computing science and automatic control (cce) mexico city, … · 2019-10-15 · aircraft: i) the...
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2019 16th International Conference on Electrical Engineering,Computing Science and Automatic Control (CCE) Mexico City,
Mexico. September 11-13, 2019
Design and Construction of a UAV VTOL inDucted-Fan and Tilt-Rotor Configuration
Juan M. BustamanteUMI-LAFMIA
CINVESTAVMexico City, Mexico
Cindy A. HerreraUMI-LAFMIA
CINVESTAVMexico City, Mexico
Eduardo S. EspinozaUMI-LAFMIA
CONACYT-CINVESTAVMexico City, Mexico
Carlos A. EscalanteCENTA
CONACYT-CIDESIQueretaro, Mexico
Sergio SalazarUMI-LAFMIA
CINVESTAVMexico City, [email protected]
Rogelio LozanoUMI-LAFMIA
CINVESTAV-UTCMexico City, Mexico
Abstract—This paper presents the development of an hybridUnmanned Aerial Vehicle that combines the capabilities ofVertical Takeoff and Landing (VTOL) as well as the hover flightof an helicopter, with the autonomy, load capacity and speedoffered by an airplane. We start from an aircraft concept thatuses the Tilt-Rotor configuration to perform the transition phasefrom one flight mode to another and simultaneously we usedDucted-Fans for increasing the flight performance during thevertical flight phase. The aerodynamic design is developed usingcomputational tools, such as XFLR5 for preliminary design andANSYS FLuent®for the verification of the preliminary fixed wingdesign. While for the vertical flight, experimental tests wereconducted in order to determine the lift system requirementsby using Ducted-Fans. Afterwards, the manufacturing processderived from the aerodynamic design is presented. The avionicsdeveloped is described for handling the control surfaces duringthe airplane mode and the main lift system during the verticalflight as well as the integration of mechanism and sensors used.Finally, the obtained results from the conducted vertical flighttests at outdoors environments are presented in order to showthe effectiveness of the aircraft concept.
Index Terms—Fixed-wing VTOL, Ducted-Fan, Tilt-Rotor, UAV,Aerodynamic Design, Convertiplane
I. INTRODUCTION
The unmanned aerial vehicles (UAVs) have been convertedin versatile aircraft that helps to does not expose the lifeof a pilot on board. Due to this versatility, recently a lotof applications for UAVs have been developed. For instance,aerial photography and cartography, patrol, load transport,recreation, among others. The UAVs have been also used toperform search and rescue missions as well as transportationof food and medicines to affected areas as a support foremergencies services. So the landing in this kind of situationsis limited to aircrafts with VTOL capabilities.
This work has been partially supported by the Mexican Conacyt ProjectLN299146.
Despite the fact that more than 70 years have passedsince vertical flight has been development, there are onlythree operational types of vertical take-off and landing VTOLaircraft: i) the helicopters, ii) the V-22 Osprey tilt-rotor, andiii) the AV-8V Harrier jump jet [1]. Although the advancesthat have been reached with these aircraft, it has not beenpossible yet to develop an aircraft capable of carrying heavyloads over long distances at a high speed as airplanes can doand perform vertical flight. The problem lies in achieving bothvertical flight as horizontal flight without sacrificing efficiencyat each flight mode. Nowadays, there are configurations thatattempt to address this challenge, but all of these solutionsconverge in a series of problems in common, starting withthe difficulty of building this type of aircraft, looking forsimplicity of manufacture, reduction of weight and vibrations,as well as the ability to perform a stable flight. In the literaturewe can find different types of convertiplanes, among whichare: the Tilt-Rotor, the Tilt Wing, the Tail-sitter, the Dual-System and the Ducted-Fan.
