CoG Centre of Gravity

If an aircraft is going to fly successfully the CoG must be in the right place. Too far back and the plane will never fly again! Too far forward and it may.
On a parallel wing (width the same all the way along) the CoG shouls be 25 to 33% of the width back from the front edge. 30% is good for Leccy's. 33% for Ic's since putting fuel in will always bring it forward.
Do not be tempted to go further back that 33%, it's unlikely you'll ever fly that model again.

With ARtFs' and Tupperware you don't have much leaway with placing equipment, so it's easy to see that you might have to add weight in order to correct it, however err on the side of forward, then you won't have to add so much weight to the tail, (them levers again)

With a Radio Queen I made up two supports, then marked the CoG on the fuselage, from there I could play nicely with the equipment to get the CoG right, no extra weight for me!

Note the Tank position, I'm hoping this model will have a duration of about 5 hours (2.5pint tank + petrol conversion) It's right on the CoG so there'll be no CoG change during the flight.

NOTE This method is not accurate! It gives you a good safe point to start from, ie slightly nose heavy MOST OF THE TIME!!!




The first one is easy, a parallel chord wing, merely take one third of the chord, and that is your safe start point.
Another way is to assume the spars are at one third chord, and take that as the c of g.

The second one a swept leading edge/straight trailing edge.
Take one third of the root chord, and mark it. Then find one third of BOTH tip chords, and put a straight-edge across. Mark on the ROOT chord where the lines cross.
Then take 50% of the difference between the two marks, and go forward a bit, and that is your start point.

The third one is a straight leading edge, forward swept trailing edge.
Do just the same as before, but note that the difference range is smaller, therefore more critical.

The fourth one, swept leading AND trailing edges.
Exactly like before, the one thirds rule.

For a biplane, do exactly the same for the appropriate planform - FOR EACH WING, then split the difference between top and bottom wing start points.

This SHOULD work for a Delta planform as well, but there are too many variables, ie wing sweep, tip chord, span, etc, to accommodate all options.

As I put at the beginning - this usually gives a safe START POINT for finding the c of g by flying the model.

Hi Dave, mine hasn't flown yet (pressure) but the plan shows 120mm back from the leading edge for the CoG. There is provision for about 18^o of travel for the Rudder and elevator which will be more than enough for a model of this type.

The CoG on an aircraft is so important, the CoG MUST be forward of the centre of pressure (the lift centre created by the main wing). Depending of the foil section this CoP can actually move backwards and forwards with speed. Using you fingers at the wing tips to see where it balances is in my opinion, not good enough! The are a number of balance checkers on the market but it's easy to make one for a few pence that's just as good. You just need two short pieces of plank say, 1' x 3" (300 x 75mm) and two old style pencils with the eraser (can't call them rubbers any more) in the end. Drill a hole in the centre of each plank that's just big enough (interference fit would be nice) to push the point of the pencil in. Cut the eraser to a chisel point, 90^o will do it, and not too sharp. Push and glue each pencil into each plank with the length of the point across the planks. When you want to check a CoG, place the planks a suitable distance apart, place your planes recommended CoG point on the erasers, if it's nose down move weight rearwards, if it's tail down move weight forwards until she's straight and level. Treat the manuals recommendations with a pinch of salt, I've seen too many errors in print. Take the wing chord measurement, multiply by 0.28 and aim for the CoG to be that far back from the leading edge. If you're close to that figure, your plane will always be flyable, you can fiddle with the final point later.

That works with a 'plank' wing, but not for a tapered or swept one.
I've got a drawing I did ages ago, showing and describing how to find the 'best start' (or ball-park) balance point.
When I find it again, I'll put it on, or I'll re-draw it.

Ha! After hunting through all my files and hard-drives, I found it - on here, about eight posts and four years back!

Anyway - as I have re-drawn it, here it is again!


NOTE - this gives you a good START-POINT, not the best or final C of G position!
Remember the golden rule - an AFT c of g flies once - - it's a crash waiting to happen
A forward c of g flies safely - the elevators may be soggy, but it won't flick on you.