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Influence of propeller configuration on propulsion system efficiency of multi-rotor Unmanned Aerial Vehicles

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Influence of propeller configuration on propulsion system
efficiency of multi-rotor Unmanned Aerial Vehicles
B. Theys
, G. Dimitriadis, P. Hendrick, J. De Schutter
KU Leuven, Leuven, Belgium
ABS TR ACT
Multi-rotor Unmanned Aerial Vehicles make use
of multiple propellers, mounted on arms, to pro-
duce the required lift. This article investigates
the influence on propulsion system efficiency
in hover due to the configuration of these pro-
pellers. Influence of pusher or puller configura-
tion of the propeller, number of blades, shape and
dimensions of the arm, coaxial and overlapping
propellers, is presented. A dedicated test bench
that allows testing of various experimental setups
is designed and built in order to realistically rep-
resent multi-rotor arms. Test results show that a
two-bladed pusher configuration is most efficient
and slenderness of the arm has more influence on
efficiency than shape. A coaxial propulsion sys-
tem approaches the efficiency of a single-prop
system at high disk loadings. Finally, interfer-
ence effects due to overlapping propellers are
discussed.
1 INTRODUCTION
Multi-rotor Unmanned Aerial Vehicles capable of Verti-
cal Take-Off and Landing (VTOL UAVs), are increasingly
deployed for various applications such as surveillance, au-
tonomous parcel delivery, oil and gas spill detection and fire-
fighting. Some of these applications demand long flight times
which are often hard to achieve with the battery powered elec-
trical propulsion system most multi-rotors use [1]. In order to
extend the flight time, energy losses must be minimized. En-
ergy losses occur in the battery due to internal resistance, the
electronic speed controller (ESC) and the motor, as presented
in [2]. Also the propeller and propulsion system configuration
present losses due to interference with the multi-rotor arms
and mutual interference between propellers. All these losses
are schematically presented in figure 1. While many papers
focus on the blade geometry of the propeller [3] or propeller-
wing interaction losses in case of fixed-wing UAVs [4], this
paper focusses on the effects of propeller configuration on
the propulsion system efficiency of the multi-rotor. Five de-
sign choices that are often made during the design process
of a multi-rotor are discussed in this paper, and investigated
Email address: bart.theys@kuleuven.be
by a set of experiments. The first choice is between mount-
ing the propellers downstream of the multi-rotor arms or up-
stream, often referred to as ‘pusher’ and ‘puller’ configura-
tion respectively. A second design choice is the number of
blades of the propeller; to this end a comparison between a
two-blade and a three-blade propeller is presented. The in-
fluence of shape and thickness of the multi-rotor arm is dis-
cussed as a third design choice. Not much research has been
published on this topic, however Fernandes [5] experimented
with different arm shapes and found a small influence on the
performance of the UAV. Finally, the mutual influence of pro-
pellers in coaxial and overlapping setups is presented. A simi-
lar study was performed by Nandakumar [6]: propellers were
mounted with an overlap and a vertical offset, and different
combinations were tested. In [7], a mathematical model and
a computational-fluid-dynamics analysis is discussed for pro-
peller wake interference on multi-rotor UAVs in high-speed
forward flight.
