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

tive drag stabilizer because of ii:S iong relative towing
distance and :1egligible adced air mass.
Systems in Steady Descent
The generalized canup'y of Fig. 6. 33 develoQs B'l
ae, odynaTlic force. represented by the \lector the
direction of which ::hanges as the relative airflow over
the canopy changes. I n three di mansions the ai rflow
separates at the canopy lip erratically at various
points around the r:eriphery and eddies into a number
of attached vortices which grovv to some characteristic
size and energy before being shed into the turbulent
wake- Mcst circular canopies have no surface
feature that would stabilize this flow aattern and
either prevent unsymrretricai growth and separation
of the vortices or causs tram to remain 8ttached in
cOltrarotating pairs Consequently, with parachute
ax,s constr-ained by the suspended weight moment to
a smal angle of attack , the aerodynamic force vector
of such canopies is constantiy shifting in direction
with varYing degr;:es of rardOl1ness, and being unable
to enter a stable giide, tha sY3tem osciJlates instead.
However , it has been observed that a large enough
angular deflection can throw the system into a steady
conin9 Ilotion , which may be viewed as a stGble glide
in a "tight spira!.
The ciffe' enees found between dif+erent types of
circula parachutes is reflected by the way in which
the momsnt generated by the shifting aerodynamic
terce varies with the angle of attack of the canopy
(see Figures 6. 72 and 6.73) fhe angle of attack at
which lda changes sig;) and produces a restoring
moment with further angular displac8r)1ent is identi.
fied 482 as the stable glide angle At this angle 01
attack the canopy is staticall'y stable, and when a
suitable strll::tural dissymmetry is introduced, the
parachute is able to take on a stable glide. A steble
flow pattern over the canopy is then established
which may be likened to that over a thick win!; of
low aspect retio witt' attached tip vortices. Since the
angle of attack ranges from a = 35 to 15 degrees, such
a wing must be gilding in a stalled condition and
transverse oscillations remain potential in the operation.
Dvnamic Stabilty of Gliding Systems, The dynamic
stability of a descending syste'l1 gliding steadily
with the canepy at was the subject of a three.dime'lsional
ana:ysis by White and Wolf4S::
System geometry and cQordinate systems used are
shown in Figure 7.40- The fOllowing simplifying assumo!ions
were made:
The system consists of an axi-symmetric parachute
rigidly connected to a neutrai body.
The aerodynamic force and hydraulic inertia of
:he body are negligible.
380
There are five degrees-af-freedom, with tile 1'011 of
the parsch'Jte about its axis of sY1'metry bein.
ignored.
Earth
Trajectory
Figure 7.40 Parachute System Geometry and
CQordinate System
18 added air ma"s and apparent moment of inertia
tensors of the conopy may be approximated by
single scalar values a' '
The aerodynamic forces may be treated as quasistate
based on the instantaneous angle of attack
of the C8'lCPY.
The canopy certer of pressure lies on t,le canopy
centroid.
Flat E:arth and no wind conditions prevail.
A stability critel'ion for small distJrbances was derived
in the fQrm of a set of linearized equations of
motion about
o' They show the longitudinal and
lateral moti:Jns to be uncoupled , as in the study of
aircraft linearized stabilitv. In the analysis . a side
for::e coefficiert is used which varies wich (J in the
same way that varies, the quantitative C ifferen:e
being merely
M (D//) 7- 130
when the distance be ween the canopy center of pressure
and t:lS system reference poin;, /, is differs'1t
from the chal acteristic diarreter, D, as is Jsually the
case. For small disturbance;" linear variation of force
coefficients in the vicinity of is assuMed as sholJln
in Figure 7.41