We continued the climb to 3,000 feet, looking for signs of a healthy aircraft before the plane got above our planned flight envelope. With all telemetry “in the green,” we continued the climb. By design, Aquila does nothing fast: It climbs slowly, descends even slower, and when flying upwind moves only at 10-15 mph over the ground. We designed Aquila this way because it is meant to stay in the same area for long periods of time to supply internet access. Aquila is solar-powered and extremely power-efficient —- running on the power equivalent of three blow dryers.
This second flight was all about data. We flew lengthy test points at constant speed, heading, and altitude to measure the airplane’s drag. The data from these “trim shots,” as they’re called, will be used to refine our aerodynamic models, which help us predict the energy usage and thus optimize for battery and solar array size. We also undertook extensive instrumentation of the airplane’s structure, adding hundreds of sensors to the aircraft to understand how Aquila's shape responds to flight in real-time. These included hundreds of strain gauges and three-axis inertial measurement units (IMUs.) These tools serve to verify and refine our structural model, which predicts both the static shape of the airplane — designed to be very flexible to respond to wind gusts and maneuvers.
Throughout the flight, we also continued to verify the drag created by new “spoilers” that we added to Aquila at various angles. Spoilers are movable surfaces on the wing of an aircraft that help create drag to reduce speed and decrease lift. We also tested the two radio links’ signal strength from various aspect angles.
After testing the landing algorithm with an elevated landing, we committed the airplane to a complete, successful landing on the designated site.
The Aquila aircraft has no landing gear in the traditional sense. It lands on Kevlar pads bonded to the bottom of the motor pods. The rationale is twofold: 1) We land at very low groundspeed and descent rates, so we can save the weight and drag of struts and wheels, and 2) much of the aircraft’s mass is concentrated in the motor pods, since this is where the batteries are installed; once the batteries land, stopping the descent of the rest of the aircraft imposes little load on the structure.
For the landing pad, we created a 500 foot circle of level gravel, about 6 inches deep and with the consistency of rough sand. Aquila flies autonomously, with the exception of manual interventions in cases such as lining up with the wind. Therefore, shortly before landing, the flight crew uploads a landing plan based on the wind direction such that the airplane lands upwind, thus respecting the crosswind limit.
When landing, the Aquila aircraft follows a 3 degree path — the glideslope — that starts a few hundred feet in the air and ends on the ground. A feature of this class of airplane is low drag — it’s the only way to fly on the limited power sunlight can provide. But while drag is the archenemy of flight, it is the staunch ally of landing. The spoilers that we added to the aircraft are controlled by the autopilot. When the autopilot senses that the aircraft is above the glideslope, it opens the spoilers more, and when it senses that the aircraft is below the glideslope, it closes them. Meanwhile, as it does throughout the flight, the autopilot lowers or raises the nose using elevons, which help to increase or decrease airspeed.
A few seconds before landing, the autopilot stopped the propellers as planned in order to lock them horizontally. The propellers are meant to lock in the horizontal position to avoid damaging them when touching down. In this flight, the motors all stopped, but only one propeller locked horizontally. The aircraft settled onto the landing surface very gently and came to a stop in about 10 meters. It was absolutely perfect. Similar to driving a car on a gravel surface, landing a plane on gravel causes a few minor, easily-repairable dings, but otherwise, Aquila landed in great shape.
