Design Selection - Purdue University



AAE 451

System Definition Review

Team 1

John Horst

Jared Odle

Keith Fay

Boyce Dauby

Andrew Kovach

Akshay Raje

Manish Handa

Jason Darby

Executive Summary

Tactical communications for today’s armed forces and disaster recovery organizations are vital to their success and can make the difference between life and death. The ability to have continuous, clean communications between different units instantly after a need arises and consistently throughout the duration of the operation is the fundamental principle of the Unmanned Communications Relay System (UCRS), which consists of multiple Unmanned Aerial Vehicle (UAV) units controlled autonomously with manual override option and a control ground station.

The primary target market for the UCRS is the United States Department of Defense (DOD) and the Federal Emergency Management Agency (FEMA). From the Systems Requirement Review (SRR) submitted earlier in the year, the projected market share is $164 million with 42 units produced at the end of the first year and up to 100 units produced each year by the 10th year of production. The competition to the UCRS includes stationary broadcast towers, communications satellites, lighter than air vehicles, and other UAVs such as the Hunter and the Predator. The designed capabilities of the UCRS will meet all of the customer attributes for the DOD and FEMA, providing a definite advantage over the competition by providing all the benefits of extended communications relay with a minimum of 24 hours continuous coverage anywhere in the world within a 60 nm radius deployed up to 100 nm to the desired location.

The current concept of the UAV is a high aspect ratio, low wing design with 2 wing mounted, propeller driven internal combustion engines. The aircraft cruises at a speed of 100 knots and will loiter around the target area at a speed of 80 knots. The aircraft flight plan will be modifiable in flight, giving the operator the ability to quickly change the coverage area if required. The UAV is equipped with the Adaptive Joint C41SR Node (AJCN), a 270 lb tactical communications relay payload used for transmitting and receiving communications from air and ground units and is capable of performing electronic warfare, signal intelligence, and information operations. An externally mounted camera is included for navigation and manual operation purposes in daytime and nighttime missions. An optional missile defense system is provided for additional survivability in high threat situations.

The gross takeoff weight of the UAV is currently at 3,100 lbs and an empty weight of 1,900 lbs that translates to a medium sized UAV. At this point, the UCRS is on track to meeting most of the compliance requirements with a current endurance time of 20 hours, combat radius of 100 nm, and a service ceiling of 33,500 feet. Takeoff and landing distance requirements will loosen because of their strong effect on gross takeoff weight of the aircraft. Initial trade studies, sizing, and constraint analysis utilized MATLAB with manually input performance equations. Transitions to more advanced sizing codes are underway. At this point, the design concept is quite pronounced, but more iterations and modifications are in order.

Table of Contents

Introduction…………………………………………………………………...……Page 4

Concept of Operations……………………………………………………………..Page 5

Major Design Requirements………………………………………………...…….Page 8

Design Selection…………………………………………………………...……….Page 9

Current Concept………………………………………………………………….Page 13

Payload Description………………………………………...…………………….Page 16

Internal Layout…………………………………………………...………………Page 20

Propeller Analysis………………………………………………………….……..Page 23

Constraint and Performance Analysis………………………….……………….Page 26

Trade Studies……………………………………………………………………..Page 31

Requirements Compliance………………………………………………….……Page 34

Sizing Codes………………………………………………………………………Page 35

Future Work…………………………………………………………..…………..Page 36

Conclusions………………………………………………….…………………….Page 38

References……………………………………………………………..…………..Page 39

Appendix………………………………………..…………………………………Page 41

Introduction

According to the Unmanned Aircraft Systems Roadmap compiled by the Office of the Secretary of Defense, the need for a more expansive and effective communications network is urgent, and will only become more so in the future. In fact, “by 2010, existing and planned capacities are forecast to meet only 44 percent of the need projected by Joint Vision 2010 to ensure information superiority.” A lack of available bandwidth as well as limited connectivity are but two of the many deficiencies plaguing the satellites that comprise the military’s global communications network. Satellites require line of sight (LOS) in order to send and receive information, which is substantially difficult to achieve at times from a low-earth orbit. It is also difficult and expensive to alter their trajectories in order to cover a specified area. Finally, satellites simply do not possess the necessary amount of bandwidth required to convey the magnitudes of information that today’s military requires.

The primary mission objective for the Unmanned Communications Relay System (UCRS) is to provide continuous area coverage in the form of a communications relay. The reason that Unmanned Aerial Vehicles (UAVs) are ideal for this particular mission is due to the long endurance times they can achieve without a human pilot as a limiting factor. It will likely be deployed in hostile foreign territory in support of the armed forces or in search and rescue operations. The UCRS will address the ever-growing need for tactical military communications bandwidth as well as possess the capability to provide continuous relay coverage over any specified area in a fraction of the time required by satellites.

