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Preliminary Analysis of the Lip Wing System for the ALIPTERA VTOL Concept Aircraft

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This paper presents a simple and direct method for calculating the thrust performance of the new lip wing concept. The system is composed of a propeller and a curved wing whose trailing edge matches the leading edge of the propeller shroud. The total thrust of the shrouded propeller is thus augmented with the lift provided by the wing. The performance of the shrouded propeller is calculated in function of the shroud air flow diffusion parameter, while to calculate the wing generated lift, several simplifying hypotheses are made. The configurations for which the lip wing system becomes more efficient, in terms of power consumption, then a simple shrouded propeller are investigated. Some preliminary experimental tests were performed to check the performances of the proposed system, and the numerical results are validated against the experimental results. NOTATION = wing chord [m] = propeller diameter [m] = gravity field accelerations [m/s 2 ] = wing lift [N] ̇ = Mass flow rate through the propeller [kg/s] = aircraft mass [kg] = pressure variation [Pa] = Power [W] = area of jet cross-section at a given station [m 2 ] = propeller thrust [N] = fluid flow speed at a given station [m/s] = angle between wing chord and horizontal [deg.] = air density [kg/m 3 ] = diffusion parameter = angle between propeller axis and horizontal [deg.]
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1
Preliminary Analysis of the Lip Wing System for the ALIPTERA VTOL
Concept Aircraft
Oliviu Şugar Gabor
Ph.D. Student
LARCASE Laboratory, Ecole de
Technologie Superieure
Montreal, Quebec, Canada
Ruxandra Mihaela Botez
Professor
LARCASE Laboratory, Ecole de
Technologie Superieure
Montreal, Quebec, Canada
Dusan Stan
Founder
ALIPTERA Aircraft
Stoney Creek, Ontario, Canada
ABSTRACT
This paper presents a simple and direct method for calculating the thrust performance of the new lip wing
concept. The system is composed of a propeller and a curved wing whose trailing edge matches the leading edge of
the propeller shroud. The total thrust of the shrouded propeller is thus augmented with the lift provided by the wing.
The performance of the shrouded propeller is calculated in function of the shroud air flow diffusion parameter, while
to calculate the wing generated lift, several simplifying hypotheses are made. The configurations for which the lip
wing system becomes more efficient, in terms of power consumption, then a simple shrouded propeller are
investigated. Some preliminary experimental tests were performed to check the performances of the proposed
system, and the numerical results are validated against the experimental results.
NOTATION
= wing chord [m]
= propeller diameter [m]
= gravity field accelerations [m/s2]
= wing lift [N]
= Mass flow rate through the propeller [kg/s]
= aircraft mass [kg]
 = pressure variation [Pa]
= Power [W]
= area of jet cross-section at a given station [m2]
= propeller thrust [N]
= fluid flow speed at a given station [m/s]
= angle between wing chord and horizontal [deg.]
= air density [kg/m3]
= diffusion parameter
= angle between propeller axis and horizontal [deg.]
INTRODUCTION
The addition of a shroud can significantly increase the
propulsive performance of a propeller. Although the shroud
increases the overall weight and mechanical complexity of
an aircraft, a good design compensates these drawbacks with
the greatly increased system efficiency. Authors have
investigated applications of shrouded propellers for aircraft
and have proposed different methods for calculating the
power consumption, generated thrust and overall efficiency.
Baltazar and Falcao de Campos (Ref. 1) investigated the
effect of the gap between the blade tip and the shroud using
an inviscid numerical panel method approach. Lind et al.
(Ref. 2) also used a panel method to calculate the thrust
force and shroud pitching moment in various flow
conditions. Chung Chand and Rajagopalan (Ref. 3)
developed an axisymmetric, incompressible Navier-Stokes
Presented at the AHS Sustainability 2015 Conference,
Montreal, Canada, 22-24 September 2015. Copyright ©
