FI - Primeiro Kfir 1975 - 1254
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Transcript of FI - Primeiro Kfir 1975 - 1254
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7/25/2019 FI - Primeiro Kfir 1975 - 1254
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8
FLIGHTInternational 7 July
975
IAI Kf i r
M i r a g e
successor
B y Y E H U D A B O R O V I K
O
UTWARDLY
the Kfir i s markedly
similar to the Mira ge family , espec
ial ly the 5J60 del ta wing, central
fin, twin semi-circular air intakes with
variable-geometry spikes , unders lung
pi tot tube and ventral fuel tank pro
t ruding from the rear fuselage.
A m ajo r d rawback o f Mi rage I l l s
and 5s , however, i s thei r Sn ecma A tar
9 eng ine, which develop s relativ ely low
thrust and has a high specific fuel
consumption (s . f .c) .
Is rael i A ir Force M irage IIIBs and
IIICs are engined with the A tar 9B,
whereas later models such as the HIE
and the 5 have the more advanced
A tar 9C. Both develop the same maxi
mu m thr ust b ut the 9C ha s a s l ight ly
lower s.f.c. , an improved compressor
being used.
T he Kfir engine is practic ally iden
tical to the J79-17 which powers the
F-4E, possibly with minor modifica
tions.
T he J79 's s .f .c . i s bet ter tha n the
A tar 's and is about half-way to tha t
of a mo dern en gin e: A tar 9C, 1-01;
J79 , 0-84; F100, 0-70. T he A tar 's s .f .c .
i s 17 per cent h igher than the J79 's in
dry thrust and 3 per cent h igher in
afterburning.
Kfir has the 6 delta wing of the Mirage
SJwhich would have been Israel s ground-
attack aircraft had France not embargoe d its
sale
and employs no leading-edge high-lift
devices
T he Gen eral Electr ic engine therer
fore burns 17 per cent less fuel on a
pound-for-pound basis . T he J79 is
more efficient for a number of
r e a s o n s :
M ass flow is abou t 11 per cen t
higher, necess i tat ing a redesign of
the in t ake duc t s .
Com press ion ra tio is significantly
higher, 12-4:1 compared with only
5-4 :1 on th e A ta r 9 .
T u rb ine in l e t t em pera tu re on th e
J79 is h igher at 954C, thanks to the
use of bet ter heat-res is t ing al loys and
more efficient cooling.
T he increased tu rb ine in l e t t em
perature manifes ts i t sel f in a much
h igher ex te rna l wal l t empera tu re (up
to 700C against 350C for the A ta r) ,
and this posed one of the major prob
lems which the Kfir developers had to
overcome.
T he J79 exha ust sys tem and after
burner, wi th a variable nozzle, are
mo re eff icient than thos e of the A tar .
T he in take flow rate in both th e
Mirage and the Kfir i s varied by
means of a spike moving forwards and
backwards wi th in the duc t , t hereby
chang ing the th roa t a rea and the
Shockwave pat t ern . A s noted above,
the J79 requires a f low rate about
11 per cen t h igher tha n tha t of th e
A tar 9, bu t apar t f rom the increased
inlet area, which obviously entai ls a
modified spike displacement ruleno
other changes in the Kfir in take duct
can be discovered from an outs ide
igf
| F f | L
" - " i Sk
jfF
*.
H I
W mm^
_ w # ~* "5
inspect ion. To increase the f low rate
during take-off the Kfir, l ike all Mir
ages, is fi t ted with auxiliary blow-in
doors behind the in let .
T he J79 is about 64cm shorter tha n
the A tar 9C, i t self so me 30cm s horte r
than the A ta r 9B. S ince the var i ab le
exhaust nozzle must protrude from
the fuselage to provide operat ing
clearance the Kfir af terbody had to be
shortened and modified.
T he a f te rbody mu s t be des igned
for minimum drag as wel l as to pre
ven t in t e r fe rence be tween the j e t
efflux and the flow passing over the
afterbody and the wing control sur
faces . To reduce base d rag the c l ear
ance between the port ion of exhaust
nozzle enclosed within the fuselage
and inner fuse lage wal l mus t be kep t
to a min imum . A t the sam e t ime t he
afterbody contours must not vary
dras t ical ly , s ince th is would resul t in
increased d rag .
The J79 's ex te rna l wal l t empera tu re
is about 300-350C higher than for
the A ta r . T he A ta r is p rov ided wi th
a thermal shield fit ted directly on the
engine but the-J79 is not . In other
J79-powered aircraft such as the F-104
and the F-4 the fuselage port ion en
clos ing the engines is bui l t of mater
ials such as s teel superal loys and t i
t an ium. T he Mi rage , however , i s bu i lt
ent i rely of aluminium al loys , which
are l iable to develop creep at such
h igh t empera tu res .
Viewed from the outs ide, the Kfir
fuselage appears to consis t ent i rely of
aluminium. I t can therefore be as
sumed that some other type of solu
t ion has been found for engine cool ing
and /o r thermal in su la t ion o f the fuse
lagethe f i t t ing of an extra thermal
shield around the J79 and provis ion of
external cool ing ai r . In the Mirage,
engine cool ing ai r i s suppl ied through
the boundary-layer separat ion ducts
wedged be tween the a i r i n t akes and
the fuselage w al l . T he quan t i ty of ai r
thus provided was adequate for cool
ing th e A tar , bu t insufficient for the
J79. A n exam inat ion of the Kfir re-
Instal lat ion
of a General Electric J79 turbojet
in place of the Mirage s Snecma Atar saves
64cm in engine length and has resulted in a
new design of afterbody being adopted for Kfir