RE: Center of Gravity "CG" Computations

RE: Center of Gravity "CG" Computations
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RE: Center of Gravity "CG" Computations - 5/14/2008 9:03:34 PM

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Joined: 5/11/2005
From: Bryn Mawr, PA, USA
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Read all the comments about cg. I am buiklding a self-designed electric bi-plane. it weighs 16 oz. in total. I have only a guess as to where the cg will end up. I have started at 1/3 back on the upper wing. I read a blurb that said you could "balance" the plane by putting in a sling ( with no electronics installed except motor) until it is level. Hang the plane from that balance point and install the rest of the gear to balance the plane. The first two i designed were very tail heavy, so I made the nose longer. I'm hoping this is the solution. i am not an engineer, nor do I want any headaches, I'm just having fun.

(in reply to xtruckerjim)

RE: Center of Gravity "CG" Computations - 5/15/2008 1:27:59 AM

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Joined: 12/23/2006
From: Zachary, LA, USA
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cjmarch,

I believe you should work with the bottom wing not the top. The sling method can work. You could also stick it on a balancer supported at the target cg point. You can then add ballast to bring it into balance. I made my balance stand with a board and wooden dowels. Making the nose longer will add mass forward.

Jim

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RE: Center of Gravity "CG" Computations - 5/22/2008 3:29:48 PM

PSJDBF

Posts: 56
Joined: 1/24/2008
From: PannawonicaWestern Australia, AUSTRALIA
Status: offline

Hi all ...is it correct low wings should be balanced up side down
from the top wing ? when using a balancer.
Cheers Steffan.

(in reply to cjmarch)

RE: Center of Gravity "CG" Computations - 5/23/2008 12:00:58 AM

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Joined: 12/23/2006
From: Zachary, LA, USA
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quote:

ORIGINAL: PSJDBF

Hi all ...is it correct low wings should be balanced up side down
from the top wing ? when using a balancer.
Cheers Steffan.

Steffan,

Yes, a low wing should be balanced up side down. I am confused about your statement "from the top wing". If you meant from the top side, then yes.

Jim

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RE: Center of Gravity "CG" Computations - 5/25/2008 12:06:57 PM

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Joined: 1/6/2005
From: Melbourne, AUSTRALIA
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I'm lazy and haven't read all the responses. I have a good idea some people have quoted some general figures and others have recommended so-called 'expert' calculators.

Take the calculators with a large pinch of salt. If they are excel spreadsheets or otherwise disclose the formula's, you will find 99% of them doing nothing more than estimate somewhere between 25-30% of chord. Very few even make use of the MAC or any other inputs. They are quite misleading.

Those that give general suggestions are pretty much right on. Unless you go for an extreme design, 25-30% of chord generally works well.

(in reply to RC Specialties)

RE: Center of Gravity "CG" Computations - 5/25/2008 1:01:57 PM

digital_trucker

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Joined: 2/4/2006
From: Dorr, MI, USA
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Interesting thread, I've always used the fingertip/eyeball method. Keep in mind that the balance point shown on a set of plans isn't always correct. I have a GP Cub 60 that if it's balanced where the plans show it's horribly tail heavy (will tend to snap on takeoff). Why? Who knows? Just as important as the CG is the thrust line of the engine. I'm now flying a Cloud Ranger that I know is nose heavy, I switched out engines so I need to move the battery pack. I wasn't afraid to give it a test flight because nose heavy is usually not much of a problem to deal with unless it's excessive. Here's the problem; with a smaller engine I had hints that it didn't have enough down thrust. When I went to a more powerful engine, I had that confirmed beyond all doubt. At full throttle I had to dial all the down trim I had just to keep it close to level. By the way, it was built according to plans. The real fun began when I throttled back to land and it tried to a good imitation of a lawn dart due to nose heaviness. Insufficient down thrust+nose heavy=interesting times. Giving it a couple of degrees of down thrust tamed it instantly (a piece of 1/32 ply jammed between the firewall and engine mount). It's still nose heavy, but it no longer needs all the down trim to fly level at WOT. Turned from a real handful at landing to one that merely comes in pretty hot. Once I get the battery pack shifted I'll have a sweetheart flyer.

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RE: Center of Gravity "CG" Computations - 5/25/2008 1:33:51 PM

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Joined: 12/23/2006
From: Zachary, LA, USA
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Trucker,

You are absolutely correct about thrust line importance. Most ARF's are close enough where it isn't a problem. However, if a plane is balanced correctly and there are bad flight characteristics the thrust line is a good place to look. I try to set up my planes where only slight elevator up pressure is required to establish a nice rate of descent and manage the throttle on the approach. Good input. Thanks.

Jim

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RE: Center of Gravity "CG" Computations - 5/26/2008 12:04:33 PM

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Joined: 5/11/2005
From: Bryn Mawr, PA, USA
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I HAVE FINISHED BUILDING THE ELECTRIC BI-PLANE. IT WILL FLY ON THURSDAY MAY 29TH. IVE SET THE CG AT 1/3 THE WING CORD ON THE BOTTOM WING. THE PLANE WEIGHS 20 OZ AND I HAVE A 950KV MOTOR. WITH A 10X3.4 SLOW FLYER PROP I AM GETTING 116 WATTS OF POWER. IT HAS PLENTY OF POWER AND IS STRAIGHT AND TRUE. I'LL LET YOU GUYS KNOW WHAT HAPPENS. THANKS FOR ALL YOUR HELP!

(in reply to RC Specialties)

RE: Center of Gravity "CG" Computations - 5/26/2008 12:43:30 PM

Posts: 255
Joined: 12/23/2006
From: Zachary, LA, USA
Status: offline

Great! Let us know.

Jim

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(in reply to cjmarch)

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