An investigation is summarized of the aerodynamic principle of boundary layer control for nonrigid LTA craft. The project included a wind tunnel test on a BLC body of revolution at zero angle of attack. Theoretical analysis is shown to be in excellent agreement with the test data. Methods are evolved for predicting the boundary layer development on a body of revolution and the suction pumping and propulsive power requirements. These methods are used to predict the performance characteristics of a full-scale airship. The analysis indicates that propulsive power reductions of 15 to 25 percent and endurance improvements of 20 to 40 percent may be realized in employing boundary-layer control to nonrigid airships

Pake, F. A.

Pipitone, S. J.

English

NASA Technical Reports Server

BOUNDARY LAYER CONTROL
N7 6 - 15 0 2 8
F. A. Pake*
S. J. Pipitone *_
ABSTRACT: This paper summarizes an investigation of the
aerodynamicprinciple of boundary layer control for non-
rigid LTA craft initiated under the Office of Naval Re-
search, Contract NOnrl412(00)LI. The project included a _ t
wind tunnel test on a BLC body of revolution at zero
angle of attack. Theoretical analysis is shown to be in
excellent agreement with the test data. Methods are e-
volved for predicting the boundary layer development on
a body of revolution and the suction pumping and propul-
sive power requirements. These methods are used to pre-
dict the performance characteristics of a full-scale
airship. The analysis indicates that propulsive power i
reductioLs of 15 to 25 percent and endurance improvements
of 20 to 40 percent may be realize_ in employing bound-
ary-layer control to non-rigid airships.
%
INTRODUCTION
The investigation of hhe application of boundary-layer control to non-
rigid LTA craft was initiated by Goodyear Aerospace Corporation in
Mnrch, 1954 under Office of Naval Research Contract NOnrl412(00)LI.
The project stretched over a 3 1/2 year period primarily because of a
20-month delay during which all effort was suspended while awaiting
the availability of the 7' x i0' transonic wind tunnel at NSRDC (then
called the David Taylor Model Basin). The scope of the study included
the evaluation of the drag characteristics of an airship hull which
employed either suction slots or an auxiliary air foil as a means of
preventing turbulent boundary layer separation. The drag results
were predicted by theoretical methods presented in References 1 and 3.
Comparative drag values were obtained for one body configuration in
the wind tunnel tests reported in Ref. 2.
*Flight Dynamics Section, Goodyear Aerospace, Akron, Ohio 4'4"315
**Technical Staff Goodyear Aerospace, Akron, Ohio 44315 U.S.A.
147 _._
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https://ntrs.nasa.gov/search.jsp?R=19760007940 2020-03-20T01:38:50+00:00Z
_ t BOUNDARY LAYER CONTROL :_
This discussion of boundary layer control will be limited to bodies
of revolution with flow at high Reynolds numbers. Therefore, turbu-
lent flows are assumed. With fluid flow about a body the friction
occurring on the forward portion consumes much of the initial energy
of the fluid adjacent to the body. The fluid so affected is termed
/lj the boundary layer. When this relatively low energy fluid reaches
_! the stern, the fluid is confronted with an unavoidable region of in-
creasing surface pressures due to the increasing static pressure of
_: the fluid external to the boundary layer being impressed upon it.
: / If the rate of pressure increase is relatively large, the boundary
- i layer fluid will not contain sufficient energy to flow against such .
! a high "back pressure," so to speak. This then results in consider- ,,_
able thickening of the boundary layer with possible flow separation.
Although it is possible to design a body of revolution having a fa-
vorable pressure gradient over essentially the entire length of the t _
body, generally such a body must have a relatively blunt after end.
This design produces a correspondingly adverse pressure gradient that
_ tends to cause boundary-layer separation and consequent drag losses.
This problem can be approached passively by lengthening the body <in-
- creasing the fineness ratio) thereby reducing the adverse pressure
gradient and delaying boundury la_r thickening so that the azea
affected by the reduced pressure is small and hence tend to r_duce
the pressure drag. For bodies of constant v_lume, however, an in-
! crease in fineness ratio is accompanied by an increase in friction
drag due to the consequent increase in surface area. Altering the
