Flight investigation of wing-gun fairings on a fighter type airplane

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Material Information

Title:
Flight investigation of wing-gun fairings on a fighter type airplane
Series Title:
NACA WR
Alternate Title:
NACA wartime reports
Physical Description:
9 p., 11 leaves : ill. ; 28 cm.
Language:
English
Creator:
Nissen, J. M
White, M. D
Langley Aeronautical Laboratory
United States -- National Advisory Committee for Aeronautics
Publisher:
Langley Memorial Aeronautical Laboratory
Place of Publication:
Langley Field, VA
Publication Date:

Subjects

Subjects / Keywords:
Fire control (Aerial gunnery) -- Equipment   ( lcsh )
Aerodynamics -- Research   ( lcsh )
Genre:
federal government publication   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Summary:
Summary: Flight tests were conducted on a Navy fighter airplane to determine methods for fairing the wing-gun installation so as to retain the maximum lift of the clean wing insofar as possible. The unfaired-gun installation increased the stalling speed over that of the clean wing by approximately 5 knots with flaps down, power off and by approximately 3 knots with flaps down, power on. Two arrangements of fairings were developed that restored the lift of the wing. Once arrangement consisted of engine cowl-type fairings for both projecting and submerged guns. This arrangement provided an annular opening between the gun barrel and the fairing lip for cooling the guns. The flush arrangement consisted of the engine cowl-type fairings for the projecting guns and faired wing openings for the submerged guns. Successful operation of this latter type of fairing, however, required that no air be admitted around the submerged guns. All arrangements of fairings as well as the unfaired guns improved the stalling characteristics of the airplane as compared with the clean-wing condition. It also appeared that the gun-fairing arrangements eliminated the ground-looping tendencies of the airplane that were attributed to wing stalling. This was evidenced by a series of landings made with the wing guns faired and the small tail wheel installed in which no ground-looping tendencies were noted. On the basis of data from the full-scale wind tunnel, it appears that no reduction in top speed need be anticipated with the four projecting fairings ventilated for cooling as compared with the unfaired gun condition. With the combination of projecting fairings and faired wing openings with no air admitted, the top speed may actually be increased 3 miles per hour as compared with the unfaired gun condition.
Statement of Responsibility:
by J.M. Nissen and M.D. White.
General Note:
"Originally issued October 1941as Advance Confidential Report."
General Note:
"NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of advance research results to an authorized group requiring them for the war effort. They were previously held under a security status but are now unclassified. Some of these reports were not technically edited. All have been reproduced without change in order to expedite general distribution."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 003805073
oclc - 123905297
System ID:
AA00009466:00001


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CitA 2917


NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS





WARTIME REPORT
ORIGINALLY ISSUED
October 1941 as
Advance Confidential Report

FLIGHT INVESTIGATION OF WING-G1N FAIRINGS
ON A FIGHTER TYPE AIRPLANE
By J. M. Niseen and M. D. White

Langley Memorial Aeronautical Laboratory
Langley Field, Va.

UNIVERSITY( OF FLORID.A
OCU N!TS DEP-3RI .~.-
1 Ws.CTCI- SC-iECE LIBRARY
-.'.. Ui 117011
G ,.JESi LLE, FL 32611-7011 USA


WASHINGTON


NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of
advance research results to an authorized group requiring them for the war effort. They were pre-
viously held under a security status but are now unclassified. Some of these reports were not tech-
nically edited. All have been reproduced without change in order to expedite general distribution.


L 247







































Digitized by the Internet Archive
in 2011 Will landing Irom
University ol Florida. George A. Smalhers Libraries wiliI support from LYRASIS and Ihe Sloan Foundation































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FLIGHT INVESTIGATION OF WIITG-GUN FAIRIITGS

Oil A FIGHTER TYPE AIRPLAIZ;

By J. 1.1. EIis.sen and M. D. White


SU:.:MARY


Flight tests were conducted on a Navy fighter airplane
to determine methods for fairing the wing-gun installation
so as to retain the maximum lift of the clean wing insofar
as possible.

The unfaired-gurn installation increased the stalling
speed over that of the clean wing by approximately 5 knots
with flaps 1aown, power off and by approximately 3 knots
with flaps down, po-7er on.

