After finishing the CG calculations for my plane at 'AIRCRAFT PROVING GROUNDS', I've got some questions that are puzzling me.

1. After moving my CG, does the Np move as well, or is it governed by the crafts measurements/physical attributes?

2. My aircraft has a flat plate style of horizontal stab, would having an airfoil section as a stab put the Np in a different position?

3. When deciding on a CG, am I safe as long as I stay forward of the Np, or, do bad thing start to manifest themselves at a point before the Np, ie, Does the Np signify a point where bad things 'start to happen' or do they start before that point?

4. If I wanted a plane that was totally neutral, inverted as well as right side up ( needing no elevator corrections whilst inverted), would the Np be the place to put the CG?


I know that it's a lot of questions for one post, but I suppose they are all inter-connected

1. NP does not move for a given aircraft design. It is independent of CG.

2. NP is also independent of the horizontal stab design. Unless you are VERY sensitive to flight nuances, you will not notice a difference in stab design.

3. You are always safe forward of the NP. How close you can get without becoming unstable in pitch, I have no experience. You can also be safe behind NP, BUT it will get very risky if you miss by a small amount. Many full scales fly with CG behind NP.

4. This is for others to answer.

Have fun!

Bedford

4. Something has to create the lift upright or inverted. That's usually angle of attack, which means trimming just a tad on a properly setup "neutral" plane, between upright and inverted.

Hi Paul. I take it you mean, that If it flies hands off inverted, it will nose down when flying the right way up and vice versa? Sooooo...... it will always need a tiny bit of down elevator, no matter how close I get it. Is this right? I assume then, that there is no such thing as a neutrally handling airplane?

The closest I've come was with a Lanier Laser 200, and a similar plane, their plastic ARF TR260. These planes have symmetric airfoils and the prop shaft is on centerline.

Both planes, when rolled inverted, need just a thought of down elevator. More than that makes it pitch up. I attribute this characteristic to (1) the design, with symmetric airfoils and centerline thrust, and (2) balancing properly. Or perhaps putting the CG awful close to aft of the proper location.

Good luck,
Dave Olson

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The trim for level upright won't work for level inverted. It shouldn't take much, usually of about the same amount in the opposite direction, for a plane balanced close to the aft c.g. limit.

A lot of the foam 3D models are flying around with the CG slightly behind the Np to enhance their stunting. BUt then they seldom try to fly in a non stalled "normal" manner for more than a few feet. Technically if the CG is right on the Np it will fly inverted with no input required but other things such as center of overall drag come into the picture as well. The center of drag in normal flight can be a de- or pro- stabilzing effect every bit as much as elevator trim. When you get down to this level of finickyness the little things will bite you.

Before starting this thread, I had always thought that if you put your CG aft of the Np it was destined not to survive long. Some of the posts are suggesting that in some circumstances, it can be placed behind the CG, as in the 3D foamies that you mentioned. I take it then, that all the common info regarding 'sticking to the manufacturers reccomendations', is definately not written in stone. I assume that most of the manufacturers use the old ' balance it at 25% chord', and it'll fly, scenario??
Take for example, my SuperAir. Manufacturers reccomendations were for a CG of 100mm from leading edge. After using the software on Aircraft Proving Grounds, it reccomended 124mm. This change really transformed the plane, especially the landings which were no longer at supersonic speeds.

Superb piece of software by the way..... highly recommended!!! Because I'm intending using this plane for pattern practice, should I try to get the CG as near to the NP as I can, without it becoming too twitchy??

Thanks for the suggestion and the link. Sounds like an excellent tool to use. There are others. For example: http://www.geistware.com/rcmodeling/cg_super_calc.htm

Actually, it'd be smart to use the advice you just gave everyone. Use the software and get it's recommendation.

And then take the airplane out and let it show you where it wants the CG. Dive test. Inverted trim. Snap ability. All these things tell you more than internet keyboards can.

I gotta suggest that those ideas aren't really "common info". Most experienced modelers aren't going to tell you to that mfg recommendations are written in stone. Most of them have seen how mfg CG recommendations often came from nowhere. Of the ARFs I've assembled this year, just this year, I've yet to see but a couple that had a CG recommendation that gave me the impression the mfg knew squat. It's a puzzle what the mfg's use for their CG "scenario." Sometimes I think they're just trying to always insure their customers start out with a very safe CG. But some locations are so "safe" they degrade the design so badly, that can't be the case. And some aren't safe at all. So who knows.......

But you got it..... don't take the mfg's recommendations as gospel. When it's time to work out the CG, do what you recommended and check what the CG software applications tell you. After you've used them a couple of times, you'll find out which ones are reliable (they're probably all going to be more reliable than the everchanging mix of mfg's) and you'll get a feel which app gives you what you like to fly. BTW, the application http://www.geistware.com/rcmodeling/cg_super_calc.htm suggests that you input your desired safety margin. After you've flown and adjust the airplane from the flight testing results, go back into that application and figure out what the safety margin is for the flight result's CG location. Then use that for your next airplane before flight testing it. After doing that for a few airplanes, you'll have a good feel for using the app to help you cut down the flight test time to locate the CG. And after all, the way to locate the CG is in the air.

