Re: Center of Gravity Position

From: Bill Gowen <b.gowen_at_earthlink.net>
Date: Sun, 15 Jul 2007 20:10:14 -0400

Leo
In my opinion the CG is determined largely by the static stability margin that you are willing to accept. The farther back you can push the CG and still have a flyable model the better off you'll be in theoretical efficiency - in other words flight time. You can use the Excel programs developed by Don S. and Bernie Hunt to get a handle on the SSM of your design with different CG locations. The problem that you usually run into with too rearward CG's is the model becomes too difficult to trim. For instance you'll find that you have a stall and as you keep reducing the decalage to remove the stall your model will start diving in. It won't be possible to find the sweet spot where the model flies normally.

A further problem with rearward CG's is that the model will be very sensitive to different speeds. My F1L has a fairly low launch torque that it's happy with - around .25 in-oz. At Lakehurst that isn't enough to get to the ceiling unless I find a gigantic thermal, but at USIC that launch torque is just about perfect. When I flew my F1L at Lakehurst recently I had to use a lot of tricks to get the model launched successfully - things like putting washin in the right wing panel and twisting the bejesus out of the motor stick just before launch. A more forward CG would probably help that situation but changing it is not a simple matter.

  ----- Original Message -----
  From: leop12345
  To: Indoor_Construction_at_yahoogroups.com
  Sent: Sunday, July 15, 2007 4:01 PM
  Subject: [Indoor_Construction] Center of Gravity Position


  Being just first year indoor endurance fliers, SO's and ministicks,
  my daughter and I have become comfortable, but not expert, with
  adjustments to the wing incidence, decalage, warps, and prop thrust
  angles for a given CG position (as specified by the plans). In
  designing our own planes, we can use various formulas for determining
  the initial CG position but we want to learn how to determine,
  through our test flights, the best position for the CG. We have read
  how the GC position affects the climb out under higher torques,
  stability, and other things. We have had some small success (flight
  times) with moving the CG from the plans' positions. However, aside
  from testing all possible reasonable CG positions with flight times,
  we do not yet have sufficient experience and wisdom to set up a
  course of adjustments that will hopefully converge on a good CG
  position for our own designs.

  We realize that the optimal CG position, just like other adjustments,
  will be a compromise between climb, glide, and descent. Can anyone
  give us tutorial (or a pointer to one) on determining, through flight
  testing, the optimal CG position?

  Thanks,

  Leo
  Bloomington, IN



   

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Received on Sun Jul 15 2007 - 17:10:38 CEST