Here ya' go.  http://www.aeromaniacs.com/Benchtrim.htm

You can get close enough on most conventional configurations by just taking the average of the root and tip chords to get the Mean Aerodynamic Chord (MAC). If your wing has a lot of taper, that method will be inaccurate. Why not do it the easy way? Use this to get your MAC and c.g. position:

http://www.palosrc.com/index.php?option=com_content&view=article&id=50:cg&catid=41:ic&Itemid=50

...or others like it on-line. Just plug in your wing's measurements and it spits out the answers for you. The rest is more complicated.  First, unflapped planes have (need) smaller horizontal tails and further-forward c.g. positions. I generally see c.g.'s for flapless planes at 15% to 18% back from the leading edge of the MAC. Flapped plane c.g.'s range more often from 19% to 25% MAC, depending on aft fuselage length and horizontal-tail area. The larger the tail moment coefficient, proportional to these , the further back the c.g. may be placed - in general.

Ted Fancher gave a rule that I often call "Ted's Rule," stating that for flapped planes, a good c.g. can be placed at a percent of the MAC equal to the percent the tail area is of the wing area. So divide horizontal tail area by wing area and convert that to a percent. That would be about how far back along the MAC you can position the c.g. I posted a couple times here and on SSW forum a graph of how that compares with a more theoretical value. It's a good approximation.

Concerning model proportions...that's tougher. I think we mostly go by experience of where similar models have balanced before - experience. It is possible to come close by computing, using weights of materials and test finishes, but that's a tedious** design thing that I doubt has been done often. I've done it without great perfection. On the plane I just finished, I guessed that by substituting an LA-.25 for an LA-.40, lengthening nose and aft fuselage and removing my 3/4 oz of tail weight, my new plane would come out about right. I was wrong. It takes that 2.5-oz stock muffler replacing a tongue muffler to balance it where I expected. It's better to be nose heavy than tail heavy, because it takes less ballast in the tail to correct things.

Final comments.

1) You should always use the search function here and at SSW forum to find the many posts on this topic that have made such great archives.

2) I have the most comprehensive collection of equations and derivations for MAC's and aero centers that I have found anywhere. It was a lot of work, but my equations work for all tapered, elliptical, and parabolic wing variants. Anyone can have them via e-mail.

** If you only want to re-position an engine/prop for a finished plane, that's an easy problem of moments (leverages).

SK

I find that formula to be a bit confusing. When you build a scale model an have to use blind nuts an a filler block in the nose, how do you guys plan were to mount the engine? Without having the plane finished you really dont know how much weight paint an covering will add.

I don't think it's realistic to expect that you're going to get things exactly right the first time around.  You're just going to end up adding balance weight somewhere.

There's a shaft extension on my Atlantis.

My goal for setting the CG on the first flight is to not crash -- after that first flight I know plenty about what I need to change.

The original post asked about the easiest way to find the CG.  Here is a picture of set up from another thread.   For me, this is the “easiest way”.  Again, that doesn’t mean it is exactly where it “needs” to be, just where it “is”.  It will get you close so crashing on an early flight is less likely.

Here is the other thread. http://stunthanger.com/smf/building-techniques/cheap-balancing-fixture/

As an aside, the airplane Steve mentioned is one that I built.  He’s right, it was way out of whack when he got it.  Apparently Tim took care of the whacking.

It's funny you mention a cockeyed clip.  I just mentioned on the Open Forum that I now use surgical tubing to limit that sort of brain fade.  I know of several others that do the same thing, not my idea.

It's funny you mention a cockeyed clip.  I just mentioned on the Open Forum that I now use surgical tubing to limit that sort of brain fade.  I know of several others that do the same thing, not my idea.

I should be doing that.

The original post asked about the easiest way to find the CG.  Here is a picture of set up from another thread.   For me, this is the “easiest way”.  Again, that doesn’t mean it is exactly where it “needs” to be, just where it “is”.  It will get you close so crashing on an early flight is less likely.

