Aerodynamic and Mechanical Design of a Morphing Winglet for a Quadrotor Biplane Tail-sitter

Abstract

Multi-mode micro air vehicles with hover and high cruise speed capabilities can serve a wide range of mission requirements but are limited in their performance in either mode due to the design compromises that come from operating across a wide speed range. This paper describes a morphing winglet for a Quadrotor Biplane Tail-sitter (QBiT) that is proposed to improve the efficiency within a broad flight envelope. The ability of the morphing winglet to increase lift at low speeds and decrease lift at high speeds is quantified in three ways. First, a vortex-lattice method solver (VLM) that accounts for vortical interactions between wing surfaces was used to determine lift characteristics and aerodynamic loads on the vehicle at various dihedral angles and aircraft angles of attack. Through this investigation, it was found that a dihedral angle of 10° generates maximum lift regardless of angle of attack. Second, a VLM analysis was used again to understand the lift distribution along the span of the upper and lower wings at two specific dihedral angles:-90° in box wing mode and 20° in open winglet mode. Third, experimental flight tests showed that open winglet mode provided a 30% power difference at 4 m/s forward flight versus box wing mode. Additionally, for the same 175W power consumption, box wing mode supports a higher cruise airspeed of 7m/s compared to 5.5m/s in open winglet mode. Thus, it is envisaged that a 20° dihedral angle can be useful for low speed cruise mode and-90° dihedral angle for high speed mode when conducting a mission.

Full text

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Aerodynamic and Mechanical Design of a Morphing Winglet for a Quadrotor
Biplane Tail-sitter
Peter Ryseck
Undergraduate Research Assistant
Derrick Yeo
Assistant Research Scientist
Vikram Hrishikeshavan
Assistant Research Scientist
Inderjit Chopra
Alfred Gessow Professor and Distinguished University Professor
Director of Alfred Gessow Rotorcraft Center
Department of Aerospace Engineering
University of Maryland, College Park, MD 20740
ABSTRACT
Multi-mode micro air vehicles with hover and high cruise speed capabilities can serve a wide range of mission requirements
but are limited in their performance in either mode due to the design compromises that come from operating across a wide
speed range. This paper describes a morphing winglet for a Quadrotor Biplane Tail-sitter (QBiT) that is proposed to improve
the efficiency within a broad flight envelope. The ability of the morphing winglet to increase lift at low speeds and decrease
lift at high speeds is quantified in three ways. First, a vortex-lattice method solver (VLM) that accounts for vortical interactions
between wing surfaces was used to determine lift characteristics and aerodynamic loads on the vehicle at various dihedral
angles and aircraft angles of attack. Through this investigation, it was found that a dihedral angle of 10° generates maximum
lift regardless of angle of attack. Second, a VLM analysis was used again to understand the lift distribution along the span of
the upper and lower wings at two specific dihedral angles: -90° in box wing mode and 20° in open winglet mode. Third,
experimental flight tests showed that open winglet mode provided a 30% power difference at 4 m/s forward flight versus box
wing mode. Additionally, for the same 175W power consumption, box wing mode supports a higher cruise airspeed of 7m/s
compared to 5.5m/s in open winglet mode. Thus, it is envisaged that a 20° dihedral angle can be useful for low speed cruise
mode and -90° dihedral angle for high speed mode when conducting a mission.
INTRODUCTION
1
Morphing aircraft attempt to expand their operational
capabilities by tailoring their configuration to different flight
regimes. Today, morphing technologies are being
investigated for small rotorcraft unmanned air vehicles
(UAV) for military, package delivery, and search and rescue
applications. Current advancements in morphing schemes
show that aircraft size, flight range, and flight performance
envelopes can be further improved over baseline
characteristics (Ref. 1).
Organisms such as birds and flying insects often morph their
wings in order to execute different flight modes. Inspired by
nature, vehicles such as the Variable Gull-Wing Morphing
Aircraft mimic the variable wing geometry of a seagull
allowing the glide ratio to change from 1 to 11 depending on
the geometry of the wings (Ref. 2). Thus, the morphing
capability of the vehicle increases the mission capabilities by
allowing for a stable descent with both a minimized and
maximized horizontal glide distance.
Morphing technologies have also been investigated as a
means of providing roll control and lateral and directional
stability. Mills and Ajaj studied the performance
characteristics of folding wingtips to serve as control effectors
for a micro-air vehicle (MAV) and found that varying the
Presented at the 2019 8th Autonomous VTOL Technical Meeting &
6th Electric VTOL Symposium, Mesa, AZ, USA, Jan. 29-31, 2019.
Copyright © 2019 by the Vertical Flight Society. All rights reserved.
winglet angle improved stability and could augment existing
ailerons for roll control (Ref. 3).
Figure 1. Current version of the Quadrotor Biplane Tail-
sitter (Ref. 4)
In this work we design a morphing winglet system to expand
the speed range of a Quadrotor Biplane Tail-sitter QBiT.
Figure 1 shows a current version of the QBiT. It uses four
rotors for thrust in both hover and forward flight mode. In
hover, the trailing edges of the biplane wings point down with
the rotor thrust vector pointing up. Once in forward flight, the
vehicle uses its biplane wings to provide the necessary lift,
while using RPM variation to produce control moments of
roll, pitch and yaw. As a result of the lift that is produced by
the wings in forward flight, the thrust and power required for
steady level flight is significantly reduced compared to a
conventional quadrotor.

