Portachiavi CNC Foldable CG Scale

Getting your CG just right empty and with ballast is important for our F5J models and CG scales make it super easy.

Splash

Bling Bling

Getting your CG just right empty and with ballast is important for our F5J models and CG scales make it super easy. We've seen 3D printed versions of these scales and now Portachiavi has released a CNC metal version that looks hot! It's not going to be for everyone, but if you need a little bling in your life, it's worth checking out. Here's more on the CG scale from the manufacturer.

"I have been working on our new CG scale since the end of summer 2019. The need for a better scale, with regulation on non flat tables, foldable for delivery on smaller boxes, harder and with a better finish made me develop this new model with love for our customers. I made the first prototype on October of 2019 and lots of problems were detected, after numerous prototypes, changes, I hope this is then the best design I could have made. I make it with CNC, Pieces manufactured in the Portachiavi workshop, with lots of aluminum, a bit of acrylic, inox, 3D printed parts and carbon. The first ten units have a launching price to help us better calculate the costs and work in production per unit. Price might increase later on. The fabrication be start next week, and first deliverys on end of September."

Check out the Portachiavi CNC Foldable CG Scale Here

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Oct 01, 2020, 09:19 AM
Team Futaba
Silent-AV8R's Avatar
Very cool. Lots of the soaring guys use a similar, and much smaller, scale to set the CG on their gliders. This should be a useful tool!
Oct 01, 2020, 07:01 PM
Overkill is underrated
elac2az's Avatar
I don't know about the rest of you, but I would love to see an actual comparison article regarding these devices.
I believe Jason announced the Glider CG Machine when it first became available. Given that they now sell a range of sizes, people must like their stuff. BUT... I have a Glider CG Machine and find it irritatingly hard to get repeatable CG readouts regardless of whether I'm measuring a DLG or an F5J plane. The weight readings WILL remain constant I have found. So that aspect of the the Glider CG Machine appears dialed in.
But regardless of how carefully I fuss the glider into position, and try to carefully draw the leading edge against the pins, I can have results that can vary by a millimeter or more sometimes- which I just don't get. Such variances result in my performing LOTS of rechecks so I can at least generate a common result and have some confidence my CG is "X".
I don't know if the 3D printed nature of my machine is the culprit or not (meaning the type of material used to print it allows for flexure when it shouldn't.) But now that we are presented with a CNC from metal and carbon device I think a comparison would be awesome. Jason states there is some 3D parts involved but until one is further dissected in a RCG user review, I will assume the cradle and "stops" the leading edge is drawn up against don't flex.
Jason, if you have one of any of the other 3d printed devices and a few hours (?) to spare, would you mind doing a side by side test with maybe a DLG and tell us at a minimum:
1- I got the same results from both from both devices
2- I did not get the same results from both devices
3- Device "A" gave consistently the same CG regardless of how I set it on the device, Device "B" did not
4- Device "B" gave consistently the same CG regardless of how I set it on the device, Device "A" did not
5- Neither device gave consistent CG results

Inquiring minds would like to know. And if you lack the time or inclination AND you have this new unit from Portachiavi, send it to me and I will perform the test and return it. Seems fair to offer up to do the work when I'm the one asking.

Thanks, Ed

Thanks for your reply Jason.
Last edited by elac2az; Oct 01, 2020 at 10:04 PM.
Oct 01, 2020, 09:43 PM
RCG Admin
Jason Cole's Avatar
Thread OP
Hi Ed, I only have an older 3D printed scale so I can’t compare it to this CNC one which I would love to have some day. If anyone snags one of these I’d also be interested in the comparison results.
Oct 02, 2020, 06:48 AM
Registered User
Same problem with inconsistent cg with my cg machine.
Oct 02, 2020, 08:53 AM
Registered User
I am perfectly happy with my 3-D printed scale even with a +/- 1 repeatability issue. There is no comparison to the old way of trying to balance on some pencil erasers or a couple of sticks or god forbid your thumbs. I know that I do not have the skill to tell a 1mm CG difference - maybe someday I'll be that good...
Jim
Oct 02, 2020, 06:06 PM
Wayne Wimbish
wdwimbish's Avatar
Just an educated guess, (I used to do load cell testing in a research lab with load cells costing tens of thousands of dollars,) but I would think that the inherent repeatability problem with calculating the cg with these devises lie in the spacing of the load cells at the center of the beam and the quality of the load cells themselves. Devices with three load cells spaced about an inch apart essentially form a teeter totter. Tolerances and quality will play a big part in the results.
(photo credit to Aaro in the Geek thread https://www.rcgroups.com/forums/show...8&postcount=34)

