Go Back  RCU Forums > RC Helicopters > Multi Rotor Helicopters
Reload this Page >

CG for "dead cat" type quad

Notices
Multi Rotor Helicopters Discuss Multi rotor RC Helicopter's here!

CG for "dead cat" type quad

Old 07-18-2015, 05:20 PM
  #1  
jester_s1
Moderator
Thread Starter
 
Join Date: Dec 2006
Location: Fort Worth, TX
Posts: 7,030
Likes: 0
Received 8 Likes on 8 Posts
Default CG for "dead cat" type quad

I've been into RC for about 9 years, but just now getting into multirotors. I am setting up a spider frame quad, and I haven't come across any resources for setting the CG. I've seen some threads where people drew a line diagonally across the motors which gives a pretty tail heavy location. Is that right? Or should I measure the location of both motors fore and aft and put the CG between them which puts me about a half inch farther forward? Or is CG really much of a concern for these vehicles?
Old 07-18-2015, 06:35 PM
  #2  
randall1959
Senior Member
My Feedback: (11)
 
randall1959's Avatar
 
Join Date: Aug 2004
Location: saint joseph, MO
Posts: 1,253
Likes: 0
Received 0 Likes on 0 Posts
Default

What I did with mine was to pinch the top plate between my fingers and try to locate the point where it balances that way. It came in about one inch ahead of where the rear arms bolt on. Depending on the flight controller it does make a difference. I use a DJI Naza M Lite and it will "toilet bowl" if you don't get it at least close. It's not as big of an issue if you're using it without gps. Mine flies pretty well.

Last edited by randall1959; 07-18-2015 at 06:48 PM.
Old 07-18-2015, 07:13 PM
  #3  
jester_s1
Moderator
Thread Starter
 
Join Date: Dec 2006
Location: Fort Worth, TX
Posts: 7,030
Likes: 0
Received 8 Likes on 8 Posts
Default

Mine is an APM 2.6 Ardupilot with GPS, so I guess it does matter. Do you have any tips for locating the right CG? How am I supposed to determine where it should be?
Old 07-18-2015, 07:25 PM
  #4  
randall1959
Senior Member
My Feedback: (11)
 
randall1959's Avatar
 
Join Date: Aug 2004
Location: saint joseph, MO
Posts: 1,253
Likes: 0
Received 0 Likes on 0 Posts
Default

First you want to put everything in it except for the flight controller. Even put the battery in. Then hold the top plate on each side between your thumb and index finger and find the point where it balances.
Attached Thumbnails Click image for larger version

Name:	DSC00002.JPG
Views:	198
Size:	2.78 MB
ID:	2109886   Click image for larger version

Name:	DSC00003.JPG
Views:	175
Size:	2.72 MB
ID:	2109888  

Last edited by randall1959; 07-18-2015 at 07:28 PM.
Old 07-18-2015, 08:13 PM
  #5  
jester_s1
Moderator
Thread Starter
 
Join Date: Dec 2006
Location: Fort Worth, TX
Posts: 7,030
Likes: 0
Received 8 Likes on 8 Posts
Default

RIght, I got the method for determining where the CG actually is. What I'm asking is how do I figure out where it is supposed to be?
Old 07-19-2015, 04:12 AM
  #6  
randall1959
Senior Member
My Feedback: (11)
 
randall1959's Avatar
 
Join Date: Aug 2004
Location: saint joseph, MO
Posts: 1,253
Likes: 0
Received 0 Likes on 0 Posts
Default

CG on a plane and CG on a quadcopter are two totally different things. CG on a plane will affect it's flight performance due to aerodynamics. Quadcopters do not have any real aerodynamic forces other than the props and the flight controller needs to be in the center of the lifting force so it's balanced out and the flight controller can keep the quad level. Deadcats are especially tricky because everyone assumes you have to draw a line from the right front to the left rear and then from the left front to the right rear and you put the flight controller in the middle of that "X" but you don't.
You put the flight controller where the center of mass is and in this case it will be further back. If you were flying an F450 or some other symmetric frame, the flight controller would most likely be right in the middle. If you add a heavy gimbal to that frame the center of mass will shift and you will more than likely have to move the FC forward.
On a Deadcat it should be around 1 inch ahead of the furthest bolt forward on the rear arms.
Attached Thumbnails Click image for larger version

Name:	DSC00004.JPG
Views:	180
Size:	2.73 MB
ID:	2109918  

Last edited by randall1959; 07-19-2015 at 05:42 AM.
Old 07-19-2015, 05:00 PM
  #7  
jester_s1
Moderator
Thread Starter
 
Join Date: Dec 2006
Location: Fort Worth, TX
Posts: 7,030
Likes: 0
Received 8 Likes on 8 Posts
Default

It makes sense that the "draw an X" method won't work on a deadcat because the extra width of the front arms will push the intersection of the lines back. You said an inch forward of the bolt- so why there? Is this a ballpark thing that doesn't really make much difference, or is it actually important to get the quad balanced between its four props? My initial thought was to get it balanced at the midpoint between a line drawn across the front rotors and the back rotors. That would take the width difference out of the equation. Is that the right way, or is something else the right way?

