Novel Considerations on the Negative Pressure Adhesion
of Electric Ducted Fans: An Experimental Study
Angelica Brusell, George Andrikopoulos, and George Nikolakopoulos
Abstract— In this article, the potential of utilizing an Electric
Ducted Fan (EDF) as an adhesion actuator is investigated
in detail, where an experimental setup is implemented for
evaluating the EDF’s ability to adhere to a test surface through
negative pressure generation. Different design variables and
modifications to the original EDF structure are tested, while
their impact on the adhesion efficiency is experimentally eval-
uated. The presented investigation acts as a preliminary study
to the goal of incorporating the resulting optimized negative
pressure-based actuation method in a wall-climbing robot for
inspection of aircraft fuselages.
I. INTRODUCTION
Electric Ducted Fans (EDFs) have been traditionally pop-
ular for their utilization as a propulsion method for both
big and small-scale aircrafts, mainly due to an increased
thrust efficiency by the ducts reduction of tip vortices and
the pressure drops at the blade tips, when compared to their
open air equivalents [1]. A typical EDF design, as graphically
displayed in Fig. 1, consists of a motor and an impeller,
encased by a cylindrical duct and shroud. A rotor and aft
cone are usually incorporated to reduce the turbulence of
the air flow through the duct around the motor.
However, the utilization of EDFs has not been limited to
propelling a craft through air, as their ability to generate a
negative pressure and actively adhere to a surface, when their
ducted structure is placed at a close proximity[2] [3], has led
to different application areas, with the most recent being the
Wall-Climbing Robots (WCRs) [4].
Although the WCRs presented in the related literature have
been utilizing a number of different methods for adhesion,
from passive [5] [6] and active suction cups [7] [8] to vortex
chambers [2] [9] and magnetic attraction [10] [11], the use of
an EDF-based design, as an adhesion method, provides im-
portant advantages that could increase the potential usability
of WCRs, has not been addressed yet at full extent.
Specifically, EDFs do not require the adhesion mechanism
to be in contact with the target surface, which alleviates the
design challenges of adhesion in cases of curved, rough or
non-magnetic target surfaces [2]. In addition, such a design
technique has an impact on the overall cost, since no external
equipment (e.g. compressor, filters, tubes etc.) have to be
used, which produces a untethered-friendly solution.
This work has received funding from the European Unions H2020
Framework Programme under the call FET-OPEN, grant agreement No.
665238.
A. Brusell, G. Andrikopoulos and G. Nikolakopoulos are with the
Robotics Team at Control Engineering Group, Lule
˚
aUniversity of Tech-
nology, SE-97189 Lule
˚
a, Sweden.
Corresponding Author’s Email:angbru@ltu.se
Impeller
Rotor Cone
Shroud
Duct
Motor
Aft Cone
Fig. 1. Typical EDF design in assembled and exploded view
In the small number of applications found in related
literature that utilize an EDF-based design, the main research
aspect has been either the negative pressure generation [2], or
thrust [12] [3] generated by the EDF. From this related work
it becomes apparent that the experimental evaluation has
been constrained on investigating the effect of the distance
of the EDF shroud to the target surface, on the adhesion effi-
ciency. Thus, there is an identified gap on the investigation of
important design parameters, such as the EDF’s placement,
the length and dimensions of the duct, the dimensions of the
shroud and its distance from the surface, which complicates
the modeling and control of such a system.
The main contribution of this article stems from the experi-
mental evaluation of the potential of utilizing a commercially
available EDF as an adhesion mechanism, while providing a
novel insight on the analysis of the adhesion nature related to
negative pressure and thrust force generation against a target
surface. To this goal, a novel experimental setup for the EDF
testing is proposed for acquiring important properties such
as adhesion force and generated pressure during the EDF’s
operation when placed against a test surface. In addition, this
article will also contribute towards the novel evaluation of
different design variables and modifications to the original
EDF structure for their effect on the adhesion efficiency and
the dependence of the adhesion nature to the EDF’s distance
from the test surface.
