July 8, 2022, ©. Leeham News: We will start the analysis of the market’s most prominent VTOL variants by looking at the simplest version, multicopters.
When we analyze the multicopters, we can go through some fundamentals of how VTOL operates and the technology used.
The VoloCity, Figure 1, and EHang 216, Figure 2, are multicopters that have been flying for several years. Their flight technology is an outgrow of the drone industry’s findings on how to make a simple yet functional vertically starting vehicle using several electric motors that can drive RPM-controlled rotors.
By spinning the motors in different directions and differentiating their RPM, you can control the altitude, the pitch angle, the roll, and the yaw. By tilting the multicopter forward, it will fly forwards.
As these vehicles are battery-powered and batteries are heavy, their capacity is strictly limited. Therefore, the lift systems energy consumption is critical for multicopters.
The amount of power needed to lift the VTOL vertically depends on the disc loading, which is the VTOL’s mass divided by its total rotor area. Low disc loading results in less power consumption, whereas higher disc loading increases the power requirements. The VoloCity has a disc loading of 217kg/m2 or 4.1lb/ft2 when fully loaded with two persons whereas the EHang disc loading loaded with two persons is 178kg/m2 or 3.4lb/ft2.
Disc loading is not something special for VTOLs or helicopters (where the term is used). As explained in Part 16, it’s just another name for what we know as specific thrust from turbofans or propeller loading from a propeller aircraft.
All propulsion systems are air pumps, and they work more efficiently if they accelerate a large mass of air to a small overspeed than vice versa. If your rotor capture area is large, then you only need to give the air a modest overspeed through the rotor to generate the thrust to hover.
The drone-like architecture of the multicopters is simple, but as the rotors are responsible for keeping the unit at a constant altitude with hover power, they represent an energy-intensive version of VTOLs. Their range is, therefore, short, typically less than 20 miles.
For forward flight, the rotors must deliver more thrust as some thrust must compensate for the drag of the forward motion, Figure 3. The wedgewise forward flight limits the achievable speed with acceptable rotor thrust. The top speed of the VoloCity is 60kts.
To make the mulitcopters safe, substantial redundancy is needed. The VoloCity has 18 rotors with motors, where the motors are fed from nine inverters and batteries in a scheme that gives the unit a high degree of redundancy from rotor, motor, electronics, or battery failure.
The EHang 216 has 16 rotors and motors with a multitude of electrical drive paths for the same reason.
The method of forward flight (Figure 3) is a variation of vertical flight. As the rotors’ thrust is only a bit higher than for hovering, the airflow variation through the rotors stays modest.
Therefore, fixed rotor blades can be used. It makes the aerodynamics of the rotors simple; no pitch change mechanisms are needed.
An area that must be considered is the risk of flutter in the rotors. During forward flight, the load on the rotors changes, and rotors can fly into the wake of other rotors. Flutter problems with rotors constitute a significant problem, and this is why the regulators require a lot of evidence the subject has been thoroughly analyzed before test flights.
During test flights, the OEM has to demonstrate that the vehicle can safely fly with different weights and centers of gravity through all possible maneuvers and speeds to ensure there will be no rotor flutter problems in operations.