曝光台 注意防骗
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the ascending blade when the airplane is operated
under power and at positive angles of attack. The
descending propeller blade of the right engine is also
a greater distance from the center of gravity, and
therefore has a longer moment arm than the descending
propeller blade of the left engine. As a result,
failure of the left engine will result in the most
asymmetrical thrust (adverse yaw) as the right
engine will be providing the remaining thrust.
[Figure 12-19]
Many twins are designed with a counter-rotating right
engine. With this design, the degree of asymmetrical
thrust is the same with either engine inoperative. No
engine is more critical than the other, and a VMC
demonstration may be performed with either engine
windmilling.
In aircraft certification, dynamic VMC is determined
under the following conditions.
• Maximum available takeoff power. VMC
increases as power is increased on the operating
engine. With normally aspirated engines, VMC is
highest at takeoff power and sea level, and
decreases with altitude. With turbocharged
engines, takeoff power, and therefore VMC,
remains constant with increases in altitude up to
the engine’s critical altitude (the altitude where
the engine can no longer maintain 100 percent
power). Above the critical altitude, VMC
decreases just as it would with a normally aspirated
engine, whose critical altitude is sea level.
VMC tests are conducted at a variety of altitudes.
The results of those tests are then extrapolated to
a single, sea level value.
• Windmilling propeller. VMC increases with
increased drag on the inoperative engine. VMC is
highest, therefore, when the critical engine propeller
is windmilling at the low pitch, high
r.p.m. blade angle. VMC is determined with the
critical engine propeller windmilling in the
takeoff position, unless the engine is equipped
with an autofeather system.
• Most unfavorable weight and center-of-gravity
position. VMC increases as the center of gravity
is moved aft. The moment arm of the rudder is
reduced, and therefore its effectivity is reduced,
as the center of gravity is moved aft. At the same
time, the moment arm of the propeller blade is
increased, aggravating asymmetrical thrust.
Invariably, the aft-most CG limit is the most
unfavorable CG position. Currently, 14 CFR
part 23 calls for VMC to be determined at the
most unfavorable weight. For twins certificated
under CAR 3 or early 14 CFR part 23,
the weight at which VMC was determined was
not specified. VMC increases as weight is
reduced. [Figure 12-20]
• Landing gear retracted. VMC increases when
the landing gear is retracted. Extended landing
gear aids directional stability, which tends to
decrease VMC.
Figure 12-19. Forces created during single-engine operation.
C L C L
D1 D2
Arm Arm
Inoperative
Engine
Inoperative
Engine
Operative
Engine
Operative
Engine
(Critical Engine)
Ch 12.qxd 5/7/04 9:55 AM Page 12-28
12-29
• Wing flaps in the takeoff position. For most
twins, this will be 0° of flaps.
• Cowl flaps in the takeoff position.
• Airplane trimmed for takeoff.
• Airplane airborne and the ground effect negligible.
• Maximum of 5° angle of bank. VMC is highly
sensitive to bank angle. To prevent claims of
an unrealistically low VMC speed in aircraft
certification, the manufacturer is permitted to
use a maximum of a 5° bank angle toward the
operative engine. The horizontal component of
lift generated by the bank assists the rudder in
counteracting the asymmetrical thrust of the
operative engine. The bank angle works in the
manufacturer’s favor in lowering VMC.
VMC is reduced significantly with increases in bank
angle. Conversely, VMC increases significantly with
decreases in bank angle. Tests have shown that VMC
may increase more than 3 knots for each degree of
bank angle less than 5°. Loss of directional control
may be experienced at speeds almost 20 knots above
published VMC when the wings are held level.
The 5° bank angle maximum is a regulatory limit
imposed upon manufacturers in aircraft certification.
The 5° bank does not inherently establish zero sideslip
or best single-engine climb performance. Zero sideslip,
and therefore best single-engine climb performance,
occurs at bank angles significantly less than 5°. The
determination of VMC in certification is solely concerned
with the minimum speed for directional control
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