Vanessa c.g. Rig: How to.
 

What follows is a copy of info related to a c.g. finding rig that may be of use to some. As requested I'm attempting to post the complete package. If this is deemed against the forum rules by all means the moderators may delete it with no hard feelings on my part for sure. So, for what it's worth here's part 1#. First the boring text stuff. This is what I wrote up and packaged with the units I produced for my local flying groups use.



Gents,
Some time last year the question was asked on the RCO forum in regards to the Great Planes C.G. machine. That question in essence was, “Should I buy the G.P. C.G. Machine? How well does it work? Who’s used it?” The replies to the posting were varied but most were complimentary, other then mine I think. I am an owner of said machine and I rather wished I’d saved my money. It sat in its box for the longest time, as I had no need of a c.g. finding device at that moment. With the completion of my SIG Magnum build I then had a need, so out of the box it came. Don’t get me wrong here, it’s a fine piece and comes with “sticky weights” and “spinner weights” all of which one might need. And the unit itself is of very good construction, no faults here. But what really didn’t rock my world was the manner in which the model would have to sit on top of this stand for it to work. Yes, I knew this when I bought it, but seeing and then doing, as in many things, are very different in deed. I believe that you would have to be close to balanced to get the model to set up there in order to have any chance of making full use of it. What if the model is out in left field related to C of G concerns? I had visions of “one hand on the model, one hand doing the adjustments”. Its manner of operation just did not set to well with me. (Personal preference to be sure here) To much work to be done on the underside of the wings for my liking (to get the model situated), but what other way is out there for finding the c.g.? Oh yes the two-finger trick. Yep I like that better then the C.G. Machine in truth. (Just a bit better but not much) But yet there must be a better way still. I think I’ve found at least one. Read on.

The C.G. Rig: How to’s and why for’s.


Firstly, this is not an invention of my own making, it’s comes from the Internet. The version I’ve built is based on the work of Jim Archer, and could be found on his web site. He calls it Vanessa for some reason or other. See www.jimarcher.cwc.net for the home page this came from, but you’ll not find the actual “Vanessa” file there, ..it’s no longer posted it seems.

Any way, changes that I’ve made included the following:

1) There are two sets of support loops, consisting of two short loops, and one longer loop. The two short loops are to be used as a pair while suspending the model via the wings only. The single longer loop is for use as a fuselage suspension system only, to support fore and aft ends of the fuselage. Profile models seem to work best with the two shorter wing support loops, and full body planes seem to work better with the fuselage loop (which is the longer one). Any thing can be made to work on any model with enough tweaking. See the diagram for the different set ups.
2) I’ve included a SliderLock bar on both the main “Inverted Tee” bar support line, and on the plumb bob support line. These SliderLock bars will enable easy adjustment of the system to suspend the model, and or adjust the height of the plumb bob above the model. Should the lines of either prove to be to long simply pull the required excess through the slider bar by the knot tied in it’s end, and then tie a new knot at the shorter line length as needed. The line lengths are sized to work in my basement via suspension from the ceiling/floor joist. These may be to long, or short for your area of operation. I tend to work with the model sitting on the floor hence the longer line at the Inverted Tee support.
3) The cross bar dowel of the Inverted Tee components is a friction fit, but I’ve added a tension screw that’s filed flat on it’s pointy end (tension screw is optional). A simple turn of the screw will bring more friction to the Tee, and thus stop the dowel from rotating freely. The dowel will need to be rotated during the C.G. finding process, so do not put this screw to tight.
4) Also included: An “S” hook, and a very short loop of rope. These are for hanging the pulley from any handy spot overhead, and/or securing the main Inverted Tee support rope to a stationary object so adjustments can then be made via the slider bar, for model height and such. Use them for either or both jobs.
5) Part of the process of finding and adjusting the model C.G. includes setting the models centerline, or thrust line to the neutral level position. Included is a small line level to help do this job. Simply place the level on the models fuselage at a point where its location is equal to the thrust line or zero/zero incidence line of the model then rotate the Tee dowel until the bubble is centered in the glass. Remove the line level after doing so, as its weight will effect C.G. location. Like wise: as the line level affects C.G. due to its weight, the actual model “level attitude” will be only close, but its better then the Mk 108 eye ball system I think.
6) Some small adjustment to the loops may be in order, check for wings level when equal turns are wrapped on the dowel and adjust as needed, they should be very close in any respects.

