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Jim Cunningham sent me some comments on my "BCF related" post from
yesterday. He asked me to forward his comments to the list, which I have
gladly provided in full below:
Norton, Patrick wrote:
> Dinogeorge@aol.com wrote:
> >> Airfoils must be asymmetric, because it is the difference between
> airflow over the top and bottom surfaces that provides the lift; a
> symmetric wing would have the same kind of airflow over top and bottom
> and thus generate near-zero lift.<<
> To which James Cunningham replied (in part):
> > This is false.<
> I interpreted George as simply saying that an airfoil, by definition,
> asymmetrical in cross section.
Me too, but of course airfoils are not asymmetrical by definition. As
an example, consider the NACA 0006 which is symmetrical, has a t/c of
6%, and is a fairly good airfoil, though too thin to contain a
structurally efficient spar. Or the NACA 0012, which is used in a
number of aircraft, including one of mine.
> George ?
> > Competition aerobatic airplanes have perfectly symmetrical wings so
> that they can fly equally well rightside up or upside down.<
> Flight control in stunt planes is achieved almost exclusively by using
> flaps to deflect the airstream in various directions, thereby pushing
> aircraft in various directions.
I'd call them ailerons, but yes, this is true. Combined with rudder and
elevator or stabilator forces, it is how flight CONTROL is achieved, but
not how flight itself is achieved.
> Even level flight in such planes is achieved with flaps partially down.
Usually this is not the case. In level flight, they usually fly at a CL
of 0.2-0.3 with an angle of attack of about 1.8-2.7 degrees and the
control surfaces in trail. They do the same when flying level and
inverted. My Cherokee (which is not aerobatic and has a 65(2)-415
section rather than a symmetrical wing) has both flaps and ailerons
deflected slightly upward in level flight. When lightly loaded (CL~0.2),
it flys at a slightly nose down angle of attack. When heavily loaded
(CL~0.3), the angle of attack is slightly nose high, the difference in
angle of attack being approximately 1.1 degree. When rolling into a
turn, the aileron on the down-going wing moves further up than the
aileron on the upward-going wing moves down, in order to minimise
differential drag and adverse yaw. Needless to say, once the turn is
established, the controls are neutralized again and remain so until
opposite deflection is required to stop the turn.
> This is necessary because a plank or
> any type of symmetrical wing generates no lift at a zero angle of
Very true, but symetrical wings usually aren't flown at zero angle of
attack and usually aren't flown with the flaps or ailerons down.
> This type of flight creates a great deal of drag and is extremely
> inefficient--as demonstrated by the substantially larger engine
> requirements in stunt planes.
It is more inefficient, and does produce a bit more profile drag
(induced drag is a function of lift and velocity only, and remains
unchanged by airfoil section), but the engines are larger to enable more
vertical penetration, not because of airfoil drag.
>In fact, an asymmetric wing would be a
> distinct _disadvantage_ in an up-side-down attitude since the pilot
> need to use flaps to compensate for the downward force created by the
> Bernoulli effect.
Very true, and very noticable on a Citabria, except for the bit about
the flaps. Check out the extreme fuselage angle when the Citabria is
flying upside down. But he doesn't do it with flaps. Think about what
would happen to the pitching moment and control authority if he popped
the flaps while was upside down. He level flight by flying at a
different angle of attack when inverted than when upright, compensating
for the fact that in an asymmetric wing, zero lift is accomplished with
a negative angle of attack when right side up and a quite positive angle
of attack when upside down.
> But an asymmetric wing design is the _only_ way to get aerodynamic lift
> at a zero angle of attack.
Also very true (in non-flapping flight). But there are pitch-rotation
mechanisms that can establish circulation and transient unsteady lift at
zero angle of attack. The clap and fling technique can even create
significant positive lift from a symmetric wing while the angle of
attack is negative and going more so.
> The advantage of an asymmetric wing moving
> through an airstream at a zero angle of attack is that it minimizes
> while providing lift. This is energetically the most efficient mode of
> flight and it seems reasonable that such a design would offer some
> adaptive advantages during the evolution of flight.
I agree whole-heartedly with this. And it's my whole point. The
mechanisms don't HAVE to pre-exist (some did). Those that don't can
develop during the evolution of flight. Their lack doesn't prohibit
beginning flight. However, I do think some of them did pre-exist. I
agree with George about the tendency toward asymmetry in the shape of
the theropod forearm. On the other hand, I don't see any particular
need for an asymmetric flight feather until flight, gust, or display
loads produced a need and reward for increased structural efficiency.
All the best,