My CG Machine
 

While I was in Hong Kong last week, I found some cheap digital scales (A$12 each).

I decided to try the method used in light aircraft to find the Centre of Gravity.

I put the model on the scales as shown and measured the distance between the main and nose wheels. A simple Excel spreadsheet gives the CG as a distance from the Main wheels or any other point such as the leading edge. Works for a taildragger but the model needs to be level.
It is a simple matter to add or subtract weight from the nose to get the correct cg.

Ok I know it is a bit of overkill, but I found it interesting.

These scales have a max of 5 Kg, so the model weight would be limited to just over 10Kg. Big enought for all my stuff.

Regards

RE: My CG Machine
 

Tom, and can you give the formula to put in the excel spread sheet ?

RE: My CG Machine
 

Tom, looks very interesting.

Is that how they check the CG of a full size ?

RE: My CG Machine
 

I have a friend who flies full size and he sent me a photo of a Piper on scales just like this. They had little ramps next to the scales.

RE: My CG Machine
 

Can you also use this setup to balance the laterally ? You would therefore add weight on a wingtip until you have the exact same readings on both left and right leg scales...
Would that be at least as sensitif as the fingertip method ?

RE: My CG Machine
 

Yann,

I guess so, the left and right main weights are never the same. I think however the distance the wheels are from the centre will make it difficult to get an accurate measurement. I think the suspende upside down is probably more accurate.

This method of CG depends a lot on the accuracy of the scales and also the accuracy of the placement of the wheels.
And as I said it is probably overkill for our purposes, but fun to do, and makes CG corrections easier.

Regards,
ps

I cant seem to upload the excel spreadsheet. Any ideas?

RE: My CG Machine
 

I agree with you about the relative accuracy of wheel position.
Your idea is still great since you don't have to use the wheels if you don't want to.
You can have the scales under nose tip and wing tips more more accuracy.
I really want to give this idea a try.

Yann

RE: My CG Machine
 

Yann,

I have thought of this idea since my friend showed me the full size photo. I tried with some regular kitchen scales but it was very hard to position the model accuratly and of course the accuracy of the scales was suspect. I would be interested to see how you make out. I am still to fly the little Savex. Just a bit more work to do. I am really looking foreward to it.

Regards

RE: My CG Machine
 

Yeah I love that plane too. Mine will be on display downtown for the whole week end, so I won't get to fly it...[:o]
I still have to try my new oleos out as well as my onboard video system...
About the scales, I have the same kind as yours, 5kg max, with 1g accuracy. I just think it would be easier to place them in areas you are sure of the symmetry of, like panel lines, wing tips, servo pockets...
You them would just have to carve small "beds" out of foam for the chosen areas, for the plane to rest nicely on.
You would not forget not to count the weight of the beds of course.
On my scale, when I turn it on, no matter what is on the plate, it initialises to 0.
So I'll turn them on with the beds on, then I'll add the plane...

RE: My CG Machine
 

Yann,

I place the model on the scales and THEN turn them on. They will show zero. When you lift the model off they will show a negative number equal to the weight. Makes placement easier.

Regards

RE: My CG Machine
 

Mmmh !!! That's a good idea too...

My question now is: was your reading close to what you extimated with your finger tips ?

RE: My CG Machine
 

Tom, with your method, you can't do live adjustments...
I'll try both ways of doing it.

RE: My CG Machine
 

email spreadsheet to [email protected] and I'll post a web link with it,

Thanks

Sean

RE: My CG Machine
 

http://seanreit.dynalias.net/planes/...ads/cgcalc.xls

RE: My CG Machine
 

That was quick, I just clicked on "send" !!!
Thanks,

Yann

RE: My CG Machine
 

As Long as you put the wheels as close to center of the scale, scale accuracy should be fine. There will always be a small amount of hysteresis (repeatability). I moved my batteries slightly off center to improve lateral balance based on what I read off the scales.

