rgm762

My Feedback: (4)

ok i'm totaly lost and need help on cg. i know it's critical and why, guess what i want to know is are there warning signs when balancing? i.e. your putting an arf together and using all the equipment the manufacture calls for, correct size enginge/motor, servos, retracts etc etc, then when you try to balance it needs 10 oz's of weight added to the nose, you've already moved things around in the fuse but still won't balance out, when do you draw the line at adding weight and start looking at the cg? or is it a personel preferecnce? i've read on other threads where someone has added 1lb of weight to the nose or 5oz to the tail. i know this is probably covered on another thread and if someone knows which one?

grimmy55

the best and surest way is to get a peice of string ,tye one end to the end of tail and the other end to prop nut ,pickup plane by the string so that you form a triangle get the plane level this way and then drop a peice of string vertical down from where you are holding it and at the point where the vertical is on the plane that is the balance point ,,hope you get my drift

Dorsal

My Feedback: (17)

Whenever possible, you want to shift any and all items to get the CG to the proper position on a plane, whether that means you need to remount the servos, add a box to move the battery, whatever. After all, dead weight is just that - dead. But if you have repositioned everything as much as possible, add weight as far forward or as far aft as physically possible, to reduce the amount of weight required. btw, a piece of lead stuck to the nose of the plane won't be visible from 40 feet away.

opjose

To clarify grimmy55's response tells you where the C.G. CURRENTLY is, but does not address the original question...

The manufacturer's SUGGEST a C.G. point that is assured of producing a decent flying plane.

If a plane is too tail heavy it will be uncontrollable. This is why you should always START with the manufacturer's recommended C.G.

As Dorsal said, your best approach is to AVOID adding any weight what-so-ever to the plane. The best way to do this but at the same time move or adjust the C.G. "balance point" is to shove things around.

A good candidate is the battery pack since it tends to be relatively heavy.

If your plane is tail heavy, you can move the battery forward.

I put my battery pack in the cowl/nose of my Funtana s90, after I put in a new LIGHTER Saito four stroke engine... this was done to move the C.G. forward.

Likewise I've put my battery pack after of the cowl on a couple of planes that were originally too nose heavy.

If you have NO other recourse and you absolutely MUST add weight ( which should always be the LAST thing you do ) try to add weight as far "out" as possible...

That is, a small 2oz weight at the extreme tail of the plane can offset as much as 8 oz on the nose, because of the long lever moment ( leverage ) it has.

Conversely if you need to add weight to the nose, a great place to put it is in the spinner... dubro sells spinner weights, that have the added benefit of improving engine idle!

rgm762

My Feedback: (4)

thanks guys, the string thing is very helpful, but is there a limit on how much weight you should add? 1lbs sure sounds like somethings out of whack

rgm762

My Feedback: (4)

thanks guys, the string sling is very helpful, but what i'm wondering is, is there a limit to how much weight you can add, maybe i'm overthinking this, but if add 1lbs of weight to the nose or tail of a plane that already weighs 6lbs, aren't you changing the cg again

opjose

Any time you add weight you are affecting the C.G.

What you are shooting for is to have the plane show NO preferance at the suggested c.g.

In other words, it will tip forward or backward just as easily and as often.

1lb sounds excessive.



First, what plane are you talking about?

Link please?

What engine are you using?

What size RX battery pack and where is it mounted?

combatpigg

My Feedback: (3)

A 6 pound model should not need more than 2 ozs of dead weight to balance it. Any more than that and something is fundamentally wrong. Why build a model out of the lightest materials then desecrate it with a bunch of lead? Consider those strips / chunks of lead as badges of shame, emblems of poor judgement and design.

da Rock

First off, the CG is critical, but not much. Why?

Because it's EXACT location isn't critical at all. Not at all. What is critical is that you do an accurate job of placing it within a sensible range on the airplane for your first flight. And you need to know that if it's toward the front of the range you might need to increase your elevator throw, and if it's toward the rear you might need to lessen the elevator throw. Beyond that, it's really not very critical at all.

Accurately locate it.
Within a reliable prediction of where it ought to go.
Understand that a forward location deadens the airplane's response and an aft makes the elevator more effective.
And the first flights will be perfectly safe.

da Rock

The exact CG location isn't exactly important.

