Crigler, John L

English

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NACA ACR NO. L5A11
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
ADVAltiCECONFitiTIti -REPORT ““ “‘ ‘“- -
!lTIEFFEOT OF TRAILING-EDGE EXTENSION FLAPS
ON PROPELIMI CHARACTERISTICS
By John L. Criglen?
STIMMARY
k analysls was made to determine the effect on
propeller performance of extension flaps added to the
trailing edge of a proneller blade. A method of calcu-
lating the chcrgas fn the Ideal an~le of’attack, the
an@e of zero 1.if2,and the design lift coefficient of
a propeller blade section having a trailing-edge exten-
sion flap was utll?.zedto calculate the performance of a
stx-blade dllal-rotatingpropeller with extension flaps
varyin~ up to ~.Opercent chord. ThG method was l15edto
determine the arqjlethat the flap oxtonsion must maka
with the chord in order to obtain a particular load
di~tribution. Althou@ tha analysis in this report was
mti’?efor a wind-tunnal ~ropeller dosi~ned to cperate at
low alivunce-diameterratio, the motlmd is directly &ppli-
cable to any propeller section under any operating
condition.
INTRODUCTION
Inasmuch as the production of a propeller of a
given design Is an expensive manufacturing procedure,
it Is expedient to make each existing design useful for
as many applications as possible. For this reason,
several choices of diameter have been available with a
given blade design. This flexibility of design has
recently been increased by providing a prooedure for
adding an extension along the blade. me selection
of’the width of extension pcmmlts a choice of’ pro eller
solidity for a given blade design and diameter. %2
addition of the tralllng.edge extension changes thee
Seotton airfoil characteristics by an amount dependent
2 NACA ACR NO. L5AM
on the length and angh of extension. Some choloe
In the airfoil charaote~lstics Is therefore permitted -
when the extension flap is added to the trailing
edge. Reference 1 presents a method of analyzing the
change In airfoil section charaoterlstics accompanying
changes in extension length and angle, and the present
report applies the methcd of referenoe 1 to tbm calcula-
tion of propeller characteristics.
The calculations givm heretn were made for a six- “
blade dual-rotatln propeller for values of advance-
diameter ratio (V~nD) that are encountered in the
operat?.onof a propeller used to drive a wind tunnel.
The same methods are applicable, however, to propellers
for any operating condfl.tion.Preliminary calculations
were first made on the propeller with the original
blades in order to study the distribution of loading
along the blade fdr a power coefficient Cp = C.31 at
an advance-diameter ratio V/nD = 0.33. Pbr this value
of v/ilD, the inboard sections of the propeller were
found to stall before the outboard sections and,
furthermore, the whole proneller wss found to stall
before a power coefficient of 0.31 was absorbed. In
order to make the design suitable for these operating
conditions, it was necessary to increase the solidity
of the original propeller. Trailing-edge extenslen
flaps were used for this purpose and were attached in
such a manaer as to change the angle of’zero lift along
the blade to increase the load on the outer sections.
Inasmuch as the effect of such extension flaps is
applicable to both tunnel and aircraft propellers, the
method given herein for use in tunnel-propeller design
may also be used Zn determining the effect of trailing-
edge extension flaps on propeller sections for aircraft
applications.
The method used to calculate the changes in the
ideal angle of attaok, the design lift coeff~cient,
and the angle of zero lift resulting from a flat sheet
attached to the trailing edge of an airfoil section is
outlined in reference 1. This method was used to
calculate the lift as a function of angle of attack for
the sections of a six-blade dual-rotating nropeller
having Curtiss blades 836- and d3T-lc2-13 with trailing-
odge extension flaps. The calculated thrust- and
torque-distribution curves for the propeller with a
@=percent-chord trailing-edge flap are presented for
two operating conditions.
\ NACA ACR No. L5A11 1~) 3
SB!BOLS
chord of propeller blade element
section llft coefficient
()
*..
section design lift coefficient; lift coefficient
at Ideal angle of attack
power coefficient fp/pn3~5)
torqre ?oefflc~ent (Q!W2D5)
(L)d dxelerl.ert:orqu9 coef’f~ci9nt \Pn2D5
*
()
element thrust coe.fflclent -
pndd+
thlclmess of nroneller b?.ad.element
lift of blade section
Mach number
propeller rotational spead
geometric pitch of propeller
input Dower to propeller
torque of proneller
radius to any blade element
tip radius
thrust of propeller
4v
x
a
al0
‘I
P
e
P
airspeed-. .
radial location of blade element
angle of attack
angle of zero lift
ideal angle of attaak
?WCA ACR i~O..L~ll
(r,&)
propeller blade ar.gleat 0.75 radius .
propeller blade anglo at radius r
mass density of air
Subscripts
~ front propeller
R rear propeller
0.7 at 0.7 raldiua
The propeller analyzed is a six-blede dual-rotating
f’36-1c2-15 (front, rightpropeller with Curtiss blades ~
hand) and 8~~-lc2-13 (rear, left hand). Blade-form curves
for the propeller are given In fi~9 1. “me propeller
cond~.tionsanalyzed vary fro~L a v&lU~ of Cp = ().31
at V/nD = 0.33 to a value of Cp = 0.095 at v,41D= 0.26.