: pressure drag by varying the fineness ratio gives rise to a change in
' friction drag of opposite and approximately equal magnitude for the
i common airship fineness ratios. When the pressure drag is efficient-
_. ly reduced, accompanied by a lower fineness ratio, the total drag can
be significantly reduced as illustrated in Figure i.
_ /-CONVENTIONALBODIES(MODELR,N.)
_"_FINENESS RATIO FCURRENTAIRSHIPS
_-FRICTIONDRAG(AllTURBULENT)
,..,= TA PRESSUREDRAG
1,0 5 10,0
FINENESSRATIO
Figure 1
Pressure Drag Versus Fineness Ratio
148
1976007927-157
LKowever, through proper body-contour design, the adverse gradient can
5e located at one longitudinal body station or for a short longitudi-
nal 5ody station or for a short longitudinal distance to produce a
favorable pressure gradient extending to the 100-percent body station.
By applying the air-flow suction at this longitudinal body station (or
area of velocity and pressure discontinuity), energy will be supplied
to stabilize the boundary layer and prevent air-flow separation. A 4
drag economy can be realized if the reduction in the external drag of
the body is greater than the equivalent suction drag.
Confi@uration Selection:
The first decision to be made in the selection of a boundary-layer
control airship configuration was the suction system. The distribut- _
ed type suction systems made up of many perforations or slots were _ i,
discarded as not feasible for the non-rigid airship application. Thus
the single slot system was chosen and it remained to choose an airfoil
shape. The available selection could be categorized in two groups - ! _
the conventional airfoil and the Griffith type airfoil. The Griffith
shape has several advantages for BLC applications. Although designed
for laminar flow, it possesses the favorable pressure gradients neces-
sary to any type of boundary layer control. The localized adverse
pressure gradient is compatible with the single slot control system.
Also, the slot location is well aft _or the lower fineness ratios.
The Griffith type airfoil was therefore chosen. The specific contour
used in the study was a 34 percent thick Lighthill shape. This was
selected on the basis of the potential flow characteristics as deter-
mined by a series of electrostatic tank tests. The selected airfoil
shape and velocity distribution are shown in Figure 2. As can be seen
the adverse flow region is quite local between X/£ = 1.6 and 1.7.
This shape was used in the theoretical drag estimates, the wind tunnel
test and the full-scale performance studies.
D_a_ Estimates and Wind-Tunnel Tests:
i _ A method of calculation was evolved to predict skin friction, equival-
• ent suction drag, and propulsive efficiency of this type of airship
I hull. Local skin-friction coefficient values were determined for the
forward stagnation region, the laminar boundary-layer region under a
favorable pressure gradient before the suction slot, and the turbulent
boundary-layer region under a favorable pressure gradient behind the
suction slot.
Equivalent suction drag was based on the mean total-head loss in theboundary-layer suction flow at the slot entry. This did not include
duct losses since such losses can be evaluated only after the prelim-
inary design of a specific ducting system. Hence, the suction drag
was evaluated for an idealized system where duct losses were small
. compared with boundary-layer losses.
The wind-tunnel tests were carried out in the 7' x I0' transonic tun-
nel. The Reynolds nu2_ber was varied from 4.4 x 166 to 107 . Due to
the model size restriction and the relatively high test Reynolds num-
bers, a powered model with force measurements was not possible and
therefore drag quantities were determined from the momentum deficit
in the wake. Artificial stimulation at I0 percent of the model length
149 _
1976007927-158
i__ VI
0.8
U0
,} O. 4
._.L___.._____.
O 0.4 0.8 1.2 1.6 2.0 r
_" X
; /- TURPqLENTBOUNDARY-LAYEREGION
/ UNDERFAVORABLEPRESSUREGRADIENT
LAHINAR BOUNDARY-
LAYERREGION----__
STAGNATION .,,_"19- I .......
L
Figure 2
Favorable Velocity Distribution and Corresponding
Regions over a Boundary-Layer-Controlled Airship
was utilized to obtain turbulent flow. The final test consisted of
one BLC configuration at zero angle of attack with the sole objective
being whether or not the theory predicted the reduction in drag real-
istically. A model of ZP2G-I airship hull was also tested under the
same environment to ensure a true comparison of drag change between
the conventional and BLC airships. The actual comparison of the ex-
perimental and test data is shown in Figure 3. The drag coefficier_ts
