[ Pobierz całość w formacie PDF ]
flight loads on the aircraft. Do not
pilot attention and will be difficult to fly, especially
exceed the design maneuvering speed
on instruments.
or any other airspeed limitation.
NOTE: Friction in the aileron control sys-
(1) To check lateral and directional stabil-
tem can completely mask the inherent spiral
ity, the aircraft should be trimmed for level flight characteristics of the airframe.
51
AC 90-89A 5/24/95
SECTION 4. A WORD OR TWO ABOUT FLUTTER
Stay up on the edge of your seat. Scott Crossfield, Test Pilot
1. OBJECTIVE. To understand the causes and c. If the center of mass of the aileron is not
cures of the condition known as flutter. exactly on the hinge line, it will tend to lag behind
the wing as it bends upwards.
2. DESCRIPTION. Flutter in an aircraft struc-
d. In a simple, unbalanced, flap-type hinged
ture is the result of an interaction between aero-
control, the center of mass will be behind the hinge
dynamic inputs, the elastic properties of the structure,
line and the inertial lag will result in the aileron being
the mass or weight distribution of the various ele-
deflected downwards. This will result in the wing
ments, and airspeed.
momentarily generating more lift, increasing its
a. To most people, the word flutter suggests
upward bending moment and its velocity relative to
a flag s movement as the wind blows across it. In
the fuselage. The inertia of the wing will carry it
a light breeze, the flag waves gently but as the wind
upwards beyond its equilibrium position to a point
speed increases, the flags motion becomes more and
where more energy is stored in the deformed struc-
more excited. It takes little imagination to realize
ture than can be opposed by the aerodynamic forces
if something similar happened to an aircraft struc-
acting on it.
ture, the effects would be catastrophic. The parallel
e. The wing bounces back and starts to
to a flag is appropriate.
move downward but, as before, the aileron lags
b. Think of a primary surface with a control
behind and is deflected upwards this time. This adds
hinged to it (e.g., an aileron). Imagine that the air-
to the aerodynamic down force on the wing, once
plane hits a thermal. The initial response of the wing
more driving it beyond its equilibrium position and
is to bend upwards relative to the fuselage.
the cycle repeats.
f. Flutter can happen at any speed, including
take-off speed. At low airspeeds, however, structural
52
5/24/95 AC 90-89A
and aerodynamic damping quickly suppress the flut- i. What can be done about it? Having
ter motion. But as the airspeed increases, so do the described how flutter happens, the following sugges-
aerodynamic driving forces generated by the aileron. tions should help reduce the possibility of it happen-
When they are large enough to cancel the damping, ing to the amateur-builder s aircraft:
the motion becomes continuous.
(1) Perform a mass balance of all flight
g. Further SMALL INCREASES will controls in accordance with the designer/kit manu-
produce a divergent, or increasing oscillation, which facturer s instructions.
can quickly exceed the structural limits of the air-
(2) Eliminate all control free play by
frame. Even when flutter is on the verge of becoming
reducing slop in rod end bearings, hinges, and every
catastrophic it can still be very hard to detect. What
nut and bolt used in attaching flight controls.
causes this is the high frequency of the oscillation,
typically between 5 and 20 Hz (cycles per second).
(3) Ensure that all rigging and cable ten-
It will take but a small increase in speed (1D 4 knot
sion is set accurately to the design specifications
or less) to remove what little damping remains and
using a calibrated cable tensiometer.
the motion will become divergent rapidly.
(4) Re-balance any flight control if it has
h. Flutter also can occur on a smaller scale
been repaired, repainted, or modified in any way.
if the main control surface has a control tab on it.
NOTE: If the pilot experiences flutter, or
The mechanics are the same with the tab taking the
believes he did, reduce power immediately
place of the aileron and the aileron taking the place
and land as soon as possible. Do not attempt
of the wing. The biggest difference are the masses
further flight until the aircraft has been
involved are much smaller, the frequencies much
thoroughly inspected for flutter induced
higher, and there is less feed-back through the con-
damage. This inspection should include all
trol system. This makes tab flutter more difficult to
wing/tail attach points, flight controls, their
detect. The phenomenon known as buzz is often
attach points/hinges, hardware, control
caused by tab flutter. Since flutter is more prevalent
rods, and control rod bearings for elongated
at higher speeds, it is not recommended that the flight
bolt/rivet holes, cracks, (especially rod end
test plan call for high speed runs within 10 percent
bearings) and sheared rivets.
of red line.
