Vertical CG, Horizontal CG, etc. meanings?

[RknRusty]

I know how to bench trim a plane before ever taking to the air, but I am confused with the terminology.

For instance, I hold my fingers under the wing to see where it balances. I may add nose weight or tail weight.

I hang it from the leadouts and look for a slight nose-down attitude(yaw), and adjust the leadouts.

I hold it supported at the centerline nose to tail and see if it rolls toward belly-up or belly-down. I make the wheels lighter or heavier, or whatever it takes, as needed.

Then I re-check it all until it's where I want it to start flight trimming.

So, please tell me which is which; Vertical CG, Horizontal CG and what is the third one.
I know how to do it, just not what to call it. I should know this by now.
Thanks,
Rusty

[Tim Wescott]

I know how to bench trim a plane before ever taking to the air, but I am confused with the terminology.

For instance, I hold my fingers under the wing to see where it balances. I may add nose weight or tail weight.

I hang it from the leadouts and look for a slight nose-down attitude(yaw), and adjust the leadouts.

I hold it supported at the centerline nose to tail and see if it rolls toward belly-up or belly-down. I make the wheels lighter or heavier, or whatever it takes, as needed.

Then I re-check it all until it's where I want it to start flight trimming.

So, please tell me which is which; Vertical CG, Horizontal CG and what is the third one.
I know how to do it, just not what to call it. I should know this by now.
Thanks,
Rusty

You are confused because the stunt community doesn't use the same terminology that you learned when you took all of those physics and mechanical engineering classes.

There is only one center of gravity of any rigid object, and it is a point in space (hopefully inside the fuselage).  If you could suspend the airplane from this point then it would be perfectly and neutrally balanced.

I'm guessing here, but I suspect that the different terms came about because we all grew up only caring about the fore-and-aft position of the CG.  Then someone got good enough to notice that if the leadouts were too high or low then Bad Things would happen, and instead of talking about "the vertical location of the CG" they just said "the vertical CG", and they said "the horizontal CG" to differentiate the two terms.

When we talk about "horizontal CG" we mean the fore and aft position of the CG.

When we talk about "vertical CG" we mean the position of the CG vertically on the airplane.

I don't think there's a term for the position of the CG from left to right, but if you start calling it the "side to side CG" and it catches on, then the term will be credited to Paul Walker or Igor Burger or someone else famous, and will become part of their fame.  But you'll know where it really came from.

[Brett Buck]

I don't think there's a term for the position of the CG from left to right, but if you start calling it the "side to side CG" and it catches on, then the term will be credited to Paul Walker or Igor Burger or someone else famous, and will become part of their fame.  But you'll know where it really came from.

   Lateral CG - what you adjust by changing the tip weight.

     Brett

[Tim Wescott]

   Lateral CG - what you adjust by changing the tip weight.

     Brett

Yes, using the 9381^st most common word instead of the 260^th and the 4^th is definitely, uh, better?

[Brett Buck]

Yes, using the 9381^st most common word instead of the 260^th and the 4^th is definitely, uh, better?

I have no idea what you are trying to say, I have heard it called the lateral CG for 40ish years now, and it seems technically correct enough for me. Bill Netzeband was talking about lateral CG and CP movement since the mid-50's at least.

   Would you prefer the Y position of the CG? And then have to explain what X, Y, and Z refer to every time?

     Brett

[Tim Wescott]

I have no idea what you are trying to say, I have heard it called the lateral CG for 40ish years now, and it seems technically correct enough for me. Bill Netzeband was talking about lateral CG and CP movement since the mid-50's at least.

   Would you prefer the Y position of the CG? And then have to explain what X, Y, and Z refer to every time?

     Brett

Just failing to yank your chain, Brett.

[Howard Rush]

Would you prefer the Y position of the CG? And then have to explain what X, Y, and Z refer to every time?

I think so and just once, respectively.

[Chuck_Smith]

Let's do it in polar coordinates.

[RknRusty]

Okay, as long as I know how to do it, that's what counts.

[Tim Wescott]

Let's do it in polar coordinates.

Yes!  Centered of a virtual navigation light on the tip of the rudder, or on the glow plug connection (thereby rendering the CG location on electrics as "undefined").

Or maybe we should do it in spherical coordinates centered on the CG -- that way you always know the CG is at 0, you just don't know where the rest of the plane is.

[Brett Buck]

I think so and just once, respectively.

