Vanessa CoG Rig

[Tony B-622]

Evening everyone once again I'm after the use of the vast collective knowledge of you all, what I would like to know is does anyone have or has ever used the " Vanessa rig " to locate the CoG on a model ? also am I right in thinking that is what it is intended for.

I know you can use the finger tip method or pencils placed in wood even use mathematics to work out where it should be (maths was never my strong point) but as I'm more of a tinkerer I find it more enjoyable to make devices that would aid me in such tasks, if anyone has any info regarding this rig I would love to here any and all opinions so thanks in advance.

Tony

p.s this is referring to the Escale Seafire as I followed the instructions as to where the CoG should be and on first attempt for it's maiden the nose dropped like a brick so either I've done some thing wrong which is most likely or it confirms my first thoughts it was in the wrong place.

Thanks again.

Edited By Phantom 622 on 18/11/2013 22:51:56

[John Privett]

I've never used one, but the Vanessa rig is indeed intended to show where the c. of g. actually is on a model. I don't know if he devised it, or named it, but it seems to have been popularised mainly by the late Jim Archer - one of the real 'characters' that this hobby tends to produce - who sadly passed away around 10 years ago.

Suggested c. of g. locations for kits or plans are often a bit further forward than most people would choose - in line with the old saying that a model with the c. of .g. too far forward flys badly. One with the c. of g. too far back flys once!

The Vanessa rig though won't help you find where the c. of g. should be, only where it is.

[Bob Merrett]

Phantom,

John's final comment says it all really. If you are building from a plan or have ARTF type kits, then the CG should be already designated. If you are scratchbuilding, then you need to put some mathematics behind it to establish a safe CG for flight. I use the formula that Gordon Whitehead published in his book from 30-odd years ago and out of print now.

The Vanessa rig will allow you to adjust the CG from it's current as built position to the correct should be position safely, without slipping all over the place, falling off stands etc. The model is safely suspended, just make sure the flying attitude is correct. So, the bottom line is, you need to know where the CG of your model is, then use the Vanessa rig to balance the model accordingly. there are some clips on you tube showing this.

Bob

[Phil Green]

'building a vanessa rig' makes it sound rather involved - all you're doing is hanging the model and a plumbob from the same point!

[cymaz]

I'm going to build one- seems a good idea. I watched the YouTube video and it looked very accurate. I get the principle I just want to see it in action for myself. Will report back in a day or three.

[Phil Green]

If you're going to town on it, you can get plumbobs that mark with chalk powder, or even laser plumbobs!

[Chris Bott - Moderator]

Danny Fenton's scale column in the November issue featured C of G measurements and the Vanessa rig.

[Martin Roberts]

One thing that most people forget is that the COG is a theoretical point of aerodynamic balance for the design of the aircraft...some aircraft are designed to be dynamically unstable (stunt and fighters), some dynamically stable transport and civilian - Cessna's etc.), some (mostly the powered variety) are a little bit tail heavy flying with a high angle of attack (AOA) to create lift... great while you have a running engine, whilst most gliders are nosey with a low AOA to induce speed to create lift.

This just means that the COG (bad terminology) should be just about the centre of lift (COL)- calculate all you want but if you don't know where the COL is then it's just trial and error until the aircraft flies the way the designer wants.

As a typical ARTF goes the Park Zone Radian COG is far more forward than it could be, so the tail plane is angled down to generate nose up, this combined with speed increase causes the AOA to increase and generate the lift, generate more drag, followed by a theoretical stall - nose down - more speed - nose up - stall (porpoising) add a motor with down thrust to overcome the drag, reduce the stall.

You can modify the radian move the COG back flatten the tail reduce the drag which increases the speed creating better lift etc...

Vanessa allows you to adjust the harness until correct AOA is achieved, then if the tailplane is level (if the design is correct) the COG and the COL should be about the same place.