53
AC 90-89A 5/24/95
SECTION 5. SPINS
Go from the known to the unknown -- slowly! Chris Wheal, Military Test Pilot
1. OBJECTIVE. To determine if spin testing is aircraft is designed for, and will be routinely flown
required. in, aerobatic flight.
c. During all spin tests, it is strongly rec-
NOTE: All FAA spin tests for type certifi-
cation require a spin chute attached to the ommended that the pilot wear a parachute and that
aircraft. Even though amateur-built aircraft a quick release mechanism to jettison the canopy
have no such certification requirement, use
or door be installed. If the pilot is unable to exit
of a spin chute during testing should be
the aircraft because of the design restraints, it is rec-
considered.
ommended that intentional spins not be conducted
even though the design has successfully dem-
2. CAUTION.
onstrated spin recovery.
a. If the manufacturer/designer of the aircraft
d. If any modifications or alterations have
has not demonstrated satisfactory spin characteristics
been made to the airframe s original design or
and safe recovery, avoid all types of high angle of
configuration (e.g., adding tip tanks or fairings), it
attack flight testing and placard the aircraft: spins
is not safe to assume that the aircraft still has the
prohibited.
same spin recovery characteristics as the prototype
b. If the prototype aircraft has satisfactorily aircraft. Spins in a modified aircraft should not be
demonstrated spin recovery and the builder s aircraft attempted without consulting a qualified test pilot
is identical to the prototype aircraft, the pilot may and/or flight test engineer.
confirm the aircraft will recover promptly from
e. The pilot who conducts the spin tests should
inadvertent spin entries. Further tests to prove that
have experience in entry into and recovery from fully
the aircraft will recover from a fully developed spin
developed spins, preferably in makes and models
(three turns or more) are not necessary unless the
similar to the aircraft being tested. If the pilot needs
54
5/24/95 AC 90-89A
additional experience, aerobatic training with an (1) As the aircraft stalls, APPLY FULL
emphasis on spins from a qualified instructor is RUDDER in the desired spin direction, followed
highly recommended. immediately by full aft movement of the control stick
keeping the ailerons neutral.
3. PLANNING THE FLIGHT. At this point,
(2) The transition from a horizontal to a
nearly all the preparatory work for spin testing has
vertical flight path takes approximately three or four
been accomplished. Planning the next flight should
turns and is referred to as the incipient stage of the
be identical to planning for the first flight through
spin.
stalls. IT IS EXTREMELY IMPORTANT THAT
THE CENTER OF GRAVITY OF THE AIRCRAFT
(3) During the incipient spin, the dynamic
IS AT THE FORWARD CG LIMIT AND ANY [ Pobierz całość w formacie PDF ]
zanotowane.pl doc.pisz.pl pdf.pisz.pl ocenkijessi.opx.pl
flight loads on the aircraft. Do not
pilot attention and will be difficult to fly, especially
exceed the design maneuvering speed
on instruments.
or any other airspeed limitation.
NOTE: Friction in the aileron control sys-
(1) To check lateral and directional stabil-
tem can completely mask the inherent spiral
ity, the aircraft should be trimmed for level flight characteristics of the airframe.
51
AC 90-89A 5/24/95
SECTION 4. A WORD OR TWO ABOUT FLUTTER
Stay up on the edge of your seat. Scott Crossfield, Test Pilot
1. OBJECTIVE. To understand the causes and c. If the center of mass of the aileron is not
cures of the condition known as flutter. exactly on the hinge line, it will tend to lag behind
the wing as it bends upwards.
2. DESCRIPTION. Flutter in an aircraft struc-
d. In a simple, unbalanced, flap-type hinged
ture is the result of an interaction between aero-
control, the center of mass will be behind the hinge
dynamic inputs, the elastic properties of the structure,
line and the inertial lag will result in the aileron being
the mass or weight distribution of the various ele-
deflected downwards. This will result in the wing
ments, and airspeed.
momentarily generating more lift, increasing its
a. To most people, the word flutter suggests
upward bending moment and its velocity relative to
a flag s movement as the wind blows across it. In
the fuselage. The inertia of the wing will carry it
a light breeze, the flag waves gently but as the wind
upwards beyond its equilibrium position to a point
speed increases, the flags motion becomes more and
where more energy is stored in the deformed struc-
more excited. It takes little imagination to realize
ture than can be opposed by the aerodynamic forces
if something similar happened to an aircraft struc-
acting on it.
ture, the effects would be catastrophic. The parallel
e. The wing bounces back and starts to
to a flag is appropriate.
move downward but, as before, the aileron lags
b. Think of a primary surface with a control
behind and is deflected upwards this time. This adds
hinged to it (e.g., an aileron). Imagine that the air-
to the aerodynamic down force on the wing, once
plane hits a thermal. The initial response of the wing
more driving it beyond its equilibrium position and
is to bend upwards relative to the fuselage.
the cycle repeats.
f. Flutter can happen at any speed, including
take-off speed. At low airspeeds, however, structural
52
5/24/95 AC 90-89A
and aerodynamic damping quickly suppress the flut- i. What can be done about it? Having
ter motion. But as the airspeed increases, so do the described how flutter happens, the following sugges-
aerodynamic driving forces generated by the aileron. tions should help reduce the possibility of it happen-
When they are large enough to cancel the damping, ing to the amateur-builder s aircraft:
the motion becomes continuous.
