Stingray: The latest in flight control R&D

The Stingray was built by Boeing to support flight tests for advanced control-system techniques at Georgia Tech Research Institute (GTRI) funded by the U.S. Government Defense Advanced Research Programs Agency. The program's official name is "AVIA," for "adaptive virtual aerosurfaces" for UAV (unmanned aerial vehicle) applications, but everyone calls it "Stingray"; I wonder why!

This is not a small aircraft: its wingspan is 2.79 meters (110 inches) and it's 1.83 meters (72 inches) long. An AMT Olympus turbojet engine provides ample motive force for this 43-kilogram (97pound) package by providing 187 newtons (42 pounds of thrust).

Guidance is provided by an interesting mixture of off-the-shelf and custom gear. A Futaba 9Z transmitter talks to two independent receivers, each powered by its own 2000mAh battery through Jaccio regulators. Each receiver is designed to "drive" portions of both the left and the right sides of the plane, so reduced-authority control may be maintained even if one receiver is lost.

Its control scheme is not simple. Both inboard and outboard elevons are set up to work in the usual tailless-model way, but there's a complication: each elevon has a top surface and a bottom surface that can be commanded to move together or separately. The outboard elevons are split for yaw control (just like the B-2 bomber's) while the inboards are split for airbrake function during approaches and landings. There aren't enough programmable mixers in the 9Z for all of this, so a mechanical mixer is provided for each elevon, and yaw is handled by special mixer yaw electronics from Wray Associates, which also provided channel splitters and gear-door sequencing units for the retract system.

A bunch of servos (21, in fact!) is needed to manage all these functions, and I haven't yet mentioned retractable gear (from Robart), wheel-braking systems (from Glennis Aircraft), a rudder, and turbojet engine control. JR 4721 servos were selected for their speed and torque.

The Stingray, which is made out of fiberglass and foam, was designed and built in only six months. The builders used molds made by a boat moldmaker in Washington state and made the wing skins with a slurry of epoxy and microballoons that filled the foam and adhered to the skins. For minimum drag and ease of setup, the Stingray's control surfaces use "living" hinges (one skin is cut to allow control movement, and the other skin flexes and so acts as a hinge [see photo lower right]).

Flight testing has shown the Stingray to be very stable in cruise and quite fast: at 1/2 throttle with wheels down, it was clocked at 145kph (90mph). Landings were difficult because the Stingray wanted to continue flying! More effective airbrakes will help to "dirty" up the airframe to achieve satisfactory landings.

To record what happens while the Stingray flies, GTRI provided a custom, onboard data-and-downlink system to measure and transmit information on speed, acceleration, turning rates and angles and control deflections. A video downlink from Plane Talk shows the view from the air.

This is the first phase of the AVIA test program. Another Stingray will eventually be built to allow the investigation of how the controls may be made to work more efficiently and to improve its "stealth" characteristics.

I'll keep you posted on the progress of this fascinating flying model! For more photos and video clips of Stingray's test flight, go to www.modelairplanenews.com/video/stingray.

Copyright Air Age Publishing May 2000
Provided by ProQuest Information and Learning Company. All rights Reserved