of the body are plotted versus the suction quantity coefficient The
plots shown are for a Reynolds number of 4.2 x 10 D. The wake drag and
suction drag are plotted separately. They are then added toae_her and
plotted as total drag. The experimental data is presented in the same
manner. It can be seen that good agreement exists between the theo-
retical and experimental work. This agreement is further borne out
by the pressure distribution. The measured pressure coefficients are
plotted with the theoretical values in Figure 4 for a Reynolds number
of I0 x 10 6.
150
1976007927-159
t.03
TEST
* _ o air II .--
, _ ._'f THEORY,.s *
COVE/$ _ / RL - 4,5 X I06 4.31 X 104 - RL - 4.)1 X 106
CDToT u CDToT r_
..... co. A co. ._
.oi _ _'_CD| o CDs _ _-
_z _. FLAGEDSYNBOLSDENOTE
SHROUDCONFIGURATION J --,t
.OZ .03 .04 .OS .OS
Co
Figure 3
Test & Theoretical Drag Comparison
BLC Model - RL = 4.5 x i0
! 1.00_ ,
0.012
.80 _
.40 0
.20 ----
Cp O --
°.4C --DTM| TEST LR-_-.gra
! '-.6G QToTo LOti SUCTION E9.44
-.8( --a REDSUCTION 34.3Z
NOT RECORDEO
x I t
"0-0_ .! ,4 ,4 ,0 1.0
l- 1
Figure 4
Pressure Distribution Comparison Test
and Theoretical BLC Body
151
1976007927-160
. [
The drag of the BLC airship at all Reynolds Numbers and slot widths,
as determined from the rake, were in excellent agreement with the the-
oretically predicted values. The ideal suction drag also indicated
close agreement although theory appears to be somewhat greater than
the measured values. Other comparisons ef BLC test parameters with
theory also showed excellent agreement. These preliminary tests vali-
dated the drag reduction predicted by theory. The tests not only
showed this excellent agreement with theory, but also demonstrated
this agreement over a sufficient range of Reynolds Numbers to give
credence to full-scale theoretical estimates.
Comparison of Full Scale Performance
In order to compare the performance of a BLC airship with that of a
conventional (ZP2N) airship, a preliminary design was required in
order to consider the impact of all the features associated with each
type that had a bearing on drag besides the hull drag alone. The
,_-'_ scope of this program does not permit comparing airship sizes and the
associated power requirements based on missions but does compare mRs-
sior capability based on an airship size of one million (10 6 ) cubic
feet. Figure 5 compares the total power requirements for the two con-
figurations. A i0 percent reduction in component drag for the BLC
. configuration can be attributed primarily to the fact that outriggers,
nacelles and empennage cables (fins are cantilevered) are not required.
--RL¢ CONFIGURATIONWZTHONE1S FT PROPELLER
_--CONVERTIONAL CONF/GURAT[ONVZTHTWO16.S FT PROPELLERS
..... CORVERTXONALCONFIGURATZONH|TH ORE1G.S FT PROPELLER
OPERATING
1600 A CONVENTIONALCONFIGURATIONWITHTWO1G.S FT PROPELLERS-ANDPLC CONFIGURATIONWITHORE15 FT PROPELLERTOWING
_ 3000 L$ DRAGLOAD
• GLCCOflFIGURATZONWXTHPROPELLERDISC AREAE®AL TO
THATOF CONVENTIONALCONFZGUUTZONTOU|NG3000 LR
._ lllO0" DRAGLOAD
• ZPG-2 FLIGHT ,EST DATA(REP. 34) _/
0 COH_TEOVALUES /I I //|| //
iii/
600 ._/ //2/
,/
,t7
i I I
._ .,_..,",, v,,_ W_TH_,_0,P - 75.S_So I I I I
0 10 _ _ _ _ _ 70 80
VI_OT$
Figure 5
Horsepower Requirements vs Flight Velocity
For BLC & Conventional Airships W = 10 6
152
J _- ° I
1976007927-161
When considering various operational conditions such as single engine _
cruise (normal conventional airship operation) with the corresponding
diffezences in SFC and propeller efficiencies, the BLC airship would _
offer an endurance improvement of between 20 and 40 percent at most _
operating velocities. With a p_opeller comparablc in size to those
used on conventional airships, the improvement in endurance for ASW
towing would be i0 percent when the tow drag is 3000 pounds or 25 per- _ :
cent if the tow load was I00 pounds.
A complete evaluation of the advantages of a BLC airship must encom-
pass many factors including a comparison of the general operational
characteristics of each configuration and the weight allowable for
fuel. Although such an evaluation was beyond the scope of this study,
it is cf interest to briefl_ discuss some of the major BLC operational _ '':
characteristics as they differ from the conventional airship's char-
acteristics.
,
(i) Static instability of an airship is due almost entirely to the
hull and is a function of fineness ratio; C. decreases with decreas-
ing fi_-ness ratio and consequently will re_ire less in the way of a
stabilizing system. As shown in Figure 6 the tail length is substan-
tially the same and due to structural considerations the aspect r_tlo