Current UAV market forecasts reflect the scale of this need for unmanned aerial systems. According to the market research performed for the Systems Requirements Review, government spending on UAV systems is on the rise. From the results of previous market forecasts, the UAV market will undergo significant expansion after 2010, especially in the military sectors. This expansion is beneficial for the UAV because it will allow an opportunity to capture market share that UAV manufacturers currently dominate. Initially, most of the revenue for the UCRS will come from the DOD, with domestic civilian customers making up the rest of the sales, as FAA regulations allow.

Expectations are that the UAV will be able to market to the entire spectrum of DOD contractors as well as an additional 5 percent to Civilian customers, giving access to 85 percent of the total US market for UAVs, totaling about $5 billion. Taking into account that the Joint Vision 2010 report projects that its goal of Information Superiority expects that only 44 percent of its need will be satisfied, this leads to a final market share of about $2.5 billion. This is created as a result of the expansion and the demand by the DOD for its Information Superiority needs. Assuming an upper limit of 10 percent of this market share and a lower limit of 3%, the UCRS stands to captures $164 million. This value is an estimate of first year revenues and is independent of production costs, overhead costs, and recurring expenses.

Concept of Operations

The purpose of the proposed unmanned aerial vehicle is to provide a continuous communications relay to an area unsuitable for more conventional communications systems. It is important to envision how the customer will use the vehicle. This concept of operations drives the design of the vehicle and provides a summary of the system’s intended use.

The primary customer is the military. The military will most likely deploy the vehicle in a situation where they are sending combat units forward into hostile territory. This hostile territory may not be able to support the communication needs of the troops in the area. Therefore, the military decides to deploy the UCRS to act as a communications relay. The relaying of communication is for tactical uses only. This means the use of the communication payload is for ground troops to contact other ground troops in the coverage area. They could also contact air support units overhead and relay final target coordinates. The UAV relays any messages to the strategic headquarters via satellite or another aircraft in order to contact outside of the communication range of the payload.

Using a base nearby, the UAV takes off from a conventional runway by remote control. The UAV deploys from a location such that the coverage area will be within 100 nautical miles (shown in Figure 1). The operator will be able to put in a waypoint for the coverage area and the UAV will fly there autonomously. Upon reaching the coverage area, the UAV will loiter autonomously above at 80 knots for up to 20 hours in a circular loiter pattern of 10 nm. The effective communication range of the communication payload reduces to 50 nm, due to this 10 nm loiter pattern (shown in Figure 2). At any time, the operator can gain manual control or input a different waypoint for the area to be covered. This allows the UAV to adapt to a changing battlefield. In order to provide the greatest coverage area, the UAV will loiter at a constant altitude of 15,000 feet AGL. The area covered has a radius between 55 and 100 nautical miles for ground communications and 60 nautical miles for air communications, as seen in Figure 2. The communications node will be able to transmit from one ground troop to another or from the ground troop to a command station. Not only can the communications node transmit friendly information, but also it can disrupt unfriendly communication through electronic warfare.

When the deployed UAV is starting to run low on fuel, another UAV prepares and launches to the area. A second UAV will join the first UAV above the coverage area and communications traffic will transfer to the second UAV. The first will then begin its return flight to a landing area. The UAV operator will manually land the aircraft on a conventional runway. Ground crews inspect and refuel so that it will be ready to take the place of the current UAV that currently loiters above the coverage area. This method of UAV replacement ensures continuous area coverage over the designated area. When involved in an especially high-risk area, twice the number of active UAVs operates in the system. Two UAVs will be loitering over the coverage area while two others wait at the base, ready for deployment. In both cases, if the destruction of a UAV occurs, a backup UAV is waiting at the base.

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Figure 1: Communication range and combat radius

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Figure 2: Effective Communication Range From Coverage Center

The secondary customer market is the Federal Emergency Management Agency (FEMA) and other disaster relief agencies. While the demand in this market is not as high as it is for the military, there is still an opportunity to seize a potential market segment. The scenario for FEMA is the case of a natural disaster. If communications were to go down in an area, the emergency rescue personnel would utilize a UAV-housed ACN to communicate. Similarly, the victims of the disaster would be able to use their cell phones to call for help. For FEMA, an alternate payload package is necessary.

