I have been flying for about 2 1/2 years and gone through lots of planes. I know about CG and how important it is to flying a plane successfully. My question stems from terminology. I hear people talking about moving CG and I get confused.

If I have an airplane that is perfectly balanced and I move the CG forward, won't that make a plane tail heavy, since I am moving the point to which the plane is balanced toward the front, I now have more weight in the rear and the tail will drop? I have heard it discussed this way and the opposite, meaning someone has said if you move the CG forward that will make the plane nose heavy, and that makes no sense to me. Can anyone explain this to me? Thanks a lot.

Moving the CG forward will indeed make the plane more nose heavy. Moving the CG back will make the plane more tail heavy. When you move the CG, you are not simply picking a new spot on the airplane and calling it the "CG". You have to add or shift weight to effectively move the CG. If you select a location forward of the existing CG and place the airplane on a balancer at that new location, what happens? The tail of the plane will fall and you will need to add weight to the front to get it re-balanced thereby making the plane more nose heavy.

I think part of the problem comes from the double-meaning of CG, and the fact that "nose-heavy", and "tail-heavy" are *relative* terms. A plane is only nose heavy if the *actual* CG is in front of the *intended* CG. Because people talk about the actual and expected CG's interchangable, it is possible to get confused about where the weight actually is ... ;-)

So, planes have the abiolity to fly when the CG is in a suitable range. The plane flies differently depending on where the CG is. If you want a stable plane, you have a forward *intended* CG, if you want a more acrobatic, unstable plane, you aim for a rearward *intened* CG. You then have to move weight around, or add weight to shift the *actual* CG so that it co-incides with the *intended* CG. If the actual CG is in front of the intended CG, the plane is "Nose-Heavy" and weight will have to be moved toward the tail. If the actual CG is behind the intended CG, it is tail heavy, and eight will have to be moved toward the nose.

In pure terms, you can not change the CG of a plane without adding, removing, or moving weight in the plane.

gus

I have wanted to ask this same question for a long time, but couldn't think of the best way to ask the question. Thanks for asking.

Technically, the CG IS where the plane balances. But if the designer says it should be at 2", and your's is at 3", then you have to add weight to the nose to "move" the CG forward.

Pretty easy.
Dennis-

DownSideUp,
Well what's the conclusion? Any of this info help ya out?

I'd like to know whether or not any of this helped too.
Dennis-

The CG is where the plane balances, not some point picked by the designer of the plane. When you see the "CG" marked on plans or listed in the instructions, that's not the CG location, that's where they want you to put the CG. Just as a picture of the engine isn't the engine, it's where they want you to mount the engine.

A plane is tail heavy when the CG is too far back, nose heavy when the CG is too far forward.

"too far" is the tricky part. To far from what? It doesn't depend on what a designer put on the planes, it depends on the configuration of the plane, the aerodynamic forces that affect the stability of an airplane.

I hear that the standard for CG is between a quarter and a third of the cord from the leading edge. Anything from this overall guideline should be considered "too far"?

Now on this nose-heavy, tail-heavy thing. As gus explained this nose-heavy and tail-heavy is relative to the intended CG. Lift your plane at the intended CG. If it is balanced, it means that the plane should fly as the designer had intended it to. If the nose goes down, then it is nose heavy (relative to the original design), which means that it will fly with a nose down attitude relative to the original design. Conversely, if the nose goes up, the plane is tail-heavy and will fly with a nose up attitude relative to the original design. That is it. Finished, end of discussion.

I'm going to put something down for those that are interested and knowledgeable to comment on the following:

1. Each plane is designed with a certain distance between the CG and a point I will refer to as the Center of Lift, which is the point where at a neutral attitude (zero or predetermined angle of attack) the average of lift is. Depending on the nature of the plane the position of the CG relative to the Center of Lift will change:

Trainer - CG will be forward the center of lift so that forward momentum via gravity is maintained
Pattern plane - CG should be near or at the center of lift for more neutral handling
Aerobatic plane - CG may be at or behind the center of lift so that the plane is twitchy and extremely sensitive to inputs and must be flown

2. Nose heavy planes are those planes with CG forward the Center of Lift whereby the plane is always wanting to go forward. In a deadstick these planes will not require the pilot to be too worried about stalling.