Two arran-,ements of fairings were developed that re-
stored the lift of the wing. One arrangement consisted
of engine cowl-type fairings for both projecting and su'b-
merged guns. This arraneer..ent provided an annular open-
ing between the gun barrel and the fairing lip for cool-
ing the gurs. The flush arrangement consisted of the en-
gine cowl-type fairings for the projecting .guns ani faired
wing openi:,gs for the submerged guns. Successful opera-
tion of this latter type of fairing, however, required
that no air be admitted around the submerged guns. All
arrange-ments of fairings as well as the unfaired guns im-
proved the stalling characteristics of the airplane as
compared with the clean-wing condition. It also appeared
that the gun-fairing arrangements eliminated the. ground-
looping tendencies of t:-e airplane that were attributed
to wing stalling. This was evidenced by a series of land-
ings made with th.e wing guns faired ar.d the small tail
rheel installed in which no grouend-looping tendencies "ere
no t e,..

On the basis of data from the fu-llt-scale wind tunnel,
it appears that no reduction in top speed need be antici-
pated with the fox.r projecting firings ventilated for
cooling as compar-red -7ith'the unfaired gunr cor-dition. With
the combination of projecting fairin.gs and faired wing
openings with no air admitted, the top speed ma;- actually
be increased 3 miles per hour as compared with the unfaired
gun condition.










'*

INiTRODUCTIOU


At the request of tne Bureau of Aeronautics, flight
tests have been conducted oi* a fighter type airplane by
the ITACA at Langley Field., The purpose of these tosts
was to determine the modifications required to correct
certain undesirable characteristics of the airplane. The
investigation started on April 15, 1941 was suspended only
for an interval from Lay 28 to Juno 16, during which chock
tests and necessary structural changes were made on the
airplane by the ?avy nt Anacostia and i'orfolk.

The present report covers the flight tests of gun
fairings designed to correct the detrimental effects of
the projecting and submerged wing guns on the airplane.
These effects, a 5-":i.ot increase in stalling speed as
compared with thec clean-wing condition, and a more pro-
nounced tenioenccy of tl-ae airplane to ground-loop in land-
ir.gs, were believed to be duo to early and unsymmetrical
wing stalling produced by tho -7ir.g-contour irregularities
of the gun installation. The belief that wing stalling
influenced the ground-loopI.g tendencies is based on
flight tests of othar airplanes that showed that violent
ground-looping tendencies wero caused by unsymmetrical
wing stalling in a three-point attitude.


THE AIP.PLAI:E A::. IiTSTRUiE1.T IITSTALLATIOI1


The airplar.e on which the investigation was carried
out is a Grumman F4-53 single-place midring monoplane
fighter (fig. 1). Airplane .o. 2538, rhich was delivered
to the 2TA1A for the test, was a standard service model
except for the following modification. The tail wheel of
the test airplane a s equipped with a pneumatic tire that
raised the tail approximately 3 inches as compared with
the hard-rubber tail -heel used on service models. By
this substitution the airplane ground angle was reduced
about 20. This r,:odification, which apparently is not
suited to deck operation, was adopted during early tests
to prevent grou:.d-looping until specific investigation of
that problem was undertaken. Theel brakes of greater
capacity than those in service models were also substitut-
ed on the test airplane to provide additional ground con-
trol,













During the tests the airplane center-of-gravity posi-
tion was nainLained at approximately 28.5 percent .I.A.C.,
the location at which it was generally flown in service.
The starting weight for each flight varied from 6425 to
6735 pounds, the greatest part of the variation. (275
pounds) being due to the removal or replacement of the
four guns. Tnis weight variation corresponds to a liffer-
ence in stalling spood of approximately 1.5 knots; for
simplicity in analysis all the stalling speeds reported
have been corrected to a -ross weight of 6725 pounds.

The instruments used in all the tests were a record-
ing air-sneed meter installed on the airplane air-speed
line and a t'ire-element control-position recorder record-
ing the movements of the r.jcvator, rudder, and ailerons.
Tufts were installed or.n the upper surfaces of the wings
and in some casoF in the ii.-mediate vicinity of the gun
fairinrs to aid in the study of the behavior of the air-
plane at the stall.

The locations of the two 0.50G-aliber machine guns
in each wing are illustrated in figure 2. Figures 3
through 7 are uhoto*:raphs of t.ie various tyres of gun fair-
ings tested. In figure 3(a) are shown the submerged gun
in its unfaired condition and the projecting gun fitted
with the fairing sDabrn'.tted by the Gru,-.:an Ccm-pany. The
Grumman fairing resembles ar. engine cowl in appearance
except that the spaco bct-wocn the gun barrel and the fair-
ing -as sealed w-ith rubber ;rommet. figure 3(b) shows
the -projectinz gun in its unfaired condit'.o:L -ith the sub-
mervol gua remov,'d. The Grumman fairir.g which was the
only fairi:-g used on the projecting -n.:s was at first
tested as zubuAittei. In later Lests the gro:mzet was re-
moved and the edge of the opening was bent in so as to
provide ar. annular space about 1/8 inch in ridth around
the *un barrel fzr tae entry of cooling air.