Here is the other thread. http://stunthanger.com/smf/building-techniques/cheap-balancing-fixture/

As an aside, the airplane Steve mentioned is one that I built.  He’s right, it was way out of whack when he got it.  Apparently Tim took care of the whacking.

So essentially this plumb bob fixture just tells you where your airplane is at an you need to know the exact CG point to dial it in perfect?

So essentially this plumb bob fixture just tells you where your airplane is at an you need to know the exact CG point to dial it in perfect?

Yes.  The plumb bob is a way to measure what's there, not a way to prescribe what you need.

So you guys use the formula to find CG then the plumb bob to make it perfect. Once you make it perfect I take it the plane is perfectly balanced so that it wouldn't tilt tail or nose heavy?

No.

The CG from a formula is only and ever approximate.  You use the formula and plumb bob (or whatever) to get it close.  Then you go fly, and adjust it to perfection based on how the airplane acts.

Even if you're trying to duplicate someone else's plane, the best CG for you may well be different.  You set the CG, then you go fly, then you adjust it to perfection based on how the airplane acts.

Whats the best way to attach weight during testing?

If you thought about it during the construction of the plane, some small boxes were built into the model at strategic locations to contain things like weights.

Phil

Whats the best way to attach weight during testing?

On my latest profile trainer for my wife I added tail weight by drilling a hole through the back end of the fuselage under the stab and putting a bolt in.  If it needs more tail weight it'll get washers, if it needs less it'll get a smaller diameter bolt, or I'll get a nylon bolt that size.

For nose weight you can use brass spinner nuts, you can use the heaviest stock muffler you can find, or you can put brass or lead weights as far forward as you can get them.

There's really no one good answer.

Ok can someone clear something up for me? Lets use a Sig Skyray 35 for an example. Now using that cool CG calculator with 15% the CG is 1.35 and using 18% the CG is 1.62. If you look at the plans the CG is around 2.37-2.75 , so what is what here?

Last one.

As I've stated on some other posts,  if the pic doesn't get big enough when you click on it,  click on the address below it.   It will come up on another page and you can enlarge or shrink it.   Mine do the same thing lately.   Must be some us computer illiterates don't know.  I shrink my pics down so I can load as many as 10 pics at a time.

Wow!

I have two pretty much destroyed SkyRays that Jerry and Merry Phelps gave me for practice years ago. So I'd never seen the plans. That c.g. range on the plans surprises me, because almost everyone on the forums has suggested 15% - 18% for flapless planes, and your Palos calculator gave you the right figures for your input. Since my SkyRays are pieces in baskets now, I can't check to see where they balanced, but from where Jerry had his leadout exits permanently set, he seems to have put the c.g. at about 2.25", which is 24% back on my 9.25" chords. The plane did fly fine there.

In contrast, Mike Pratt's Primary Force plans (I built one of the first ones) show the c.g. at about 18% of the root chord of that tapered wing, which probably is about 18% MAC (haven't computed it in recent years). He may have changed it over the past years. SO,...? I dunno. This one would have fooled me into greater conservatism in c.g. placement, because I have always trusted the 18% number for flapless planes. I'm wondering now whether I can replace that boat-anchor of a stock muffler on my P-Force derivative, with its bigger tail, to move the c.g. back. I'll learn that soon.

The Palos calculator told you what you probably already knew though, since the SkyRay wing is constant-chord with no sweep to move the MAC back or forward. It will be very handy for tapered and swept wings though. One thing about rules of thumb and plans is that you may or may not prefer the suggested c.g. positions, and some earlier plans have had c.g.'s and leadout positions both poorly placed to compensate for each other. Mike though tests his designs and refines them so that you can trust his suggestions. Depending on your skill level though, you might want to balance it, at least for now, a little ahead of that spar.