can be considerably greater.
(2) Low speed control is a prime consideration for airships and with
the BLC airship it can, to a considerable degree, be obtained
by vectoring the outlet air from the duct. This would have i_s great-
est effect during a towing operation such as sonar array towing. :i_
-SLOTklR INTAKEFOR
B_NDARYLAYERCONT_L
.... - ,:
............. .,.. UL
-_,-_.'_-:.-._._'_..._s ,.'CL..- -'_ AhOFIN BASE
Figure 6 ,_
Comparison of BLC Airship with Conventional
For Equal Volume
153
1976007927-162
9(3) Propeller and engine noise interferes not onl).with crew comfort
but also with the effectiveness of the mission equipment; sonar oper-
& ations as an example. The BLC configuration is inherently conducive
_ . to quiet operation; the propeller is shrouded and the distance between
the propulsion unit and the crew is considerably greate_ than is the
case with the conventional design. The aft location of the BLC power _
plant also represents a noise reduction to the crew.
• (4) Other advantages of the BLC configuration are in the areas of
/ elimination of variable pitch protection from physical damage.
I
I CONCLUSIONS ,
The findings of this limited investigation into the boundary-layer- ,e
control airship show sufficient increase in the airship performance _
to warrent further study. The following conclusions are offered:
._.-- (i) The NSRDC wind tunnel tests confirm the ability of the theoret-
ical methods described in this report to predict the boundary layer
control of a body of revolution at zero angle of attack.
(2) The t_eory confirmed by the NSRDC wind tunnel tests together with
: allowance for inlet and duct losses predicts that the bare hull power
requirements for a full scale BLC airship hull of fineness ratio 3.0
at zero angle of attack can be expected to be i0 to 20 percent less
than the power requirements of a conventional airship hull of equal
volume.
(3) The differences in the components other than the hull associated
with the two configurations, offers an additioDal 5 to i0 percent re-
duction in power requirements for the BLC non-rigid airship
L configuration.
' (4) A BLC configuration of fineness ratio 3.0 can be expected to re-
_ _ duce the total propulsive power requirements of a con,,entional non-
rigid airship of equal volume 15 to 25 percent.
(_) If both configurations have equal fuel quantities available, BLC
can be expected to increase the endurance 20 to 40 percent.
(6) Indications exist that the fineness ratio of 3.0 selected for
this investigation may not be optimum for a BLC airship.
The predicted theoretical increase in performance, together with the !
operational advantages, indicated a significant advance in airship
design and led to the initiation of the BLC program. This program,
although limited in scope, has confirmed the validity of the predict-
ed performance improvement. To take full advantage of the results
thus far and fully exploit the potential of the BLC confiquration,
this contractor recommends the following program to continued effort
be initiated:
(i) To refine the merits and limitations of applying boundary-layer
control to air_hips, the following investigations should be initiated:
a) Theoretical power requirement studies for bodies with fineness
ratios less than 3.0 which necessitate further electrostatic tank
testing.
b) Wind tunnel testing to determine the effect of angles of at_ _k.
c) Preliminary design studies to define an operational corA;quration
would, in conjunction with items (a) and (b) above, permit t_e s._iuu-
tion of an optimum and praetleable configuration.
i
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1976007927-163
I(2) To obtain data for the design and fabrication of a BLC airsh!_,
a wind tunnel test of a self-powered model at reasonabl% large
l_eynolds numbers shoul _ be conducted upon completion of Item i above.
REFERENCES:
_ i. Goodyear Aerospace Corporation, A Theoretical Aerodynamic Anal_sls _ °
of a Boundary-Layer Controlled Airship Hull, GER-6251, Akron, ohio _
_14 September, 1_54). " _'
2. Cerreta, P. A., Wind Tunnel Investigation of the Drag of a Propos- _ _
ed Boundary Layer Controlled Airship, Navy Dept., David _ay!or Model "*
_i_ _ Aerodynamics Laboratory, Aero Report 914 (March, 1957).
3. Horshe, M. L., Pake, F. A., and Wasson H. R., Air Investigation
of a Boundar_ La_er Controlled Airship, GER 8399, Goodyear Aerospace
.- Corporation, Akron, Ohio (15 October, 1957}j,- •
i
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1976007927-164


Boundary layer control for airships

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