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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
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
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
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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.
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)
T= 2 ρ A (vi)2(4)
2ρ A (5)
Pi=T V1(6)
ηplr =Pi
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
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 losses
ESC losses
cable losses
propeller losses
propeller configuration losses
motor losses
Fig. 1: Overview of losses from energy source to kinetic en-
ergy in the air.
Fig. 2: Momentum theory applied on a propeller in hover
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.
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
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: and
0 50 100 150
Disk Loading [N/m²]
Propeller efficiency [%]
2−blade puller
2−blade pusher
3−blade puller
3−blade pusher
0 50 100 150
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
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.
10 cm
15 cm
single propeller
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
Disk Loading [N/m²]
Propulsion system efficiency ηprop[%]
single prop
0 50 100 150 200
Disk Loading [N/m²]
Relative ∆ ηprop with standard configuration [%]
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)
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.
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-
In order to evaluate the influence of propeller overlap on
the propulsion system efficiency ηprop two approaches are
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
distance [%]
overlap [%]
Relative ∆ Pele [%] for 5N
−100 −50 0 50 100
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
distance [%]
overlap [%]
Relative ∆ ηprop [%] for 5N
20 40 60 80 100
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
distance [%]
overlap [%]
Extra power required [%] for 5N
20 40 60 80 100
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
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.
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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The latest trends of Urban Air Mobility pushed the aeronautical industrial sector towards the eVTOL concept, i.e. electrical vertical take-off and landing. Electrical power, tilt-wing configuration and multiple propellers in tandem configuration, i.e. with the propellers placed one after the other, are the key features of such concept. In particular, the presence of multiple propellers working at close range introduces a new challenge: the investigation of the rotor-rotor aerodynamic interaction between front propeller slipstreams and rear propellers. This topic is rather new, thus a lack of experimental literature is noticed. The present work aims to partially fill the gap through an extensive experimental activity which investigates the main physical aspects of the phenomenon in a typical eVTOL configuration. A dedicated wind tunnel testing campaign is performed to investigate deeply the interaction between two co-rotating tandem propellers at fixed axial distance and variable lateral separation. The performance of the tandem propellers were compared with an isolated configuration both in terms of thrust and torque measurements and Particle Image Velocimetry (PIV) surveys. The experimental results are the first step in the creation of a reference database for the validation of numerical codes implemented during the design phase of such vehicles. Load measurements showed a significant loss in the rear propeller performance as a function of the overlapping ratio between the propellers. Furthermore a dedicated spectral analysis of wind tunnel thrust signals outlined high amplitude fluctuations in partial overlapping configurations. In parallel a numerical activity was performed using a mid-fidelity aerodynamic solver relying upon Vortex Particle Method (VPM) in order to enhance the comprehension of the phenomenon. The analysis of the numerical results allowed to access the flow behaviour involving the front propeller slipstream and the rear propeller disk, which is responsible of the massive losses experienced by the rear propeller.
Coaxial rotors are widely used for large-sized multi-rotors to achieve the efficiency of spatial layout. However, as the wake effect of the upper rotor affects to the lower rotor, performance degradation of the lower rotor is common. In this paper, a model of performance degradation for the coaxial rotor configuration is proposed based on experiment results on a given combination of lower and upper rotor pulse width modulation (PWM) signals. For the performance reduction model, loss of actuator effectiveness with respect to the given PWM signal of the lower motor is estimated comparing with the single rotor configuration. For the coaxial rotor experimental data, the PWM percent of the upper rotor is fixed with a constant value in each scenario, while the lower rotor angular velocity is changed with a specific interval in the coaxial rotor configuration. Also, the PWM percent of the upper rotor is changed with another specific interval for each of the scenarios. In each scenario, thrust, torque and angular velocity of the rotor are measured. For comparison, these same measurements were taken for the single rotor configuration with the same PWM percent interval of the upper rotor in the coaxially configured rotor. By comparing the coaxial rotor and single rotor configuration, thrust and torque efficiency degradation with respect to the PWM signal combination of the upper and lower rotors is derived. To evaluate the performance reduction of thrust and torque, the loss of actuator effectiveness with the function of the upper and lower rotor PWM signal is derived with the ratio between the single rotor and coaxial rotor configuration in the same PWM percent of the upper rotor. This can be applied to precise multi-rotor modeling for simulation or controller design.KeywordsCoaxial propeller configurationLarge-sized multi-rotorExperiment-based modelingLoss of actuator effectiveness
Multiple propellers arranged in overlapping configurations can reduce the sizes of unmanned aerial vehicles for compact architectures. However, the resulting aerodynamic interaction between the propellers could lead to extra noise and calls for extra studies on acoustic physics. In this work, we conducted aeroacoustic tests of two overlapping propellers in an anechoic chamber, focussing on the influence of the axial separation distance and lateral tip spacing at different tip Mach numbers, ranging from 0.18–0.23 and Reynolds numbers ranging from 5.2 × 10 ⁴ to 6.7 × 10 ⁴ . The results indicate that, compared with the coplanar configurations, the propeller thrusts of the overlapping propellers are deteriorated. The rear propeller under the impingement of the upstream wake flows is significantly affected, leading to the mean value of the thrust being reduced by up to 8.2% and the thrust fluctuation being increased. Consequently, the tonal noise is intensified by up to 10 dB at the low observation angles (referenced to the rotation axis). However, the broadband noise is slightly changed within 3 dB. Also, the lateral tip spacing has a primary impact on the noise characteristics, while the effect of the axial separation distance is secondary.
