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 > RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off. > RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off.
