Proceedings - edge.rit.eduedge.rit.edu/edge/P15462/public/Final Documents/Tech… · Web...

Multidisciplinary Senior Design ConferenceKate Gleason College of Engineering

Rochester Institute of TechnologyRochester, New York 14623

P15462: Airborne Wind Energy Plane

Devin BunceMechanical Engineer

Matthew KennedyMechanical Engineer

Carl StahoviakMechanical Engineer

Matthew MaginnMechanical Engineer

Matthew ZebertMechanical Engineer

1: PROBLEM DEFINITION

This project and related research is motivated by the need to develop a more efficient and more cost effective method for harvesting wind energy than what current wind turbines are capable of. Wind turbines have been able to generate more power by becoming taller, heavier, and consequentially, more expensive. There is a limit to how large these turbines can become, and out of the need to address the future generation of wind energy, the concept of the Airborne Wind Turbine (AWT) was developed. An AWT will be much smaller, and more cost effective to implement. And just as importantly, these AWTs will be capable of harvesting wind energy in high altitude environments where wind speed is faster and more sustained than near the ground where conventional wind turbines must operate. These AWTs can take the form of a powered glider, or kite. Companies such as Makani , Ampyx Power, and KiteGen have all developed versions of these systems. Also, Senior Design team P14462 (our predecessor) attempted to develop a glider and successfully created a base station.

Therefore, the goal of this project is to design, build, and reliably test a powered, human-controlled tethered glider specifically for future application as an airborne wind energy system (or AWT). While our system will not generate power, it can be used to gain an understanding of the dynamics of tethered flight for future applications. The major deliverables for this project include designing and manufacturing a tethered glider, which will integrate with our predecessor’s base station. From this design we aim to achieve multiple tethered flights, allowing us to employ the base station. This base station is able to measure the tension in the tether and position of the system using a data acquisition system. With the data recorded, a deeper understanding of the influence of each parameter of this system will be obtained. Lastly, this data has the potential to aid in the creation of automated controls for this or similar systems in the future.

2: DESIGN

2.1: FUSELAGEThe fuselage design was split up into three separate sections, the 3D printed part, the foam section, and the

tether connection system. The 3D printed part required certain characteristics including withstanding the tether tension load through a designed wall thickness, allowing for a connection section for the wings, a hatch for the electronics bay, a motor mount, and containing the foam fuselage. The interior foam fuselage required enough room for the electronics, holes to accept the boom and spars, and enough extra foam to take impacts from flight tests. The tether connection system needed to distribute the tether tension load as much as possible to not create areas of high stress on the fuselage, and have a connection point to the tether.

Copyright © 2015 Rochester Institute of Technology

Proceedings of the Multi-Disciplinary Senior Design Conference

2.1.A: TETHER CONNECTION POINTIn order to maximize the distribution of the tether tension load, a wrap was designed to extend around the

entirety of the external section of the fuselage. The tether would then be connected at a singular point on the bottom by a knot tied to a metal ring. The intended material is a spandex so that it will stretch as the tension increases. This wrap would then transform the point load of the tether to a pressure load across the entire fuselage surface. Due to the stretchy nature of this connection point, the jerk experienced by the impulse of the tether becoming taut should be decreased so as to not pull the plane out of the sky.

2.1.B: FUSELAGE FOAM INTERIORThe foam interior was designed to have a 4” outer diameter with three separate sections. The first section

would be the front to take the impact loading of crashes in flight that would extend from the front of the 3D printed part. The middle section should be hollowed out to contain the electronics bay that has a cut out the same size as the 3D printed part in order to insert the electronics. Holes were placed in locations that match the position of the wing spars on the 3D printed part as well as one in the rear of the section for the boom. The third section just adds additional surface to take impact loading and to hold the boom.

2.1.C: FUSELAGE 3D PRINTED PARTFrom the quasi-static tether tension analysis, the

maximum tether tension load that could potentially be seen by the fuselage was found to be 2021 lbf. Due to the weight and cost constraints this value was far too high to be designed for so one of the minimum values of 350 lbf was used. This load was assumed to be a pressure load over the external features. Using the stress produced by this pressure the load the wall thickness was chosen to be .15”. The connection point for the wings was designed to extend 1” from the external surface and maintain the same shape and size of the wing airfoil at the root. This connection creates a press fit .5” deep. The electronics bay was designed to be a cut out in the bottom of the cylindrical length that was 4.5” long and 4” wide. This was believed to a large enough hole to fit in all of the electronics while allowing for the inner cavity to be larger. The motor mount was designed to have an inner diameter equivalent to the external diameter of the specified motor while also angled such that the thrust would be directed through the center of gravity determined through aerodynamic analyses. The inner diameter of the cylindrical section was set to be 4” so that the foam interior would fit through.

