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c of g suspended from ceiling thingy!


Den Moran
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Hi! Can anyone remember the name of the home-made suspender unit to check C of G, please? I think it was a girls name, but a lot of time has passed. From a ceiling hook it used two underwing loops, with a central plumbob from the hook. I've got one, just want to check I'm using it correctly. Cheers, keep safe all,

Den, Abertawe.

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Well Done Alan & Steve..................https://www.youtube.com/watch?v=afvfShdOuck

 

Looks all rather OTT to me and prone to error. Fingers are fine for the type of model being shown and there are far better methods of finding the CG on large/awkward models. The busted up wing in the background is not too encouraging!

Edited by Cuban8
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Spurious accuracy.  You cannot determine the optimum CG position without flying the model.  Get it to a safe position and then go and fly and determine when it's in the air whether you have a forward or aft CG and then adjust it to suit.  Once you have identified the optimum CG position by flying the model, then measuring it accurately makes sense.  Otherwise, its a waste of time IMHO.

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Nothing spurious or OTT about the Vanessa rig (Diana's sister). For larger / heavier models it is a godsend, I've used mine on my Bearcat, Tigercat, Super Sportster, Giant Revolver, SU26 to name but a few, all of which would have been very difficult to balance without it. During the build stages it gave me indications of where weight was needed which in turn helped me locate the flight batteries etc. Obviously the maiden flights allowed me to refine the C of G locations but Venessa helped make sure it was roughly correct to start with.

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2 hours ago, Ron Gray said:

TBH I can't see another way of determining the C of G on the Tigercat bearing in mind its size and weight!

Perhaps the same way that full size aircraft have their CGs determined by weighing and a bit of simple maths? Shouldn't be too difficult to adapt to modelling needs.

http://avstop.com/ac/weightbalance/ch3.html

Edited by Cuban8
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Following on from Cuban 8 above, if you have a digital scale then proceed as follows.

Set the aircraft into its flying position.  This can either be by eye or by using a spirit level if the fuselage has a flat surface that should be horizontal in flight.  Alternatively, if you have an incidence measuring device set the wing at +1 deg.  You should also have wings level.

 

You then need to weigh the aircraft at each of its undercarriage points i.e. both main wheels and either nose or tail wheel.  You need to make up some spacers that are the same thickness as the weighing device so that the aircraft remains level at all times.  Swap the machine and spacers around so that you record the weight at each of the 3 wheels.  The diagram attached shows that we need also to measure one distance and that is the horizontal distance between the main wheels and either nose or tail wheel.  

Since we need to find out the value of the distance from mainwheel to CG there is only one unknown and we have one equation so it can be solved very simply as in the diagram.  W1 needs to be the sum of the two mainwheel weights and W2 the tail or nose wheel weight.  The answer will be how far behind the main wheels the CG is located for a tail wheel aircraft or how far in front of the main wheels for a nose wheel aircraft.

 

 

 

Calculating CG Position.pdf

 

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I meant to post a diagram in the above post but it apparently has to be a jpeg to show as a diagram.  So, here's what I meant to post and be visible in the post:

 

754360747_CalculatingCGPosition.thumb.jpg.2083f60d40c19ba6beb925c6a13450a6.jpg

 

Where W1 is the total of the main wheel weights and W2 the tail or nose wheel weight.

 

The length L is the horizontal measurement of the tail/nose wheel to main wheel axis when the aircraft is in the flying position.  Draw a line, place the main wheels on it and then with the fuselage in the flying position use a plumb bob (weight on a piece of string will do) to get the nose or tail wheel position.  Then measure the distance at right angles to the main wheel line.

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I have to say Ron, when I'm looking for the CG on my 2 mtr, I mark the desired CG position on the bottom of the wing at the root. Stick my fingers on the marks and lift.  I did buy a Hangar 9 CG machine.  I used it once or maybe twice.  It's gathering dust in a drawer somewhere now.

 

Provided the CG is forward of the rear CG mark I fly the aircraft and check the CG position when flying and adjust accordingly to achieve the desired flying characteristics.

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Try balancing a 6m moulded glider weighing 16 to 18kg on your fingertips and you'll be shouting for someone to drive you to A&E with broken fingers. 

Vanessa rig is highly accurate, cost me nowt made out of the scrap box and does for all my bigger models and I use the MPX balance beam CG device for all the smaller models.  Once they are balanced at the kit, or plan CG, then fine tune in the air by moving tanks or batteries etc. 

The only one which gave a problem was the Precedent Stampe as the CG on the plan is 30mm or so too far rearward and so the maiden flight was rather lively.  Once corrected it was fine, but that was not a problem caused by the Vanessa rig, but by the incorrect location on the plan.

John M

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I don't have any 'sky-hooks' left to keep Vanessa happy, and as already said, keeping two fingers in good condition can be important. Then there are my other weird checks, rolling backward and forwards on a dowel or pencil, (I like to use a pencil with the hex flats).

 

So I decided finally to build a simple balance rig with what I had to hand.

 

But as always I use a CG Calculator to get a starting point (for my 103" Lancaster).

Ray

 

CG Rig 1a.JPG

CG Rig 2.JPG

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Maybe worth remembering with a big plane on a Valerie rig that it’s possible to sling the cords under the fus, where they are less likely to slip off or stick in the aileron gap etc although from recollection the cord then needs to be in one piece.   It goes under the front, winds around the fulcrum then goes under the back.   Kinks might be an issue depending on the cord.

 

I found some “soft rope” on a curious website which I mistook for the folks who taught me to drive, BSDM or similar.   Takes some beating.

 

if I grasp the physics correctly, as long as everything swings freely then with the model level, the dangly bit (clunk weight works well) will inevitably find the c of g.

 

Caravanners mini spirit levels are handy too, £1 each.

 

oo err ... 3rd para might be capable of misinterpretation ...

 

BTC

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