A Look Inside Vahana Flight Testing
We regularly publish about milestones we reach on our flight test journey, from first flight in January 2018 to full transition in May 2019. We recently completed our 100th flight and to celebrate we’d like to “take you” to Pendleton, Oregon for a deep dive into our full-scale flight test process.
To start, we’d like to touch on why we flight test Alpha One — our full-scale technical demonstrator — in the first place. Simulation can tell us a lot about the behavior and performance of the aircraft; subscale vehicles can demonstrate that the general configuration and control logic are sound; but only full-scale flights prove that our designs and systems perform as well as expected, especially when considering scaling and aeroelastic effects. In addition, the data collected in flight helps improve models and design tools to more closely match real-world behavior.
When flight testing a new aircraft, there are numerous issues that can crop up anywhere in the flight envelope (i.e. the operational limits such as airspeed, altitude, turn rate, etc. within which we can fly safely). To ensure we address any issues we gradually expand our operating envelope so that we can adapt the vehicle or control logic before issues become hazards. For instance, after completing our extensive ground test campaign, Vahana flew at 12 intermediate speeds to demonstrate stability and performance between hover and its design cruise speed of 100 kts (~115 mph). In traditional flight test terms, this is called a ‘build-up’ and while this approach may be relatively normal, there are two main distinctions that make testing Vahana unique.
First, Vahana is all electric. Batteries store significantly less energy than fuel and need to recharge between flights. To minimize the operational impact of this constraint, we designed our batteries to be swappable in less than 10 minutes, which allows us to perform multiple flight tests a day. Two 300-pound lithium-ion batteries slide into the vehicle on rails using a custom cart. The batteries are then charged off aircraft with a charging station normally used for electric cars.
Ground crew loading batteries
Second, Vahana is self-piloted. No one is ever in direct control of the vehicle. Instead, Vahana follows a predefined flight plan for each flight, defined as a series of small segments with the desired vehicle state including position and speed. These flight plans can include climbs and descents, turns, or maneuvers designed to reveal handling qualities and information about the performance of the flight software. All flight plans are validated with nonlinear simulation, hardware-in-the-loop simulation, Monte Carlo simulation, and subscale demonstration prior to being flown on the full-scale aircraft.
10,000 runs of a flight from our Monte-Carlo simulation
A crew of at least seven engineers and technicians are present for each flight to monitor systems via telemetry, inspect the aircraft, and conduct the actual test. In the event of an unexpected behavior during flight, each flight plan is designed with several contingency maneuvers. These contingencies include having the aircraft return to its starting position to land, landing immediately, or activating a parachute.
Our test team includes engineers and technicians living in Pendleton full-time, as well as engineers rotating in weekly from our headquarters. The team includes at a minimum:
- A test director to coordinate and execute the test.
- A pilot-in-command to issue a limited set of instructions to the aircraft: such as loading the flight plan, executing pre-flight checks, and commanding take-off. He is also responsible for conducting emergency procedures should the need arise in flight.
- A range safety officer to provide oversight of the airspace in use.
- Ground crew to tow the vehicle, set up cameras and other equipment for the test.
- Visual observers to watch the flight and advise on abnormalities and nearby air traffic.
- Responsible Engineers to monitor critical parameters during flight including,
- Flight controls to oversee vehicle performance and handling
- Avionics to oversee system-level health and low voltage systems
- Propulsion to oversee motors, variable pitch fans, and batteries
The image below shows where the team might be located during a cruise test event that follows the white line.
An example positioning during flight testing in Pendleton
So what does a typical test day at the Pendleton Unmanned Range look like? Test days all follow the same sequence of events and every action we take strictly follows established checklists.
On the day prior to the test, the flight software, generated in our headquarters in California, is loaded onto the vehicle. The maintenance team installs batteries and performs the pre-flight inspection.
The test director holds a mission brief to align the test team on why, who, what and how the test will occur. Test cards detailing the upcoming flight plan are distributed as part of this brief. We generally brief several plans in one sitting, with the intention of flying multiple times in the same day.
A sample flight test card
On the day of the flight, the team gathers at the test center at dawn (winds tend to be low in the mornings, which we favor from a data quality perspective). The test director verifies that our flight is deconflicted with other users of the range, and makes a go/no-go call for the test based on weather and team readiness. If weather conditions look favorable, the pilot-in-command performs a vehicle walk around, the ground crew tows the aircraft to its take off location, and sets up sound and video recording equipment. Visual observers get in position along the flight path, and the engineering team situates themselves in the mobile command center where all telemetry can be observed and commands can be sent.
Our mobile command center
When the team is ready, automated pre-flight tests verify nominal operation of low voltage (wing and canard tilts, variable pitch fans and control surfaces) and high voltage (battery management systems, motors, high voltage bussing) systems. The pilot obtains clearance from the control tower and sends the takeoff command. The aircraft takes off vertically, then transitions to forward flight. During the flight, the Responsible Engineers monitor telemetry and take note of any discrepancies that arise. Once the flight is completed, the aircraft decelerates and transitions back to a hover, then lands.
Once the aircraft is safely back on the ground, the team runs automated post flight checks and begins to download flight data recorded on the aircraft. The data are sent to a server to be accessible by teams in both California and Oregon. If flight operations are complete for the event, the team sends a command to open battery contactors and discharge the high voltage distribution bus. This step ensures the safety of the ground crew as they conduct post flight inspections and tow the aircraft back to the hangar.
The test director runs the mission debrief. After a brief synopsis for the benefit of engineers that did not witness the test event(s), the test crew shares observations and discrepancies observed during the test. The engineering teams analyze test data while the maintenance team turns the aircraft for the next flight by swapping the batteries, and performing a pre-flight inspection. Findings of the data analysis and preflight inspection are presented to the test team prior to conducting the next flight, and the process repeats.
At the end of the day, good communication is key for successful flight tests. Luckily for us, we get plenty of it! We’ve all been sharing a bed and breakfast in downtown Pendleton since we established operations in Oregon. We quickly found out that there isn’t a better way to bond than living together, day in and day out. That being said, the entire Vahana team has always been one big family, and we stay in very close and frequent communication with our colleagues back in California.
The Pendleton House Historic Inn B&B that we call home
We hope you enjoyed this quick look into our flight test operations and would love to hear your comments.
- By Matt Deal and Nico Kokocinski