The rest of this article is structured as follows. II covers
the fundamental negative pressure adhesion principles, while
Accepted Version
Section III presents the experimental setup’s design specifics
and utilized components. In Section IV the experimental
results are presented in detail and finally, the concluding
remarks are provided in Section V.
II. NEGATIVE PRESSURE ADHESION PRINCIPLE
Negative pressure adhesion works on the principle of
generating and maintaining a low pressure zone P
a
inside
a cavity compared to the surrounding outside pressure P
b
.
As depicted in Fig. 2, the difference in pressure will induce
a force F
s
across the projected cavity area onto the target
surface, A, by the high pressure region [13] as:
F
s
= A(P
b
− P
a
). (1)
Considering an EDF-based adhesion mechanism, the low
pressure zone lies within its shroud, hence the active pressure
area is A = π(r
2
s
−r
2
d
) mm
2
with r
s
and r
d
denoting the outer
shroud radius and the duct’s internal radius, respectively.
In this definition, the effect of the shroud’s curvature is
neglected for simplification purposes. Area A will from now
on be referred to as the active area. In this configuration,
an increase in applied force can occur via a decrease in the
shroud’s pressure, or an increase in the active area.
Furthermore, air flow through the low pressure zone P
a
is introduced by the distance between the shroud’s end and
the test surface, h, as displayed in Fig. 2. The flows Q
in
and
Q
out
entering and exiting the chamber are defined by,
Q
in
= A
in
v
in
,
Q
out
= A
out
v
out
,
(2)
where A
in
and A
out
denote the in- and outlet areas, while
v
in
and v
out
denote the air velocity into and out of these
areas. The outlet area is A
out
= π(r
2
d
− r
2
m
) mm
2
, with r
m
denoting the radius of the motor’s cylindrical case, while the
inlet area A
in
= 2πhr
s
mm
2
depends on the gap height h.
In order for a negative pressure zone to be generated, the
flow out of the chamber has to be higher than the flow in,
for a specific period of time. As P
a
lowers, air is pulled
in through the gap and after a time t the flow reaches an
equilibrium, with the lower pressure zone being maintained.
The steady state operational point is defined as:
A
in
v
in
= A
out
v
out
, (3)
In other words, a mass flow imbalance occurs for an increase
in the EDF’s rotational speed. As air is drawn out from
A
out
at a faster rate than in through A
in
, this results in
an additional pressure decrease in the inlet area, which is
defined as the air volume constrained by the shroud and its
projection to the target surface. This decrease in pressure will
speed up the air flowing in through the gap until a steady-
state is reached, while as the rotational speed is kept constant
the negative pressure and flow rates remain constant as well.
In addition to the adhesion force produced through the
negative pressure, the EDF will generate a thrust as it pulls
air through the duct. At small h, the air density will not
be enough to create a big thrust force, but as the gap
increases and airflow is less restricted, the generated thrust
rs
rd
Pb
Pa
Target Surface
Shroud
h
F
Fig. 2. Shroud detail in half-section view with highlighted shroud pressure
zones and geometrical properties.
will increase. Ultimately the thrust generated will converge
to its free flight equivalent [14].
III. EXPERIMENTAL SETUP
A. Conceptual Design Properties
The experimental setup was designed with the main goal
of measuring the adhesion force F exerted from the EDF to
the test surface. To achieve this, as represented in Fig. 3, the
EDF was mounted to a legged support structure where each
of the four legs was connected to the baseplate via a load cell,
resembling the rectangular formation of commercial weight
scales. In this configuration, the total measured force F will
be derived by the summation of the force measured by the
four load cells, displayed in Fig. 3 as F
1,...,4
.
In the setup’s conceptual design presented in Fig. 3, a
vacuum sensor array was placed underneath the test surface
with the sensing tips placed coincidently through inlet holes
of the surface. Provided the general symmetry of the EDF
structure, the flow distribution along circular sections around
the EDFs longitudinal axis in its interior and exterior space
was assumed to be symmetrical, while the aerodynamic
effect of the legged structure was assumed negligible. For
these reasons, the sensor array was placed radially from the
surface center, which is defined as the intersection of the
EDF’s longitudinal axis and the target surface. In this way,
the setup provides with a series of pressure sensor points,
denoted in Fig. 3 as P
1,...,8
. A ninth sensor P
9
was placed
at the top of the EDF duct for reference purposes and to
provide an insight on the generated thrust.