RE: My CG Machine
 

Hi Yann,I tried it with the finger tips and it was almost exaclty as the spreadsheet said. Difficult to get it exact with finger tips however.
At 92.5 I still need to add some nose weight. By the way , How much does your Savex weight? and where is the CG?.
I will try my Reaction 54 next, I can't hold it up on my own on my finger tips!!!.



Regards

RE: My CG Machine
 

Tom,

My L39 balances out between 85 and 90mm back of LE. Like you said, hard to say with finger tips...
A friend of mine who is a way better pilot than I am, felt it could be balanced a little more aft. He flew it several times and still feels this way. Last time I checked, my L39 was 3.7 kgs dry. Probably a little more now with the oleos I just fitted. No nose ballast was required.
Let me know how CG check turns out on your reaction 54.

Yann

RE: My CG Machine
 

Overkill is good ... I've been playing with a similar spreadsheet approach :D

Mike

RE: My CG Machine
 

G'Day Mike,

Wow, that sure leaves my Spreadsheet for dead. Any chance of a copy?.

I just put my Reaction 54 on the scales. and got some good repeatable data. The CG is in the rearward quarter of the Plan CG range and the model flies great. The Stb side is 200 grams heavier than the port side. A bit of work needed here.


Regards

RE: My CG Machine
 

[quote][Tom asked ...
... Any chance of a copy?/quote]

I wanna hang on to it for a while longer just to make sure I get repeatable data compared to my usual POS way of checking things. Bob Parks helped refresh my memory on the formulae. As you know, (a) the scale(s) has to be fairly accurate, (b) wheel placement on the scales is important. And if you've only got one scale then the remaining wheels need to be at the same height as the scale. I was using a BobCat side view since there really *are* a lot of 'Cats flying but I was unable to get permission. Hence the ubiquitous Eagle. Thanks for checking it out.

Mike

RE: My CG Machine
 

Re MrMike's spreadsheet - I believe that the calculation can be carried out just by measuring the distance between the mainwheels and the nosewheel. Using his data:

----------------------------------0---------------------------0
........................................262.4<-------------25------------>
.................................................2 70.4<---------x----------->

Moment about the CG = moment about the mains

270.4x = 262.4*25
x = 264.4*25/270.4
CG = 24.26

So CG is 24.26inches behind nosewheel or as given by MrMike 44.26 from nose of the aircraft

RE: My CG Machine
 

I can't believe this was so easy and accurate. I'll never try balancing 28 lb models on my fingertips again.

One difficulty I found was accurately converting measurements on the table top (between wheel contact points), up to the fuse. I ended up using a plumb line which worked well. Seeing the more recent posts I'm now going to make all measurements from the nose, using a square to align the nose with marks on the table.

This is the first time I have had enough confidence in the measurements to start pulling lead out of the nose.

Very valuable thread. Thanks TommyWatson.

RE: My CG Machine
 

I like the overkill of MrMike's spreadsheet, however, I think we can simplify this down if I did my algebra right.

Cg (forward of the main gear axle) = Gear separation (NG axle to MG axle) X Weight on NG / Total airplane weight

Think that tells you where the actual Cg is in front of the main gear. Additional arithmetic to use other reference points.

What you really want to do is set the Cg to the correct location. So if you change the above around you can figure the weight required on the nose gear to set the Cg. Take the plans and measure the Cg location relative to the main gear axles. Weigh the entire airplane accurately (any method). Then level the plane with the nose gear on the scale. Adjust objects in aircraft until weight on nose gear is:

Weight on NG = Cg distance x total airplane weight / Gear separation

If you don't have any plans as might be common with many jets, you may have to do additional measurements to get the right numbers. All I need now is a nice digital scale.

RE: My CG Machine
 

Hi all
If anyones got an old stock of RCM JUNE 1992 page 96 very good artical
for both trike or tail drager simple and it works I've used it on half scale

Phil