2015 by the American Helicopter Society International, Inc.
All rights reserved.
2
solver, together with a specialized mesh generation code,
capable of accurately predicting the performance of
shrouded propellers. In Ref. 4, Wang and Zhang presented a
fast and powerful algorithm for the design and optimization
of ducted propellers. The moving propeller was analyzed by
a vortex lattice approach, while the duct was analyzed with a
panel method, the two methods being coupled by an iterative
process.
Abrego and Bulaga (Ref. 5) performed a wind tunnel
experimental investigation of the performance characteristics
of a ducted fan, for various airspeeds, duct angles of attack
and duct chord lengths. Li et al. (Ref. 6) also performed an
experimental investigation of a ducted fan system in ground
effect. The lift and drag forces generated by the duct system
were measured for various flow conditions and duct
geometries. Most of the simple ducted geometries analyzed
presented forward flight drag forces that were too high to be
efficient without a detailed optimization process.
Guerrero et al. (Ref. 7) developed a semi-empirical
aerodynamic method for calculating the performance of a
shrouded propeller during vertical take-off and landing, and
applied it to the multidisciplinary design optimization of an
unmanned aerial vehicle concept. In Ref. 8, Quackenbush et
al. present a smart shroud concept that could direct the flow
behind the propeller using a Shape Memory Alloy (SMA)
design. The shroud concept is applied to an Unmanned
Undersea Vehicle (UUV) and the numerical computations
are validated with experimental testing. Gossett (Ref. 9)
performed an extensive analysis of the performance of a
cross-flow ducted fan propulsion system for a lightweight
Vertical Take-off and Landing (VTOL) aircraft. The fan was
used only for providing the necessary vertical thrust, while
the forward flight thrust was provided by an independent
system. The study concluded that the VTOL system is viable
with a proper design of the ducted fan.
Pflimlin et al. (Ref. 10, Ref. 11) presented a linearized
aerodynamic model of a VTOL unmanned aerial vehicle that
used a shrouded propeller system. The model is used to
develop an attitude control system for dealing with cross-
wind gusts, and the experimental testing demonstrated its
effectiveness. In Ref. 12, Walton presented a concept for a
VTOL small aircraft that was powered by four ducted fan
systems. Each system used two counter-rotating propellers,
and could be moved between the position needed for
providing VTOL thrust, and the position required for
providing horizontal thrust. Graf et al. (Ref. 13) presented
duct geometries that improved the performance of the
system during horizontal flight of a Unmanned Aerial
Vehicle (UAV), by reducing the duct drag coefficient and
pitching moment coefficient. Experimental tests were
performed, and validated the improved performance and
controllability.
A typical fixed wing aircraft has a horizontal flight lift
to drag ratio of about 15. The thrust needed for level,
constant speed flight is about 15 times less than the aircraft’s
weight. This represents a challenge for designing an efficient
Vertical Takeoff and Landing (VTOL) aircraft, where the
thrust needs to balance its weight. A bigger engine and
propeller are needed, leading to increased weight, fuel
consumption and drag. This design conundrum is the main
reason there are very few VTOL aircraft today.
The ALIPTERA lip wing aircraft was imagined as a
VTOL aircraft capable of providing a viable door-to-door
transportation alternative to current means of transport,
designed by ALIPTERA Company (Ref. 14). It features a
variable configuration design that modifies between VTOL
mode and horizontal flight mode, achieving more efficiency
for both these modes. The concept aircraft features a lip
wing and shrouded propeller system located at the rear of the
fuselage and another shrouded, variable pitch, horizontally
mounted propeller at the front, providing balancing thrust
and control at lower speeds. Figure 1 presents the
ALIPTERA concept aircraft in the two possible
configurations, VTOL and horizontal flight.
Figure 1. The ALIPTERA VTOL concept aircraft
featuring the new lip wing system.
LIP WING CONCEPT
