By Inspection: Senior Design Turbine-Powered Low Radar Cross Section RC Aircraft

Christopher Colletti1, James Harvey2, Nick Kuzma2, Casey Smith2, Sanjay Jayaram3

Parks College, Saint Louis University, Saint louis, M0 63103

I. AbstractBy Inspection is designing an aircraft to compete in the Oklahoma State sponsored Speedfest competition. For

the Statement of Work, SpeedFest requests an aircraft as a mock proposal for target drone to train US defense personal with the tracking and identification of small, fast moving, low signature UAS threats. This competition necessitates a highly maneuverable aircraft using a KingTech K-45 turbine engine with a minimal radar cross section. The aircraft design is finalized with: a GTOW of 9 lbs, a tapered trapezoidal wing with a 42 in span, a v-tail, a cruise speed of 150 KTS, an endurance of 4 minutes at wide open throttle, and a “potted” turbine providing 11 lbs of static thrust. In order to survive the high loading during maneuvers, the aircraft is designed to be manufactured with both carbon fiber and fiberglass composites using wet layups and prepreg materials. Radar cross section is accounted for with aircraft geometry as well as manufacturing materials. Aircraft material is chosen to mask the internal components of the fuselage and allow radar to pass through the lifting surfaces. Engine testing verified the manufacturing specifications, while giving exhaust data to make better informed design decisions for the v-tail and engine deck. Load testing verifies the manufacturing method for the lifting surfaces, demonstrating the ability to survive over 60 lbs of distributed loading. Manufacturing for final components has been started and first flight testing occurred during the middle of March. Initial flight testing alowed for an investigation in the takeoff and landing distance, installed thrust, along with verifying all components are integrated properly and work together. Later testing includes flights to verify maneuverability and endurance as well as the potential for radar testing to collect data on the aircraft radar cross section.

II. IntroductionEach year there are numerous aircraft design competitions with a variety of unique requirements and challenges,

Speedfest is one such competition, sponsored by Oklahoma State University. While the specific statement of work (SOW) has changed each year, one requirement has remained the same: the need to have the fastest aircraft at competition [1]. This year’s competition is no different: each team must design a target drone, powered by a Kingtech K-45 turbine, to perform various maneuvers at high speeds while also minimizing the radar cross section. This target drone could be used to train military personnel to locate and track small unmanned aerial vehicles in preparation for potential threats. During competition, teams will also be presenting commercials and detailed design reports for their aircraft as if they were bidding to contractors.

This competition provides a unique challenge that requires the design teams to consider non-physical elements in addition to flight optimization. Over the course of this competition, standard procedures were followed during the design process: trade studies for conceptual design, Gantt charts to develop a project schedule, flight performance analysis, drag build up, stability analysis, etc. Due to the unique challenge for this competition, there were multiple additional factors that affected the course of the design. These requirements for the aircraft to be powered by a turbine engine and have a minimal radar cross section challenged the aesthetics and overall manufacturing process for the aircraft. This paper details the competition requirements and general methodology, while delving into the process for designing with a miniature turbine and with radar cross section in mind.

III. Competition ObjectivesThe 2016-2017 Speedfest competition, sponsored by Oklahoma State, is designed around building a turbine

powered, remote controlled, target drone for possible military clients. The drone must be easily operated, low cost, and reliable, while featuring a small radar cross-section.

From these competition requirements, design drivers were selected to narrow and direct the design space. The main requirements considered for competition include the low radar cross-section, maneuverability, flight speeds, and total aircraft weight. More specifically the aircraft must:

Have a gross takeoff weight (GTOW) of between 7.5 and 9lbs. Present the smallest possible radar cross-section using designs informed by open sources. Fly at speeds of 120 - 173 kts.

