C of G Which way up and why?

[Chris Walby]

If its high wing mark C of G on upper wing surface and turn upside down then measure

Mid wing mark C of G on upper wing surface and turn upside down then measure

Low wing mark C of G on underside and measure right way up

Why and what's the benefit doing it this way as opposed to marking and measuring just the underside?

Answers please

[Chris Walby]

PS How come "Vanessa" is used with the model the right way up

PPS won't two tanks full of fuel move the C of G further forwards (as they as well forward of the C of G point) and then gear down moves it forwards. Too many permutations.....off to the shed to wire the wing up!

[Peter Miller]

I mark the top surface on a low wing model and the bottom on a high wing. I use about three layers of Solartrim. THis way you can feel where the marks are with your finger tips supporting the model.

One dodge is to get two pencils with those chisel shaped erasers. Drill two holes in a plank of wood,insert the pencils and support the model on them with the erasers on the CG Mark.

Yes, the fuel will move the CG forward making the model safer. If you balance with the fuel as it burns off the CG will move back behind the designed location. A forward CG is safe. a rearward one could make the model uncontrollable.

Just for your amusement.

One club member with years of experience was given a Piper Cub. He installed all the grear and engine. In flight the model was completely unstable and eventually crashed.

I took a photo of the model in flight which showed the underside clearly including the CG marks.

Our hero had put the mark at 47% of the chord. It was a miracle that it flew as long as it did.

Edited By Peter Miller on 19/08/2018 08:12:26

[Frank Skilbeck]

I've always checked the c of g the opposite way round to Chris, a low wing is balanced upside down and high wing the right way up, this way the models c of g is below the balance point and it will either sit nose down (model nose heavy) or tail down (tail heavy).

[sharpy1071]

I use the little bump stops that you use on kitchen doors. Half spherical, self adhesive, about 20 to a sheet in B&Q.

[Paul C.]

My preference is to use the Vanessa rig on all of my models, large and small. Always have the model the right way up (how it will be flying) I have never been able to get an accurate c of g measurement with my fingers could be half inch either way. You can get away with a model being slightly nose heavy but tail heavy is always going to end badly, once you are happy with the initial flight it's easy to start with fine adjustment to set the c of g to your individual liking.

Paul.

[Ron Gray]

If the main mass of the airframe is above the balancing point (eg high wing held upside down) then it will always try to rotate to below the balance point. This means that if you are slightly off with the balance point then the airframe won’t just point nose or tail down, it will try and rotate. With a Vanessa balance rig the airframe is supported at 2 points either side of the balance point so it will be prevented from rotating, in fact it is the rig that allows you to rotate the airframe to get it level and then check where the resultant balance point is.

So after all that waffle, if using ‘fingers’ to balance, high wing airframes balance right way up, low wing balance upside down, mid wing whichever works for you!

[Former Member]

[This posting has been removed]

[Ronos]

What about a model with the wings halfway up the fuselage, like the Seagull Mosquito, normal or inverted?

[Biggles' Elder Brother - Moderator]

David is right of course, the CoG is the CoG - either way up!

But having said that some things are just easier than others - so given you get the same result why make things hard for yourself! So I go for:

High Wing - right way up

Low Wing - inverted

Shoulder Wing - please yourself!

Always balance, initially at least, in the worst posible configuration - so that's usually fuel tank empty as it usually in front of th CoG so when full can only help matters and empty is its worse case. Same with retractable U/C, put it in the worst case position where it will push the CoG back.

If you do that, and its level, or very slightly nose down, then you'll be fine for a maiden. As others have said you can then tweak it a little as you get to know the aeroplane's characteristics and to suit your flying style better. With "tweaking" though remember that some models are very sensitive to CoG position and 5mm can make massive difference. With others, 20mm can hardly seem to make a change! Until you know which you've got be cautious when moving the CoG

BEB

Edited By Biggles' Elder Brother - Moderator on 19/08/2018 10:07:47

[Keith Evans 3]

Nose down ! ! ! I've never really questioned this in the past , but how can a model "balance " nose down ? I've seen this and do it myself . If I were to try and balance a long piece of wood on a sharp fulcrum it would either balance or tip one way or the other .It would not "balance" nose down. Is it something to do with friction between the balance fulcrum and the wing surface ?

