# Preliminary Propulsion and Power System Design of a Tandem-Wing Long-Range eVTOL Aircraft
## 2 Propulsion System Design Methodology
### Propeller Positioning and Sizing
#### 2.2.1 Number of Propellers  
The selection of the number of propellers is based on assessing the benefits of having fewer and bigger propellers versus more and smaller propellers. These choices have an effect on several performance characteristics of the aircraft:  
1. **Disk loading:** Bigger propellers allow for a larger total area and hence a lower disk loading which improve efficiency. This directly translates into a lower required peak power which can decrease the size (and thus weight) of the electric motors and the power system [2];
2. **Ground clearance:** More propellers means smaller radii, which therefore allows for more ground clearance when the wings are in horizontal position;
3. **Propeller-wing interaction:** Bigger propellers have a higher slipstream height which results in a higher increase in lift due to the propeller slipstream for the same slipstream velocity [12]. On the other hand, bigger propellers would reduce the slipstream velocity;
4. **Propeller-propeller interaction:** For the same front and rear wing separation, smaller front propellers with lower slipstream heights allow one to more easily place them such that the propellers on the downstream wing are outside the slipstream of the propellers on the upstream wing. This slipstream ingestion can lead to big losses in thrust from the second row of propellers [7] and increase noise emissions;
5. **Blade rotation mechanism:** The propellers need a mechanism to alter the pitch of the blades. With very small propellers, implementing such a mechanism becomes more difficult.
6. **Safety in OEI conditions:** More propellers mean more redundancy, and with more and smaller propellers, a failure of one of them has a smaller effect on the controllability in hover and on total thrust.  
The proposed aircraft architecture features two wings. For the present framework, it was decided to place the same number of propellers, all with the same size, on each wing, for two reasons. First, to evenly distribute the thrust and to minimise differences in propeller loading. Here, it is worth noting that due to the fact that the CG of the aircraft is closer to the front wing, the front propellers need to provide higher thrust during hover. However, (due to reason 4) in the previous list, having less but bigger propellers in the front wing than in the back wing is deemed not necessarily beneficial overall; due to the fact that hover is a very small part of the mission compared to cruise, and the potential slipstream ingestion caused by bigger front propellers would lead to thrust reduction during the cruise and hence a need for more power during the entire cruise phase of the mission. Secondly, having all propellers with the same size is simpler and cheaper for manufacturing and maintenance, since only one electrical motor and two different blade designs are needed (due to the fact that the propellers are rotating in opposite directions on each side of the fuselage, as further discussed in Section 2.2.2). In more advanced stages of the design process (not covered in this design framework), where more information about the performance of the aircraft is known, this assumption can be revised, and the relative sizes of the propellers can be adjusted.  
Based on the previous considerations, the possible number of propellers considered were 8, 12, and 16, which, respectively, translates to 2, 3, and 4 per half wing. Having only 1 propeller per half wing was discarded due to the lack of redundancy and having more propellers would mean having very small propellers which would therefore be too highly loaded, leading to low propulsive efficiency.