- Mandatory Requirements
The design requirements and limitations for my project and the full aircraft can be found in [1]. The rules are more of a base line when it comes to the aircraft design providing minimum and maximum dimensions and values for different sections of the aircraft. This allows for team creativity and innovation while keeping aircraft performance relatively the same.
Rules for propulsion for the advanced teams did not change this year so propulsion power draw remains limited to 750W with a minimum battery specification of 6S, 3,000mah at 30C. The way officials ensure we stay at or below 750W is with the use of a specific power limiter we are required to use. The power limiter must be connected between the battery and the ESC. The final requirement for the propulsion electrical loop is an arming switch. The arming switch must be mounted and clearly visible on the outside of the fuselage 12in behind the propeller. The use of this switch is to provide a factor of safety as with wirelessly controlled electric motors once they have power then can spin up to full throttle without warning. This is a very uncommon occurrence as technology has greatly improved but it’s still a risk in using this technology.
- Primary Design Goals
Now that the rules set by SAE, I need to accommodate my team members and design a propulsion system tailored to their aircraft design. The Team wanted the aircraft to be roughly 25lbs fully loaded with a wing span of 10.3ft. The aircraft will be fairly heavy. Based on last year’s aircraft the team estimated 10-15lbs unloaded. In addition the aircraft is going to be hauling several gliders, footballs and water bottles adding about 10lbs to the overall weight.
Now that we have a base line for the weight of the aircraft the team calculated minimum takeoff speed of roughly 17mph and a maximum speed of at least 33-40mph. Minimum take off speed refers to how fast air needs to be moving over the airfoils to produce enough lift to fly. Next we chose 33-40mph as a top speed because you should have at least double your takeoff speed as a cruise speed to ensure a safe and stable flight.
With a takeoff speed of 17mph we need the takeoff distance to be less than 120ft or half the runway distance to be used at competition. We only used half the runway distance because it’s a good rule of thumb. If there is an issue during the takeoff roll, it will be noticed by the time the plane is half way down the runway. This allows adequate time to stop the aircraft safely.
Finally the aircraft must have the ability to be airborne for at least 6mins. While flying, the aircraft will be required to drop the three types of items being carried. It is not a good idea to drop everything at once because of weight shift. The plane is carrying at least 10lbs of extra weight and if it is all dropped at once, the aircraft can respond violently. Resulting in a dangerous situation where the aircraft is uncontrollable and crashes.
- Secondary Design Goals
The goal is at least 10lbf of thrust from the chosen motor/propeller combo. While this is not necessary it would be very nice to achieve. The more thrust that can be produced at lower wattages, the more weight the plane can carry.
The optimum propulsion setup is to be as light as possible. There is no weight restriction as of the writing of this paper but we will be making one during prototyping in the coming months. The ideas we took from last year’s design was we wanted to use less “beefy” components. Last year’s team over designed the propulsion electronic loop and we feel quite a bit of weight could be saved from that alone. First of all we will be using a 60A ESC instead of a 90A. The reason for this is we are limited at 750W and are using a 6S battery which has a recommended minimum use charge of around 22.8V. Using ohms law we show that the system will only be drawing around 33A maximum. Obviously this can be exceeded with voltage sag and due to inconsistency in the watt limiter so we chose a safe option in which is roughly double the expected value. Even though this may seem excessive to some current can spike extremely quickly in these setups. When LIPO batteries drop below roughly 3.7V per cell there voltage sag greatly increases. The ESC wants to keep pulling the same amount of power, but voltage has decreased, so in return current must increase. This has an increasing exponential effect and can cause damage to components or even result in a fire. We would like to avoid this and still save weight.
Another way we plan to save some weight is through the battery. Batteries have a mah rating and a C rating. For weight saving purposes we will be ignoring C rating as it does not affect battery weight. What a mah rating is how many milliamps a battery can dissipate per hour. With a higher discharge rate the battery needs to store more energy so the higher the number the bigger the battery cell size. Since our power and amp draw is relatively low we didn’t see a need to continue using the 5,000mah batteries the team used last year. Instead we are considering going to a lower mah battery for their smaller cell size and lighter weight.
Finally, the connectors and wire used last year were over designed. They were using xt90 connectors with 10AWG wire. This was unnecessary because the mandatory watt limiter uses xt60 connectors which are much smaller. A comparison of the two plugs is shown in figure 7. They were downsizing plugs and wire gauge, to just increase them again. This configuration seemed a bit pointless and a waste of weight.
Figure 7: XT90 to XT60 Plug [4]
[4] OliYin 3pcs XT90 Male to XT60 Female Connector plug, Accessed on Nov. 24, 2019. Available: https://www.amazon.com/OliYin-Connector-Converter-Adaptor-Electric/dp/B07C7SVT8G/ref=asc_df_B07C7SVT8G/?tag=hyprod-20&linkCode=df0&hvadid=309770211034&hvpos=1o3&hvnetw=g&hvrand=10216417250334753516&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9004717&hvtargid=pla-644215743566&psc=1