The system is composed of an elliptically curved wing - the
lip wing - and a propeller enclosed into a shroud. The
trailing edge of the lip wing matches aerodynamically the
leading edge of the shroud, creating a smooth and
streamlined continuous surface when the shroud/propeller is
tilted at a specific angle. The lip wing increases the area of
the lower section of the shroud. The airstream is forced over
3
top of the wing and the lift is created by accelerating a
volume of air so the pressure over the wing upper surface
decreases. A curved streamline will develop a pressure
gradient perpendicular to the direction of flow, with the
lower pressure on the inside of the curve. An alternate
explanation could be: the addition of the lip wing causes
more air to accelerate downwards, increasing the lifting
force, consistent with Newton's third law.
In VTOL configuration total lift is the result of lift
created by the lip wing added to the thrust created by the
propeller and shroud. As a result, the slip stream is not
vertical, but oriented at such an angle that may impede or
delay the formation of the vortex ring state, a well-known
hazardous situation for the helicopter and other VTOL
aircraft. The non-verticality of the slip stream may also
reduce the speed at which the aircraft enters transitional lift,
increasing the lift capacity and efficiency. At slow speed, the
lift capacity increases as a result of higher circulation around
the wing, improving the STOL characteristic of the aircraft.
Figure 2 presents the VTOL configuration of the lip wing
system.
Figure 2. VTOL configuration of the lip wing
concept.
The tilting angle between the shrouded system,
specifically the leading edge of the shroud and the lip wing
chord can be varied, enabling a computerized control system
to match the performance of the aircraft, its lift/drag, to the
aircraft flight condition, its airspeed, ensuring the ability of
the system to provide efficient high lift at slow or zero
forward speed where more lift is needed, conventional wings
being unable to provide the required lift. As speed increases,
the conventional wings starting to generate lift, enabling the
computerized control system to tilt the shroud system to an
angle that ensures the remaining needed lift is provided by
the lip wing/shroud system and at the same time increasing
the available horizontal thrust and decreasing drag. When
the speed reaches a value that all the required lift is provided
by the conventional wings, the computerized control system
has tilted the shroud system to an angle that might generate
maximum horizontal thrust and ensures the minimum drag
possible. In horizontal flight configuration, the lip wing and
the shroud propeller system are tilted at such an angle that
provides low drag in forward flight. Figure 3 presents the
horizontal flight configuration of the lip wing system.
Figure 3. Horizontal flight configuration of the lip
wing concept.
LIP WING SYSTEM PERFORMANCE
Before performing a preliminary analysis of the lip wing
concept, the basic performance of the shrouded propeller
must be investigated, in order to clearly identify the
parameters of interest and establish the framework for the
4
comparison between the basic design (only shrouded
propeller) and the proposed design (shrouded propeller and
lip wing system).
All the calculations are made using the assumption of
non-viscous flow. Figure 4 presents a cross-sectional view of
a typical shrouded propeller functioning in static conditions.
Station (4) is assumed to be sufficiently downstream of the
propeller so that the local flow velocity is uniform and
parallel to the propeller axis. The ratio between the fluid
stream area and the propeller disc area is called the
diffusion parameter , and is considered and important
design variable (Ref. 15).
Figure 4. Shrouded propeller is static conditions
(reproduced from Ref. 15).
The mechanical power provided by the engine on the
shaft is equal to the kinetic energy (in effect power, since it
is calculated per unit of time) of the fluid mass flow at
station (4):
(1)
From Equation (1), by introducing the diffusion
parameter, the expression of the fluid velocity at station (4)
can be obtained, as a function of the propeller design
parameters:
  