A reliable CG range is.

The good mfgs usually provide a range. It is important for you to accurately place your CG into that range. Whatever equipment shifting required or dead weight added is important. But you need to do an adequate job of balancing.

da Rock

If your mfg only gave you ONE CG location, it's time to learn some more about aerodynamics. And it's time to place that mfg into the "makes Barbie dolls when not making ARFs" category.

If you get locked into the belief that only one CG matters and that you've got to get the plane balanced on it for the plane to fly safely, you're at the mercy of any toy manufacturing ARF mfg you're unlucky enough to encounter. And your next model will probably come from one.

Aircraft pitch stability isn't easy to understand, but it's not beyond most people. It's basically a function of how big the wing is, how long the wing chord is, and how big the horizontal tail is and how far back that is. There are a number of other modifiers but they aren't major players. Take a look at the measurements needed to figure a CG location and you'll see a bunch of formulas. They boil down to finding the above 4 things. Look at http://www.geistware.com/rcmodeling/cg_super_calc.htm and you'll see all that stuff.

It unfortunately scares lots of people off.

All that stuff basically figures out how big the tail is and how far back. And how big the wing is and it's chord and "where it is on the airplane". And all you have to do to get that all figured out for you is to provide 9 measurements you can do with a yardstick.

da Rock

But back to your problem...................

Do you need to add all that weight to get your model to balance at ONE magic place?

If the mfg gave you only one place, the answer is that you need to double check his advice. That mfg really isn't to be trusted. Check that geistware application and get a CG range for your model. Then decide for yourself (with an absolutely reliable and safe range to work with) how much weight you need to add and how far into the range you want your CG. Then go fly and decide if it needs to be moved or not. Probably not. You might want to slow down your elevator throws but you'll be maidening an airplane with safe pitch stability.

When you need to move a CG, most good experienced modelers move stuff inside the model first. Then they balance again.

Knowing a range for the CG gives the modeler much more intelligent control over the problem.

da Rock

The lines you draw at adding weight start with an understanding of many, many things.

One pound to the nose of a 35% IMAC is insignificant. Five ounces to the tail of a 40size model is huge. So an answer to your question would have to be some percentage. And in 50 years of modeling, I've never heard anyone offer ANY realistic rule of thumb for what you want.

It's personal preference.

Jetdesign

My Feedback: (8)

For someone who hasn't done research on this topic, is it safe to assume that the CG should be somewhere near the thickest part of the wing? Will that always be a safe place to start with our models? In the 4 models I've assembled this year, all the CG's were near the (cord?), with two of them being slightly behind the thickest part of the wing (and these would have flown safely if the CG were moved up to that point).

JohnW

My Feedback: (6)

"is it safe to assume that the CG should be somewhere near the thickest part of the wing? Will that always be a safe place to start with our models?" - No. It may seem there is a correlation between wing thickness location and CG, but there isn't.

Ideal CG location is not magic and can be easily computed.

http://adamone.rchomepage.com/cg_calc.htm

RCKen

My Feedback: (9)

John is correct. And making an assumption like this is a good way to get in trouble quickly. There is a saying that goes "A nose heavy plane flies poorly, a tail heavy plane flies - once". Getting the CG wrong because of an assumption is a good way to take off with a tail heavy plane that is quite likely uncontrollable.

Ken

rgm762

My Feedback: (4)

OPJOSE
the plane i'm wanting to build is the h9 f4u corsair, i'm researching info on it and in one of the threads someone said they had to add 1lbs to the nose, a lot of the guys are saying the cg is off, guess my question is, if you balance the plane where the manufacture says too, what shoukd i look for that might indicate the manufacture's cg is off?

da Rock

You could find out what a fullscale manufacturer would use for a CG.

As for interpreting how much lead it's taking to balance, that won't tell you a thing about whether the mfg was off on his CG. Because it just as easily tells you he blew his wood sizes and choices or the construction details.

da Rock

You could find out what a fullscale manufacturer would use for a CG for that model.

Use http://www.geistware.com/rcmodeling/cg_super_calc.htm or http://adamone.rchomepage.com/cg_calc.htm and you'll have that.

rgm762

My Feedback: (4)

so it would be wiser/safer to refigure cg and not really use manufactures?

