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the rudder combat asymmetrical thrust.
Figure 12-16. Wings level engine-out flight.
Rudder Force
Yaw
String
Fin Effect
Due to Sideslip
Slipstream
Wings level, ball centered, airplane slips toward dead engine.
Results: high drag, large control surface deflections required,
and rudder and fin in opposition due to sideslip.
12-23
Ch 12.qxd 5/7/04 9:55 AM Page 12-23
2. Engine inoperative flight using ailerons alone
requires an 8 - 10° bank angle towards the operative
engine. [Figure 12-17] This assumes no
rudder input. The ball will be displaced well
towards the operative engine. The result is a
large sideslip towards the operative engine.
Climb performance will be greatly reduced by
the large sideslip.
3. Rudder and ailerons used together in the proper
combination will result in a bank of approximately
2° towards the operative engine. The
ball will be displaced approximately one-third
to one-half towards the operative engine. The
result is zero sideslip and maximum climb performance.
[Figure 12-18] Any attitude other
than zero sideslip increases drag, decreasing
performance. VMC under these circumstances
will be higher than published, as less than the
5° bank certification limit is employed.
The precise condition of zero sideslip (bank angle and
ball position) varies slightly from model to model, and
with available power and airspeed. If the airplane is
not equipped with counter-rotating propellers, it will
also vary slightly with the engine failed due to P-factor.
The foregoing zero sideslip recommendations apply to
Yaw
String
Excess bank toward operating engine, no rudder input.
Result: large sideslip toward operating engine and greatly
reduced climb performance.
12-24
Rudder Force
Yaw
String
Bank toward operating engine, no sideslip. Results: much
lower drag and smaller control surface deflections.
Figure 12-17. Excessive bank engine-out flight. Figure 12-18. Zero sideslip engine-out flight.
Ch 12.qxd 5/7/04 9:55 AM Page 12-24
12-25
reciprocating engine multiengine airplanes flown at
VYSE with the inoperative engine feathered. The zero
sideslip ball position for straight flight is also the zero
sideslip position for turning flight.
When bank angle is plotted against climb performance
for a hypothetical twin, zero sideslip results in the best
(however marginal) climb performance or the least rate
of descent. Zero bank (all rudder to counteract yaw)
degrades climb performance as a result of moderate
sideslip. Using bank angle alone (no rudder) severely
degrades climb performance as a result of a large
sideslip.
The actual bank angle for zero sideslip varies among
airplanes from one and one-half to two and one-half
degrees. The position of the ball varies from one-third
to one-half of a ball width from instrument center.
For any multiengine airplane, zero sideslip can be confirmed
through the use of a yaw string. A yaw string is
a piece of string or yarn approximately 18 to 36 inches
in length, taped to the base of the windshield, or to the
nose near the windshield, along the airplane centerline.
In two-engine coordinated flight, the relative wind will
cause the string to align itself with the longitudinal axis
of the airplane, and it will position itself straight up the
center of the windshield. This is zero sideslip.
Experimentation with slips and skids will vividly display
the location of the relative wind. Adequate altitude and
flying speed must be maintained while accomplishing
these maneuvers.
With an engine set to zero thrust (or feathered) and the
airplane slowed to VYSE, a climb with maximum power
on the remaining engine will reveal the precise bank
angle and ball deflection required for zero sideslip and
best climb performance. Zero sideslip will again be
indicated by the yaw string when it aligns itself vertically
on the windshield. There will be very minor
changes from this attitude depending upon the
engine failed (with noncounter-rotating propellers),
power available, airspeed and weight; but without
more sensitive testing equipment, these changes are
difficult to detect. The only significant difference
would be the pitch attitude required to maintain VYSE
under different density altitude, power available, and
weight conditions.
If a yaw string is attached to the airplane at the time
of a VMC demonstration, it will be noted that VMC
occurs under conditions of sideslip. VMC was not
determined under conditions of zero sideslip during
　
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