(2)
The thrust developed by the shrouded propeller is equal
to the product between the fluid mass flow and the fluid
velocity, calculated at station (4), and can be expressed as
follows:
    



(3)
Equation (3) also provides the link between two
important variables, the disk loading  
and the disk
power loading  
. It also shows that for a given shaft
power value, the static thrust can be increased by increasing
the diffusion parameter through a careful design of the
shroud exit section.
 
(4)
The advantage of using a shrouded propeller over a
simple free propeller becomes clear with the help of
Equation (3). According to Froude’s momentum theory (Ref.
16), the diffusion parameter is equal to 0.5 for a free
propeller. Thus, for equal available shaft power and equal
propeller disc area, the thrust ratio between the shrouded and
free propeller is given by Equation (4) and plotted in Figure
5.
Figure 5. Thrust ratio between shrouded and free
propeller as function of the diffusion parameter.
The lip wing concept versus traditional VTOL
In the case of a traditional VTOL aircraft that uses a
shrouded propeller, all the weight must be balanced by the
thrust produced by the propeller. Thus, if is the total
aircraft mass, and assuming that the propeller axis is
oriented vertically:
  
(5)
1.00
1.10
1.20
1.30
1.40
1.50
1.60
0.50 0.75 1.00 1.25 1.50 1.75
Thrust Ratio
Diffusion Parameter
5
By inserting Equation (3) into Equation (5) the power
required to balance the aircraft in the case of a traditional
design is equal to:
 

(6)
For the lip wing concept aircraft, the weight is balanced
by the sum of the thrust produced by the shrouded propeller
and the lift produced by the wing, as illustrated in Figure 6.
Figure 6. Forces developed by the shrouded
propeller and lip wing system.
To allow for a more general assessment, no assumptions
are made concerning the directions of the thrust and lift
forces. The angle between the propeller axis and the
horizontal direction is (it will be named propeller pitch
angle) while the angle between the wing chord and the
horizontal direction is (it will be named wing mounting
angle). The following equation can be written:
(7)
Again, by inserting Equation (3) into Equation (7) the
power required to balance the aircraft in the case of the lip
wing design is equal to:
  


(8)
Some limitations on the propeller pitch angle and the
wing mounting angle can be determined from the desired
condition that for equal aircraft weight and equal shrouded
propeller characteristics, the shaft power required for the lip
wing system is smaller than the power required for the
traditional VTOL design. Thus, from Equations (6) and (8):

 

  
(9)
Performing some mathematical operations of Equation
(9) leads to the following equation linking the minimum
pitching angle of the shrouded propeller to the ratio between
the lift generated by the lip wing and the weight of the
aircraft:
   

(10)
The minimum angle obtained with Equation (10) is
plotted in Figure 7 against the lift to weight ratio, for several
fixed wing mounting angles. The angle between the
shrouded propeller and the horizontal direction is primarily
determined by the lip wing’s ability to generate lift in the
VTOL condition, since it is the propeller thrust that must
provide the remaining force required for take-off/landing.
Figure 7. Minimum necessary propeller pitch angle
to satisfy power reduction requirement.
Lift generation on the lip wing
Because the lip wing is placed in the inlet region of the
propeller, then it will develop some lift, due to the pressure
difference between the wings’ lower and upper surfaces. On
the lower surface, the pressure will be equal to the
atmospheric pressure , while on the upper surface, the
pressure will be slightly lower , the pressure in the flow-
field ahead of the propeller disc. By applying the Bernoulli
equation for the flow-field around the wing leading edge, the
pressure difference between the wings’ lower and upper
surfaces can be written as:
 

(11)
0.00
15.00
30.00
45.00
60.00
75.00
90.00
0.00 0.20 0.40 0.60 0.80 1.00
Minimum necessary propeller angle
Lift to Weight Ratio
Theta 0 deg Theta 10 deg
Theta 20 deg Theta 30 deg
Theta 40 deg Theta 50 deg
6
Equation (11) cannot be applied for the entire chord-
length of the wing, because the suction velocity induced by
the propeller vanishes with distance ahead of the disc. An
assumption is made: the suction velocity induced by the
propeller ahead of the disc decreases proportionally with the
distance from the disc. In addition, we assume that this
velocity will be zero at the wing leading edge point, and
equal to the full value at the wing trailing edge point,
point that coincides with the propeller shroud leading edge.
If the    coordinate is placed at the wing leading edge
point, and the    coordinate is the wing trailing edge
point ( being the wing chord), then the suction velocity and
the pressure difference along the wing chord are given by
Equations (12) and (13):
 
