Vahana Configuration Trade Study — Part I

At the beginning of Project Vahana, we had an idea of the capabilities the vehicle should have, but we lacked concrete data on the best configuration to achieve those capabilities.

The highest level requirements were for a low-cost, single-passenger, electric VTOL aircraft that could provide utility to a large number of people.

This is a broad set of requirements, which is both good and bad. They could be satisfied by many different configurations: helicopters, multi-rotors, tilt-rotors, tilt-wings, or configurations with separate forward and lifting propulsion systems. There is a rich history of VTOL concepts (see below) and each of these configurations has advantages and disadvantages that may make them better suited to electric propulsion and to different mission requirements.

V/STOL Wheel (American Helicopter Society)

De-scoping The Problem

We elected to perform a trade study on two of the most compelling configurations instead of investigating all of the conceivable VTOL configurations. .

  • Electric Helicopter: The first configuration we investigated was an electric helicopter. Helicopters are by far the most common VTOL transport aircraft in use today. They are efficient in hover due to their low disk loadings, and cruise at reasonable speeds. We expected that helicopters would be compelling for short range missions where a larger fraction of the flight time is spent in hover.
  • Eight Fan Tilt-Wing: The second configuration we investigated was a tandem tilt-wing with eight fans. The addition of wings improves cruise aerodynamics compared to a helicopter. In hover, a tilt-wing has lower power requirements than a tilt-rotor as the download on the wings is substantially reduced. An additional benefit of a tilt-wing is that the induced airflow behind the fans reduces the angle-of-attack on the wing in hover and in low-speed flight. This has the benefit of keeping the flow attached to the wing and canard surfaces throughout most of the flight envelope, simplifying aerodynamic analysis and flight control. Eight fans were selected for the tandem tilt-wing configuration with redundancy in the case of a failed motor and to keep the vehicle footprint small.

The figure below illustrates how the overall vehicle footprint changes for designs with different numbers of rotors/fans. It’s easy to see that an 8 fan configuration appears most compelling.

Comparison of vehicle footprint as the number of fans is varied for a fixed disk area. Green = 1 rotor, Cyan = 4 fans, Blue = 8 fans, Magenta = 12 fans, Red = 16 fans.

Arriving at Sizing

Sizing a vehicle cannot be divorced from the actual configuration as it drives critical performance parameters: aerodynamic efficiency, structural weight, maintenance requirements, and more. As such, one of the first steps we took was to define the mission and to then “size” the vehicle so that it is capable of executing that mission. Sizing generally refers to parameters such as wing area, aspect ratio, engine size, and fuel weight, predicting the performance for these , and then iterating on the values until the design and the mission requirements align.

This sizing process is difficult to do manually, especially with many parameters and mission constraints. For Project Vahana, we turned to multi-disciplinary optimization (MDO) techniques to size the vehicle. This approach allowed us to benefit from numerical optimization techniques that found the parameters (or design variables) that satisfied all the constraints.

Defining Our Mission

The concept of operations (CONOPs) for Vahana has yet to be defined, though we have lots of intriguing ideas. At this early stage, we define the typical mission as: a vertical takeoff, transition to forward flight, cruise for a specified distance, a transition back to a hover, and a vertical landing. The hover and transitions are not assumed to contribute to the total distance travelled, and are assumed to each be limited in duration to about 90 seconds. This five phase mission is illustrated below.

Additionally, Federal Air Regulations (FARs) have requirements for all aircraft and helicopters that ensure they can handle emergency situations. The FAR requirements vary and those for Vahana will be determined down the road as we propose a certification basis that draws on both helicopter and fixed wing requirements. For the purposes of our trade study we used the reserve requirement of 20 minutes at the minimum power speed.

Early in the design process it was also not obvious what the design cruise range should be, as the market for such vehicles does not yet exist. As such, the trade study was setup to size both configurations for a range of different distances in order to identify which offered the biggest advantages.

Technology Assumptions

To make a fair comparison between the two configurations the same technology assumptions were used whenever possible. For instance, we assumed the same specific energy for the batteries and specific power for the motors. Additionally, for automated flight without a pilot it was assumed that both configurations required the same sensors and computing resources.

This trade study was intended to explore the performance of both concepts in the timeframe of a production vehicle that would enter detailed design about 3 years down the road. That meant we needed to extrapolate to work with where the various technologies would be at that time.

For example, our current lithium polymer battery technology has a specific energy of just under 200 Wh/kg at the installed pack level. Projecting out 3 years (5–6% improvement per year) this gives a pack level energy of 230 Wh/kg.

Similarly, extrapolations were made based on current trends for the specific power of electric motor systems, which include the motor, motor controller, cooling system, and other accessories. With current off-the-shelf technology a specific power of about 3.5 kW/kg is achievable. Motor controllers with higher switching frequencies are being developed that will achieve motor system specific power of 6 kW/kg or better within 3 years.

Though difficult to quantify, assumptions were also made to give both configurations similar levels of propulsion redundancy and to provide safety in the case of complete power loss.

The helicopter was assumed to have redundant electric motors driving the main rotor through a transmission. Additionally, we required the kinetic energy in the blades during an autorotation to be high enough arrest the descent rate when flaring at touchdown.

The tilt-wing achieves redundancy by distributing the thrust generation over the eight motors that are physically separated. To handle single motor failures and allow sufficient control authority, each motor was sized to generate 1.7 times the thrust required in a typical stationary hover. Due to a lack of autorotation capability, provisions were also made for a ballistic parachute for the tilt-wing. It should also be noted that the varying inflow and thrust requirements between hover and cruise require the tilt-wing to have variable pitch fans, requiring an additional actuator. Cyclic is not required as the tilt-wing achieves hover control like other multi-rotors by varying the fan speeds.

Stay tuned for more details on the models and the final results of our trade study in Part II…

- Zach Lovering