Preliminary calculations showed l%at the propeller with the
ori~inal blades would stall at an advance-diameter ratio
of 0.33 before absorbing a power coefficient of 0.31. In
order to use the available propeller for this condition,;+
it was necessary to increase the propeller solidity by the
use of extension flaps attach.edto the trailing edge.
Extension flaps cause a change in the angle of zero lift,
which results in an effective c-nangein the propeller
pitch distribution. It was necessary, therefore, to
calculate the lift of the sections as a function of angle
of attack for use on tb.epropeller. The method of
. .—— , ,- -, .,-. , ,,, mm — ,, ,
.reference 1 was used to calculate the change In lift
oharaoteristlcs caused by extension flaps, and the results
show the angle that w1ll be required between the extension
flap and the ohord line of the original airfoil to
produce zero change in pitch distribution for several
sections along the blade.
Certain assumptions regarding the atrfoil character-
istics of the propeller were necessary In order to make
the calculations. Experimental data are usually used in
analyzing propeller performance. Ihasmuch as the Curtiss
blades 836- and 83Y-1c2-13 are of NACA 16-series airfoil
section, the section lift characteristics (fig. 2) for
the original propeller were obtained by extrapolating
tk experimental data of reference 2. “The design llft
coefficients and the operating Mach numbers for several
sections along tb blade for the limitlng condition
of operation are shown in figure 2. The calculations
for the sections with extension flaps were made on the
assumption that the addition of flaps did not change
the slope of the lift curve for a given section.
Inasmuch as no experimental data were available
for the airfoil sections with extension flaps, it was
necessary to use theoretical calculation In analyzing
the performance of propellers with these sections.
The calculated and experimental values of ato for
the original sections are not in perfect agreement.
Since experimental data were used for the original
sections, the differences between the calculated
values for the original and the extended airfotl section
characteristics were applied to these experimental
data. The corrected values were then used in calculating
the perfommnce of the propeller with extension flaps.
RESULTS AND DISCUSSION
Computations were made to determine the effect of
the trailing-edge extension on the lift characteristics
of an airfoil as a function of angle of attack. Curves
showing the results of some of these computations are
. presented In figures 3 to 6. The calculated ahgle of
zero lift azo, the ideal angle of attack aI,
and aI - azA (on which the design lift coefficient
depends) are”given for the propeller section at the
6 NACA ACR Ho. L5.111
0.45 radius in figure 3. The calculated angles are
measured from a straight line joining the extremities
of the mean camber line of the extended airfoil seotlon
but are plotted against the angle between the extension
flap and the strai@t line joining the extremities of’
the mean camber line of the original airfoil section,
as was done in reference 1. The effects of a
10-percent extension, a 20-percent extension, and a
4.O-percentextension are compared.
In the use of this information for propeller
calculations, it is more convenient to refer the
calculated angles to the line joining the extremities
of the mean camber line of the original airfoil section.
The angular difference al between the two reference
llnes is given by the following formula:
( )Extension lengthChord sin (Angle of extension)tan-lal =
1+
F )
tension len,gth
Chord
cos (Angle of extension)
The results for the 0.45 radius plotted in figure 3 are
replotted in figure ~but in figure 4.the calculated
angles are measured from the chord line of the original
airfoil section. The calculated and extrapolated
experimental values of
=2o and aI - al. for the
original section (without flap) are shown in figure 4..
The points on the curves also show the calculated
angles at which the extension flap must be set to the
chord llne of the orighal section to give the same value
of al. or aI - =20 for the extended section as for
the original section. If the values of azo for the
sections with the extenston flap are the same as for
the orlglnal sections, the pitch distribution is
unchanged. Since al for the original section
(16 series) is zero, the crossing of the aI mrves
with the zero ordinate gives the angle of extension
for an unchanged aI. Figure 5 shows similar curves
for a 20-percent-chord extension flap and for a
40-percent-chord extension flap at the 0.95 radius.
In figure 6 the curvas at several radii are compared
for the 20-percent-chord extension. From curves of
this type, any desired changa in the pitch distribution
of a propeller may be made by properly setttng the
extension-flap angles.