Now I'm also hearing that the flight controller should be at the CG, which throws another wrench into the project. I bought this quad already built, but it wouldn't be that hard to move the flight controller if I need to.
Old 07-20-2015, 02:41 AM
  #8  
randall1959
Senior Member
My Feedback: (11)
 
randall1959's Avatar
 
Join Date: Aug 2004
Location: saint joseph, MO
Posts: 1,253
Likes: 0
Received 0 Likes on 0 Posts
Default

The reason you find the balance point is because that's where you need to put the FC. My experience with the Deadcat frame has told me that it balances at the point ahead of the rear arms and this is where you need to get the FC close to.
Old 07-20-2015, 04:47 AM
  #9  
jester_s1
Moderator
Thread Starter
 
Join Date: Dec 2006
Location: Fort Worth, TX
Posts: 7,030
Likes: 0
Received 8 Likes on 8 Posts
Default

So if I'm understanding you right, it doesn't matter much if the load is balanced between the props? It's more about putting the flight controller on the CG so it doesn't get weird accelerometer readings?
Old 07-20-2015, 06:46 AM
  #10  
randall1959
Senior Member
My Feedback: (11)
 
randall1959's Avatar
 
Join Date: Aug 2004
Location: saint joseph, MO
Posts: 1,253
Likes: 0
Received 0 Likes on 0 Posts
Default

Getting the flight controller near the balance point is the most important thing but it's also good to try to get the frame to balance close to the mid point between the front motors and rear motors. It doesn't have to be perfect. I have a DJI Naza M Lite and the GUI will compensate for an offset of the CG. The Deadcat can give you fits depending on which flight controller you use. A symmetric frame is much easier to set up but I really like the aggressive look of the Deadcat. I'll post a video later today of mine flying. It's been a while and I've made some changes. A lot of people try to put the FC in the middle of that "X" and the intersection of the lines is way to far forward and they fly really squirrely. Put the FC back near where the rear arms bolt on and it's a much tamer bird.

Last edited by randall1959; 07-20-2015 at 01:21 PM.
Old 07-22-2015, 03:19 PM
  #11  
randall1959
Senior Member
My Feedback: (11)
 
randall1959's Avatar
 
Join Date: Aug 2004
Location: saint joseph, MO
Posts: 1,253
Likes: 0
Received 0 Likes on 0 Posts
Default

I did a little adjusting and now have the FC and the GPS pretty much about an inch behind the center of the body plate.
https://www.youtube.com/watch?v=aBOWhVNB_7Y
Attached Thumbnails Click image for larger version

Name:	DSC00008.JPG
Views:	181
Size:	3.53 MB
ID:	2110615  

Last edited by randall1959; 07-22-2015 at 03:21 PM.
Old 07-22-2015, 07:17 PM
  #12  
jester_s1
Moderator
Thread Starter
 
Join Date: Dec 2006
Location: Fort Worth, TX
Posts: 7,030
Likes: 0
Received 8 Likes on 8 Posts
Default

Ok, yes. That's how I want mine to fly. Don't be surprised if I span your inbox after my first couple of flights!
Old 10-03-2015, 11:18 AM
  #13  
cooke86
Junior Member
 
Join Date: Jan 2007
Location: la trinité porhoet, FRANCE
Posts: 7
Likes: 0
Received 0 Likes on 0 Posts
Default

The proper way to determine the CG on a Spider type frame ( or dead cat ) is to draw a line between the front and rear motors on each side, then draw a second line accross the frame at the center point of the two previous lines. That line is where Your copter should balance, normaly a little in front the point you get if you draw diagonals across the frame/motors. However as previously stated it doesnt need to be perfect. It can be quite far out and the FC will sort it out by spinning the Front or rear motors faster or slower, this can have an negative effect on flight performance though.

Check out the website for my open source project for more helpfull tips: WWW.EXPLORER4-QUADCOPTER.COM

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.