CG Shifts When Doubling Wt of Plane?

[AZ_Astro]

Question for the die-hard modeling experts. You haven't let me down yet!

I have built three fan-fold foam models in the past few weeks and have come up with a CG question.

The airframes come out pretty light - about 5-10 oz. Once the airframe is complete, I tape on bolts, washers, whatever, usually to the front of the plane, to provide ballast for glide testing. After numerous throws with each set of ballast, I mark where the CG is on the bottom of the plane. The bottom usually looks like this:

___!___!___!___!___!___! Rear of Plane ==>
___F___F__O___O__B___B

where F indicates Forward (the nose dropped quickly during glide tests, indicating nose-heavy), and B indicates Backward (the plane balloons up during flight, indicating tail-heavy). O stands for OK.

After glide tests are complete, I put in servos, Rx, and battery, aligning everything so that the CG ends up between the Os. The AUW weight usually comes out to something like 16-22 oz.

During the first few flights of each of these planes, I have found that the O-marked CGs are too far back. I.e., the CG needs to be further forward, in some cases almost an inch!

How can the glide testing be off so much? Does more than doubling the weight of the model cause the CG to shift, when the airframe is basically unchanged?

Any thoughts you may have would be much appreciated.

[Ollie]

The CG location should NOT be used for pitch trim adjustment!!! Pitch trim adjustments should be made with the elevator.

The CG location should be used only for stability adjustments. See:
http://www.polecataero.com/
Select Articles from the menu and go to the article on CG by Dr. Mark Drela.

[AZ_Astro]

<<<The CG location should NOT be used for pitch trim adjustment!!! Pitch trim adjustments should be made with the elevator.>>>

Perhaps I didn't express myself clearly. I am not trying to 'trim' the plane. I am trying to find a CG (or range of CGs) at which the airframe flies well.

Or are you saying that I can't really find the CG using glide tests at all but would need to do a 'dive test' as indicated by Dr. Drela.

My glide-test findings indicate that when the plane is very light (under 10 oz), the best CG is in one location. And when the plane is doubled in weight, the 'best flying' CG appears to significantly shift forward from the glide-test CG. The trim settings are neutral and remain unchanged.

For example, when the plane was light, I found a CG that worked well. But when the plane weight is doubled, that same CG doesn't work - i.e., the plane is unstable.

Does this make sense? It really doesn't to me as the airframe is exactly the same. But, on the other hand, doubling the weight is a significant change.

Appreciate your input. Thanks.

[Sparky Paul]

If the added weight is EXACTLY on the old c.g., there will be no difference in the total c.g.
See the picture..
BUT.. you now need a LOT more alpha to generate the Cl to keep the thing in the air.
Or more airspeed.
Or more wing area.
What you're seeing isn't increased "sensitivity".. it's just stalling!

[vintage1]

I think everyne has worked out part of the picture, but the whole of the picture goes something like this (always assuming I gotr it right, which is questionable:))

As you add weight, the wing loading goes up. As the wing loading goes up, the glide speed goes up.

As the glide speed goes up, the stabiliser exerts more downward pressure on the tail. Now this *ought* to be counteracted by the fact that there is more weight ahead of the center of pressure, but I doubt the relationship is linear, AND it may well be that the center of pressure doesn't stay in the same place either, as the speed changes.

In fact thats the standard way to tune CG isn't it? If the nose pitches up in a dive (equivalent to adding weight in your case) the CG is too far forward.


So the inference is youir CG is already too far foward, and you have a lot of 'decalage' betwen the main wing and the stabiliser to compensate. Move the CG back, and use a tad of down elevator to re-trim, and then adding weight won't have such an effect?

Does that sound right?

[Ollie]

Variations in wing loading do not much affect the pitch trim if the location of the CG is fixed.

AZ Astro's e observations are undoubtedly caused by a change in elevator angle when the servo is connected. Not by any wing loading variation.

[KillerWatt]

Why bother with all the "glide test" stuff....wasted bother, at best all they do is beat-up your model... just establish the correct finished CG, move it foward about 1/4" for extra stability insurance, load the model up to minimum all up flying weight (maybe smaller battery) and just FLY the thing with the radio..assuming the fuse dosen't resemble too much of a banana and the wing a propeller, you ought to get a good flight in or that's how i do it.................. kw

[AZ_Astro]

Thanks for all the responses. I think I have gleaned the answer.

Killerwatt: There is no 'correct finished CG' as these are scratch built models. Nice try though.

Beyond that, I think the following comments hit close to the mark:

<<<As you add weight, the wing loading goes up. As the wing loading goes up, the glide speed goes up. >>>

and

<<<If the added weight is EXACTLY on the old c.g., there will be no difference in the total c.g. See the picture... BUT.. you now need a LOT more alpha to generate the Cl to keep the thing in the air. Or more airspeed. Or more wing area. What you're seeing isn't increased "sensitivity".. it's just stalling>>>

At the same CG location, the airplane glided nicely when it was light but was a brick with the added weight. It really needed a lot more speed for lift.

I don't know what 'alpha' is but I sure do recognize stalling when it occurs.

Thanks everyone.

Jim (AZ_Astro)

[KillerWatt]

well , there are existing general CG rules/guidlines for any fixed wing aircraft design that are "close enough" to get you first powered test flights done in a safe, non-damaging manner unless your designs are some kind of 1/2 wing, reverse triangle flying saucer or something equally weird, but what the heck, smash away... it's just a shame to knock the wings off before the first real radio controlled powered flight........... kw