SK

You're surprised an I'am surprised, I'am just surprised the opposite way you are. All the airplanes I've build have had the CG way further back than suggested on here. I'am use to checking it about a third of the way back from leading edge. Like all my prints suggest. My busted up Banshee suggests 3in back for CG (calculator 1.91), Twister suggests 2 3/8 back (calculator 1.73), Super Clown Suggests 1 7/8 back (calculator 1.71) closest one yet. Flite Streak suggests 3in back (calculator 1., Ringmaster suggests on the position of the down leadout line where it exits which on mine is 1 7/8 (calculator 1.69) I used 18% for all of them.

Ive never messed with changing the leadouts, heck back when I built them I didn't even know you should have them adjustable. So all of mine are glued in place. Where is suggested that you want your leadouts possitioned in relationship to the CG point? Does having your leadouts adjustable help with the plane turning tighter? Or what do you gain by fine tuning them?

The whole point of my asking about CG I was going to go back through my airplanes an try to make them not so nose heavy. Just to make trimming them a little easier and not have to hang a small truck off the tail to balance them.

All the airplanes I've build have had the CG way further back than suggested on here. I'am use to checking it about a third of the way back from leading edge. Like all my prints suggest. My busted up Banshee suggests 3in back for CG (calculator 1.91), Twister suggests 2 3/8 back (calculator 1.73), Super Clown Suggests 1 7/8 back (calculator 1.71) closest one yet. Flite Streak suggests 3in back (calculator 1., Ringmaster suggests on the position of the down leadout line where it exits which on mine is 1 7/8 (calculator 1.69) I used 18% for all of them.

Ive never messed with changing the leadouts, heck back when I built them I didn't even know you should have them adjustable. So all of mine are glued in place. Where is suggested that you want your leadouts possitioned in relationship to the CG point? Does having your leadouts adjustable help with the plane turning tighter? Or what do you gain by fine tuning them?

The whole point of my asking about CG I was going to go back through my airplanes an try to make them not so nose heavy. Just to make trimming them a little easier and not have to hang a small truck off the tail to balance them.

I don't know why all of us took your original question to be for an unknown plane, but yes, if you have plans and if the planes have good reputations, then put the CG where the plans say.  Of the planes that I know personally, all of those points sound fair or too far back (the Flight Streak, in particular, was originally designed for combat-ish flying, and does stunt best with the CG 1 5/8" back from the LE).  But if it works for you, it works for you.

But read and re-read what Steve said: this is all to get a starting position, which you then move around to suit.  Assuming that you have adjustable leadouts (because the correct leadout position is relative to the CG), feel free to move the CGs around and see how the planes fly.

Another thing, which may not have come up in this thread: it's better to have a brick with the correct CG than a feather with the CG wrong.  If you need to add weight to move the CG, just do it.

So you really don't need to have a starting point, because you already have planes that fly.

I'm thinking at this point that if you don't have adjustable leadouts, and if you don't want to hack up a bunch of wings to make them so, that you should mess around with each plane a bit, and be happy with whatever seems best.

If you do have adjustable leadouts, and if you're looking for "stunterish" flight characteristics, then find Paul Walker's trim articles and follow them -- he tells you how to separate the effects of CG from the effects of leadout position and handle spacing.

Keep in mind that with a lot of the older planes with small tail volume that they're just plain limited by the aerodynamics -- you can almost always make a plane better with trimming, but none of the planes you mention are going to out fly an Impact.

Paul Walker's Impact.  Many-times Nats winner, and inspiration to us all.





Here's a profile Impact:

Paul Walker's Impact.  Many-times Nats winner, and inspiration to us all.





Here's a profile Impact:

Are there different tail configurations?
I like the green one! How can I aquire one?

Well, first you have to understand that these are all electric powered.  The PVC stand is for placing the airplane in an upside down position for changing batteries and other maintenance.  Howard calls it his “UDT”, or “Upside Down Thing”.  You have to know Howard.

