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

. .
LID cot e
/sin e
Because (LID)max occurs close to the angle of attack
where a rapid drop in LID is imminent. it is sometimes
desirable to design for LID at for Vv minimum.
Sizing the Gliding Parachute, Given the recoverable
weight W, and haviny established the operationi!1
LID requirec. selection of the type of canopy to be
Jsed is guided by the following critcria
(L/D)max:? LID (required),
at LID (recuired) is close to the best attainable.
The ratio of the canopy ptanform area to the total
fabric area is large.
Complexity of reefing reqlmerl\ents is acceptable
Turning rate satisfies minirrum maneuverabiiity.
Since it is uSI.Jaily desirable to have a canopy of mini
mum bulk and weight. C must be consid.
Rane
ers? togetrer. O her things being equal , the canopy
design of least weight will be obtained when S IC
5 () minimUm, I. , the least arca aT fabric is needed to
e the etfect :e I ifting surface. Area necessarily
Includes all ribs , gussets , and flares, as weli as
the upper and lower swfaces. because these control
thc: airfoil orofi:e ot the canopy and are essential to
its aerodynamic performance. In :his respect, sirgle
surface canopies have a'1 advantage. Significa'll differences
in susr:ension line riggi:lg and reefing require.
ments will irfluence the final selection.
The required planform . area (by Equation 8- 10) is
Sw
W/C
Numerical Examples
Methods of selecting and szing a steerable gliding
canopy for a giver application are illustrated oy the
following numerical examples:
System D is fer an RPV to be recoveree at
:he conclusion of its mission bv being flown
under full control to a designated landi1g area
where a spot landing is made after initiation of
recovery at or above 5 000 ft MS L (2 500 Tt
abcve ground leve!)- At the design altitude of
500 ft MS L the nominal rate of descent is to
be ''V - 15 fps \Itith a landing weight W= 1500
Ibs. The;Jfiding system shall be able to penetrate
winds U:: to vH 20 kts(TAS) (33. Ips).
As a minimu'T the glide ratio in straight"8wav
flight should be vHlvv 25. Selection of the
simplest steereble parachute design capable of
meeting the requirements is generally the most
econornicaJ approach. ExaminatiQn of Figs.
36A and 6.36B suggests U',at either 1) Parafoil
419
of AR 5 or a single-keel Parawing could be
expected to satisfy this requirement, with a
margin for err:Jr, by designing to
2.4. On this basis:
(LID) max
Vv 15fps
vH 15(2.4)=36fps
cot a 2.4 () - 22. sin 0 0,385
15/.385 39 fps (T AS)
At 2500 f: fiS L, 002208 sl/ft and the
design dynamic pres ure is
001104 (39;2 ""
68 psf. Then the canopy effective area requ
ired is
'" 1500/1. 68= 892.
Determination of at (L/D)max requires
perfornarce data obtained, ideally, from large
scale free flght tests , othecwise fron wind
tunnel testing of carefully constructed models.
The comparative calculations summarized in
Table 8. 7 tor Pamwing and Paratoil are based
on data derived from the most recent source
references containing large model test measure.
ments in a "orm suitable for t1is evalua-:ion.
g. Fig. 6. 36A.
When decelerator sy'stem weight and bulk
are the dominant criteria. usually the case, ,hen
the flexible wing havin9the srlallest aerodynam-
IC area ratio re::resented by would be
selected, provided there is not a large difference
,n the strength of materials required. Otherwise
differences in reefing requirements. maneuver.
ability and the useful range of LID modulation
may govern. Pertinent characteristc data of
the kind and quality needed are not equally
available for the different fleidble wing designs.
This fact, as well as the rela ive weight factor,
would iustify selection of the single. keel Parawing
for further study as the primary component
of Svstem D.
System E is for a payload of 1415 t::s to be
spot-dropped with E target.seeking steerable
parachute system for whiCh the allowable
weights ara 150 Ibs and 55 Ibs for the control
package Olnd the parachute pack respectively.
The systen1 shall be capable of gliding at
against a 25 kt (42.2 fps) headwind while
descending at 30 fps at sea level.
The forward glide veloeitv must be at least
VH 30 + 42. 2 '" 12. fps
At any altitude above sea level the TAS will be
greater than t'1 is fi ;jure.
The minimulT glide ratio in straight-away
flight must be
'"