Bell OH-58A Technical Manual page 95

.
NOTE
Steel and aluminum are incom-
patible materials and normally
require special precautions.
However, for BDAR, this is an
.
acceptable usage.
Refinements to patch repairs,
such as countersunk fasteners,
chamfered edges, anti-corrosive
treatment, and radius corners
of the patches are unnecessary.
e. External patch repairs generally
will meet strength criteria for effec-
tive BDAR. Sometimes these repairs must
take into consideration the clearance of
moving parts (e.g., control surfaces,
In low stress areas,
doors, etc).
patches may only be necessary to provide
aerodynamic shape to prevent ram air
effect or to keep water out.
these areas can be covered with thin
gauge metal or Army green tape. In some
areas, negligible damage can be ignored.
Use environmental protection for exter-
nal repairs if possible. Where an
access hole is large or can be made
larger for internal repair, an internal
or external repair may be used.
External repairs should be aligned
within 15° either side of the direction
of air flow.
f. General Repair Requirements.
Restore structural continuity by
restoring original structure or by
bridging the damage. Original strength
should be restored if possible. Weight
and appearance are unimportant in battle
Structural stiffness
damage repair.
should be sufficient to assure loads are
properly distributed and to avoid
serious vibrations. Use shims to avoid
Avoid hand formed and fitted
joggles.
parts. Where a stringer/stiffener is
light (1-1/2 X 1-1/2 X 0.060 inches or
less) and the stringers on either side
of the damage or fracture are sound,
apply a skin patch 2 gauges thicker over
the damaged area.
g.
method to create a complex curve in a
normally straight angle or T - Profile
member is to place appropriate cuts and
stop drill holes to allow bends, as
given in Figure 4-16. This field expe-
dient procedure will allow former repair
from stray stock.
4-16. SKIN-STIFFENER DAMAGE.
GENERAL INFORMATION: These repairs are
applicable to any skin-stiffener/
stringer structure such as the fuselage,
fairings, and many secondary structures
other than sandwich construction.
OPTION 1:
Stiffener/Stringer.
LIMITATIONS: None - Condition 1. Inspect
after every flight for damage growth.
Damage in
PERSONNEL/TIME REQUIRED:
2 Soldiers
2 Hours
MATERIALS/TOOLS REQUIRED:
Substitute Stiffener or Stringer
Section
Skin Patch
Cherry Rivets (item 37, Appx C)
PROCEDURAL STEPS:
1.
and stiffener/stringer containing the
damage.
Cleanup ends of the stiffener/stringer.
Stop drill any remaining cracks, Figure
4-17.
2.
thicker allowing overlap for at least
two rows of rivets, Figure 4-18.
Special Repair Techniques. One
Patch Plate and Substitute
Remove sections of the aircraft skin
Smooth and round the cutout.
Cut and fit a skin patch one gauge
TM 55-1520-228-BD
AIRFRAME
4-51