In order to expand the evaluation span of the EDF’s
adhesion properties, different design variables in the setup
were incorporated. Specifically, the distance of the EDF
from the test surface h
j
was conceptualized as variable
and to this purpose the legged structure was equipped with
multiple inlets placed at predetermined intervals denoted
by the subscript j. In addition, EDF shrouds of different
outer diameter r
s
were designed to be interchangeable via
mounting brackets for evaluating the effect of the active
surface and volume change on the attraction between the
EDF and the test surface for a given distance. For the needs
of this study, the design of the shrouds followed the outward
curvature profile utilized in commercially available EDFs,
while their height l was selected as constant and predefined.
Force Sensors
Vacuum Sensor
Vacuum Sensor Array
EDF
IMU
Interchangeable Shroud
h
rs
Test Surface
Legged Support
P9
rd
l
F
rs
P1P2P3P4P5P6P7
P8
F2
F1
F4
F3
Fig. 3. Graphical representation of the Experimental setup: side (left) and bottom semi-transparent (middle) view with highlighted sensor components,
(right) half-section view with highlighted design variables.
Load Cells
Vacuum Sensor Array
Vacuum Sensor
Data Acquisition Cards
Load Cell Amplifiers
IMU
Fig. 4. Experimental setup prototype with highlighted sensor and data
acquisition components.
B. Setup Prototype and Utilized components
The setup prototype, which is displayed in Fig. 4, was
3D printed from polylactide (PLA) following the required
design properties for the execution of this study. The di-
mensions were adjusted to the needs of the selected test-
EDF and sensors. Specifically, the test surface was printed
as a 190×190×2 mm (Length×Width×Height) plate, with
incorporated inlet holes for the pressure sensors and mount
extrusions for the load cells. The gap height h can be set
between 3 and 21 mm in 3 mm increments. This is achieved
via the inlet holes of the structure’s four legs, placed in a
140×140 mm formation for uniform force distribution.
The test-EDF selected for the needs of the presented
study is a commercially available model developed by Dr.
MadThrust Co. with inner duct diameter at 92 mm, overall
weight 0.67 kg and 12-blade fan actuated via a brushless DC
motor of 2350 W maximum power at 41000 maximum rpm,
which provides a maximum thrust of 3.8 kg. The Electronic
Speed Controller (ESC) selected for this EDF unit was the
Turnigy AE-100A with continuous current capability of 100
A and is characterized by a fast and precise throttle response.
The EDF’s aluminum shroud was replaced by three dif-
ferent EDF shrouds, which were 3D printed via polylactide
(PLA) with r
s
= 60, 70 and 80 mm, following an outward
curvature profile and constant shroud thickness 2 mm, while
all the shroud heights were predefined at l = 22 mm. For
measuring the force F generated from the EDF and acted on
the test surface via the legged structure, four TE Connectivity
button-type load cells, with 11.34 kg maximum range, were
utilized and properly incorporated in both the test-surface
and respective leg tips in order to ensure proper uniformal
tension on the active button element of each force sensor and
thus more reliable measurements.
The acquisition of the negative pressure measurements
was achieved via multiple NXP USA Inc. MPXV5050V
differential pressure sensors with a sensing range of up to -50
kPa. Due to the differential principle of their operation, the
tip sensing point targets the test surface while their capsule
reference point remains in a part of the setup unaffected by
the flow. Specifically, a sensor array of eight sensors was
placed in a single radial formation along the test surface;
starting from the surface center, i.e. the intersection of the
test surface with the EDF’s longitudinal axis, the sensors we
placed radially at a 12.5 mm interval that was dictated by
the sensor dimensions. A ninth sensor of the same type was
also placed at the top of the EDF duct.