2.2: WINGS

2.2.A: AIRFOILAirfoil selection was based on targeting those which were designed for high lift generation at low Reynolds

numbers. Final comparison and selection fell between three airfoils: An Eppler E207 low Reynolds number airfoil, an N-22 airfoil, and the Selig S1210 high lift low Reynolds number airfoil. After analysis and comparison of all three airfoils using XFLR5 with respect to lift coefficient, drag coefficient, pitching moment coefficient, and efficiency, the S1210 airfoil was selected for the wing. Wind speed was assumed to be an average of 30 mph, due to previous experience with RIT Aero club design assumptions and operating conditions. The final selected design of the aircraft had (at zero angle-of-attack and relative speed of 30 mph), according to the software tool XFLR5: a lift coefficient of 0.751, a drag coefficient of 0.042, a pitching moment of 0.078, and an efficiency of 1.031

2.2.B: SIZING AND LOCATIONOperating primarily on observed performance of the previous aircraft flown, general wing dimensions

chosen were to err on the larger side. Wingspan was made to be five feet, with a root chord of ten inches narrowing down to a chord of six inches. This change in chord length is achieved by means of a linear taper in the wing. The taper was applied to the trailing edge of the wing, to help boost stability of the aircraft via the generation of a rolling moment in addition to the wings’ pitching moment. Through pitching moment analysis in XFLR5, it was determined


Figure 1: 3D Printed Fuselage


that in order to achieve stable flight, the center of gravity would need to be located three and one half inches aft of the leading edge.

2.3: HORIZONTAL AND VERTICAL TAILS

2.2.A: AIRFOILFrom previous experiences with RC aircraft design, the decision was made to use a symmetrical airfoil, as

it would not generate any considerable moments during steady level flight. A relatively standard airfoil for tail components is a NACA 0012, with no camber and the maximum thickness occurring at 12% of the chord length. For the vertical tails, a NACA 0012 was cut in half along the chord line for easier integration with the horizontal tail. With a symmetrical airfoil chosen, no lift would be generated at zero angle-of-attack while minimal drag would be induced. (Incorporated into full analysis, see wing).

Figure 2: Airfoil Performance Curves

2.2.B: SIZING AND SHAPEUsing XFLR5, an appropriate horizontal tail size and location was determined through an iterative process

to minimize necessary chord length, tail span, and boom length to generate enough pitching moment to counteract that of the wing and achieve static stability. An H-Tail design was elected, providing for twice the vertical tail area of a normal configuration while only requiring half of the height. As there was no set sizing requirement for the vertical tail, estimations were made based on real world and previous RC aircraft experience to be able to generate enough of a yawing moment as might be necessary to fly the prescribed flight path.

2.4: ELECTRONICS

2.4.A: ELECTRONIC SPEED CONTROLLER, BATTERY, AND TRANSMITTER/RECEIVERThe speed controller (ESC) was selected primarily upon its capacity to handle power draw loads received

from the motor. Because of the maximum current requirements of the new motor, a 55 amp speed controller was required.

Battery size was scaled up from early stage testing, with a need for increased flight time over the original batteries used as well as a need to withstand an increased draw from a now larger motor.

Transmitter and receiver were both provided courtesy of Dr. Kolodziej, who had provided them to the previous project group (P14462).

3: MANUFACTURINGManufacturing of the aircraft can be divided into 5 subsections: cutting the wings, fuselage, and tail out of

EPP foam, post-processing of all of these parts (including the fabrication of control surfaces, creating the electronics bay, and cutting holes for the wing spar and boom), fabrication of the carbon fiber spars/boom, fabrication of the motor-mount, and the post-processing required to bring the 3D-printed fuselage to specification.



3.1: WINGSCutting the fuselage, wings and tail from EPP foam requires the use of the foam cutter in Dr. Gomes’ lab.

