Im currently building a Kyosho P-40 and I need to balance her out. The manual gives a certain range, such as "the center of gravity can be anywhere from X to Y back from the leading edge". Should I balance the plane further back towards the trailing edge or closer to the leading edge? Pros and Cons? Thank you in advance for any input.

Bouch620

closer to the LE as a nose heavy plane will live to fly another day ........

The safest range is usually somewhere between the middle of the manufacturer's specified range to the forward most point, the one closest to the leading edge. I would steer clear of the rear location until I knew the plane.

On a scale plane, I would guess that anywhere would be fine, especially since the planes is aimed at the sport flier and since I have heard that Kyosho is a reputable manufacturer.

I would still put the CG forward for the first test flight.

Put the C of G to the mid point of the range (measured dry of course). Afterall you will be taking off with several oz. of fuel up front.

If the C of G is too far forward you risk running out of up elevator on the landing approach.

The P40 needs to be balanced with the wheels retracted and the plane inverted . As stated balance from the leading edge , a nose heavy plane will live to fly another day and tail heavy plane will usually live for one flight .

Thank you all for the input. After I wrote this thread I did a little more research and found out alot more about this topic, so I believe my question has been answered. Thanks again.

Everyone who asks where to locate their CG should learn to use http://www.geistware.com/rcmodeling/cg_super_calc.htm

Matter of fact, even people who don't ask should take the time to learn about Neutral Points and Margin of Stability. And an excellent place to learn that in one sitting is: http://www.geistware.com/rcmodeling/cg_super_calc.htm

There are probably other free online applications out there, and they'd also be worthwhile, but I happen to know about geistware.

Nowadays, the ARF mfg's have proven time and again that their understanding of their products is the understanding of a toy manufacturer who has simply expanded their product line. I've built well over 20 ARFs in the last year or so, and way more than half of them had mistakes. The mistakes ranged from silly ones to fatal flaws. To expect that industry to recommend correct CGs for every ARF is not a good bet. They've produced ARFs with wing attachment designs that would not have stood up to everyday flying (the Hobbico Sukhoi didn't stand up to 3 flights). One Decathlon fuselage had such weak fuselage stringers the top of the fuse aft of the wing was failing after the first outing, an outing that was only 3 trim flights long. Balsa where spruce should have been. I've seen two ARFs blow horizontal stabs that were made out of crap balsa when they should have had twice the balsa and it hard balsa, or spruce. A recent favorite ARF was sold with gear that placed the main wheels DIRECTLY under the CG. Needless to say, I bent new struts before the stupid ones were even installed. etc etc etc etc.........

So you're going to accept the ARF mfg's CG recommendation? Without at least checking it out yourself. It takes maybe 10 minutes to measure the airplane and enter the numbers into the geistware application.

A recent 60size ARF manual recommended a 1/4" (that is one quarter of an inch) CG range and was emphatic that you not to go outside that range. And their location was pretty much a joke. It's a joke to think a 60size would have such a critically small CG range in the first place. And the manual's text was obviously the compilation of previous, half-done descriptions from the mfg's earlier models of other models. It's great to keep the price down by using already done work, but not if the previous recommendations aren't correct, or sensible. And are probably the recommendation for an earlier model.

It takes 10 minutes to find out where the CG can be from an application that works very, very well and is absolutely trustworthy. CG is a function of the airplane's planform, nothing else. Measure that planform and do the math and the answer is GOOD no matter what the airplane is or who manufactured it. That's not worth doing?????

Back when all we had were either plans or kits, all of those came with information that you could count on. And some of us still found the formulas and did the measuring and math to see how far the CG could be moved. Today there is good reason NOT to trust the mfg's information, and doing the math is dead simple. You don't have to. It's done for you. All you need is a yardstick and pencil/paper. Back then, you had to learn lots of things to build models that lasted for you. Today is no different. Only difference is that most of the work is already done for you, good or bad. It's still up to you to look at the bits and pieces and parts and angles and decide if that landing gear is stupidly placed or not. Balancing the airplane is just one of the things that separate an experienced modeler from a beginner. So isn't it time to start working at becoming an experienced modeler.

jeez......... didn't mean to preach.............

