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Jun 30, 2017, 10:42 AM
solastagia
kcaldwel's Avatar
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Discussion

3D printed CG locator, fixed, single digital scale


I made a new CG locator that holds the glider stationary, and can detect the CG movement from adding 0.5g to the nose. It doesn't involve rocking gliders or needle points on your wing

It uses a low cost digital scale that most of us have anyway. It does require a simple calculation to find the CG, one multiplication, and one division. I Have made two versions, one with 300mm arms that will fit most gliders, and a smaller, more sensitive version for lightweight DLGs.

It is much easier and the glider is more secure than all my other methods. It basically has knife edges at the LE, and then the scale measures the force from the glider weight aft of the knife edges. If you know the glider weight, it is a very simple calculation to find the CG location (for 300mm arms):

Xcg = (300mm x scale reading) / glider weight

I have two versions, for what I hope are the most readily available arrow shaft sizes. 11/32" (8.73mm) seems to be the cross-bow bolt shaft size, and I have read that 19/64" (7.54mm) is a common arrow shaft size.

There are small holes in the yoke and knife parts that you can use to lock the arms in place, either with a drop of thin CA, or small screws. The holes in the rear yoke are also useful for measuring the arm length from the LE stops on the knife parts, since they align with the adjustment screw that rests on the scale.

You need an accurate all-up weight of the glider as well. There are files for 3D printed parts to make a stand to hold the glider on the scale for weighing. The stand uses 4 short pieces of arrow shaft that would be glued to the printed parts.

You can either print the front towers, or I have V-blocks that can be mounted to wood blocks to save some printing time and plastic. I have printed them in both PETG and PLA. I love PETG, but PLA actually makes a stiffer yoke for the rear. I don't think that is too important though.

The CAD files are publicly available on OnShape if you want to modify the hole sizes for other shaft diameters. I find making the hole sizes 0.3mm larger than the shaft size, to account for the straight line approximation of holes done by the printing process, makes a pretty good fit with my printer. Going larger than than the 9.05mm holes I used for the 8.73mm shafts will likely require other changes. Hopefully this link will lead to all the CAD files:

https://cad.onshape.com/documents/09...dc491d14e9f804

I have an adjustment screw in the bottom of the rear yoke, to level the arms with different scale thicknesses, or to adjust the attitude of the glider which will change the CG location slightly. Levelling the arms isn't super critical. Small angles make very little difference.

I have ordered one of these scales for weighing the glider and using with the CG locator:

https://www.amazon.ca/dp/B06XR9PQ5L/...I1ISBMVI9TOIAZ

For a 1600g glider, a 1mm CG change will be about 5g on the scale with 300mm arms. The larger scale will work with even DLGs, but the smaller DLG version (Hand Launch forum) will be better suited to them.

Kevin
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Jun 30, 2017, 10:52 AM
solastagia
kcaldwel's Avatar
Thread OP
Files for the two versions for different arrow shaft diameters, 11/32" (8.73mm) and 19/64" (7.54mm).

You will need:

2x Balance knife in the appropriate size
1x Balance rear in the appropriate size

Either: 1x Balance Rt front and Balance Lt front
or 2x VBlock V2 and some wooden blocks 109mm high to mount the VBlocks on

The piece of plywood I used for a flat base is about 9" (220mm x 16" (400mm), with a centreline drawn lengthwise.

The cradle parts for holding the glider on the scale for getting the all-up weight are optional, but you would need 2x of the appropriate size.

Kevin
Last edited by kcaldwel; Jul 13, 2017 at 10:19 AM.
Jun 30, 2017, 04:21 PM
Professional sink locator
Very nice Kevin.
I'm going to find the time to print one over the holiday weekend.

Garry
Latest blog entry: Repairing a boom break
Jul 01, 2017, 12:34 PM
solastagia
kcaldwel's Avatar
Thread OP
Here is an assembled cradle to hold the glider on a scale to get the all up weight. I used 100mm long tubes in the bottom, 150mm long in the top, but whatever works for your scale should be fine. I chamfered the ends of the tubes, to avoid a sharp spot that might damage the wings. I discovered I had a bunch of 10mm aluminum tube, so I made another cradle version for 10mm tubes.

One thing I forgot to note above, is that some of the parts will have to be rotated in your slicer so the longest flat side is down. I have a tendency to draw in CAD in the orientation the parts will be used. The knife and rear balancer parts have to be rotated. Everything prints fine without supports for me.

There are also 3mm holes through the knife parts, so you could put CF tubes that extend closer to the fuse if you have wings with significant LE sweep and/or a skinny fuselage.

Kevin
Last edited by kcaldwel; Jul 01, 2017 at 11:48 PM.
Jul 01, 2017, 01:19 PM
Registered User
Looks good Kevin !

