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should be conducted with the aircraft in the forward
of center CG. Climb to at least 6,000 feet AGL and
trim the aircraft for zero stick force in straight and
level flight at low cruising speed. (Note: Do not
retrim the aircraft once the test has begun.) Apply
a light ‘‘pull’’ force and stabilize at an airspeed about
10 percent less than the trimmed cruise speed. At
this reduced airspeed it should require a ‘‘pull’’ force
to maintain the slower speed.
(i) If it requires a ‘‘pull’’ force, pull
a little further back on the stick and stabilize the
airspeed at approximately 20 percent below the initial
cruise trim speed.
(ii) If it requires a still greater ‘‘pull’’
force to maintain this lower airspeed, the aircraft has
POSITIVE STATIC LONGITUDINAL STABILITY.
(iii) If at either test points, no
‘‘pull’’ force is required to maintain the reduced airspeeds,
the aircraft has NEUTRAL STATIC
LONGITUDINAL STABILITY.
(iv) If either of these test points
require a ‘‘push’’ force to maintain the reduced airspeed
then the aircraft has NEGATIVE STATIC
LONGITUDINAL STABILITY.
(2) Repeat another series of static longitudinal
stability tests using a ‘‘push’’ force on the control
stick. At an airspeed 10 percent above the trim
cruise speed the control stick should require a
‘‘push’’ force to maintain the airspeed. If a ‘‘pull’’
force is required, the aircraft has NEGATIVE
STATIC LONGITUDINAL STABILITY.
WARNING: If the aircraft exhibits
negative static longitudinal stability,
seek professional advice on correcting
the problem before further flight.
(3) After confirming the aircraft has positive
STATIC longitudinal stability, the pilot can
check for positive DYNAMIC longitudinal stability
(short period). First, trim the aircraft to fly straight
and level at normal trim cruise speed. With a smooth,
but fairly rapid motion, push the nose down a few
degrees.
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5/24/95 AC 90-89A
(4) Quickly reverse the input to nose up
to bring the pitch attitude back to trim attitude. As
the pitch attitude reaches trim attitude, release the
stick (but guard it). The aircraft with positive
dynamic longitudinal stability will oscillate briefly
about the trim attitude before stopping at the trim
attitude position.
(5) To test the aircraft for positive
DYNAMIC longitudinal stability (long period),
begin from trimmed, straight and level flight. Without
re-trimming, pull (or push) the stick to a speed
about 5 mph/knots off trim and release the stick.
There is no need to stabilize at the new speed. Expect
the aircraft to oscillate slowly about the trim airspeed
a number of times before the motion dampens out.
If there is significant friction in the control system,
the aircraft may settle at a speed somewhat different
from the original trim speed.
(6) If the amplitude increases with time,
the dynamic longitudinal stability is negative or
divergent. This is not necessarily dangerous as long
as the rate of divergence is not too great. It does
mean, however, the aircraft will be difficult to trim
and will require frequent pilot attention.
(7) An aircraft with ‘‘NEUTRAL’’
dynamic longitudinal stability (long period) will continue
to oscillate through a series of increasing/
decreasing airspeeds and never return to the original
trim airspeed.
f. Lateral-directional Stability Control Tests.
Lateral (Dihedral Effect) and directional stability
tests are to determine if the aircraft can demonstrate
a tendency to raise the low wing in a sideslip once
the ailerons are freed. They also determine if the
rudder is effective in maintaining directional control.
CAUTION: This test may impose high
flight loads on the aircraft. Do not
exceed the design maneuvering speed
or any other airspeed limitation.
(1) To check lateral and directional stability,
the aircraft should be trimmed for level flight
at a low cruise setting and an altitude above 5,000
feet AGL. Slowly enter a sideslip by maintaining
the aircraft’s heading with rudder and ailerons. The
aircraft should be able to hold a heading with rudder
at a bank angle of 10 degrees or the bank angle
appropriate for full rudder deflection. The control
forces and deflection should increase steadily,
although not necessarily in constant proportions with
one another (in some cases, rudder forces may
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