When is the CG too far aft?

[target]

I know this will get a hot debate going:

For F3F:
When is the CG too far aft?
What are the symptoms?
Is an aft CG better than a slightly more forward one?

Target

[F3X]

I find it too far aft when the model starts to hunt in pitch along the straights. Changing the CG adds or reduces trim drag, once you go past neutral (model goes where pointed) you start adding more trim drag. Also if the little guy pitches on the flats you stop being efficient.

Our F3F models fly at a CL of about .13 when not turning, that's not much stab angle. Moving the CG further back won't make it go faster from neutral trim.

[timmig]

I've also found that slow and high speed "snaps" are possible with the CG too far back when pulling hard pylon turns. Plane gets' overly sensitive to elevator inputs. And like Tom said-- it tends to "hunt" around neutral elevator--and pulls down as speed increase's. All bad things!!
T

[djklein21]

The CG thing is very pilot dependent. Everything you guys have said is true. As the C.G. moves back, the stability of the lpane decreases. This makes the plane more sensitive to control input. This is good and bad. If the plane is more sensitive, it takes less input to make it turn, thus it takes less drag to make it turn. As it gets too far, the stability margin of the tail is decreased, so it becomes more sensitive to flicking. The tighter your servos and linkages are, the more you can move the C.G. back. Remember to reduce your throws as the C.G. is pushed back, this will help you reduce the risk of flicking the plane. Tom has a good article about keeping your plane from flicking on his website under the how to page.

For thermal flight, you will notice that your glider will become more likely to tip stall when flying slow in a thermal. So this is another trade off for a rearward C.G.

So the answer to this question is to move the C.G. back untill you are no longer comfortable flying your pane, then move it back to the spot where you liked your plane the most.

To fly consistently in a competition, you must have a plane that is relatively easy to fly. So spend some time to set up your plane the way you like it. Don't show up at a competition to test fly your plane, you won't learn much that way.

[Jantar2A]

I notice on my planes that are stable with an aft cg that they have a tendancy to squat or sit down in the turn easy,,especially when pulling hard,,as well as they seem to need to be pushed harder just to go slower,,,as well as all the above mentioned stuff...

Nick,,,,who would love to make it out to the aug. grass mtn mom.