Will enlarging wing area relative to tail feather change CG?

[dw1122]

To add ailerons to foamies I used to cut out a trip of the trailing edge, bevel it and tape it back. Not wanting to cut the wing this time I just taped two strips of 1/8"x16"x2" balsa to the original trailing edge of my Multiplex Easystar. The result is a widened wing. Should I do anything to compensate for this change? Are there other issues I need to be aware of?

[HELModels]

My limited opinion is that you will be fine. If you notice anything adverse, it might be reduced glide. If you notice a boost, it might be improved roll ability. :) You are changing the mysterious lift distribution. You might want to experiment with mixing to see if there is a sweet spot for thermaling. It will still turn with rudder only. :) Experimentation and tuning is rewarding.
The pup probably wont mind either way and just enjoy the outing.

[ThinnedDope]

By adding more wing area, you have effectively moved the aerodynamic neutral point forward. This will act similar to having moved the CG back making the plane more pitch sensitive.

Pitch stability requires that the CG be forward of this neural point. Therefore at the extreme, you could move the plane from stable to unstable. However, I suspect you won't be moving the neutral point forward very much. So unless this plane was already borderline unstable, it will probably be fine.

Just in case, tape a small weight (like a penny) to the nose to temporarily move the CG forward. If it flies okay and doesn't seem too pitch sensitive, take the penny off and see how it goes. The point being, err on the side of making it too stable for the first flight, and then move CG back to where you like it best.

Clin

[vintage1]

..however adding area at the BACK of the wing tends to move the neutral pont rearwards..so chances are the two effects will nearly cancel out.

[ThinnedDope]

I think this depends on how you look at it. In the new configuration, the wing area is larger and it's center of lift has in fact moved back relative to any fixed point on the airframe (as you pointed out). This does two things. It shortens the moment arm of the stab and makes the stab smaller as a percentage of the wing. Both of these move the neutral point forward in terms of it's percentage of the wing cord.

I agree though, that the center of lift of the wing has in fact moved back in absolute terms of where it was on the fuselage. In fact, maybe the neutral point did also, but in relative terms, the neutral point has in fact moved forward.

For example, if the neutral point was at 40% of the wing cord (as measured from the leading edge), it would I think, always be less than this after this type of change.

Also, given that the added structure itself affects the CG, the new CG should be positioned as a percentage of the wing cord wherever it happens to be.

Of course, I had 5 hours less sleep than normal last night, so I’m not going to bet the family fortune on this.

Clin

[Sparky Paul]

It takes a LOT of difference in area to make a noticeable difference in performance..
My Goldberg Mirage with the kit wing, and a GL wing of a lot more span/area.
There's nothing much to choose between the wings.. the larger one carries a camera up faster than the kit wing.

[dw1122]

I did not change the CG. The original aileron neutral position was flat with the wing bottom but it nosed up too much. It flew very well after I dialed in some flaperon so they follow the downward curve at the trailing edge of the top of the wing.