I am confused by some post I have been reading, I hope some one can straighten me out. Several have stated that at the recommended 100MM from the leading edge their model was tail heavy. Then they state that moving the CG to 90 MM from the leading edge corrected the tail heavy problem. In my mind if its tail heavy at 100MM from the LE and you move the CG to 90MM from the LE it is going to be even more tail heavy. What am I missing here? Thanks

Hi Silver Flyer
It is a bit confusing. You have the CG at 100MM back from the wing LE. Now you add some lead weight, say to the engine mount. You re-check the CG and you will see that it has moved toward the engine. OK? Instead of adding weight, you may have been able to move the battery or the engine forward.

You would have to shift LESS weight to correct a tail heavy condition at a 100mm location than at the 90mm location (fulcrum point).

You are confusing moving the pivot point (in this case from 100mm forward to 90mm) WITH actually moving the CG.

If you have a tail heavy condition at 100mm, then slide your fingers forward to the 90mm location (pivot point) and lift the plane again, the tail heavy condition worsens!

Confused? Me too...Hope I got this right!

I'm going to suggest that the person you quote added weight to the nose. That moves the balance point (the CG) toward the nose (in this case, 10 mm closer to the nose.)

I see the plane balanced on a support at 100mm behind the leading edge, and a few ounces added to the nose. The nose tips down. In order to get the plane level, the support is moved ahead 10 mm (to the new balance point, 90 mm behind the leading edge.)

Does that help?

Best wishes,
Dave Olson

If the plans call for the plane to balance at 100MM and it is tail heavy there, then I add enough weight to make it balance at 100 MMM, wheather it be by moving the engine forward or the battery or maybe a larger engine. The balance point is still at 100 MM. (I think). Thanks for the answers, I am still mulling this over.

If its tail heavy at 100mm then its tail heavy no matter whether you add weight or shift weight to achieve 100mm. The downside of adding weight is increasing wing loading (which will make a tail heavy plane even worse)

Remember......(borrowed from Gordon Banks).....

Nose heavy planes my fly poorly but tail heavy planes may fly only once.

Silver Flyer:

You are correct!
What has been called the CG, is not!
What we do is to locate that point along the fuselage where there is no torsion force or torque trying to pitch the fuselage, in any direction.
When the wings are supported at that point, the fuselage remains horizontal, and, if disturbed, it always returns to that position.
This happens because the forces and levers are balanced, in front, and behind that point.
By adding or taking away weight at the nose or the tail, we can make ANY point in between the CG or balance point.

Now, any model will fly at its best if it balances around a "magic point".
The location of that "magic point" has to do with the location of the lifting force produced by the wings and the tail, which only appears while the model is flying.
That is the reason it is referenced to a point of the wing, generally, the leading edge next to the fuselage.

By playing with the distribution of the weights along the fuselage, we make the CG and that "magic point" to coincide.

In your example, that post you have been reading, has stated that at the recommended by the designer "magic point", the model was flying like tail heavy models fly. Then it states that adding enough weight to the nose moved the CG (or balance point) closer to the leading edge, where the "magic point" was verified by experimentation to be.

Two things may have happened there:
1) A design or typo mistake. The "magic point" was assumed, calculated, located or described wrongly.
2) Some deviations from the original drawings were introduced during the construction of the model, enough to cause a 3/8" slide of the "magic point".

Thanks, Lnewqban, I did some playing around with my model, the Edge 540T with an OS 91Fx today and by moving the 2 batteries as far forward as they would go the model balanced at exactly 100 MM using a home made balance rig similar to the small one Tower sells. I coiuld have gone out more with the engine but then the cowling would not have fit. I have built a few models and have always put a swivel in the bottom of the fuse exactly where recommended and hung it from a ceiling hook, upside down, thus being able to balance in all directions before the plane was finished and covered. Always worked out well and never had to re balance. This is an ARF and only has ultrakote on the bottom so that couldn't be done this time. After reading page after page of posts regarding the Seagull Edge 540 like mine I realize lot of folks seem to have a different idea about what exactly is meant by CG and balancing a model. Your answer covered it very well and thanks for "agreeing" with me.

Phoenix Super Decathlon? Much better starting at 90 then moving back. I think my sweet spot was about 93mm.

Like others have said above, move something toward the front of the plane, effectively moving the CG closer to the front edge of the wing. Can move the battery forward, the engine forward if need be...

Last thing you want to do is add any weight. That plane flies pretty nice (again assuming it's the Decathlon) but I flew it with an added smoke system and the extra weight took all the grace out of the plane.

Adding dead weight is never desirable, but it is always better than flying a model that is balanced off the “magic point”. A few additional ounces will make the model flight a little faster, while an improper balanced model will never reach its peak performance. Using bigger batteries is a way to make the extra weight useful.

May be you could try the ingenuous CG machine described in this link:
Vanessa CG machine:
http://home.mindspring.com/~the-plum...%20Machine.htm

If interested, you could read more about CG and balancing and trimming a model here:

Animation describing the theory behind the CG location:
http://www.geistware.com/rcmodeling/...rorgravity.htm

http://www.hebertcompetitiondesigns....ngulation.html

http://findarticles.com/p/articles/m...g=content;col1

http://www.flyrc.com/articles/03-16-...airplane.shtml

http://fatlion.com/sailplanes/divetesting.html

http://www.charlesriverrc.org/articl...GMarkDrela.htm

http://www.hebertcompetitiondesigns....ngulation.html

http://www.seniorpattern.com/pdf/BalancingAct.pdf

http://www.geistware.com/rcmodeling/...calc/index.htm


A model that is at balance at any CG machine, is supported at its CG.
A model that is flying, is supported by the air at the NP (neutral point) or CP (center of pressure).
For stable flight, there should be a distance between the CG and the NP, and the NP (point of support during flight) must be behind the CG (unique point around which the model rotates).
This distance is called stability margin.
Big stability margin: High pitch and yaw stability – low maneuverability (Like an arrow).
Small stability margin: Low pitch and yaw stability – high maneuverability.

If the CG is forced to relocate just behind the NP, the model becomes as instable as an arrow flying with the feathers at front.
Hence, there is a safe margin for CG located way ahead (a nose heavy model will fly poorly), but the margin is smaller and dangerous for CG located to far back (a tail heavy model will fly once).

Hence, the “magic point” or best location of the CG, as measured in a CG machine, is a matter of pilot’s preference and skills.
The designer will experiment with the model and will establish a range of CG location that is suitable for most of the pilots and most of the models built exactly as his drawings specify.
This should be taken as a first approach to proper balance.
The only way to find the “magic point” for YOUR model and for YOUR preferred compromise stability–maneuverability, is by experimenting and trimming.