The UAV departs from a nearby airport not affected by the disaster. As seen in Figure 3 below, there are many public-use airports available, so there are many viable locations for using a conventional runway [1]. The UAV would takeoff under the control of the operator and fly up to 100 nautical miles to the coverage area. It would autonomously loiter at 80 knots at an altitude of 15,000 feet AGL for upwards of 20 hours in a circular loiter pattern of radius 10 nm. During this time, it would act as a communications node for emergency response workers and victims of the disaster. When the UAV begins to run low on fuel, a second UAV takes off to replace it. Once the second UAV reaches the coverage area, it will take over the communications load and the first UAV will return for inspection and refueling. Once the second UAV’s fuel supply diminishes, the first one will be re-launched to replace it in a similar manner. In such a fashion, no loss of continuous area coverage will occur. Should a UAV malfunction or be damaged for any reason, a spare UAV is always waiting in reserve to assure continuous coverage.

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Figure 3: Location of public-use airports [1]

Major Design Requirements

The mission outlined in the previous section drives what the design will look like. Table 1 outlines these design requirements.

|  |Target |Threshold |

|Endurance |24 hr |14 hr |

|Range |100 nm |50 nm |

|Ceiling |30,000 ft MSL |15,000 ft MSL |

|Takeoff |500 ft |1,000 ft |

|Landing |500 ft |1,000 ft |

|Transportation Volume |Fit inside C-130 (55’x10’x9’) |Fit inside C-5 (143’x19’x13.5’) |

Table 1: Major Design Requirements

The selection of each requirement arose from the mission objectives that the aircraft must accomplish. Endurance has a lot of leeway built in from the operational use of the UAV. Because the UAVs operate in pairs that rotate in and out, the endurance of each individual aircraft can be sacrificed somewhat in order to reduce cost.

The ceiling target arose from the operational altitude of the communication package. The transportation volume is important because these UAVs will be operating overseas and will need to be transported.

Design Selection

The level of success an aircraft design achieves depends significantly on the initial concept. The process of concept selection begins with the generation of concept sketches. By creating a large number of sketches, a wide range of potential solutions is available from which to make design choices. Additionally, the level of variation between the candidate designs increases the probability that the ideal aircraft configuration appears in some combination amongst the concept sketches. Variations amongst the candidate concepts included the main wing configuration and placement, tail configuration, number and placement of engines, as well as landing gear arrangement. Since the evaluation of one concept to another can be a very subjective process, a method known as Pugh’s method ensured an objective comparison between the concept sketches.

Pugh’s Method

Pugh’s method is an analysis tool used to evaluate potential concepts to one another according to a list of design criteria. This list of criteria, generated from the design requirements supplied from the voice of the customer, represents design constraints that the candidate concepts should meet to facilitate the execution of the intended mission. Concepts are compared according to each criterion to a datum design that exhibits a high probability of accomplishing the desired design criterion. Either this datum can be a system currently in production that meets all or most of the mission requirements or the generated concept that appears to most successfully meet the design criteria. A matrix of each concept’s ability to achieve these criteria relative to the datum design is generated with three possible scores. The concept receives a plus sign (+) if it fulfills a given criterion more effectively than the datum, a negative sign (-) if it is less effective, or a same (S) if there is no difference. Once all concepts are compared to the datum for all criteria, the +’s, -‘s, and S’s for each concept are summed yielding a numerical score. This score, while helpful in objectifying an otherwise subjective comparison, does not immediately expose the best initial concept. After consolidating successful concepts to create hybrids and refining them to reduce their deficiencies, they are once again evaluated according to the design criteria against the datum and new scores are computed. Only after several iterations and numerous concept modifications will the best scoring design approach the ideal concept, although this is not guaranteed since the effectiveness of Pugh’s method depends on how successfully the datum meets the design requirements. The selection of the datum design is a very important step in Pugh’s method, since a merely satisfactory datum will produce only a slightly better than satisfactory initial concept.

Team 1 used a concept with a high chance of accomplishing the intended mission as the datum for comparison via Pugh’s method. The datum design featured a high-mounted, unswept main wing, a conventional tail, two wing-mounted engines, and tricycle landing gear. For the first iteration of Pugh’s method, nine different concepts were created and compared. Based on those initial results, new concepts were created while others were modified to better meet the design criteria. For the final iteration of Pugh’s method, ten different concepts, shown below in Figure 4, were analyzed and the totals computed. Results showed that the most beneficial design features to achieve the mission objectives include unswept, tapered wings to aid endurance and lower manufacturing costs as well as easily accessible engines to reduce turn around and repair times. Two concepts scored higher than the rest, and using the beneficial design features of both, the initial UCRS concept was created. The list of design criteria, comparison matrix, and the sum total of the overall proficiency of each concept is shown in Table 2.