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Current versions of the QBiT have fixed wing geometries
which are inevitably sub-optimal for diverse mission
scenarios. Hrishikeshavan, et.al found that the sizing of a
QBiT wing and aspect ratio significantly impact flight
performance and power efficiency at various operating speeds
(Ref. 4). Thus, a morphing winglet applied to the QBiT
configuration could broaden the flight capabilities while also
consuming less power (Fig. 2).
The goal of this research is to improve and further expand the
multi-mission capabilities of the QBiT platform. In this paper,
we present the design and fabrication of the morphing QBiT
and evaluate aerodynamic advantages through the use of
experimental test flights as well as a vortex lattice-method
(VLM) solver. The paper first discusses the aerodynamic
design of the winglet using VLM and the performance
differences between wing geometry with and without
winglets. Next, the mechanical design of the morphing
winglet and vehicle platform is discussed. Lastly, key
findings from experimental flight data are presented.
AERODYNAMIC ANALYSIS AND DESIGN
A Vortex Lattice Method (VLM) 3D solver was used to
determine two metrics: First, estimate the winglet dihedral
angle (Γ) (Fig. 3) for maximum lift and then second,
determine the lift distribution of the upper and lower wings at
two fixed dihedral angles as well as a baseline case without
winglets. The VLM solver, Tornado (MatLab based), offers a
first order analysis based on the theory of an ideal flow (Ref.
5). The solver assumes 1) incompressible, inviscid and
irrotational flow, 2) lifting surfaces are thin and 3) angle of
attack is small. These assumptions are suitable for the low
angle of attack cruise regime.
Figure 3. A simplified diagram of the vehicle defining the
upper main wing, lower main wing, winglet and dihedral
angle
Maximum Lift at Cruise Configuration: Results and
Discussion
Our initial objective was to determine the dihedral angle of
the winglet for maximum lift. To determine this, linear mesh
types with dihedral angles varying from -90 to 90° at angles
of attack from 0 to 15° were tested. Figure 4 shows three
example geometries.
Figure 4. Vortex Lattice Method Geometry Examples.
Dihedral angles from -90 to 90° were tested.
The results yielded coefficient of lift (CL) values of the main
wings and winglets. These values were then used to determine
the total lift force (L) of the vehicle using Eq (1). A velocity
(u) of 15 m/s was chosen based on previous QBiT flights. The
surface area (S) of each wing was used to determine lift.
(1)
The values of CL generated from VLM were used to produce
Total Lift vs. Dihedral Angle, as shown in Fig. 6. As shown
by the dotted line, maximum lift occurs when the dihedral
angle is set to 10°. This result was consistent from 0 to 15°
angle of attack.
Figure 5. Lift vs. Dihedral Angle at AoA of 5°. Maximum
lift obtained at a positive dihedral angle of 10°.
This result is in agreement with Gerontakos and Lee who
found that a wing tip extension mounted at a dihedral angle
induced additional lifting circulation not present on a planar
Γ = -90°
Γ = 90°
Figure 2. Morphing Quadrotor Biplane Tail-sitter beginning from hover and transitioning to forward flight