(12)



(13)
The fact that the wing is circular, following the shape of
the propeller shroud, means that the pressure difference
between the upper and lower surfaces depends only on the
distance to the propeller face, while the direction of the
generated lift force depends only on the wing shape
expressed in curvilinear coordinate around the propeller
circular shape. The overall lift generated by the wing shall
be:
  


 



(14)
In Equation (14),
is the vector normal to the wing
surface and pointing towards the center of the propeller disc,
while  is the differential length element (as the wing
extends in a circular fashion around the propeller shroud).
By combining Equation (14) with Equation (13),
performing the calculation of the integrals and extracting the
modulus of the lift vector, the following lift force value is
obtained:
 


(15)
The velocity at the propeller disc station (1) that appears
in Equation (15) can be expressed in function of the velocity
at the downstream station (4), presented in Equation (2) in
order to obtain Equation (16):
 
 

(16)
Advantage of the lip wing system
If the traditional VTOL system and the lip wing concept
system are used for an aircraft of the same weight, then the
forces presented in Equations (5) and (7) must be equal:
   
(17)
Expressing the thrust according to Equation (3) and the
lift according to the proposed Equation (16), the following is
obtained:




 
 
(18)
Performing the mathematical calculations permits the
calculation of the power ratio:


 
(19)
In Figures 8 to 11, the ratio expressed in Equation (19)
is plotted against the propeller pitch angle for several
values of the diffusion parameter and for four values of the
ratio between the wing chord and the propeller diameter
( . For all plots, the wing
mounting angle was set equal to 0 degrees.
Figure 8. Power ratio plotted against the pithing
angle for C/D = 0.25.
0.50
0.75
1.00
1.25
1.50
0.00 15.00 30.00 45.00 60.00 75.00 90.00
Power Ratio
Shrouded propeller pitch angle
Sigma 0.75 Sigma 1.00
Sigma 1.25 Sigma 1.50
7
Figure 9. Power ratio plotted against the pithing
angle for C/D = 0.50.
Figure 10. Power ratio plotted against the pithing
angle for C/D = 0.75.
Figure 11. Power ratio plotted against the pithing
angle for C/D = 1.00.
PRELIMINARY EXPERIMENTAL DATA
AND COMPARISONS
In order to gain some further insight on the lip wing concept,
three sets of experiments were performed using an electric
2208 outrunner motor and an 8 inch diameter propeller. The
thrust developed was measured, using an electronic scale,
for the open propeller, the propeller encased in a shroud and
the shrouded propeller with a lip wing attached. All tests
were done at approximately the same power, and multiple
measurements were taken and averaged, in order to reduce
possible reading errors. The data collected during the three
experimental sets if presented in Tables 1 to 3.
Table 1. Measured performance for the open
propeller.
Intensit
y [A]
Voltage
[V]
Power
[W]
Thrust
[g]
Thrust/Pow
er [g/W]
9.7
11.1
107.7
118
1.096
12.11
10.85
131.4
148
1.126
10
10.97
109.7
123
1.121
10
10.8
108
120
1.111
10
10.76
107.6
115
1.069
10
10.7
107
121
1.131
10
10.48
104.8
124
1.183
Average Thrust
112 @ 100 W
Average Thrust/Power
1.12
% from reference
100
0.50
0.75
1.00
1.25
1.50
0.00 15.00 30.00 45.00 60.00 75.00 90.00
Power Ratio
Shrouded propeller pitch angle
Sigma 0.75 Sigma 1.00
Sigma 1.25 Sigma 1.50
0.50
0.75
1.00
1.25
1.50
0.00 15.00 30.00 45.00 60.00 75.00 90.00
Power Ratio
Shrouded propeller pitch angle
Sigma 0.75 Sigma 1.00
Sigma 1.25 Sigma 1.50
0.50
0.75
1.00
1.25
1.50
0.00 15.00 30.00 45.00 60.00 75.00 90.00
Power Ratio
Shrouded propeller pitch angle
Sigma 0.75 Sigma 1.00
Sigma 1.25 Sigma 1.50
8
Table 2. Measured performance for the shrouded
propeller.
Intensit
y [A]
Voltage
[V]
Power
[W]
Thrust
[g]
Thrust/Pow
er [g/W]
9.7
11.38
110.39
190
1.721
12.4
11
136.4
209
1.532
10
11.15
111.5
170
1.525
10
11.04
110.4
180
1.63
10
10.87
108.7
150
1.38
10
10.83
108.3
180
1.674
10
10.75
107.5
170
1.604
Average Thrust
156 @ 100 W
Average Thrust/Power
1.56
% from reference
139
Table 3. Measured performance for the shrouded
propeller with the lip wing.
Intensit
y [A]
Voltage
[V]
Power
[W]
Thrust
[g]
Thrust/Powe
r [g/W]
9.7
11.13
107.97
219
2.029
12.7
10.92
138.68
257
1.853
10
11.07
110.7
207
1.87
10
10.9
109
200
1.835
10
10.83
108.3
197
1.819
10
10.78
107.8
201
1.865
10
10.61
106.1
192
1.81
Average Thrust
186 @ 100 W
Average Thrust/Power
1.86
% from reference
166
The preliminary experiments performed show that the
simple lip wing system used provided an increase of 66%
over the free propeller and of almost 19% over the shrouded
propeller. The setup of the experiment is presented in Figure
12.
The data is also used to perform a quick comparison
with the theoretical approach. Knowing that the addition of
the shroud provides a thrust ratio  
 ,
from Equation (4) the value of the diffusion parameter can
be determined, and a value    is obtained. During the
experimental testing, the shrouded propeller kept its axis
vertical, but when the lip wing was added, the entire system
rotated from the vertical direction, so that the shroud reached
a pitching angle of approximately   , while the lip
wing chord line had an angle that is approximated to be
  ., with reference to the horizontal direction. The
ratio between the lip wing chord and the propeller diameter
was approximately  
 .
Figure 12. Setup of experimental tests for the lip
wing concept.
The ratio between the total forces developed by the lip
wing system and the traditional shrouded propeller can be
obtained with Equations (17) and (18) by maintaining the
power input equal for both systems:
9
  

