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Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects Figure 21: Single Motor Aircraft However, in order to get the thrust required to meet the 100ft takeoff distance requirement with a single motor, a propeller size of 20 in was necessary. In order to accommodate a 20 in propeller with adequate ground clearance, the landing gear would need to be at least 11 in long. This would allow for 10 in or half the propeller blade to spin under the wing with 1 in of ground clearance. This added landing gear size would require more structure and thus added weight and drag from the long landing gear. A system level requirement states that the aircraft must fit within a 2 ft x 2 ft x 4 ft box. With 11 in landing gear, an aircraft thickness, and 13 in winglets that fold 15 in in from the tip of the wing, it would be impossible to fit the aircraft vertically in a box under 2 ft tall. This drove the design of the propulsion system to utilize dual motors. Other disadvantages of a single motor include less directional stability due to the large amount of p-factor coming off one motor. In order to use a single motor rated for the required power necessary, the motor itself would weigh 0.25lbs more. That means the overall weight savings from using a single motor in this case would only be 0.25lbs. Because of these characteristics, a dual motor configuration was chosen. Once dual motors were selected, the exact configuration of the motors was analyzed. Three configurations were analyzed. These included dual inline pusher puller motors, dual rear mounted motors, and dual front mounted motors. The advantage of using dual inline motors is that if an engine were to fail, asymmetric thrust would not be an issue. A conceptual design of dual inline motors is shown in Figure 22. Figure 22: Dual In-Line Pusher Puller Motors The major disadvantage of using dual inline motors is that it would add length to the aircraft. The front motor would need to be placed in front of the nose of the aircraft, adding an additional 3 in 40
Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects to the aircraft length. The aircraft is already 22.5 in from the nose to the most rearward location of the wing after the wingtip is folded. If any additional length were added in front on the nose, the aircraft would not be less than 2 ft long, and therefore would not fit into the box. Another big disadvantage would be that the propellers would interfere with the water bottle placement on the CG. In order to keep the aircraft static margin the same during all flight missions, the payload CG must be placed on the aircraft CG. The wing chord at the root is 20 in, and the length of the water bottle with aerodynamic fairings is 15 in long. Depending upon where the aircraft CG ends up, placing the water bottle on the CG may be impossible without the propeller striking the bottle. As for dual rear mounted motors, the primary advantage is that the wake from the propellers does not interfere with the lift and efficiency of the wing. A conceptual design of dual rear mounted motors is shown below in Figure 23. Figure 23: Dual Rear Mounted Motors The major disadvantage of having dual rear mounted motors is that there is additional ground clearance required for rotation on takeoff. As the aircraft nose pitches up for takeoff, the rear of the aircraft moves down about the CG. Estimating that the CG was approximately ½ way down the chord, or 10 in, and assuming a 10° angle of attack on rotation, the rear propellers would move down 1.5 in. This means that the landing gear would need to be 1.5 in taller when compared to front mounted motors. This added landing gear length and structure would add unnecessary weight and drag. Two other disadvantages to having dual rear mounted motors are weaker structure and interference with other aircraft systems. The airfoil chosen for the wing becomes very thin towards the rear. If the motors needed to be mounted on the rear, the structure would need to be strengthened significantly to handle the weight of the motors, as well as the stress caused by the force from the thrust, and any torque from the motors. As for interference, there are many components in the rear of the aircraft. All of the control surfaces will be at the rear of the aircraft. In addition, all of the servos and wiring required to move them will need to be integrated in the rear. The two main landing gear struts will also need to be placed at the rear of the aircraft. The 41
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