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wing (Ref. 6). At a dihedral angle of 10°, the tip-vortex flow
is diffused, reducing the induced drag (Ref. 6). Beyond 10°
however, this effect is diminished by the reduction of
effective wing span that accompanies a larger tip angle.
Lift Distribution at Cruise Configuration: Results &
Discussion
Two dihedral angles were further explored to analyze the lift
distributions of the upper and lower wings. First, a dihedral
angle of -90° representing box wing mode. Second, a dihedral
angle of 20° representing open winglet mode. A third
geometry was also created, representing a baseline case with
no winglets. The lift sharing between the upper and lower
wing changes based on the configuration type, with all three
configurations producing more lift on the upper wing than the
lower. Figures 6(a), 6(b), 7(a) and 7(b) show the results of
these cases at an angle of attack of 0 and 10° of the lower and
upper main wings (upper wing refers to the upper main wing
without winglets), as is outlined in Fig. 3. Tables 1(a) and 1(b)
show the specific CL of these same upper and lower wings in
their respective flight mode and angles of attack.
At both 0 and 10° angle of attack, the baseline configuration
has similar lift distributions on the upper and lower wings,
with 6% more overall CL on the upper wing than the lower at
both angles of attack. This effect is further illustrated with a
delta CP distribution (Figs. 8-9). As was found in the CL
distribution plots, the pressure distribution (delta CP) of the
baseline case is similar on the upper and lower wings.
This symmetry between the upper and lower wing lift
distribution is affected when winglets are added to the
baseline wing system. Box wing mode results in
approximately 54% more overall CL on the upper wing than
the lower for 0° angle of attack. Open winglet mode produces
approximately 33% more overall CL on the upper wing than
the lower for 0° angle of attack. In box wing and open winglet
mode for 0° angle of attack, the CL distribution of the upper
wing is mostly consistent at a CL of 0.7, whereas the lower
wing more closely represents the baseline case. This
asymmetry is observed again in the delta CP distribution,
which shows that the delta CP along the span of the upper
wing is mostly constant whereas the baseline configuration is
not, as shown by the lighter regions on the wing tips.
At 10° angle of attack, the trends change for box and open
winglet mode. Overall, box wing mode produces
approximately 25% more overall CL on the upper wing than
the lower for 10° angle of attack. Open winglet mode
produces approximately 38% more overall CL on the upper
wing than the lower for 10° angle of attack. Open winglet
mode’s upper wing CL distribution remains mostly constant
as was found at 0° angle of attack, but the box wing’s upper
wing begins to taper off at the wingtips and instead follows a
trend skewed more similarly to the baseline case as shown in
Fig. 7(a). On the lower wing, the CL distribution at 10° angle
of attack show that all 3 cases follow the same trend and
remain at the same values of CL. The delta CP distribution
shows the same results. In open winglet mode, the distribution
of the upper wing is consistent along the span at both 0 and
10° angle of attack, whereas the distribution in box wing
mode and the baseline case is not.
Figure 6a Upper Wing Lift Distribution at AoA of 0°:
Open, Box and Baseline Geometries. Box and open
winglet mode have greater CL distributions compared to
the baseline configuration.
Figure 6b. Lower Wing Lift Distribution at AoA of 0°:
Open, Box and Baseline Geometries.
Figure 7a. Upper Wing Lift Distribution at AoA of 10°:
Open, Box and Baseline Geometries. Box and open
winglet mode have greater CL distributions compared to
the baseline configuration.
Figure 7b. Lower Wing Lift Distibution at AoA of
10°: Open, Box and Baseline Geometries. All three
geometries have similar CL distributions.

