Re: [Phys-l] unusual areodynamics

[John Denker <jsd@av8n.com>]

On 12/13/2011 01:00 PM, Josh Gates wrote:
> The weight is being supported mainly by the lift of the 
> *fuselage*.

Yes.

> Very neat - so the lift generated by the fuselage is
> substantially greater than the lift generated by the tail?  

Yes, very "substantially" so ... typically more than
infinitely so, since the lift from the tail is likely
to be negative.

  This means the fuselage lift is *greater* than 100%
  of the weight.

There is a lot of physics going on in this system.  We need
equilibrium of the forces *and* equilibrium of the torques
*and* various kinds of stability ... et cetera.

> It seems
> like the torque generated by the tail's lift about the CM could be
> significant, even if the lift itself is small.

Yes, more-or-less by definition, a "tail" is way, way 
back there where it has a lot of leverage.

>  The fuselage's lift
> would seem to have a pretty small or possibly zero moment arm about
> the CM, though, right?

Actually it is a reasonable bet that the center of lift
is *aft* of the center of mass.  That means the aircraft
is in some sense "nose heavy".  This is almost (but not
quite strictly) necessary, in order to provide stability.

The strict rule is that the thing in back (usually the tail)
flies at a *lesser* angle of attack than the thing in front
(usually the main wing).  This rule applies even to canard
designs, where the thing in back (the main wing) flies at
a lesser angle of attack than the thing in front (the 
canard).

The rule is explained in detail at
  http://www.av8n.com/how/htm/aoastab.html

One way to guarantee lots of stability is to move the 
CM sufficiently far forward that the tail is producing 
negative lift.  The wing has a positive AoA, so if the
tail has a negative AoA that certainly counts as "lesser"
in accordance with the aforementioned rule.  Since an 
RC plane doesn't have a pilot on board, it needs even 
more stability than a piloted aircraft would ... so
it needs either fancy electronic/gyroscopic stability 
augmentation, or lots of built-in aerodynamic stability, 
i.e. a forward CM location.