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Die STAB-Jahresberichte werden normalerweise bei den alternierenden Veranstaltungen Symposium und Workshop als Sammlung der Kurzfassungen (Mitteilungen) der Vorträge an die Teilnehmer verteilt. Da das STAB-Symposium in diesem Jahr aufgrund der Corona-Pandemie nicht stattfindet, sondern nur der Band "New Results in Numerical and Experimental Fluid Mechanics XIII" veröffentlicht wird, erscheint der diesjährige STAB-Jahresbericht lediglich in digitaler Form. Der Bericht enthält 83 Mitteilungen über Arbeiten aus den Projektgruppen und Fachkreisen der STAB. Den Mitteilungen vorangestellt ist ein Inhaltsverzeichnis (Seite 14 bis 19), das nach Projektgruppen/Fachkreisen gegliedert ist. Innerhalb der Rubriken ist alphabetisch nach Verfassern sortiert. Die Beiträge verteilen sich (bezogen auf den Erstautor) zu 41 % auf die Hochschulen und zu 59 % auf Forschungseinrichtungen (DLR, DNW, ISL). Auf Seite 186 sind die Autoren und Koautoren dieses Berichtes aufgelistet.
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The functioning of a state is a complex process that primarily depends on the macroeconomic national system. During the functioning of the state, obstacles such as corruption arise in the system, reducing the effectiveness of the state. Therefore, the purpose of this research is to analyse the effects of corruption on the effectiveness of a state, observed through the results of measurable state activity. The aim of this research is to assess the direction, form and intensity of corruption effects on indicators of state activity. In order to assess the direction, intensity and form of the impact of corruption on indicators, correlation will be used for each of the segments/indicators of state activity. Furthermore, in order to get the final result and explanations of the relationship between corruption and state activity indicators, a comparison of one developed EU country - Germany - with a country in transition- Croatia - will be made. Given the research methodology, in addition to the classical methods of analysis and synthesis, the desk research method will be used, along with correlation coefficients. In order to get the final result and explain the connection between corruption and indicators of state activity, the method of comparison will be used to reach a final conclusion. Using this methodology, the conclusion will demonstrate the role of corruption in the state activity system. Also, the research is expected to show which segments of state activity are affected more by corruption and which are affected less or unaffected, and what is the effect of the impact of corruption on the success of state activity. In particular, it is expected that this research will identify the impact of corruption on the indicators of state activity in the field of entrepreneurship, which is the backbone of a national economy. It is difficult to predict the results before calculating the correlation, but the research hypothesis can be related, with great certainty, to the more visible impact of corruption on those segments of the state activity in which a part of the state budget is located.
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Tax expense makes up a significant part of the company’s total costs, which are directly related to the company’s competitiveness on the market. Besides direct costs, tax burdens also create indirect costs because increasingly complex tax regulations demand specific and specialized knowledge as well as being familiar with them. Research of existing Croatian practice through appropriate case studies will confirm the thesis that tax planning and optimization are finding ways to reduce tax payments, and at the same time have a role in achieving tax savings through various opportunities for tax relief and other savings. The aim of the research is to show based on concrete case studies the application of the scientific approach in tax planning. The obtained results show that ignorance and misapplication of legal regulations can lead to significant expenses in the form of penalties for committing a tax offense or even a possible criminal offense. Therefore, entrepreneurs can achieve savings in paying taxes only if they correctly apply tax regulations through tax optimization and planning and at the same time, they can reduce the risk of reckless entry into the area of tax avoidance. It is, therefore, important to be well acquainted with the tax system and tax regulations, but also with the procedures and legal possibilities in the case of tax supervision, through which proper tax planning is controlled. VAT optimization is a complex process and involves the use of various methods, approaches, and strategies to achieve a useful result without ultimately looking like an attempt to avoid paying taxes. Knowledge of tax reliefs and exemptions enables efficient tax planning and facilitates the company’s management to make optimal business decisions. Tax revenues make up a significant part of state budget revenues, which makes the controlling in this area of tax planning and optimization frequent, thorough and comprehensive.