The tilt-rotor aircraft are categorized into three branches:the bi-rotor, the tri-rotor, and the quad-rotor. These onesemploy two, three or four tilting rotors, respectively, to providelift in hover mode and thrust in cruise mode. The bi-rotordesign concept was first introduced in 1993 by the BellHelicopter Inc with the Bell Eagle Eye UAV. The Tri-rotorconvertiplane generally features the followings advantages:the lift generation requirement on each rotor can be reduced,rotor configuration is geometrically symmetric with respectto the aircraft center of gravity, which can effectively easethe attitude stabilization of the tri-rotor convertiplane in hovermode, and the rear rotor is constantly fixed to the fuselage andthus causes minor additional mechanical design complexity.The first remarkable success of the development of a tri-rotoraircraft was the Panther UAV. The third tilt-rotor configurationis the quad-rotor. The most representative aircraft with thisconfiguration is the Quantum Tron which integrates a quad-978-1-7281-4840-3/19/$31.00 ©2019 IEEE
rotor configuration and retractable rotor blades[2].The main feature of a tilt-wing is that the wing needs to be
tilted together with the rotors during flight mode transition.Compared with the tilt-rotor, a tilt-wing generally featuresa more complicated and sophisticated design in on-boardcomponents such as the tilting drive train. Furthermore, inlow-speed operation the wings of the convertiplane need to bepointing upwards, which makes the aircraft more vulnerableto cross wind and thus it increases effort in developing controlmechanisms to reach attitude stabilization. An example of thisconfiguration is the Greased Lightning (GL) VTOL Dronewhich adopts a substantially different control mechanism byemploying ten fixed-pitch rotors along the airframe. Anothervery successful example is the DHL parcelcopter which istargeted for parcel delivery in mountainous regions [2].
The other type of hybrid UAV is the tail-sitter whichperforms a VTOL on its tail and the entire airframe tilts toachieve cruise flight. Its principal advantage, the reduction enmechanical complexity because it is not necessary any extraactuators to perform the transition. Although this advantage,it is an unstable aircraft for VTOL that requires a complexcontrol systems to achieve the transition. Some example ofthis configuration are the Wintrag One and the Mark II.
The dual-system utilizes two sets of propulsion systems:one contains rotors mounted pointing upwards for verticaloperation and another adopts propeller functions for cruiseflight. The dual-system has a simplified mechanical design,compared with the two aforementioned convertiplanes. How-ever, during cruise flight, the multiple non-operational rotorsfor VTOL cause extra aerodynamic drag. The Arcturus JUMPis a representative dual-system example which includes ahybrid energy system that consists of a source of electricenergy for vertical propulsion and a combustion motor forcruise flight [2].
The UAVs that use Ducted-Fans for the propulsion systemhave several advantages compared with other rotary wingsUAVs. Its rotors are compact and covered with a duct thatminimized the hazardous area for both operator and the envi-ronment. Furthermore, it also provides more lift and it emitsless noise that the free rotors [3]. There are few representativeprototypes using the Ducted-Fan configuration. One of themis the Allied Aerospace’s iStar 29, a concept introduced byGeneral Dynamics in the 80s. Another Ducted-Fan vehicle isthe Phantom Swift developed by Boeing.
In this work, we developed an aircraft by using a ducted-fan configuration to increase the aircraft lift during the verticalflight. The configuration presented in this work, consist oftwo ducted-Fan inside the airplane wing and a counter-rotatingtilting motor on the front to perform the transition betweenboth flight modes. The main contributions of this paper canbe summarized as:
• The development of a VTOL aircraft based on ducted-fans and a tilt-rotor configuration.
• The development of a comparative study between theXFLR5 and ANSYS Fluent® CFD software in order
to verify the precision of the results obtained with theXFLR5 as a preliminary software design.
• The development of an Experimental study of the ducted-fan in order to increase the generated lift.
The remaining of the paper is organized as follows: Insection II, the development of the aircraft is described underthe desired configuration, starting from the development ofconceptual design. Continuing with the aerodynamic designof the aircraft. Where the wing is designed based on theanalyzes developed by XFLR5 and obtaining a verificationthrough ANSYS Fluent. In section III, Experimental tests wereconducted to compare the performance of the embedded rotorinside a duct and without it. In section IV, the construction ofthe prototype is described and in section V, the vertical flighttests were carried out to evaluate the concept of ducted-fanand Tilt-Rotor UAV VTOL.
II. AIRCRAFT DEVELOPMENT
In this section we describe the design process of the VTOLvehicle, starting from the desired requirements through theconceptual design to the construction of the experimentalprototype for the evaluation of the VTOL concept.