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Figure 4: Final Pugh’s Method Concepts

Table 2: Pugh’s method selection matrix

Current Concept

The current initial concept for the UCRS, shown below in Figure 5, utilizes a high aspect ratio, unswept, low-mounted main wing. Since the loiter velocity of the UCRS is under a mach number of 0.1, a high aspect ratio is required to produce sufficient lift at such a low speed. Historically, aircraft achieving cruise speeds less than mach 0.5 do not require wing sweep, since airspeeds approaching the sound barrier are never reached which negates the need for wing sweep to offset the rapid escalation in drag [2]. Due to the high costs of producing elliptical planform wings, rectangular planform wings are often tapered to mimic the ideal load distribution of those with elliptic planforms. The taper ratio (the ratio of the root chord length versus the tip chord length) that achieves a load distribution closest to that of the elliptic wing is 0.45. However, the weight savings granted by tapering the wing yield a taper ratio closer to 0.4, which is the value used for the current UCRS concept. Vertical placement of the main wing is a critical early design decision, since its positioning affects numerous other parameters such as engine nacelle configuration, propeller diameter, tail placement, and landing gear length. The current UCRS concept incorporates a low-mounted main wing to avoid the structural issues inherent in mid-wing designs, which necessitate extra support in order to carry the wing box through the fuselage. The low-wing configuration also reduces the necessary landing gear length providing additional weight savings over high-mounted wings.

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Figure 5: UCRS Initial Concept

Survivability of the UCRS is a principal design requirement for the Department of Defense. Two engines placed over the wing mounted in nacelles will serve to ensure continuous coverage over high-threat theatres of operation, that is, areas where there is a high probability of anti-aircraft attacks. By mounting the engines on opposing sides of the aircraft, the chances of a single attack affecting both engines and resulting in a total power loss is greatly reduced over an inline-fuselage mounted configuration, thus increasing survivability. Specific engine and propeller selection is still pending until further sizing calculations produce a more accurate estimate for the total power required.

The fuselage design of the current UCRS concept mimics that of an unmanned submersible vehicle designed at the Applied Physics Laboratory at the University of Washington [3]. Studies showed that the fuselage geometry shown in Figure 6 yielded the lowest drag coefficient and maintained laminar flow over its surface. Since the majority of the UCRS’s payload will be located toward the front of the fuselage, the rear portion of the fuselage will serve only to provide mounting point for the tail, so its interior volume should be the minimum required to withstand the associated moment. Since the fuselage geometry of the unmanned submersible supports the requirements of the UCRS’s fuselage while minimizing drag, it was used as an initial guide for this concept.

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Figure 6: University of Washington Seaglider Fuselage

As the name implies, the conventional tail is the most commonly used tail configuration in past and current aircraft. Comprised of two horizontal stabilizers and one vertical stabilizer, the conventional tail provides exceptional stability at the lightest weight. Since all of the tail components attach directly to the fuselage, the additional structural support required by other configurations to brace the stabilizers is not necessary. The current size of the tail in the initial concept model is merely an estimate, and will most likely increase in order to compensate for the yaw moment produced when only one engine is functioning.

The landing gear configuration for the initial concept of the UCRS is of the tail-dragger arrangement. The advantages of this type of configuration include allowance for a larger diameter propeller and a higher takeoff angle of attack. However, aircraft with tail-dragger landing gear are difficult to land, requiring nearly perfect alignment with the runway on touchdown due to the location of the center of gravity. If the rear landing gear is not fully aligned with the direction of motion, the aircraft can become unstable and veer off the runway. Due to the challenges already involved with remotely landing UAVs, implementing a more stable landing gear configuration to the UCRS may be necessary. Tricycle landing gear is a reasonable alternative due its greater stability during unaligned landings.

Payload Description

The UCRS will carry three payloads to enable full mission capability. The payloads consist of a camera, tactical communication relay, and an optional missile defense system. The team carefully selected the payloads to provide high mission performance to the customer while weighing the trade-offs with weight, power, and cost.

The visualization system on the UCRS will consist of two cameras produced by MicroPilot. While the cameras are interchangeable with the UCRS high endurance, day will change to night before landing to refuel [4]. Therefore, UCRS will carry both the Dayview and Nightview thermal imaging camera. The two cameras provide the operator with visuals allowing for manual flight operation in day and night conditions. Designed for small UAVs, the cameras are durable, light-weight, and operate with little power. While the UCRS is closer to a medium sized UAV, the intent of the camera is not surveillance. Instead, the cameras allow increased flight responsiveness and awareness during autonomous and manual flight. With a 25x zoom and a video resolution of 320x240 pixels, the camera is capable of providing valuable real-time video during critical mission stages such as takeoff and landing [4]. The addition of the cameras provide substantial benefits with little weight and power costs, each weighing only 2.2 lbs and consuming only 3 W of power during operation [4]. With a diameter of 4.3 inches and height of 8.3 inches, finding room for the two cameras was not difficult and a schematic of the internal layout will be provided and discussed later [4].

