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Old Aug 03, 2004, 02:34 PM
Valid8r
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Boston and Belgium
Joined Jul 2004
943 Posts
Help w/ SS CoG determination

I've tried several times to find the sections of this massive thread that deal with determining CoG and have given up going back and finding it manually (even search doesn't seem to work - tried cg, CG etc. - nothing) so I am resorting to asking you all for help.

Can anyone provide a basic primer on determing CoG? With, without batteries? From where to where?

Thanks for humoring and my apologies for giving up on searching this thread, but I can't find it. I know Boomer posted several times on the subject but I'll be darned if I can find which thread 1-5 it was in and where!

Thanks in advance.

I've finally assembled my plane, been playing with my Tx and Sim and am getting ready for my first ever RC maiden flight this week or weekend and I'd like to go out with as many cards stacked in my favor as possible.

Jon
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Old Aug 03, 2004, 05:48 PM
PGR
Low AltiDude
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United States, CA, Costa Mesa
Joined Jun 2004
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Quote:
Originally Posted by Valid8r
Can anyone provide a basic primer on determing CoG? With, without batteries? From where to where?
The object isn't to 'determine' CG. The object is to 'set' the CG to the proper point. Assuming that you have a basically stock SS with an EPS300 power system, that CG should be about 4-1/8 inches back from the leading edge of the wing. On the SS, close is good enough, btw.

Measure back along the center rib on the underside of the wing and make a mark where the CG should be. Then (with the battery pack mounted to the plane and using the mark you just made as a guide) pick the plane up by the underside of the wing with your fingertips on each side of the rib at CG point you just marked. Let the plane teeter-totter from your fingertips.

The plane should hang level. If it doesn't, slide the wing and/or battery pack around on the aluminum tube until it does.

That will be a good starting point. Once you become familiar with how your SS flies, you can adjust the CG forward or backward to tailor how the plane flies.

May the Force be with you.

Pete
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Old Aug 03, 2004, 07:08 PM
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Alan Ford's Avatar
Ridge, NY
Joined May 2004
321 Posts
Jon,

I checked the manual.

If you are in Boston, the CG should be 4 1/8" from the leading edge of the wing.
If you are in Belgium, the CG should be 95 - 105 mm from the leading edge of the wing.

I'd start at 4" or 100 mm from the leading edge.

If you find it difficult to keep a steady altitude, then move the wing backward a bit. (This will move the CG forward.)

If you find it difficult to change altitude, the move the wing forward a bit. (This will move the CG backward.)

In both cases, "a bit" should be approximately 1/8 " or 2 - 3 mm.

When you are happy with your altitude control, mark the wing saddle settings on the stick, as the wing saddles tend to move forward with every less than perfect landing.

PM me if you need more information.

-a.
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Old Aug 03, 2004, 07:15 PM
Been There! Done That!
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Eugene, Oregon, United States
Joined Sep 2001
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Go with what PGR told you!
boomer
www.boomerseflight.com
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Old Aug 03, 2004, 09:25 PM
Actually NW Ga.
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Chattanooga Lovell, Tennesse, United States
Joined Jun 2004
160 Posts
It'll help if you start with your front wing saddle 6.25 inches from the back of the motor.
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Old Aug 03, 2004, 09:33 PM
Been There! Done That!
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Eugene, Oregon, United States
Joined Sep 2001
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Quote:
Originally Posted by jlankf1
It'll help if you start with your front wing saddle 6.25 inches from the back of the motor.
That distance can be anywhere from 4" to 7-1/2" so not important. Most fly at 6 to 7" depending on servo size and location. I always balance WITHOUT the battery and then place the battery directly under the CG. That way you can change battery pack sizes at will and the CG never changes!
boomer
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Old Aug 04, 2004, 05:46 AM
Valid8r
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Boston and Belgium
Joined Jul 2004
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Thanks to all who replied. I think I have the basic idea now. Geez this forum is an amazing place.

Thanks everyone.

Jon
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Old Aug 04, 2004, 08:51 AM
PGR
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United States, CA, Costa Mesa
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Quote:
Originally Posted by boomerace
That distance can be anywhere from 4" to 7-1/2" so not important.
The distance between the wing and the tail feathers can have as dramatic an effect on a plane's flight envelope as the CG. Typically, moving the wing closer to the tail will increase elevator response and tighten the turn and loop radius. A plane rigged like this is often referred to as "close-coupled."

Combined with a tail-biased CG, close-coupling can make a plane extremely pitch-sensitive which, in the right hands, can be a desirable attribute. I flew my symmetric Stick that way for quite a while but recently moved the wing forward again when I started playing with elevator-flaperon mixing.

My personal recommendation for a newcomer to RC flight would be to keep the wing as far forward as possible and still be able to set the CG. I'd also recommend a slightly nose-biased CG (4-1/4 inches or so) for the first few flights. Rigged like this, a SS will be more forgiving of the natural tendency a newcomer has to over-correct and it also helps negate the tendency for the plane to loft under high throttle settings.

That's the magic of a SS: It's so adjustable! I have several more advanced planes to fly but I still have the most fun with my Stick.

It's the perfect "what if" plane.

Pete
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Last edited by PGR; Aug 04, 2004 at 08:54 AM.
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Old Aug 04, 2004, 10:30 AM
Been There! Done That!
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Eugene, Oregon, United States
Joined Sep 2001
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Pete,
I agree with what you said 100% in theory however I have never found any pitch sensitivity even back to the 7" I quoted. A STOCK SS just flys too slow and has too much of that big undercambered wing to be pitch sensitive. I have several students on the SS and none have had any problem controlling the plane with the wing within the range I quoted.
boomer
www.boomerseflight.com
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Old Aug 04, 2004, 11:38 AM
PGR
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United States, CA, Costa Mesa
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Quote:
Originally Posted by boomerace
Pete,
I agree with what you said 100% in theory however I have never found any pitch sensitivity even back to the 7" I quoted. A STOCK SS just flys too slow and has too much of that big undercambered wing to be pitch sensitive. I have several students on the SS and none have had any problem controlling the plane with the wing within the range I quoted.
I think we're both right, Boomer. Stock, the difference is negligible at 7" but there is a difference.

Now if you want to have some fun, wack 7" off of the tube, put the leading edge 7" back from the new front of the tube, and set your CG at around 3-7/8 inches. I suggest reinforcing the horizontal stab too because it -really- tightens up the loops and turns, even with a stock wing and motor.

As you can well imagine, the plane will no longer be a good student trainer.

Pete
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