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C of G Question

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David Pearce 402/07/2019 19:30:30
313 forum posts
37 photos

I have a Seagull Challenger ARTF which I have had 4 trimming flights with. When the build was complete I balanced to the recommended C of G of 60mm from the leading edge. Two things struck me, one was that it required about 100g of nose weight to achieve this and the other was that, with a chord of 270mm, 60mm was only 22% of chord. It looked too far forward and was certainly outside the normal position.

Other users on another forum said 70mm worked and when I did a calculation on a C of G app it came out as 72mm. So my test flights have been removing most of the weights to move it back bit by bit. I now have it at 72mm but find I have two conflicting attributes:

  1. It still flies as if nose heavy, diving and needing up trim.
  2. When I do the usual test of diving at 45 degrees and closing the throttle it tucks its nose under which signifies that it’s tail heavy.

Otherwise it flies well with none of the twitchiness that being tail heavy usually causes. I have no down thrust or side thrust as none is recommended in the instructions.

I’d be grateful for any advice. I can fly it with permanent up trim but I am wondering if there’s another answer?


Jon - Laser Engines02/07/2019 19:43:50
5413 forum posts
263 photos

I have never put any faith in the dive test, especially with ic.

c/g is as much down to preference as anything else so just play with it until you feel it is how you want it. Most manufacturers play safe so you can normally move it back by quite a bit. If the model starts to get twitchy take 10% off of your elevator rate and try again.

Most of my warbirds are 'tail heavy' and fly on very low rates.

David Pearce 402/07/2019 19:54:08
313 forum posts
37 photos

Thanks for that John. I should have mentioned, it's 4S electric, although that may not effect your advice. So you recommend I remove more nose weight, a little at a time, until it flies without up trim, or gets twitchy?


Denis Watkins02/07/2019 19:54:49
4321 forum posts
104 photos

My ballpark start on these types is to measure from leading edge to the centre of the wing tube David

On the side of the model with no wing fitted

The balance is usually not very far from there

What mm is that distance?

Edited By Denis Watkins on 02/07/2019 19:55:32

David Pearce 402/07/2019 20:06:24
313 forum posts
37 photos

Hi Dennis

That's 98mm which would be about a third of the chord. Perhaps too far but it does show that there's some way to go.


Robert Welford02/07/2019 20:14:05
183 forum posts
4 photos

Assuming normal tailplane area and tail moment 30% of mean wing chord is a good starting point for CofG.

See recent "CofG" thread for in-flight testing for appropriate CofG position.

David Pearce 402/07/2019 20:25:24
313 forum posts
37 photos

Thanks Robert.

I'll read that with interest.

Jon - Laser Engines03/07/2019 11:35:39
5413 forum posts
263 photos

I dont think i have really measured a c/g position in years. I dont quite understand why some guys get so hysterical about 5mm movement of the c/g position or trying to get it accurate to the last 10g of lead. Being an i/c flyer, i loose over 1lb of weight from the front of some models during a flight as the fuel us used. Given the apparent lack of trim change due to this fuel burn i think its safe to say that models are not as sensitive to c/g changes as many would think.

I will qualify that a bit in that i fly scale and sport aerobatic, and that my 23lb sea fury uses about 1.25lbs of fuel during its flight so its a relatively small change vs its overall weight. Things like gliders and f3a are more sensitive so it depends on your mount.

In any case, as robert suggests 30% is a good starter for 10 and after that its just a case of playing with it until you get it the way you like it. Be aware that it might be impossible to get the model to behave perfectly. My acro wot XL is an example of this as its very nose heavy due to my installation of a large and heavy engine. I added some weight to the tail but i think its still nose heavy according to the plans. I would re-balance it, but in normal flight its perfect so dont want to touch it. For landing though its nose heaviness shows and i run out of elevator authority. My solution was to enable an elevator-flap mix (even though i have no flaps) which effectively trims the model for landing speed and means my normal elevator deflection is plenty for a nice landing.

I could have re-balnced it to land nicely, but as it flew perfectly as it was i would expect a balance change to ruin its normal flight performance and that is why i left it alone.

David Pearce 405/07/2019 17:23:40
313 forum posts
37 photos

As I asked the question I thought I'd give an update. I've followed the advice and have been moving the C of G slowly back.

Now all the nose weight is gone and I moved the battery back and the C of G is 82mm instead of the recommended 60mm. She now takes off easier and flies well with no vices.

I've learnt something from this, thanks for all your help.

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