Tips:

1) The farther away the weight is from the C.G. location the more effect it has on the resulting change. If the model is tail heavy it will take less weight to balance it if you can get the required weight as far out front toward the nose as is possible. The moments of force are greater the further they get from the pivot point.
2) I’ve found that the technique of duct taping the rear fuselage loop to the underside of the full body fuselages works very well, the wing flaps are not fouled by the support loop this way. I’ve not tried this with a profile fuselage model yet, but it could be made to work. Flapped ships tend to be a bit harder due to flap-fouled loops, unless a means of locking the flap position can be found, and the flaps will support the load placed on them by the models weight.. Remember that if the flaps are locked, the locking medium could be part of the weight and balance of the model. Be aware of anything that you put on the model during the balancing, and where it is your putting it!
3) A neat way of adding weight without adding weight is to take a sandwich lunch bag, or Glad “Cheap Ware” container and pass a loop of string through the sides so as to form a handle of string. This handle can be hung over the engine crank shaft at the front, or tail wheel strut at the rear, and you just pile the weight into the container or baggy. With a known amount of weight arrived at, you can then attach this weight on the model to effect the desired balance. Helps to stop them lead bars sliding off your fancy paintwork and such.




Addendum: for SSW posting.

A Brief run down of the parts and cost follows.

1) 15/32” x 36” dowel stock: $1.45 (You only need about 14” as per my dimensions)

2) 1”x 2”x 8’ Pine: $4.72 (You only need 12”)

3) 1 Nail Setting punch: $2.98 (Optional, for plumb-bob, you can make your own from lumber)

4) 1 Line level: $3.98 (Optional, you can use the Mk-108 eyeball)

5) Cord for support lines: $4.29 (This gets you a 36’ pre-packaged roll, similar nylon braid cord is available for $00.12 per foot, or a slightly larger size at $00.18 per foot.)

6) Pulley: $2.86 (Optional, but I like the smooth operation myself)

7) Snap Latch: $2.00 (Optional, but works well with the pulley)

8) Carpet Thread $00.07 per foot (for plumb bob line)


(All amounts are in Canadian funds.)


That’s all, enjoy, ….I hope.

Vanessa c.g. Rig: How to.
 

What follows here is Jim Archers own words related to Vanessa.

Centre of Gravity

Locating the exact C.G. position on your mode is one of the most difficult parameters to measure, as anyone will attest who has tried to balance a low wing monoplane on a pair of pencils stuck in a 2 x 4.

If you want to know where the C.G. is located on almost any configuration of model plane accurately, and I mean within 1/16”, make this device! It works on any model, from a 1/2A two channel to a 30-lb. 1/4 scale Tiger Moth.
It works every time, is simple to use, and it’s accurate.
It can be made of almost anything kicking around the workshop - a piece of 1 x 2 (or 2 x 2) wood about a foot long; a piece of 1/4 or 3/8 dowel also about a foot long; some flexible but strong cord about 1/8" in diameter; a pulley you can hook up to the ceiling (preferably over a bench where your plane can sit), and a little plumb bob.

Construction:
Drill a hole parallel to the long dimension near the end of the 1 x 2. It must be snug enough so that you can just turn the dowel by hand. If it is too loose put a saw-cut through the hole and a bit beyond it, and a bolt with a wing nut (as shown in Fig. A) so that it can be adjusted by hand. A hole in the other end for the "hanging rope," a pulley, and two nails in a stud to snub the "hanging rope," and that part is ready.

Next, make two equal loops of soft rope or cord to support the model. The pair I use are 40" in circumference, and they handle most 40 to 60 size models. Cut two pieces of soft cord 40" long and tie the ends together. Pull them taut over your index fingers to make sure that the lengths are equal, and then put a drop of CA on each knot to assure they stay forever.

Now make a small plumb bob. I made one out of a 2" length of 3/8" dia. aluminium rod. I chucked it in a drill press and filed a point on one end. A small axial hole at the other end and a cross-hole made a neat place to tie a piece of string or thread. I discovered later that wallpaper stores sell little lead bobs that are perfect. About a foot and a half of thread tied in a slipknot will allow you to adjust the height of the plumb bob.

