Air Force the Official Service Journal - Air Force Historical Studies ...

technique •••
CO]\;Tl:"UED
New Method of Static Testing
By E. R. Weaver, Fuginccring Di\'ision, ATSC
As larger aircraft with more criticallv designed wings have
been developed for the AAF, methods of ground testing the
strength of the planes under simulated flight conditions have
undergone radical changes.
1'\e\\est method of static-testing aircruft structures climina
tes the tradition;!! S~'stclll of pil ing shot-hags ;llId 1c;!d
,,'eights on \\'ings, fuselages and engine mounts to measure
critical loading limits. The new system ntilizcs rubber-tometal
adhesive tcnsiou patches that enable engiueers to
simulate and measure flight stresses and strains more accurately.
Actt;al loads arc appl icd by series of adhesive tension
patches on the top-side of an airfoil and soft sponge pads on
the under-side of the \\iug. l Ivclranlic jacks then proportion
the applied loads through a system of heams and 1cn:rs to
the patches and pads in accordance with the correct aerodynamic
data for the airfoil sections incorporated in the specific
design.
Essentially, the tension lifting patch is a flat or curved
plate (depending upon the surface to which it is applied) of
steel, dural um in or plastic material. It may range in xi:«:
from a. six-inch square to a 6 hy 24-ineh rectangle, or even
an irregular shape. One surface which is applied to the structure
under test is covered with a piece of tough, high te!lSile,
rubber or neoprenc sponge. The thickness of the ruhberized
patch or pad varies from YI of an inch to I ~~ inches and
forms a lifting element on the structural skin by adhesion.
'The lifting pads or pressurc pads arc the same size and composition
hut they arc applied to the underside of the airfoil.
II und reds of tension pa tchcs and pressure pads arc needed
to run a single test, far less, however, than the number of
shot-hags required for the dead \\'eight method prcviouslv
used. A single operator can load and unload the test struc-
Shot-bags and lead weights were piled on wings. cn!;ine mounts
and f11ScL1~cs to UW:l'\UT critical lO:ldin~ limits unclcr the old
mot lrocl of .,t;\tic tnt iu~ .iirrr.rf t struct urc-. The operation was a tedious
one, several da\s heing required to test an airplane completely.
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turc in a few minutes. By comparison, it formerly took days
to pile on the shot-hags for simulating loads, since an average
of 27 different tvpcs of tests arc required to static test a
military airplane completely.
In the test of a C- 54 airplane, for instance, 300 tension
patches and 300 pressurc pads were used. By this number
of tension patches sufficient lifting forcc was exerted to
cause complete failure of the wing.
The adhesive tension patch docs not reinforce the plate
stringer combination of the wing surface. \Vrinkle patterns
clue to shear lag, which form under the patches in their
natural ,,'ay, can be visunlly inspected and photogmphed
during the tests. The sponge adheres tighth' to the surface
deforlllations anc] has no influence on the structure.
This method of appl~'ing load is purticul.ulv effective in
the testing of cmdings, canopies, bomb doors, hatch doors,
trim tubs and control surfaces. In testing a curved surface
the patch bases arc moulded to the curvature so that the
sponge sheet distributes the load cvcnlv to the structure.
\\'ith cements now in usc it is possible to develop a load
c.rpacitv of 2,000 pounds per square foot on any structure
for nearly fi\"Chours.
\\'rig!;t held's Aircraft LilJOratory 110\1' has tIHI of the
new static test machincs-one with a lifting capacity of
100,000 pounds, the other with a lifting capacity of
1,000,000 pounds. The smaller machine is used for testing
trainers, fighters or small glidcrs, while the larger machine
tests aircraft with gross wcights cxcccd ing 1"3tons.
By employing the new system, the complete airplane
structure including the fuselage and horizontal tail lIlay be
tested simultaneously. Controls and control surface operation
em he studied during tests since there is no dead load
inside the fuselage or piled on the wing to endanger personnel
and interfere with control ,,'iring and operation.
Absence of dead loads in the fuselage 111akcs it possible to
inspect visually the stress reactions of the fuschge inside and
outside during tests. All hydraulic lifting equipment and
beams, lever systems, electric dr ivcn pump, high press nrc
hvdr.iulic cylinders and auxiliary pumps and gaugcs arc portable.
and thcv can he rcnclilv disassembled and couvc nicntlv
stcnee!. . .
Rubber-to-metal adhesive tension patches and prcssure pads, used
with new stat ir-t cst machine, enable ell~illeers to measure flight
stresses more .iccuratclv rh.ru thcv could for nu.rlv. Speed, too, is
gained. One m.ui can load .md unload test structure in few minutes.
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