 
(20)
With the numerical values detailed before, the
theoretical ratio between the lip wing system total generated
force and the traditional shrouded propeller thrust becomes
equal to 1.17, thus being a very good approximation of the
experimentally obtained ratio value of 1.19. There is a close
agreement between the numerical and experimental values,
thus providing a preliminary validation of the simplifying
hypothesis used in the theoretical development presented in
the paper.
CONCLUSIONS
The lip wing system was designed to provide increased
efficiency for a VTOL aircraft, in both take-off and
horizontal flight configurations. The system combines a
shrouded propeller and an elliptical wing in a single design
that can be modified according to the flight condition. The
performance of the shrouded propeller is calculated in
function of the power provided on the shaft and of the
diffusion parameter. The addition of the wing in the
propeller inlet region permits the generation of a lift force
that adds to the thrust developed by the propeller. A simple
analytical formula is proposed to approximate the lift force.
This allows the calculations of the static thrust conditions for
which the lip wing system becomes more power efficient
than a simple shrouded propeller. A preliminary
experimental investigation was performed in order to verify
the validity of the concept. The addition of a simple lip wing
has increased the total thrust by 19% compared to the simple
shrouded propeller. The numerical result obtained with the
performance estimation method is compared to the
experimental result, and a very good agreement is obtained.
Author contact:
Oliviu Şugar Gabor: oliviu.sugar-gabor.1@ens.etsmtl.ca,
Ruxandra Mihaela Botez: ruxandra.botez@etsmtl.ca,
Dusan Stan: dusan@aliptera.com
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