2 THEORY AND DEFINITIONS
The propulsion system of a multi-rotor UAV consists of
a battery, ESC, motor and propeller. Motor and ESC convert
electrical power into mechanical power, delivered at the shaft
of the motor. The efficiency of motor and ESC combined is
therefore calculated as:
ηmot+esc =ω Q
U I =ηmot ηesc (1)
For the test setup in this paper, angular speed ωand torque
Qare measured at the motor shaft, while voltage Uand cur-
rent Iare measured at the power source. The propeller uses
the mechanical power to accelerate the air going through the
disk area Adescribed by a rotating propeller. In hover, the
efficiency of the propeller ηplr (also referred to as Figure
of Merit in helicopter theory) is calculated using momentum
theory as described by Rankine - Froude [8]:
V0= 0 (2)
V1=vi(3)
T= 2 ρ A (vi)2(4)
vi=sT
2ρ A (5)
Pi=T V1(6)
ηplr =Pi
Pmech
=T(3/2)
Q ω 2ρ A (7)
Here, V0is the airspeed ahead of the propeller, V1is the air
speed at the propeller disk and V2is the velocity behind the
propeller disk after contraction of the flow tube as illustrated
in figure 2. Tis the thrust produced by the propeller. viis the
induced velocity: the increase in velocity, induced by the pro-
peller disk. The efficiency of the propeller ηplr is calculated
as the ratio between the induced power Piand the required
mechanical power Pmech.Piis the theoretically required
power to achieve the increase in air speed. A larger propeller
diameter increases the disk area and decreases the required
induced power. An innovative high-endurance multi-rotor us-
ing this principle is described by Verbeke [9]. The efficiency
of the total propulsion system is calculated as:
ηprop =Pi
Pele
=T(3/2)
I U 2ρ A =ηplr ηmot ηesc (8)
Here, Uis the voltage of the power source and Ithe delivered
current to the propulsion system. In this paper the disk area
Ais defined as the projected surface of the turning propeller.
For a coaxial pair of propellers, this area is the same as for a
single propeller of the same diameter.
Battery
Propeller
Arm
battery losses
ESC losses
cable losses
propeller losses
propeller configuration losses
motor losses
Motor
ESC
Fig. 1: Overview of losses from energy source to kinetic en-
ergy in the air.
2
V
1
V
0
V
T
Fig. 2: Momentum theory applied on a propeller in hover
condition.
3 EXPERIMENTAL SETUP
Thrust and power, as well as rpm and torque are measured
in order to calculate the efficiency of the motor with ESC and
propeller separately using equations (1) and (8). Compara-
ble test setups were described and used by Kotwani [10], As-
son [11] and Hossain [12]. All these setups, however intro-
duce a relatively large interaction between the propeller and
the setup that is representative for a fuselage of a fixed-wing
UAV but not for a multi-rotor arm. Figure 3 presents the ex-
perimental setup used in this paper. The setup is designed in
a way that the addition of sensors introduces only a minimal
change in shape compared to a typical multi-rotor arm. To
this end, the torque of the motor is transmitted through a lever
inside the arm and measured with force cell 1. This force
cell is positioned at a lower point on the arm, out of the pro-
peller slip stream and allows for measuring up to 1Nm with
a resolution of 0.001Nm. The arm with motor, propeller and
torque sensor is mounted onto a pivot point. Thrust is mea-
sured by force cell 2, at the other end of the pivot. Force cell 2
measures forces up to 50Nwith a resolution of 0.01N. Rpm
is measured by analysing the three-phase current between the
ESC and the motor. The number of pulses per minute is di-
vided by the number of poles of the DJI 2212/920KV motor
in order to obtain the rpm. Voltage and current are measured
at the DC power source, therefore power losses in the cable
between the power source and ESC are incorporated in the ef-
ficiency measurement of the ESC+motor. The voltage of the
power source is kept constant at 11.1Vthroughout all experi-
ments. This corresponds to the nominal voltage of a three cell
lithium-polymer battery and is a suitable voltage for the com-
bination of the motor with a Graupner 9x5 E-prop. The air
density is taken into account, as calculated from a barometer,
thermometer and humidity sensor measurement.
pivot point
force cell 1
force cell 2
contact point
pivot axis
Fig. 3: Basic overview of the setup of the experiments.
4 EXPERIMENT
A two-bladed Graupner 9x5 E-prop is used for all experi-
ments described in this paper, except for the comparison with
a three-bladed Graupner 9x5 E-prop. All experiments fol-
low the same procedure: while holding the input voltage con-
stant at 11.1V, the PWM (Pulse Width Modulated) signal to
the ESC is increased in steps of 100µs from 1200µs up to
1900µs and then again decreased by the same steps. This
procedure gives two measurements per data point and aver-
ages out any hysteresis effects.
