Paper Number Here: 05008

Proceedings of the Winter KGCOE Multi-Disciplinary Engineering Design Conference Page 6

Paper Number Here: 05008

Proceedings of the Winter KGCOE Multi-Disciplinary Engineering Design Conference Page 6

Airborne Platform for wildfire detection system

Ben Wagner/ Project Leader / Christina Alzona/ Project Engineer
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Proceedings of the Winter KGCOE Multi-Disciplinary Engineering Design Conference Page 6

Abstract

Airborne sensing platform was designed to meet the needs for a high endurance electric propulsion UAV. Other objectives to be achieved included: (1) easily repeatable modular composite construction, (2) flexibility to integrate up to three pound “black box” style payloads, (3) ability to easily integrate telemetry and stability augmentation systems with the possibility of autonomous flight.

introduction

Rochester Institute of Technology College of Imaging Arts and Sciences had expressed an interest in the creation of an unmanned airborne sensing platform to assist their Wildfire Airborne Sensor Program (WASP). In an effort to assist CIAS, two design teams were assembled. One will design and build the body of the UAV, and another team will design and integrate onboard telemetry and stability augmentation. This UAV approach is scheduled to be an ongoing project in coming years. Current mission objectives include flight ranges of at least two miles from base station and an endurance of at least one hour. CIAS also requested a fairly large payload capacity and ease of repeatability in design and construction. CIAS has requested a UAV capable of a 3.2 km range, able to carry a 1.5 kg payload, and adaptable to various unspecified mission requirements.

UAV Construction Methods

Propulsion System

Due to the possibility that payloads may include sensitive visual equipment, considerations had to be made to minimize vibrations throughout the aircraft. For this reason an electric power plant was chosen over a more traditional glow style combustor engine. This provided the added benefits of not having center of gravity shift due to fuel consumption and there would be no problems with exhaust gases and oil diversion. Care was taken in choosing a commercially available motor that would meet the power consumption requirements of the project while taking special consideration to keeping weight down. After researching potential venders it was decided lithium polymer batteries would be chosen because they offered the highest energy capacity per weight for the size of batteries required for this project. A Model Motors AXI 4120/18 outrunner style brushless motor was chosen and paired with an 8000mAhr 18.5 Volt lithium polymer battery from Thunderpower. A JETI 70A speed controller was chosen to moderate power consumption.

The AXI 4120/18 boasted an engine efficiency of 86% to the propeller and a weight of 0.3 kg. The battery system weighed .862 kg and was less than 1/3 the weight of NiCad batteries of comparable energy storage.

An APC 13x10F folding propeller was chosen to maximize motor output. A folding propeller was chosen to help protect the motor and propeller in the event of a landing mishap. The propeller pitch would allow 30 revolutions per meter considering no slip and a propeller diameter of .33 m.

Power calculations indicated a maximum power output requirement of 148 W during takeoff. Mission loiter power consumption was estimated to be 78 W. Loiter mission power requirements were determined using the assumption that the drag created by the aircraft at cruise speed and altitude would be equal to the thrust required to keep the aircraft airborne. Preliminary battery and motor tests indicated power output of well over 300 W is possible depending on the propeller and obstruction around the control volume of the actuator disc.

Fuselage

The fuselage was design and constructed around the payload requirements of the sponsor. The payload objective was to fully enclose a .15x.15x.3 meter black box with a mass of 1.35 kg. It was also uncertain as to the size of the telemetry and flight control system to be integrated later. A relative shape of .15x.15x.8 meters was chosen for the main section of the fuselage. Due to the simple geometry of the rectangular box, two .013 rails were molded into each side of the fuselage. These railings offered increased bending and torsional stiffness compared to a standard box.

Composite construction techniques were employed. The fuselage skin was constructed from a layer of (0, 90) weaved carbon cross-ply sandwiched between two layers of (0, 90) weaved fiberglass cross-ply in an epoxy matrix. The carbon weave was chosen to provide stiffness in the event of a landing mishap to protect sensitive onboard equipment as well as to limit deflection in the fuselage due to aerodynamic forces. The laminated fiberglass provided a smoother outer finish to decrease parasite drag as well as to negotiate the geometry of the molded railings during composite lay-up.

The mold for the fuselage was a female mold. Because of two axes symmetry only one half of the fuselage had to be represented in the mold. The mold was constructed from .02 m thick MDF. The railing and corner contours were formed using a router directly into the MDF. Imperfections in the MDF mold were smoothed out with Bondo and wet sanded. When the desired shape was achieved, mold smoothness was achieved using primer and spray paint. A thin layer of epoxy was painted over the layer of spray paint. To prevent the epoxy from the lay-up from bonding to the sides of the mold, several layers of Partall #9 mold release were buffed onto the mold surface and a layer of PVA was painted over the mold release. After initial tests were conducted with limited success this method of mold conditioning proved to work best for this purpose.

Completed fuselage halves were trimmed leaving .03 m excess material on the top and bottom. This .03 m overlap was roughed up with sand paper and provided extra gluing surface to reinforce the fuselage construction. .05 m wide strips of (45,-45) woven carbon angle ply were epoxied over the exterior glue joint of the fuselage to further reinforce this mating surface as well as to provide additional torsional stability.

Interior of the fuselage contains 3 payload compartments separated by 5 bulkheads. Bulkheads were constructed of .003 m thick birch plywood. A .045 m diameter hole was centered in each bulkhead to decrease weight as well to allow cables to be routed throughout the fuselage. Bulkheads were epoxied into the fuselage before the fuselage skin was epoxied together.

