"lfk f; \"A Lt. - Airborne Systems

Blocking effects arise when the ratio of the model
projected area to the test-section area is large. For
subsonic testing, the maximum ratio of model area to
test section area should not be larger than approximately
fifteen-percent if blocking effects are to be
minimized. This area ratio applies primarily to opening-shock
measurements of canopies- For the determination
of aerodynamic coefficients, smaller models
should be utilized. For best results. the projected
area of the inflated aerodynamic decelerator model
should not exceed 10 to 15 percent of the tes:
sert:on area.
Wind tunnel testS are generally conducted above
critical Reynolds number in order to avoid scale
efieGts. As in aircraft we,rk, an 2ttempt is made to
achieve Reynolds number equivalent to actual fullscale
test or operational conditions; however, this
has seldom been feasible because o the la-ge canopy
diarleter3.
The requirements for wind tunnel testing at supersonic
speeds are more stringent. The positioning of
the decelerator model' the test section is critical
and mounts must be rigid. Trailing models are limited
to a range of downstream positions that will not
be intersected by mount and body shock waves
reflected from the tunnel walls.
Decelerat:lf models in a supersonic wind tunnel
must be structurally strong enough to withstand
sustained operation far beyond the demands of an
operational item. Instability pulsation and fluttering
will tend to limit the maximum dynamic pressure of
the tests, or the use of over-strength models will
introduce unfavorable sl:ale factors with respect to
stiffness and elasticity. Wind tunnel facilities currently
used for testing supersonic decele'ator tvoes are
located at the Arnold Air Force Station , Tennessee.
Support Testing
Support tests are those conducted on recovery system
components and materials to obtain, at low cost,
functional verific8liCln or spec ;tic data in support of
(and usually prior to) expensive major development
operations such as flight testing. Ground tests or
bench tests are often used to demonstrate functional
integrity of a decelerator or a component of the
system. Other tests are made to learn exact charac.
teristics of materials, for example, in the investigation
of failures to determine cause- Tests of this nature
employ laboratory methods and equipment. Simple
laboratory tests, such as the results of chemical analy.
sis, determining weathering qualities, establishing
abrasion resistance, or determining the friction coef.
ficient of a material , are made with reference to their
effect on the primary characteristics. Since required
characteristics for textiles in other fields are somewhat
similar, the testio;J equipment for recovery system
textiles has been borrowed or adapted from these
other tields. However, chfJr"cteristics such as air
perneabilit' and elongation IJIder stress are more
thoroughlV investigated. The methods described in
the fOllowing paragraprs are primarily for textile test-
207
ing. Some testing is done on aerodynamic decelerator
hardware, but this follows the same pattern as any
metal testing. such as bending and hardness testing
and radiographic analysis. A number of instruments
are commercially available for all of these tests.
Both static and dynamic labOFatory test equiprnept
are needed to adequately evaluate mechanical
properties, functional characteristics and performance
of every part of a recovery system. Measurement of
the mechanical properties of textiles and energy
absorbing materials are a normal function of the
manufacturers of decelerator and landing systems.
Complox testing, such as the discrete vibration spectrum
or extreme vacuum , sometimes required in
flight environment simulation, may be
delagated tQ
an independen: test laboratory or an available government
facility.
Simulated Deproyment.
Sa'.teral different kinds of
deployment tests are performed with varying degrees
of s::phistication dependirg on the purpose.
The static extraction force required to strip a
deployment bag fr8m a packed decelerator is usually
measured as a function of displacement on a long
smooth .table.
The mo1ion of a deploying decelerator may be
evaluated to a degree und8r static conditions by impu!sive
extraction or ejection of the pack from its
comps:tment in the vehicle or in a partial dummy
vehicle. A stretched elastic "shock-cord" has been
used for this pl.rpose to sirrulate the drag of the pilot
chute or prior-stage drogue.
Static mortar firings and ejection tests may be performed
with either dummy or actual decelerator
packs to evaluate muzzle velocities and the behavior
of the decelerator and dep! oyment bag during the
stretchou t sequ ence .
Recovery system deployment sequencing may be
eva\uated with actual or dUMmy vehicles at rest on
the ground for detailed instrumental and photographic
coverage of the complete series of events in re61
time. Reference 324 describes a dynamic sinulation
technique which utilized a moving truck to achieve
stretch out of a packed parachute. The packed parachGte
is mounted on the truck bed and the main riser
anchored :0 the ground, As the truck accelerates, the
parachute deploys,
Deployment impact loads and bridle failure modes
are dup!ica:ed with several different kinds of dyrwmic
loading equipment, some highly specialized.
The release and inflation of airbags for impact
attenuation or flotation, and the deployment of other
landing devices may be checked for function and
operating time prior to or in conjunction with appropriate
drop impact tests