What is it?
The C of G or Centre of Gravity is the balance point of a body (aircraft) at which the total mass can be assumed to be acted on by gravity.
The C of G is correctly located in 3-axes but with our models we are usually concerned with the 2 dimensional effects in a plane as depicted by the side-view on our plans. The position of this is a major factor in whether a plane will fly well or not. We’ve all heard about approximate measurements. A monoplane trainer needs a C of G position at about 25 to 30% of the mean chord of the wing measured back from the leading edge. This can vary quite a lot depending on other model geometry and should be measured a little more precisely than with a couple of podgy fingers on a windy day at the field!
A useful guide
Moving C of G forward (nose heavy) will leave the model very flyable and generally controllable by most people although it will run out of elevator authority at lower speed.
A rearward C of G will result in disaster for most of us and even the experienced club flyer who flies the model off for you without knowing if the C of G is correct will be observed to have ceased his relaxing banter and will have his teeth clenched as he tries to prevent the demise of your unstable model.
Check the quoted position in the plans/instructions
A good thing to start with is to set up the C of G as recommended in the plans, but these are not always correct! Good kits and plans of well designed models will indicate a correct C of G position erring on the side of added stability for initial flights and quoting a more rearward position as proficiency increases. Not so good kits give little information and some positions are so wrong they can only have been included in the instructions so that you wreck the model first time out and have to buy another……
My short-lived Piper Cub fell into the latter category. One of the Far-Eastern ARTFs, the C of G was stated to be 100mm back from the leading edge. With the mean chord being 250mm would you be able to spot the error? The position stated puts the C of G at 40% mean chord – impossibly unstable for this type of model which requires a starting position of about 50mm (25%) or less.
On the other hand my Spitfire, which is posing in the photograph, quotes a C of G which is correct and leans toward stability for first flights. The kit is by RBC in the Netherlands and comes from a designer who knows what he is doing – you pay your money and you take your chances on your BMFA insurance!
And then, just so you know it can lean the other way, I had a problem with an Easy Pigeon I bought. Its C of G was in such a safe position (even for a glider) that it was proving very difficult to climb and maintain height long enough for sustained flight before it was heading for the ground again. It can be difficult to move enough equipment in a built model to adjust the C of G and I had to rearrange the flight pack and mount the servos directly to the tail surfaces to get it into a good position. It turns out that 45% mean chord works well and even with very modest power the climb is now reasonable and the glide is now fairly flat and extended.
Some quite excellent and simple to use calculators are available from the internet which will help confirm the validity of a manufacturer’s C of G position. I will try to explain how to use one in a future article, but if you haven’t got one then the old methods still work.
Measurement
I have 2 rigs which I use for measurement and photographs of these are below.
The first of these is nice and easy to use because it all hangs from the garage roof. This method allows weight to be shifted about while the model is still hanging in the rig. Two loops of cord are required which must be the same size. Sit the wings in these loops as near to the root chord as possible. The upper part of each loop is wound round a short length of dowel (say 3/16” or ¼” diameter) in the same direction. The dowel is then lightly clamped in a rectangular section of wood which has a sawn slot at one end. Clamping is achieved by pinching the slot with a small bolt. By rotating the dowel the attitude of the model can be altered. A plumb bob (cotton thread tied to a nail) hangs from the dowel. The whole apparatus hangs from the ceiling.
To find your model’s C of G, hang it in the wing loops and attach these to the dowel keeping the dowel parallel with the leading edge of the wing. By turning the dowel adjust the attitude of the model so that it is sitting in its level flight position. Adjust the length of the plum bob so it is almost touching the wing. When it all stops swinging about it will be pointing at the model’s C of G position.
There are difficult models where the hanging balance won’t work, or where it is inconvenient in use. In particular, delta wings cause the loops to slip off and small models are not practical. Such models are more easily dealt with using a table-top device like the one in the photograph below. This consists of two short dowel fitted into a 4”x2” block, long enough to keep the bottom of the fuselage clear of the block. The dowels are fitted with round-headed pins to increase the sensitivity.
article02.htm
In-flight checking
The C of G can also be assessed during flight (assuming the model is flyable in the first place), by means of a procedure sometimes referred to as a “dive test”.
The theory goes that if your model is flying around and trimmed at your cruise speed and the C of G is not exactly in a neutral position (and many won’t be for good reasons) then a change in speed will either put the model nose-up or nose-down. This can be readily assessed from a dive test, which dives the model to produce an increase in speed independent of any motor thrust angles which may also be present.
The dive test
Take your model up to a good height, until it’s a speck in the sky because you will need to observe what it does next. Settle down to cruise speed, correctly trimmed in level flight. On an upwind pass, push the nose down until the dive is about 45 degrees (don’t touch the throttle) and then centralise the elevator.
This set-up phase may take a few attempts before you are comfortable with it!
If the C of G is spot on the model will continue to dive at 45 degrees.
If the C of G is slightly forwards (which I prefer because of the extra forgiveness of the added stability) then the dive will slowly flatten out of its own accord.
If the C of G is way too far forward the nose will pull up sharply to the point where the model starts climbing and may stall near the top!
If the C of G is too far back then the dive will steepen as the nose tries to “tuck under”.
Whatever the outcome of the test make sure you leave enough room to pull-up or otherwise return the model to level flight!