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Figure 8a. Delta CP distribution at 0 degrees AoA: Open
Winglet. Delta CP is constant along the span of the upper
wing.
Figure 8b. Delta CP distribution at 0 degrees AoA:
Box Wing. Delta CP is constant along the span of the
upper wing.
Figure 8c. Delta CP distribution at 0 degrees AoA:
Baseline. Delta CP is symmetric on upper and lower
wings.
Figure 9a. Delta CP distribution at 10 degrees AoA:
Open Winglet. Delta CP is constant along the span of the
upper wing.
Figure 9b. Delta CP distribution at 10 degrees AoA: Box
Wing. Delta CP begins to taper off at wingtips of upper
wing.
Figure 9c. Delta CP distribution at 10 degrees AoA:
Baseline. Delta CP is symmetric on upper and lower
wings.

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Table 1a. CL of Upper and Lower Main Wings at 0° AoA
Baseline:
CL
Upper Wing
0.52
Lower Wing
0.49
Box Wing Mode:
CL
Upper Wing
0.70
Lower Wing
0.32
Open Winglet Mode:
CL
Upper Wing
0.71
Lower Wing
0.47
Table 1b. CL of Upper and Lower Main Wings at 10° AoA
Baseline:
CL
Upper Wing
1.13
Lower Wing
1.049
Box Wing Mode:
CL
Upper Wing
1.31
Lower Wing
0.98
Open Winglet Mode:
CL
Upper Wing
1.59
Lower Wing
0.99
As shown in tables 1(a) and 1(b), the CL difference between
the upper and lower wings for the baseline case is marginal
and may be attributed to the wing interactions of the upper
and lower wings. However, the difference between the upper
and lower wings is much greater for the box and open winglet
modes compared to the baseline case. In open winglet mode,
the winglets extend the lift distribution of the upper wing, as
shown in Figs. 6(a) and 7(a). For box wing mode, the
asymmetric lift distribution between the upper and lower
wing is in good agreement with the findings of Gall and Smith
who noted a similar lift distribution for their box wing study
(Ref. 7). It is to be noted that this asymmetry may also be
impacted by the choice of winglet airfoil (Ref 8.)
The lift distribution difference between open and box wing
mode, as shown in Figs. 6(a), 6(b), 7(a) and 7(b), can also be
attributed to the interaction of the winglet with the main wing.
The addition of the winglets either in box or open winglet
mode significantly impact the lift distribution over the wings.
This is also illustrated in the pressure distributions over the
wing surfaces as shown in Figs. 8-9. The altered lift
distribution due to the presence of winglets in either the open
or box wing modes serve to reduce induced drag penalties,
and hence, also impact the trailing vorticity distributions (Ref.
6).
MECHANICAL IMPEMENTATION
The mechanical design of the morphing winglet mechanism
was influenced by the results found in the aerodynamic
analysis and design section. First, the dihedral angle limits for
the experimental vehicle were set to cover from -90 to at least
10° in order to achieve box wing mode at -90° and maximum
lift at 10°, as was estimated from the VLM analysis.
Furthermore, the wing loading requirements of the winglet
were estimated from the load predictions in Fig. 5, setting the
minimum torque requirement of the winglet at 2.5 Nm. In
terms of the geometry, the dimensions of the winglets had to
be large enough in order to achieve box wing mode using the
pre-existing biplane wings of the vehicle. Lastly, the chord
length and airfoil were chosen to be the same as the pre-
existing biplane wings for consistency. These parameters are
tabulated in Table 2.
Table 2. Winglet Design Parameters
Parameter
Value (units)
Wing Span
0.5 meters
Chord
0.254 meters
Min. Dihedral Angle
-90°
Max. Dihedral Angle
10°
Winglet Min. Torque
2.5 Nm
Airfoil
Wortmann FX 63-137
Worm Gear Transmission
A worm drive is capable of high torque with a 10:1 gear ratio,
reducing most of the strain on the servo. The torque
requirement of the servo is further reduced since the dynamics
of the gears do not allow for back driving of the servo. In the
end, the HITEC D955 servo was chosen for its high torque
and compact size. This transmission system is able to transfer
approximately 25 Nm of torque, setting the mechanism’s
overall safety factor at 10. With this setup, a full transition
from -90 to 20° can be accomplished within 2.5 seconds.
Figure 10. Worm Drive Mechanism labeled
Figure 11. Fabricated Version of the Winglet Mechanism
Upper Wing
Winglet

