Aviation
Today 1.6 billion people live in regions that don't have access to a mobile broadband network. Connecting these remote parts of the world with existing technologies such as buried optical fiber or microwave links on towers is often cost-prohibitive. As part of our commitment to Internet.org, the Facebook Connectivity Lab is developing many new technologies to bring affordable internet access to more people, more quickly. One of the technologies we are building is a fleet of solar-powered aircraft called Aquila. Once they are fully operational, these high-altitude planes will stay airborne for up to 90 days at a time and beam broadband coverage to a 60-mile-wide area on the ground, helping to open the opportunities of the internet to people in underconnected regions.
After several months of flying scale models, our team has reached an important milestone: We successfully completed the first full-scale test flight. The low-altitude test flight lasted for 96 minutes — more than three times our planned mission length — and provided our aeronautics team with data on numerous aspects of Aquila's performance, including the autopilot, motors, batteries, radios, ground station, displays, basic aerodynamic handling, structural viability, and crew training. This post details some of our early learnings from the test flight and the data we collected, and looks ahead to some of the challenges we're working on next.
Testing Aquila
All flight tests are intended to answer one question: How good were our models at predicting actual behavior? The flight test phase for our first full-scale aircraft started with a functional check flight designed to verify our mathematical models and overall aircraft structure. Here's what we tested:
Takeoff: Aquila's design is optimized for minimal mass, so it does not include traditional takeoff and landing gear. The first part of our test involved orchestrating a new kind of takeoff. We attached the airplane to a dolly structure using four straps, then accelerated the dolly to takeoff speed. Once the autopilot sensed that the plane had reached the right speed, the straps were cut simultaneously by pyrotechnic cable cutters known as “squibs.” While the ground-based crew can command the plane's heading, altitude, airspeed, and GPS-based route from a control computer, there are no joysticks involved — takeoff is on autopilot. A great deal of simulation and analysis were performed prior to first flight to understand the dynamics of takeoff and to choose the aircraft's pitch on the dolly, the speed at which it would be released, and the initial elevon angle. The specifications based on our simulations resulted in a successful takeoff.
Aerodynamic model: Aquila will need to operate in both cold, thin air at high altitude and warm, thick air at low altitude. The air is 10x more dense at sea level than at cruising altitude, and the aerodynamics of the plane's wings and propellers vary greatly over that range. When designing the aircraft, we built computer models based on computational fluid dynamics that helped us understand how much power is required to propel the plane for the different altitudes and speeds at which it will operate. We then ran the model through thousands of tests, using different parameters, to ensure the plane would meet our performance, stability, and control requirements. We also performed a coarse verification of our drag model, which measures the force of air opposing the moving aircraft. Both the climb rate and the battery usage were close to the predicted values. We tested the autopilot and the aeroservoelastic model on which it is based when we flew our 1/5-scale aircraft earlier this year. Our initial analysis from the full-scale flight suggests our aerodynamic models are in line with what we observed.
Battery and power performance: While Aquila's power system will eventually include solar cells, our first few test aircraft are powered only by batteries to give us early indications about their aerodynamic performance, handling qualities, and autopilot performance, and to verify our structural models. Given the low drag and slow speed of 25 mph, Aquila flew on less than 2,000 W of power during its first flight. This matched well with our prediction, which suggests that the propeller efficiency, motor efficiency, and drag were also in line with predicted values.
Autopilot performance model: Like many modern UAVs, Aquila is controlled by a full-time autopilot. To make our large, flexible aircraft controllable requires that we understand how the aircraft deforms under the aerodynamic and inertial loads, and what the structural modes and frequencies look like. Our first flight verified that the autopilot design is correct at low altitude and low true airspeed. During all maneuvers, the autopilot was stable and accurate, and it brought the aircraft to the commanded condition in the predicted amount of time. The automatic landing algorithm also performed well, tracking the glide path and centerline with expected accuracy.
