PROPELLER - AIRFRAME AERODYNAMIC INTERFERENCE ON

ICAS 2000 CONGRESS
PROPELLER - AIRFRAME AERODYNAMIC
INTERFERENCE ON TWIN ENGINE AIRCRAFT
Sergey G. Derishev
Federal State Unitary Enterprise "Siberian Aeronautical Research
Institute" – SibNIA, No 21, Polzunova Str., Novosibirsk, 630051, Russia.
Keywords: twin engine aircraft, propeller, aerodynamic interference, engine positioning,
aerodynamic perfection, thrust, power, propeller efficiency, lift, drag, longitudinal stability.
Abstract
Rational integration of the propulsion system–
airframe allows quite obviously improve the full
economizing of propeller aircraft and to arise
safety of the take-off and landing modes. The
article presents the series of experimental
results, got during the low-speed wind tunnel
testing of some typical configurations of twin
engine aircraft. It’s shown here quite
substantial airframe influence on effectiveness
and normal force of air propellers and also
influence of propellers streams on lift, drag and
longitudinal static stability. On the basis of got
data indexes of aerodynamic perfection for
examined configurations of twin engine aircraft
have been determined. The most potential flight
range may be realized with the traditional
configuration with engines on the wing and
pulling propellers. The best take-off and landing
performances are achieved while engines were
installed on pylons at the tail of the fuselage.
Nomenclature
V∞ , q∞ CL
CD
Cm
-
undisturbed air speed and dynamic
pressure;
lift coefficient;
drag coefficient;
pitching moment coefficient;
T
Ty
-
thrust (axial component);
propeller normal force;
P
ρ
ns
-
power;
air density;
propeller angular velocity, rev/s;
α
η
-
angle of attack, deg;
propeller efficiency;
λ
-
advance ratio ( λ =
B
-
relative
(B =
αx
-
βx
-
αy
-
α αy
-
XF
-
X c .g . δf
-
HS-F HS-T -
V∞
);
ns D
thrust
of
propeller
4T
);
πD 2 q ∞
T
);
ρns2 D 4
P
power coefficient ( β x = 3 5 );
ρn s D
normal
force
coefficient
Ty
( α y = 2 4 );
ρn s D
normal force coefficient due to
angle of attack, deg-1;
dimensionless aerodynamic focus
position relative mean aerodynamic
chord nose;
dimensionless center of gravity
position relative mean aerodynamic
chord nose;
flap deflection angle, deg;
thrust coefficient ( α x =
Horizontal Stabilizer position – on
the Fuselage;
Horizontal Stabilizer position – T –
tail unit (above the fin).
Introduction
Nowadays all over the world it’s developed and
serviced the great amount of twin engine
123.1
Sergey G. Derishev
aircrafts, using different versions of power plant
position.
The most widespread version at the present
time is one with engines on the wing and pulling
propellers. Sometimes engines with pushing
propellers are installed on pylons in the tail of
the fuselage. Aircrafts with engines on the wing
and pushing propellers are also known. The
variety of power plant different positioning on
twin engine aircraft is connected with the
variety of tasks which are carried out by these
mentioned aircrafts and also with absence of
correct assessments for effectiveness of used
variants. How should we choose engine's
position to intensify merits and to weaken lacks
of aircraft configuration or in other words
speaking to provide positive aerodynamic
interference?
Researches of this problem have been held
for many years. However, the effectiveness of
different engine's positions for twin engine
aircraft hasn't been still assessed. The closest
with this work’s goal investigation are the
Williams, Johnson and Yip study of the twin
engine aircraft model with three different
variants of engine's positions [1]. But in this
study there aren’t data of aerodynamic drag, lift
to drag ratio and also effectiveness of propellers,
operating in the airframe constitution.
Analyzing of the longitudinal stability,
effects due to propeller's stream and normal
force on focus location haven’t been separated.
In a whole that study, done at the highest
technical level (in comparison with earlier ones)
doesn’t allow to make any conclusions
regarding the perfection of considered variants.
The aim of present work was experimental
research
of
major
propeller-airframe
aerodynamic
interference
in
typical
configurations of twin engine aircraft and
comparing assessment of the effectiveness in
taken versions in cruise and take-off modes.