A. Flight profile
The flight profile is the set of tasks that the aircraft shouldbe able to perform from the takeoff to landing phases. Weconsidered six stages during a typical mission which canbe modified depending on the mission requirements. In ourcase, the mission to be performed is the cruise type, which isintended for load transportation from a point A to a point Bby considering a stall speed of 10 m/s.
Fig. 1 depicts the desired flight profile: 0) at the first stagethe aircraft should perform a vertical takeoff, then 1) it shouldascend to a height of 30 m, after this point it should 2) performa transition to airplane mode, and then 3) start the cruisingstage. At the end of this stage the vehicle should reduce itsspeed in order to perform 4) a transition to vertical mode andthen 5) start the descending phase to finally 6) complete themission with a vertical landing.
Fig. 1: Desired flight profile for the VTOL aircraft.
B. Description of the concept aircraft
We decided to use a Conventional Airplane Configurationsince in order to perform cargo transportation missions thevehicle won’t reach speeds greater than 20 m/s. By definingthe wing as the main component of the aircraft that ensures
lift and compensates the weight, then the vehicle can performa stable flight. For the development of this aircraft we decidedto use a straight wing due to: its ease of construction, its goodperformance at low speeds. Since this type of wing does notmodify the length of the airfoil chord along the wingspan itallows a better arrangement of the engines that will be insidethe wing.
While a horizontal stabilizer provides longitudinal stabilityto the aircraft, the vertical stabilizer provides stability in theyaw movement. For the considered airplane, we decided to usea V-Tail configuration since it is composed of 2 surfaces andhas less drag compared with the conventional configurationwhich has 3 control surfaces.
A Ducted-Fan is a hybrid system that is characterized bysurrounding the rotor blades inside a duct. In this propulsionsystem an increase in lift is obtained by increasing the down-stream flow area, while it is obtained a reduction in energyconsumption compared with traditional rotors. In addition, aDucted-Fan potentially mitigates the generation of rotor noiseand protects the blades from damage.
Based on the above information, we obtained the conceptaircraft shown in Fig. 2a, which has been sketched by usingCATIA V5 software as a result of several iterations that startedfrom the conceptual design through the detailed design andmanufacturing of the prototype. This concept has a configu-ration based on a conventional aircraft with a high straightwing and stabilizer in V-Tail for the plane mode, while forvertical mode it has two ducted-fans located inside the wing,in such a way that them do not affect the aerodynamicsin the airplane mode. In the front part of the aircraft thereis a tilting coaxial motor, which together with the ducted-fans create a multi-rotor configuration known as a Y-4 whichsimultaneously, is a variation of a quad-copter, since it has 4rotors pointing upwards. What can be seen from the proposal
(a) VTOL aircraft concept basedon a system of Ducted-Fans anda Tilt-Rotor.
(b) Final dimensions of the developedvehicle.
Fig. 2: CAD and final dimensions of the proposed VTOLvehicle.configuration in the VTOL mode, is that the Ducted-Fanssupport the greater amount of the aircraft weight since themare completely devoted to work only in for vertical flight. Inthe same way, the coaxial motor will have to be able to helpwith the remaining weight in the vertical mode and will also beresponsible of performing the transition to the airplane mode,which is the main function of the Tilt-rotor configurations bytilting the coaxial motor from a vertical position to a horizontal
one, where the aircraft will gain speed reaching the stall speedand completing the transition to airplane mode from which theaircraft will be propelled by the front engine by using only oneof its propellers.
C. Aerodynamic Design
The first phase for the wing design consists on the selectionof the airfoil. In order to characterize a profile by its loadcapacity or speed, according to the type of mission thatthe aircraft will should perform, there are two fundamentalparameters which help in the selection of the profile. The firstof these is the Lift Coefficient (CL) defined as the lifting forcenormalized by the wing surface and the dynamic pressure(q = 1/2ρV 2
∞). The CL increases with the angle of attack(α) for a given profile [7]. The second parameter is the dragcoefficient (CD) defined as the drag force normalized by thewing surface and the dynamic pressure, is also dependent onthe angle of attack. It should be noted that these coefficients aredimensionless and are defined mathematically in the followingway, (1) and (2).
CL =L
12ρ V
2∞ Sw
(1) CD =D
12ρ V
2∞ Sw
(2)
where: Sw: wing surface, L: lift force, D: drag force,ρ: wind density, V 2
∞: relative speed.