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Figure 7: The MicroPilot Dayview/Nightview mounted in a gimbal [4].

The reason for selecting the developmental system known as the AJCN or Adaptive Joint C4ISR Node is that it meets the mission needs of the military [5]. AJCN is a scalable communications relay that is currently under development by BAE systems [6]. AJCN is a multi-function, reconfigurable, and platform independent payload. The payload will have additional capabilities beyond secure communication relay including signal intelligence (SIGINT), electronic warfare (EW), and information operations.

Currently, the AJCN package under development for middle to large sized UAVs weighs 270 lbs and requires 1500 W additional power [7]. Additional weight savings may decrease the payload weight to less than 200 lbs [8]. Since the system is scalable, it is possible to increase the capability of the system rather than redesigning the platform [9].

The communication capabilities include providing beyond line of sight range extension, bridging of dissimilar waveforms, automatic voice and data routing, and reach back. At 15,000 ft AGL, the system provides an air-to-air communication range of 60 nm and between 55-100 nm for air-to-ground relay [7]. Four 25 MHz tuners would provide a total of 100 MHz receiver bandwidth over the frequency range of 20-3000 MHz. The system has a transmit frequency range of 40-500 MHz, 902-928 MHz, and 1800-2000 MHz with the capability to simultaneous transmit on up to five channels. AJCN contains the ability to communicate on multiple waveforms to increase the interoperability between the Armed Forces, where units in different branches can have incompatible communication equipment. The following waveforms will be available on the AJCN system fielded on the aircraft: VHF AM/FM, SINCGARS, UHF AM/FM, LMR/ Public Service, Wireless IP Crosslink, EPLRS CRP, and GSM. AJCN creates an “ad hoc” network where users automatically add and drop as they come in and out of range allowing for an autonomous relay system [10]. The system also will provide reach back capability as an additional form to tie waveforms together by allowing tactical communications to make SATCOM links.

The system provides signal intelligence over the full range of the receiver frequency range, 20-3000 MHz [7]. The AJCN SIGINT provides the ability for electronic reconnaissance, auto recognition, geolocation, as well as exploitation. Electronic reconnaissance is the ability to scan a wide or narrow range of frequencies over the mission location. Auto recognition and geolocation are the identification of the waveform in use and the determination of its location. The final piece of SIGINT is exploitation, which refers to the real-time or playback of copied voice and data communication. Together these permit enhanced real-time situational awareness allowing a precise location of the target and reducing the sensor to shooter time.

The AJCN system is also capable to conduct electric warfare and information operations (EA/IO) missions [6]. Electric warfare is composed of two segments, electromagnetic jamming and electromagnetic deception [7]. Jamming is defined as the block or hinder of enemy communications, where deception is when misleading communication signals are purposely emitted to confuse the enemy. A UAV mounted with the AJCN would allow for close range jamming of targets and boost or supplement other electronic attacks.

The AJCN is still undergoing development and some technical changes remain before completion, but the system appears on pace. The system began flight demonstrations in 2003 and will incorporate SIGINT capability into the communication package by 2010 [7]. The AJCN communication relay package fielded on the high-endurance UAV will help ensure information superiority over the enemy long into the future.

Due to the export compliance and the classified nature of a developmental communication package, information on the specifics of the AJCN system is difficult to acquire. The main issues are the system’s overall volume as well as antenna placement and integration. Although, these are significant design considerations, work must continue without exact numbers. The team intends to be cautious in estimations relating to the AJCN. Continuing this trend, the volume was estimated using an average avionic density of 0.02 - 0.03 ft3/lb was computed to be between 6 and 9 ft3 for 270 lbs of communication equipment [2]. Using the worse case scenario for the volume, 9 ft3, the team computed conservative sizing estimates and payload configurations. A contact at BAE systems also provided the team with a publicly released picture of a prototype of the AJCN system fielded on a Hunter UAV [11].

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Figure 8: Developmental AJCN mounted on a Hunter UAV [11]

From the picture of the Hunter with the AJCN system, Figure 8, it is difficult, but possible to make out small antennas protruding from the bottom of the fuselage and along the aircraft’s wings. While developing the sizing for UCRS, the team under-predicted the lift-to-drag ratio to offset some of the drag penalty of a “dirty” aircraft due to the integration of the antennas with the system.

For the remainder of the primary target customers, it is believed that a non-militarized communication package of similar weight and power requirements could be adopted to provide similar, if not more, bandwidth and channels for disaster relief agencies. This is a reasonable assessment, since the communication data security could be relaxed along with the elimination of the electronic attack and signal intelligence aspects of the AJCN system.

