Friend asked a simple question (to which I have no answer)!

If the CG is 25% of a wing's chord and you slide the wing back say 1", where is the CG now? Same 25%?

If you slide the original configuration foreward, same question!!!!

Jerry

When the wing shifts forward an inch, the CG is at a larger percent of the chord than before. When the wing shifts aft an inch the CG is at less of a percent of the chord than it was before. To get a more specific answer, you must specify the weight of the wing, the weight of the rest of the plane and the wing chord.

Which CG are you referring to...the original?

Jerry

When the wing shifts an inch, the shifting weight of the wing shifts the CG by a fraction of an inch. That fraction of an inch is the ratio of the wing weight to the weight of the whole plane. That fraction of an inch represents some percentage of the wing chord which depends on the chord’s actual measurement. That shift in CG is in the direction of the wing shift.

When the wing shifts it shifts the position of the wing chord by an inch. Since the CG is referenced to the chord before and after the shift, we must take that into account by adding or subtracting (depending on the direction of the shift) the percentage of chord that one inch represents.

If you specify the wing chord, the weight of the wing and the weight of the whole plane, I will run through the calculation for you step by step.

BTW you can tell when you have arrived by the feeling of acomplishment if you are an explorer by temperament.

So, if i am understanding you correctly, the change in CG is affected only by the shifting WEIGHT of the wing and not the amount of coupling (is that the right term?). (distance between say, wing and horiz. stab.)

The original question was one that dealt with the measurement of CG as a percent of chord and did not address the consequences of such a shift. Now you raise quite a different question which is how the wing shift affects the tail moment arm. The second question has implications for the location of the neutral point. The distance from the neutral point to the CG is called the static margin which determines the static stability.

Since the first question uses the wing as a reference, it is the shifting of the weight of the rest of the plane relative to the wing that must be considered. One can easily be confused if one thinks in terms of a shifting reference. You must look at the problem from the point of view of a fixed wing and a shifting fuselage and tail because the wing has been chosen as the reference and the reference must be fixed to avoid confusion.

Assuming you are talking about a typical size RC model, the rough answer is that the CG should be adjusted to keep it at the same % of wing chord.

yes, if you move the wing back, you loose a bit of tail moment arm, but 1" out of a few feet is not much of a change.

Bob Parks

It is the same 25% no matter where the wing goes

Without citing any names, let me say that there are some Sukhoi SU 31 models out there that just will not balance at the suggested CG point. I suspect that they were designed by looking at 3 views of the actual plane which has a very heavy radial engine. When converted to models, the ratio of engine to aircraft weight is different from the full size airplane. So instead of being able to balance the kit where suggested with a 6lb engine, an engine weight of 8lbs or more may be required to acheive correct CG without adding dead weight to the nose somewhere. If the wing tube is placed further aft, the engine moment becomes greater and the tail moment less and correct CG can be acheived. If the movement of the tube is about 2 inches, will that effect cause the flying characteristics be noticably changed?? What is the concensus fo opinion of the panel about this method of avoiding carrying dead weight into the air???? I would probably solve the problem by using a larger engine before I would haul 5lb of lead around, but that is just my preference.