3. Tail heavy planes are those planes with CG behind the Center of Lift whereby the plane is inherently unstable. In event of a deadstick these planes needs to be flown all the way or else they will stall.

4. This also explains why high wing is the trainer of choice. Not only does the high wing offer a CG that is below and forward the Center of Lift so that it maintains upright. Imagine the Center of Lift is the pivot and the CG is the weight. In a dive, the CG will want to swing the plane back so that it is relatively below the Center of Lift. Same thing in a climb. Thus stalls are more difficult, they have to be induced. Now look at a low wing acrobatic plane where the CG is above and on top of the Center of Lift. It's like trying to balance a wrench on your fingertip. In a dive, the CG will want to push the plane further into a dive. In a climb the plane will want to pull it further into a stall.

Anyway... just my thoughts that were inspired by this CG discussion.

Looks like the original poster is MIA guys. [sm=sleeping.gif]

Sure, but that doesn't mean the thread has no value.

The 25-33% of chord rule works well for most conventional airplanes. However, the best CG range is determined by a lot of factors. The biggest is the size and moment arm of the horizontal stab. A plane with a very large stab located a long way from the wing can have a CG much farther back, and, in general, will have a broader range of "safe" CG. A plane with a short tail and/or a small horizontal stab will need a farther forward CG, and will be more sensitive to small CG changes.

The center of lift of the wing is important, but it's not the whole story. The real issue is the neutral point of the airframe. Finding the neutral point is quite hard. There are some short hand ways to estimate it based on the aerodynic centers of both the wing and the tail, and the distance between them. (many other factors also affect the NP to a smaller degree)

You want the CG in front of the Neutral point for stability. The distance between the CG and the NP is the static margin. If the CG is on the NP, you have 0 static margin, and have neutral stability. CG behind the NP is unstable. (You can fly an unstable plane, but it's hard)

The vertical location of the CG in relation to the wing is only a small part of pitch stability, I'd say it's actually a negligable part for our purposes. Just having the CG over the wing does not imply that a plane will want to stay in a dive or "tuck". The aerodynamic forces from the wing and stab are much greater than the CG being over the wing. And acrobatic planes tend to have mid- and low-wings for many reasons, the CG location's effect on pitch is just a small part of it.

I'm trying to avoid getting long winded or overly technical. What Fourstroke said IS workable in the real world, even if it's technically not accurate and is overly-simplistic.

If you really wan to know about the CG, go browse through the aerodynaics forums.

Anyway, the point of all that is this. What the plane or instructions say is a good place to start. But it's not "The Law". In fact, some kits have come with incorrect CG locations given. Most trainers have the recommended CG very far forwards, and the plane will fly a lot better with the CG moved back. Compared to the recommended CG, you'd call it "tail heavy", but in the air, it would still be plenty stable, and even exibit the traits of a "nose heavy" plane, since the CG would still be well in front of the NP.

My knock-around trainer has the CG set about 1" behind the recommended CG. It's still a little nose heavy, but I do use it for training once in a while, so I leave it there. The recommended CG made it VERY nose heavy, actually harder to fly than it needed to be.

Thanks Montague... that's precisely what I needed to hear... Neutral Position was the term I was looking for instead of center of lift. Anyway it's good to know that I'm not completely wrong but overly simplistic.

sure helped me. thanks for the question and the thorough answers

You guys have helped me too.
- I bought a Dynaflite Super Decathlon secondhand, it was flown with an OS four cylinder fourstroke, and the guy who flew it said that it was very nose heavy.
Now I want to fit it with a Stihl chainsaw engine which is even heavier than the OS.
I moved the firewall back, but still had problems balancing at the manufacturer's reccomended CG point.
Now I understand ( I think) that I can move the CG back a little from the manufacturer's stated setting.
Anyone with experience on this model!

