AOPA Pilot Magazine - May 2021 - 91

DRAG POLAR FOR CESSNA CARDINAL

FLAPS 00

1.2

LIFT COEFFICIENT (CL)

1.0

FLAPS 300

0.8

0.6

0.4

0.2

0.0
0.0

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

DRAG COEFFICIENT (CD)
THE BEST GLIDE RATIO (CL/CD)MAX is the slope of a line that goes through the origin and is

tangent to the drag polar curve. For the Cessna Cardinal with flaps set at zero, the factor
Figure
1. DRAG POLAR
FORflaps
CESSNA
CARDINAL
The best
glide ratio (CL/CD)MAX
the slope
is
0.59/0.053=11.1
and with
set at
30, the factor
is 0.75/0.089=8.43.
In is
other
words,
of a line that goes through the origin and is tangent to the drag polar curve. For the Cessna
gliding
in still air at the respective best glide airspeeds, for every 1,000 feet, the clean airCardinal with flaps set at zero, the factor is 0.59/0.053=11.1 and with flaps set at 30, the factor
plane
will travel 11,100
feetwords,
horizontally
flapped
airplane
will
travel
only 8,430
is 0.75/0.089=8.43.
In other
gliding inand
still the
air at
the respective
best
glide
airspeeds,
for feet
horizontally.
Thus
clean
airplane
will 11100
travelfeet
32 percent
farther.
every 1000 feet,
the the
clean
airplane
will travel
horizontally
and the flapped airplane
will travel only 8430 feet horizontally. Thus the clean airplane will travel 32% farther.

surfaces on the outboard trailing edges of the
wings that could be used to turn the aircraft.
The downward deflected aileron increased
the camber and lift of its wing while the
upward deflected aileron decreased the lift
on the other wing. As we know from our
first flights as student pilots, the wing with
the greater lift also incurs a greater drag force
and, for the attendant adverse yaw, we compensate with rudder in the direction of the
turn to maintain coordinated flight.
Anderson explains that flaps extended
downward increase the coefficient of lift
and, all other factors such as airspeed and
wing area remaining the same, means that
the lift increases. But they also cause an
increase in drag and with both quantities
raised, it begs asking about their net effect.
For example, if both my salary and living
expenses increase, it's not clear if I'm doing
any better; the difference (increased salaryincreased expenses) might be a helpful
metric. For lift and drag, a traditional metric is L/D or lift-to-drag ratio since that's
also our glide ratio when the engine quits.

Let's review some of the effects of
extending the flaps during flight.
Adding flaps raises the coefficient of lift
as well as the wing area itself for certain
flap designs. Assuming that other factors
such as airspeed stay the same, that means
a greater lift force. Extending them also
reduces stall speed which means that we
can rotate earlier in the takeoff roll and
get off the ground sooner, which is helpful
especially on unimproved strips. A lower
climb speed can provide a better climb
gradient to clear an obstacle although we
convert to a lower pitch angle as soon as
possible for better engine cooling and an
improved climb rate.
A lower stall speed can be an important
ingredient in the famous canyon turn when
a tight turn is necessary to reverse course
in areas of rising terrain (see " Take it to
the Bank, " May 2020 AOPA Pilot). In a nowind situation, reducing the airspeed by 15
percent cuts the turn radius by almost 28
percent, so lowered flaps are typically part
of the recipe for an optimal canyon turn.
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AOPA Pilot Magazine - May 2021

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