AOPA Pilot Magazine - May 2019 - 88

P&E

PROFICIENCY
V-n Diagram
(Maximum
Gross
andand
Lighter
Weights)
V-n diagram
(maximum
gross
lighter
weights)

LoadFactor
factor
Load
4
3
2
1

100

150

200

KCAS

-1
THE CURVES from the graph on p. 87 form the upper left part of the maneuvering (or V-n)
diagram. Note that the lighter aircraft has a larger envelope and is shown using the lighter
green shading.

that your shoulder harness should have been
fastened more tightly.
Exactly how strong are the airplanes
we fly? The FAA requires that aircraft be
designed to withstand a range of load factors with no damage or deformation. For the
three most common aircraft categories, these
ranges are minus 1.52 to plus 3.8 (Normal),
minus 1.76 to plus 4.4 (Utility) and minus
3 to plus 6 (Acrobatic). Because Acrobatic
category aircraft routinely incur high load
factors, they are equipped with accelerometers to ensure that G limits are not exceeded.
An aircraft wing stalls when it reaches
its critical angle of attack and the airflow
across the top of the wing separates. Beyond
the critical angle of attack, the lift created
by the wing decreases drastically. A stall
can occur at an airspeed that is slower or
faster than the published stall speed, indicated by the bottom end of the green arc
on the airspeed indicator. This published
(1 G) stall speed is determined by slowly
decelerating the aircraft, typically at maximum gross weight and full forward center
of gravity, and noting the speed at which the
stall occurs. An abrupt pull on the yoke will
induce a stall at a higher airspeed and result
in a load factor greater than 1 G. Loading
an aircraft below maximum gross weight or
with a center of gravity aft of the forward
limit will result in a lower stall speed. You
may notice with your own airplane that the
88 | AOPA PILOT May 2019

stall rarely occurs exactly at the published
value. We'll refer to VS as the speed at which
a stall would be achieved after a slow deceleration in its given configuration.
Interestingly, the load factor incurred by
abruptly pulling back on the yoke at a particular airspeed turns out to depend only
on VS. It's n=V2/ VS2, where V is the calibrated airspeed at the start of the maneuver.
The graph on p. 87 shows this relationship
and you might recognize it as forming the
upper left part of the maneuvering diagram
(above). Note that as airspeed increases, the
load factor rises sharply. For example, an
aircraft with a published stall speed of 46
knots that abruptly stalls at a speed of 90
knots will register a load factor of 3.8 Gs. So
90 knots will be the maneuvering speed, the
maximum speed at which the load factor at
the stall will not exceed the (Normal category) limit load factor of 3.8 Gs.
Note that if your pilot's operating handbook includes one maneuvering speed, it
corresponds to the published stall speed.
Since we are almost never flying at maximum
gross weight or at the forward CG limit, VS
is less than the published value-so maneuvering speed is as well. Here is a simple rule
of thumb for calculating maneuvering speed
based on weight changes: For every 10-percent weight decrease, the maneuvering speed
decreases by 5 percent. If a 3,300-poundmaximum-gross-weight aircraft with a

102-knot published maneuvering speed is
flown at 2,500 pounds, a 24-percent weight
decrease, then the new maneuvering speed
would be the published value reduced by 12
percent, or 89.8 knots. (The actual value using
the formula above yields 88.8 knots.) This
same rule applies to stall speeds, too.
The maneuvering envelope provides a
critical tool in planning for the safe execution of snap rolls. Abruptly applied loads
are harder on aircraft structures than those
gradually applied. For this reason, I plan a 3-G
snap roll even though the acrobatic aircraft
is certified to 6 Gs. Knowing that using the
wrong entry airspeed could result in an excessive load factor, Louise and I used our weight
and CG position to find that a snap roll with
entry speed of 80 knots would incur a load
factor of 3 Gs. During the flight, we climbed
to altitude, cleared the area, and pitched the
nose of the aircraft slightly upward. When
the airspeed reached 80 knots, we abruptly
pulled back on the yoke to stall the wings,
applied full left rudder and watched the
world go around. After 270 degrees of rotation, we recovered from the spin with forward
pressure and opposite rudder. Louise smiled
when she saw that the accelerometer read 3.0
Gs, exactly what our calculation predicted.
Although your next flight will almost
certainly not involve snap rolls, the topic
offers important information for proper
aircraft care. Any maneuver that requires
authoritative control movements should
be performed at or below maneuvering
speed-which almost always is less than the
published value in the operating handbook.
The Airplane Flying Handbook warns that
aircraft strength requirements do not cover
multiple control movements, in the same
or different axes of rotation. To be kind to
your aircraft, know its actual maneuvering
speed and choose control inputs with care.
List the maneuvering speed for three different weights on your checklist so that you can
quickly and easily slow to an appropriate airspeed before any abrupt control movements.
Doing so will help to ensure many happy
years for both you and your airplane. AOPA
CATHERINE CAVAGNARO owns Ace Aerobatic

School in Tennessee (www.aceaerobaticschool.com), and is professor of mathematics at Sewanee: The University of the South.

http://www.aceaerobaticschool.com/ http://www.aceaerobaticschool.com/

AOPA Pilot Magazine - May 2019

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