4.1 Pusher vs.Puller
The arm of the VTOL UAV supports the motor with pro-
peller and can be mounted upstream or downstream of the
propeller, respectively referred to as puller and pusher config-
uration. Both configurations are illustrated1in figure 4. This
figure shows that the ‘pusher’ configuration requires a more
complex landing gear in order not to obstruct the propellers.
The efficiency of a two-blade and three-blade Graupner 9x5
Fig. 4: Left: a multi-rotor with propellers in puller configu-
ration. Right: a multi-rotor with propellers in pusher
configuration.
propeller is measured up to full throttle for both pusher and
puller configuration. Results are presented in figure 5. The
left graph shows the propeller efficiency as a function of the
disk loading of the propeller. The graph shows a strong in-
crease of propeller efficiency that flattens out for higher disk
loadings for all four configurations. A quadratic fit is made
through the data points and presented as a solid line. For
clear comparison, the relative difference with the two-blade
puller configuration is plotted in the right graph of figure 5.
For both propellers, the pusher configuration results in an effi-
ciency improvement of roughly 3% for disk loadings between
25N/m and 120N/m.
4.2 3-blade vs. 2-blade
Most multi-rotors make use of propellers with two blades
and these propellers are also most widely available. Some
manufacturers also offer three-bladed versions of some of
their propellers. In the following experiment a comparison
is made between a two-blade and a three-blade Graupner 9x5
E prop. The data presented in figure 5, show that the three-
bladed propeller is less efficient with an average drop in effi-
ciency of around 4%. However, more blades result in a lower
1Source: http://www.dji.com/ and http://aeryon.com/.
0 50 100 150
30
35
40
45
50
55
60
65
70
Disk Loading [N/m²]
Propeller efficiency [%]
2−blade puller
2−blade pusher
3−blade puller
3−blade pusher
0 50 100 150
−6
−4
−2
0
2
4
6
8
10
12
Disk Loading [N/m²]
Relative ∆ ηplr to 2−blade puller [%]
2−blade puller
2−blade pusher
3−blade puller
3−blade pusher
Fig. 5: Propeller efficiency comparison between pusher and
puller configuration in combination with a two- or
three-bladed propeller.
rpm for the same thrust, as presented by the graphs in figure
6, and can therefore be interesting for reducing the noise.
1000 2000 3000 4000 5000 6000 7000 8000
0
1
2
3
4
5
6
7
rpm [−/min]
Thrust [N]
2−blade puller
2−blade pusher
3−blade puller
3−blade pusher
Fig. 6: Thrust as function of the rpm for a two-bladed and
three-bladed propeller.
4.3 Arm interaction
For multi-rotors, a motor with propeller is mounted on an
arm that is connected to the frame. This arm is subject to
the airflow that the propeller induces and partially obstructs
this flow, leading to a loss in thrust and therefore propulsion
system efficiency. In the following experiment, three differ-
ent types of arm are used in order to quantify their influ-
ence on the propulsion system efficiency. The first arm is
a25mm cylindrical carbon tube, also used for the other ex-
periments in this paper. The second arm is equipped with a
3D printed aerodynamically shaped nacelle that also incorpo-
rates the motor. The third arm is a thin 10mm square carbon
tube. The three arms are presented in figure 7.