From the UIUC Airfoil Database, we obtained the set of (x,y) points that define our chosen wing airfoil, the S1210. This series of data points is on an “airfoil scale” (i.e. having a chord length of 1). From this file, typically imported as a .txt, a .dat file is created to specifications that are detailed in the Manufacturing Process Document. This .dat file is imported into the Foam Works software and is scaled to the chord lengths that correspond to the Root, Mid, and Tip section of the wing. The Root wing section is 3.5” wide, has a 10” chord, and is not tapered. The Mid section has a root chord length of 10” and a tip chord length of 8”. The Tip wing section has a root chord length of 8” and a tip chord length of 6”. Both the Mid and the Tip wing sections are 12.25” wide. In total, each wing is 28” long, yielding a full wingspan of 61”. Practice cuts of all 6 wing sections were made from spare EPO. Having this spare

foam was crucial to the learning process associated with operating the foam cutter hardware and software.Figure 3: Foam Cut Wings

3.2: HORIZONTAL AND VERTICAL TAILThe horizontal tail is also made from EPP and has a span of 26 inches, uses a NACA-0012 symmetric

airfoil with a chord length of 6” and is made of two separate sections. With EPP foam, we had problems maintaining a sufficient level of structural rigidity when the section of foam was cut too thin (less than 1/8”), and this would have been the case for the vertical tail. So in place of EPP foam, we chose to use EPO for the vertical tail as EPO does a much better job of maintaining sufficient rigidity even when the material is very thin.

3.3: FOAM FUSELAGEFinally, the last piece of the plane to be fabricated using the foam cutter was the fuselage. The foam

fuselage has a diameter of 4” and was cut from EPO and is comprised of 3 sections, nose, mid and tail, and have lengths of 5”, 6”, and 5” respectively. The nose section has a rounded front end and is solid aside from the section that has been hollowed out for the LiPo battery pack. The mid section has been hollowed out for the electronics bay and has a remaining wall thickness of ¼”. The electronics bay houses the ESC, receiver, receiver battery back, and all associated wiring. The tail section has a hole thru the center of it for the ½” diameter boom. A section near the motor has also been removed to allow for proper clearance of the propeller. After fabrication of each section is complete, all three sections are connected using foam-specific glue.

3.4: POST PROCESSING OF FOAM COMPONENTSOnce the wing sections were fabricated and glued together to create the left and right wings, post-

processing on the wings began. This included milling slots to mount the servos, cutting out the control surfaces, and cutting the root of each wing to fit into the wing slot of the 3D-printed fuselage. Milling of the servo slots was done with a small end mill on a drill press in the aero club room. The areas that the control surfaces would fit into were cut out of the wings using a Dremel tool spinning at low rpm which created the cleanest cut near the trailing edge of the airfoil where the foam is the thinnest. Then the last ~3” of the root wing section were removed by a tapered cut to ensure a snug fit with the 3D-printed fuselage that was printed on the low end of tolerance. Finally, due to complications with the fiber glassing process we had to temporarily forego our plans to use fiber glassed EPO control surfaces, and in their place we have used corrugated cardboard as a cheap, reasonably rigid, and easily replaceable alternative.

3.5: MANUFACTURING OF NON-FOAM COMPONENTS



The last of manufacturing process concerns the carbon fiber frame, the motor mount and the 3D-printed fuselage. The only post-processing that the carbon fiber rods that provide a frame for our aircraft required was to cut them to length and drill holes at the ends of the wing spars to provide a way to fasten the tip of the wing to the rod. In addition to this, the left and right wing spars are fused within the fuselage by a ¼” steel drill bit blank. Our initial design did not provide an adequate method for mounting the motor to our aircraft. To solve this problem, our team came up with a motor mount system that required the fabrication of two additional 3D-printed parts. One to be glued in place to the fuselage and the other to be fixed to the motor. These two parts would then be joined by screws and would successfully mount the motor to the aircraft. Finally, the 3D-printed fuselage had to undergo some additional machining to meet our needs. Spar holes were brought to the proper dimension, wiring feed-thru holes were widened as necessary and the motor mount area was increased in diameter to allow a snug fit for the motor.

4: ASSEMBLYOnce the parts were manufactured, the assembly process began. A major aspect of the design was allowing for

removable parts for quick repairs from damage in flight testing. During the assembly process it became evident that not all the parts could be easily removed to the design and instead, some minor parts must be destroyed for disassembly.

4.1: TAIL ASSEMBLYTo properly assemble the tail a primary spar runs down

the length of it. Three secondary spar segments connect the left rudder to the left horizontal tail, the left horizontal tail to the right horizontal tail, and the right horizontal tail to the right rudder. These second set of spars prevent the tail from rotating about the primary spar. The spar connects the tail to the boom by running through a hole in the boom. The vertical tail is connected to the wing by strips of metal and plastic washers which press everything together. Two servo motors control the tail. One has two long push rods which push the rudders in sync. The other has a short push rod to control the elevator. The servos are located in the center of the wing to prevent and roll from the offset mass. A small strip of foam connects the two halves of the horizontal tail so that the servos have material to sit on. The servo wires are fed down the boom on the inside to protect the wires from any damage upon impact. All the control surfaces are connected to both the vertical and horizontal tails through plastic hinges. The most challenging aspect was connecting the boom to the fuselage. This required feeding servo wires down the length of the boom, then fitting the secondary spar through the boom without interfering with the wires to secure it in place.

