CG changes
 

I all the time read about people saying, move the CG forward or backward. I thought the CG stayed the same. Do they mean to just balance the plane at a different spot? If a plane was balanced at the correct spot and you 'moved the CG forward' without rebalancing the plane it would be tail heavy.So when the advice to move the CG forward or back is given they are saying to rebalance at a new spot. Am I right or wrong on this?

There are two ways to think about "CG". The first is the place shown in the builders manual or plans. This is the "design CG" and is where the plane is intended to be balanced. The other view is the place on the model where it balances fore and aft. This is the "actual CG"....if you hang the plane by a string it will balance level. When someone says "move the CG" they mean the actual CG.

OK, this means to balance the plane at another spot forward or back from the "design CG" correct?I would have to add weight it the CG was moved forward and take out weight if it was moved back. Right?

Correct, if you add weight to the nose the actual CG moves forward. If you add weight to the tail the actual CG moves aft.

OK, this is what I always thought but was worried I was missing something. People say moved the cg forward or back to change flight differences but just assume everyone knows what they mean.Thanks.

CG isn't one absolute spot, it's a range where the plane will fly, If the plans call out one spot,, that's usually the center of the range.

Some people like to fly a more aft CG,, "Aft in the range", typically for a more aerobatic snappy performance,, some prefer a forward CG on their planes, that typically makes the plane fly more stable. It's a personal preference.

If a plane is balanced outside this range then it wont fly, correct? Is the range defined by the airfoil?

The C/G is determined by the mean aerodynamic cord (MAC). The MAC is the average cord dimension of the wing. If the wing is a simple straight, non tapered wing then measure the wing from the front of the leading edge to the rear of the trailing edge and that is the wing cord. 25-33% of that cord dimension (MAC) is the C/G range that is normally used (aft of the leading edge). If the wing is tapered, measure the widest part (usually at the root by the fuselage), then measure the narrowist part (that would be outward just before the tip) and average those two numbers to get the MAC for a tapered wing. The percentage number within that 25-33% range that you pick as your C/G spot to balance at is influenced by the type of aircraft, it's intended flying style, and the experience of the pilot. It is generally accepted that if you go outside the 25-33% area you may not have a flyable airplane. Yes, some hot dogs go past the 33% (in a tail heavy direction) with some aerobatic planes but they are not flying in the normal range and will either be very skilled or cleaning up a crashed airplane. Maybe both. For beginners, stay close to the 25% end of the range for a friendly flying airplane.

Thank you very much for your definition. This gives me a clearer picture of the CG and changing of it. Before the answer to just move the "cg"did not give a clear picture of the overall concept of the CG. There were no limits and now there are.Thanks.

Every plane has a center-of gravity(CG), where the plane balances level. The CG location is very important to the flight of the plane, to get optimum performance, and a flyable model.

The suggested CG location is derived from the designer/engineer based on aerodynamics of given airfoils/shapes all dealing with pressure/gravity and effects of flight...That info is on the plans, or in the instructs.

The actual CG location is just that, based on where its found on the actual model itself... That can and will vary depending on the engine/motor, radio, and everything to do with the finished plane. Usually it won't vary too much, or be extremely off from the suggested CG, but when it does, and its evident that its too far from the suggested(or desired) CG, the CG point must be changed(moving the weight within the plane or adding weight when needed(last resort))... to get the actual CG closer to the suggested(or desired) CG, to hopefully make the model perform as it should reliably...

As someone stated, there is a CG "range", which is the small area on either side of the exact CG(we'll say suggested CG in this case)..that is still a relatively safe area for test purposes... I recall about a 1/2"-3/4" range on about a .40 size plane to be safe for test purpose.

The CG of a model can be fine tuned, on high performance planes, aerobats, and every plane that one might want to get better handling and flight character. But all in all, the CG of the actual model should be what the suggested CG is in the manual for starters... and if it's not then... You have to move the CG... which is what I'm getting your question is... so.. yes.. you move the CG via moving components and/or adding/removing weight until the plane balances where you want it too.

In a nutshell.... the constant here is... There is always a CG... the variable is... is it in the correct place? the answer is... if its not.. move it. :)

PS... I've always wondered why we call it center of gravity, I think its actually center of longitudinal balance... and don't forget to balance laterally..(side to side).. the plane should sit level side to side... and obviously the CG point is center of fuselage there.

mashp39, BTY, I neglected to mention that sometimes the C/G stated in the instructions are just flat out wrong! It seem like that happens more with ARFs than stick built kits but be aware that what is printed in your instructions MIGHT not be correct. Now that you know how to figure the C/G you can (and should) double check the position called out by the manufacturer. I don't know if it is a problem with language translation or what, but it happens. Be aware.

