American Institute Of Aeronautics And Astronautics Engineering Essay

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The objective of the Phoenix Team from POLITEHNICA University of Bucharest is to design a remote controlled aircraft to compete in the 2012-2013 American Institute of Aeronautics and Astronautics (AIAA), Cessna Aircraft and Raytheon Missile Systems Design Build Fly competition. The presented design, has been created for optimal performance under the provided competition rules and restraints. It will be designed to complete the three missions of this year's competition so as to maximize points earned.

The two payloads consist of as much as possible Estes's Mini Max rockets internal stores for mission 2, and a combination of internal and external stores as required by the rules for mission 3. This document presents the detailed design, analysis, testing, manufacturing, aircraft performance and management plan employed to ensure a successful flight system.

Several aircraft configurations were considered for the design of the aero model including conventional monoplane, flying wing, canard, and tandem wing. The conventional monoplane configuration was ultimately selected due to the available wealth of knowledge about the design and manufacturing processes, as well as the high volume payload capacity. The conventional design also allowed for a high wing placement, allowing less structural interference with the internal payload volume. A conventional tail layout was also selected for its longitudinal stability and simplicity. The structure weight of the aircraft is critical to the scoring for each mission. Therefore, the aircraft was designed with lightweight construction in mind.

Analyzing the requirements it has been found that the most restrictive is the 30ft takeoff requirement, in the conditions of an underpowered aircraft due to NiCd/NiMH 1.5lb propulsion battery. For such type of batteries the specific energy is under 40Wh/kg for NiCd and 60Wh/kg for NiMH as seen from Greg Covey's article in RC Universe. Thus the energy available will be only about 41Wh, which means a mean 489W propulsion power for 4 minute flight using a 0.8 efficient motor-propeller combination. For a typical 10lb RC plane at 65ft/s cruise speed this gives a thrust of about 5.6lbf i.e. 0.56 T/W ratio. This estimations have to be penalized by the higher energy needed for takeoff and the efficiency of the propulsion system.

The ferry flight and loaded missions require that the system allow for set up and loading in a five minute time period. The aircraft must also carry internal and/or external payload while withstanding a

2.5g load maneuver, must takeoff in less than 30ft runway, and must land on the runway after completing each flight. Mission two requires the aircraft to complete three laps while carrying maximum internal stores. Mission three requires the aircraft to complete three laps while carrying a combination of internal and external stores as stated in Table 3. bellow totalizing 3lbs.

A sensitivity analysis was performed on the mission scoring equations. It was determined that the most sensitive parameter of this competition is the number of internal stores flown in mission two. The Numbers of Laps Flown in mission one is the second most important parameter. The Time Flown in mission three is the third most important parameter.

After considering different configurations, team have decided that the maximum payload should be 3lbs, which, for mission two means 12 internal Mini-Max 0.25lbs stores. The payload restraint system for the stores was designed to securely hold them while allowing for the installment in less than 5 minutes. Following these requirements, in addition to constraints imposed by the 1.5lbs NiMH battery limitation, the aircraft was designed to have a low stall speed of 29.5ft/s and a maximum cruise speed of 68.9ft/s at maximum gross take-off weight.

The aircraft is designed to have the lowest possible structure weight while remaining sufficiently rigid to endure the stresses of flight. The estimated flight weight of the aircraft for Mission 1 is 3.44lb. For Mission 2 and Mission 3 it is 7.1 and 7.3lb, respectively. Mission 1 consists of an empty aircraft. The Mission 2 weight includes the weight of 12 internal Mini-Max stores, and the Mission 3 weight includes the weight of the combination of internal/external stores.

High lifting S1223 airfoil was selected for the wing in order to achieve the 30ft takeoff requirement, allowing smaller wing surface, lower stall and takeoff speed

Management Summary

In order to accomplish the desired objectives of the competition, Phoenix Team aims to use the resources efficiently. The project is highly modular, requiring a wide array of individual skills and knowledge. Every section of the project was assigned to proficient students who are led by the same goal: to Design, Build and Fly an aircraft which will be able to successfully finish all the Missions from the competition.

Organization of Design Team

We divided our work into fundamental sections: Flight Dynamics, Aerodynamics, Structures, Propulsion, Manufacturing and Testing. All the activities are rigorously coordinated by our team leader and by our advisor.

