🚀 go-pugleaf

In article <1993Jan20.232439.14173@bmerh85.bnr.ca> mkfeil@bcrki9.bnr.ca (Max Feil) writes:

>As I understand it, the aerodynamic center of all airfoils is very
>close to the 25% chord point when operated subsonically. My question
>is: Just how far does it vary for the different kinds of airfoil
>sections, eg. conventional; flat (sometimes used for fins/stabs);
>reflexed; laminar flow (high point at 50%); etc.

I believe that this is a result of lifting lione theory, that the AC
will always be at 25% chord.

>As an example, in data I have the a.c. of NACA 2412 ranges between
>23.9% and 24.7% at reynold's numbers between 3.1E6 and 8.9E6 (this is
>a semi-symmetrical airfoil). All the other airfoil's that I have data
>for are also within a percent or two of 25%, but none of these are
>exotic.

Even the more exotic airfoils (eg. supercriticals, GAWs, Eppler,
Wortman, Liebeck, Drela types, etc) behave in this way.  I think as
long as you maintain reasonably attached flow on the airfoil, it
should hold.  If you do something stupid, like a cusp on the trailing
edge that is so strong that it never attaches the flow, then I'd
expect it to not be well behaved.

>Another question: In supersonic flow the a.c. is at 50% chord, as a
>result of (as I understand it) fairly equivalent shock wave effects at
>the leading and trailing edge. How far does this vary among airfoils?
>What does the transition (trans-sonic) area look like when comparing
>the a.c.'s of different airfoils?

I think this is explained by Busemann's theory.  And in most of the
cases I have seen, it behaves very close to the theory as well.  This
was the idea behind unstable aircraft.

If you make the CG farther back so that the aircraft has a negative
static margin subsonically, then supersonically, the static margin
will be less positive than if you has a subsonically stable aircraft.
This should reduce your transonic trim drag and give better supersonic
performance.  Unfortunately, you also incur problems with high alpha
recovery and trim drag at subsonic conditions.

--
Al Bowers                       bowers@rigel.dfrf.nasa.gov
NASA F-18 High Alpha Research Vehicle, Lead Aero
NASA, Dryden Flight Research Facility, Edwards, California