Several fairings for t.s.e submerged gun, designated
for brevity "o. 1, "'. 2, :'o. 3, ani faredd on .ning, "
are illustrated, respectively, in figures -z, 5, 6, and 7.
Fairing o10. 1 is a modifie version of the C-rumm:an fairing,
being somewhat more oval in cross section as compared with
the flat sides of the latter. Fairin"s o-,o 2 a.d 3, which
are shorter versions of the .o. 1 firing, differ from each
other only in -i.th, 'o. 3 being the ;:arro'er. The faired
openinCg sho-n i.1 figure 7 is faired into a tube that en-
circles the gun barrel for a distance 'ac' from the gun
muzzle of about 6 inches, c.nd an annular space about 1/8












inch wile is provided between this tube and the gun barrel
to permit the passage of cooling air. Similar annular
spaces are provi-ed between blast tube and fairing for the
other fairings.

TESTS, RESULTS, AIJD DIS3USSIOT

Stall Characteristics


The results of the stall tests with various arrange-
ments of -un fairinjs are presented in table I for the two
Ilirt conditions investigated. These flight conditions
were the landing condition, roweor off, flars down, and
gear idon, and tie c rrier-approach condition 23.5 inches
of mercu-ry manifold pressure and 2350 rpm, flaps down and
gear down. In these flight conditions continuous records
were obtained of stalls aproach:od gradually in P. lateral-
ly lovel attitiio, the pilot notirn the violence of the
stall, the rosporrn of the sirnlran to the ailcrons and to
poncr application in teo stall, .nd the tuft behavior in
the stall a-.ro.c-h,.

Tho results tabulated ir. table I may be briefly sum-
marized as follows:

1. In the pcwer-off, flap-iown flight condition
each set of unfaired guns, projecting or submerged,
alone effected a 3--not increase in stalling speed
over the clean-wing condition while in combination
the increase was 5 knot3. With o-owcr on, flaps
down all arra:gmc:c.nts of .unfaired fr-ns increased
the stallir.g spcc;ds by about 3 knots tests 1, 2,
3, and 5).

2. A fairing arrangc.rent consisting of the
Grumman .airing on the projecting gui- and the :To. 1
fairing on the submerged run (fig. 4 and test lMo.
7) effected an improvement over the runfaired-gun
condition in the followin;- respects:

(a) Th.. stallin,; speed in he ca.rrier-
a]T.proach condition was reduced b, 1 or 2 knots
as con-.pared with the ur.faircd-Purn condition
and tLe landing-condition stalling sped was
rei-.acd to the clean.--ing values.













(b) Tho stalling characteristics worc im-
proved over the cler.n-wing condition as indi-
cated by the ..iildcr roll c.t the stall and the
increased responsiveness of the airplane to
aiclron y.ovc.mont or power application at the
stall. Conclusions regarding the controllabil-
ity in the stall as listed in table. I are based
on tests in which, ir.-reiiately- after the stall,
ailerons were applied against the roll or power
wac applied and the stick moved forward only
enough to prevent a sharp rise of t-c nose,

3. Arothor gau.-fairi.ng arrangement consisting
of the Grucinan fai.-ir.g 'n the projecting gun tnd
the faired. !ir.Cg -por.ing for thc subr.orgod gu.n (fig.
7 and tests :_os. 14 ari 15) savc roeslt- similar
to those listed :-.drlr pa-agraph (2), unicr restrict-
ed con dit ic.s; that is, the e'fcctivc functioning
of this ."rra-n-oiont reoqircd that no air flow be
perL:itteo: through the firings. (Ccmpare tests
1os. 11 i .:a 14,) If it is n-cossary that air bc ad-
titted tr, co l the r'u:..s i.urin firintz, then in or-
der to utilize this nrrai.gement in service, provi-
sion woul.i -nave to be mnile for opening and. closing
an i:r seal around the .,un in fli-nt.

4. NTore oL the other arrangements listed in
table I was considered satisfactory. It is of in-
terest to note, ho-over, that at least one of the
otner fairings for the s'aubrerged guns was effective
rhen teste.i alonrc -ith tiec orojecting gun removed,
but wvas cntiroly ineffective i. combiination with a
fairing o0: the projecting -un (tests 'os. 8 aid 9).
Apparontl.- dtrizontal i:.tcrfcrcn:-c effects result
from the close T-roxinity of the tvw gu-ans to each
other, especially ,-ith po-cr on.

