Center of gravity - RC Groups
Jul 28, 2001, 04:24 PM
Registered User

# Center of gravity

How do you find center of gravity and how would you ajust it?
 Jul 28, 2001, 05:15 PM Ascended Master 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. . Sparky Paul PJB's Seriously Aeronautical Stuff
 Jul 28, 2001, 08:45 PM Registered User 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
 Jul 29, 2001, 02:36 AM Registered User 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.
 May 31, 2008, 05:51 PM alex4orly 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
 May 31, 2008, 10:14 PM Pushin' the Limits 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
 Jun 01, 2008, 08:34 AM An itch?. Scratch build. 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.
 Jun 01, 2008, 08:39 AM An itch?. Scratch build. 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.
 Jun 01, 2008, 12:23 PM Pushin' the Limits 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
Jun 01, 2008, 01:21 PM
Boffin
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.
Jun 01, 2008, 01:21 PM
Suspended Account
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
Jun 01, 2008, 01:25 PM
An itch?. Scratch build.
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.