Flight dynamics represents an essential part of our project. Our team studied the performance, stability and control of our airplane. We are doing our best to balance the conditions of three missions of the competition and to accomplish all the tasks (to build a fast, stable and powerful aircraft to be able to make as many laps as possible in 4 minutes, to takeoff from the perimeter required and to efficiently carry internal and external stores).

Students concerned with the aerodynamics section performed different types of flow analysis around airfoils to find the most efficient one. They also designed the external configuration of the plane.

Structural study was carried by our team to outline the effects of loads on the physical structure and to choose different materials for the aircraft.

Propulsion plays a key role in this competition; thus, students worked to find the most efficient motor-propeller combination for the plane, to optimize the aircraft propulsion and to select the electronics.

We manufacture several prototypes in order to choose the best solution. We also searched for sponsors to buy the necessary components. Moreover, testing signifies a major step in a real success, so we let approximately one and a half month to test and improve the plane that is now ready for the competition!

Figure 2.. Organizational Chart

Project Schedule

Phoenix Team is aware of the importance of prioritizing and organizing. Therefore, we developed a milestone chart to graphically describe the key events of the contest. Specific points are added into the chart to check if the project respects the deadline (they are marked with a triangle). We accept that a delay is not permitted in our work schedule, therefore we worked to respect the timetable, and as it can be seen, many activities were finished before deadline.

The milestone is divided into four stages depicted above: Design, Manufacture, Testing, Report and Searching for sponsors. Most of the tasks are interconnected and were performed in the same time.

logo final (cu transparenta).pnglogo final (cu transparenta).pnglogo final (cu transparenta).pnglogo final (cu transparenta).png

Sept.

Oct.

Nov.

Dec.

Ian.

Feb.

Mar.

Apr.

DESIGN

Conceptual Phase

Preliminary Phase

Extensive Design

Final Design

MANUFACTURE

Prototype Aircraft

Final Aircraft

TESTING and SELECTION

Batteries

Materials

Motors

Final Testing

REPORT

Draft

Final report

SPONSORS

Fund-raising activities

Conceptual Design

During the conceptual design phase, the team considered competition requirements along with the mission scoring formulas to guide general understanding of how to maximize the overall score. By analyzing each mission score independently then calculating the total score, the relative sensitivity of various aircraft characteristics on score were determined.

Mission Requirements and Rules

This year's aircraft is required to successfully complete three missions in order to complete the competition. Aircraft will use ground rolling take-off and landing. Missions will simulate take-off from a small austere field. The aircraft will be placed such that all ground contact points are completely inside an approximately 30 x 30 ft square marked on the runway. Aircraft must successfully take-off before crossing any edge of the square. Aircraft must complete a successful landing at the end of each mission for the mission to receive a score. After completing each mission, the team will receive a score based on the performance of the aircraft during that mission. The final score is calculated from team's Written Report Score, Total Flight Score and Rated Aircraft Cost using the following formula:

The total flight score is the sum of the individual mission flight scores: ; RAC is a function of empty weight and size factor: ; Empty weight will be measured after each successful scoring flight: where is the post flight weight with the payload removed. Size factor is computed with: , where is the longest possible dimension in the direction of flight and is the longest possible dimension perpendicular to the direction of flight.

Mission and Scoring Summary

Mission 1 - Short Take-off. Take-off within the prescribed area. Must complete a successful landing to get a score. Maximum number of complete laps within a 4 minute flight time is the goal. Mission performance will be normalized over all teams successfully completing this mission. Mission score is:

Mission 2 - Stealth Mission. Take-off within the prescribed area. Must complete a successful landing to get a score. 3 Lap internal-stores flight. Stores must be carried internal to the aircraft in the main fuselage (not a secondary pod) or completely inside the wing. Access to the stores for loading must be through the lower surface of the aircraft. Stores must be aligned to the direction of flight (tails aft, body along flight axis). Internal stores must be secured to a mounting structure/rack that is a permanent part of the aircraft structure. They must be secured and positioned such that they "could" be released "down" one at a time. Stores must not contact each other or any part of the aircraft structure except for the specified mount/rack. Store mounting points must secure the store by/on the store body. Mission performance will be normalized over all teams successfully completing this mission. Mission score is:

Mission 3 - Strike Mission. Take-off within the prescribed area. Must complete a successful landing to get a score. 3 lap mixed-stores flight. Payload will be a random draw of internal and external stores (Roll of 1 dice) as seen in Table 3.. Internal stores must follow requirements outlined for Mission 2. External stores must be wing pylon mounted and fully external to the wing profile (no submerged stores). Store fins must be below the wing lower surface trailing edge height. Store mounting points must secure the store by/on the store body. External stores may not overlap/block the access/deployment area of internal store location(s). Stores must have a minimum store-to-store separation of 3" on centerline. The most inboard store(s) centerline must be at least 3" from the aircraft centerline. Mission performance will be normalized over all teams successfully completing this mission. Mission score is:

;

Table 3.. Payload Configurations for Mission 3

Flight Course

All missions are carried out on the flight course, shown in Figure 3.. The course consists of 8 maneuvers of which take-off and landing are only preformed once per mission. After takeoff the aircraft is supposed to do a 500ft upwind straight flight, a 180o degrees turn to the right, 500ft downwind leg, a 360o degrees turn, another 500ft downwind leg, another 180o degrees turn to the right, a 500ft upwind leg, and after passing the departure line, the plane must land safely on the paved runway. The length of the course is estimated to be 3860ft.

Figure 3.. Flight Course

1. 30 foot takeoff

5. 360° turn to the left

2. 500 foot upwind leg

6. 180° turn to the right

3. 180° turn to the right

7. 500 foot upwind leg

4. 1000 foot downwind leg

8. Landing on paved runway

Design Requirements

To maximize the flight score, the design must be light, carry many internal stores, and fly fast. These design goals are interconnected and must be balanced to maximize the score. To prioritize the design requirements a score sensitivity analysis was conducted and figures of merit assigned to each goal.

Scoring Sensitivity Analysis

Sensitivity analysis was conducted considering a conventional configuration capable of 30ft takeoff (configurations not meeting this criteria were ruled off). Wing span, fuselage length between 10ft to 20ft, and number of stores flown in mission two, for SF between 4 to 12 were considered for determining Score plotted in Figure 3..

Figure 3.. SF and number of internal store flown influence on Score

Increase in Score due to variations of parameters is shown in Figure 3.. Considered variations were: empty weight reduction in steps of 5% up to 55% of the initial weight (case 1 to 10); X_max reduction in steps of 5% up to 55% the initial value (case 1 to 10); Y_max reduction in steps of 5% up to 55% the initial value (case 1 to 10); number of laps flown in mission one from 5 to 10 (case 1,3,5,7,9,10), N_Stores_Flown in mission two from 6 to 12 (case 1,3,5,7,8,9,10), time flown in mission three from 10 min. to 5.5min in steps of 0.5min. (case 1 to 10).

Figure 3.. Parameter influence on Score

Mission Goals

From the preceding analysis the relative importance of the goals were determined. Figures of merit were assigned according to Table 3..

Table 3.. Mission Goals

Goals

Figure of merit

Payload - N_Store_Flown

40

Speed - N_Laps_Flown/Time_Flown

25

Empty Weight

20

Size Factor - 2*Y_max + X_max

15

Configurations Review

The configuration selection process evaluates and compares component for the aircraft design based on their relative attributes. In order to design the mission optimizing aircraft that provided the potential for the maximum achievable mission score, a variety of generic aircraft configurations were compared and contrasted against one another in the form of a trade study. A basic description of the advantages and disadvantages provided by each aircraft configuration is described below:

Conventional: Often used in large payload applications, an extensive knowledge base is available, and the configuration of such an aircraft involves simple and reliable fabrication techniques.

Canard: Employs a horizontal stabilizer forward of the aircraft's main wing. The canard may be designed to provide positive lift in trimmed flight, however, canard designs can be less stable.

Flying Wing: Without a fuselage, this configuration has little potential to carry a high-volume internal payload. There is a limited knowledge-base for this configuration, and the manufacturing is complex.

Dual Wing: This configuration places a lifting wing section both above and below the aircraft's fuselage, allowing for a shorter wingspan at the expense of increased drag.

