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Old 07-24-2014, 01:04 PM   #1
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Default setting CG

Talking to some pilots some like their planes a little nose heavy others like a little tail heavy I though you had to set it up to the manufactures CG What advantage is it in making it nose or tail heavy
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Old 07-24-2014, 01:12 PM   #2
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In general, tail heavy planes have a tendency to fly....once. Usually the manf CG is the best place to be. It's possible to get a more nose heavy plane if you change the size of batteries used. Some pilots of 3D planes will go for a more tail heavy setting as it enhances some of the moves they can do. Overall it's much harder to control a tail heavy plane than it is a nose heavy one.
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Old 07-24-2014, 03:18 PM   #3
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But remember, even the manufacturers get their cg's wrong at times. Even the almighty ParkZone with the infamous Extra 300, remember that whole thing? Then there were the FMS 1400mm size Corsairs. Many of them didn't survive long enough for the owner to realize the mfr cg recommendation was too far aft. So to me, that recommended cg is a good start, but not the final say. If it's a popular plane, I'll see what others use for cg in the forums. Personally I lean towards a little nose heavy, as most of my stuff is scale and I'm a casual flyer. But the stunt planes benefit from a more aft cg.
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Old 07-24-2014, 03:24 PM   #4
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The reason we don't want our planes tail heavy is because
it will tent to make the wing stall at slow settings. Stall = crash if too close
to the ground to recover.

Helicopters don't really fly.......
They're just so ugly, that the earth repels them.
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Old 07-24-2014, 04:39 PM   #5
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Manufacturers' recommended CG is usually somewhat on the nose-heavy side to enhance stability so anywhere in the indicated range should be fine (usually) for the initial flight.

There are a lot of factors involved in where you end up setting CG after test flight confirms the aircraft flys acceptably. Trainers we typically leave in the conservative range well ahead of center of lift.

Center of lift vs CG is what determines how stable the airplane is in pitch. The more forward CG is from center of lift the more force is needed to hold the nose up and the more change in force is needed to cause a change in attitude. When you get the CG behind center of lift the force needed to change attitude gets very small and the slightest wing gust can cause an extreme pitch excursion.

Note that center of lift includes all horizontal flight surfaces with a conventional aircraft (looks a lot like a Piper Cub) often having the tail contributing negative lift. A "Canard" has a small forward horizontal surface and large rear main wing with both providing positive lift and the center of lift might be in front of the main wing, forcing the CG to be between the fore-plane and main wing...
(just examples showing that many factors can be involved)

It really doesn't affect the speed or attitude where the plane will stall... CG will change how the airplane responds when it stalls.
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Old 07-24-2014, 09:02 PM   #6
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What fhhuber said, good explanation.
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Old 07-24-2014, 11:01 PM   #7
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Yep, there is no 'optimal' CG position. The manufacturers recommendation is usually a safe starting point that will fly ok, but many like to fine tune their models to their own preference.

Generally nose heavy makes the plane more stable but less responsive and also more sensitive to changes in airspeed. A nose heavy plane will climb a lot when it flies fast and dive when you slow up. It will also needs lots of down elevator when flying inverted. Providing it's not taken too far 'nose heavy' is a 'safe' set up but not optimal for more advanced flying. Some advise adding more nose weight as a panacea for all ill's, this is a big mistake. More nose heavy is not always better a very nose heavy plane can be a real problem to fly.

As you move the CG back you reduce all of the above tendencies. The plane becomes less speed sensitive and more responsive to elevator input which is good for advanced flyers but makes it easier to stall for the less experienced.

Personally I usually end up moving the CG back from the manufacturers recommended position, but there are exceptions. I've a couple of 3D models where the manufacturers CG gave a really tail heavy trim that was a real handful to fly.

Other than the maiden flight I tend not to worry much about where the CG is but more about how the plane flies.
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Old 07-25-2014, 04:06 AM   #8
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Originally Posted by JetPlaneFlyer View Post
Generally nose heavy makes the plane more stable but less responsive and also more sensitive to changes in airspeed. A nose heavy plane will climb a lot when it flies fast and dive when you slow up. It will also needs lots of down elevator when flying inverted. Providing it's not taken too far 'nose heavy' is a 'safe' set up but not optimal for more advanced flying. Some advise adding more nose weight as a panacea for all ill's, this is a big mistake. More nose heavy is not always better a very nose heavy plane can be a real problem to fly.
This.

Very generally speaking, forward CGs are more stable / less responsive. Rearward CGs are less stable / more responsive.

Even if we have identical planes, you might prefer yours to be a bit more responsive than I do, so you might favor a CG farther back. Your plane will also be more challenging to fly, but you might be perfectly willing to make that trade-off.

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