For the aircraft that was developed, one of the most im-portant parameters is the maximum relative thickness (emax)because the wing should have enough space to contain the twoDucts.The calculations developed in XFLR5 were made using theatmospheric conditions of Mexico City, at an elevation of2,250.0 m. Taking into account the stall speed Vs = 10m/sand a airfoil’s chord c = 30 cm, a number of ReynoldsRe = 171, 399.0 is calculated.Based on the calculations obtained from the software, theGOE425 airfoil was chosen, whose characteristics are: Max-imum Lift Coefficient (CLmax = 1.6) at 12o, zero-lift angle(α0L = −4), maximum thickness (emax = 0.16c) andminimum CL/CD = 40.
D. Verification of the CL vs α graph obtained with XFLR5using ANSYS Fluent ®
For the first calculation of the wing’s CLmax, the XFLR5software was chosen because it is used for preliminary designdue to the less time to solve, giving the possibility to evaluatedifferent proposals airfoils, but reducing the accuracy ofthe calculations. As a reason for the verification of the dataobtained with XFLR5, a more robust CFD software calledANSYS FLUENT® was used to have more certainty aboutthe calculations made.What is sought with this analysis is to obtain the curve CL
vs α and also to be able to visualize the detachment of theboundary layer at the different angles of incidence.To obtain accurate results, a meshing methodology was
proposed for the discretization of the problem as appropriatein the regions of interest taking care not to consume muchcomputational cost.The SST(k − ω) model was used to solve the problem, thewhich according to the literature [8] is a robust model suitablefor modeling turbulent flow, the principal disadvantage is thatthis model consumes many computational resources.
A mesh sensitivity analysis was developed, which ensuresthat the results are independent of the mesh. For this analysis,we considered the case of zero angle of attack of the airfoil(α = 0), obtained in the XFLR5, the results obtained forLift coefficient (CL) were evaluated considering differentnumbers of mesh elements. Finding that from 350 thousand
Fig. 3: The CL vs Number of elements graph is used toperform a sensitivity analysis of the mesh to ensure theindependence of the mesh. In this case considering α = 0.
elements, the mesh no longer affects the results of the CFDanalysis see figure 3. So, when a mesh over that number ofelements it is used, the results of the simulation no longervary much and also the simulations take longer, because thecomputational cost increases.From the calculation made to the goe425 airfoil, using a meshof 350 thousand elements, from an angle of attack of -5◦ to12◦, the curve of CL vs α shown in Figure 4 was obtained. Inthis graph, it can be see the curve corresponding to the resultsobtained with the XFLR5 and its verification with ANSYSFLUENT. From the direct comparison corresponding to eachangle of attack, its proximity can be observed, obtaining amaximum error of 12%.
E. Wing dimensions and stabilizers
The dimensions of the wing start from the equation referringto the lift defined in equation (1) of which the wing surface(Sw) is unknown. The parameters used for the calculationof the Sw are given by the parameters of a total weight(WT = 2kgf ), the stall speed (Vs = 10 m/s), a density(ρ∞ = 0.98151kg/m3) of 0.98151 kg/m3 and a CLmax =1.58.
Based on the above data, the span of the wing can bedetermined since the wing being rectangular the span isobtained by dividing Sw between the chord c, this gives us awingspan bw = 0.8434m. For security reasons and to increasethe ARw to 5. The dimensions of the wing are as follows:
-6 -4 -2 0 2 4 6 8 10 12 14
Angle of attack (α) [degrees]
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Lift C
oeffic
ient (C
L)
ANSYS FLUENT
XLFR5
Fig. 4: CL vs α graph. Comparison between ANSYS FLUENTand XFLR5.
wing surface (SW = 0.45m2), an aspect ratio (ARw = 5),the taper ratio (λ = 1) and the chord (c = 0.3m).
The design of the stabilizer is similar to the design ofa wing. The sizing of the stabilizer starts from the dataalready obtained from the wing, using the volume coefficientmethod [9] and [10]. Using this method the dimensions ofthe V-Tail was obtained as: V-Tail surface SV T = 0.16m2,V-Tail span bV T = 0.74m, V-Tail chord crV T = 0.2m V-Tailaspect ratio ARV T = 3 and V-tail angle = 42.The finaldimensions of the aircraft are shown in figure 2b. Thesedimensions are those used during the manufacturing phasefor the construction the prototype for tests.
