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ARF's and CG, motors and engines

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Old 04-20-2014, 05:45 AM
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chuckk2
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Default ARF's and CG, motors and engines

Many of the current ARFs are designed for either/both engine or electric power. Even so, they tend to have the same CG specified in the manual.
The CG may be given as a single point, or a range.
Some obvious things to remember - - -
Engines use fuel, so the nose will be lighter at the end of a flight when the fuel tank is empty, and a bit nose heavy on takeoff with a full tank.
Electric motors and batteries, despite rumors to the contrary, weigh the same at landing as they did at take off. (Unless something fell out!)

Engine powered ARFs CG is normally checked and specified with an empty tank, as that would be the most tail heavy condition.
Electric powered ARFs CG is normally checked with the battery present, thus showing the actual CG.

A bit of dichotomy, perhaps?

When you are lucky enough to have a maximum forward and aft CG given or found, should the common practice on engine powered ARFs be used? (Split the difference, and go from there.)

Or, instead, should some other "rule" be used for electric motor powered ARFs?

Instead of just splitting the difference, perhaps this might be better.
Mark the most forward, center, and aft recommended CG points when they are specified or otherwise found.
Split the difference between the center point and the aft point, and use that for the desired CG on a motor powered ARF.
This should result in a less nose heavy condition, and likely a bit better behavior.

Last edited by chuckk2; 04-20-2014 at 09:56 PM.
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Old 04-29-2014, 09:45 AM
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aeajr
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CG is not an absolute number. This is more noticeable in gliders than in powered aircraft. I have no idea how it applies to helis or multi rotors.

Since I am primarily a glider pilot I normally set the CG at the specified point then go fly. I know it will fly at this position. Then I "tune to taste" to put it where I want it to have the glider fly the way I want It to fly. Usually I move it back from the starting point.

For my electric airplanes I do the same thing. Start at the given spec, then tune from there. However because you are flying under power virtually all the time it is not as critical unless you are a highly aerobatic pilot. For aerobatics I believe the tendency is again to move the CG back to make the aircraft more responsive to the elevator.

For trainers I think there is a tendency to keep the CG a bit forward to emphasize stability over maneuverability.

So, my advice is to set it per the instructions and "tune to taste."
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Old 04-30-2014, 01:42 AM
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"So, my advice is to set it per the instructions and "tune to taste.""

That's the common method, and it works! All I'm trying to say is that there may be a way to
narrow the starting point range a bit.

When I've in flight trimmed, and so forth, I usually check to see if there is any noticable
elevator deflection from the horizontal stabilizer when the model is back on the ground.
If not, well and good. If there is, then I'll consider a change in CG.

Naturally, there are other considerations as well.
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