With the load cells spaced so close together, reproducibility of the overall weight should be relatively good as it is just the sum of all three load cells being calculated. CG however would be based on a differential where a slight variation in readings would be magnified when expanding the calculation to the length between the pads. For the quality and price of the load cells being used, the reproducibility is actually pretty good.

Before I purchased a ready made unit like this I was using a rigid frame sitting on two .01 gram scales ($10 ea from Harbor freight.) Even with the pads of the frame located directly over the scales, there still were some variations in my readings.

IMHO, (and only just an opinion,) changing the frame from 3D printed plastic to CNC aluminum will not change the inherent problem with the location and quality of the load cells. If we want the reproducibility to be improved to actually correlate to the 0.1 mm readout we currently have, I think the design and cost of components would need to be vastly increased by at least an order of magnitude.

Still, the unit I have is so convenient to use, I accept that there is going to be some relative error in the readouts simply due to the design. I find that the accuracy correlates well though with any other devices that I have used. i.e. pencil erasers, knife points, screwdrivers, hanging from strings, etc. It does the job needed and that's good enough for me.

Wayne
Latest blog entry: Paying for Purchases by PayPal
Oct 03, 2020, 02:45 AM
Registered User
Aaro's Avatar
Hi,
I do not like to hijack or clutter this thread, but I it feel necessary to elaborate that the picture Wayne used with he's argument shows 3pcs of 6x6 mm push buttons used to calibrate the scale, not load cells
Aaro

Quote:
Originally Posted by wdwimbish
Devices with three load cells spaced about an inch apart essentially form a teeter totter. Tolerances and quality will play a big part in the results. (photo credit to Aaro in the Geek thread https://www.rcgroups.com/forums/show...8&postcount=34)

With the load cells spaced so close together, reproducibility of the overall weight should be relatively good as it is just the sum of all three load cells being calculated.

Wayne
Last edited by Aaro; Oct 03, 2020 at 02:56 AM.
Oct 03, 2020, 04:41 PM
Wayne Wimbish
wdwimbish's Avatar
Oooppps on the photo. Point me to a load cell photo and I'll update it. I would still think the short spacing and relative load cell quality available in this size is the limiting factor for cg precision.

Wayne
Latest blog entry: Paying for Purchases by PayPal
Oct 05, 2020, 10:25 AM
Registered User
320pilot's Avatar
I've got this printed one. It's wide enough for my Xplorer X-1 and smaller stuff. Seems to display repeatable values for CG and weight. This one also works with any 3S LiPo.

The Portachiavi one on SUSA's website can only handle up to a 2" wide fuse, (without adapter) so wouldn't work with my X-1.
Oct 05, 2020, 11:55 AM
DS Junkie
Screamin' Eagle's Avatar
I’ve had the same repeatability issues with at least two 3D printed machines. One was significantly better than the other. But I would still pay good money for a machine that will give me the same correct reading every time.
Oct 12, 2020, 02:18 PM
RCG Admin
Jason Cole's Avatar
Thread OP
Alex from Portachiavi messaged me today and it looks like I will be getting one of these CNC units to review. He said there are not any printed parts on structural members and is full metal chassis and arms which is more solid and stable than 3d printed parts. They went through 20 prototypes to get this right. I'm super excited to get one and report on it for you guys. Alex also said that Soaring USA will have these in the states for sale.
Oct 12, 2020, 05:03 PM
Registered User
I've made a lot of the 3D printed scales and found that they usually don't provide the same measurements. My solution....Lift and Replace the plane a few times and take the average. Usually, the numbers are within a millimeter or two, sometimes three. Take 5 measurements and average them, you should be really close to the real measurement.

Again, I'm only really concerned about the CG for setting up the plane after a repair or playing with ballast. The initial measurement is to simply get me into the ballpark and I adjust after flying.


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