Looking at the plans and rulers pictured, I see that all but one (Flight Streak - see below) of the wings are flapped. The c.g. positions shown appear to me to approximate those %'s I gave for these flapped types (20% - 25% MAC). Expecting that the flaps are not swept forward the same amount as the leading edges are swept back, some could actually be a bit conservative. IOW, MAC aero center may be behind the root aero center.

The rules of thumb I've heard (See earlier post) and seen used are that for flapped planes, the range should be 19% to 25% of the MAC, and for these, a good guess for how far back you can go is the percentage the horizontal tail area is of the wing area. That is, if you divide the tail area by the wing area and convert that answer to a per cent, then you can reasonably put the c.g. back that % on the MAC. This position is the kind you'd want to compute with the Palos calculator. For big tails as on the Impact, the c,.g. would then approach 25% MAC. I have found by theoretical mathematical comparisons that within the ranges of more modern conventional configurations, this rule of thumb becomes the most accurate.

I've seen many posts on the Flite Streak, and most say the plane is much too twitchy with that c.g. position - for the people who posted. The few who liked it may have been experienced combat pilots or  - maybe - more talented. Most responses were to move it forward. Edit: The stationary Flite Streak "flaps" are counted as part of the chord (see below) but do not put the "Streak" into the flapped category. The rules of thumb are different for flapped and flapless wings (they both allow you to use the Palos calculator and only use wing-dimension input, even though the tail area and arm are factors. They're short-cuts to reasonable starting points..

Before writing off the c.g. position advice, I'd take one of those flapped and tapered wings and enter it's data into the calculator and compare the answer to what the plans suggest. I think you may have done this (using all requested dimensions), but did you use the flapped - wingrecommendations? Most important, did you include the flaps in your chord? They are part of the wing and must be included.

I put adjustable lead-outs on my planes and do use them. Changing a c.g. position calls for adjusting the lead-out exit. At different site altitudes, the plane may need lead-out adjustment to keep line tension when flying high in maneuvers (and often moving them forward, rather than the intuitive backward). The same goes for hot vs. cold days. In each case the air density changes, and that is part of what dictates the best lead-out rake. If I can recall them, the parameters of line rake include model mass (weight), line length (including body measurements, like your arm length) and lead-out length, speed, air density, line diameter (important; line drag is not inconsequential), and maybe something else. There are a couple calculating soft wares that you can get from fliers on this forum that tell you how far to exit the lead-outs behind the c.g. (again as a starting place) so that they don't yaw the aircraft. For your planes, that would be about 3/4" to 1", but it's greater for larger, heavier planes.

Remember that with reasonable power (and we modelers have it in abundance) you can make almost anything fly. To fly well, you need to keep it balanced (trimmed). The better the trim, the better the flight characteristics. We used to maintain line tension by just cocking the rudder and thrust line out, but when we did aerobatics, if we didn't balance those right with c.g. and lead-out positions, the plane bounced or wallowed around or "hinged" a lot. The better we set things up, the smoother and nicer it felt. Some of us didn't care. It's really what's fun for you, but sometimes just understanding ball-park figures and what causes what flight characteristics makes things more fun when you hit a snag.

So you really don't need to have a starting point, because you already have planes that fly.

I'm thinking at this point that if you don't have adjustable leadouts, and if you don't want to hack up a bunch of wings to make them so, that you should mess around with each plane a bit, and be happy with whatever seems best.

If you do have adjustable leadouts, and if you're looking for "stunterish" flight characteristics, then find Paul Walker's trim articles and follow them -- he tells you how to separate the effects of CG from the effects of leadout position and handle spacing.

Keep in mind that with a lot of the older planes with small tail volume that they're just plain limited by the aerodynamics -- you can almost always make a plane better with trimming, but none of the planes you mention are going to out fly an Impact.

Tim do you have a Impact? If so whats yours look like?

No.  I was all set to build one when a Brodak Legacy kit was thrust under my nose -- that's my current build.

Will you be flying at Brodaks this year?