Bulkheads were constructed to maximize the module capability of fuselage. The rear most bulkhead is flush with the edge of the fuselage. Four ¼-20 threaded holes were tapped into the corners of the bulkhead and reinforced with basswood to provide at least .02 m of effective threading. Threads were reinforced using a cyanoacrylic style super glue. These bolt locations provide a best placement to distribute the aerodynamic forces from the empennage along the load lines of the fuselage. These bolts allow from the easy removal and replacement of the empennage of the aircraft. Removal of the tail also provides an additional access point for the rear most payload pay.

Two bulkheads were placed under the wing saddle to support the wing loads. Wing chord is .2 m, and bulkheads were placed at the leading and trailing edges of the wing. The top of the fuselage at the wing saddle was cut away .03 m to create wing mounting surface that was flush with the top of the fuselage. A .15x.3 m sheet of .003 m thick birch plywood was mounted to the fuselage railings and the tops of the bulkheads to strengthen the wing opening. A .1x.15 rounded rectangle hole was cut into the wing saddle to provide access to the payload bay below. Four .03 m blocks of basswood for wing mounting were tapped to a ¼-20 threading and thread reinforced with thin cyanoacrylic glue. These tapped blocks were epoxied to the plywood wing saddle and bottom surface of the top of the fuselage. Mounting blocks in this manner allows the fuselage skin to receive the majority of the wing loading forces.

The front two bulkheads in the fuselage provide a mounting point for the motor. The two bulkheads were cut from .003 m thick birch plywood and have a .06 m square mounting point centered along the tip surface. One bulkhead is mounted flush to the front of the fuselage, and the second is mounted .05 m aft of the first bulkhead. The centerline of the motor is mounted .05 m from the top of the fuselage. Minimal moment force was incurred by mounting the motor above the aerodynamic center of the aircraft. .007x.025 balsa sticks were glued between the mounting points of the bulkheads to provide compression strength. The screws in the motor mount are mounted through both bulkheads to distribute thrust loading over a greater area of the fuselage.

The motor cowling was vacuum formed from .17 mm thick styrene plastic. A balsa plug was carved to represent half of the motor cowling. The cowling was created large enough to cover the entire motor and most of the mounting portion of the bulkheads. Because the main purpose of the motor cowling was to provide assistance to minimize drag from aerodynamic forces and little actual structural support, the strength of the styrene was not particularly important. The motor cowling did stiffen considerably when it was mounted to the motor bulkheads and because it was molded in halves, still provided easy access to the motor. Other benefits of the motor cowling were to protect UAV operators from the spinning casing of the outrunner style motor and it was determined that the motor cowling provided some aesthetic improvements to the overall look of the UAV.

The fuselage cowling was vacuum formed using .43 mm thick clear PVC plastic. A balsa plug was carved and used to vacuum form the plastic over. These vacuum forming plugs aid in the module design of the aircraft and the ability to easily construct new parts and assembly. It was found that when the PVC was stretched over the balsa plug, it was too thin to provide structural support. The nose of the fuselage is a particularly vulnerable portion of the UAV. The fuselage cowling could not extend past .075 m from the front of the fuselage because of the propeller clearance. It must not deform under the prop wash and must be able to withstand minor landing mishaps. A video camera to stream real-time data to the aircraft pilot is planned to be mounted under the fuselage cowling and it would need protection from these forces. As a consequence of these concerns it was decided that the fuselage cowling would be reinforced. Several layers of fiberglass in varying directions and layer of unidirectional carbon fiber were used to stiffen the fuselage cowling. It is anticipated that a hole will have to be drilled into the fuselage cowling to provide field of view for the telemetry camera. The fuselage cowling is mounted to the front bulkhead using small wood screws and .007x.025 m balsa blocks.

The weight of the finished fuselage was approximately 2.25 kg.

Empennage

The tail cone of the empennage was constructed using .003 m thick balsa sheets reinforced with .006 m square balsa sticks. The balsa sticks created a truss underneath the balsa sheets. The tail cone tapers up from the .15x.15 m dimension of the fuselage bulkhead to a .025x.025 m square at the rear of the tail cone. The tail cone is hollow and has a hatch screwed onto the bottom front to allow easy access to servo lines and nylon bolts securing the tail cone to the fuselage. The front bulkhead of the tail cone is constructed of .006 m thick birch plywood.

The horizontal stabilizer is .5 m wide and has a mean chord of .2 m. It is .25 m at the largest chord at the root. The horizontal stabilizer utilizes a NACA 0006 airfoil. It was cut using a hot wire passed through foam blocks. These foam cores were sanded and laminated with a single layer of (0, 90) woven fiberglass cross-ply impregnated with epoxy and black pigment. The foam cores and fiberglass were vacuum bagged to eliminate air bubbles and delaminating points.

The vertical stabilizer was constructed in the same manner as the horizontal stabilizer. The vertical stabilizer was constructed using a NACA 0006 wing planform. The mean chord is .2 m and the height is .33 m.

The vertical stabilizer was formed to the horizontal stabilizer. Both the horizontal and vertical stabilizers were epoxied to the tail cone. The tail cone was primered and painted to provide limited protection from moisture and punctures.

Control surfaces were cut from the trailing edge of the horizontal and vertical stabilizers. Control surfaces on both stabilizers are .05 m wide. The leading edge of the control surfaces was beveled with a razor to allow freedom of movement while hinging. Hinging of the control surfaces was completed using clear packaging tape on the top and bottom of the mated surfaces of the stabilizers and control surface. This provided a more simple design and prevents air from passing through the hinge line and limiting the effectiveness of the control surfaces.

The empennage section was bolted to the main fuselage using ¼-20 nylons bolts through holes in the bulkhead. Calculations show in the event of a mishap, the nylon bolts should fail before tail cone failure. If this proves to be incorrect, fewer bolts may be used without adversely affecting aerodynamic loads from the empennage through the fuselage.