III. EXPERIMENTAL TESTS OF ROTORS
During the vertical flight phase, the lift force will beprovided by the rotors. Starting with the engines used forthe Ducted-Fan. These are restricted in terms of dimensionsbecause they must be contained within the wing. Rotors witha high pitch angle were chosen to produce enough lift to raisethe whole vehicle, selecting this kind of rotors can compensatethe small diameter of the blades due to the confinement insidethe Ducted-Fan.For the Ducted-Fan, two different designs were used (figure5) in order to determine which design would be most efficient.These ones are based on facilitating the entry of air into theducted by smoothing the surface of the duct lip. The guidevanes was modified their shape and quantity. These modifi-cations affect the efficiency of the Ducted-Fan efficiency. [5].The tests of the motors and rotors were carry out in a test
(a) Duct 1, with inletlip and 4 guide vanes.
(b) Duct 2, with inletlip and 6 guide vanes.
Fig. 5: Ducts used in the tests to get more lift.bench. Monitoring the current, voltage and electrical power
of the system. These parameters are important to determinewhich combination turns out to be more energy efficient.
A. Tests Results
With the experimental data obtained, the graph that relatesthe traction to the electrical power was constructed for eachof the combinations of motor, rotor and duct. After a selectionprocess based on its traction-power ratio, the best rotor testedin the two ducts was taken for each engine.From the data analysis it can see that the MR2205 motor standsout with the rotor 5045 of two blades using a duct with 4 guidevanes for having the best traction-power ratio.In order to confirm the hypothesis of the increase of tractionwhen the blades are inside a duct [6]. The data obtained withthe MR2205 engine was evaluated separately using the 5045rotor with two blades inside the ducts of 4 and 6 guide vanes,as well as the free rotor. And it can see from figure 6, that thereis an increase in lift as well as a lower energy consumptionwhen using the ducts, mainly the 4-blade guide duct is theone that has a better performance, increasing up to 20 % thetraction generated for the rotor compared with not using theduct.
0 50 100 150 200 250
Electric Power [W]
100
200
300
400
500
600
700
Tra
ctio
n [
grf
]
5045/2 Blades, MR2205 D6
5045/2 Blades, MR2205 D4
5045/2 Blades, MR2205 Free
Fig. 6: Comparison between free rotor vs rotor with duct usingthe MR2205 engine.
IV. CONSTRUCTION OF THE PROTOTYPE
Starting with the design in detail, the next phase isthe construction of the prototype, in which the wingmanufacturing stands out. Expanded Polystyrene (EPS)was used to build the wing and the V-Tail, to give greaterrigidity to the wing, carbon tubes were added. For theconstruction of the fuselage, a semi-monocoque type structurewas made using plywood frames and balsa wood spars. Theconstruction of the Ducted-Fan was carried out using additivemanufacturing technology, in this case, using PLA material.
For the propulsion system, the Himax contra-rotatingmotor CR2816 series was positioned in the front of thevehicle. It uses 2 propellers, one of 10×5 inches and theother of 9×7 inches. The DYS MR2205 is a precision enginedesigned for multirotor vehicles and optimized for 5 inchespropellers. This engine was chosen to be contained withinthe ducts that go in the wing due to its size and performance.
The autopilot used is the Pixhawk 1, which is a popularautopilot for general-purpose flight controller and the firmwarecharged in the board is the px4fmu-V2 which is an opensource autopilot development project that is part of DroneCode. It has been developed with an architecture that can beeasily migrated from one platform to another. Once all thecomponents are integrated, it can seen in figure 7 the finishedprototype of the proposed aircraft.
Fig. 7: Finished prototype VTOL aircraft including avionics.
V. TESTS AND EXPERIMENTAL RESULTS
A series of experimental tasks were carried out todetermine the functionality of the VTOL aircraft conceptunder the Ducted-Fan and Tilt-Rotor configuration. Thetests and experimental results obtained only considered theperformance during the vertical flight evaluating the VTOLcapacities as well as its stability in stationary flight.