(1) Perform a mass balance of all flight
g. Further SMALL INCREASES will controls in accordance with the designer/kit manu-
produce a divergent, or increasing oscillation, which facturer s instructions.
can quickly exceed the structural limits of the air-
(2) Eliminate all control free play by
frame. Even when flutter is on the verge of becoming
reducing slop in rod end bearings, hinges, and every
catastrophic it can still be very hard to detect. What
nut and bolt used in attaching flight controls.
causes this is the high frequency of the oscillation,
typically between 5 and 20 Hz (cycles per second).
(3) Ensure that all rigging and cable ten-
It will take but a small increase in speed (1D 4 knot
sion is set accurately to the design specifications
or less) to remove what little damping remains and
using a calibrated cable tensiometer.
the motion will become divergent rapidly.
(4) Re-balance any flight control if it has
h. Flutter also can occur on a smaller scale
been repaired, repainted, or modified in any way.
if the main control surface has a control tab on it.
NOTE: If the pilot experiences flutter, or
The mechanics are the same with the tab taking the
believes he did, reduce power immediately
place of the aileron and the aileron taking the place
and land as soon as possible. Do not attempt
of the wing. The biggest difference are the masses
further flight until the aircraft has been
involved are much smaller, the frequencies much
thoroughly inspected for flutter induced
higher, and there is less feed-back through the con-
damage. This inspection should include all
trol system. This makes tab flutter more difficult to
wing/tail attach points, flight controls, their
detect. The phenomenon known as buzz is often
attach points/hinges, hardware, control
caused by tab flutter. Since flutter is more prevalent
rods, and control rod bearings for elongated
at higher speeds, it is not recommended that the flight
bolt/rivet holes, cracks, (especially rod end
test plan call for high speed runs within 10 percent
bearings) and sheared rivets.
of red line.
53
AC 90-89A 5/24/95
SECTION 5. SPINS
Go from the known to the unknown -- slowly! Chris Wheal, Military Test Pilot
1. OBJECTIVE. To determine if spin testing is aircraft is designed for, and will be routinely flown
required. in, aerobatic flight.
c. During all spin tests, it is strongly rec-
NOTE: All FAA spin tests for type certifi-
cation require a spin chute attached to the ommended that the pilot wear a parachute and that
aircraft. Even though amateur-built aircraft a quick release mechanism to jettison the canopy
have no such certification requirement, use
or door be installed. If the pilot is unable to exit
of a spin chute during testing should be
the aircraft because of the design restraints, it is rec-
considered.
ommended that intentional spins not be conducted
even though the design has successfully dem-
2. CAUTION.
onstrated spin recovery.
a. If the manufacturer/designer of the aircraft
d. If any modifications or alterations have
has not demonstrated satisfactory spin characteristics
been made to the airframe s original design or
and safe recovery, avoid all types of high angle of
configuration (e.g., adding tip tanks or fairings), it
attack flight testing and placard the aircraft: spins
is not safe to assume that the aircraft still has the
prohibited.
same spin recovery characteristics as the prototype
b. If the prototype aircraft has satisfactorily aircraft. Spins in a modified aircraft should not be
demonstrated spin recovery and the builder s aircraft attempted without consulting a qualified test pilot
is identical to the prototype aircraft, the pilot may and/or flight test engineer.
confirm the aircraft will recover promptly from
e. The pilot who conducts the spin tests should
inadvertent spin entries. Further tests to prove that
have experience in entry into and recovery from fully
the aircraft will recover from a fully developed spin
developed spins, preferably in makes and models
(three turns or more) are not necessary unless the
similar to the aircraft being tested. If the pilot needs
54
5/24/95 AC 90-89A
additional experience, aerobatic training with an (1) As the aircraft stalls, APPLY FULL
emphasis on spins from a qualified instructor is RUDDER in the desired spin direction, followed
highly recommended. immediately by full aft movement of the control stick
keeping the ailerons neutral.
3. PLANNING THE FLIGHT. At this point,
(2) The transition from a horizontal to a
nearly all the preparatory work for spin testing has
vertical flight path takes approximately three or four
been accomplished. Planning the next flight should
turns and is referred to as the incipient stage of the
be identical to planning for the first flight through
spin.
stalls. IT IS EXTREMELY IMPORTANT THAT
THE CENTER OF GRAVITY OF THE AIRCRAFT
(3) During the incipient spin, the dynamic
IS AT THE FORWARD CG LIMIT AND ANY [ Pobierz całość w formacie PDF ]