Calculating c/g

[Jeffrey Cottrell 2]

Hi Guys

Do you have a favourite way to calculate c/g of a model?

Reason I ask is I have bought a second hand model with sharply tapered wings on both LE and TE. I do have a program which will calculate c/g, but it seems to take little account of the TE taper when filing in the dimensions.

I did manage to find part of the manual for the model, which shows a c/g position of 105mm from root LE. The model does have some marks on it which look like c/g position, but they are some 135mm back, quite a difference.

So, I need to calculate a c/g position for myself.

How would you do it?

Thanks

Jeff

 

[Paul De Tourtoulon]

50 ways of doing it, I work out the surface and then 25% from the leading edge for a first flight,  I don't worry about the tail surface however Canards are very different.

[MattyB]

2 hours ago, Jeffrey Cottrell 2 said:

Hi Guys

Do you have a favourite way to calculate c/g of a model?

Reason I ask is I have bought a second hand model with sharply tapered wings on both LE and TE. I do have a program which will calculate c/g, but it seems to take little account of the TE taper when filing in the dimensions.

I did manage to find part of the manual for the model, which shows a c/g position of 105mm from root LE. The model does have some marks on it which look like c/g position, but they are some 135mm back, quite a difference.

So, I need to calculate a c/g position for myself.

How would you do it?

Thanks

Jeff

 

 

Simple answer - use a better calculator that does take into account TE taper!

 

https://www.ecalc.ch/cgcalc.php

[Mike Mc]

I like RC Planes calculator. They also have a Canard and Flying Wing calculator.

https://rcplanes.online/cg_calc.htm

https://rcplanes.online/cg_canard.htm

https://rcplanes.online/cg_wing.htm

 

[Jeffrey Cottrell 2]

Hi Guys, thanks

 

Paul

Not quite clear what you are suggesting here. What does 'work out the surface' mean? If you're suggesting 25% of the root chord, this comes out to 85mm. Maybe ok for straight wings but these are very sharply tapered, see photo.

Even the manual suggests 105mm and this is probably conservative.

 

Mike

This is the one I use. Found it very handy over the years, but concerned that is has no provision for wings that have a sharp taper on the trailing edge. Can't be sure whether this is significant or not, but looking at the wing planform, that's a recipe for tip stall if I get it wrong.

 

Matty

That's the same as the one Mike suggested, albeit with a more complex interface. Can you point out where the provision is for TE taper?

 

Thanks anyway for your thoughts, most welcome

 

Jeff

[MattyB]

29 minutes ago, Jeffrey Cottrell 2 said:

Matty

That's the same as the one Mike suggested, albeit with a more complex interface. Can you point out where the provision is for TE taper?

if you know the LE sweep plus the chord and span between each point that the wing shape changes, the TE taper is automagically calculated.

 

Example - Imagine you have a modern aerobatic model, something like an Edge 540 with a die straight LE. All that is needed is to set sweep at zero and input the span, plus the root and tip chords - the TE taper is effectively defined by the other measurements.

Edited September 29, 2022 by MattyB

[MattyB]

PS - Your wings are really that not sharply tapered, and they still have very friendly root and tip chords. It should be pretty tolerant on CG position provided the tail surfaces are decently sized. It would need to have a much higher a/r before tip stalling became something to worry about. 

Edited September 29, 2022 by MattyB

[Jeffrey Cottrell 2]

Hi Matty

Good point, well taken.

Thanks for explaining. Obvious really, when someone else points it out.

Jeff

[MattyB]

Np. Re: the question of taper, it’s far less of an issue than you might think - a/r is way more influential over stall behaviour (for example, look at the extreme example of a delta, they normally have a very benign stall).
 

I have a 2.5m Standard Libelle glider with a much lower taper ratio, but a high a/r and a tip chord of about 4cm. Now that does have some exciting low speed characteristics!

 

[Paul De Tourtoulon]

Hi Jeffery, I just measure the whole surface and divide by 25% ( a quarter ) and work out from the front of the wing where it is, a bit rough but a couple of rubbish photos of my Mirage 2000 one of the photos of it last week at the model field, a 'Like' will be posted to who recognises it.🥳

[Nigel Dell]

Is that a LMC kit Paul?, IIRC it was also produced by Peterborough model centre, can’t remember who produced it first, but one bought the range from the other.

[leccyflyer]

On 29/09/2022 at 10:31, Jeffrey Cottrell 2 said:

Hi Guys

Do you have a favourite way to calculate c/g of a model?

Reason I ask is I have bought a second hand model with sharply tapered wings on both LE and TE. I do have a program which will calculate c/g, but it seems to take little account of the TE taper when filing in the dimensions.

I did manage to find part of the manual for the model, which shows a c/g position of 105mm from root LE. The model does have some marks on it which look like c/g position, but they are some 135mm back, quite a difference.

So, I need to calculate a c/g position for myself.

How would you do it?

Thanks

Jeff

 

First off for a conventional model I'd initially opt for 28% of the wing chord. A more accurate measure would be 28% of the Mean Aerodynamic Chord. For less conventional models, such as a twin model, with a tapered wing and a T-Tail, I'd weigh it all up and then opt for 28% of the MAC. Or ultimately, for that first flight, go for 28% of the wing chord. For deltas and such like - I'd just ask a grown up. 🙂

 

Did I mention that 28% ;)?

[Martin Harris - Moderator]

These rules of thumb are fine for conventional planforms but use them at your peril on anything other than plank wings and reasonable sized tailplanes. 
 

I’ve know models to fly well at 10% or less (tailless) and over 50% (vintage lifting tail section).  I’ve also test flown models for clubmates with C of Gs at positions that seemed reasonable and have been almost unflyable. 

[Jeffrey Cottrell 2]

Hi Guys

leccyflyer

Thanks for that, good advice, but I would suggest probably only for parallel chord or slight taper. Think the steep taper on mine has more effect than you might think.

My root chord is 340mm, so 28% would be 98mm. Even the manual, which generally errs on the safe side, suggests 105mm as start point.

Going even further forward seems like a step back.

Anyway, starting to get a clear idea of what I have to do.

Thanks to everyone who contributed

Jeff