4.2: WING ASSEMBLYThe wing subsystem proved simplest to

assemble and disassemble. The wing spar hole created an interference fit between the carbon fiber and the foam to prevent sliding. The wing then is press fitted into a section in the


Figure 4: Motor Mounting Plates

Figure 5: Tail Assembly


3D printed fuselage to prevent rotation. The servo cavities are milled into the wing to minimize the disturbance of airflow. The aileron and flap control surfaces are also connected to the wing through plastic hinges. Like the tail, the control horn is glued to the control surface and a bronze fixture connects the control horn to the push rod via set screw. The servo wires run along the length of the wings, passing through holes in the fuselage into the electronics bay. The wing assembly must be done after the tail assembly due to the difficulty of inserting the secondary spar through the boom.

4.3: FUSELAGE ASSEMBLYOnce the tail boom was fixed to the fuselage through

the secondary spar and the wings are press fit into the 3D printed case, the plane assembly quickly reduces to electronics assembly. The wing servo wires fit into a small cavity in the side of the fuselage which feeds into the electronics bay. In addition, the tail wires are already in the electronics bay since they run along the inside of the boom. Lastly, the motor wires feed into the back of the electronics bay. From there all the electronics are ready to be connected. The battery connects to the speed controller, the speed controller connects to both the battery and the receiver. All the servo motors also feed into the receiver in their designated channel.

5: FLIGHT PATH ANALYSIS FOR TETHERED FLIGHTThe flight path is defined as a 3D circle that is tilted some angle, φ, from the horizontal with a specified

diameter. The two sides of the triangle in Figure 9 represents the tether which remains constant throughout the circle. This therefore forces the circle to be tilted some angle from the vertical. In order to ensure the tether connection on the plane is pointing towards the origin some key maneuvers need to be made. Based off of the four key points in the flight path, these maneuvers were defined as seen in Figure 8.

Based on the four key points defined in Figure 8, a quasi-static analysis was performed to determine tether tension at these points in relation to the angle from the ground and the flight path diameter. Table 1 shows the key results from this analysis. For this analysis the key assumptions were the velocity was constant throughout, the tether length was constant, the flight path diameter was constant and a perfect circle, and the incoming wind velocity is constant and in the x-direction only.

Phi, Diameter Point 1 Tension Point 2 Tension Point 3 Tension Point 4 Tension


Figure 8: Flight Path Maneuvers Figure 9: Flight Path 3D View

Figure 7: Full Assembly


Phi = 10, Dia = 50 ft 324.9 lbf 965.8 lbf 365.5 lbf 763.2 lbfPhi = 30, Dia = 50 ft 344.9 lbf 1035 lbf 365.9 lbf 694.2 lbfPhi = 10, Dia = 25 ft 1036 lbf 1872 lbf 556.6 lbf 1584 lbfPhi = 30, Dia = 25 ft 1092 lbf 2021 lbf 334.3 lbf 1435 lbf

Table 1: Tether Tension in Relation to Phi and Flight Path DiameterFrom this analysis it was determined that tether tension increases as the flight path diameter is increased

and the angle from the ground is decreased. The maximum tether tension occurred at point 2 with a maximum phi and minimum diameter. The minimum occurred at point 1 with a minimum phi and maximum diameter. From this analysis it was determined that to most easily be able to perform the flight path, the angle from the ground should be minimal and the diameter as large as allowed by FAA constraints.

6: TEST RESULTS

6.1: UNTETHERED FLIGHTBeginning in MSD I, a ready-to-fly foam plane was

purchased to aid in controls familiarization and enable experimentation with design ideas to be applied to the yet-to-be constructed glider. Additionally, before attempting tethered flight with the designed glider, performance was evaluated for untethered flight. Several launch methods were attempted, with results showing the most reliable to be a proxy winch system using a longboard and a set of wooden blocks forming an incline for the glider to rest upon (Fig 10). This system provided an initial velocity to assist in takeoff as well as an increase in initial lift from the now non-zero angle-of-attack. It does, however, require the presence of another individual, as well as an additional tether to be attached to the longboard itself.

