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Help needed: CG of formation of 3 planes

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Help needed: CG of formation of 3 planes

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Old 04-10-2018, 04:11 PM
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penthane
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Default Help needed: CG of formation of 3 planes

I'm building a formation of 3 airplanes and don't know how to calculate CG. Planes are 22" long each. The middle plane is almost 5" ahead of the other two. Can I just use standard CG per plane (some 14" from the nose) and add 2/3 of the 5", or am i way off then?

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Old 04-11-2018, 06:00 AM
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Has nothing to do with the length of the fuse. Here is a CG calculator.
Center of Gravity Calculator
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Old 04-11-2018, 11:36 PM
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I'm familiar with CoGC and eCalc and such. Problem is that the 3 planes are interconnected and I would not know for the world how to calculate CG for this contraption with CoGC.
I've made a photo to clear things up: ibb.co/mm2tux
Sorry I didn't do that in my first post, but as rcuniverse won't let me post pictures (<10 posts), I had to find some other place to host.

Last edited by penthane; 04-11-2018 at 11:45 PM. Reason: changed interpunction
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Old 04-14-2018, 10:38 AM
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Originally Posted by penthane View Post
I'm building a formation of 3 airplanes and don't know how to calculate CG. Planes are 22" long each. The middle plane is almost 5" ahead of the other two. Can I just use standard CG per plane (some 14" from the nose) and add 2/3 of the 5", or am i way off then?

Greetingz,
Pen
Welcome, Penthane!

I suggest using the same method than for finding the CG of tandem wings or even biplanes.
Please, take a look at this:
C/G for Tandum Wing

In essence, you have three centers of pressure (one for each main wing of each airplane in the formation.
Assuming equal areas and incidence angles and airfoils of each wing, the three forces can be considered pretty close in magnitude to each other.
If that is the case, I would find the geometric center of that angle, which is the point at which the total lift of the formation is excerted.
As you have the total negative lift of the three tails being applied aft the previous found point, your center of balance will need to be enough ahead of the total lift point in order to achieve in-fligth balance.

You will also need to be careful about lateral balance of the formation, as the airplane on one side may be creating slightly more lift than the one on the opposite side.
I would also suggest a Vanessa balancer for this case:

Vanessa c.g. Rig: How to.

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Old 04-16-2018, 12:49 PM
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@ Lnewqban Thank you for the info. Have have done what you suggested, though I used another similar method. I have a CC3D installed, so lateral balance is taken care of. I've tested the plane last Sunday and it still flipped backwards inspite of the FC, so I needed to shift the balance even further. Which meant I had to enlarge the battery compartment. I also gave the motor a bit more down angle. Won't test 'till next Saturday, cause I had a problem with a bunch of ESC's. Bought 5, 2 caught fire mid-air, one blew just before launch and yesterday number 4 went up in smoke during a stationary test. A lot of repairs to make and I lost a whole weekend of beautiful flying weather
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Old 12-22-2018, 07:49 AM
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Hi ppl,
Took me a while but, after much trial and error, I finally have the Tripljet as I've called it, flying. All CG calculators were off by inches. It flies like a truck, but nevertheless. Everyone that has seen it flying irl says it's a spectacular sight, so here's a video of it flying. Recording isn't the best, but my friend couldn't see anything on screen cause of the sunlight. I cut the worse parts, so this is what's left
photos.app.goo.gl/YkJ1a24VnKh1MwRY9

Merry Xmas and lots of good flying weather in 2019

Last edited by penthane; 12-22-2018 at 04:30 PM.
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Old 12-22-2018, 10:31 AM
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Originally Posted by penthane View Post
Hi ppl,
Took me a while but, after much trial and error, I finally have the Tripljet as I've called it, flying. All CG calculators were off by inches. It flies like a truck, but nevertheless. Everyone that has seen it flying irl says it's a spectacular sight, so here's a video of it flying. Recording isn't the best, but my friend couldn't see anything on screen cause of the sunlight. I cut the worse parts, so this is what's left
photos.app.goo.gl/5SV9WVcxDoAubHxVA

Merry Xmas and lots of good flying weather in 2019
Congratulations on making it fly successfully!
on what direction was the CG off?
Your link to the video is not working for me for some reason.
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Old 12-22-2018, 04:30 PM
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Created a new link in a new public map, so should work now. Put the Http-stuff in front, cause I took the Http off. Not enough posts to post links. photos.app.goo.gl/YkJ1a24VnKh1MwRY9
CG was off by some 7cm. It was very tail heavy and I had to modify the middle fuse to shift the battery forward to avoid adding lead. The first time I let it go, it almost hit me on the head, cause it did a backloop as soon as it had some speed.
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Old 12-24-2018, 09:14 AM
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Originally Posted by penthane View Post
Created a new link in a new public map, so should work now. Put the Http-stuff in front, cause I took the Http off. Not enough posts to post links. photos.app.goo.gl/YkJ1a24VnKh1MwRY9
CG was off by some 7cm. It was very tail heavy and I had to modify the middle fuse to shift the battery forward to avoid adding lead. The first time I let it go, it almost hit me on the head, cause it did a backloop as soon as it had some speed.
Thank you!
Many more happy flights.
I am posting the link to the video for you:
https://photos.google.com/share/AF1Q...J2S282Y094YTR3
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Old 12-25-2018, 06:57 AM
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Did you figure out how to do the calculations? I'm curious how the math works out. I would think calculating the MAC for each plane and then using that as a line for the MAC for the group would work, but that's just a guess. Very neat project. I've always wanted to make a diamond formation set like that.
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