[SG Symposium] FuelCellUAV CSU 1

8/8/2019 [SG Symposium] FuelCellUAV CSU 1

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Design and Test of a 24 Hour Fuel Cell Unmanned Aerial Vehicle

(FCUAV)

Derek Keen, Grant Rhoads, Tim Schneider, Brian Taylor, Nick Wagner

Colorado State University

Faculty Advisor: Dr. Thomas Bradley

Abstract

Long endurance unmanned aerial vehicles

(UAVs) have increasing value as a low cost,

autonomous reconnaissance and remotesensing platform for research, commercial

and military missions. Current multi-

disciplinary optimization techniques andfuel-cell technologies have the potential toincrease the endurance of such systems

significantly. Research performed by Dr.

Thomas Bradley while at Georgia Tech.University showed that significant gains

over current systems were possible. This

aircraft, powered by a polymer electrolytemembrane (PEM) fuel cell, with compressed

hydrogen storage, and integrated

conditioning systems, is an effort to verify

and continue his research. The flight testresults will be compared with the

optimization research leading to this aircraft

design and flight tests, as well as topublished results of similar 0.51kW long-

endurance unmanned aircraft. As per the

research analysis, the flight tests will verify

the increased endurance of greater than 24hrs of flight time. Further improvements to

the system and planned future work will

possibly include switching to a liquid

hydrogen storage system for greatlyincreased endurance. The practical

implications of this effort are wide reaching

and pertinent both to further research workand current UAV customers.

1 Airframe DesignThe research and aircraft demonstrator

undertaken by Dr. Thomas Bradley at Georgia

Tech University provided the set point for the

airframe that was constructed during the

summer of 2009. As noted above the goal ofthis aircraft is to demonstrate the use of a

gaseous hydrogen supplied PEM fuel cell

system. Based on Dr. Bradleys research, and a

custom designed 600 W fuel cell from United

Technologies Research Center (UTRC), we had

an optimal threshold in terms of weight, size,

and aerodynamics that had to be met in order

to achieve the predicted 24 hour flight later

on[3]. The design decisions made as a result

are discussed in the following sections.

1.1 Wing AssemblyAll of the lifting surfaces on this aircraft are

originally from the Blue Explorer 5m composite

sailplane sold by Northeast Sailplane Products

. This approach allowed for a shorter

development time, while providing a high

quality, aerodynamically efficient and stable

wing to begin the design process. To maintain

the aircraft stability, care was taken to ensure

that the center of gravity was directly beneath

the quarter chord of the wing. The quarter

chord refers to the position one quarter of the

distance between the leading and trailing

edges. The existing fastener attachment points

were used to connect the wing to the carbon


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fiber spine via custom ASTM 6061 aluminum

mounts. This carbon fiber spine is discussed in

further detail below.

The wing is a three piece spar and monocoque

composite structure, with eight internal servomotors controlling split ailerons, flaps, and

spoilers. The airfoil is a modified HQW 2.5 for

high lift at moderate speeds and low Reynolds

numbers. The lifting capacity of this wing was

determined sufficient based on the coefficient

of lift and wing area as compared with the

computational design tool [2,3] developed at

Georgia Tech as well as the published metrics of

the acceptable G-loading.

1.2 Tail AssemblyThe empennage of this aircraft was taken from

the Blue Explorer sailplane mentioned above. It

utilizes a traditional configuration with the

elevator positioned very close to the horizontal

datum plane of the main wing making an upside

down T with the rudder. Using a traditional

configuration allowed for the application of

previously developed autopilot flight controls.

The rudder and elevator are controlled by

separate servo motors located in front of the

structural hydrogen tank. These are connected

to their respective control surfaces via graphite

control rods along the carbon fiber spine. As

stated above, the empennage assembly was

taken from a pre-constructed sailplane. It is

bonded to the carbon fiber spine that extends

from the fuselage structure using wood

buttresses and epoxy. Plastic body filler was

used to ensure a premium surface finish and

smooth spine-empennage transition.

1.3 Fuselage StructureDue to the large frontal surface area of the

hydrogen storage tank, much of the fuselage

shape was dictated by this tank. Acting as a skin

between the internal components and the

environment, a thin layer of fiberglass was

manufactured to enclose all components except

the infrared sensors used by the autopilot

telemetry. Due to the shape and size of the

hydrogen storage tank, a cylindrical fuselage

shape was used with conical shapes to

transition from the nose to the tail. Using hose

clamps and custom fixtures, the hydrogen tank

is secured to a one inch diameter hollow carbon

fiber tube. This serves as the spine of the plane

providing structural support along the length

from the front motor mount all the way to the

empennage in the rear. As the main structure of

the aircraft, everything stems form the carbon

fiber spine. The wings, servo motors,

electronics, propeller motor and hydrogen

storage tank are attached to this spine via ASTM

6061 lightweight aluminum brackets that were

manufactured using a computer numeric

controlled (CNC) milling machine. All structural

components were computationally tested

against theory using finite element analysis.

