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Old Jul 28, 2001, 03:24 PM
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Center of gravity

How do you find center of gravity and how would you ajust it?
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Old Jul 28, 2001, 04:15 PM
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For a conventional plane, wing, tail.. the c.g. for stable flight is between 25 and 35 percent of the mean aerodynamic chord.
go to http://www.palosrc.com/instructors/cg.htm
for an explanation of this term, and how to find it and the c.g.
.
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Old Jul 28, 2001, 07:45 PM
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You adjust the model to get it to balance around the CG by moving equipment around. In electric models, we tend to do this by moving the battery.
..a
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Old Jul 29, 2001, 01:36 AM
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I just bought a Great Planes CG locator and it has helped me out a lot. A lot of guys here built their own but for the price ($16) I think this one is worth it. As a newbie myself I think I am getting more air time and much more comfortable flights now.
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Old May 31, 2008, 04:51 PM
alex4orly
Australia - Melbourne
Joined Feb 2005
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Hi there,

I must wonder (following Sparky's method), is the only thing we need to know for this is the wing details?

Is the horizontal satbilaizer, size and distance from the wing not playing in part in the equation?

Cheers
Alex
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Old May 31, 2008, 09:14 PM
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Joined Jan 2008
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Hey Mike, where did you get your COG Machine from? I only found one so far online and it's like $22.99 (at Tower Hobbies) plus what ever they charge for shipping, so around $30.00. I'd pick one up if I could find one for $16.00 though. Thanks

Bryan
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Old Jun 01, 2008, 07:34 AM
An itch?. Scratch build.
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Alex

If you want to take into account the tail area have a look at - Full Area CG Calculator

Though most people will probably use the 'between 20% and 30% of the wing chord on a parallel chord wing' guess.

Very few people will bother to actually 'trim' the CG, (try moving it), to suit there flying if the model actually flies first time.

We live in a 'plug and play' world.
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Old Jun 01, 2008, 07:39 AM
An itch?. Scratch build.
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Bryan

The easiest CG machine is one finger on each hand, (saying two fingers sounded rude )

The easiest one to make is a piece of wood and two pencils with those rubber tips. Drill the wood to take the pencils about about 4" to 6" apart, rubber end up, and balance the model on them.
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Old Jun 01, 2008, 11:23 AM
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eflightray, thanks for tip. Hate to ask but I'm a real "visual" kind of guy. Do you happen to have one of these built that you could show a picture of or possibly sketch it out and post that so I can see exactly what we are talking about? That would be great. If not that's ok to, I'll figure it out eventually.

On the larger picture of the COG scene I have a question.....Even though I understand what COG is, what we are trying to achieve etc., what I don't understand is how it can vary so much. Let me explain. If say 4 of us were all talking about 4 different styles/models of planes I can see where each one would indeed be different dependent on the Wing Chord size, length of Fuselage etc., and all the various other components. But what I don't understand is if 4 of us were all talking about the same plane (and I see this all the time) such as the HobbyZone Super Cub how can one person say for them the COG is 2 1/2" from LE and one person say for them it's 2 1/4" from LE and one person say it's 2 15/16" and so on? Follow me? If we are dealing with the same plane shouldn't our "reference" point all be the same and then according to the weight/size of the motor or the weight/size of the battery or any other accessories then determine whether to move those items backwards or forwards or to add more more weight in front or back of the COG to balance things out properly?

To me it seems like on a particular model (again let's say the HZSC) that we should be able to say okay, with the stock set up the COG (where I'm going to place my finger tips) is say 2 1/4" from the leading Edge and say 1" out from the cockpit. From that starting point I can then make sure my plane is properly balanced. Then, if I decide to use a different motor or heavier battery or what have you to me it only makes sense to still use that same starting point and if after you added say a heavier battery either move that battery backwards or forwards to have the plane balanced again by having my fingers in the same exact spot. Does this not make sense? Please tell me what I'm missing. Thanks

Bryan
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Old Jun 01, 2008, 12:21 PM
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rpage53's Avatar
Victoria, BC, Canada
Joined Apr 2001
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Quote:
Originally Posted by Olias
From that starting point I can then make sure my plane is properly balanced.
There isn't a single location for "proper balance". A rearward CG will make the plane unstable and difficult to control but might be what an expert pilot wants for 3D aerobatics.

A CG that is too far forward leads to sluggish behaviour but provides a saftey factor for maiden flights. A lot of planes have been lost on maidens when the builder was hesitant to add enough weight to the nose. A 1/2-1" range in flyable CG locations is not unusual.