1Undergraduate Student, Aerospace and Mechanical Engineering, Student Member AIAA2Undergraduate Student, Aerospace Engineering, Student Member AIAA3Associate Professor, Aerospace and Mechanical Engineering, ____ AIAA

Demonstrate 3 acrobatic maneuvers, the Horizontal 8, Cuban 8, and the Immelmann turn.Other requirements which drove the design include: ground-ops radius, unit cost bid, and endurance. More specifically the aircraft must:

Cost no more than $10,000 per unit as determined by specific cost analysis guidelines. The per unit cost is based on 100 aircraft and 25 launch/recovery systems.

Operate within a 100 foot ground ops radius for both launch and recovery, including any launch/recovery systems, and ground roll

Operate at wide open throttle (WOT)  for at least 3 minutes and up to 5 minutes.

IV. Design MethodologyIn the early phase of design, the team analyzed competition rules to produce a feasible design to maximize the

score. The rules where organized requirements, design impacts, and scoring factors. A quantitative analysis was performed to weight the respective point contribution in the final ranking for each team. Mission requirements were also compiled into a list that detailed what aspects of the design they would be primarily affected. Each requirement has a set of goal and threshold expectations as detailed by the competition governing body. Scoring and rules where used to synthesize a list of primary and secondary requirements that the team would design the aircraft to meet. Through this process, the primary requirements the team decided to focus on included weight, speed and maneuverability, and radar cross section, while the secondary requirements include the cost/unit bid, ground operations radius, and endurance.

With a firm idea of what requirements the aircraft must meet, a trade study was initiated. This allowed the team to evaluate various geometric parameters for the aircraft. This process would be repeated multiple times as the rules were released and updated until a conceptual aircraft design was finalized. Once an aircraft configuration was settled on, detailed analysis began for the aerodynamics, performance, stability, propulsion, and structures subsystems. In the detailed subsystem design, in house developed codes and other software, such as XFLR5, where utilized to evaluate and finalize the aircraft characteristics. As sizing of the aircraft was completed, detailed research and design was filtered into all subsections of the design to consider the additional limitations that would be added by the need for a low radar cross section. This requirement heavily affected both the aircraft aesthetics as well as manufacturing materials and process.

Testing was also a paramount part of the aircraft design. Tests include both verifying manufacturer specifications on different components, such as the engine, and testing the sizing and strength of different subsections of the design, such as an inlet sizing so the engine has enough airflow.

This aircraft was designed to be competitive in the Speedfest competition. The design process blended general RC aircraft design with non-typical elements in the turbine and radar cross section in order to meet the challenging requirements set by competition organizers.

V. Conceptual DesignThe initial concept was based upon requirements for a previous competition year. These requirements made use

of the KingTech K-45 jet turbine engine, and formed a very typical aircraft with twin vertical stabilizers, unswept trapezoidal wing, and narrow cylindrical body.

Figure 1: First conceptual aircraft, designed for the previous year’s rulesThe second iteration, using the first set of updated design requirements, did not stray far from the initial concept. This second concept employed a low unswept trapezoidal wing and a traditional tail with one vertical stabilizer. The

main difference between iterations was the motor placement.. The motor was moved forward and “potted” below the tail boom allowing for a more forward center of gravity and larger static margin with smaller control surfaces. This second conceptual design was made to minimize the frontal cross sectional area, which would lower the frontal radar cross sectional area. Figure 2 illustrates the second conceptual design.

Figure 2: Second conceptual aircraft, designed for frontal radar cross sectionThrough suggestions from mentors and requirement clarifications from the competition governing body, the

concept was changed for a second time, ending in a very different looking design. The tail was transformed into a V-tail configuration; the wing, an unswept trapezoidal planform, now employs a much lower aspect ratio; the “potted” engine design exhausts over a deck on the upper surface of the tail boom. The fuselage was also modified to employ angled surfaces, leading to a hexagonal cross section. These design changes in the wing, fuselage, and tail geometry were made with the 3D cross sectional area kept in mind. This final conceptual design closely emulates current designs including the YF-23 and F-22 Raptor. In this final concept, the aircraft appearance was heavily influenced by the reduction of the radar cross section.