Tandem Wing: This configuration places a lifting wing section both in front and back of the aircraft's fuselage payload volume, allowing for a shorter wingspan at the expense of increased drag and stability problems.

Configuration Selection Process and Results

Selection Process

The selection process for the configuration of the fuselage, wing, empennage, propulsion, and landing gear was made based on the weighted goals from Table 3.. The configuration choices were quantified by multiplying the design figures by a component weighing factor and a configuration weight for each design goal. The final score for a component is the summation of this product for all goals. The highest score is then selected. The design goals are enumerated in the previous section. Component weighting factors from 15 to 40 determine the importance of the component on the whole design and configurations figures determine the relative strengths of a particular configuration.

Configuration Selection

The configuration concepts that pass initial screening are scored against each other using FOM criteria as shown in Table 3.. The conventional concept scores the highest and is used for further development of the aircraft.

Table 3.. Configuration Selection

Figure of merit

Weight

Conventional.jpg

Conventional

Canard.jpg

Canard

FlyingWing.jpg

Flying Wing

Biplane.jpg

Biplane

Tandem wing.png

Tandem

Payload

40

5

4

3

5

3

Speed

25

5

4

5

3

4

Empty Weight

20

4

4

5

3

3

Size Factor

15

2

2

5

4

4

Total

100

435

370

420

395

340

Fuselage

The fact that the payload is the largest contributor to the flight weight for M2 and M3 makes payload configuration selection critically important. Payload configuration has many implications for structural layout and component sizing. Generally, the fuselage role is to house its major components (its payload, batteries, electric actuators and other electronic devices (sometimes its motors) and it is the base on which the other parts of an aircraft are mounted. The fuselage must withstand the forces during flight and landing so it must be strong enough and shaped in such manner that offers the least drag.

Table 3.. Fuselage Selection

Figure of merit

Weight

Conventional

Low Tank

High Tank

Discrete Pods

Payload

40

3

4

3

5

Speed

25

5

4

3

2

Empty Weight

20

3

5

5

3

Size Factor

15

4

5

2

3

Total

100

365

435

325

355

Table 3. provides a critical analysis of the Fuselage selection. With the conventional configuration directional stability is difficult to maintain at high angle of attack because a considerable amount mass in our aircraft is concentrated in the middle of the body. The High Tank Fuselage has a high center of gravity which diminishes stability. The Low Tank Fuselage offers a significant stability due to its low center of gravity. Also, the landing gear can be mounted on its lower side, thus reducing the length and weight of the landing gear struts. The Discrete Pods Configuration provides more internal space for payload, at the expense of speed and weight. This configuration is suitable for carrying missions but have a problem with achieving a short and fast take-off therefore it isn't suitable for the contest requirements.

Wing

The shape of a wing deals with airflow in 3 dimensions and is very important to understand the wing performance and airplane flight characteristics. Taking into consideration the Mission 1, the wing selection shall be correlated with a high speed. An elliptical circulation distribution produces a constant downwash and has a minimum induced drag. However, there is less area on tip of the tapered wing, thus it produces less drag than the elliptical wing, for example. If you consider distributing the weight evenly over the entire area of the wing, a square wing is a good choice, offering stability to the model. However, it does not exceed the performance of a tapered wing. A swept wing offers a maximum lift, control for landing and take-off and stability. In fact it is frequently necessary to use zero or negative dihedral on a swept wing to avoid excessive stability. For high-speed flight, a swept wing is desirable. For cruise as well as take-off and landing , an straight wing is desirable. Also, the wing sweep and aspect ratio together have a strong effect on the wing-alone pitch up characteristics. "Pitch up" is the highly undesirable tendency of some aircraft, upon reaching an angle of attack near stall, to suddenly and uncontrollably increase the angle of attack.

After researching the advantages and disadvantages of the wing configurations in the chart below we chose the straight tapered configuration as being the most fitted for the missions requirements.

Table 3.. Wing Selection

Figure of merit

Weight

Square

Elliptical

Tapered

Swept

Payload

40

5

4

4

4

Speed

25

4

3

4

5

Empty Weight

20

5

4

4

5

Size Factor

15

3

4

2

1

Total

100

445

375

370

400

Empennage

Tail surfaces are used to both stabilize the aircraft and provide control moments needed for maneuver and trim.

