Union College has been sponsoring an AERO team for over ten years and it has changed greatly since its inception. When founded it consisted of one mechanical engineering student and now we have over ten members across three majors. Six of those students make up the advanced team and the rest make up the micro team. The club’s purpose is to participate in the SAE AERO event.
SAE Aero is an event held twice a year where college teams from around the world design and build RC aircraft to specifications outlined in the rules put out by the organization [1]. In addition the event has a set theme which the competitions are modeled after. This year’s and last year’s theme is colonizing Mars. The aircraft is responsible for dropping colonists, habitats and supplies safely to the surface. Basically we are dropping autonomous gliders carrying colonists (ping pong balls), nerf footballs (habitats) and bottles of water (supplies). We understand that the theme isn’t exactly practical but it adds a fun aspect to the competition.
2019’s AERO competition saw a drastic rule change which stated teams were required to use electric motors instead of internal combustion engines. This rule change turned the design portion leading to the event on its head. Teams usually start preparing preliminary designs before the rules are released and use previous year’s designs as starting points. With this new rule, almost every aspect of the aircraft needed to change. Electrical motors are much lighter weight than internal combustion engines and take up much less room. This changes CG and how much space is needed at the nose of the aircraft. Electric motors also need ESCs and batteries to operate adding more to think about when selecting a motor and propeller. On the bright side electric motors are much more reliable and pretty much plug and play where gas engines can take a lot of tuning and adjustments to run smoothly. This allows for much more consistent thrust numbers allowing for tighter tolerances. This is not 100% true as with time battery voltage will drop allowing the motor to draw less power.
In my opinion, last year’s AERO team was not well prepared for the propulsion rule change and used a low thrust and low efficiency motor/propeller combo. I spoke with some of the graduating seniors before the end of spring term 2019 to get some ideas of where to start. We discussed resources available to the team and how to use them. The main resource available to me was the RCbenchmark series 1580 thrust stand and dynamometer shown in figure 1 (motor and propeller unrelated) [2].
Figure 1: RCbenchmark Thrust Stand [2]
After some tinkering and setup I was able to test last year’s motor/propeller configurations. The motor being tested was an E-flite 60 Brushless Outrunner, 400KV shown in figure 2 [3].
Figure 2: E-flite 60 Brushless Outrunner, 400KV [3]
The propellers being tested were APC propellers size 14x10in shown in figure 3, 16x10in shown in figure 4, 17x10in shown in figure 5, and 18x8in shown in figure 6. Note propeller size is denoted by diameter in inches then pitch in inches (diameter x pitch). Diameter is how long the propeller is from tip to tip and pitch is the angle each blade is given. More pitch means more air is being grabbed by the propeller.
Figure 3: E-flite 400KV/14x10in Propeller
Figure 4: E-flite 400KV/16x10in Propeller
Figure 5: E-flite 400KV/17x10in Propeller
Figure 6: E-flite 400KV/18x8in Propeller
It is clearly shown the thrust increase with propeller diameter and pitch. This is to be expected but thermal data is unclear due to the fact we only flew the aircraft with the 16x10in propeller which was recommended by last year’s team. The reason for that is the higher diameter and pitch the more power it takes to turn the propeller causing more heat generation which could damage the motor. The other reason is that the motor is only rated up to a 16x10in propeller. This data helped tremendously with my research for a better motor/propeller combination. I knew we needed to go bigger on the propeller but I had to find the right motor for the job.
[2] RCbenchmark Series 1580 Thrust Stand, Accessed on Nov. 24, 2019. Available: https://www.rcbenchmark.com/products/dynamometer-series-1580?variant=13071468527728
[3] Power 60 Brushless Outrunner Motor, 400Kv, Accessed on Nov. 24, 2019. Available: https://www.horizonhobby.com/power-60-brushless-outrunner-motor–400kv-eflm4060a