To increase aircraft survivability in the high threat operational environment for the military customer, UCRS offers an optional integration of a missile detection and countermeasure system. The ALQ-156 missile warning system working in conjunction with AN/ALE-47 provides automatic response to protect against Air Interceptor (AI), Anti-Aircraft Artillery (AAA), and Surface-to-Air Missiles (SAMs) [12,13]. Increased survivability will decrease the number of aircraft losses increasing the aircraft’s reliability and effectiveness for the troops depending on the UCRS communication relay.

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Figure 9: Illustration of ALQ-156 and AN/ALE-47 system placement and integration [13]

Developed by BAE systems the AN/ALQ-156(V) Warning System requires 425 W and weighs 50 lbs and the AN/ALE-47 Dispensing System weighs an additional 20 lbs [12,13]. Proven reliable in the field, both have flown on multiple platforms across the globe. In fact, the AN/ALE-47 is flying on over 1,300 aircraft. The warning system is a single box, with dimensions of 20.4 in × 10.2 in × 7.6 in or a volume of 0.92 ft3 [12,13]. The dispensing system comes in multiple components, and Table 3 provides a component list being fielding along with the dimensions and weights.

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Table 3: AN/ALE-47 Dispensing System Component Breakdown [12]

The combination of these payloads is essential in providing the customer with the desired performance to meet all mission requirements. The payloads selected do add considerable weight and expense to the UAV, but the each payload was individually evaluated based on its role in the concept of operations and deemed an integral part to achieving the customers’ expectations. A fully outfitted aircraft will have 345 pounds of payload that consumes 1930 watts during operation, or 2.6 hp. Table 4 below provides a quick summary of the payload aboard a fully outfitted UAV.

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Table 4: Payload Summary

Internal Layout

Many characteristics dictate how the internals of an aircraft are located and oriented. Figure 10, shown below, details a top-down view of the internal configuration of the initial concept model. The AJCN communications node, the primary payload component of the UCRS system, is the largest and heaviest component to reside within the fuselage. Locating it near the nose of the aircraft promotes longitudinal stability and ensures a clear line of sight with ground relays as well those from other aircraft. While exact dimensions and specific information pertaining to integration and antenna placement are unavailable at this time, since the system in still under development, the current sizing of the AJCN is at the maximum estimated dimensions for the initial concept model.

The MicroPilot UAV camera is located in the nose of the aircraft to provide maximum visibility to the environment and to ensure other UCRS components do not compromise line of sight. Due to the length of the projected loiter time of the UAV, it is likely that two separate cameras will be necessary: one to function for daytime navigation and one to provide nighttime thermal imaging. Due to their small size and minimal power consumption, integrating an additional camera in the nose of the aircraft should not create a significant problem.

As earlier stated, the engine nacelles are positioned on opposing sides of the main wing. While this does enhance the system’s survivability, an additional motive for placing the engines away from the fuselage pertains to the lack of specific information about the AJCN. Since it will not be operational until 2010, details describing its sensitivity to engine vibrations are unknown at this time. In the case that the communications relay is later determined to be vulnerable to these oscillations, it is beneficial at this stage of the design process to separate them as much as possible.

The fuel tanks and avionics equipment are currently located in the optimum locations for preserving a beneficial center of gravity. Since the weight of fuel onboard the aircraft will steadily decrease over the course of a mission, it is important to account for this in the tank placement so that static stability is not lost because of the center of gravity’s location shifting. Additionally, since the lateral placement of the avionics system is not critical, it can be treated as ballast in order to locate the center of gravity in an ideal position.

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Figure 10: Internal Layout of Major Components

Figure 11, shown below, better illustrates the three-dimensional internal layout of the UCRS. Only the main components described above are shown, in addition to our initial landing gear arrangement, since dimensions of other components such as missile defense, countermeasures, antennas, etc. are still unknown at this time.

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Figure 11: Major Payload Components and Initial Landing Gear Arrangement

Propeller Analysis

Propellers are of immense importance to the performance of the aircraft. It is ideal to have an engine that provides sufficient power to drive the propeller as well as have a propeller that produces the ideal amount of thrust to perform the mission capabilities. Therefore, propeller selection and design becomes a very important issue. Engine and propellers work hand in hand, which makes their performance and selection interdependent. Moreover, the propeller depends on the engine so it becomes very crucial to have accurate engine requirements before propeller selection and design.

The two most common types of propellers include variable pitch and fixed pitch. The selection of these depends on mission requirements and the aircraft capabilities. For the case of this UAV, it is desired to give optimum results while loitering and during takeoff and landing. The mission requires the UAV to have an endurance of 20 hours, which demands that the engine and propeller assembly perform efficiently and provide optimum thrust for the fuel intake.