Vertical empennage mounted on the bottom side of the airplane give lateral static stability during flight. Also, the wing adds a positive torque, and the horizontal empennage adds a negative torque, which is stabilizing.

We opted for a conventional configuration since roots of both horizontal and vertical surfaces are attached directly to the fuselage. In this design, the efficiency of the vertical tail is large because interference with the fuselage and horizontal tail improve its effective aspect ratio.

Also, for the horizontal empennage, we shall take into consideration the disturbances created by the wings, the deceleration procedures and the deflection of the airflow. A T-tail is sometimes used to reduce the aerodynamic interference. The disadvantages of this arrangement include higher vertical fin loads, potential flutter difficulties, and problems associated with deep-stall.

V-tails mix functions of horizontal and vertical tails. They are sometimes chosen because of their increased ground clearance, reduced number of surface intersections, or novel look, but require mixing of rudder and elevator controls and often exhibit reduced control authority in combined yaw and pitch maneuvers.

Table 3.. Empennages Selection

Figure of merit

Weight

Conventional

Cross Tail

V Tail

T Tail

Payload

40

3

2

1

3

Speed

25

4

1

3

2

Empty Weight

20

2

1

4

3

Size Factor

15

4

2

1

3

Total

100

320

155

210

275

Propulsion

Propulsion system location depends on aerodynamic features, airplane design and power consumption. First of all, the contest rules prevent us from using a motor that exceeds the maximum of 20 A per motor, battery pack combination.

Using a tractor propulsion system the UAV would be more stable than in the case of a pusher configuration, allowing us to have more stability during the flight. A tractor system is also lighter than a pusher system, because the pusher configuration requires an additional mounting structure.

A dual propulsion system needs two smaller propellers instead of a big one to provide the thrust needed. The main drawback of dual propulsion system is the additional weight coming from extra wires, two propellers instead of one, two motors. The dual propulsion system was selected because of the overall UAV height. Other weak points of the dual motor system are several extra failing phases (the point in which an aircraft collapses/is damaged) and more drag.

Taking into consideration Table 3. as well as the strong and weak points listed above, we chose the Dual Tractor configuration as being the best option for the technical designing and missions requirements.

Table 3.. Propulsion System Selection

Figure of merit

Weight

Single Tractor

Single Pusher

Dual Tractor

Tractor Pusher

Payload

40

3

3

4

3

Speed

25

4

4

3

3

Empty Weight

20

4

2

3

1

Size Factor

15

2

1

4

1

Total

100

330

275

355

230

Landing Gear

Landing gear forms the principal support of an aircraft on ground. We have chosen the landing gear taking into consideration the air model general design requirements, stability, performance, maintainability and efficiency during the three missions from the competition. Moreover, we shall consider the impact energy absorption in landing gears on taxing on the runway.

A tricycle landing gear represents an arrangement that puts the nose gear well forward of the center of gravity on the fuselage and the two main gears slightly aft of the center of gravity. It allows more forceful application of the brakes during landings at high speeds without causing the aircraft to nose over.

Bicycle landing gear, as the name implies, has two main gears, one aft and one forward of aircraft. This geometry is unstable while on ground.

Table 3.. Landing Gear Selection

Figure of merit

Weight

Tricycle

Tail Dragger

Bicycle

Payload

40

4

3

2

Speed

25

2

3

4

Empty Weight

20

3

4

2

Size Factor

15

4

2

2

Total

100

330

305

250

As shown in table 3.8, the tricycle landing gear is a safe choice for the mission goals. With the tricycle landing gear, the centre of gravity is ahead of the main wheels so the aircraft is stable on the ground and can be landed at a fairly large angle of attack thus a low landing speed.

Final Design Configuration

Based on the FoM analysis the final conceptual design fitted for this competition is a conventional monoplane aircraft with conventional tail arrangement with all-moving horizontal tail.

This configuration is powered by a dual tractor propulsion system with symmetric propellers and has a low-tank fuselage that provides ease of access to the cargo bay and to the electronics and a tricycle landing gear for a better weight balance.

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