Changing plane's weight > affect on CG?

[stevem1928]

How does changing the planes weight affect the CG placement?

I have been flying a custom cut Projeti-like wing 32" wingspan on a Johnson 250 and Lipos. AUW was 12.5 oz. I changed it over to a speed 400 and 8 cell 1100 HE cells. It now weights 18.5 oz. I extended the motor mount backwards about 1" to get the CG where it was before. The thrust line remains the same. On the test glide, the nose went straight up, even with the elevons slightly lower than they were with the light AUW. Does the weight change affect where the CG needs to be? Or, is the CG just more sensative with more weight. Maybe the CG was too far back on the lighter version, and it just did not affect the flight as much as the heavier wing.

[Sparky Paul]

A small change on a small plane with a restricted c.g. range can do what you observed.
Ideally, if ALL that was changed was the weight.. c.g. the same, no change in surface position, then the plane should have nosed down.
More weight requires more trim.
As it went the other way, something else has changed in addition to the weight, espcially since you say you lowered the surface position from that which had worked.

[Sail 'n Soar]

How does changing the planes weight affect the CG placement?

I have been flying a custom cut Projeti-like wing 32" wingspan on a Johnson 250 and Lipos. AUW was 12.5 oz. I changed it over to a speed 400 and 8 cell 1100 HE cells. It now weights 18.5 oz. I extended the motor mount backwards about 1" to get the CG where it was before. The thrust line remains the same. On the test glide, the nose went straight up, even with the elevons slightly lower than they were with the light AUW. Does the weight change affect where the CG needs to be? Or, is the CG just more sensative with more weight. Maybe the CG was too far back on the lighter version, and it just did not affect the flight as much as the heavier wing.

With the trim and CG just the same normally you would expect the ~50% weight increase to result in ~20% required flight speed increase and slightly better L/D due to the higher Re, but with no other noticeable impact. Flying wings are more sensitive to CG shifts than conventional configuration aircraft. I'd guess your CG is slightly aft of the previous location, but not detected by the method you used to measure the current position (I use my stubby finger tips with questionably repeatable results.) Did your Projeti-like wing hand glide previously OK?

[stevem1928]

I fixed the problem. I moved the CG 1" farther forward from where it was with the light motor. It is amazing that the thing flew with the CG that far back on a plane with only 240"sq and a root cord of 12". I obviously compensated with the trim, more than I had realized.

Sail n' Soar,
I use the same unscientific finger tips to check for the CG. Yes, it did glide well before. I always start my test glides early in the building process when the plane is light. I tape the elevons on, and get an approximate CG. The test glides seemed to find the CG way behind where I had calculated it. It looks like the calculations were more accurate.