The left graph of figure 8 shows the propulsion system ef-
ficiency as a function of the disk loading for the three different
(a) 25mm tube (b) nacelle (c) 10mm square tube
Fig. 7: Propeller and motor mounted to three different shapes
of arm.
arms. Since in this experiment there is no space to include the
torque sensor on the aerodynamically shaped and thin arm, it
is not possible to measure propeller efficiency separately. The
right graph of figure 8 shows the relative difference in propul-
sion system efficiency due to the different shape of arm. From
these graphs, a noticeable efficiency increase is observed for
the aerodynamically shaped arm and the thin arm. It is inter-
esting to notice that the thin arm performs even better than the
aerodynamically shaped arm, which was a somewhat unex-
pected result. Overall, the thin arm is clearly the best design
choice, since in addition it is lighter than the other arms.
Fig. 8: Influence of arm dimensions and shape on propulsion
system efficiency.
4.4 Coaxial propellers setup
It is often a topic of discussion whether a counter rotat-
ing pair of propellers, also referred to as coaxial propellers,
is more efficient than a single propeller. Since coaxial pro-
pellers rotate in the opposite direction, swirl losses can be
minimized compared to a single propeller. On the other hand,
the downstream propeller operates in an air stream that is dis-
turbed by the upstream propeller and can therefore lose effi-
ciency. A carefully designed coaxial setup requires a down-
stream propeller with a slightly smaller diameter due to con-
traction of the flow, and a slightly higher pitch angle of the
blades due to operation in an already accelerated flow. On
most coaxial multi-rotors used nowadays however 2, the up-
stream and downstream propellers are a counter rotating pair
of identical propellers. Four different setups with coaxial
Graupner 9x5 E propellers, as presented in figure 9, are tested.
The performance resulting of each of these setups is presented
in figure 10. The left graph does not reveal any large dif-
ferences in propulsion system efficiency. However, from the
right graph, showing the relative difference in propulsion sys-
tem efficiency compared to the conventional coaxial setup,
it becomes clear that a single propeller propulsion system is
significantly more efficient than the coaxial setups, but the
difference becomes smaller at higher disk loadings. At the
highest disk loading of the single propeller at full throttle,
the coaxial setups even become slightly more efficient. The
right graph also shows that the conventional coaxial setup is
more efficient compared to the other coaxial setups. There
is no noticeable difference between 10cm and 15cm of spac-
ing between the propellers, however for the setup in which
the coaxial pair is very close at 3.5cm, there is a clear loss
in efficiency. From these experiments, it is concluded that
the propellers require some space in between and this space
is best used to incorporate one arm, supporting both motors
and propellers. It is also clear that coaxial propellers can be
also interesting in terms of efficiency when high disk load-
ings are required for example if the UAV has to be compact or
wind resistant. The additional weight due to the extra motor,
ESC and propeller for the coaxial setup compared to a single
propeller system, was not taken into account in the evalua-
tion of the efficiency. In practice, this extra weight has to be
subtracted from the produced thrust of the propulsion system
since the system itself becomes heavier.
A
B
A
B
A
B
conventional
10 cm
15 cm
A
single propeller
A
B
3.5 cm
Fig. 9: Four different coaxial setups and a single propeller to
compare with.
2For example: Aerialtronics Altura Zenith, Harwar Mega V8, 3DR X8+
0 50 100 150 200
15
20
25
30
35
40
45
50
Disk Loading [N/m²]
Propulsion system efficiency ηprop[%]
conventional
10cm
3cm
15cm
single prop
0 50 100 150 200
−6
−4
−2
0
2
4
6
8
10
12
14
Disk Loading [N/m²]
Relative ∆ ηprop with standard configuration [%]
conventional
10cm
3cm
15cm
single
Fig. 10: Performance of different coaxial propeller setups.
4.5 Partially overlapping propellers setup
Larger propellers are able to deliver the same thrust for
less power and are therefore preferred if long flight times
are required. Since there is often a restriction on the di-
mensions of the UAV, as the diameter of the propellers in-
creases, the tips of different propellers approach each other.
