AOPA Pilot Magazine - September 2019 - 93

P&E TECHNIQUE

Don't worry,
be happy...feet
Active use of the rudder pedals is crucial
for avoiding ground loops

SEE FOR YOURSELF
View these YouTube videos to
see how crosswinds and swerves
can cause a ground loop in an
instant.
It even happens Down Under. A
Boeing Stearman PT-17 ground
loops because of crosswinds in
Australia.
youtu.be/i8oqVNgA6fI

BY DENNIS K. JOHNSON

A Cessna 170 ground loops at
California's Santa Monica Airport
in crosswinds.
youtu.be/mukmCGDvFlQ

A Red Bull T-6 Texan, flown by
an experienced pilot, ground
loops after losing the tailwheel
steering because of a broken
spring.
youtu.be/3xx0pV_AUQE

I WAS TAUGHT, "Never stop flying the airplane until it has stopped, completely." My
dad would add you're not finished until you
write the check.
If you've ever seen an airplane off a runway, resting on a bent wing and flat tire, you
may have seen the results of a ground loop.
"Swapping ends," as the taildragger pilots
call it, is when the tail of an aircraft swings
far enough to one side to spin the airplane
around. The momentum, if great enough,
will often tilt the airplane and cause a wing
to strike the ground. The airplane can be
seriously damaged, including bent wings,
blown tires, and collapsed landing gear.
With less speed, this uncontrolled turn
may cause no aircraft damage, only giving
a good jolt of adrenaline to the pilot.

GOING LOOPY

An airplane's motion about its vertical axis
is called yaw, and it's controlled by the
rudder, which is controlled by the rudder
pedals, which are controlled by your feet.
That's why happy landings and the prevention of ground loops start with happy feet
actively working the rudder pedals.
Ground loops are caused by uncontrolled yaw, and although most occur on
landing, they can happen anytime an airplane is moving on the ground, including
during takeoffs and taxiing. During a perfect landing the airplane's heading (the
direction it's pointed) corresponds precisely with its track (the direction it's
moving over the ground), and this track
is aligned straight down the runway.

Unfortunately, landings aren't always perfect. Crosswinds can cause an airplane to
land with a slight yaw, inept piloting may
start a swerve, or a hole in a grassy field
can send the airplane askew-and then
the pilot better be quick to get it straight
again. Mechanical problems, such as a broken tailwheel spring, a tire failure, or stuck
brake, also can cause ground loops.
Ground loops occur more often on
tailwheel aircraft because their center of
gravity is behind the main wheels. When
this rearward CG is yawed to one side,
momentum wants that CG to continue
in its original direction of travel. That's
part of Newton's first law of motion: An
object in motion stays in motion with
the same speed and in the same direction unless acted upon.
A physicist I know speculated, "I'd say,
when the plane touches down, the front
wheels define the center of drag. The further out of alignment the center of mass
[not precisely the same as CG, but close
enough in this instance] is from the center
of drag, the greater the tendency will be to
spin the aircraft in a ground loop. I realize I'm mixing force and momentum, but
think of the drag from the wheels as force
aopa.org/pilot AOPA PILOT | 93


https://www.youtube.com/watch?v=i8oqVNgA6fI&feature=youtu.be https://www.youtube.com/watch?v=mukmCGDvFlQ&feature=youtu.be https://www.youtube.com/watch?v=3xx0pV_AUQE&feature=youtu.be http://www.aopa.org/pilot

AOPA Pilot Magazine - September 2019

Table of Contents for the Digital Edition of AOPA Pilot Magazine - September 2019

Contents
AOPA Pilot Magazine - September 2019 - Intro
AOPA Pilot Magazine - September 2019 - Cover1
AOPA Pilot Magazine - September 2019 - Cover2
AOPA Pilot Magazine - September 2019 - 1
AOPA Pilot Magazine - September 2019 - 2
AOPA Pilot Magazine - September 2019 - Contents
AOPA Pilot Magazine - September 2019 - 4
AOPA Pilot Magazine - September 2019 - 5
AOPA Pilot Magazine - September 2019 - 6
AOPA Pilot Magazine - September 2019 - 7
AOPA Pilot Magazine - September 2019 - 8
AOPA Pilot Magazine - September 2019 - 9
AOPA Pilot Magazine - September 2019 - 10
AOPA Pilot Magazine - September 2019 - 11
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AOPA Pilot Magazine - September 2019 - 14
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AOPA Pilot Magazine - September 2019 - 16
AOPA Pilot Magazine - September 2019 - 17
AOPA Pilot Magazine - September 2019 - 18
AOPA Pilot Magazine - September 2019 - 19
AOPA Pilot Magazine - September 2019 - 20
AOPA Pilot Magazine - September 2019 - 21
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AOPA Pilot Magazine - September 2019 - 30
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AOPA Pilot Magazine - September 2019 - 111
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AOPA Pilot Magazine - September 2019 - Cover3
AOPA Pilot Magazine - September 2019 - Cover4
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