Conference Paper
View Video Presentation: Experimental studies were conducted to investigate the aerodynamic and aero-acoustic performance of a dual-rotor configuration applied to small UAVs. The aerodynamic performance, including thrust and power consumption, of a dual-rotor with a contrasting rotational direction (anti-rotor) was better than that of a dual-rotor with the same rotational direction (co-rotor), with the thrust of the downstream propeller decreased because of the wake of the upstream propeller. A parametric study with different rotor distances revealed that both the broadband and overall noise of the dual-rotor system increase as rotor distance decreases. In addition to exploring the influence of rotor distance on thrust and noise, this study also used PIV technology to examine flow-field characteristics for different incoming flow velocities.
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This paper presents the design and analysis of a quadrotor with a novel configuration, VOOPS-Vertically Offset Overlapped Propulsion System. The objective of this configuration is to increase the payload capacity of a quadrotor without increasing the overall dimension and without compromising endurance. This has been achieved by vertically offsetting the propellers and allowing propeller overlaps so that an increase in propeller size can be achieved without any increase in the overall dimensions. The design details and the dynamic model of the VOOPS configuration are presented. The constraints on the design parameters such as propeller offset and overlap are determined using propeller deflection model and geometry respectively. The effects on change in the design parameters of VOOPS configuration were studied using bench tests and performance simulation of quadrotor with VOOPS configuration were carried out. Practical implementation of VOOPS configuration is also discussed.
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The paper presents an analytical framework for addressing the hovering time prediction of rotary-wing aircraft, with a particular focus on multi-rotor platforms. By imposing the balance between required and available power, the endurance expression is derived as a function of airframe features, rotor parameters, and battery capacity. The best endurance condition is also obtained in terms of optimum capacity and hovering time, by means of two approximate closed-form solutions. The proposed methodology was validated by means of numerical simulation and flight testing. Results show the effectiveness of the proposed approach.
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We have conceived a novel compound multicopter (helicopter type utilizing multiple different size propellers for separate lift and attitude control) configuration specifically for flight through narrow corridors. Its design combines the contradictory requirements of limited width, high agility and long endurance while carrying a significant payload. This configuration can be scaled for both indoor and outdoor applications. The development is part of a doctoral research in which an autonomous unmanned rotary helicopter is designed, constructed and flight tested for inspecting fruit orchards and vineyards while flying in between the tree rows in outdoor conditions such as wind and gusts. The compound hexacopter configuration combines two large lift propellers, with a constant rotational velocity, with four small control propellers commanded by an autopilot. The autopilot is configured as a quadcopter commanding only the control propellers as only these change the attitude and overall thrust of the hexacopter. The benefit of using large lift propellers is their lower disk loading (thrust divided by disk area) which results in a higher Figure of Merit and lower power consumption compared to the smaller control propellers, while the latter are better suited for outdoor (windy) conditions due to their fast reaction time in spooling up and down. Compared to a standard quadcopter with the same width, payload and battery capacity, the endurance of the compound hexacopter is potentially up to 60% higher. As a concept validator, a small-scale prototype has been designed, constructed and successfully flight tested.
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The propulsion system plays a key role in each unmanned aerial vehicle. Considering different multirotor platforms we may observe that mostly the dynamics and performance of the particular vehicle is strictly depended on the drive unit. In this paper we focus on the problem of modeling and identification of the propulsion system consisting of an electronic speed controller, electric brushless direct current motor and propeller. We propose a multiple-input and multiple-output nonlinear model of the propulsion unit based on the block oriented modeling with the static nonlinearities at the input and linear dynamical part at the output. Such model enhanced with the aerodynamics theory of a propeller should be suitable and detailed enough to provide the simulation, control algorithms prototyping and solution verification under the circumstances of designing the vertical take-off and landing unmanned platform.
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In recent times, mini and micro aerial vehicles have shown significant potential as miniature unmanned aircraft for surveillance and reconnaissance purposes. Selection of correct combination of engine and propeller is very crucial step in the design of this class of vehicles. The objective of present study is to establish measurement system and obtain performance maps for available mini/micro vehicle class propellers and engines. Wind-Tunnel facility was developed and different experimental set-ups were designed for measuring thrust and power of different power plant systems. Results of experimental facilities were validated with those available in literature for particular power plant. Established measurement system complements for optimizing propeller design and selecting best engine or motor for given geometry of vehicle with given mission requirements.