Figure 3: Final conceptual aircraft, designed for 3-D cross section

VI. Detailed DesignA. Wing Sizing

The main lifting surface was designed using an iterative process, which includes updating the drag polar and constraint analysis to determine the necessary amount of lift to meet the ground ops radius requirement. First, a plethora of airfoils were analyzed using an in house developed MATLAB code and the software, XFLR5, to determine the two dimensional lifting qualities. Airfoils ranging from high camber to reflexive were analyzed until a slightly reflexive airfoil, the Martin-Hepperle 44 was chosen for its two dimensional lifting coefficient and its moment coefficient. Through advice of the team pilot, the wing transitions from a symmetrical airfoil at the root to the MH-44 at the tip. A NACA 0008 was chosen for the symmetrical airfoil, due to its thickness at the root, see

figure 4. The composite wing was found to have a CLalpha of 0.1089 1/rad, a stall angle of approximately 14 deg, and the CLmax was found to be 0.95.

Once an airfoil combination and wingspan estimate were chosen, an iterative constraint analysis was used to determine the final span, taper ratio, and aspect ratio. Only combinations which would provide at least 10 lb of lift at cruise were considered. After many iterations the combination of a 42 inch span, taper ratio of 0.5, and aspect ratio of 3 were determined to provide the necessary lift to complete the mission and meet the competition requirements for ground ops radius.

A drag polar was updated with the finalized wing parameters to determine the flight speeds which could be experienced. Using the drag polar, it was concluded that the aircraft and wing combination chosen would operate in the velocity range specified by the competition requirements, and required less than 10 lbs of thrust to do so. This is significant because the KingTech K-45 jet turbine engine is not rated to produce more than 10 of thrust [2].

Figure 4: Side view of the wing with a NACA 0008 at the root and a MH-44 at the tip.

B. Stability and Control Once the final aircraft configuration was chosen, the static stability was estimated using a predicted position for

the center of gravity and predicted weights of each component within the aircraft. The quarter of the mean aerodynamic chord of the aircraft was chosen as the reference point for the center of gravity estimates. The static margin was designed to fall in the range of 4-15% to provide enough stability, yet maintain a maneuverability during flight. During the design of the aircraft, an in house-developed MATLAB code was primarily used to assist in the sizing and location of the wing and tail. The stability trends calculated from the MATLAB code where confirmed with additional analysis from XFLR5. The developed MATLAB code used an analytic solution to calculate a select few of the static stability derivatives, while XFLRF uses the vortex lattice method to analyze the aircraft and output various flight characteristics. The following table summarizes the calculated static stability derivatives.

Table 1: Static Stability Characteristics of the Final DesignCLα 3.514Cmα -0.168Clβ -0.015Cnβ 0.094Cmà -0.861Cmq -1.421Clp -0.198Cnr -0.118

Once the aircraft lifting surfaces where sized, control surfaces were also sized. Control surfaces where designed as the servos were also chosen; this was done using an iterative process, balancing the torque provided by the servo of interest and the torque necessary to move a control surface of a given size under competition flight conditions. For this competition, the Turnigy TGY-S712G was selected, providing a torque up to 50.6 kg-cm, and the control surfaces are 16 in2 for the v-tail, 15 in2 for the flaps, and 14 in2 for the ailerons.

C. Radar Cross SectionAircraft radar cross section is a unique part of this competition that has differed from previous years. The

requirement to minimize total aircraft radar cross section, while not affecting the main design parameters, was kept in consideration when developing the overall appearance and material selection.

When looking at the radar cross section of an aircraft, it is useful to imagine the radar cross section as a “fuzz-ball” surrounding the entire aircraft, see figure 5 [3]. When viewing the aircraft at all angles, there is a certain amount of radar that would be reflected back to the sources. Based on current open source materials, it was determined that the best way to minimize the radar cross section was to manipulate the locations of spikes in the “fuzz-ball”. By manipulating the angles of the aircraft body and lifting surfaces, the “fuzz-ball” could be manipulated to have large spikes at very specific angles, while maintaining a low radar cross section for the rest of the aircraft. Additionally, further RCS control can be obtained by adjusting the material selection of the aircraft to shield components of the aircraft that would scatter radar, like an engine, and let radar pass through other components with few metal parts.

