Brown** » Mon Apr 07, 2008 9:44 pm

Send me a PM with your email & I'll send you a sample s/s

Steve

bencmaster · Post by **bencmaster** » Tue Apr 08, 2008 8:12 am

Thanks for the input. It's beginning to come into focus!

tnowak · Post by **tnowak** » Tue Apr 08, 2008 8:32 am

I have an Excel spreadsheet for my Vagabond which you can have. However, you need to enter in your own specific fixed data for it to provide meaningful results.

What about jacking the tail of your aircraft to the W&B measuring position and weigh the tail using bathroom scales (drain your fuel first). Reposition your battery and then see what difference in weight is shown. Adjust your ballast as necessary to revert back to the original reading.
TN

Mark A · Post by **Mark A** » Tue Apr 08, 2008 3:05 pm

You're not an accountant, by any chance?

I take that as a slur on my character - an engineer if you must know

I've spent about 5 minutes knocking up a spreadsheet to do that with some example data, and will e-mail it if you PM me an address.

Pete · Post by **Pete** » Tue Apr 15, 2008 12:20 am

Mark,

I suspect that Rob's observation was meant as wit...

I like the wob/dob approach - Redwing, I guess you are a painter and decorator