Use:
To use this marvellous device, place the model aircraft on the bench with the centre of the wing more or less below the pulley in the ceiling. Lower the dowel and support to a couple of inches above the wing. Place the 40" loops around the wing as shown in Fig. C or D, and wrap them around the dowel 4 or 5 times (same on both sides). Since the C.G. is normally well forward of the centre of the chord of the wing, there will be more weight on the forward side of the, loop than the rear, and the angle of dangle will be different fore and aft. Thus the rope would like to slip forward, but the wraps around the dowel provide enough friction to prevent this. Put the plumb bob string over the dowel. Usually it is more convenient to have the plumb bob on the inside of the loop, hanging over the fuselage, or wing root.

Carefully hoist the model a couple of inches off the bench. Steady it until it reaches a state of equilibrium. By hand rotating the dowel in its hole, adjust the model to a level flight attitude. An important detail in using this device is to have the dowel parallel to the wing spar.

Adjust the slipknot on the plumb bob so that it hangs just above the model and - Voila! - It points to the Center of Gravity. Since all of the weight of the model is being supported by the dowel, the C.G. will be directly beneath the dowel (where- the plumb bob is pointing).

Put a piece of masking tape on the model where the plumb bob is pointing, and mark the spot with a felt tip pen. Or better still; put the tape on before you hoist the model, mark where the C.G. should be, and then get the good/bad news when you hoist it. While it is still up there, you can add weight to the nose or tail, adjust the attitude to level flight again by rotating the dowel, and see before your very eyes where the C.G. has moved to. After a couple of tries you should know exactly how much weight to put where.

Versatility.
Fig. C shows a high wing trainer, but the device works equally well with a low or mid wing pattern type aircraft (Fig. D). With a biplane, just put the loop around both wings (Fig. E). You say you have a J3 Cub with wing struts that get in the way! No problem; make a pair of 40" ropes with a loop at one end and a little wire hook at the other end. Feed this under the wing, around the dowel, and put the hook in the loop.

If you have flaps or strip ailerons that come close to the fuselage, and may not support the weight put on them by the ropes, a couple of things can be done. Make a single large loop of rope and sling it under the fuselage fore and aft of the wing as shown in Fig. F. Tape the rope to the bottom of the fuselage, far enough behind the wing to keep the rope off the trailing edge. Or cut a piece of balsa an inch or so wide and a bit longer than the chord of the wing at the root. Place this under the wing with the rope beneath. Make sure they stay in place as you hoist the model. A delta can be tested with the single loop fuselage sling method (Fig. F) by putting a strip of 3/32" balsa beneath the fuselage. The strip must be long enough to be held by the front sling, and protrude behind the wing for the rear sling.

Vanessa c.g. Rig: How to.
 

Related drawings as per Mr. Archer.

Vanessa c.g. Rig: How to.
 

This is the last of Jims Archers drawings related to Vanessa.

Vanessa c.g. Rig: How to.
 

My pictures start here. There will be no more commentary on my part, the pictures include text.

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Part#17 The last one. I hope this is of use to some of you. Cheers.

Tony

RE: Vanessa c.g. Rig: How to.
 

I know this is an old thread but I would like to try this balancing setup as it looks like it would be fairly easy to do by ones self. Just wondering how big of a model does this work well with?
Thanks,
Bob

RE: Vanessa c.g. Rig: How to.
 

Bob,
The SIG Magnum C/L stunt model pictured is of 60" span. The techniques shown here could/should be able to service any size model you like. Size the hardware and lines to suit your models size a weight.


Tony

RE: Vanessa c.g. Rig: How to.
 

Thanks for the quick reply, I am really tired of using the GP balancer and this looked like a suitable substitute for all the larger aircraft. The finger lift method works ok but can't balance the plane while alone.Thanks for confirming my thoughts. I will try this setup on a 1/4 scale and see how it goes. I have no doubt that it will work well.

RE: Vanessa c.g. Rig: How to.
 

A buddy of mine uses this system on his 40% scale Edge 540 ect.He has a cordera belting rather than string, but much the same otherwise.