1.4 Landing GearThis is the one aspect of the airplane that has

caused a number of problems during the testing

stages, though it will be replaced by a skid plate

for the final 24 hr flight. The difficulties

presented with this aircraft are its large size and

weight, and the ground clearance needed for

the large diameter propeller (20+ inches).

The initial landing gear setup was a composite

two-wheel tail-dragger configuration which,

while lightweight, was structurally unstable and

turned out to be too narrow. Following this a

mono-wheel configuration with wing skids was

employed, but proved to be too unstable for

use on a multi-flight aircraft. The landing gear

design has since moved to a traditional tricycle

configuration with two wheels of a large


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wheelbase behind the center of gravity and a

single wheel directly behind the propeller with

steering controlled by the rudder servo motor.

Designed into this configuration is a lower angle

of attack to increase the acceleration during

initial take off. Angle of attack refers to the

difference between the horizontal datum plane

and the angle made by the wing in which zero

lift is produced. While this reduces lift

temporarily, it also reduces drag significantly

allowing the plane to achieve a higher velocity

in a shorter distance. Once the desired velocity

is achieved, the elevator is moved quickly to

induce high lift for take off. This tricycle landing

gear configuration provides more stability and

control while permitting lower induced drag.

These advantages come with the minor cost of

additional weight.

2 Autopilot SystemIntegration

For the hands-free control of this aircraft and

optimal flight management we have integrated

the open source Paparrazzi autopilot developed

by Ecole Nationale de lAviation Civile in France

and used by a number of other research UAVs

(USU-OSAM, USU Aggie Air Remote Sensing,

UCSD, U of Arizona Autonomous Glider, Team

UAV UALR). This flexible ARM7 based system

uses IR (Infrared) Thermopiles for horizon

sensing on the pitch and roll axes of the aircraft.

For the flight pattern and altitude control of the

aircraft, a small uBlox LEA-5H GPS receiver is

used. With the included transceiver system,

waypoints and other

commands can be

given and

performance data

obtained from the

aircraft throughout the flight.

2.1 IR SensorsThe use of IR sensors for attitude (pitch, and

roll) control is based on the principle that the

ambient temperature IR signal from the ground

and the sky are distinctly different. While

terrain, and weather can have an impact on this

form of sensing, it is remarkably robust, and all

of our flight testing will be performed over

virtually flat terrain. Yaw control is provided

primarily by

the GPS

waypoint

commands

and any

coordinated

flight control

schemes

written in the

controller.

2.2 GPS ReceiverThe GPS Receiver is a combination of the u-Blox

chipset with Sarantels SL1206 helical antennato produce an incredibly sensitive 50 channel

GPS receiver. Some of the advantages of this

receiver is the 2 Hz update rate, low power, and

small form factor. The Sarantel

antenna also has its own filtering

giving high immunity to RF

interference.

2.3 Transceiver SystemThe transceivers used for communicating

between the ground station and the aircraft are

the Digi XBee Pro 900 RPSMA and

allow a very reliable and simple

Figure 2 - Diagram of IR sensing

Figure 1 - Autopilot board

Figure 3 - GPS receiver/ante

Figure 4 - XBee transceiv


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communication. This low power, high data rate

wireless module allows for up to 6 miles line of

site communication and have been tested to

work well with other wireless modules on the

aircraft.

2.4 Processing and Servo ControlThe processing of sensor readings and

outputting servo control is based on common

PID control. The desired closed loop dynamics

of flight are tuned by changing proportional,

integral, and derivative gains in the autopilot

software either permanently in the code or in

flight using the ground station software. The

critical core of the autopilot code has been

tested formally using Lustre.

2.5 Graphical InterfaceThe ground station interface for the autopilot

runs in a linux environment. Currently our

ground station consists of a laptop running

Ubuntu linux with the Paparazzi Center

software installed. When a flight is executed, a

satellite image of the current aircraft location

and flight plan is loaded. Here we are able to

keep track of important aspects of the plane

like battery voltage, GPS signal, altitude,

location, and autopilot mode (manual, wing

leveling, fully autonomous). The software also

records the flight for future playback.

3 Battery Power SystemThe battery power system in use is to readily

and safely provide multiple flights for flight

testing and data acquisition. This data will be

used to determine the final setup of the fuel cell

power management.

The current heavy-duty power system in the

aircraft uses 2, 5000 mAh Lithium polymer

batteries to provide power to a Hacker A60-18L

motor through a Phoenix 110 speed controller.