     I have attempted to use standard coordinate frames multiple times and it always resulted in a chorus of "huh?"

    BTW, everyone in the space industry uses the same standards. Except for the people who wrote the most recent version of our flight software, where they somehow ended up with X in the local vertical direction, and similarly sick and wrong things. We are now rewriting it, including, putting it back the way it had been since the 1959.

Brett

[Howard Rush]

These names aren't as bad as some nonstandard nomenclature that stunt people use, but perhaps more understandable would be:
fore-aft CG position
lateral CG position
vertical CG position

Whenever you use them for absolute, rather than relative, measurement, you gotta say from what point you are measuring. 

[RknRusty]

I hold my fingers under the wing to see where it balances. I may add nose weight or tail weight.

I hang it from the leadouts and look for a slight nose-down attitude(yaw), and adjust the leadouts.

I hold it supported at the centerline nose to tail and see if it rolls toward belly-up or belly-down. I make the wheels lighter or heavier, or whatever it takes, as needed.

Then I re-check it all until it's where I want it to start flight trimming.

So, please tell me which is which; Vertical CG, Horizontal CG and what is the third one.
Thanks,
Rusty

These names aren't as bad as some nonstandard nomenclature that stunt people use, but perhaps more understandable would be:
fore-aft CG position
lateral CG position
vertical CG position

Whenever you use them for absolute, rather than relative, measurement, you gotta say from what point you are measuring. 

Seems our aeromodelling esoterica doesn't have any hard-fast terms for these balancing points, but I'll stay with these whenever showing someone what to do or what I did.
Thanks,
Rusty

[Tim Wescott]

These names aren't as bad as some nonstandard nomenclature that stunt people use, but perhaps more understandable would be:
fore-aft CG position
lateral CG position
vertical CG position

Making sure to say the word "position" will certainly keep the engineers from getting unhappy.

[Howard Rush]

Making sure to say the word "position" will certainly keep the engineers from getting unhappy.

Engineers have different terminology which serves them well.  I suggested adding "position" so an outsider might understand private stunt jargon.   

[Steve Helmick]

It's ok, Howard. They also use that word in the Kama Sutra.  Steve

[Ted Fancher]

Typically, the issue has devolved into a debate over terminology which allows pontification but little elucidation.

As some wise one said above, there is only one Center of Gravity for any object with mass (go ahead, beat the words to death without answering the original question) including your Nobler.

What is important to the lay stunt person is that that "singular" CG location has effects on a stunt ship with respect to its three axes; fore and aft, wingtip to wingtip and top to bottom.  Ultimately, the location of the "singular" CG with respect to those axes and the point at which the leadouts exit the aircraft will need to be satisfactorily resolved so as to allow the airplane to be stable but responsive (nose to tail location), fly with the (unwarped) wings level upright and inverted by virtue of the lift being more or less equal on both sides of that wingtip to wingtip "singular "CG location; and fly "tethered" with the wings level due to the point of tether (where the leadouts exit the wing) being in line with the vertical location of the "singular" location of the one and only Center of gravity.

Aerodynamically, both the fore and aft and wingtip to wingtip location of the CG may be adjusted by weight distribution (tip weight and nose or tail weight adjustments) to achieve desired response in pitch with the wings level.  The vertical location of that singular CG location with respect to the leadouts is best resolved by placing the leadout exit point in close proximity vertically to the designed/expected/divined CG in the vertical axis.  For the vast majority of nominally normal stunt ships (Nobler derivatives over the decades of stunt history) very little adjustment will be necessary.  As a wise Al Rabe made crystal clear, however, once you start bending the wings in the middle (dihedral/anhedral) you've got to pay close attention to where you place the leadout exit point vertically so that the exit point will closely align with the vertical location of that singular center of gravity.

If you're a "modern" Nobler kind of guy like me you'll find time spent worrying over vertical CG trim issues to be time wasted that could have been better spent on a few more appearance points.  I flew "modern" Noblers for several decades pretty successfully and don't recall ever finding it necessary to actually "trim" the location of the CG in the vertical axis.  If you're a more inventive designer employing wing dihedral (as many are) paying close attention to major "mass items" (engines, batteries, etc.) during the design process will be time well spent and the "oddity" of the tether/vertical location of the CG may well require some attention to detail.

