[essi]

HI GUYS
Is that true that as you move the CG towards the back,plane would be more aerobatic!
i did it on 300 extra, flew ok but such a hard landing almost crash...

[chuebner]

Yup it does but moving it back to far will make certain airframes almost uncontrollable. Highly aerobatic, fun fly etc etc planes handle it better....in planes like warbirds it would be disasterous.

There is a saying..... Forward CG planes fly, tail heavy planes fly once.

[JoeT]

Generally 30% back from the MAC is a good starting point. The farther rearward you put the CG the more unstable the plane gets. The elevator sensitivity goes way up, and depending on the size of the horizontal stab / elevator, may stall first during slow flight.

There's a correct CG "RANGE" where it's safe to move to get the appropriate response, anything out of that range = major compremise.

The old saying goes: A nose heavy plane flies poorly, a tail heavy plane flies ONCE!!

[Mike Sebastien]

Why did you move the CG to the rear? Bad landing isn't indicative of a rearward CG it's more about the pilot.

[Cougar429]

The reason there is the saying a tail heavy plane flies only once is that most aircraft are designed to be stable and controllable within a defined CofG range. Move it outside that and unless changes are made to the control system you will run out of authority. In most cases this occurs when slowed down for aeros or landing. As speed lessens so does airflow over the tail and if the plane departs from controlled flight and cannot recover the end is pretty well guaranteed very quickly, usually from the wing stalling and remaining that way.

Design can play a part, as well, as some aircraft can hide the tails behind the wing at high alpha, meaning the tail moves into the airflow shadow of the main wing and therefore loses complete control. In tail heavy conditions it remains that way and unrecoverable.

Ducted fans, pushers and turbines can suffer the same effects since they have no way to accelerate airflow over the controls with propwash and unless the CofG is forward enough to pull the nose down or they have enough thrust to accelerate away they don't live long either.

Having said that there are systems that now mimic what has been available in full size military aviation for quite a while. As said, tail heavy can make the plane more maneuverable, a condition you would want in a fighter aircraft. Unfortunately, the closer to the ragged edge, (and beyond) you push this condition the more you rely on external stabilization and limitation systems. The F-16 is a perfect example of this, if not the first to require external systems to the pilot to remain controllable in flight. The plane is so tail heavy that without those systems it simply will not remain airborne.

These systems work by recognizing and correcting changes too small and quick to be noticed by us mere humans and inputting control corrections before these changes can grow too large to be reigned back. Lately the same can be said of stab systems available to modellers in the form of Futaba and Demon multi-axis gyros.

[Flypaper]

One good way of finding the rearward limit on an aerobatic plane is to use the inverted flight method. Keep moving the CG back a bit at a time and retrimming for level flight each time you go upright. When you go inverted, it will normally drop the nose when going inverted. Keep moving the CG back until you find the magic spot, the plane will fly level inverted or upright with no trim change and no elev. stick movement.
Another point. As you move the cg back, you need to lessen the control throws to keep the same loop rate as with the CG further ahead.

[essi]

thanks Gord
you've been help full thanks for your info...

[moo]

Originally Posted by Cougar429

Lately the same can be said of stab systems available to modellers in the form of Futaba and Demon multi-axis gyros.

If you want something really fast, stable, and bleeding edge get yourself over to cleanflight running on a Naze32

about $30

Just switch it on for take-off and landing
You can also use it to run your string of RGB LEDs

P.S. they are not gyros, Naze32 uses a 10 degree-of-freedom accelerometer and air pressure sensor. You can use a compass and GPS (another $25)

A Naze32 mini is under 10 grams

Much older board (three years ago): https://youtu.be/3KFCbe_FRwE?t=24m
but you will get the idea