AOPA Pilot Magazine - May 2021 - 92

P&E

PROFICIENCY

DRAG POLAR FOR LANCAIR 360 MKII

for that emergency landing in a confined
area, it's probably best to leave them
retracted until a touchdown in the desired
spot can be assured.
AOPA

1.2

FLAPS 00

FLAPS 40

LIFT COEFFICIENT (CL)

1.0

CATHERINE CAVAGNARO is an aerobatics
0.8

instructor (aceaerobaticschool.com) and

FLAPS -70

professor of mathematics at Sewanee: The
University of the South.

0.6

FLAPS AND DRAG POLARS
Visualizing the effects on
glide ratio

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 CLEAN BEST GLIDE RATIO for the Lancair 360 is 16.4 and with flaps extended 4 degrees

downward, that value is reduced slightly to 16.3. But this aircraft has a reflex feature and
Figure 2. DRAG POLAR FOR LANCAIR 360 MKII The clean best glide ratio for the Lancair
with
degrees
upward,4the
glidedownward,
ratio becomes
18. In
this configuration,
the
360 isflaps
16.4 set
and 7with
flaps extended
degrees
that value
is reduced
slightly to 16.3.
Lancair
will glide
percent
farther
thanflaps
it would
with noupward,
flap extension.
But this aircraft
has 10
a reflex
feature
and with
set 7 degrees
the glide ratio
becomes 18. In this configuration, the Lancair will glide 10 percent farther than it would with no
flap extension.

The ability to use a low approach airspeed is especially important for those
who fly aircraft that are primarily designed
with cross-country cruise speed in mind
as the stall speed tends to be high as well
(see " The Perfect Aircraft for the Mission, "
September 2019 AOPA Pilot). Converting
from an aerodynamically clean wing to
one with higher camber allows those efficient aircraft to alight with the grace of a
bird. Because stopping distance is a function of the square of ground speed, even
a modest reduction in approach speed
can steepen the approach angle and profoundly shorten the landing roll.
Depending on aircraft configuration,
extending flaps can change the pitch angle
of the aircraft in a favorable way. For example, lowering the flaps in my Beechcraft
Bonanza while maintaining the same
approach speed provides for a lower pitch
attitude and an increased likelihood of seeing the runway complex upon breaking out
of the clouds on an instrument approach.
Nothing in life is free, and extending flaps exacts the toll of greater drag.
92 | AOPA PILOT May 2021

Correctly weighing the benefits and disadvantages of this simultaneous increase in
lift and drag is important for each particular phase of flight. Upon engine failure (in
a no-wind condition), using the best glide
airspeed will ensure that the aircraft glides
the greatest distance per unit altitude loss,
or (L/D)MAX (or (CL/CD)MAX). Generally
speaking, lowering the flaps hurts cruise
performance and glide distance. Some aircraft such as Lancairs and Maules feature a
position called reflex that deflects the flaps
in a negative direction that allows a higher
cruise speed with a decrease in drag. This
reflex position can also be used to advantage in an engine-out, gliding situation. Lift
and drag simultaneously decrease but the
increased lift-to-drag ratio allows better
glide performance (see " Flaps and Drag
Polars, " right).
While extending flaps can reap benefits in takeoffs, maneuvering, and landings
in tight areas, a better knowledge of their
aerodynamic effects can help us use them
wisely. If you're coming in low for that
power-off one-eighty or, more important,

A drag polar for a gliding
aircraft is a way to compare
the efficiency of the aircraft
with changes in lift and drag.
In particular, to find the most
efficient configuration, connect a line from the origin to a
point on the curve so that the
slope of the line is the same as
the slope of the curve. (Mathematicians call this a tangent
line and its slope is (CL/CD)MAX.)
That combination of lift and
drag will result in the greatest
distance forward per unit of
altitude loss. The drag polars
shown at left for the Lancair
360 MKII performed by C.
Zavatson (n91cz.com/
Performance/Cruise_Flap_
Report.pdf) as well as that for
the Cessna Cardinal by NASA
(NASA-CR-2337, " Flight Test
Data for a Cessna Cardinal " )
on p. 91 were redrawn from
the respective references.
The reduction in glide ratio
with positive (downward) flap
deflection means you are not
likely to find flap extension as
part of an engine-out procedure to extend the glide. In
fact, reflexed flaps (deflected
upward) can significantly
boost glide distance.
-CC
For more details on the
effects of flaps, see the video:
Catherine Cavagnaro

http://www.aceaerobaticschool.com http://www.n91cz.com/performance/cruise_flap_report.pdf http://www.n91cz.com/performance/cruise_flap_report.pdf http://www.n91cz.com/performance/cruise_flap_report.pdf

AOPA Pilot Magazine - May 2021

Table of Contents for the Digital Edition of AOPA Pilot Magazine - May 2021