Updated: Sep 8, 2021
The AeroVironment Helios Prototype made its first flight on this day in aviation history.
The Helios Prototype was the fourth and last aircraft in a series of aircraft powered solar cells and fuel cells. It was an unmanned aircraft. Its predecessors were the NASA Pathfinder, Pathfinder Plus and NASA Centurion (see the drawing below for comparison). These four aircraft were developed under NASA's ERAST program (ERAST = Environmental Research Aircraft and Sensor Technology). The purpose of this program was to built as research and develop aircraft for research into long-term, high-altitude aircraft which could serve as atmospheric satellites.
The different aircraft of the ERAST Program
The Helios Prototype was built by heavily modifying its predecessor, the NASA Centurion, a 41 ft wing section was added, as well as a fifth landing gear and systems pod. With the added wing section the span of the aircraft reached 247 feet (75 meters), larger than the wingspan of a C-5 Galaxy (222 ft - 68 meters). It was constructed using mostly composite materials, Carbon-fiber, Graphite-epoxy, Kevlar and Styrofoam. It had a thin plastic skin. It had a tubular wing spar which was made of carbon., wrapped in Nomex and Kevlar for added strength. The flight controls consisted of 72 (seventy-two!) trailing edge elevators operated by tine servo-motors which received deflection commands from the aircraft's Flight Control Computer. Yaw control was achieved by applying different power to the motors. The aircraft consisted of 41-foot sections, which at the joints had an under wing pod, containing landing gear, batteries, Flight Control Computers and data instruments.
The ERAST program had two research goals for which the Helios Prototype was to be used;
Sustained flight at altitudes of ~100.000 feet (~30.000 meters)
Endurance of at 24 hours, of which at least 14 hours above 50.000 ft (15.000 meters)
The Helios prototype could be specially configured to adapt to the required research goal.
Configuration HP01, for achieving the altitude target powered by batteries and solar panels.
Configuration HP03, for optimized endurance, using a combination of batteries, solar panels and a hydrogen-air fuel cell for power at night. During daylight operation, the aircraft would use 14 engines, and for nighttime operation only 10 engines would be used.
The aircraft set an world altitude record in its class, on the 13th of August 2001. Sustained level flight by a winged aircraft at an altitude of 98.863 feet (29.524 meters). crushing the previous record by 11.000 feet (3.400 meter). During this flight the aircraft spent more then 40 minutes above 96.000 feet (29.000 meters)
Sadly the aircraft was lost on the 26th of June 2003 during a check flight in preparation for an endurance test flight. When climbing through 2800 ft (850 meters, 30 minutes after take off the aircraft got in to turbulent air which caused an excessive dihidral configuration. This let to an unstable flight path especially the pitch. The aircraft Vmo was exceeded and the The aircraft breaking up mid flight dynamic pressures caused the wings to fail. Nearly all of the wreckage was recovered from the surface of the ocean, with the exception of the hydrogen-air fuel cell pod and two of the motors.
The full report on the crash can be found here.
Debris floating in the ocean after the mid air break-up
Below a comparison of the specifications of the different aircraft of the NASA ERAST program. Not all specifications are public knowledge Those unknown are indicated with a "?";