Model of Twin Engine Aircraft
providing examination of typical versions for
twin engine aircraft. Horizontal stabilizer was
installed in two typical positions: on the
fuselage and T-typely above the fin. The series
of propeller's blades have been chosen according
to the range of flying speed for the regarding
types of aircraft. Transition stimulators were
installed on all models' surfaces.
After preparing the model, several series of
balance and visual tests were carried out. The
aim was “adjusting” of all tested versions and
the improvement of local aerodynamics.
Through of these tests results the vortex
generators on the engine's nacelle were chosen
together with the fairing in adjoining of nacelle
with the wing were installed. These
undertakings allowed to prevent flow separation
on the wing and to linearize the major
aerodynamic characteristics at subcritical range
of angle of attack. “Adjusting” took place also
in propeller: an angle for propeller's blades had
been choosing, which provided the best
efficiency at cruise mode.
Tests Equipment
The major series of wind tunnel tests was
carried out with using of “disconnecting” power
plant scheme (Fig.2). The model was fixed on
the external electromechanical balance by wire
suspension. In empty-bodied nacelle with a gap,
which excluded mechanical contact, electrical
engine with power 12 kW was installed, and
there was propeller on it’s shaft. The engine was
fixed on the holder of a strain-gage balance,
which was installed on a special following-up
system, providing the stable mutual position of
the model and engine while the angle of attack
was changing during testing.
Used
scheme
of
testing
with
"disconnecting" power plant, allowed to
measure aerodynamic loads, which were on the
model and loads, produced with the propeller,
simultaneously but separately and with the high
accuracy.
The subject of research was a parametric model
of twin engine aircraft (Fig.1). Engine's nacelles
were installed on the model in three positions,
123.2
PROPELLER - AIRFRAME AERODYNAMIC INTERFERENCE ON TWIN ENGINE AIRCRAFT
Tests Conditions
Testing of the model has been conducted in the
low-speed wind tunnel T-203 SibNIA with open
test section 2.33 by 4.0 m. A speed was 40…60
m\sec, which corresponds to Re number
(0.7…1.0).106, determined through the mean
aerodynamic chord of the wing. Re number of
the propeller, determined with the chord of
blade, which is away from the rotation axis in a
distance of 75% of propeller radius, was 0.3.106
during the cruise mode.
The necessity of correct comparing the
results of wind tunnel tests of model's different
versions determined some limits, which were
satisfied during the testing. Comparison of
aerodynamic characteristics through the
investigated configurations was done at the
following parameters:
- Re, M number – idem;
- idem (fixed) position of the laminarturbulent transition lines;
- idem lift coefficient at
cruise mode – C L .cruise = 0.5;
take-off mode - Ñ L .lim , corresponding to
the speed which was 10% more than
stalling speed;
∂C m
= -0.1,
- idem the static stability:
∂C L
determined at C L = 0.5 and zero relative
thrust of the propellers.
The last condition was satisfied with the
definite choosing of center of gravity position.
Relative propellers thrust at cruise mode was
taken from the equality of thrust and drag in
horizontal flight. At take-off mode the relative
thrust was taken according to statistic B=1.5.
It is known when air compression doesn’t
influence a lot, similitude of aerodynamic loads
on a propeller and kinematic similitude in its
stream for wind tunnel model and full-scale are
provided with equality of relative thrust and
advance ratio - a specific form of Strouhal
number. Idem Re number for all compared
propellers allowed to satisfy with the only
criteria – relative propeller thrust and to
determine
changing
of
aerodynamic
characteristics as a function of this parameter.
In used approach, each series of testing
contained two stages. At the first stage
aerodynamic coefficient ( C int ) were determined
for the model with the imitator of power plant –
nacelles and propeller's spinners (without
blades) were done solid together with the model.
At the second stage testing of model with
operational propellers was conducted and the
increments of aerodynamic coefficients vs.
relative thrust were determined. After this the
increments were summed with the coefficients
C int .
In the testing with the operational
propellers the following parameters was
measured: axial thrust, normal force, reactive
moment and propeller's angular velocity. With
these measured loads thrust, power and normal
force propeller's coefficients, relative thrust and
efficiency of propellers were calculated.
