C of G contration - does it work?

[rcKeith]

Hi

I came across this some time ago and wonder if it does the job. http://www.utahvalleyaeromodelers.com/framer.html?url=/articles/glitch/09_2003/glitch_sept_2003.pdf

Its on page 5. I've built one and it seems to get it right on my other models but I'm a bit unsure.

I have a Green Air Design spitfire and that I just built and when I put in on normal multiplex C of G tool with the C of G at the recommend setting of 35mm from L.E its very tail heavy. Putting on the contraption it seems to point to a C of G arounfd55mm.

To get it right on the recommendations I'd need to add some lead. I can't shift the battery its right up as far as it will go.

Any thoughts

Keith

[birdy]

Add a link.

[rcKeith]

OK Link added

http://www.utahvalleyaeromodelers.com/framer.html?url=/articles/glitch/09_2003/glitch_sept_2003.pdf

[birdy]

Thanks for the link! I would think it would work, because the COG should go directely under where the model (or anything else for that matter) is hanging from. I wouldn't matter where you hanged it from.

[260 Flyer]

I prefer to use my digital CofG indicator. It is much simpler to use. Just place one digit from each hand either side of the fuselage and adjust back and forth until the balance point is found. If I can lift it I can balance it!

[Brian Parker]

Seeing the model hanging by two triangles in the article invites a pun regarding equilibrium of forces whose conditions are only true if the forces act in one plane. 

Why build a rig when you have fingertips?

[Former Member]

[This posting has been removed]

[Brian Parker]

Eric,

As a man of great experience you will be aware that once you have used your fingertips and balanced and flown your first model your instinct will compensate (as in your ‘nose heavy’) to give the precise fingertip position for any subsequent CofG regardless of the ‘fatness’ of the finger. It’s all about being in contact with the model.

[Myron Beaumont]

Passing on someone elses tip I've tried success fully ( in fact the only digital device ever-first time)Try sticking a couple of cocktail sticks-tubing-whatever ? on the wings with masking tape -cellotape -glue -whatever .Then using digits-whatever  ! you can feel them spot on for checking .No fancy doodahs costing the earth.  This can be done upright or inverted (the model that is )

[Myron Beaumont]

I use straws that yobbos have thrown into the street  No cocktail sticks here I can assure you

[Mike Rolls]

I find that cocktail sticks can sink into the fingertips (even with fairly bony fingers like mine) so most or even all of the pad of the finger is in contact with the wing. I use 1/8" dowel or bamboo skwer of similar size. Couple of pieces an incch or so long, 4 scrps of Sellotape or similar - job done.

Mind you, I don't have any 1/3 scale Lancasters to balance....

Mike

[001]

When building sports or 'ordinary '  aerobatic i.c. models, if I know the recommended c of g (balance point) I build in a small block of pine on the top of the wing on a high wing model or bottom of a low wing model and screw a small cup hook or curtain hook in at the point of balance, then hang it from the ceiling or a tree with a cord! 

This way you are checking lateral balance at the same time as fore and aft and it is very easy to place a piece of ballast where it is needed before deciding where and how to fix it. 

[rcKeith]

Thanks for all you comments, I think perhap I didn't explain the issues very cleary.

I suspect the CofG is wrong on the model and using the contraption is suposed to locate where the Cof G is.  When I know where the Cof G is then I use my normal balancer.  I 've also in the past purchased models without any plans/intstruction(e.g. Ebay) and its a bit hit and miss getting the Cof G right.  On a straight wing such as trainer its fairly easy to get even calucalating it with some of those freely availble tools on the net.  But for something like a spitfire with and ellepitical wing its not so easy. 

My dilema is that using the contrapion I get a CoG around 55mm from the LE where the plan indicates 30-35mm.  At 35mm I need to add at least 3 ounces of lead, which on a depron model of 24" wingspan is quite a lot of extra weight.  At 55mm CoG I don't need to add any weight.  

Thanks

Keith 

[Grasshopper]

Fingertip error when checking C of G?

Not for me - I always insert a couple of plastic headed pins (6mm diameter head) into the underside of the wing at the recommended C of G position. Then after a trimming flight when I feel the position is correct - I glue them in place.

This way, before each flight you can quickly and accurately check the C of G. 

Also after a prang - and subsequent repairs, correct balance can be quickly checked.

 

Oh - and like the new site - but how do you check spelling before posting?

[Myron Beaumont]

Wots the euse of ay spel cheque inn frans

[Alan Dunstan]

I can see you guys have never had a plank glider. I have a NCFM M60 and I can tell you that moving the C of G by 1mm totally alters the characteristics of that aeroplane. 1mm behind the sweet spot and its darn near unflyable. On the sweet spot it is a magic aeroplane. 1mm in front and it is soggy, horrible and very slow. To get to that level of accuracy I use the Multiplex C of G machine which does give that accuracy. I also record where the sweet spot is so on any subsequent repair, maintenance or refurbishment I can reproduce the exact same sweet spot.

 

I gave up using fingers years ago when I 1st became involved with gliders. They are so much more sensitive than pudgy fingers can ever provide. For their modest price they are they undoubtedly one of the best VFM tools you can have.

 

Alan