PFR - Aerospace Engineering Sciences Senior Design Projects ...

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Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects a battery voltage of 0.9V/cell on takeoff, there must be at least 22 batteries in order for the aircraft to make the 100ft takeoff distance. Next, the amount of battery cells required to make the four lap range with the full water bottle payload was calculated. The cruise power draw required for the full water bottle payload was assumed to be 350W. With an average lap time of 1:00 per lap, the battery must be able to run at cruise power for 4:00. Although the Elite 1500’s have 1500mah of capacity, the batteries cannot discharge this amount when discharged under high amp draw. Therefore, it was assumed that only 1000mah of actual usable power could be drawn from the battery pack before the power would drop off below a usable level. The rest would be dissipated as heat from the internal resistance of the battery cells. 1000mah of usable power means the batteries can be discharged at 1 amp for an hour, or 15 amps for the required 4:00. While the Elite 1500’s are 1.2V batteries, the battery voltage drops while under such high amp draw. It was assumed that each battery would have an average voltage of 1.0V. In order to get 350W of power at a maximum average amp draw of 15 amps, the equation P=I*Vwas used to calculate the required battery pack voltage under load. The battery pack voltage must be at least 23.33V. Therefore, there must be at least 24 batteries in order to make the four lap range at full weight. From this analysis, the optimal battery pack size was determined to be 24 cells. With 24 cells, the battery pack weighs 1.3lbs. A picture of the battery pack is shown in Figure 78. Figure 78: Battery Pack Overview The battery pack was designed like this in order to fit in the very nose of the aircraft. This allows the center of gravity to be moved as far forward as possible, increasing the aircraft static margin and overall aircraft stability. 9.1.3 Electronic Speed Controller Electronic speed controllers are required for each motor in order to control the amount of power flowing from the battery pack to the motor. The speed controllers must be able to handle 33.6V and up to 40 amps each. The best speed controllers suggested for Neu Motors are Castle Creations Phoenix series. Within the Phoenix speed controller line up, three different speed controllers were considered. 102
Project Final Report – CUDBF April 30 th , 2009 ASEN 4028: Aerospace Senior Projects Table 21: Speed Controllers Options Speed Controller Maximum Maximum Weight (g) Voltage Amps Phoenix 45 19.2 45 30 Phoenix HV-45 50 45 60 Phoenix 60 35.0 60 58 Because of the high voltage requirement, the Phoenix 60 speed controller was chosen. The Phoenix 60 was chosen over the Phoenix HV-45 speed controller because it was 2g lighter and is rated for higher current. The higher current rating means the speed controller will run cooler and will run less risk of overheating. An image of the speed controller is shown in Figure 79. Figure 79: Speed Controller 9.1.4 Wire Gauge In order to connect the battery pack at the center of the aircraft to each motor, approximately 1ft of wire must be run in each direction. This wire must be able to sustain the amount of current flowing without overheating or causing a significant voltage drop, while keeping weight down. The wire gauge sizes considered were 16AWG, 14AWG, and 12AWG. The higher the wire gauge, the lighter the wire. However, the lower the wire gauge, the more amps the wire is rated to handle. This means the wire will not get as hot, and there will be less voltage drop across the wire at a given voltage. The wire gauge was chosen based upon current rating, voltage drop, and weight. 16AWG wire is rated for 22 amps, while 14AWG wire is rated for 32 amps, and 12AWG wire is rated for 41 amps. These ratings are for continuous current, and can be exceeded for short power surges. Each motor may draw up to 20 amps of continuous current and up to 40 amps in short bursts for takeoff. Next, the weight of one foot of wire and the voltage drop over that span were calculated for each wire. The voltage drop was multiplied by the average current in order to get the average power lost in the wire. This value was compared to the equivalent weight of a battery required to provide that amount of power. The extra weight of the wire was compared to the equivalent weight of the extra batteries required to provide that power. 103
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