Elliptical wing C of G

[Tony B-622]

Hi all could some explain to me in lamens terms how I find the C of G in an elliptical wing as I would like to double check the destructions suggestion I've scoured the net but can't come up with any thing fool proof, if it helps it's for the Escale Seafire and at the current setup which is.......

Manual has cog at 90mm from leading edge I have an ASP 1.20 with a 6v nimh at the front and not that it weighs much an on board glow, now if this is correct I would need approx 11 ounces in the spinner.

Any help from you lovely people would be greatly appreciated as I'm now desperate to give this bird it's maiden, thanks

Tony

p.s I was fairly tight with the glue at tail end

[Martin Harris - Moderator]

I would use a straight edge on a plan of the wing to estimate a mean straight line for the leading and trailing edges, do the same at the tip and than apply the resulting trapezoid shape to something like this calculator,

[Tony B-622]

Thanks for the reply Martin only problem is I don't have a plan so I was hoping some one knew of way to work out it by wing area, span and width of the wing.

[Mark Stevens 1]

Might be worth having a read of this review featured in RCM&E - I presume this is the one you have.

http://www.jperkinsdistribution.co.uk/downloads/4495120a.pdf

 

Mark

Edited By Mark Stevens 1 on 14/09/2013 03:21:06

[Mark Stevens 1]

Ps - the plans I have for one at the same wingspan as above has the CG marked at about 122 mm back from the leading edge of the wing.

Hope this helps

Mark

[Martin Harris - Moderator]

With a bit of old wall lining paper and a pencil you soon could have a good enough plan for the purpose. You only need a half span plan so it may even fit on a single piece with the model assembled so that you can capture the tail area and moment on a single drawing.

However, it doesn't look as though the reviewer above found any real problem with the maker's recommended position - unless the control sensitivity, which he resolved by reducing the movements, was due to a rearward position? I'd use his experience as a starting guide...

 

Edited By Martin Harris on 14/09/2013 10:29:09

[PatMc]

The chordwise lift distrubution on a Spitfire/Seafire wing is the same as a parallel chord wing. This means that cg can be calculated/estimated by simply using a % of the root chord.

[crispin church]

**LINK**

try here

[Lindsay Todd]

You could take a very simplistic view on this and simply find the mean (average) wing chord across the span and set the cg between 25 and 30% this will give you a pretty safe starting point and can be fined tuned later to personal preference. This assumes a Spitfire types plan view and no sweep. For the tip I would suggest measure the chord length a few inches inboard from the tip say 3 or 4 inches, measure the centre chord, add the two togther and divide by two, then fined a chord that length on the wing that corresponds to that answer and mark the 25 to 30% position. Linds

Edited By Lindsay Todd on 15/09/2013 09:12:34

[PatMc]

Lindsay, there's no need to get that complicated. The elipse of a Spitfire wing is such that simply measuring at the root is all that's necessary to determine the aprox cg position for the initial flight.

[Lindsay Todd]

True enough PatMc but nobody ever believes that lol. Linds

[Martin Harris - Moderator]

I can't quite see the logic here - if the Spitfire had a true eliptical wing that would be the case but as the (theoretically non-existant) tip is at about 1/3 chord, doesn't that mean that the mean chord isn't valid at the root?

[Lindsay Todd]

Thats why I suggest measure root chord and tip chord to establish mean chord position, Patmc's comment assumes true eliptical plan layout which would be correct. Linds

[PatMc]

No Lindsay, I didn't.
I assumed that the asymmetry of the elipse is designed to distribute the chordwise centre of lift equally from tip to root to tip - i.e. in a straight line from tip to tip.

Martin is correct if it's just considered from the geometric POV. However taking the cg simply as a % of root chord will bring it forward a fraction compared with calculating the true mean chord. But the difference is so insignificant (and in the "safe" direction) that it's not worthwhile calculating what is an aproximation anyway & then using that as the basis for an estimated initial cg.