AOPA Pilot Magazine - June 2024 - 90

RUDDER & WRENCH
FLY ING SMART /
It's complicated
The wing's on-again, off-again relationship with lift
BY CATHERINE CAVAGNARO
5
o
Attached flow over upper surface of the wing produces lift.
As angle of attack increases, so does lift.
attack increases. When the separation
bubble envelops most of the airfoil, the
angle of attack becomes critical, and lift
drops precipitously. I showed Jack flow
visualizations by Alexander Lippisch,
similar to those in the illustration here,
that confirmed the information in my
drawings ( " No Equations Required, "
November 2019 AOPA Pilot.)
Now, what makes a wing stall is far
more difficult to explain than the same
for an airfoil.
A wing is a bunch (OK, infinitely
20
o
When the separation bubble envelops most of the airfoil
and the angle of attack becomes critical, a further
increase in angle of attack results in a decrease in lift.
A B L A R I NG S TA L L horn, except right
before touchdown on landing, is a sign
that everything isn't right with the world,
or at least in the relationship between the
wing and the lift it generates.
Despite the high-pitched wailing,
my student-and son-Jack kept pulling
back on the Cessna's yoke in the hopes
that the nose would drop. Once it did, he
would then complete the stall recovery
by releasing back-pressure, adding power,
and commencing an efficient climb away
from the terrain. Sometimes, that's exactly
how it went. Other times, despite quick and
appropriate rudder movements, one wing
dropped, and he risked deviating from ACS
standards by allowing the nose to wander
by more than 10 degrees laterally.
90
AOPA PILOT / June 2024
In our post-flight chat, Jack asked
why it seemed tough to keep the wings
level during stall practice. I understood
his frustration and explained, " Well, it's
complicated. " I suggested we grab a bite
to eat and enjoy a leisurely discussion on
exactly how a wing produces lift.
I drew diagrams of an airfoil, a
2-dimensional slice of the wing, and
helped him visualize the flow around it as
the engine propels the airplane through
the air. At low angles of attack, the flow
is attached to the airfoil. As the angle of
attack increases, so does the lift, but the
flow starts to separate at a point before
the trailing edge. This separation bubble
features backward flow along the top
of the airfoil and grows as the angle of
many) of these airfoils glued together,
starting at the fuselage, and ending at the
wing tip. For our purposes, thinking of
each wing section as an inch wide with
about the same airfoil-shaped cross section
and dimension works just fine. The
total lift produced by the wing is the sum
of that produced by each of these sections
and should be at least the weight of the
aircraft to sustain flight. But lift generated
by the wings has other effects, too.
If airflow across the wing were perfectly
parallel to the longitudinal axis,
analyzing a stall would be much easier.
The vortex action around the wing tip
means that flow below the wing moves
outward toward the wingtip and that
above the wing travels inward toward
the fuselage. For the simplest rectangular
wing, that means the sections near
the wing root achieve their critical angle
of attack before the outboard sections.
And that's a good thing since the pilot
feels a buffet that warns of a stall while
the ailerons, located near the tips, are still
effective. The downside of a rectangular
wing is the inefficiency of that relatively
large vortex action.
The most efficient planform is
elliptical, but it comes with a high manufacturing
price. By using straight
CHARLES FLOYD
AOPA Pilot Magazine - June 2024

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