( Don't mean to hijack the thread)

Be careful with that Decathalon. Trainers almost always have the CG marked really far forwards, and they have big tail surfaces. Decathalons aren't trainers, and are usually more sensitive to CG. Some Decatholons that I've seen have been very close to tail heavy even on the suggested CG range (One, I think it was a GP kit, was down right "snappy"). But if the previous owner had it on the suggested location, and the plane flew like it was nose heavy, then yes, you should try moving it back. Just do it in small increments.

Just to clarify, I make a first flight with the CG where the plans say unless I have a reason to do otherwise. It's just that you can feel free to adjust it from there.

Once the plane has been flown, I ignore the instructions and make small adjustments until the plane flys like I want it too, or is close enough. Think of the plans as a good, safe, place to start, rather than the absolute final CG location.

Thanks Kirk,
I've been told before that the Decathlon could be tricky, but couldn't see why.

Now I think I'm beginning to understand the difference between centre of lift and centre of gravity.

As I see it, - the centre of lift (neuteral point), is inherent with the design of the wing, and stays put as the permanent pivot point of the wing, whereas, the CG position can be moved, giving the aircraft a nose up, or nose down, attitude. ( back for nose up, and forward for nose down).

I realise that my perception is a bit simplistic, but is it basically correct?

No replies!,
- nobody said it is incorrect, so I'll assume I've got it right.
Thanks for clearing things up, - looks like the Decathlon is designed to be a little nose heavy.

Well, kinda, and, I think close enough.

The NP of the airplane can actually change in flight (depending on the airfoil), but it's reasonable to think of it as fixed.

The nose-up/nose-down thing, while not inaccurate, is not the way I'd describe it. Nose up/down only applys when you're suspending the airplane on your finger tips or a balancer at some arbitrary point. If you pick a point far enough forward, the plane will always go nose up, and vice versa. When I check a plane's CG, I try to find the point where the plane balances level. That tells me where the CG is at the time. Then I look at where that is in realtion to the wing chord, and where I'd expect it to be (25-33% of MAC to start with).

Once the plane has flown, I adjust the CG by moving it a little bit at a time, again finding the point where the plane balances level. Depending on how the plane flew, I'll usually move it 1/4" at a time or so (depends on the plane of course). The problem with suspending the plane so that it's nose-down or tail-down is that you then have to worry about the angle. It's easier to always balance level, mark that balance point, and measure it's location.

In the air, flying nose up or down depends on a lot of factors, the CG just being one of them.

Thanks Kirk,
I think I'll start with the CG a little back from where it says on the specs, but maybe I'd be better to get someone with more flying experience than I have, to try it first. ( if I crash this one, it won't survive like my old foam trainer), - not to mention the borrowed engine!

Willdo, doesn't it make most sense to try at the recommended CG? You can put some lead weight on the tail and remove it as you become more familiar/comfortable with the plane. Many people start with self-adhesive lead weights to test the plane. When they find the amount of lead they are comfortable with, they replace the lead weights with a more permanent solution. But then, if you are installing a stihl and moving back firewalls, you should've already known that :-)

As for getting someone with more flying experience, that is usually a good call if you're not completely comfortable with the plane yet.

Yes, you're probably right, but as I said the plane has already been flown with the forward CG position and the guy who flew it had to use full up elevator trim, and was still struggling!

I have just learnt to fly with a cheap resilient scratch built trainer, and have not worried too much about crashing, so I have been a little cavalier with my flying attitude to say the least, - now I realise this one is much more serious stuff and requires a little thought.

Someone has kindly lent me a little Honda GX 25 four stroke and I think that it will suit the plane better until I have the flying sorted out, being much less powerful than the Stihl. However I've got to be careful not to total the engine!