CG Calculations

[Peter Smedley]

With an airframe of Approximately 17kg how do I do the CG calculations to find the correct position?

I know of two methods...

1) weigh the main wheels and tail wheel and do some quick maths and measuring

2) use one of the on line CG calculating spread sheets.

Which one works the best to give the most accurate result. I know to my cost having the CG in the wrong place . I nearly lost a 1/4 scale Stampe due to it being tail heavy.

I may only get one chance to get this correct with something this heavy

Any ideas would be real help, thanks

[Peter Siggins]

Hello Peter not clear whether you just have to balance at a known position
Anyway if you know where it should be Google 'Vanessa balancing machine' for an easy way to balance each a heavy model

Hope it helps

[Peter Smedley]

I have purchased the model from a model builder, the plane has never flown. I have the original Bud Nosen Mr Mulligan plans that show the CG. However, the wing span and empennage have been increased slightly- off plan. This means I can't rely on the plan CG.

The plan shows the CG at 4.5" from the LE. Using a few online spread sheets, the results vary from 3.5-5.5".

Have I confused myself by looking at too many spread sheets?

I have a Vanessa rig and have found this useful. Though space is at a premium in the shed.....and I don't know if the roof beams would be up to it

[Rob Buckley]

You appear to be asking two questions- How do you measure where the CofG is and how do you know where it ought to be?

Has the model been completely scaled up or just bits of it?

If just the wing and tail, have the chords been changed or just spans?

The online calculators will do a rough calculation based on areas and moments, but if you've put the revised data in and they've provided a range with the plan position in the middle of the range, use the plan position.

[Peter Smedley]

The wings have been lengthened and the all the tail surfaces have a slight surface area increase ..

I have used this online calculator. This says 5" from the LE. The original plan is 4 1/2
http://adamone.rchomepage.com/cg_calc.htm

I also found this...though I have yet to try it, ( near the bottom of the page).
https://www.rcgroups.com/forums/showthread.php?t=744632

It got me thinking as the Vulcan et al that fly at Lma shows must have had the CG calculated.

[Rob Buckley]

When using the first calculator, what did you set the static margin to? That will effect the C of G and change how 'stable' you want the model to be.

[Peter Smedley]

I put the Static Margin at 12%. Not right at the very safe end of the scale ( a very nose heavy model has reduced elevator authority, as you probably are aware).

[Rob Buckley]

Sounds reasonable. So that has given you whee the C of G ought to be. You now need to weigh and measure the model to see where it is at the moment & move things around to shift it if necessary to where it needs to be.