Figure 5. Depiction of an Aircraft Radar Cross Section as a function of the azimuth angle

In order to manipulate the radar cross section, the wing surfaces and v-tail surfaces were all designed to be trapezoidal with the same angles, see figure 6. Additionally, the body was designed to have a hexagonal shape to continue to control the direction radar would be reflected. The fuselage side was set an angle so that the v-tail would be normal to the fuselage surface, see figure 7. The angled surfaces act to focus feedback spikes in the radar “fuzz ball” to be at specific locations and in specific directions instead of being spread more evenly in all directions. The entire geometry of the aircraft was also selected so that there would be no overlap between the tail and wing and their wouldn’t be any closed angles, which would allow the radar to “bounce around” in and then reflect in random directions, amplified.

Figure 6. Top view of the aircraft, illustrating the consistent angles of the wings and tail.

Figure 7. Front view of the aircraft showing the fuselage angles. Notice side fuselage side surface and v-tail normal to each other.

Finally, materials were chosen to minimize the radar cross section of the aircraft. Carbon fiber composites, which reflect radar, will be used in the manufacturing of the body so as to shield the engine and other internal components that could reflect radar in non-ideal directions [4]. Fiberglass composites, which allow the radar to pass through, will be used for the wings, which house no major components, so as to continue to minimize the total radar cross section [4]. Figure 8 shows the two separate materials used for the manufacturing of the aircraft, with the fuselage made as a carbon fiber structure and the wings manufactured out of fiberglass composites.

Figure 8. Assembled fuselage, showing the two different materials used for the manufacturing due to RCS concerns.

VII. Manufacturing ProcessAs the third concept was becoming finalized, a manufacturing plan was created detailing the steps necessary to

create the aircraft. Materials were also analyzed for strength and stiffness during this time. Based on the necessity for a light aircraft that could withstand up to 9gs of loading during maneuvering at high speeds, carbon fiber was chosen as the optimal material for the aircraft body. For the wings, fiberglass wrapped foam with custom built Z-beam style spars running the quarter and three quarter chord was chosen to minimize weight while maximizing strength. Carbon fiber composites will reflect radar, shielding the engine from view, while the fiberglass composites allow the radar to pass through it, minimizing the visible cross sectional area. These materials offer high stiffness and strength, while also meeting the necessary radar properties for competition.

In order to maximize strength and stiffness the fuselage section shall be created in 1 piece of carbon fiber, employing a 2 piece mold for use with the wet layups. A plug was made out of CNC and MDF profile cuts, body filler, and primer, see figure 6a.The plug is then covered with gelcoat and fiberglass to create the mold, see figure 6b. Once the mold was finished, the fuselage can be made out of carbon fiber composites, see figure 6c.

Figure 6. a) Plug drying after being coated with primer. b) Plug covered in wax, waiting for the carbon fiber. c) Completed carbon fiber fuselage structure with slots cut out of it.

The wings will use a much simpler method; a foam core was cut out with a CNC and a Z-beam was made using aircraft grade plywood and fiberglass strips at the ¼ and ¾ chord of the wing, see figure 7a. Figure 7b shows the final test flight wing, manufactured with a layer of fiber glass and a central strip of carbon fiber to reinforce the

joints of the two halves. The central carbon fiber strip will be hidden inside the fuselage, so it will not have a negative impact on the aircrat RCS.

Figure 7. a) Foam core layed out with cuts made for the spars at the ¼ and ¾ chord. b) Final wing for test flight, with a central carbon fiber strip.

VIII. Component TestingDuring the course of the design processes, testing was performed on the KingTech K45 turbine engine for

competition. A static test was performed by first constructing a test stand using aluminum, and a small load cell controlled with an Arduino micro-computer. The engine is free to slide securely into the load cell, which exports the force applied into excel for easy data reduction, see figure 8. Using this set-up it was determined that the engine was capable of up to 11 lbs of static thrust, see figure 9, more than the manufacturer specifications.