A LSM9DS0 Inertial Measurement Unit (IMU) was uti-
lized for the measurement of the vibrations on the legged
setup via its integrated accelerometer, while its temperature
sensor was used to monitor the thermal levels of the setup
and ensure the EDF’s safe operation during the experimental
sequences. Finally, the EDF’s operation and the acquisition
of the setup’s sensorial data were achieved via two National
Instruments USB-6008 cards and one Arduino Mega, while
the setup’s programming was carried out in MATLAB.
IV. EVALUATION OF VORTEX ADHESION
PROPERTIES
In this section, experimental sequences were performed for
different combinations of r
s
and h values, with the throttle
T input signal being a stepping function of 5 % increments
every 5 seconds, until a peak throttle T
max
= 80 % was
reached and the procedure then reversed down and reaching
T = 0 %. Ten seconds of data was collected before and after
the throttle step signal to ensure the calibration integrity of
the sensors during every experiment. The upper limit of 80
% was set via the ESC for safety against overheating. Force
measurements are acquired as the summation of the four load
cells, while subtracting the weight of the EDF equipment
acting on the force measurements points in order to properly
extract the adhesion force measurement.
The points P
i
, i = 1, . . . , 8, denote the vacuum sensors
radially and outwards from the surface center, starting with
P
1
as the center sensing point. It is important to note that
for the variable shroud radii r
s
, the sensors lying under the
EDF and shroud differ. Specifically, for r
s
= 60, 70 and
80 mm the sensors P
1,...,5
, P
1,...,6
and P
1,...,7
lie within the
outer radius, respectively. In all aforementioned cases, P
1,...,3
are positioned under the EDF duct, while the respective
remaining sensing points are positioned under the shroud.
The test setup was properly designed so that there are always
sensors outside the affected surface area, in order to provide
an insight on how the pressure is affected in these cases.
An indicative experimental test was executed using the r
s
= 70 mm and h = 6 mm, while the acquired throttle input
T , negative pressure P
i
, force F and supplied current I are
presented in Fig. 5. A clear increase in negative pressure
is observed with the increase in throttle, which becomes
progressively greater for the pressure sensors closer to the
surface center, thus reflecting the generation of a vortex.
The maximum negative pressures were recorded for the
center point P
1
, reaching a maximal value of approximately
6.5 kPa for T = 80 %. The neighboring sensor P
2
is
providing similar measurements but its larger range profile
appearing as noise, reveals the different effect from the
vortex generation. On the opposite side, the sensors P
7
and
P
8
, which are positioned outside of the affected area of the
chosen shroud, measure a smaller pressure change as the
air volume is bigger and the inlet area restricted, but the
entirety of the low pressure zone is thus not confined to just
the volume between the shroud and the surface, but extends
further and outside the enclosed volume. The measured force
F shows a behavior proportional to the throttle, following
the quick response characteristics of the negative pressure
signals with absence of transient phenomena. As expected,
the maximum force was acquired for maximum throttle and
was measured at around 21 N.
The aforementioned experimental trial provided a clear
indication of the generated vortex and the resulting adhesion,
as already recorded in related literature [2]. The experimental
cases presented in the sequel were performed in order to
a) find the design variables that provide this EDF setup’s
optimal performance, and b) investigate whether the vortex
is the evident and sole factor for generating adhesion in all
shroud and gap height cases.
Specifically, the aggregated P
n
and F results for h = 3,
..., 12 mm and r
s
= 60, 70, 80 mm were acquired and are
displayed in Fig. 5 for the same maximum throttle input
0
2
4
6
8
Pn [kPa]
0
5
10
15
20
25
F [N]
0
15
30
45
60
I [A]
0
25
50
75
100
T [%]
P1
P2
P3
P4
P5
P6
P7
P8
100 120 140 160 1800 20 40 60 80
100 120 140 160 1800 20 40 60 80
100 120 140 160 1800 20 40 60 80
100 120 140 160 1800 20 40 60 80
Fig. 5. Experimentally acquired negative pressure, force and current
responses along with the input throttle signal T for the case of r
s
= 70
mm and h = 6 mm.
signal. For all presented cases of different gap heights and
shrouds the negative pressure and measured force follow a
proportional nature to the increase and decrease in throttle,
with the maximum measurements acquired at maximum
throttle (80%). An important observation is that the increase
in the gap height leads to a clear decrease in the P
n
mea-
surements, but the force does not follow the same decreasing
behavior. On the contrary, the force increases to a maximum
of approximately 55 N in the case of h = 15 mm and r
s
= 80
mm. This reveals a change in adhesion nature that reflects a
pressure distribution change in need of further analysis.