Tests Results and Analysis
Propeller
Aerodynamic characteristics of propeller depend
on its position on an aircraft. Exactly speaking
they depend on kinematic parameters of flow
field, which is formed with plane components in
the propeller's position (Fig. 3). Pulling
propeller, which is streamed with uniform flow,
has the highest efficiency.
Effectiveness of the propeller turned out to
be lower in the 2 configuration. Its maximum
efficiency was 10% less. The reason of such big
difference is strong nonuniformity of the flow,
which streamed the propeller in this case.
Propeller's efficiency in the 3 configuration
was lower than in the traditional one, but the
difference wasn’t so big.
When the propeller operates in the
dawnwash flow the force appears in the plane of
rotation. This force, called “normal” is
proportional to the angle of attack. The normal
force magnitude of the propeller is very low in
comparison with a lifting force of an airplane.
123.3
Sergey G. Derishev
But when it acts in a long distance from the
center of gravity, it can influence a lot on the
aerodynamic focus position of an aircraft, so on
the parameters of the longitudinal stability.
As the test results showed, the magnitude
of normal force coefficient at fixed relative
thrust can be quite exactly approximated with
the linear function of angle of attack.
With this reason we can consider of the
derivative of normal force coefficient due to
angle of attack as the validate estimation, which
characterize propeller normal force in the
subcritical angle of attack range, and use it for
the parameters of longitudinal stability
calculation. Normal force coefficient depends
on the real angle of attack of propeller axis, so it
has the highest magnitude in the 1
configuration.
Downwash of the wing decreases of the
normal force: more in the 2 configuration and
less in the 3 one.
Airframe
Operational propellers influence on the lifting
ability of configurations – derivative C Lα and
coefficient C L max are raising as soon as
coefficient B is increasing, especially while the
flap is deflected (Fig. 4).
It’s necessary to pay attention on increasing
of C L max in the 2 configuration, where the wing
is not washed with propeller's streams. The main
reason here is the reduction of positive pressure
gradient on the wing due to the suction of the
propellers.
Propeller's streams and suction of
operational propellers influence on the
aerodynamic drag: coefficients C D min increasing
similarly in the 1 and the 2 configurations and
less in the 3 one.
The maximum magnitude of the lift to drag
ratio is decreasing due to the relative thrust. The
most reduction is in the 1 configuration and the
smallest one is in the 3.
Propeller's streams effects on the
aerodynamic focus position. In fig. 5 changes of
the focus coordinate vs. B coefficient for three
configurations are given in cruise and take-off
modes. In the 1 configuration the influence of
propeller's streams is promoted of focus
displacement ahead. This displacement depends
on the position of horizontal stabilizer a little.
The main reason here is increasing of downwash
flow by the wing at the tail unit.
In the second configuration the horizontal
stabilizer at the fuselage is blown with the
propeller's streams, but in T-type unit isn’t. This
fact has an effect on focus displacement.
Effectiveness of blown stabilizer is raising and
focus of the model “goes” back. In T-type unit
the influence of increasing downwash dominates
and focus moves ahead.
In the 3 configuration the propeller's stream
influence on focus position is not revealed.
Normal force of the propellers, allocated in
front of the center of gravity (as in the 1
configuration) reduces of the longitudinal
stability. If propellers are allocated behind the
center of gravity so does in the 2 and the 3
configuration the propeller's normal force moves
the aerodynamic focus back and degree of
longitudinal stability increase.
Mutual influence of propeller's streams and
normal force leads to total loss of longitudinal
stability on take-off modes in the 1
configuration. It leads to raising of longitudinal
stability in the 2 configuration with the
horizontal stabilizer on the fuselage, so that to
increasing of trimmed losses of lifting force and
lift to drag ratio.
In the 2 and the 3 configurations with Ttype tail unit, propellers operation doesn't
influence a lot on stability changing.
For correct account of trimmed losses and
comparing effectiveness of the configurations
the new center of gravity position and respective
parameters of longitudinal stability was
determined for all possible flight modes. The
new center of gravity position was chosen on
some compromise between providing safety of
take-off mode and most reduction of losses in
cruise flight.
