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

Opening Forces
300
200
100
100
tf
RO
4prrR 3
200
MASS RATIO IR;')
Disk-Gap-Band
6. Modified Ringsall
HemislQ
Hypedlo
Plirasonic
Figure 18 Upper Limit Effect of Compressibility on Parachute Filing Distance (Ref, 313)
During the inflation of a parachute or similar
towed bait istic type decelerstor an aerodynamic force
is developed tangential to the flight patn that varies
with time in the characteris:ic ways illustrated in Figures
6. 19. , 6.22 and 6.23.
The opening force crlaracteristics of solid cloti
c.irculsr parachutes have been comprehensivelyevaluated
in full scale aerial drop tests and analyzed in
great detail by Berndt and DeWeese 381 using Air Force
Type C-9 28 ft personnel pErachute as a lest
model. Similar work was carried out by Watson and
DeWeese 351 with two structural versions (light Bnd
heavy) of the Type T- 10 35 ft (Do 10% flat extended
skirt parachute. Figure 6.24 utilizes data from the
9 tests to provide a correlation between canopy
shape during inflation and the corresponding instantaneous
force. One point of interest is (6) which is
near the ini tial force peak and represents the comple-
tion of tha: portion oi the infletion process vVhere the
canopy is filling from "ski rt-to-ve '1t' , (see inflation
stage (c) in Fig. 6.9A). Additional force-time histories
from the C-9 tests as shown in Figures 6. 19 and
20 exhit,it the pronounced double force pulse of
this parachute, the second peak occurring close to full
inflation.
The relative magniwde of the two peak forces,
F/F varies with snatch velocity, altitude, flight-
o,
path angle, and canopy loading as shown. The ratio
253
/( .. ""
300
400
of the initial to final peak loads is shown to be
II Figue 6. 20 a.
strongly dependent on parameter K.
By definition
N. (F/€Wb)
1/3
(PoOl 1mb) (J21.J 6-
where is the instantaneous elongation of the suspension
lines in percent, based on an assumed linear
spring constant tor the suspension line bundle defined
such trat
P uZleuls
e = FZ/"
In 51' art. the force ratio is a function of the elastic
properties of the parachute structure as well as the
system mass ratio and the density altitCide (See also
Referenoe383 ).
The effect of Frauds number on the maximuf"
opening force parameter /qs is shown to be
o.
small but significant in Figure 6.20 for differe1t
val ues of
Fr2 vl 100B sin e