Rick.
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Old Jun 01, 2008, 12:21 PM
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Quote:
Originally Posted by Olias
To me it seems like on a particular model (again let's say the HZSC) that we should be able to say okay, with the stock set up the COG (where I'm going to place my finger tips) is say 2 1/4" from the leading Edge and say 1" out from the cockpit. From that starting point I can then make sure my plane is properly balanced. Then, if I decide to use a different motor or heavier battery or what have you to me it only makes sense to still use that same starting point and if after you added say a heavier battery either move that battery backwards or forwards to have the plane balanced again by having my fingers in the same exact spot. Does this not make sense? Please tell me what I'm missing. Thanks

Bryan
You nailed it perfectly. It not only makes perfect sense, but it's exactly the same process we all use...

Chuck
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Old Jun 01, 2008, 12:25 PM
An itch?. Scratch build.
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Olias

The CG position often quoted is usually a safe starting point, though sometimes manufacturers do get it wrong.

For a conventional model, i.e. wing and a tail somewhere behind it, the best place to start is slightly nose heavy, (shortest recommended dimension back), as a nose heavy plane is usually more stable.

Conversely a tail heavy plane can be either very 'aerobatic', or impossible to fly.

All CG balancing should be in the 'ready to fly' state. (For one of those noisy things, that means with a half to full fuel tank).

Now there are ways to test your CG position while flying, (assuming it was close enough to let you fly). But the method can depend on the type of model, so don't take this a gospel for all models.
Climb to a safe height, throttle to a steady level cruise, push the nose down into a 30 to 40 degree dive.
If the CG is correct, (depending on thetype of model), the plane will continue in a straight(ish) dive.
If it's too far forward, the model will recover quickly and probably zoom.
If it's too far back, the model could tuck it's nose down further and what some call 'zero out', go into a vertical dive with no natural recovery.

As I pointed out, it depends on the model. It works better for models with symmetrical wing sections, not so good for flat bottomed sections or undercambered ones, sometimes referred to as 'lifting' wings. They are much more susceptible to a speed change in the dive increasing the lift and zooming anyway.

The CG position can also be related to how much incidence the wing has relative to the tail, a lot needs a forward CG, little and the CG can go further back.

If you do some searches on model aircraft aerodynamics, how do planes fly, sort of things, they put it much better than me.

Picture below of the 'pencil balancer', there are many more variations here on the forums.
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Old Jun 01, 2008, 01:18 PM
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Thank you for the pic. I feel kind of stupid now as that as just about as basic as it can get, but I just wanted to be sure.

Lastly, I just want to comment here (not question or debate) because I understand everything everyone is saying about CG but I think I might not be explaining myself properly. If we take the example of the home made CG balancer you show in your diagram (or our fingers for that matter) I could move the plane anywhere I want forwards or backwards on those rubber tips until I get the plane to balance out level. But that doesn't mean tht's where my proper CoG is. Follow? If I knew for instance that someone said say ok, your CG for the stock SC is 2 1/4" from the leading edge then I'd know that if I marked it there and then put the rubber points (or my fingers) right on those spots and the plane say tipped way forward then I'd have to shift my battery or electronics or what ever backwards so the plane would then balance again while still resting on those exact marks. Make sense? Otherwise heck I could balance the plane on marks only an inch from the LE (or what ever) to make the plane balance out to where all my innards are currently located instead of visa versa which is actually the correct way.

I'm probably still not explaining well enough. That's ok though because I do understand what you are saying and I really haven't had any major problems getting my plane balanced and/or trimmed out, that I know of. I say that meaning although my plane seems to be flying just fine (i.e. 1/2 to 3/4 throttle, thumbs off stick in next to no wind, she fly's level on her own) but I'm always wondering if I have it tweaked as good as it should be. But I also get it trimmed/balanced by using a standard rule of thumb starting point and in my case it has been at about 2 1/4" back from the LE. Guess I've just always wondered if I balanced it out by going back farther or up closer to the LE if it would be more accurate. I guess I'll just have to keep experimenting and see what happens.

Thanks for your explanations, pictures, and patience in listening to my rambling. It's most appreciated.

Bryan
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Old Jun 01, 2008, 01:43 PM
An itch?. Scratch build.
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olias

What you perhaps need to have a look/search on the MAC, (Mean Aerodynamic Chord).

For a parallel chord wing the dimension is pretty obvious, but what about a tapered wing?

That's were CG calculator come into their own, if you have a look at the calculator, linked in post #7, it should give you a better idea.

Any dimension quoted in any thread is for that specific model only, and the percentage figure is a much better unit to use. Also as can be seen when we quote somewhere between 20% and 30%, a CG isn't a fixed dimension, just a starting point.

Have a look at the calculator, it might explain it better, (or confuse you more ).
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Old Jun 01, 2008, 01:51 PM
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Thanks again, I'll go have a look at that calculator. BTW....what kind of wing is it on the SC? When you talk about "tapered" etc., what kind of wing is it called/considered that is flat on bottom and curved on top (I think that's what the SC has).?? Thanks

Bryan
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