Flight Training - December 2019 - 50

I feel
the need...
the need
for speed

JARGON
Flutter

We're taught early in flight training that every airplane has a speed
limit based on its structure, and that bad things happen if we exceed it.
This is the never exceed speed, VNE.
But how is this speed determined? Is the prototype flown until
its wings pop off, or nearly so, and that speed marked on the airspeed
indicator?
Actually, no. According to Al Lawless, the technical council chair for
the Society of Flight Test Engineers, a flight test engineer, and an FAA
designated engineering representative, "The never-exceed speed is
baked into the very structural design up front." Lawless explains it this
way: "Based on design goals set by the marketing department, CEO, or
whoever, out pops a maximum cruising speed for normal operations.
To certify the airplane, the manufacturer must show that if a pilot flies
along at this speed and mishandles the flight controls or hits turbulence
that pushes the plane into a dive, the maximum airspeed reached won't
be catastrophic. With this high airspeed in mind, engineers then design
the vehicle to be just strong enough to recover without damage."
FAA regulations require the structure to survive a certain percentage beyond the redline, as a safety margin. Does that mean there's
some wiggle room? Can you push the envelope on VNE? Lawless says,
"VNE isn't something to mess around with," and even professional test
pilots creep up on it. "Such testing is highly disciplined and includes
careful buildups with engineering oversight, data review, parachutes-and, usually, a new airframe."
Got it. But just out of feline curiosity, what happens
north of the redline?
The answer, it turns out, is it depends. Flutter can
set in and tear an airplane to pieces in minutes, or even
flight in the future by a different
seconds. Or, according to William F. O'Hara, an assispilot will be the straw that breaks
tant professor of aeronautical science at Embry-Riddle
the airplane's back. "There's nothAeronautical University's Prescott, Arizona, campus,
ing wrong with going right to the
dynamic pressure can turn an airframe "into confetti in
limitation," says O'Hara, "but don't
a heartbeat."
be a jerk and exceed those limitaBut other times the damage is subtler. An airplane
tions, because some other poor
can be bent, wrinkled, or weakened by exceeding the
shmuck will pay the price."
redline; and that's actually the type of damage that
worries O'Hara the most: a flight surpassing VNE by one
FAST MOVING PARTS
pilot, damaging the aircraft to the point that a normal
What about that redline on the
tach? How is the maximum speed
of an engine determined? According to A&P/IA Mike Busch, founder
and CEO of Savvy Aviation, it's not
Small vibrations can grow into violent, unstable
the speed of the engine, but the
oscillations in a phenomenon known as aeroelastic
speed of key moving parts inside of
flutter. Flutter results from a combination of aerodyit-especially the pistons and their
namic, elastic, and inertial forces and often results in
connecting components-that both
structural failure. It usually is seen at high speeds but
has occurred at speeds as low as 55 mph on homeaccelerate and decelerate as the
built aircraft-and it was responsible for the collapse
engine runs.
of the Tacoma Narrows Bridge in 1940.
As with most redlines, the actual
numbers are worked out "through a

50 FLIGHT TRAINING DECEMBER 2019

Flight Training - December 2019

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