The Helios Prototype is an ultra-lightweight flying wing aircraft with a wingspan of 247 feet (75 m), longer than the wingspans of the U.S. Air Force C-5 military transport (222 feet (68 m) or the Boeing 747 (195 feet (59 m) or 215 feet (66 m), depending on the model), the two largest operational aircraft in the United States. The electrically powered Helios is constructed mostly of composite materials such as carbon fiber, graphite epoxy, Kevlar, styrofoam, and a thin, transparent plastic skin. The main tubular wing spar is made of carbon fiber. The spar, which is thicker on the top and bottom to absorb the constant bending motions that occur during flight, is also wrapped with Nomex and Kevlar for additional strength. The wing ribs are also made of epoxy and carbon fiber. Shaped styrofoam is used for the wing’s leading edge and a durable clear plastic film covers the entire wing.
The Helios Prototype shares the same 8 feet (2.4 m) wing chord (distance from leading to trailing edge) as its Pathfinder and Centurion predecessors. The 247-foot (75 m) wingspan gives the Helios Prototype an aspect ratio of almost 31 to 1. The wing thickness is the same from tip to tip, 11.5 inches (29 cm) inches or 12 percent of the chord, and it has no taper or sweep. The outer panels have a built-in 10-degree dihedral to give the aircraft more lateral stability. A slight upward twist at the tips of the trailing edge helps prevent wing tip stalls during the slow landings and turns. The wing area is 1,976 sq ft (183.6 m2)., which gives the craft a maximum wing loading of only 0.81 lb./sq. ft. when flying at a gross weight of 1,600 lb.
The all-wing aircraft is assembled in six sections, each about 41 feet (12 m) long. An underwing pod is attached at each panel joint to carry the landing gear, the battery power system, flight control computers, and data instrumentation. The five aerodynamically shaped pods are made mostly of the same materials as the wing itself, with the exception of the transparent wing covering. Two wheels on each pod make up the fixed landing gear—rugged mountain bike wheels on the rear and smaller scooter wheels on the front.
The only flight control surfaces used on the Helios Prototype are 72 trailing-edge elevators that provide pitch control. Spanning the entire wing, they are operated by tiny servomotors linked to the aircraft’s flight control computer. To turn the aircraft in flight, yaw control is applied by applying differential power on the motors — speeding up the motors on one outer wing panel while slowing down motors on the other outer panel. A major test during the initial flight series was the evaluation of differential motor power as a means of pitch control. During normal cruise the outer wing panels of Helios are arched upward and give the aircraft the shape of a shallow crescent when viewed from the front or rear. This configuration places the motors on the outer wing panels higher than the motors on the center panels. Speeding up the outer-panel motors caused the aircraft to pitch down and begin a descent. Conversely, applying additional power to the motors in the center panels caused Helios to pitch up and begin climbing.
Solar panels, supplied by SunPower were then installed in 2000. The cells featured a rear-contact cell design that placed wires on the underside of the cells, so as not to obstruct the cells’ exposure to solar radiation.
The Centurion was modified into the Helios Prototype configuration by adding a sixth 41 feet (12 m) wing section and a fifth landing gear and systems pod, becoming the fourth configuration in the series of solar-powered flying wing demonstrator aircraft developed by AeroVironment under the ERAST project. The larger wing on the Helios Prototype accommodated more solar arrays to provide adequate power for the sun-powered development flights that followed. The aircraft’s maiden flight was on September 8, 1999.
The ERAST program had two goals when developing the Helios Prototype: 1) sustained flight at altitudes near 100,000 feet (30,000 m) and 2) endurance of at least 24 hours, including at least 14 of those hours above 50,000 feet (15,000 m). To this end, the Helios Prototype could be configured in two different ways. The first, designated HP01, focused on achieving the altitude goals and powered the aircraft with batteries and solar cells. The second configuration, HP03, optimized the aircraft for endurance, and used a combination of solar cells, storage batteries and a modified commercial hydrogen–air fuel cell system for power at night. In this configuration, the number of motors was reduced from 14 to ten.
Using the traditional incremental or stair-step approach to flight testing, the Helios Prototype was first flown in a series of battery-powered development flights in late 1999 to validate the longer wing’s performance and the aircraft’s handling qualities. Instrumentation that was used for the follow-on solar-powered altitude and endurance flights was also checked out and calibrated during the initial low-altitude flights at NASA Dryden.
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