In some designs propellers actually overlap [6]. The mu-
tual influence of this overlap is tested in a series of exper-
iments in which the axial distance and overlap of two pro-
pellers are varied. Figure 12 presents the used conventions of
axial distance and overlap, in this paper presented as a per-
centage of the diameter of the propeller. Six different val-
ues for axial distance are used, being [100%,50%,
5%,5%,50%,100%] and five different values of overlap are
used, being [10%,0%,10%,20%,30% ] of the diameter
of the propeller. This results in a total of 30 configurations.
Figure 11 shows the experimental setup. The marked area
in this figure is used to calculate the efficiency of the double
propulsion system with equation 8. The area covered by the
two overlapping propellers Ais calulated following relations:
α= 2 cos1(1 overlap)(9)
A=1
2D
22(αsin(α)) (10)
The conventions used for axial distance and overlap are
presented schematically in figure 12. Propeller ‘A’ creates the
disturbing flow for propeller ‘B’. For a positive axial distance,
propeller ‘A’ lies upstream of propeller ‘B’ and for a negative
axial distance it lies downstream.
In this set of experiments, only the power and thrust of
motor ‘B’ is measured since for the set of experiments in
which the axial distance is negative, motor ‘B’. For every
combination of distance and overlap, the power of motor
‘B’ is measured for producing both 2.5Nand 5Nof thrust,
Fig. 11: Experimental setup that allows variation of axial
distance and overlap between two counter rotating
multi-rotor UAV propellers.
A
B
upstream
downstream
diameter
overlap
axial distance
Fig. 12: Used conventions for the experiment with overlap-
ping propellers.
which represents approximately 40% and 80% of the maxi-
mum thrust of the propeller in this setup. The relative dif-
ference in power compared to a single propeller without in-
teraction of another propeller is presented in figure 13. Both
graphs show an increase in required power for higher over-
lap. There is a large gradient between 5% and 5% axial
distance visible: the propeller requires less power when there
is a propeller directly in front of it. This reduction is visible
on both graphs and most pronounced for an overlap of 10%.
These results suggest that a propeller with a small overlap,
located downstream of the other propeller at a small distance,
has some benefit of this configuration since less power is re-
quired.
In order to evaluate the influence of propeller overlap on
the propulsion system efficiency ηprop two approaches are
used.
Overlapping propellers caused by increasing the pro-
peller diameter:
For a multi-rotor with a fixed arm length, the diame-
ter of the propellers can be increased in order to in-
crease the disk area of the propellers Aand therefore
distance [%]
overlap [%]
∆ Pele [%] for 2.5N
−100 −50 0 50 100
−10
−5
0
5
10
15
20
25
30
−4
−2
0
2
4
6
distance [%]
overlap [%]
Relative ∆ Pele [%] for 5N
−100 −50 0 50 100
−10
−5
0
5
10
15
20
25
30
0
2
4
6
8
10
12
14
Relative
Fig. 13: Cubic interpolated contour plot of relative power in-
crement for a single propeller in proximity to an-
other propeller compared to a single propeller with-
out interference, as a function of distance and over-
lap for 2.5N(left) and 5N(right) of thrust.
reduce the power required, resulting from equations 2
to 7. In order to calculate the propulsion system effi-
ciency with equation 8, the combined disk area of the
two propellers is calculated with equations 9 and 10.
The relative difference in propulsion system efficiency
compared to two propellers that are not in interaction
is presented in figure 14. The left graph shows a clear
decrease in efficiency for increasing axial distance be-
tween the propellers. From this graph, it can be seen
that a slight overlap of 10% to 15% even increases the
efficiency of a pair of counter rotating propellers when
placed at a minimum axial distance. On the right graph,
the same zone, 10% to 15% overlap and minimal ax-
ial distance, also shows the least decrease in efficiency.
The highest decrease in efficiency is observed for high
levels of overlap and increasing distance. Comparing
the right graph the to left shows that the setup to pro-
duce 10Nof thrust has a higher overall efficiency de-
crease compared to the 5Nsetup. From these graphs it
can be concluded that the propulsion system efficiency
of a pair of overlapping propellers decreases for higher
loads and higher axial distance between the propellers.