A. Vertical flight tests
The vertical flight tests were carried out at Mexico City.The maximum takeoff weight of the aircraft (MTOW ) was1.7kgf . The first tests consisted of checking the correctfunctioning of each of the aircraft systems, especially theswing mechanism for the transition. The following was tocarry out flight tests inside the laboratory, restricting thetranslational movements due to the reduced space of the place.These tests consisted in evaluating the take-off capabilitiesof the vehicle, the stabilization in orientation of the vehicle.From these tests, the load capacity was evaluated, findingdifficulties during the takeoff, since the vehicle presentedproblems to stay in the air. For that reason, the the V-Tailstabilizer was removed to reduce weight allowing the aircraftto takeoff during the inside’s tests with a MTOW = 1.4kgf .
The following were the flight tests in exteriors. Theatmospheric parameters during the performance of theflight tests is very important, because the performanceof the aircraft depends on several factors but mainly onthe temperature, humidity and atmospheric pressure whichtogether or separately affect the density of the air causingthe aircraft to have better VTOL capabilities one day incomparison with another. Similarly, the wind speed thataffects the performance of the VTOL was taken into account.
The flight test was carried out at 12 hrs (local time),under a temperature of 15oC, at a relative humidity of 77.1%
and an atmospheric pressure of 1026 HPa and a wind speedcomponent 11.1 Km/h NE. During this test the aircrafttraveled 31.5 m to a maximum height of 4 m, managedto take off with 50% power of the motors. In this test, theaim is to stabilize the vehicle in orientation through thedefault control, provided for the PX4 firmware charged to theautopilot, which consists of a PID controller. The flight logsshow that the vehicle is kept following the reference in rollangle (Fig. 8a). The pitch angle already has fewer oscillationswhen it begins to follow its reference see Fig. 8b while forthe result yaw angles are shown in Fig. 8c.
As for energy consumption, the vehicle’s motors required anaverage current of 24.2A, with peaks of 43.7A, which causedthat when using a 3S Lipo battery with 3.6 mAh, these willnot last more than 5 minutes of vertical flight. Consideringthe flight profile presented in section II, it is estimated thatthe VTOL phase and transition will take one minute in orderto save energy for the airplane phase which it will take around20 minutes. Below is the link of the video where the verticalflight of the test is shown https://youtu.be/egCvkzUrZX4.
0 2 4 6 8 10 12 14 16 18
time [s]
-20
-10
0
10
20
Ro
ll a
ng
le [
de
gre
es]
Roll
Roll setpoint
(a) Tracking for Roll angle
0 2 4 6 8 10 12 14 16 18
time [s]
-30
-20
-10
0
10
20
30
[gra
dos]
Pitch
Pitch referencia
(b) Tracking for Pitch angle
0 2 4 6 8 10 12 14 16 18
time [s]
-80
-70
-60
-50
-40
-30
-20
Ya
w a
ng
le [
de
gre
es] Yaw
Yaw setpoint
(c) Tracking for Yaw angle
Fig. 8: Flight test tracking result of the vehicle’s attitude.
VI. CONCLUSIONS
An aircraft with VTOL capabilities was built combiningthe Ducted-Fan and Tilt-Rotor configurations. Starting from anaerodynamic design of the aircraft based on the methodologyproposed by Raymer [9] and Roskam [10], using the compu-tational tools of XFLR5 for first approximations and ANSYSFluent for the verification of the results. At the same time, theperformance of the confined rotors in comparison with the freeones was checked experimentally, obtaining a 20% increase inthe lift generated at full thrust using a Ducted-Fan.The developed experimental platform allowed to conduct verti-cal flight tests at outdoors environments achieving successfultake-off and landing. Furthermore, the vehicle was kept inhover flight by means of the stabilization of its orientationangles.Finally, based on the obtained results, it can be deduced thatthis aircraft is functional as a VTOL vehicle by using theproposed configuration.
VII. FUTURE WORK
As future directions of this research we consider to performa deeper analysis of the ducted-fans in order to increase itsperformance during the vertical flight as well as to considerthe effects that occur on the wing during the airplane modecaused by the ducted-fans.Similarly, we are going to verify the aerodynamics of theaircraft through CFD to characterize the vehicle and also wewill perform optimizations of the proposed design. Finally,we consider to evaluate the flight capabilities of the aircraft incruise mode and to perform the transition between both flightmodes.
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