Therefore, it is most beneficial to utilize a propeller of variable pitch. Control of the pitch of the propeller in flight provides improved performance in each phase of the mission. Typically, the aircraft would take-off with a fine pitch allowing the engine to develop reasonable revs, before increasing the pitch during acceleration to the cruising speed. At cruise, with the propeller at a relatively coarse pitch, this variation in pitch allows for high velocities without straining the engine.

A variable pitch propeller provides many performance advantages including:

• Reduced take-off roll and improved climb performance

o At lower air speeds, a fine propeller pitch allows the engine to reach maximum speed and power. This is crucial for take-off, climb, and go-around maneuvers on landing.

• Improved fuel efficiency and greater range

o With a coarse pitch, the desired aircraft speed can be maintained with a lower throttle setting and slower propeller speed. This results in better engine efficiency and increased range.

• Higher top speed

o A coarse pitch will ensure your engine does not overspeed while the propeller absorbs high power, producing a higher top speed.

• Steeper descent and shorter landing roll

o With a fine pitch and low throttle setting, the slower turning propeller is able to increase the aircraft's drag, which enables a quicker, steeper descent when landing.

Variable pitch propellers actually come in a variety of versions based on how they are controlled, including Two-position propeller, In flight adjustable propeller, Automatic propeller, and Constant speed propeller.

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Figure 12: Propeller Efficiency and Cost Tradeoff [14]

Considering the cost and mission requirements, the UAV could have ground adjustable or in-flight adjustable propellers. In order to have ground adjustable propellers a lot of testing would be required to set the blade angle of the propeller depending on the mission.

Choice of blade angle produces maximum performance at a particular flight condition, for example:

• Vy climb i.e. a climb propeller

• Vc cruise i.e. a cruise propeller

• High speed.

Figure 13 illustrates how performance changes for various requirements airspeeds.

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Figure 13: Changes in Performance of the Aircraft at Different Blade Angles [14]

When choosing a ground adjustable propeller, it becomes crucial to know the precise effects of the blade angles on the propeller’s performance. A modification to the ground adjustable could be made by making it in-flight adjustable. This would yield better performance for the aircraft at the various stages of the mission. For this reason, in-flight adjustable propellers would be ideal for this UAV. With only a small increase in the cost, the performance from ground to in-flight increase drastically.

The next step in the selection process is choosing the number of blades and the blade diameter. This is still under consideration since it depends heavily on the engine selection. The preliminary analysis suggests that the UAV will have either 2 or 3 blades. A basic tradeoff has to be performed for the performance of the engine with both a 2 and 3 blade propeller. After deciding the number of propeller blades, the blade diameter can then be decided. No consideration in this part of the propeller analysis has been made, since this is highly dependent on the number of blades as well as the engine performance.

Constraint and Performance Analysis

Several parameters were held constant when analyzing the vehicle's performance. Many parameters were held at the values used during initial sizing for the SRR. Namely, L/D was held at 12, AR at 10, CL,max;Landing at 1.8, CL,max;Takeoff at 1.6, and We/W0 was determined from the a regression formula derived from the UAV database. As discussed previously, the low value of L/D is used to allow for greatly increased drag due to the unknown number and arrangement of AJCN antennae. In addition, a component weight buildup was used to find the final predicted gross weight. The general aviation component weights from Raymer Chapter 15 were used. Specific fuel consumption was assumed to be 0.4 lb/hp-hr for cruise and 0.5 lb/hp-hr for loiter. As discussed in the Concept Selection section, Λ was held at 0. Values of W/S and P/W are discussed below.

Three necessary maneuvers for the vehicle come directly from the system requirements definition: Takeoff in 500 ft; Land at the same airport; Ceiling of 30,000 ft.

The three maneuvers must be translated to constraints

1) Takeoff ground roll of 500 ft from a 15,000 ft airport on a Air Force Hot day (+45°)

2) Landing ground roll of 500 ft from a 15,000 ft airport on a Air Force Hot day, with all fuel burned

3) 100 fpm climb at 30,000 ft on a Air Force Hot day

In addition, a turning constraint was added. This constraint was meant to symbolize a collision avoidance maneuver.

4) 2 g sustained turn at 8,000 ft, gross weight, on a Air Force Hot day

The constraint diagram shown in Figure 14 was generated using the equations developed in lecture. The green area signifies acceptable performance, while the red area indicates a failure to meet all the performance constraints. This constraint analysis and the carpet plots assume a 10,000 foot supercharger on each engine. The best design point appears to be a wing loading of 20 and a power to weight ratio of 0.13.