The best configuration for a pair of counter rotating
and overlapping propellers is in a zone between 10% to
15% overlap at a minimal axial distance at low loads.
Overlapping propellers caused by decreasing multi-
rotor arm length:
For a multi-rotor with a fixed propeller diameter, the
arm length can be decreased to make the UAV more
compact and resulting in overlapping propellers. In or-
der to evaluate the impact of this overlap on the propul-
sion system efficiency, the relative power increment is
plotted on figure 15. These graphs show an increase
in required power for all data points. The interesting
zone between 10% to 15% overlap and minimal axial
distance [%]
overlap [%]
Relative ∆ ηprop [%] for 10N
20 40 60 80 100
−10
−5
0
5
10
15
20
25
30
−4
−3.5
−3
−2.5
−2
−1.5
−1
−0.5
0
distance [%]
overlap [%]
Relative ∆ ηprop [%] for 5N
20 40 60 80 100
−10
−5
0
5
10
15
20
25
30
−2
−1.5
−1
−0.5
0
0.5
1
1.5
Fig. 14: Relative propulsion system efficiency increment
(calculated based on the combined area of both over-
lapping propellers) for an overlapping pair of pro-
pellers compared to two single propellers without
interference, as a function of distance and overlap
for 5Nand 10Nof combined thrust.
distance shows to require not more energy than pro-
pellers that are non-overlapping but in close proximity
to each other at 10% overlap. For a compact multi-
rotor design, letting the propellers overlap with 10%
while keeping the axial distance minimal is the best de-
sign choice.
distance [%]
overlap [%]
Extra power required [%] for 10N
20 40 60 80 100
−10
−5
0
5
10
15
20
25
30
1
2
3
4
5
6
7
8
9
distance [%]
overlap [%]
Extra power required [%] for 5N
20 40 60 80 100
−10
−5
0
5
10
15
20
25
30
0
1
2
3
4
5
6
Fig. 15: Relative required power increment for an overlap-
ping pair of propellers compared to two single pro-
pellers without interference, as a function of dis-
tance and overlap for 5Nand 10Nof combined
thrust.
5 CONCLUSION
This paper discusses the effects of propeller configuration
on the propulsion system efficiency of a multi-rotor. Five de-
sign choices are studied. A pusher configuration proves to
be preferable in terms of efficiency in hover conditions. An
increase of 2to 4% in efficiency is measured. This increase
is small, however, and requires a taller more complicated in-
tegration of the landing gear, resulting in more weight. A
three-bladed variant of the tested propeller results in a lower
efficiency in the order of 2to 6% but can be beneficial to re-
duce noise and risk due to its lower required rpm.
Tests with three different arms on which the propulsion
system is mounted, show that a thin rectangular arm is more
efficient compared to a slightly thicker but aerodynamically
shaped arm and can improve efficiency compared to a thick
arm with 4to 8%. The difference between the three arms
becomes less pronounced for higher disk loadings. At low
disk loadings, the propulsion system setup with a coaxial set
of propellers is less efficient compared to the setup with a
single propeller. However this difference becomes smaller
at higher disk loadings at which a single propeller propul-
sion system almost reaches its maximum thrust. For a pair
of overlapping propellers, power demand increases with the
percentage of overlap. The best configuration for overlapping
propellers was observed to be in a zone between 10% to 15%
overlap while keeping minimal axial distance. This configu-
ration allows to use larger propellers or decrease the length of
the multi-rotor arms without decreasing the propulsion sys-
tem efficiency. The overlapping configuration is also more
interesting for propellers with a low disk loading.
ACKNOWLEDGEMENTS
A special thanks to Menno Hochstenbach, who imple-
mented the rpm measurement based on the three-phase cur-
rent between motor and ESC and Jon Verbeke for the fruitful
discussions on the test results.
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Article
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