Happy flying

Marty

[Dave Miller]

Whilst use of a Vanessa Rig will only show the actual balance point its use is a great help in finding how much weight is required to shift it to the calculated position.

[Tony B-622]

Thanks for all the replies after knocking up a quick rig I know understand its use, it takes a while but it's finaly sunk in thanks John for your comments explaining what it does. Chris it was Danny's column that actually got me thinking about building a rig I just didn't fancy the thought of using scales, so thanks once again one and all I really appreciate it.

Tony

[Bill Brown 3]

Hi P622, Would this be a Vanessa Rig?

[Tony B-622]

Yes Bill thats basically what you would need or items similar to the same effect but that would make a Vanessa rig.

Tony

[Danny Fenton]

Thanks for mentioning the scale column guys I was starting to think nobody had read it when I initially saw this thread. I sometimes wonder how many regular readers frequent this forum....
cheers
danny

[Tony B-622]

Thanks to you Danny for doing it I've gained a little more knowledge all because I read that little tit bit in the mag and it put the idea in my head

Tony

[cymaz]

Here is the feedback as promised. It only cost about £6 to make so I reckon this experiment is worth a go, you never know I might win the Nobel Prize for Physics-not

I used my artf Wot 4 as the trail plane as I know it flies slightly tail heavy by the fact that when inverted I barely have to touch the elevator and it's off to the sun. I assume three things with the plane 1) the horizontal stab is parallel with the datum line of the plane 2) the stab is not twisted 3) 3/14" c of g was correct off the instructions.

The first thing was to set the level and put the plane in the rig...

Next was to get the tail exactly parallel with the level and see where the plumb bob went - at this point I had no idea where the actual c og g was apart from using my fingers when first setting it.

The plane shows that its c of g was behind the factory recommendation.

I then added some 25g of lead re adjusted the balance and checked again. I was surprised that the adjustment need very fine movements.

Not a great deal but getting there. Next was 230g of lead....

I took this from the other side of the line so the plumb bob is in fact ahead of the c of g. I then tried 145g of lead and readjusted again...

I think that is spot on.

The next thing to do is to fit the ballast and go and fly it and come back to tell you how this extra nose weight and the moving the c of g forward has changed the flying habits of this plane. The way an individual pilot sets up a plane will vary though. So far I have been surprised on how fine you can make the adjustments and how much lead is needed to put the c of g back to the manufacturers point.

[Danny Fenton]

Nice one, should have got you to do the article I think they work very well, and as you say, very little outlay

Cheers

Danny

[cymaz]

They can copy and paste it if they want

[Chris Bott - Moderator]

Nice one cymaz, some good pics.

Might be a little difficult to cut and paste them into the November issue now though

[cymaz]

Any time to suit you Chris

[Mike Freeman]

I've been using one of these for a few years to great effect. One thing I noticed whilst balancing a very light model - a Speedo Electric in my case - the weight of the ropes can have an adverse effect on the results. My rig uses thick rope loops like Cymaz but I had to swap them for thin string to get an accurate balance. The reason is, on a typical layout model like the Speedo, the COG of the rope loops ends up behind the models COG. The combination of the two gives an artificially rearward COG indication especially with relatively heavy ropes.

Cheers

[Bill Brown 3]

Cymaz, at the risk of sounding a bit thick, how did you set the plane level ? was it by setting the tail plane level ?

[Dave Miller]

Posted by Bill Brown 3 on 21/11/2013 17:31:43:

Cymaz, at the risk of sounding a bit thick, how did you set the plane level ? was it by setting the tail plane level ?

That would be the normal way.

[cymaz]

Yes it was

[Rick Tee]

You don't need to do much maths to calculate the initial C of G for a model, just go here put in a few dimensions. **LINK**

[cymaz]

Yes you're right Rick. I use both- one to check the c of g in theory the rig shows actual c of g. A few people on theRCUniverse forums have missed the point that the rig does not show theoretical but actual c of g .