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This paper details the design and development of a small scale air propeller dynamometer based on thin beam strain gauge load cells. The dynamometer will be used to characterize the performances of small propellers for Unmanned Aerial Vehicles (UAVs) in order to obtain an accurate system model and design an appropriate controller for hovering and smooth flight. A brief description of the design concept and calibration procedure along with test results is presented here. A static calibration was performed to determine thrust/torque measurement sensitivity as well as cross-sensitivity. Measurement data was captured and processed using a sigma-delta data acquisition board. Test results confirm that the dynamometer can be used to reliably measure thrust and torque produced by UAV propellers up to 10 inch in diameter with an accuracy of ±1% of full scale.
In this report, we present the theory on aerodynamics of quadrotors using the well established momentum and blade element theories. From a robotics perspective, the theoretical development of the models for thrust and horizontal forces and torque (therefore power) are carried out in the body fixed frame of the quadrotor. Using momentum theory, we propose and model the existence of a horizontal force along with its associated power. Given the limitations associated with momentum theory and the inadequacy of the theory to account for the different powers represented in a proposed bond graph lead to the use of blade element theory. Using this theory, models are then developed for the different quadrotor rotor geometries and aerodynamic properties including the optimum hovering rotor used on the majority of quadrotors. Though this rotor is proven to be the most optimum rotor, we show that geometric variations are necessary for manufacturing of the blades. The geometric variations are also dictated by a desired thrust to horizontal force ratio which is based on the available motor torque (hence power) and desired flight envelope of the vehicle. The detailed aerodynamic models obtained using blade element theory for different geometric configurations and aerodynamic properties of the aerofoil sections are then converted to lumped parameter models that can be used for robotic applications. These applications include but not limited to body fixed frame velocity estimation and individual rotor thrust regulation [1, 2].
The traditional aerodynamic model of the quadrotor is almost entirely based on the modeling of a single rotor, ignoring the wake interference among the four rotors in close proximity. Because of the oversimplification of the aerodynamic model, the attitude control and trajectory tracking of the quadrotor is unsatisfactory in high-speed flight. To accurately and effectively control the attitude and trajectory of the quadrotor, this study proposes a quadrotor forward-flight mathematical model considering rotor wake mutual interference, which can be used in flight control. To validate the model, a series of computational-fluid-dynamics analyses were conducted. An integral quadrotor model and an isolated rotor were analyzed under the same conditions. The mathematical model and results of the computational-fluid-dynamics analyses results provided consistent trends and demonstrated the suitability of the proposed model for high-speed forward flight. © 2015 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.
The research and development efforts outlined in this paper address the aerodynamic design of micro air vehicles with hovering and vertical takeoff and landing capabilities. The tilt-body configuration of the vertical takeoff and landing micro air vehicle is proposed based on a propulsion system consisting of two coaxial contrarotating motors and propellers. Values of thrust, torque, power, and efficiency of this propulsion system were measured in pusher and tractor arrangements of propellers and compared against single motor-propeller propulsion. With comparable efficiency, the developed propulsion system has very little propeller torque. Hot-wire measurements have been conducted to investigate the velocity profile in slipstream. The lower average velocity and significant decrease in velocity in the core of the slipstream found in the tractor arrangement are mostly due to the parasite drag caused by the motors. It causes the decrease of the thrust force observed for the tractor arrangement in comparison with the pusher arrangement. Wind-tunnel testing was conducted for a motor, a wing, and an arrangement of a wing with a motor. The drag force on the wing is produced by two mixing airflows: freestream and propeller-induced pulsating slipstream. The zero-lift drag coefficient increases by about 4 times with propeller-induced speed increased from 0 to 7.5 m/s. The results of this study were realized in the design of a vertical takeoff and landing micro air vehicle prototype that was successfully flight tested.
An advanced compact propeller dynamometer measurement system has been developed that determines propeller performance values to within +/-5 percent error. Propeller thrust is found to within +/-3 percent with a linearly variable differential transformer that measures dynamometer shaft extensions, torque to within +/-1.5 percent using an optical torque transducer, and rpm to within +/-0.003 percent by an optical digital resolver. The system was tested in a low-speed wing tunnel, using marine and remotely piloted vehicle propellers to assess its operational capability. Results reveal that the thrust and torque coefficients and efficiency can be determined to within +/-3.45, +/-2.08, and +/-4.2 percent, respectively.