Finally, the fore and aft location of the leadout exit point with respect to our "singular" CG location will have an effect on the "yaw angle" of the ship with respect to its journey around the stunt sphere.  Adjusting the leadout guide so as to result in the inboard wheel obscuring all but the rearmost fraction of the outboard wheel in level flight (a function of the relationship between the leadout exit and the fore and aft location of the CG (assuming no artificial/ineffective line tension enhancements such as rudder offset) will satisfy 95+% of the fore/aft exit with respect to the CG trim equation.

[Ted Fancher]

Ooops.  This should have been the first subject in the above message.

The "Singular" location of the CG can be demonstrated in the following manner with any handy, irregular, object to which you can attach a line in a variety of locations from which to suspend it.

No matter where you attach the line to suspend it the CG will always fall immediately below the point of tether.  By hanging the object from as few as three points and extending the line described by the tether those extended  lines will intersect at a single point which is the Center of Gravity of the object.  By using an irregularly shaped object you will realize quickly the similarity to our stunt ships in that that singular location will always have a relationship to the "important toy stunt plane" parameters we've been discussing in this thread.  

The relationships can all be altered either by adding weight (thus moving the CG) or by altering the relationship of the parameters to the CG.  I.e. we add or subtract nose/tail weight to move the CG to a place where the maneuvering response rate meets our criteria.  We add or subtract tip weight so as to locate the Center of Gravity at or near to the center of lift so as to make the wing an extension of the control lines...always (to the greatest degree possible) keeping the wing from moving up or down during maneuvers as well as in level flight, etc.

When designing an airplane we adjust a variety of factors: wing/tail size and likely optimum CG for response; offset wing versus equal span wings and the expected required tip weight; dihedral yes/no and the proper location of the leadout exit with respect to the expected CG  etc. etc.

Hope this helps clarify what appears to have become and excessively complex subject.

Ted

[Howard Rush]

Typically, the issue has devolved into a debate over terminology which allows pontification but little elucidation.

Rusty said, "I know how to do it, just not what to call it,"  hence the discussion of what to call it.

[Ted Fancher]

Rusty said, "I know how to do it, just not what to call it,"  hence the discussion of what to call it.

I knew it would work eventually!

[RknRusty]

Well, I got more than I asked for, so that's a good thing. I still know how to do it. If I forget what to call it again, I'll just come back and re-read this thread.

Meanwhile Thanks, Ted, your work has made it possible for me to fly the best damn profile model I ever built. The Walter Umland Fancherized Twister, flagship of my stunt fleet. Added about 5 points to all of my stunts.

[Howard Rush]

Ted knows his stunt.

[Ted Fancher]

Well, I got more than I asked for, so that's a good thing. I still know how to do it. If I forget what to call it again, I'll just come back and re-read this thread.

Meanwhile Thanks, Ted, your work has made it possible for me to fly the best damn profile model I ever built. The Walter Umland Fancherized Twister, flagship of my stunt fleet. Added about 5 points to all of my stunts.

Awesome, Rusty.  Glad it's working for you.  

Here's another five points "per" that'll require a bit more effort and might drive you nuts at first.  When you're flying "serious" stunt, work hard to get comfortable with getting your hand up in front of you and oriented vertically.  I realize this is very difficult to do after a long period of stunting with a flat orientation at your waistline but I promise you will gain great benefits from doing so.  Check out all the "hotshots" and you'll get all the "evidence" you need.  Think of it like aiming  a gun!  Your hand will follow the shape of every trick and increase the precision the same way you hit the target better at the range!

Ted

p.s.  Good looking Twister!

[Ted Fancher]

It's ok, Howard. They also use that word in the Kama Sutra.  Steve

Now that's cute! ...but naughty

Ted

[RknRusty]

Yep you're right Ted, that picture really betrays my bad habit. I've tried sighting right down the lines, and somehow lose my control. But I am working on following the plane better with my arm. Stunt season is coming up soon in my neck of the woods.
Rusty

[Pat Chewning]

Well the location of the CG will lie somewhere along the 3 axes of the aircraft (yaw, pitch, roll) or [Vertical, Lateral, Longitudinal].   See diagram.

Of course, you also need to establish the 0 (zero) reference along each axis in order to determine the distance from the CG to the zero reference.   Common 0 references might be:   C/L of airfoil, Thrust-line of engine.  LE of wing.   25% of MAC.  etc.......

OR, you could establish 0 (zero) as being the CG position and then make measurements to other interesting places on the aircraft....