I assembled that Kyosho P-40 back in June of last year. The manual showed a range of 95-105mm. (10/25" or 2/5" range) You probably won't be able to tell the difference between a CG at the 95mm location versus the 105mm location.

I'm sorry that I seem to have lost my building notes I took when assembling that airplane, or I would have told you the geistware numbers. Mine came out to 5lb 7oz and I'm flying it with an OS50FX. I put retracts in mine. They didn't come with the airplane so I had to order them. Turns out the Kyosho retailer in the US at that time had no idea which of two retracts they had were the right one for the airplane. But they did know that one set would be for the P40 of the two sets. So I got both. Turns out one has chromed strut wire, the other unfinished wire. They both rotate opposite the way a P40 gear has to rotate in order that the wheels be outboard the struts when the airplane is parked, yet fold into the wheelwell correctly. Turns out the track is narrower because of that, but WTH, it's to be expected. It's ground handling isn't the greatest. It's a very good flyer.

I have really tried to use that formula , I dont find it easy .
Call me stupid , what % of mac ???? there are so many variables that you have yourself asking -ok from what point , I would love an example

You only enter the airplane's measurements. What measurements you ask? The ones shown on their diagram.

You measure the wing:
halfspan
tip chord
root chord
LE sweep

You measure the horizontal tail:
halfspan
tip chord
root chord
LE sweep

You measure from the LE of the wing at the root to the LE of the tail at the root.

Plug those numbers into the application's page and click the "compute" button (or whatever the button is called).

The MAC is the Mean Aerodynamic Chord. The application takes those simple measurements I listed above and tells you all that magic stuff. Truth of the matter is, all you're really interested in is where it says the NP is and where the CG is.

OH YEAH, you tell it one other thing. You also tell it a SM (static margin) to figure out for you. THEN you click the button right below. I think everything you have to tell the application is in the box at the very top of the page. And the "tell me" button is in that box too. When you click the button, all the other informational boxes instantly pop all the magic numbers. You only need one, the CG. They give you how far back on the wing root it'll be. Just like everybody does, even the ARF manual does. Only this one will be dependable as heck. You might note the Neutral Point as well.

I've used the sucker on just about every ARF I've built since getting back into the hobby a year or so ago. Think that's about 18-20 of them. It's amazing to see what the application says and what some of the ARF manuals said. Actually, is rather disappointing.

I've found that it's worthwhile to get the CG location for 20% SM, for 15% SM, and for 10% SM. That only takes 3 clicks of the "tell me" button. Ok, you gotta enter 20% and click, 15% and click, 10% and click.

Write all of them down. I usually write 'em into the ARF manual right over the manual writer's suggestion.

I've found that the geistware 15% SM works like gangbusters for me for maiden flights. The 20% geistware CG location is good for most 1st year flyers. I don't start a new airplane off at 10% only because the elevator is going to be so effective it's sometimes hard to sort the trim. However, I've got a number of models that now have their CGs at the 10% or so location. But they got their neutral elevator sorted at 15% and their elevator throws roughed out at that same 15%. THEN it was entirely safe to move the CG back, and the geistware had clued me in as to how far to go. When I do that step nowadays, it's a slam dunk to move the suggested amount, AND I also move the elevator connecting rod out one hole on the elevator horn. I go ahead and move the elevator connection to reduce the throws because the elevator effectiveness (NOT the airplane's stability) is going to change with the CG move. I've already tuned the elevator connection at 15% for good safe deflections, and know that much won't be needed nor desired with the aft movement of the CG. It's worked every time.

OK, I just brought up the geistware application to see if anything I told you above was actually true. Old age sometimes dims memory, and I haven't looked at that application in over a week soooooo.......... chuckle...........

The measurements go in the light purple box with your desired SM. The "tell me" button is in that box too. Nine easy to do measurments and your desired value for SM.

When you hit the "Refresh MAC and Neutral Point" button, almost all the other purple boxes' blanks will be filled in. You're really only interested in one answer, and it's in the box that's two shades darker purple. Calculated Results is the heading. And the answer is in that box. It's the measurement in inches of the CG back from the LE of the wing at the root.

hope this helps you. it helps me.