Ir. adiitio- to trc rc-zlts tabalat.i inr table I, in-
formation was obtained from tuft rtuiics that is consid-
ored of i..t-rcst. T ic tuft observations indicated that
oven in t.heo cloni.-wing conriition- t.-e initiall break-io-:-o
f floor occurs in the vicinity of the ,;un -locations. This
fact explains :1 Eson extant rhy the gun-fairinn design.
was critical. The tufts shore too thot, in ge-ieral, the
character of the stall correopondel *7ith tae rate and e7-
tent of spa ,rise progress of t.-he flo- breakdown. A sharp
break and fast roll in the .tall, for oxa.nplo, occurred













when the flow breakdown spread rapidly to the wing tip as
in the cl arn-.7wijg condition; on the other hand, a mild
roll resulted '.hen the flow progressed only to a station
Fo.o-rb-at irbo-irO of the ailerons as in the unfaired-gun
conditions ani as with the recommended fairings.

From the above, it is evident that the troubles ex-
perienced follo-ing the installation of unfaired guns on
the clean rving were thue not to their harmful effects on
stalling characteristics but only to the increased stall-
ing speeds that they proclaced. The mill rolls that fol-
lo-red the cerly advent of the stall would cause disturbing
moments o:n the ground which, combined -ith the inherently
unstable lar.diig-gear arraige..ient, res-.lted in violent
grou'id loops.

Tests -ith tufts in the immediate vininity of the
fairings showe-d ainl4 that it was the fairing for the
submorgei run th-:t suffered froL interference, while the
flow about tiae other fairing anpeared to be maintained
satisfactorily. Checl tests miaae with ani without the
tufts near the fairi:-.gs indicated the effects of the tufts
to be negliri'le.

Following firing tests conducted on the recommended
fairing arrangements by a squadron at :orfolk, it was re-
porte.' that a 1/8-inch-wide annular space between gun or
blast tube and fairiing gave adequate gun cooling. No fir-
ing tests .wer'e conducted -4ich openings sealed.


Dra.g Estimate

On the tasis of full-scale wini-tunnel tests conduct-
ed on another airplane, it is estimated that there will
be no reduction in top specdl due to the projecting fairing
as compared with the i.nfaired 'ins. The use of the faired-
'wing opening -7ith tao flow roalod off would actually in-
crease the top spcci by about S3 ilcs per hour as compared
with the u-nfaircd. rius.

Ground-Looping Tests

Previous tests or. other airplanes have shown that
frequently objectionable ground-looping tendencies are as-
sociated with an unsymoetrical, early stalling of the
'aing in the ground run. As was stated earlier, it was












with the idea of reducing the ground angle of the airplane
below the decreased stalling angle of the unfaired-gun
arrangement that a pneumatic tail whuel was installed on
the airplane,

This tail wheel reduced the ground angle by about 20
It is calculated that in the power-off condition the lift
rocoverel by the recommended gun fairings corresponds to
an increase in stalling angle as compared with the unfaired-
gun condition of 30. With the gun fairings on, therefore,
the ground angle could be increased by as much as 3 with-
out exceeding the stalling angle; hence, the pneumatic
tail wheel no longer seemed necessary.

To verify this conclusion, a series of landings was
made with the original hard-rubber tail wheel installed
and the guns faired with the Grumman and the ITo. 1 fairing.
ijo ground-looping tendency was noted in any of the land-
ings. From thcso landings it is evident that the aerody-
namic souriros of 7round-looping tondoncies wore eliminated
by the gun fairings. It should bo noted, howovcr, that
this modification in no -wa: affected the natural tonden-
cios of the' airplano to ground-loop; in fact, the landing-
gear rarrangcment of this airplane appears loss satisfac-
tory from this standpoint than do many others.


GENERAL RE:.IARKS


The problems associated -ith the installation of win-
guns in the subject airplane appear to be of a rather gen-
oral -nature. For example, the difficulties that necessi-
tated the proscnt investigation wore due largely to the
introduction of discontinuities in what is known to bo the
most critical portion of the wing chord, that is, the up-
per surface of the. wing in the immoliate vicinity of the
leading c.igo.