Guys, figuring the safe range in which to locate the CG is affected by way more than just the MAC. In fact, the size of the horizontal tail and how far back it is could be the most important detail (pun intended). Even the wing's airfoil affects it.

The idea there is one best % of MAC to use ignores all but one detail. That there is one perfect percentage would be a great idea, but for it to be possible would only work if every airplane and model were all exactly the same and had the same wing and tail shapes, aspect ratios, tapers, airfoils, and a few others.

Over time the aviation world has narrowed down the formulas used to estimate CG locations and the simplest single computation happens to be available online. It happens to take 9 different measurements. http://adamone.rchomepage.com/cg_calc.htm It's actually quite simple to do as it only requires a yardstick or tape measure and 4 or 5 minutes.

One reason today's ARFs aren't to be trusted to have a good CG suggestion, is they're being manufactured by people who really have little or no experience flying models. They're awesome mfg's of toys, but not so good beyond that.

It is simple.

But you do have to measure the wing:
root chord
tip chord
sweep
half span.

Then do the same measurements for the horizontal tail.

Now measure the distance from the wing LE to the tail LE.

You're done measuring. Plug in those numbers and run the app with 5% static margin, then run with 15% and you'll have the CG range for YOUR exact model.

It really does take into consideration enough of what actually matters to work out a safe starting range for your exact model's CG.

If there is any doubt err towards a forward CG rather than a rearward one.

Too far forward and you stand a pretty good chance of getting it back in one piece, this is not the case if the cg is to far aft. The model will be virtually uncontrollable in pitch and likely to spin.

See https://youtu.be/00CGAPOp7A0

The guy flying and explaining in that video did a nice job. He was clear and not overly technical in what he was saying.

Did I miss something? Looking for video here.

See post no 15

Those guys are pretty cool. I've been subscribed to their YouTube channel for years. I don't know how I missed your link... but thanks for pointing it out.

The video basically shows how to fly a tail heavy model, and demonstrates how possible it is to come out alive.

They don't seem to offer anything more than that, such as how far out the CG was. Nor did they mention how simple it is for 3D pilots to successfully fly tail heavy models, something less experienced flyers would greatly benefit from. The video actually doesn't say much more than "it's possible even when grossly tail heavy". They don't even take a couple of seconds to show where their model balanced. (although they did take the time to show a P51 about to be checked)

Actually seeing how far out the CG of their T28 foamy is would have been worth more than 1000 words.

With one exception I have found CG to be flyable in ARF and kits.
Exact CG is a pilots choice. I now perfer a neutral CG, vice slightly nose heavy.
I check CG by flying a 45 deg up line roll inverted and let go of the sticks. If she climbs she's tail heavy, if she dives then she's nose heavy.
a very nose heavy airplane will not want to slow down on approach and you're going to buy lots of props because it won't flair.
A tail heavy airplane if flyable will be sensitive on the controls.

As for a tail heavy model being sensitive on the controls.....

And adjusting the elevator throw to suit the sensitivity, dialing in exponential, setting up appropriate dual rates, or simply learning how gently to move the stick (as 3D pilots do) can work great.

The terror some posters have of tail heavy models is grounded (pun intended) in inexperience quite often.

This is true Rock

Yu may be helped by this: http://www.rcflightschool.com/SetupPDFs/cg.pdf

When you buy an ARF or kit, the instructions tell you where the designer found the CG worked the best. Or sometimes, where he figured it would be best for a less experienced flyer. That designer knows how effective the elevator (stabilizer) is when setup with the throws he found worked best, and that's usually the suggested location you find in the last couple of pages of the included instructions.

So you wind up with two CG locations, the one suggested and the result of your components and your placement of them.

Its sensible to setup the elevator throws suggested as they match up with the CG location suggested, which matches those throws.

So are you afraid the plane will be too much to handle without some insurance? Just about all transmitters nowadays have dual rates. Use them, at least on the elevator. Setup a low rate that's about 2/3 the suggested throw in the instructions. Take off with the suggested throws and if it scares you, flip to low. Too much for your flying skills? Take off on low rates then. If it's too sluggish (like it'll be if nose heavy) you can easily flip to the suggested rate.

If both these ideas scare you, it'd be a really good idea to get an experienced flyer to maiden your plane for you.

Know the old saying, "a nose heavy model flies sluggishly, a tail heavy model flies....(fill in your favorite type of doom)"... really is silly, because anyone who would blow off setting a realistic CG and winds up taking off with a tail heavy model should take up a hobby better suited to them.