The Co-Pilot and the Futaba equivilent (and now the device from hobbico in the Nextstar) can make learning both easier and harder.
They are useful in that they help the aircraft to remain level, or to get back to level quickly just by letting go of the sticks. They can save your airplane almost as well as the instructor.
The co-pilot can't see the tree you are about to hit
The co-pilot also won't help you figure out which way to turn to get that little spot to start heading back toward you instead of away....)
The Co-pilot also can't tell upright from inverted...[X(] If yuo got it REALLY messed up, and let go of the sitck, the plane could end up flying level, upside down (with a trainer that means descending VERY fast.. roll quick or its going to hit the dirt.)
You can also end up deendant on the thing to constantly correct your errors... you simply never learn to properly fly the airplane, so you never progress to the type of flying you really want to do. This is similar to the student on the buddy box who just won't ever solo, because he is too busy doing "tricks" to learn to fly level.
The last is all to common with those who are between 10 and 14... [>:]unless the instructor takes over at the start of every loop or roll.[>:] ( "Fly level around the pattern or not at all kid!"[:@] ) But you have to let the kid do some of the loops... or he loses interest[&:] ( I HAVE taught a few kids a little about flying...

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These things are tools. You can use them and they can assist you. You can abuse them... and you will not gain the experience you need to become a better pilot.