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if the main control surface has a control tab on it.
The mechanics are the same with the tab taking the
place of the aileron and the aileron taking the place
of the wing. The biggest difference are the masses
involved are much smaller, the frequencies much
higher, and there is less feed-back through the control
system. This makes tab flutter more difficult to
detect. The phenomenon known as ‘‘buzz’’ is often
caused by tab flutter. Since flutter is more prevalent
at higher speeds, it is not recommended that the flight
test plan call for high speed runs within 10 percent
of red line.
i. What can be done about it? Having
described how flutter happens, the following suggestions
should help reduce the possibility of it happening
to the amateur-builder’s aircraft:
(1) Perform a mass balance of all flight
controls in accordance with the designer/kit manufacturer’s
instructions.
(2) Eliminate all control ‘‘free play’’ by
reducing slop in rod end bearings, hinges, and every
nut and bolt used in attaching flight controls.
(3) Ensure that all rigging and cable tension
is set accurately to the design specifications
using a calibrated cable tensiometer.
(4) Re-balance any flight control if it has
been repaired, repainted, or modified in any way.
NOTE: If the pilot experiences flutter, or
believes he did, reduce power immediately
and land as soon as possible. Do not attempt
further flight until the aircraft has been
thoroughly inspected for flutter induced
damage. This inspection should include all
wing/tail attach points, flight controls, their
attach points/hinges, hardware, control
rods, and control rod bearings for elongated
bolt/rivet holes, cracks, (especially rod end
bearings) and sheared rivets.
54
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
required.
NOTE: All FAA spin tests for type certification
require a spin chute attached to the
aircraft. Even though amateur-built aircraft
have no such certification requirement, use
of a spin chute during testing should be
considered.
2. CAUTION.
a. If the manufacturer/designer of the aircraft
has not demonstrated satisfactory spin characteristics
and safe recovery, avoid all types of high angle of
attack flight testing and placard the aircraft: ‘‘spins
prohibited.’’
b. If the prototype aircraft has satisfactorily
demonstrated spin recovery and the builder’s aircraft
is identical to the prototype aircraft, the pilot may
confirm the aircraft will recover promptly from
inadvertent spin entries. Further tests to prove that
the aircraft will recover from a fully developed spin
(three turns or more) are not necessary unless the
aircraft is designed for, and will be routinely flown
in, aerobatic flight.
c. During all spin tests, it is strongly recommended
that the pilot wear a parachute and that
a quick release mechanism to jettison the canopy
or door be installed. If the pilot is unable to exit
the aircraft because of the design restraints, it is recommended
that intentional spins not be conducted
even though the design has successfully demonstrated
spin recovery.
d. If any modifications or alterations have
been made to the airframe’s original design or
configuration (e.g., adding tip tanks or fairings), it
is not safe to assume that the aircraft still has the
same spin recovery characteristics as the prototype
aircraft. Spins in a modified aircraft should not be
attempted without consulting a qualified test pilot
and/or flight test engineer.
e. The pilot who conducts the spin tests should
have experience in entry into and recovery from fully
developed spins, preferably in makes and models
similar to the aircraft being tested. If the pilot needs
55
5/24/95 AC 90-89A
additional experience, aerobatic training with an
emphasis on spins from a qualified instructor is
highly recommended.
3. PLANNING THE FLIGHT. At this point,
nearly all the preparatory work for spin testing has
been accomplished. Planning the next flight should
be identical to planning for the first flight through
stalls. IT IS EXTREMELY IMPORTANT THAT
THE CENTER OF GRAVITY OF THE AIRCRAFT
IS AT THE FORWARD CG LIMIT AND ANY
BALLAST USED SHOULD BE SECURELY
ATTACHED TO THE AIRCRAFT.
a. The aircraft should be tested with landing
　
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