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-   -   where to place the CG (http://www.Wattflyer.com/forums/showthread.php?t=72780)

egrave1 12-12-2013 01:11 AM

where to place the CG
When the airplane CG says 31/2" to 41/2" what does it make the airplane do I still have to balance it at both 31/2 and 41/2 I would think that 41/2 would make the airplane noes heavy

kyleservicetech 12-12-2013 01:20 AM


Originally Posted by egrave1 (Post 933962)
When the airplane CG says 31/2" to 41/2" what does it make the airplane do I still have to balance it at both 31/2 and 41/2 I would think that 41/2 would make the airplane noes heavy

The further back the CG is located, the more unstable the model will be. To far back toward the tail, and your model will not be controllable.

And, to far forward, and you might not be able to pull the nose up during a landing. So, the location between the suggestions by the mfg should be fairly safe. Adjust either forward or rearward to suit your flying preferences.

As for me, I just put the CG per the mfg recommendations, and go fly.

Some folks out there fly the model up high, then while reducing power, put the model in a dive while returning the elevator to neutral. If the model continues straight, the CG is pretty much neutral. If the model pulls out, this is a sign of being nose heavy. If it tucks under, look out!

This would be for an acrobatic model. It would be of less usefulness for a Piper Cub type of model.

mclarkson 12-12-2013 01:23 AM

As a general rule, the further forward the CG is, the more stable the plane. The further back the CG is, the more responsive/maneuverable the plane.

Go too far in either direction and the plane becomes unflyable. There's an old saying that a plane with the CG too far forward flies poorly, but a plane with the CG too far back flies only once.

Start with the CG near the forward-most position recommended. Then, if you want, you can experiment with pushing it farther backwards to find the sweet spot for you, personally.

egrave1 12-12-2013 01:29 AM

thanks for clearing that up for me I will get the hang of this soon I hope

mclarkson 12-12-2013 01:32 AM

Always feel free to ask. Nobody starts out knowing all of this stuff.

Rockin Robbins 12-12-2013 03:01 AM

I think it's safe to say that the plane will fly at both extremes. What they're saying is that it is your choice to pick a CG between those. If you start forward you'll find that the plane is very stable, but that you are holding the nose up with the elevator, which has to exert quite a bit of force just to keep the plane flying level. Lots of people with powered planes like a plane that flies like that. They weather vane into the wind really well and aren't upset by gusts. They feel "solid."

When you move the CG back it takes less force to change the flight path of the plane because it doesn't have that stability trying to force it back to straight and level flight. That means you'll find yourself lessening the control throws either mechanically or with your radio. The plane will respond quicker and more decisively to control inputs. It will be upset from straight and level flight by turbulence and wind gusts. It will take more input from you to right the plane and return to straight and level.

It's smart to move the CG back slowly, I'd recommend 1/8" at a time. Then give yourself time to get used to how your plane flies that way. Too fast and you'll be too busy being terrified to be able to assess whether you've really made the plane better or worse. Remember more responsive and less stable are just different ways of saying the same thing. Saying your wife looks like the end of a long hard night is the same thing as saying that she looks as fresh as a new morning. She might not take it that way!

Similarly you have to separate out having to learn your "new plane" from whether it's flying better with the new CG. That takes time. When you're comfortable with the new position and think maybe you can improve it more you might consider another move. Once you have gone too far, the plane will still be flyable because you're moving in small steps. Just go back to your previous position and call it good.

There is no "right" CG. It's a matter of personal preference depending on many factors including your skill as a pilot. I like to move CG forward if I'm flying on a windy day. Some people say I'm not so bright for doing so. It's entirely possible that's because they're a better pilot than I am.

quorneng 12-12-2013 04:19 PM

Just to add to what Rockin Robbins has said try to do your test flying is similar, and preferably calm, conditions. That way you are identifying real differences rather than the effect of different conditions.
I have a light weight plane that is hard to control in any sort of turbulence but is a stable cat in still air. Move the CofG forward 1/8" and it handles turbulence pretty well.
It took me a while to find out exactly which caused what!

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