123.4
PROPELLER - AIRFRAME AERODYNAMIC INTERFERENCE ON TWIN ENGINE AIRCRAFT
Influence of propellers on an aircraft static
longitudinal stability is determined with two
dominating
factors:
propeller's
streams
interference and normal force of propellers. The
magnitude of normal force depends on
kinematic parameters of flow field, formed with
airframe elements. In configurations, where
propellers operate in downwash flow, their
normal force coefficient is less than the same
coefficient of a separate propeller. Minimal
unfavorable influence on static longitudinal
stability has place in configurations where
horizontal stabilizer is not blown.
General Efficiency
The following characteristics of cruising mode
are given in the table: lift to drag ratio with
taking into account all of losses, requiring cruise
relative thrust, efficiency of propellers operating
in a constitution of aircraft and potential flight
range (range by Breguet).
Comparing got data it’s easy to see that the
most perfect in cruise mode is traditional
configuration with pulling propellers. The best
take-off and landing characteristics – the highest
lift coefficient, effectiveness of high lift devices,
Cruise mode
Configuration
L
 
D
(trimmed
condition)
HS-F
HS-T
1
2
1
2
3
10,7
10,1
10,6
9,9
10,6
B
(trimmed
condition)
0,151
0,161
0,152
0,164
0,152
take-off lift to drag ratio and biggest thrust
excess for acceleration and climbing are realized
in the 3 configuration, having “clear”, not
shadowed with nacelles, wing.
Conclusions
1. Substantial influence of propellers position
on twin engine aircraft was exposed. The
biggest propeller's efficiency is realized in
traditional configuration with pulling propellers,
operating in uniform flow. When engines are
located on the pylons the propeller's efficiency is
lower a little. Pulling propellers allocated
behind the trailing edge of the wing operate with
the worst effectiveness.
2. Influence of the propellers on
aerodynamic characteristics is seen in its raising
of lifting ability, raising of aerodynamic drag
and reduction of lift to drag ratio.
η
0,77
0,67
0,77
0,67
0,73
L
C L .lim
1.0
0.82
1.0
0.80
0.94
1,2
1,2
1,2
1,2
1,3
Take-off mode
L
 
D
BTO − BHF
(trimmed
condition)
7,8
8,5
8,2
9,2
9,9
1,00
1,04
1,02
1,07
1,07
3. Comparing assessments of potential
flight range for typical configurations of twin
engine aircraft were got. The most potential
flight range is achieved on aircraft with
traditional engine position on a wing and pulling
propellers. However much better take-off and
landing characteristics may be achieved on the
configuration with “clear” wing. The worst
effective in cruise flight is the configuration
with engines on a wing and pulling propellers.
Its potential flight range is only 80% of the
range with traditional configuration.
References
[1] Williams K.J., Johnson J.L. and Yip L.P. Some
Aerodynamic Considerations for Advanced Aircraft
Configurations. AIAA Paper 84-0562.
123.5
Sergey G. Derishev
1
3
2
Horizontal stabilizer
T-tail unit
on the fuselage
Fig.1. Versions of model's configuration
Electrical motor P=12kW
Wire suspension
of external balance
Elastic air-tight seal
Following-up device
Strain-gage balance