I've been mocking up my 3 scale design. Should have something to share soon.

My printer is monopolized right now because I'm trying to finish my PT17 before a club meeting in July.
Jul 01, 2017, 02:53 PM
Registered User
Im building this, almost done https://github.com/olkal/CG_scale
Jul 01, 2017, 03:41 PM
Registered User
Ironically that is the CG tool that someone is building and selling commercially. I think the selling price is 150EUR? Expensive for something that was an open source project.

They did add a printed OLED holder? That was the only thing I saw as being significantly different from the original design.
Jul 01, 2017, 05:58 PM
Registered User
Yep its very similar, i don't really know the differences because i don't have one comercial, today i saw another one comercial unit very very very similar.

Im building mine without oled display with an embed web server so i can view the data on my mobile.

The owner of the github (Olav) repo its very nice, i ask to make the supports wider to fit my f5j glider because i don't know 3d modeling and he made new files to fit.... very cool. Thanks Olav!!!
Jul 01, 2017, 11:14 PM
I can fix that ...
scottsdalejohn's Avatar
Thank You Kevin! My 3D printer was looking for a new project to keep it occupied.
Jul 02, 2017, 10:50 AM
Professional sink locator
Printed out the parts last night. They are in the tank to dissolve the support material, so another 6-8 hours before I can assemble it.
Latest blog entry: Repairing a boom break
Jul 04, 2017, 04:10 PM
Professional sink locator
Kevin,
The parts printed nicely.
Do I measure the 300mm from the edge of the knife piece to the center hole of the rear balance yoke, or to the inside edge?
Want to get it right the first time,
Garry

Edit,
The spacing directions are in his first post...
Last edited by GarryOgilvie; Jul 04, 2017 at 04:23 PM. Reason: Found the answer
Jul 12, 2017, 03:36 AM
VHO
VHO
glider misguider
I'm struggling to reconcile the CG numbers derived by this method and those obtained using the teeter / totter methodology (Great Planes CG machine).

With an 1115 g El Nino I get a CG of 103mm using the teeter / totter method and a CG of 108-110mm (from LE) using the moments method (made a machine like your DLG version with wooden blocks holding the v mounts rather than printing the whole shebang). All else is the same: 300mm moment, arms level, scale 3000g x 0.1g. I have checked the scale using kitchen weights and 1lb = 454g which is close enough

Check my calculations CG = (300mm*410g)/1115g = 110mm, another rep. (300*404)/1115 =108.7mm. There is a bit of variation between repeats, but that isn't the real issue.

Why is it so?

PS Kevin, I fly my Tweagle with CG closer to 68-70mm than the 74.5mm calculated from your photo (assumed weight 225g)
Jul 12, 2017, 10:42 AM
solastagia
kcaldwel's Avatar
Thread OP
How far apart are the arms on the GP CG machine? The EL Nino has some LE sweep in the centre panel, maybe the two methods aren't using the same LE position as a reference?

With LE sweep, you should likely use some 3mm CF rod to get the LE reference to the LE at the fuse junction, or in the case of the EL Nino, the centre LE of the panel. I'm not sure if that is enough to make 5 to 7mm difference.

I have found some of my scales are far more repeatable than others.

Kevin
Jul 12, 2017, 05:33 PM
VHO
VHO
glider misguider
Quote:
Originally Posted by kcaldwel
How far apart are the arms on the GP CG machine? The EL Nino has some LE sweep in the centre panel, maybe the two methods aren't using the same LE position as a reference?

With LE sweep, you should likely use some 3mm CF rod to get the LE reference to the LE at the fuse junction, or in the case of the EL Nino, the centre LE of the panel. I'm not sure if that is enough to make 5 to 7mm difference.

I have found some of my scales are far more repeatable than others.

Kevin
The moment CG system has arms 110mm apart (as per the stl files). The Great Planes balancing thing has arms 110mm apart (in this implementation). There is not sufficient LE sweep (there is almost none at that scale) to account for the difference.

The scale is repeatable if one puts and takes a known weight (e.g. 454 g which is about 50g more than the reading when used as intended). However, it seems there is some hysteresis in the moment weighing system. Could this be because of small deviations in the knife edge / v block due to 3D printing characteristics, or damping due to the characteristics of the application?

I happen to think that the correct CG is close to that obtained using the teeter / totter system rather than the moments machine (in its current embodiment).
Jul 12, 2017, 09:31 PM
solastagia
kcaldwel's Avatar
Thread OP
Well, something seems to be wrong. Certainly the two methods should be a lot closer than that.

If you haven't already printed parts and bought arrow shafts, I suggest you hold off until I can resolve this.

Sorry.

Kevin


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