The Vanessa rig

Does anyone here use one to sort out the C of G?

Yes I use one and also three scales to calculate the moments on larger planes.

Have you got any tips to pass on Ralph?

STF

What is a Vanessa rig?

Here is the CG gauge that I recently made, it will be part of the 3D printing article that I am preparing.

I like Ralph's method of using three scales, I think he may have got that idea from Motorsport and corner weight scales.

Regards

Barrie

It's a super accurate way of setting the CoG:
YouTube demo

Klipkopwildlife wrote: the Vanessa rig is a string system that supports the wing either side of the fuselage, ie, a bar across the plane clearing the fuselage, string from the bar, down, around the LE of the wing, underneath, back around the TE and back up to the bar. A plumbob suspended from the bar points to the CoG when you lift the whole thing up.

Blimey, that sounds a bit complicated ! I use two pencils with rubbers on the top stuck in a piece of wood. Mind you, I really like your little gadget Barrie.

I like the Vanessa rig, has a lot of Fidelity in it.

I usually use finger tips for CG measurement, I got that pylon racer at 70mm CG position on finger tips and 70.5 on the gadget.

Regards

B.

I agree, two fingers is the easy method and with care accurate for most models. But try doing that with a 2 meter F3A model, 68" Vulcan or 102" Lancaster. They are awkward to handle and at around 6kg bloody hard to hold and mark the CoG. If you build a rig then my advice is to use a light plumb bob. A heavy pointed one makes a big hole in a plane when you drop it! Ask me how I know

I pinched this from another site:

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.

No mention of a spirit level in there Ralph?

Good point ST, didn't spot that when I pinched the text from another site!! Just shows how poor quality the other forums are!!

Hehe, yep.
Do you level on the wing incidence, the tail or a surface on the fuselage that appears level?

I have seen this rig used on a full size glider after it was repaired.
To determine the CofG the glider was set level to the fuselage datum which was defined in the planes manual.
The factory determined CofG position with a fixed cockpit load was given and the new one had to be within a tolerance dimension of that by adding fixed ballast as required.
The cockpit placard weight limits then had to be adjusted accordingly and signed off in the planes service record.

One glider I flew had a particularly high minimum cockpit weight and as I was much lighter in my youth;) I could only fly it with a 20 lb ballast cushion under me.

supertigrefan wrote: Hehe, yep.
Do you level on the wing incidence, the tail or a surface on the fuselage that appears level?

Rightly or wrongly I usually set the level to the horizontal thrust line. Then the wing incidences are set to the thrust line. F3A models usually use the access cover horizontal frame. Scale models......... another story. I usually make a template to give me a horizontal surface refenced to the thrust line.