RE: Vanessa c.g. Rig: How to.
 

Hi!
What's the fuss with all these strings and gadgest when you can just use your indexfingers!

RE: Vanessa c.g. Rig: How to.
 

Try balancing this on your fingertips. We used the Vanessa Rig to balance it perfectly.

RE: Vanessa c.g. Rig: How to.
 

Try This.

Ed S

RE: Vanessa c.g. Rig: How to.
 

Major benefits with the plumb bob approach are: accuracy; convenience (once you've got your rig); one guy can handle a big plane by himself; you can hang your model with everything mounted except battery and rudder servo(s), then position them very quickly forward or aft of CG to get the balance just where you want it, with little or no ballast required; you can check the lateral balance at the same time, and add a little wingtip ballast if needed. I use nylon straps for my sling instead of cord--gentler on the model's finish.

RE: Vanessa c.g. Rig: How to.
 

Great for trainers and very light aircraft.:eek:

RE: Vanessa c.g. Rig: How to.
 

Hi!
Well! Should'nt be that difficult.

RE: Vanessa c.g. Rig: How to.
 

Some questions about the suspension system.

When the aircraft is suspended, the wing incidence has to be that which is designed for flying, Correct? How is that ensured.

If the airplane is suspended in a nose/tail down attitude does the CG pointer still point to the correct CG position?

The CG of the airframe is a function of the mass of the airframe. The flying balance point is an aerodynamic function. Are the two in the same location?

I am seeking information. I am not trying to decry the method.

Ed S

RE: Vanessa c.g. Rig: How to.
 

For the plumb bob method in general, the pointer will point down through the fuselage to the three-dimensional CG, which is located somewhere in the interior of the model. But all you can go by is the visible point on the fuselage exterior. Therefore, for reasonable precision, you want the model to be straight & level. In the sling variation I use (without the dowel and twisting cord), I just slip the sling a little forward or back as needed to have the model hanging level in pitch and roll. With the Vanessa rig, I understand this function is performed by rolling the cord one way or t'other on the dowel.

Aerodynamic neutral point and CG are typically different, although they may coincide. If they're different, you make it up with adjusting wing incidence or decalage or control surface trim. What we call 'the CG' can be understood two ways. Any mass has its natural, physical CG. In setting up your model, you decide where you want that natural CG to be, and distribute your movable masses (battery, servos, ballast, etc.) to make your 'design CG' come out where you want it--let's say at 33% of mean aerodynamic chord, laterally centered on the fuselage. The aerodynamic neutral may be forward or aft of where you arbitrarily set your design CG, which you'll discover when you do your trim flight. So you test fly, trim surfaces for straight & level, and after observing and feeling how the model is flying, you nudge the CG to take out the trim, until you like how it flies. No more measuring, no more calculating, no more plumb bob, just fly it and change little things (wood prop or composite, heavy or light spinner, shift the battery, etc.) until you like the result. So the design CG is a first approximation, chosen for the purpose of positioning the heavy components for which you have some choice as to where to put them.

The hanging and plumb bobbing is only done once, and I do it with every component installed, with the usual exception of rudder servo(s) and battery. These I simply slide around (maybe with a little patch of double tape to keep them from rolling off a rounded surface), and the new configuration will naturally swing its new CG under the pointer. When the pointer comes to rest at the spot I have marked, I then figure out how to do the final installation of the variable components.

The old way I used to do it was to just put everything where the plans said, and then add lead to bring the balance to the manual's specifications (which are reliably nose-heavy, usually at 25% of MAC). The way I do it now lets me use necessary components instead of ballast, saving who knows how many ounces and even pounds of lead on a large model. So the possibilities for conserving lightness can be very considerable. If you're lucky, and the trial CG comes out very close without the battery, you can choose a lighter LiPo or whatever and put it right at or very close to the CG. If you're way off, you can make up a lot of balance with a heavier nicad and put it out on the engine mount beams or way back in the tail or anywhere in between. Sometimes this means cutting in a hatch to mount and remove the battery, which I've done, and seen it done on other people's pattern models.

RE: Vanessa c.g. Rig: How to.
 

Thanks for the detailed explanation.