6.2: TETHERED FLIGHTThe glider purchased in MSD I was also used to gain experience with an aircraft’s reactions to a tether and its

tension while in flight (Fig 11). Through this experimentation, the impacts of tether harness weight, asymmetry, and sudden tension increases were observed, with severe impacts to aircraft performance resulting from both. These lessons were applied to the MSD II glider design, with a lighter, symmetrical tether connection harness and a method of introduction to the flight path which minimizes the initial impulse of tether tension. Once fully assembled, testing was performed to determine the system’s handling capabilities as well as the level of response from the control surfaces. In order to maximize experienced wind speeds while minimizing the risk of damage, a testing method was devised whereby the glider would be held by a short tether out the sunroof of a car, travelling at a set speed (Fig 12). From these tests, static stability was determined to be neutral, with the glider characterized neither by returning to nor deviating from a stable flight condition when subjected to a disturbance. Control surfaces were demonstrated to have enough of an impact on the glider to reliably control it in expected flight conditions, with the exception of a large and unexpected rolling moment generated due to the H-tail configuration of the rudders.


Figure 11: Purchased Aircraft in Tethered Flight

Figure 12: In-Car Tethered Glider Testing

Figure 10: Successful Untethered Flight


7: LESSONS LEARNED

7.1: FOAM CUTTINGThe CNC foam cutter proved far more difficult to use than anticipated. There were a few major issues

experienced that needed to be resolved in order to properly manufacture our key surfaces. These were as follows: applying the taper to the trailing edge of the wings, maintaining thickness on the trailing edge, and creating the spar holes.

The foam cutter had issues cutting a linear taper on the trailing edge. Even with importing the proper geometry, the taper would be cut on the leading edge. In order to combat this, one of the towers was offset a specified distance which would force the taper to be on the trailing edge.

Due to the heat of the wire, the trailing edge became very thin, especially on the EPP foam. Therefore the speed of the cutter was increased after a certain point to maintain the thickness. However, even with this speed increase the thickness was not large enough for the control surfaces. Due to this fact, cardboard was used as a replacement.

7.2: MOTOR AND PROPELLEROriginally when the motor came in, there was no method to attach the propeller. In order to allow for this

attachment, the motor shaft was threaded. Next, when the motor was powered on, the propeller unthreaded itself. The power and ground inputs to the propeller were then flipped in order to invert the polarity so the motor would spin in the opposite direction.

7.3: POST FIRST FLIGHT ISSUESAfter the first flight of the plane, many issues occurred. The front nose sheared off due to being too front heavy,

the wing and tail spars cracked, and the ESC battery eliminator circuit broke. The front nose was shortened and glued back on to the center foam section. Also, the counter weights placed in the front were removed. The wing spar was carbon fiber wrapped in the center and had a metal shaft inserted on both sides. Since the speed controller could no longer power the receiver due to a broken chip in the battery eliminator circuit (BEC), a secondary battery source was required. A regulator circuit was created to take 2 3.7 V batteries in series and output the 5V required by the receiver. This significantly added electrical weight to the system as well as made it difficult to fit all the components in the electronics bay.

7.4: LAUNCHING METHODThe hand launch method originally intended to be used proved to be not feasible without high winds. The proxy

winch system by placing the plane on a longboard and pulling it with a tether worked the best. An improved winch system that is more reliable and repeatable should be used in the future.

8: NEXT STEPS

8.1: MOVING FORWARDMoving forward a new speed controller would need to be purchased due to the failed BEC and potentially a

new receiver due to motor intermittency and control surface fluttering pre, during, and post flight. The cardboard control surfaces are not permanent. After experimentation with fiber glassing the control surfaces out of EPO foam, a better possible solution would be to 3D print the surfaces as the material would meet the rigidity, durability, and ease of replacement requirements. Currently the motor mounting plates block the airflow across the inner third of the propeller. In order to maximize the thrust from the motor, it would be beneficial to cut material away to allow for maximum airflow.

8.2: SYSTEM DESIGN CHANGESThe H-Tail design proved to be problematic during test flight. The rudders were creating an unexpected

rolling moment as well as hanging below the bottom of the fuselage so upon landing, more often than not, the vertical tail surfaces would fracture or completely fall off. If this were to be redesigned, a more typical vertical tail would have been chosen to increase the simplicity of piloting the aircraft as well as an increased stability during flight.



The current wrap is very loose on the fuselage and thus needs to be redesigned out of a different material and be a little smaller in order to more tightly wrap the fuselage as well as more evenly distribute the tether tension load.

During flight testing and assembly it was found that the wall thickness on the 3D printed fuselage was too thin. In order to better distribute the impact loading, this thickness should be increased. Also, since the foam nose broke at the connection point with the 3D printed fuselage, the 3D printed section should be extended further into this foam section to add structural integrity. Also a motor mount feature should be added so that a separate part is not required for mounting.


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