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Figure 14: Constraint analysis; (20 , 0.13) appears to be a good design point.

Carpet plots were attempted using more detailed predictions of the aircraft performance, including power available and required. This greatly changes the predicted wing loading and power to weight ratio, as seen in Figure 15. These new values seem to be more in line with other aircraft, as a 500 foot takeoff roll from 15,000 feet on a hot day is a very demanding constraint. The new optimal point appears to be a wing loading of 4 and a power to weight ratio of about 0.06.

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Figure 15: Carpet plot with 500 ft takeoff and landing constraints at a 15,000 ft airport on a hot day.

Since one goal of the aircraft is to be transportable by a C-130, operations on a field shorter than a C-130 are not required; a C-130J has a takeoff ground roll of 1,630 ft at sea level standard conditions [15]. The threshold takeoff and landing constraints are 1,000 feet at an unspecified altitude and temperature. Assuming a standard day at sea level, the carpet plot changes to Figure 16. The preferred design point from Figure 16 is a wing loading of about 19.5 and a power to weight ratio of 0.07. The predicted gross weight of the aircraft drops from about 3800 pounds to about 3,100 pounds. If the ALQ-156 and ALE-47 were removed, the carpet plot changes to Figure 17. The axes on Figures 16 and 17 are the same. The predicted gross weight is about 250 pounds less, including the 70 pounds for the systems themselves.

From the results illustrated in Figure 16, the aircraft has an approximate gross weight of 3,100 lbs. A summary of the current aircraft design is presented in Table RC1.

Finally, a preliminary flight envelope is presented in Figure 18. The service ceiling of the aircraft is approximately 33,500 ft at 115 knots, while the absolute ceiling is 36,000 ft at 120 knots. Maximum speed is 160 knots at 10,000 feet. As noted above, the aircraft is assumed to have a 10,000 ft supercharger installed in each engine. A summary of performance and requirements compliance is given in Table RC2.

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Figure 16: Carpet plot with 1,000 ft takeoff and landing constraints at sea level on a standard day.

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Figure 17: Carpet plot with 1,000 ft takeoff and landing constraints at sea level on a standard day without ANQ-156 and ALE-47.

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Figure 18: Excess power curves

Trade Studies

Preliminary trade studies were performed for the SRR. These trade studies identified reducing target endurance from 24 hours to 20 hours as a useful trade. The trade studies have been redone using the results from the initial sizing and performance analysis. In addition to the constants from the constraint analysis, the wing loading was held constant at 19.5 lb per ft2, and the power to weight ratio was held constant at 0.07 hp per lb.

The trade from 24 hours to 20 hours is still beneficial and the endurance target has been changed accordingly. With the increased payload weight for the countermeasures over the previous studies, the gross weight is 3,090 lbs. The required fuel volume is now only 142 gallons.

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Figure 19: Effect of payload and endurance on gross weight

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Figure 20: Required fuel volume to complete mission.

The other main performance parameter is the combat radius. Figure 21 shows the effect of combat radius on gross weight. Surprisingly, the combat radius can be doubled to 200 nautical miles by increasing the gross weight 100 lbs. Giving the customer double the operating field for only a 3.3% increase in gross weight appears to be a very good trade. The required fuel volume rises from 142 gallons to 153 gallons, a 7.8% jump. Since fuel storage volume is not a major concern with the concept chosen, the increase in required fuel does not reduce the usefulness of increasing combat radius.

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Figure 21: Effect of combat radius on gross weight

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Figure 22: Effect of combat radius on required fuel volume.

Requirements Compliance

A summary of the aircraft is presented in Table 6. The wing loading and power to weight ratio determined in the Constraint and Performance section were used to size the aircraft. These are not, however, the actual final wing loading and power to weight ratios wince the empty weight component buildup requires wing area and engine power to be known before the final aircraft gross weight can be predicted. As such, the empty weight buildup step raises the gross weight, slightly lowering the ratios from the intended P/W = 0.07 and W/S = 19.5. As seen in Table 7, however, all performance constraint requirements are currently met. Some additional requirements, such as transportation volume, cannot be evaluated at this time. These will be greatly dependent on future design decisions such as foldable wings.

|GTOW: |3100 lb |

|Empty Weight: |1900 lb |

|Fuel Weight: |855 lb |

|Fuel Volume: |140 gal |

|Wingspan: |37.0 ft |

|Fuselage Length: |25.0 ft |

|Engine Power: |99 hp per engine |

Table 6: Summary of Current Design

| |Current Value |Threshold |Target |

|Combat Radius |100 nm |>50 nm |100 nm |

|Endurance |20 hrs |>14 hrs |24 hrs |

|Takeoff Roll (SL, ISA) |870 ft | ................
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