More C of G questions

[Steve T]

I am sure that I read somewhere, high wing models, Cub etc. are balanced from under the wing whilst low wing, Spitfire, is balanced upside down, on top of the wing in effect? I made my own balancer, easy, big square of chipboard and two 1/2" dowels. It does the job but is this really vital. It also stated mid wing made no difference which way up. Clue me in please as I place considerable importance on C of G !

[Biggles' Elder Brother - Moderator]

You need to get the vertical location of the CoG beneath the plane of suspension that's all. Its not an aerodynamic thing - its just so that it doesn't fall off the balancing rig that's all!

So for a high wing model the vertical location of the CoG will obviously be somewhere below the wing - so you balance it right way up. For a low wing model the vertical CoG will clearly be somewhere above the wing - so you invert the model so that its now below the wing. For a mid-wing, just experiment, if it seems unstable on the balance rig right way up its probably because the CoG is above the point of support - so invert it and see if that any more stable.

BEB

PS I assume you have sanded the ends of the dowels to reasonably fine point?

[Erfolg]

The balance point or CG in the horizontal plane does not change which ever way up the model is balanced.

What does change is the stability of the system, in the vertical plane.

What am I trying to convey? It is the idea that there is a balance point in the x, y, and z planes. Normally we have no interest at all in the the "z" axis. our primary interest is along the "x" plane. However, the "y" plane balance point, sometimes makes balancing in the "x" plane awkward.

For the system to be stable the "y" CG benefits from being lower than points of contact in the "x" plane. That is why some are balanced inverted to achieve this system stability.

[Martin Harris - Moderator]

Translation of the above from Engineer to English:

 

The C of G position is a 3 dimensional point. It can be above or below where you're holding the model to balance it. It can also be offset from the centreline towards a wingtip but you can safely ignore this in almost every case for C of G balancing.

 

If it is above the point you're holding it at (low wing upright) and the model swings a little, the balance point moves away from where you're holding it and due to gravity and the whole thing topples rapidly as each bit of movement makes the situation worse.

It's unstable.

 

If the point is below where you're holding it (high wing) and the same swing occurs, the balance point moves away but gravity makes it swing back towards where it came from.

It's stable.

 

...and of course, if it happens to be exactly at the place you're holding the model (e.g. mid-wing) then it should be neutrally stable so will topple controllably.

 

Either of the last two situations will allow you to find the balance point easily.

Edited By Martin Harris on 18/03/2014 15:28:17

[Biggles' Elder Brother - Moderator]

Which is what I said!

BEB

[Martin Harris - Moderator]

...which means that he can look at it from the viewpoint of an academic, engineer or oily rag!

[Levanter]

..................... or we could keep going to see how many different ways we can come to the same conclusion. BEB's point about it not being an aerodynamic thing is spot on. It just saves you mending you model after you pick it up from the floor

[Martin Harris - Moderator]

Just trying to add some balance!

[Steve T]

Er! good job I never mentioned that my degree was in engineering...but in the early 70's ! I must be getting a bit of 'oldtimers'.

Thanks anyway, I have got the gist of it, it is as I had read, just couldn't remember which technical masterpiece of boredom it was in.

By the way, for planes with retracts....up or down?

[Martin Harris - Moderator]

It depends how they retract - going forward goes safer, back may put a marginal model into an unsafe condition. Assuming the C of G has been calculated from the planform then balance where the undercarriage is in the worst case.

Full size always do the measurements undercarriage down for practical reasons - they measure the weight on each wheel in the flying attitude (remember the original point?) and then calculate the position from weight and moment information.

Having said that, if a model designer specifies his position wheels up, then go with that.