[andyh]

Posted by Keith Evans 3 on 19/08/2018 10:33:08:

Nose down ! ! ! I've never really questioned this in the past , but how can a model "balance " nose down ? I've seen this and do it myself . If I were to try and balance a long piece of wood on a sharp fulcrum it would either balance or tip one way or the other .It would not "balance" nose down. Is it something to do with friction between the balance fulcrum and the wing surface ?

your balance fulcrum is typically above the CofG (which has a vertical position, as well as the longitudinal one which we're usually concerned with), so the model will balance with the CofG directly below the fulcrum, which may be slightly nose- or tail- down, so long as it's not too far out. when you're balancing a piece of wood, the fulcrum will be below the CofG, so it will tip unless it's perfectly balanced.

[Former Member]

[This posting has been removed]

[Keith Sharples]

I use CofG stickers a small circle quartered 2 white quarters and 2 black quarters (ebay and cheaper from Hobby King). I put them where the plan/instructions require them to be and fly the model, moving them forward or backward it till I'm happy with the performance and leave them there. It works for me!

[Dave Hess]

IMHO, on most conventional models (models with a fuselage, wings and tail), C of G is not that critical. I never even look at the plan to see where it should be. For the first flight, I check that it's somewhere around 1/4 to 1/3 of the way back from the front of the average wing chord, then adjust it a bit afterwards, depending on how the plane flies and what type of flying characteristics I want. I don’t use any measuring device. I just use my eyes and a rough judgement. Many of my planes have ended up with a C of G miles from the recommended position.

[Ron Gray]

Personally I’ve only checked c of g with U/C up on the basis that most of the flying will be carried out in this configuration. I do make a mental note of any effect lowering the u/c may have and in the case of say a P40 or Corsair this can be fair amount, especially when coupled to the additional drag and resultant nose down turning force. This is where TX mixing comes into play.

[Martin Harris - Moderator]

We have a club member who in all seriousness, walked up to a tail-wheeled model in the clubhouse, picked up the tail a couple of inches, weighed it in his hand, dropped it and pronounced that the C of G felt just right!

It might explain the odd flying characteristics of some of his models!

For ease of balancing a shoulder wing model, one way up will normally work better - often the undercarriage will locate the C of G slightly below the "thumb" location. While the explanations above are quite correct, it might be worth emphasising that the C of G is a single point in the model with a 3 dimensional location rather than simply anywhere on a vertical line drawn "n" inches from the leading edge.

Some people get fixated by measuring the C of G within a few thousanths of an inch. The location given is a point somewhere (hopefully!!!) within an acceptable range and who knows whether the designer has opted for an ultra safe position or one which he feels will give lively but safe performance?

Do you balance wheels up or down in the case of an aircraft with retractable wheels? Only the designer can tell you which way he was intending them to be when he drew the balance point on the plan although the full size world is easier in that aircraft are always weighed with their wheels down - giant thumbs are not an option! I always opt for the worst case scenario unless it has been specified.

There is an old truism that an aircraft with a forward C of G may fly badly but one with a rearward one may only fly once! Small adjustments rearwards after initial test flights with a safe starting position are the way to go.

[Bob Cotsford]

I can see some logic in the tail-wheel lifters idea. If you lift the tail then at some point the CofG will be vertically above the main wheels axles and the tail will feel weightless as the model reaches equilibrium. On some airframes you probably could judge whether it should be safe or not. I don't think I'd use that method myself!

These days I use an Mpx balancer, and as others have said the model goes on the opposite way to how Chris' first post suggested. I always thought the slightly nose down bit was to get the wing at roughly zero incidence, this being a throwback to the days of freeflight cabin jobs.