To investigate this phenomenon, the experiments were
repeated for all combinations of r
s
= 60, 70 and 80 mm
and h = 3 to 21 mm with 3 mm increments with the same
throttle input signal. The surface pressure distribution P
n
with respect to the radial distance from the surface center
(in mm), which concede with the pressure sensor points, for
all gap heights h and shroud radii r
s
are presented in Fig.
6. These results show an increasing behavior as the shroud
radius increases and indicate a clear change in pressure
distribution as the gap alters, with an evident dissipation of
the vortex phenomenon and a general decrease in P
n
for
increasing h values. This change in distribution emphasized
by the maximum pressures is rapidly progressing for all
shrouds in gaps between 6 and 12 mm, where the increase
in h shifts the dominant pressure points from the near-center
ones P
1,2,3
to the middle P
4,5,6
.
To visualize this complex phenomenon and its rapid pro-
gression, the experimentally acquired pressure distribution is
Radial Distance from Center [mm]
-1
0
1
2
3
4
5
6
7
0 10 20 30
40 50 60 70 80 90
Pn [kPa]
-1
0
1
2
3
4
5
6
7
Radial Distance from Center [mm]
0 10 20
30 40 50 60 70 80 90
(a) (b) (c)
Radial Distance from Center [mm]
-1
0
1
2
3
4
5
6
7
0 10 20 30 40 50 60 70
80 90
3 mm
6 mm
9 mm
12 mm
15 mm
18 mm
21 mm
rs =
Fig. 6. Measured pressure distributions at maximum T = 80 % for h = 3 to 21 mm with 3 mm increments and for r
s
= (a) 60, (b) 70 and (c) 80 mm.
0.8
1
1.2
1.4
1.6
1.77
0.77 (kPa)
0.72 (kPa)
1.2
1.6
2
2.4
2.66
46.7 N54.8 N51.7 N
30.0 N22.6 N13.0 N
1
1.5
2
2.5
2.91
0.54 (kPa)
0 (kPa)
1
2
3
4
5
5.52
1
2
3
4
5
6.05
0.18 (kPa)
0.44 (kPa)
1
1.5
2
2.5
3
3.5
4.05
3 mm
6 mm
9 mm
12 mm
15 mm
18 mm
Fig. 7. Graphical representation of the experimentally acquired pressure distribution on the test surface for constant T = 80 %, r
s
= 80 mm and gaps h
= 3 to 18 mm with 3 mm increments).
radially expanded to the affected test surface for the shroud
case of r
s
= 80 mm, for T = 80 % and gap heights of h = 3,
. . . , 18 mm with 3 mm increments. As displayed in Fig. 7,
the pressure values are visualized via different coloring, the
values of which are presented in color bars for all respective
cases, while the maximum mean force is also indicated. As
mentioned, there is a fast progression when the gap reaches 9
mm, where the centralized vortex pressure profile dissipates
and gets shifted from the area underneath the duct towards
the outer surface area beneath the shroud.
As the gap increases, the negative pressure distribution
gets minimized from a range of 0 - 5.32 kPa at h = 3 mm
to 0.77 - 1.77 kPa at h = 18 mm, but the force undergoes a
big increase from 13 to 46.7 N. This observation leads to the
conclusion that the occurring vortex is not the sole factor for
adhesion to the test surface. Specifically, as h increases above
a specific threshold, the inflow of air increases suddenly and
starts to get propelled through the duct, thus producing thrust.
This abrupt change was experimentally observed by a sudden
and large increase in sensed flow, which was accompanied
by increased noise and vibrations.
Thus, the produced thrust results in an additive force
component that leads to the observed increase in the mea-
sured force summation. Further increase of the gap height
would lead to the EDF reaching a adhesion force plateau
and generating thrust converging to its free flight equivalent,