Corrective measures that might logically be employed
in future designs would be: (1) to lower the gun within
the wing possibly by turning the f-un on its side so that
it would project below the stagnation point, or (2) to
provide a faired opening with an air seal that can be
opened and closed in flight if it is considered necessary
to admit cooling air to the guns. From the standpoint of
simplicity of design and installation, the former alterna-












tive recommends itself. For ready adaptation, however,
the installatiDn should be incorporated in the original
design since structural limitations will generally prevent
relocation of the guns once the airplane has been con-
structed, as in the present instance.

The second alternative has tha disadvantago that it
might require the aided complication of a novable air
seal, This disadvantage would be compensated for, to a
considcrablo extent, howovor, by tnh- produced drag of this
installation as comparc- with the first and by the protec-
tion froin adverse weather conditions that it affords the
gr. n,

Rcg.s'.rdloss of the fairing installation employed, pro-
vision must be made for bore-sighting the guns. This could
be accomplished most readily 'y first bore-sighting the
guns and the4n installing .eio firings so that the gunn are
centered in the openir.gs, Another problem that merits at-
tontion in connection with i-i:g-.c-un installations is that
of minimizing tho size of lcading-cige popning required
to cover difforont settings of tL.e gun. In this connec-
tion, consideration Lii.-ht logically be given the possi-
bility of cl.anjing the angle of the .gun a;'out the muzzle
instead of about the front support. 71Tatever the means
employed, however, it appears dieirable that some steps be
taken in this direction.

CO::CLUS IO:S


As a result of the flight investigation of wing-gun
fairings on a fighter type airplane, the following con-
clusions may be stated:

1. The installation of unfaired -'uns on tne other-
wise clean wing resulted in a premature stall that in-
creased the stalling speed in t-ic cirrier-a-proach and
landiz.g conditions of flight.

2 3y suitably fairing; the guns it was possible to
reduce the stalling speeds to very nearly the values cor-
responding to the clean win~ and at the same tirmo elimi-
nate the objoctionrablo stalling characteristics associ-
atcd with the cean-w.ig co-ndition.












3. For immediate adoption on airplanes now in serv-
ice, a gun-fairing arrangement consisting of the Grumman
fairing on the projecting gun and a modification of this
fairing for the submerged gun recommends itself largely
because of its simplicity.

4. An alternative and oquall, ocffectivc arrangomant
consisting of the Grummanr. fairing for tl..o projecting guin
and a faired wing opening for the submeorgod gun dopcnicd
for its offoctivcnoss on the scaling off of cooling air
around thc gur., so that in service m ans might have to be
provided for opening and closing an ai.: sacl in flight.

5. In a soricr of landings made -ith the original
hard-ru'-bcr tail 7.vhc;l ii.stallel and the guns faired, -o
ground-looping tcn"oncj.r was noted. The landing-gear
arrangement on this airplane, however, appears less satis-
factory frou a ground-looping; standpoint than do many
o others.


Langley" memorial Aero:iartical Laboratory,
iTational Advizory Coma.i1teo for Acrona-utics,
Langlo, FiOl, Va.








NACA TaFble 1



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Fig. 3


Figure 3a.- View of submerged gun in unfaired condition and projecting
gun with Grumman fairing. .Rubber grommets installed around
edges of fairing and wing opening.


Figure 3b.- View of projecting gun in unfaired condition with submerged
gun removed.


NACA






NACA Fig. 4














































Figure 4.- Views of No. 1 fairing on submerged gun and Grumman fairing on
projecting gun. Both fairings provide annular space about
1/8" wide around gun barrel or blast tube for cooling air.






Fig. 5


Figure 5.- Views of No. 2 (wide) fairing on submerged gun and Grumman
fairing on projecting gun. Both fairings provide annular
space about 1/8" wide around gun barrel or blast tube for cooling air.


ACA






NACA Fig. 6










a,

































Figure 6.- Views of No. 3 (narrow) fairing on submerged gun and Gruuman
fairing on projecting gun. Both fairings provide annular
apace about 1/8" wide around gun barrel or blast tube for cooling air.






NIACA Fig. 7

















































Figure 7.- Views of faired wing opening for submerged gun and Grumman
fairing on projecting gun. Both fairings provide annular
space about 1/8" wide around gun barrels for cooling air.







UNIVERSITY OF FLORIDA

3 1262 08106 492 4


>.,
UNIVERSITY OF FLORIDA
DOCUMENTS DEPARTMENT
1 L'-'RSTOII SCIE- ICE LIBRARY
-U B0.' 11,011
(.. !i. -.vli L E, FL ?2C 1 -7011 USA




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