This setup is capable of delivering over 2kW of

power. The previously attempted flight tests

using Axi motors were thwarted by an

overloaded speed controller, shorted motor

coils, and broken magnets, thus the switch to

the more durable system despite a 1lb weight

penalty.

4 Fuel Cell System4.1 PEM Fuel CellThe 33 - cell stack we will be using is developed

specifically for this application by United

Technologies Research Center. It is a 600 W

nominal system at max power and operates at

200 W for cruise performance. Its weight is

1.68 kg, providing 357 W/kg at max power with

a hydrogen utilization of 90%. See Figure 6 for

characteristics.

4.2 Hydrogen StorageThe hydrogen is stored in a 9L, 4.5 kg composite

wound pressure vessel at 5500 Psi (MCS

International). Pressure regulation is provided

by three stages of regulators. The first

regulator drops the pressure from 5500 Psi to

Figure 5 - View of Graphical Interface


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500 Psi. Second stage regulator brings the

pressure from 500 psi to 50 psi, and finally from

50 to 1 psi. On the exhaust side of the fuel cell,

an on-off purge valve is used to maintain the

proper humidity, pressure and stoichiometric

conditions inside the fuel cell. This is controlled

by the power management system discussed

later.

4.3 Air SupplyThe air supply for the fuel cell is provided by a

Micronel U51DX 51mm High Performance

Radial Blower. This fan is capable of a max flow

of 16.7 CFM and max pressure of 4,900 Pa. This

blower was chosen for its performance

specifications, power usage, and weight.

4.4 Power ManagementCurrently in development is the power

management controller for the fuel cell system.

This device, developed by our team, provides

control for the air and fuel utilization by

measuring current and adjusting the air supply

blower and the hydrogen purge rate

accordingly. Also included on this board aresensors to determine the health of the fuel cell

while in flight, a data logger to record these

details during the flight and a telemetry system

for sending the readings back to the ground.

Many of the features of the power

management controller were included due to

the results of a DFEMA completed by UTRC

engineers and our team.

4.5 ByproductsThe byproducts of the fuel cell system are heat,

water, hydrogen, and air. Cut into the nose of

the aircraft are vents to provide air to the

blower as well as to remove heat, and at the tail

of the aircraft we have a vent for the escaping

air, hydrogen, and water vapor.

5 Flight TestingCurrent flight testing is focused on achieving

level flight for verifying the aircrafts general

handling and stability characteristics. These

experimental results will allow for tuning theautopilot controls and power consumption

characteristics. Due to design iterations in the

landing gear configuration and battery power

consumption, these flights are scheduled for

the first two weeks in May 2010.

A minimum of two successful test flights will be

needed; the first to determine the aircrafts

characteristics and then set the controller for

optimal power and control scheme efficiencies,

and the second to operate at optimal conditions

and record data. This will be used to perform

accurate lab tests on the fuel cell system before

installation of the fuel cell in the aircraft.

6 Future Proposed WorkWhile we are currently working towards

achieving the fuel cell long endurance flight,

there are possibilities for future work with this

aircraft. Gaseous hydrogen systems have aslightly higher specific power than existing

boro-hydride systems [1], however cryogenic

systems have roughly 10 times the power

density. We are currently investigating

possibilities of creating an insulated tank system

for use with cryogenic hydrogen, and have

spoken with some tank and specialized

materials manufacturers about such an

endeavor. Depending on funding developed

and interest from future students and external

parties, more testing will be possible to

investigate different power schemes, and flight

envelope limits.


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7 AcknowledgementsThe team has greatly enjoyed working on this

cutting edge project, gaining invaluable skills in

a variety of engineering tasks, and providing a

useful segway into graduate school and career

work. Many thanks are due to Dr. Thomas

Bradley, the pilot, Rich Schoonover, the team at

United Technologies Center, and Dr. Azer Yalin,

with the CSU Space Grant Program.

References

1. Bradley, T.H., Moffitt, B.A., Fuller, T.F., Mavris, D.N., Parekh, D.E. "Comparison of DesignMethods for Fuel-Cell-Powered Unmanned Aerial Vehicles," Journal of Aircraft, Volume 46,

Number 6, 2009.

2. Bradley, T.H., Moffitt, B., Mavris, D., and Parekh, D.E., Development and ExperimentalCharacterization of a Fuel Cell Powered Aircraft,Journal of Power Sources, Vol. 171, 2007, pp.

793-801.

3. Bradley, T.H., Moffitt, B.A., Mavris, D.N., Fuller, T.F., Parekh, D.E. "Hardware-in-the-Loop Testingof a Fuel Cell Aircraft Powerplant," Journal of Propulsion and Power 2009, Vol 25, No 6. 2009

Documents

[SG Symposium] FuelCellUAV CSU 1