Calculate CG with scales!

[Maingear]

http://www.mediafire.com/view/t2oqddbkezlclgn/FT_W&B.xlsx

Here is a spreadsheet I made that can help you find the CG of your airplane with digital kitchen scales. There are notes inside the cells that need entries for the calculation and a link to the FAA's weight an balance manual.


Thanks John (johnmw) for your help with this!

Justin

 

[Maingear]

W&B manual in a nut shell:

Chapter 1 - Weight and Balance is important.

Chapter 2 - The further you go out on a see saw the more force it takes to counter that force because it is felt on the other side as a negative force. One side of the see saw is positive, the other is negative.

Chapter 6 - Helicopters

A datum is a fancy term for a reference point where all measurements are started from, or the zero in a number line. It is not the hinge point because airplanes do not pivot on hinges, so they measure firewalls, spinners, main landing gears, etc. It keeps all measurements relative to that point for consistency.

The arm is the distance from the datum (zero in the airplane's number line) to the location of the weight being recorded.

Some aircraft reference their CG from the LE of the wing. They are setting the datum as the LE of the wing. If you use the same datum your nose gear weight will be a positive weight with a negitive arm (positive weight at a negative number line location).

Move batteries and radio around to move the CG to the location that the manufacture recommends.

See a new battery you wanna use and want to know what it will do to your CG?
1. Weigh your aircraft without its battery.
2. Measure the location, remember that locations fwd of the datum is a negative number.
3. Enter the weight and location in the blank space below the gears on the spreadsheet

Your new CG will be calculated and the effects of the new battery will be in the new calculated CG.

Justin

 

[IFlyRCstuff]

Thank you! The way I test CG on a scratchbuild it to make an exact cardstock model. It always works, just use paperclips and tape to balance til it flies good, then apply to foamboard.

 

[johnmw]

hey Justin, no worries man.
glad to be of help.
thanks for the spreadsheet!

 

[rwehuman]

So I am taking an old styrafoam throw glider that has a 3.5' wing span(swept wing) and turning it into a three channel powered glider.
It does not have landing gear but two tooth picks stabbed in the wings and one on the nose and I have what would be gear to put on my scales.
Where should I put them? -is my biggest question.
Is it an arbitrary point under he wing?
symetrical is obvious to me but how far from the leading edge and from the center line? it sems very subjectiave....
I cant wrap my brain ariound it. everything else makes sense. any direction is greatly appreciated.
I dont wanna throw test it to find balance I'd rather do the math like an enginerd.

It has:
2 4g E-max servos
Spektrum AR400 reciever
E-max 2280 out runner -this thing is gunna kick burd butt
Kinexsis 800mAh
an ESC but dont remember the name...

I have done to it:
balsa spar for wing strength
chopped the nose 3° Por t / 3°Down
custom pcb board motor mount (soon to be 1/32" carbon fiber)
Full authority rudder
Full authority elevator
Monokote wings, rudder, elevator

 

[Craftydan]

If you're going to use measured weight to measure CG, the measuring points are arbitrary. You simply need to fix the position of your measuring points, pick one as your reference point (or an unweighed point that you know it's position relative to the others), and the CG you calculate will be relative to the reference point. At that point it's no more complicated than measuring the CG on a trailer.

There are better and worse points to work form than others -- pick points too close to the CG and you'll find your calculation becomes sensitive to measurement error. For instance -- one on the nose, and two in the wings about half-way back on the chord are probably not great spots. Ideally the CG will be near 1/3 of the chord, so the wing CG points will weigh a lot but have very short measurements, and the nose may weigh very little but have a long measurement. Fundamentally, the smaller the measurement, the more impact any error will have. Picking the points more spaced out (say, one in the nose, and one on each tail surface) will more evenly distribute the weight among the supporting points (making weight measurement errors equal at all points) and make length measurements easier (reducing the errors in the length).


. . . all that being said . . .


two fingers -- one under each wing -- can be a VERY effective CG estimator. I appreciate the desire for the precision, but for something that light and small, anything more is probably overkill.

Calculate the ideal CG (there are several calculators online), mark on the wing, measure with your fingers and move the ballast until it's just slightly forward of those marks (or leans nose-down when your fingers are on the ideal marks) . . . then fly.

This thing used to be a chuck glider -- it should still glide. Load her out for flight, get the CG about right, and run a few glide tests over tall grass (leave the prop off until maiden -- the missing weight is negligible but they can break easily in test landings). So long as the control surfaces are functioning in the right directions, you should be happy with the results

 

[rwehuman]

Thank you very much.

 

[rwehuman]

I found a very helpful video that goes over finding CG with scales.

https://www.youtube.com/watch?v=9EgC6rX24AU