Edited By Bob Cotsford on 19/08/2018 12:12:06

[Martin Harris - Moderator]

Posted by Keith Evans 3 on 19/08/2018 10:33:08:

Nose down ! ! ! I've never really questioned this in the past , but how can a model "balance " nose down ? I've seen this and do it myself . If I were to try and balance a long piece of wood on a sharp fulcrum it would either balance or tip one way or the other .It would not "balance" nose down. Is it something to do with friction between the balance fulcrum and the wing surface ?

It's called equilibrium - the forces are in balance. If the C of G (3 dimensional point - see my previous post) is below the pivot point but not exactly in line with it, the model will rotate until the points line up. If there is only a small misalignment then the model will hang nose (or tail) down.

[The Wright Stuff]

...and the nice thing is, this neatly answers the original question. This 'nose down' or 'nose up' balance only works IF the CoG is below the chosen balance test point vertically.

Technically, it would be perfectly possible to balance either way up, but to do it with the CoG above the balance point would require infinitesimal precision in where it was balanced, assuming a frictionless pivot. It's the difference between a stable equilibrium, and an unstable equilibrium. The forces all balance in both cases, but only in the first one does it come back to equilibrium if it is displaced by a small amount.

[The Wright Stuff]

Posted by Martin Harris on 19/08/2018 11:56:58:

There is an old truism that an aircraft with a forward C of G may fly badly but one with a rearward one may only fly once!

This is certainly based in truth, but personally I feel this can be a little overstated on occasion. Part of the issue is that many ATRFs and plans have the CoG marked in a position that they already feel is a bit forward (i.e. the safety margin is already built-in). If you flog the truism too hard, people end up adding their own margin on top of the designer's one, resulting in a VERY nose heavy model. Believe me, many of those only fly once, as well, particularly if hand launched!!!

[Chris Walby]

Thanks chaps, I think that answer's Ronos's question.

• As per drawing or manual
• With tanks empty and gear up unless they have adverse affects
• Check C of G anyway around you want, but best not to put your fingers through the wing skin
• Better to start forward and work back rather than rearward C of G and need a bin liner
• If it lasts beyond the maiden then adjust to suit your own preferences

IMHO you only get a very short period of time to find out how the rates, C of G and flying characteristics all work together or against you.

[Cuban8]

Just a couple of things..........one of my old club mates (now sadly departed) was a superb builder and could construct to an amazing level of accuracy. Trouble was, he'd always take the designers stated CG on the plan as gospel, cast in stone, never to be experimented with etc etc. He'd complain about a model's performance and would fiddle with just about everything else to get it flying as he wanted but could never be convinced that a simple tweak in CG might well be a fix!

In all the years I've been flying models I really can't think of any that have suffered from having too little elevator movement on a maiden flight - certainly to the extent that the model was in danger. Either just right or a bit too much usually - the number of times I've heard people worrying about not having adequate movement on control surfaces and will wind in a bit more just in case it's needed in an emergency. Couple that with a rearward CG and you're asking for trouble.

Edited By Cuban8 on 20/08/2018 09:44:12

[The Wright Stuff]

Posted by Chris Walby on 20/08/2018 08:32:21:

• Check C of G anyway around you want, but best not to put your fingers through the wing skin

Ha ha, that is indeed raising an interesting point: how to check the CoG if there's nowt there but covering. I wonder whether model designers take this into account and make sure the ideal CoG falls on a sheeted part of the wing...

Peter?

[Bruce Collinson]

All the more reason for Veronica then.

Further refinement to mine (think it was in the Seagull thread); bought new 11 mm dowel, couldn't find a tight fitting bit, bored it 12 mm using bench drill, put a 3mm hole at 90 deg and introduced an M4 machine screw which happened to be lying nearby. Now, when the c of g is close I can nip up the dowel whilst fine tuning.

BTC