Fig.2. Structure of testing rig
123.6
PROPELLER - AIRFRAME AERODYNAMIC INTERFERENCE ON TWIN ENGINE AIRCRAFT
0.2 0
0.2 5
0.1 5
0.2 0
P o w er co efficien t (βx )
T h r u s t coefficien t ( αx )
α=−4...6°; δf = 0
0.1 0
0.0 5
0.1 0
0.0 0
0.0 5
0.5
0.6
0.7
0.8
0 .9
1.0
A d v a n ce ratio
1 .1
1.2
0.5
0.6
0 .7
0.8
0.9
1.0
A d v a n ce ra tio
1 .1
1.2
0.0 06
N o rm a l force co efficien t d u e to A O A
0.8 0
0.7 5
0.7 0
E fficien cy
0.1 5
0.6 5
0.6 0
0.5 5
0.5 0
0.0 05
0.0 04
0.0 03
0.0 02
0.0 01
0.0 00
0.0 0 .2 0.4 0.6 0 .8 1.0 1.2 1 .4 1.6 1.8
R ela tive th ru s t
0.0 0 .2 0.4 0.6 0 .8 1.0 1.2 1 .4 1.6 1.8
R ela tiv e th ru s t
F ig.3. P ro p elle r ch a ra cteristics
123.7
Sergey G. Derishev
, d eg - 1
Lα
0.110
C
δ =0
f
C
L max
1.6
0.105
1.5
0.100
1.4
0.095
1.3
0.090
1.2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
R elativ e th ru st
C
D m in
0.050
δ =0
f
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
R ela tiv e th ru s t
( DL ) m ax
13.0
12.5
0.045
12.0
0.040
11.5
0.035
11.0
0.030
10.5
0.025
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
R elativ e th ru st
C
L max
δ = 20°
f
10.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
R ela tiv e th ru s t
( DL ) T -O
2.0
12.0
1.9
11.5
11.0
1.8
10.5
1.7
10.0
1.6
9.5
1.5
9.0
1.4
8.5
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
R elativ e th ru st
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
R ela tiv e th ru s t
F ig .4 . P ro p eller strea m s in flu en ce o n th e a irfra m e a ero d yn a m ics
(T -ta il u n it, trim m ed lo sses a ren 't ta k in g in to a cco u n t)
123.8
PROPELLER - AIRFRAME AERODYNAMIC INTERFERENCE ON TWIN ENGINE AIRCRAFT
HS -F
δ = 20 °; C L = C
L lim
f
0.15
0.15
0.1
0.1
0.05
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
F o cu s d is p lacem en t
F ocu s d isp la cem en t
δ = 0; C = 0.5
f
L
-0.05
0.05
0
0
0.2
0.6
0.8
1
1.2
1.4
1.6
1 .4
1.6
-0.05
-0.1
-0.1
R ela tive th ru s t
R ela tive th ru s t
HS -T
δ = 2 0 °; C L = C
L lim
f
δ = 0; C = 0.5
f
L
0.1
0.1
0.0 5
0.0 5
0
0
0.2
0.4
0.6
0 .8
1
-0 .0 5
-0 .1
1 .2
1 .4
1.6
F ocu s d is p lacem en t
F ocu s d is p lacem en t
0.4
0
0
0.2
0.4
0.6
0 .8
1
1 .2
-0 .0 5
-0 .1
-0 .1 5
-0 .1 5
R ela tive th ru s t
R ela tive th ru s t
F ig . 5 . A e ro d yn a m ic fo cu s d isp la cem en t d u e to pro p elle r strea m s
123.9
SergeyAIRCRAFT
G. Derishev
PROPELLER - AIRFRAME AERODYNAMIC INTERFERENCE ON TWIN ENGINE
H S -F
δ = 20 °; C L = C
L lim
f
δ = 0; C = 0.5
f
L
0.1
0.1
0.0 5
0.0 5
0
0
0
0.2
0.4
0.6
0 .8
1
1 .2
1 .4
1.6
0.2
0.4
-0 .1
0 .8
1
1 .2
1 .4
1.6
1 .4
1.6
-0 .1
-0 .1 5
-0 .1 5
-0 .2
-0 .2
-0 .2 5
-0 .2 5
-0 .3
-0 .3
R ela tiv e th ru s t
R ela tive th ru s t
H S -T
δ = 0; C = 0.5
f
L
δ = 20 °; C L = C
L lim
f
0.1
0.1
0.0 5
0.0 5
0
0
0
0.2
0.4
0.6
0 .8
1
1 .2
1 .4
1.6
0
0.2
0.4
0.6
0 .8
1
1 .2
-0 .0 5
X c. g . - X F
-0 .0 5
X c. g. - X F
0.6
-0 .0 5
X c. g. - X F
X c. g. - X F
-0 .0 5
0
-0 .1
-0 .1
-0 .1 5
-0 .1 5
-0 .2
-0 .2
-0 .2 5
-0 .2 5
-0 .3
-0 .3
R ela tiv e th ru s t
R ela tive th ru s t
F ig .6 . D istu rb an c e o f sta tic lon g itu din a l sta b ility
d u e to p ro p eller stre a m s a n d n o rm a l forc e
123.10
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