I gotta suggest that those ideas aren't really "common info". Most experienced modelers aren't going to tell you to that mfg recommendations are written in stone. Most of them have seen how mfg CG recommendations often came from nowhere. Of the ARFs I've assembled this year, just this year, I've yet to see but a couple that had a CG recommendation that gave me the impression the mfg knew squat. It's a puzzle what the mfg's use for their CG "scenario." Sometimes I think they're just trying to always insure their customers start out with a very safe CG. But some locations are so "safe" they degrade the design so badly, that can't be the case. And some aren't safe at all. So who knows.......

But you got it..... don't take the mfg's recommendations as gospel. When it's time to work out the CG, do what you recommended and check what the CG software applications tell you. After you've used them a couple of times, you'll find out which ones are reliable (they're probably all going to be more reliable than the everchanging mix of mfg's) and you'll get a feel which app gives you what you like to fly. BTW, the application http://www.geistware.com/rcmodeling/cg_super_calc.htm suggests that you input your desired safety margin. After you've flown and adjust the airplane from the flight testing results, go back into that application and figure out what the safety margin is for the flight result's CG location. Then use that for your next airplane before flight testing it. After doing that for a few airplanes, you'll have a good feel for using the app to help you cut down the flight test time to locate the CG. And after all, the way to locate the CG is in the air.

Would(n't ) the tail volume (especially the horiz. ) have a large impact on the ability to move the C.G. aft to or perhaps behind the N.P.?
The larger the area of the stab. the greater the ability to remain stable in pitch as the C.G. moves aft, to or behind the N.P.?

The 3D types have large tail volumes. Makes for good control authority, and stability, and allows you to shift the C.G. further aft than perhaps a more "conventional" design...correct?


I added a little wt. to the tail of my Tribute "4D" 36 and now it (when I get it trimmed neutrally ) will dive just slightly whether upright or inverted. If I read the above posts correctly, that means I have the C.G. pretty close, right? (for this type of application )

The larger tail volume will move the neutral point (N.P.) back further towards the tail. Moving the C.G. behind the N.P. will still be unstable, even with the bigger tail volume.

Exactly, the Neutral point is set by the tail volume. The Np and resulting CG range is set by the planform of the design. And as pointed out nothing is cast in concrete. Up to a point the final CG position on any model is determined by your tolerance to dodgy pitching stability or lack of stability. A model with a CG at or VERY slightly behind the Np will be flyable but it'll need constant attention. But move it too far back and it'll exceed the ability of the pilot and radio to input corrective actions quickly enough.

There's a lot of misunderstandings about all this. One area I see this in a lot is the conversion of old timer FF models to RC. In many cases the models were made to be stable with a CG at the 40 to 60% range yet many folks try to balance it at the typical 28 to 30% point that they are used to using on "standard" designs with smaller tail areas and lenghts. In some extreme cases of competition FF models from the 50's the CG was at or even slightly behind the wing's trailing edge. But the models were still stable at that point.

There was a chart around here that was made up by one of our more technically savvy guys that showed the progression of the NP location as the tail gets larger and larger. It started as a normal looking top view and progressed to a full on canard at the other end with various stops along the line to show extreme large stabs and a tandem wing. You see, there really isn't any different planforms. It's just a case of the stabilizer getting larger and larger. You can actually use the Np determination for all these various planforms and it'll work fine. In the case of the canard your "wing" is just a lot smaller than the "stabilizer".

I think the site I was using, was just a link to geistware. But yes, excellent stuff.

By definition!!

close your eyes and envision a simple monoplane with a large wing and a small stab and picture the CG at 25% on the wing
now have the wing shrink in size as the stab increases --
finally- we will have a canard having passed thru the dual wing setup (nasty SOB)
as the sizes changed , the usuable CG moved aft and ended up almost the same on the thing formerly know as the stab.
if you can't envision this , you
A --don't drink enough
B-- spent too much time in some aero school-

Thanks guys
Hey Dick...Joe Valvo sez "say hi to Dick for me if ya get the chance"...guess he bought one of your pattern planes some years back?

I know where to find the CG but where do you find the NP(Nuetral Point?) Thanx, Scott

The following application is titled as a CG locator. If you look at it's output, you'll see that it also tells you where the NP is. It's quite a useful tool. I run all my airplanes measurements through it. Used to figure all the stuff out with pencil and paper but this thing is much faster and doesn't make mistakes.

The measurements needed are very easy to do and take just a couple of minutes. The value of those few minutes simple work is amazing.

http://www.geistware.com/rcmodeling/cg_super_calc.htm

BTW, no matter what safety margin you choose and no matter what CG it suggests for that safety margin, the NP will be the same. As it should be since the NP is dependent on the planform.

Thanx for that invaluable information darock. I will find this very useful., Scott

I just eyeball it -
works for me --

cool

When you've done your 500th plane which really resembles the previous 499.. wing here, tail there... getting a feeling for the correct c.g. -still- depends on flying the thing.
And big surprise, the c.g. will be pretty much on #500 at the point it was on #499, or #333.....
As long as you pay attention to what worked on previous planes, that same point will work on anything of a similar planform.
Most of us are pretty confident with recommending the tried-and-true..,"start at 30% mac.."
Unless your design is really unusual, that will work!

Thanx