Real-world conditions: Our simulations and analysis predict how the aircraft will respond to wind, turbulence, and vertical gusts. Our first flight lasted three times longer than the minimum mission length, so we were able to gather data on how the structure and autopilot responded under a range of real-world conditions to help verify these predictions. We are still analyzing the results of the extended test, including a structural failure we experienced just before landing. We hope to share more details on this and other structural tests in the future. To prove out the full capacity of the design, we will continue to push the plane to its limits under more extreme conditions in a lengthy series of tests.
Communication
Approximately 40 percent of the world is connected to the internet. Of those who aren't, many are offline for one major reason: Connectivity is expensive.
If you think about the traditional model of connectivity, it starts with a tower that propagates radio signals to people's devices. To connect people this way, mobile operators have to build out an extensive infrastructure requiring land rights, equipment, fiber/microwave, and access to power to run it all. Using this model, connecting people in remote or low-population-density areas can be financially challenging — there are fewer potential customers, and you have to build more infrastructure to reach them. To make the problem even more challenging, one in five people globally lives in extreme poverty, existing on $1.25 per day or less. While tremendous progress has been made in connecting more than 90 percent of the world's population to 2G networks, getting to 100 percent using conventional approaches is unlikely to happen in the near term, given how unlikely it would be that operators would be able to recoup their infrastructure investments.
We started the Connectivity Lab at Facebook to see if we could change this paradigm. We are developing a range of new technologies — including high-altitude aircraft, satellites, free space optics, and terrestrial solutions — to help accelerate the process of bringing connectivity to the unserved and underserved.
Our vision is that these technologies can be used as building blocks, allowing operators, governments, and others to build networks in these regions that are at least an order of magnitude more cost-effective. Today we announced that we've reached two major milestones on our way to making this vision a reality. The first is that the first full-scale model of Aquila — the high-altitude, long-endurance aircraft designed by our aerospace team in the U.K. — is complete and ready for flight testing.
With Aquila, we've designed a new aircraft architecture, one that can support staying in the air for months at a time. Aquila is solar powered, and when launched, it will create a 50-km communications radius for up to 90 days, beaming a signal down to the people in that area. This signal will be received by small towers and dishes that will then convert it into a Wi-Fi or LTE network that people can connect to with their cellphones and smartphones.
To make all of that possible, we had to make the plane really big and really light. Aquila has the wingspan of a Boeing 737 airplane but weighs a third as much as an electric car. The monocoque wing is made from a cured carbon fiber that is stronger than steel for the same mass of material. Before it's cured, the material is flexible, so it can be molded into the right shape.
Aquila will fly at between 60,000 and 90,000 feet during the day — above commercial air traffic and above the weather. The air at that altitude is thin, about 5 percent that of sea level, so we utilized a high aspect ratio wing and an undercambered airfoil in the design to optimize its lift-to-drag ratio. During the day, the aircraft will fly at 90,000 feet to maximize its ability to charge its solar cells. At night, it will glide down to 60,000 feet, taking advantage of gravitational potential energy to consume less power.
The communication payload sits in the center of the aircraft, in the fuselage. Not only do aircraft allow us to not have to dig to lay down fiber backhaul, but aircraft have the added benefit of allowing the onboard communications technology to be upgraded at whatever rate is required to meet the market needs.
Test flights for the full-scale model should begin later this year, following the sub-scale flight tests from earlier in the year in the U.K.
The second milestone is an advancement in using free space laser communications as a mechanism for communicating between aircraft. Our optics team has designed and lab-tested optical transceivers that improve upon the state-of-the-art by approximately 10x, to data rates in the tens of Gbps. As part of this effort, the team leveraged technologies that were developed for Facebook’s data centers and backbone of traditional fiber-optic communications. The resulting throughput is similar to what you'd find over fiber-optic networks — only we can now send that data through the air. We'll be sharing more details on this technology in the near future.
We're proud of the progress we've made so far. In 14 short months, we've designed and built an aircraft from start to finish and made great strides in developing the technology required to distribute high-capacity data streams through the air. These are examples of the Facebook work ethos at play: Move fast and build things — even if it's a massive high-altitude, long-endurance UAV. Of course, there's a long way to go before this vision can become a reality, but I'm confident we have the right team in place to be able to make meaningful strides toward accomplishing Facebook's mission of connecting the world.