When you say the model should be hung straight and level do you mean the wing at 0-0 incidence?

Ed S

RE: Vanessa c.g. Rig: How to.
 

The horizontal stab will fly at 0-0 unless you force it to do otherwise. The wing will fly at whatever incidence the design has dictated.

Les

RE: Vanessa c.g. Rig: How to.
 

Maybe, but will it hang that way in the system described? I am pretty sure I could get the airplane to hang at any angle I want. Will the CG position always be the same? As yet nobody has answered that question.

Ed S

RE: Vanessa c.g. Rig: How to.
 

Ed, the answer from Les above is correct and pertinent. I don't typically get into the fine points of wing incidence unless the model is giving me problems. When I say straight & level, I just look at it as though I were flying it as in IMAC or pattern level flight, and then set my CG with the plane in that attitude. There is some illusion of precision here that doesn't really apply. Because you can measure down to 1/64" or 1 mm doesn't mean you've solved your ultimate problem. The whole point of the sling is to get me to my first approximation of CG with the least lead possible. But the final CG can only be settled on after flying the plane--easily ten or twenty flights before I'm satisfied. When all is said and done, I might be an inch away from where I started, but at least I start with a balance point somewhere in the ballpark.

My typical plane starts off at 0-0-0 for vertical thrust, wing incidence and tail incidence. If I can get it to fly the way I want while sticking to these parameters, I'm happy. A pattern or IMAC model I will usually put my initial CG somewhere between 30-33% of mean aerodynamic chord. For a long time I liked 33%, tried as far back as 35%, but my latest model seems to do better around 30%, so I'm still learning. I've never had a plane set up with CG so far back I couldn't fly it, but I have had planes with initial CG at the manufacturer's recommendation that were so nose-heavy it was all I could do to make it around once and land it.

You ask 'will it hang that way in the system described?' I have no experience with the Vanessa system specifically, but my approach uses the same principle. I just take two nylon straps with adjustable lengths, loop them, tie a piece of nylon cord connecting the two loops, then hang the cord over a screw-hook in the ceiling. The straps can go either around the fuselage fore and aft of the wing, or under each wing at the armpits. I adjust the loops as needed for everything to hang straight. If the plane is hanging nose down, I just lift the nose and the nylon cord slides over the skyhook and settles in the adjusted attitude. So you're right in saying you can get the plane to hang however you want it. But how you want it is straight and level, because you want the plumb bob to point to a mark you've put on the surface of the fuselage (top or bottom, depending on the type of model). You want the fuselage mark to be directly above the center of mass of the model, which is somewhere in the middle of the fuselage. So if you adjust the plane to a level attitude, the plumb bob will point straight down to the center of mass, because the whole thing is basically a pendulum which will come to rest with the CG straight down from the plumb bob.

This is one of those situations where it takes too many words to describe what is so simple if you just set it up and give it a try.

RE: Vanessa c.g. Rig: How to.
 

Ed. Yes you can get the airplane to hang at any angle that you want, but what you want is the airplane hanging in it's flying attitude (horizontal stab at 0) and at the same time have the plumb bob pointing at the CG point specified on the plans. If the plumb bob is not pointed there, you add (or move) weight to accomplish this.

Les

RE: Vanessa c.g. Rig: How to.
 

Thank you Les.

You said the magic words, "Horizontal stab at zero". All is now clear.

Ed S

RE: Vanessa c.g. Rig: How to.
 

If the rope doesn’t break size doesn’t matter. I have been using the balance cradle method for several years and the string broke on a 35-pound model. The fall to the floor was only a couple inches and softer than most landings. With a chain hoist and a big tree you could balance a full-scale airplane.

Bill

RE: Vanessa c.g. Rig: How to.
 

Here is my Venessa rig. Works like a charm, and the parts were cheap. The Funtana came out tail heavy with all the servos in their stock location. I got my Funtana balanced with a pull-pull setup and NO extra weight.

RE: Vanessa c.g. Rig: How to.
 

Got two of them in shop and they work fine.
I use 1" wide strap for support for wings and a section of Spyder wire fishing line for lateral balancing (prop hub to rudder hinge.
Rigged with pulleys they serve as overhead storage of planes when not in use.