C of G assumptions?

[fly boy3]

Hi all, On a normal type model, if the Cog is unknown ,a near as possible position prior to maiden flight can be assertained by using the 1/4 to 1/3 chord idea. Can this be used on flying wings. ? Cheers

[Engine Doctor]

No Try this  C of G calculator  or this one 

Edited By Engine Doctor on 05/02/2013 21:07:20

Edited By Engine Doctor on 05/02/2013 21:10:15

[fly boy3]

Thanks ED. I really liked the first one which gives choices on whether you are a beginner or expert. Cheers

[Simon Chaddock]

FB3

The simple 1/4 to 1/3 rule only works on a straight wing with a conventional tail plane.

Any tailess plane has to mimic the effect of the tailplane by special areodynamics (a delta) and/or a reflex trailing edge.

This means the effective wing chord is less than the physical one so the CofG has to be a bit further forward but how much is complex.

The calculators should get it somewhere near but be aware tailless planes are more sensitive to CofG position than conventional.

[Myron Beaumont]

fly boy

My rule of thumb on home designs especially is to go for 25% to start with .This born out by the above links .On a conventional set up (however you interpret that ),you'll probably end up around 30 to 33%.Only on free flight high climbing under timed power fast flying pylon models will you find the C ofG way back nearly to the TE .with a lifting tailplane . I'm now reminiscing again.Sorry

Edited By Myron Beaumont on 05/02/2013 23:24:03

[Greybeard]

I'm not sure where it should go but experience has taught me that it's good to have one.

[fly boy3]

Hi Simon, must say all this information is very interesting. Is the refex trailing edge you speak of, the reason why Zagi type wings have some up on elevons at all times.

For Myron, for reminicing substitute the word experience. Cheers

[Devon Flyer]

I was always taught that what we determine on a model aircraft is the 'balance point' - NOT the 'centre of gravity'.

They are two subtly different things. A true C of G is a three dimensional measurement as opposed to a one dimensional 'balance point'.

THIS makes interesting reading.

[crispin church]

 

use this is good and easy

 

Edited By Biggles' Elder Brother - Moderator on 26/05/2013 16:22:33

[crispin church]

IS IT JUST ME OR IS Support Our Partners ADDS IN THE WAY?

[Myron Beaumont]

Me too !

[Steve Dorling]

BALANCE POINT (C/G...............)

[Biggles' Elder Brother - Moderator]

Posted by crispin church on 06/02/2013 20:45:02:

IS IT JUST ME OR IS Support Our Partners ADDS IN THE WAY?

The reason it was Crispin is becuase you posted a very long URL in the post above this comment - which I have now edited. If you have not already done so (this was some time ago!) please read up in the tutorials area how to post links on here.

BEB

[Martin Harris - Moderator]

Posted by fly boy3 on 06/02/2013 15:14:31:

Hi Simon, must say all this information is very interesting. Is the refex trailing edge you speak of, the reason why Zagi type wings have some up on elevons at all times.

The explanation of tail-less theory that appeals to me is that the "tailplane" in effect overlaps the main flying surface - i.e. the ele part of the elevons coincides with the trailing edge of the wing. As the moment arm is so short, in order to stabilise the wing correctly, the C of G needs to be very much forward of the "normal" 25 - 33 per cent mean chord.

The reflex is actually the equivalent of setting the normal positive angle of attack for the wing provided by the "decalage" angle when using a conventional tailplane.

[Steve Dorling]

Balance Point - Devon Flyer is correct

Just sayin'

[Piers Bowlan]

For an unswept flying wing (planks)15-20% MAC is a good starting point for the C of G. At 15% MAC you will need a lot more reflex but 'how much' is down to trial and error. The C of G calculator posted by Engine Doctor works well for swept/tapered wings.

 

As an aside, I have scratch built a few flying wings over the years and I have found that broader control surfaces with smaller deflections tend to work better than narrow strip elevons. Just my 2p worth!

 

What are you building by the way?

Edited By Piers Bowlan on 01/06/2013 07:46:57