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OVERVIEW OF THE MORPHING
QUADROTOR-BIPLANE EXPERIMENTAL
VEHICLE
The vehicle uses four brushless motors configured in two
pairs above the upper and lower wing (X-configuration) (Fig.
12). The vehicle’s main wings span 1.0 meters with a chord
of .254 meters. The upper and lower wings have an offset of
0.5 meters from one another (Fig. 13). The vehicle uses four
fixed pitch variable RPM rotors with a diameter of .381
meters for propulsion and control in roll, pitch and yaw. The
motors are powered by two 6s 3300 mAh LiPo batteries in
parallel, providing 6600 total mAh of capacity to the vehicle
at 22.2 volts. The morphing quadrotor-biplane weighs
approximately 3.93 kg with the winglets and their supporting
mechanism contributing up to 25% of the gross take-off
weight.
Figure 12. Morphing Winglet Quadrotor Biplane
Tail-sitter
Figure 13. Front view of Morphing QBiT with key
dimensions
Table 3 provides a breakdown of the vehicle’s weight in
greater detail. In order to offset the weight of the winglet
mechanisms on the upper wing, the batteries were positioned
on the lower structural members, as opposed to inside the
fuselage, in order to keep the center of gravity at the center of
the vehicle.
Table 3. Weight Distribution of the Morphing QBP
System
Weight (g)
% Total
Batteries
1010
26
Fuselage
903
23
Winglet Mechanism
775
20
Upper and Lower Wings
447
11
Motors
371
9
Landing Gear
197
5
Winglet Wings
192
5
Avionics, Wires
35
<1
Total
3930 grams
100%
Microcontroller: ELKA-R
Onboard attitude stabilization was implemented using a
custom embedded lightweight kinematic autopilot (ELKA-
R). The microcontroller is developed in-house at the Alfred
Gessow Rotorcraft Center and weighs only 1.7 grams (Ref.
9). The Cortex-M4 microprocessor was chosen for ELKA-R
for its large memory and high clock speed. For orientation,
the board has an integrated MPU-9150 IMU which includes a
3-axis magnetometer, accelerometer and gyroscope. Due to
the large pitch changes throughout flight, a quaternion-based
feedback controller was implemented.
Flight Data Collection Electronics
The experimental vehicle was flown with a set of sensors
installed to measure power system voltage, current, and
vehicle airspeed. The avionics package uses a raspberry Pi
Zero for storage on an SD card. In total, two ELKA boards
were used. One ELKA board functioned exclusively as the
autopilot while the other was used for collecting pilot inputs,
vehicle pitch, yaw and roll attitude. A set of sensors was also
used to collect basic operating characteristics. First, the RPM
of all four motors was measured using an integrated ESC
RPM feedback. Second, a flow sensor was used to determine
the vehicle’s airspeed. Third, a voltage and current sensor was
used to measure power consumption. Similar setups have
been used on past vehicles (Ref. 9).
FLIGHT TESTING RESULTS
Figure 14. Box Wing Mode (left), Open Wing Mode
(right)
Flight Data Collection: Results
The morphing QBiT was flight tested to provide experimental
support for the first order VLM analysis. In order to determine
power draw characteristics, the vehicle was flown at two
separate and fixed dihedral angles: box wing mode, at -90°
dihedral angle and in open winglet mode, at 20° dihedral
angle (Fig. 14). The vehicle was equipped with onboard data
collection to determine power draw and airspeed in the two
flight modes.
The results from these flight tests are shown in Figs. 15, 16
and 17. Each data point represents one second of trimmed
steady level flight in its respective mode. Figure 16 shows the
power savings between both modes (Popen - Pbox), as
represented by ΔP. The figure shows that in hover, box wing
mode required a similar amount of power compared to open
winglet mode. At higher cruise airspeeds, the power draw
decreases for both modes, as is expected from previous flight
test results (Ref. 9). The cruise power savings in open winglet
mode versus box wing mode increases steadily and reaches a