Charlie P.

I would always start with the manufacturer's recommendation. Then change it if flights determined it was necessary. But from there I always base mine on flights and not the recommended range. Usually they align, but not always. Unless you fly the same equipment the designer used there will be variations.

Best test I have been shown is to trim the model for level flight. Then do a half roll and angle the plane up to a 45 climb. If it holds the climb with the controls centered/neutral it's balanced. If it falls off it's nose heavy. If it tries to climb it's tail heavy - land immediately and make balance corrections (as it will get worse as the fuel is consumed).

You'll also note that depending on how much fuel you are carrying the model will always be nose heavy compared to the dry balance - unless you've done the pattern fliers trick of mounting the tank at the C.G.. You're adding 10 or 12 ounces of fuel per flight up near the nose.

I like Combatpigg's rule of thumb of no more than two ounces with a 6 pound model. If that don't do it do something else. For tail heavy shim the engine forward on 1/2" Nylon spacers, add a brass prop hub, something. I've even cut and pulled bulkheads back 1" on nose heavy models rather than add excessive tail weight. Lighter always flies better. The advantage to kit builders is you can do a pre-covering balance and get the worst problems figured out while it's easy to make additions or changes.

da Rock

Well, yeah.

That is somewhat affected by WHO the manufacturer might be. Nowadays we're mostly assembling ARFs. How many sets of instructions have you seen that were screwed up here and there? Especially the ones that come with the "you get what you pay for" ARFs. And how many have you seen that were copied from the ARF mfg's previous models?

Like everything, it depends. How much you trust the mfg, and how much you value the work you're putting into the model. Truth is, with the importance of the CG why bother making a judgement of who the mfg is. I check my own work, why assume anything.

It takes me maybe 10 minutes to measure the model and run the numbers. And I get an absolutely reliable RANGE to work with. It might be the most valuable 10 minutes spent on the model. It often saves me a bunch of time and lead when balancing. And certainly insures that the CG location will help the model, not hurt it. And I get a feel for what I'd like to do with the mfg's suggested throws.

We often get more out of our models than we put into them.

BTW, do you know the rule of thumb, "measure twice and cut once"? Why do that? You measured it once already. Isn't that good enough? Why check anything? Same concept with taking the mfg's word for it.

da Rock

The thickest part of the wing? At the root?

Safe to assume?
If the wing isn't swept or tapered. Different taper planforms introduce a sweep.
If the horizontal tail is always in the same proportion to the wing and always the same distance aft.
If the wing and tail always have the same aspect ration.
And a few other things..........

Read up on Mean Aerodynamic Chord. It's one of the things that blows most simple assumptions about CG that ignore planform.
Also, the vast majority of airplanes flying have horizontal tails. They also blow simple assumptions about CG that don't take them into consideration.

CG's are important to PITCH stability. Pitch stability is as much a function of the horizontal tail as it is the chord of the wing generally speaking.

flyX

1lb, that's a lot...that's on top of retracts ?
You might as will install a bigger engine becuase you're going to need the power to pull it.
That's a brick, brick.

Have you moved the batteries forward or under that tank ?
Did you mount the eninge forward enough ?

When you determind the CG... was it from the leading edge of the wing or the former. On a tappered wing model it can be confusing
if the the manufacture is not especific or show it on a plan. You might want to double check where they want you to take the measurement from.
If they ment from the former instead of the LE...it'll throw you off.

CG is important becuase, if you're a noobie...it might be too much model for you. The further back the more sensitive the model will feel.

rgm762

My Feedback: (4)

there's my question, if you install a bigger/heavier engine, doesn't that change the cg? then how do you find cg when the formula doesn't ask for weight?
you guys are great and have helped a lot, and i'm sorry if i'm a pain, what i am doing is looking at the hanger 9 corsair, there's a thread on it under RTF/ARF with over 1900 posts. there's several posts about the cg being wrong, (among other things) but some say it's right and this is what's got me confused, the specs call for a 100 fs engine and they're installing 120's or 125's. so how are they finding the cg with the heavier nose weight than what ther manufacturer is calling for?