1NAC.4AC:fl~\;O. L5A11 ,~, 7
Since the inboard sections of the original propeller
stalled much earlier than the outboard sections for
low v/’rlD operation, it was decided to c’nange the ..
angles of zero lift of the blade sections in order
to shift more of the load toward the tip. This change
in the angle of zero lift is obtained by setting the
flap extension at the proper angle to the chord line.
The angles of zero lift of the blade sections were
changed by the amount shown by the solid. line in
figure 7. This curve may be shifted up or down, as
is shown by the dashed lines in fiSure 7, with no
change in the load distribution; the only changes
resulting are in the design lift coefficients and a
constant shift in the angles of zero lift of’ the
sections. In making the nropeller performance calcu-
lations, however, a shift in the angles of zero lift
results in a change in the ~ropeller Pitch setting for
constant Cp and V/nD. The only change in CT and rj
will result from the small effect of th,e change in the
drag of the airfoil sections.
~xamination of the results (see figs. 4. to 6)
shows that a 20-nercent-chord extension to the Curtiss 836-
or 337-1c2-13 blade should be set about 7.2° to the
chord line at all blade sections to give the various
angles of zero lift that would be obtained by
adding Aazo (solid line in fig. 7) to aLo of the
original section. A )+0-:percent-chord extension should
be set about 6.)+0at all blade sections. The angles
of zero lift for the sections at all radii given in
figure 2 were increasedby the amount shown by the solid
line in figure 7 for making the calculations of the
propeller performance with the extension flap.
Analyses of propeller performance for several pro-
pellers indicate that single-rotating propellers stall
at section lift coefficients of about 1.0 for most of
the blade and that the thin sections near the tip stall
at section lift coefficients of 0.8 to 0.9. The calcu-
lations presented herein show that these lift coeffi-
cients were realized for a pitch setting of 2)+0 at the
0.7 radius for operation at V\nD = 0.33 and that a
,!+O-percent-chord extension (lO-percent increase in
solidity) is required to absorb the power. Experimental
data on dual-rotating propellers, however, show that
the dual-rotating propeller can be operated without
stall at higher blade angles and at higher section lift
,, ,,, , ,,,-,. ! !m. ! .! !-.. ! ,.. .. ..
8coefficients than sin~le-rotati.ilg propellers. It is
quite possible$ there~ore, that th’e ~0-percent-chord
flap extension to the tunnel..propeller that would be
required for a single-rotating wropeller will not be
necessary to prevent stall for “~he lii;litin~ condition
of operation with dual
-~otatfi.ng propellers and. that a
lower solidity may be used. Nevertheless, in order to
obtain conservative results, the calculations for the
tunnel propeller have been .nade on the basis of a
)+0-percent-ckord flap extension and the results for
two operating con(;.itions are given.
Figure 8 s’hews the differential thrust and torque
curves plotted against x for Oneration at a V/nD
of 0.35 with the .front-uropeller” blade angle set 2)+0 and
the rear-propeller k!.ade afi~le sat 23° at the 0.7 radius.
The element lift coefficients at several section radii
are shown in this ~j.:y~~e . yj.:u~e
~ shows similar curves
for o?>eration at V/nD = 0.26 wttb.the front-propeller
blade aj;gle set 12° and the rear-propeller blade angle
set 11° at the 0.7 radius.
The solidity of a six-blade dual-rotating propeller
}]avi]lgCurtiss ~j3~- and 857-1C2-15 blades has been
increased by addin~ extension flaps .to th-e trailing edge .
The method of analyzinS the new blade-section characteri-
stics in this case was av:~lied to a particular 9ropeller
for ormration. at a very l.o-~vadvance-diameter ratio, but
the m~thod. may be anpl~ed to arty propeller section under
anj~ operating condition. The :Oitch distribution of the
propeller with flaps may be :held constant or, if desired,
may be varied for differen-t design operating conditions
by properly setting the flap ang:le.
Langley lle-mor~-alAeronautical La?]oratory
Nat5.onal Advisory Committee for Aeronautics
Langley Fieldj Va.
NACA ACR No. L5A11 ~ 9
REFERENCES
.
1. Theodorsen, Theodore, and Stickle, George W.: Effect
of a Tralllng-Edge Extension on the Characteristics
of a Propeller Section. NACA ACR NO. I@21, 1944.
2. Stack, John: Tests of Airfoils Designed to Delay the
Comprosslbllity Burble. NACA TN No. 976, hC. 19&
(Reprint of ACR, JunB 1939.)
—. — ..—. —
NACA ACR No. L5A11
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The effect of trailing-edge extension flaps on propeller characteristics

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