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local maximum of 100 watts at approximately 4m/s, at which
point the ΔP begins to decrease. Thus, open winglet mode
required 30% less power than box wing mode at a cruise
speed of 4m/s.
Flight test results also show that box wing mode allows for
higher cruise airspeeds. At 5.5m/s the power required for
trimmed flight in open winglet mode reaches a minimum of
~175W and starts to increase slightly to a maximum measured
speed of 6m/s. At the same power setting of 175W, the cruise
speed in box mode is 7m/s.
Figure 15. Experimental Power vs. Cruise Airspeed.
Open winglet mode requires less power compared to box
wing mode.
Figure 16. Experimental Δ P (Popen - Pbox) vs. Cruise
Airspeed
Figure 17. Experimental Cruise Airspeed vs. Pitch.
Box wing has a greater speed for the same vehicle pitch.
Flight Data Collection: Discussion
These results indicate that open winglet mode requires less
power for low airspeed cruise and is suited for loiter
operations. Open winglet mode has a larger lifting surface on
the upper wing and a higher aspect ratio compared to box
mode, resulting in a larger overall lift coefficient. The VLM
analysis predicted that open winglet mode would produce
more lift than box mode. The experimental airspeed vs. power
data supports this prediction.
Current flight data shows that the box wing mode supports a
higher maximum airspeed and is suitable as a high-speed dash
configuration. Folding the winglets mean they do not
contribute to the total lift of the vehicle, thereby requiring a
higher cruise speed and more thrust. Specifically, flight data
shows that for the same 175W power consumption, box wing
mode supports a cruise airspeed of 7m/s compared to 5m/s
with the open wing. These conclusions are consistent with
general aerodynamic theory and the vortex lattice method
analysis of the wing system.
CONCLUSIONS
Hybrid vehicles such as the Quadrotor Biplane Tail-sitter
(QBiT) with hover and high cruise speed capabilities can
serve a wide range of mission requirements but are limited
due to the design compromises that come from operating
across a wide speed range. The focus of this paper was to
develop a Morphing Winglet QBiT and investigate the
aerodynamic performance gains. Unlike existing QBiTs, the
morphing winglet QBiT has two variable winglets capable of
rotating 110 degrees, allowing the vehicle to be optimized for
power efficiency across a wider flight regime. The following
is a summary of some key conclusions drawn from this study:
1. The morphing winglet QBiT weighs 3.93 kg with 4
rotors at a diameter of .381 meters. It has a maximum
wing span of 2.0 meters at a dihedral angle of 0° and
a minimum wing span of 1.0 meter at a dihedral
angle of -90°. The platform successfully
demonstrated hover, transition to forward flight
mode, and sustained forward flight in box wing (Γ=-
90°) and open winglet mode (Γ=20°).
2. The winglet mechanism provided a maximum torque
of 25 Nm, 10 times greater than the estimated
maximum winglet loads.
3. A first order vortex lattice method study predicted
that a winglet dihedral angle of 10° produced
maximum lift. VLM also predicted that the upper
wing produces more lift than the lower in all three
cases: box wing, open winglet, and a standard
biplane configuration without winglets. At 0° angle
of attack, box wing mode has a CL difference of 54%,
open winglet 33%, and baseline 6%. At 10° angle of
attack, box wing mode has a CL difference of 25%,
open winglet 38%, and baseline 6%. These were in
agreement with what has been reported in current
literature.
4. At increasing steady level cruise airspeeds, both box
wing and open winglet mode power draw

8
requirements were reduced. Open winglet mode
consumed less power compared to box wing mode
throughout the transition, peaking at a maximum
power difference of 100 watts at 4 m/s, which
represents 30% less power than box wing mode.
5. At the same pitch, the vehicle’s airspeed in box wing
mode is consistently higher, supporting the idea that
box wing mode is capable of greater speeds than
open winglet mode. In flight testing, box wing mode
supported a higher max measured cruise airspeed of
7m/s compared to 5.5m/s with the open wing at
175W.
AUTHOR CONTACT INFORMATION
Peter Ryseck – pryseck@umd.edu
Dr. Derrick Yeo – dyeo@umd.edu
Dr. Vikram Hrishikeshavan – vikramh@umd.edu
Dr. Inderjit Chopra – chopra@umd.edu
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