The CG is relatded to the Mean Aerodynamic Chord of the wing, and would
vary with airfoil section, the planform is used to find the MAC.
........bc ([EMAIL PROTECTED]
http://www.widomaker.com/~conk
Williamsburg, VA 23185
On Sun, 16 Nov 2003, Harley Michaelis wrote:
> Isn't it an oversimplification to specify a certain CG location for a
> particular airfoil. Surely that location is planform related.
> ----- Original Message -----
> From: <[EMAIL PROTECTED]>
> To: <[EMAIL PROTECTED]>
> Cc: <[EMAIL PROTECTED]>
> Sent: Sunday, November 16, 2003 7:01 AM
> Subject: Re: [RCSE] on setting a CG and upsetting a Gordy
>
>
> > In a message dated 11/16/2003 7:41:18 AM Central Standard Time,
> > [EMAIL PROTECTED] writes:
> > OK, I bite. What do you put on the paper?
> > Cal Posthuma
> >
> > CAL -
> >
> > I figger out where 37% of the MAC is at the wing root. That's where the CG
> > should be on all the gliders I ever flew. That's a touch forward of
> neutral,
> > which works for me.
> >
> > Your favorite CG may be forward or aft of this, but the important thing is
> > that once you find the sweet spot for one plane it will be at the same
> spot on
> > the next plane, as long as you don't radically depart from the typical TD
> > layout and try a canard or flying wing.
> >
> > It's not hard for the typical straight trailing edge tapered wing. I am
> not
> > qualified to teach math or geometry, but here's roughly what I do.
> >
> > What I put on the paper is a rough sketch of the wing planform. Then I do
> a
> > bit of rudimentary geometry and change the shape to an equivalent size
> > rectangle. Since the TE is straight the TE of the rectangular wing is the
> same. The
> > areas of the real wing and the rectangle need to be the same. The span
> needs to
> > be the same. The root chord of the rectangle will be less, and the tip
> chord
> > will be greater. The chord of this rectangle is the MAC.
> >
> > 37% aft of the leading edge of this rectangle is the spot. 63% forward of
> the
> > trailing edge is the same spot. The leading edge of the real wing is
> forward
> > of the LE of the equivalent rectangle, but the trailing edges are the
> same. I
> > can measure from the TE of the real wing and have the right place.
> >
> > If the trailing edge is swept forward or aft it complicates the process
> > slightly, but you do basically the same thing. For elliptical shapes I
> need to drag
> > out the Machinery's Handbook and a calculator, or just pretend it's a quad
> > taper wing and call it good enough -- it's still pretty close. Close
> enough for
> > this government worker anyway.
> >
> > happy trails - Rob
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