Figure 8. Static thrust testing with step inputs.

Figure 8. Engine thrust strand with the Kingtech K-45 engine attached.Exhaust temperature was also tested, due to a concern that the exhaust temperature would melt and de-laminate

the composite structure in the tail. During initial static thrust testing, the exhaust temperature was measured to be 700°F at the outlet. In the current concept the exhaust exits over a deck between the V-tail of the aircraft, this configuration was chosen so the engine would be shielded by radar by the carbon fiber tail. To determine what thermal environment must be endured in flight, a thermal camera was employed during static thrust testing. The temperature range of the camera was exceeded, but the thermal profile of the exhaust was easily seen. Initial testing and the thermal image profile, see figure 10, showed that the temperature profile would quickly drop outside of the exhaust cone. This profile was mapped closer in a later test using a temperature probes to determine how quickly the exhaust cone temperature drops off and to get a better idea of what environment the tail would be in during flight. It was found that about an inch from the central axis of the exhaust, the temperature will drop of too about 100°F, which is well below the cure temperature of the epoxy.

Figure 10. Thermal reading during static thrust testing.Additional engine tests were performed to determine whether any flight conditions encountered would be

problematic. The first of the set of additional tests include testing inlet sizing for the motor. During maneuvers at high speeds, air flow could be cut off from the engine and choke the engine, so testing needed to be performed in order to determine what size the inlet would have to be on the aircraft. This testing was performed by creating a mock fuselage and using a high velocity air to be passed over the body, simulating the body blocking airflow during flight, see figure 11. After setting up the engine and mock fuselage with a large NACA inlet cut into it, the engine was started. The NACA inlet was gradually reduced until it was completely closed off, then other cracks in the mock fuselage where attempted to be closed off. Throughout this test, the engine showed no sign of thrust lost or performance reduction. Inlet testing confirmed initial given information, that the engine doesn’t require a large amount of air inflow and doesn’t require high quality air inflow.

Figure 11. NACA inlet sizing testing.Additional testing was performed to ensure that the turbine could not be blown out from a strong tailwind, an engine susceptibility test was performed. This was performed by using a high speed blast of air and blowing it against the exhaust of the engine, where it was found that the turbine would not blow out.

IX. ConclusionThe senior design team, By Inspection, developed a turbine-powered RC aircraft to meet the design

requirements for the Oklahoma State sponsored Speedfest competition. This competition required the development of a turbine powered target drone that could perform certain maneuvers at high speeds while also minimizing its radar cross section. At the end of the design process an aircraft was designed with a GTOW of 9 lbs, a 42 inch wingspan, a 250 fps cruise speed, and 4 minutes of flight time at wide open throttle. The aircraft design was heavily influenced by the need for the aircraft to be powered by a turbine and the need for certain materials and aesthetics to lower the radar cross section.

In order to finalize the design and verify configuration decisions by the team, numerous motor tests and structural tests were performed. These tests include analyzing the required engine inlet size, blow-out potential, and choke potential during maneuvers, as well as wing loading tests. As the aircraft flight test model is finished, the weight build continues to be updated and the aircraft center of gravity and overall stability has been finalized. Flight testing is planned to begin in mid-March, where the success of the aircraft will serve as the basis for the final verification for a successful design. Instrumentation and data collection of the aircraft during flight will help prepare future design teams work with the KingTech K45 turbine and the Speedfest competition in general.

X. References[1] “SpeedFest VII Alpha Class Statement of Work for a Turbine-Powered Advanced UAS Target Drone,” SpeedFest, 2016-17.[2] “KingTech K-45G,” Radio Control Jet International, pp 44-49, Aug/Sept 2015.[3] Skolnick, M.I., Introduction to Radar Systems, McGraw-Hill, 1980.[4] E Knott, J Shaeffer, M Tulley, Radar Cross Section. pp 528–531.