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

'"
Axial Force Coefficient
I t is customary to use the term "tangential" with
reference to vectors tangent to the flight path , the
rm " axial' is used instead when defining components
of fmce and mo:ion applied along the major
longitUdinal axis or axis of symmetry at :he decelerator
The axial force equals the decelerator drag coefficient
at angle of attack O. The axial force coefficient,
CA' presented in Figure 6.28 are calculated
from tes1 data
AlqS
obtai1ed on model wind
6.
tunnel tests
(Ref. 3891. The axial force coefficient is calculated
as follows
CA F
The speed for these tests was 1. Axial force
data for tests 391 with sirrilar canopy designs 131 M =
and are shovvn in FigJre 6.29.
Parachute Cluster Drag Coefficient
The cluster dl'ag coefficien: COC 's less than
of the individual parachutes, the ratio COe/CO be.
ing inversely proportional to tr,e number of para-
ChJleS as sl10wn in Figure 6. 30. While differences in
rate 01 descent account for some difference in CDC'
the data spread is too large to be the resuit of this
factor alone 1te divergent trends suggest 1'18 pres.
ence also of differences in both parachute and cluster
riggin ) geometry not evident in the source informJticn.
ThE' effactive rigging leng:h used for referen::e pur.
poses, and sometimes in cluster design, is based on
the cqllivalent single pa" achute length ratio
0. In terms of the nomi1el diameter of the incivid.
ual canopies of the cluster th is converts to
o =(nc) 6where
/ c is the combi ned lengtns of suspension I incs
and cluster risers to the riser confluence, and
the number of canop es in the cluster. The following
numerical '/a,ucs are obteined:
TABLE 6.3 EFFECTIVE RIGGING LENGTH FOA
CLUSTERED PARACHUTES
CID
141
For practical reasons, shorter rigging lengths 31'e used
in airdrop cluster of G- A, 100 ft and G- 12D,
64 ft parachutes: (See Table 6.4)
Emrir:cal data from differen sources on the norrral.
ized drag coefficients of clustered parachutes are plotted
in Fi gure 6.30. The relat:ve riggi ng lengths of the
individual parachutes and of the clusters are indicated
where known,
264
TABLE 6. EFFECTIVE RIGGING LENGTH FOR
CLUSTERED G- 11A AND G- 12D
PARACHUTES
11A
12D
1.2 1.4 1.6 1.8
1.1 1.1
it will be seen that ,n steady descent the individJal
canopies stand apart. as in F:gure 6. 31. They 0150
:end to wander about, constrained somewhat by the
number in the cluster. Apparently the fl owfield has a
radial component away from the sys1em axis in addition
to the familiar oatte n of flow about each can,::py,
which would tend to hold them apart, but this is
not strong enough to prevent the caropies frcm occasionally
conlac:iny each other lightly, Thus, on the
average it would appear that the mean relative flow at
each canopy has an angle of attack of roughly a'"
de'grees , provided the canopy is not gllding 390 . Sil'
the clus1er as a w'lole does not glide , the only way
the member canopies might glide is to move away
from the system axis and stand at an al1gle of attack
to the relative flow. This behavior would result in
some flattening of the outer or leecin\) edges of the
canooies and so would be visi:;le in fi, m records.
That suer evidence has not been observed tends to
support the idea t"1at on the average degrees,
and random shifting of the aerodynamic force vec10rs
driven by vortex shedding, causes the canopies to
wander, possibly seeking a stab,e angle of attack.
TI11S rei:scning justifies :he proposition that the cluster
drag coe'fficient may be rej:resented as
CD COo COS tpc 6-
where rPc is the irnegrated average angle of the cluster
risers from the system axis, The implicit assumptions
'1ere are t.18t there is l1ut nutual interference
and the dynamic pressure 1elt bV ea:h canopy is equal
to Q8' i.e.. th€ sarna as its equilibrium dyna11: c pres.
sure descending alone at the system rate of descent.
The angle
is mainly a function of
.:c
and has
its m nimum value when all the canopies of the cluster
are in contact with each other " as some rave been
rigged on occasion with tangent points on the skirt
bcund togetner. Then presumably CDC would be a
maximum ard the assllmption of n::Jn-inte'ference
may be tested by comparative evaluation.