By Bill Sweetman, Design News July 1, 2000

A remarkable aircraft is being built in Victorville, California, under a contract from the Defense Advanced Research Projects Agency (DARPA). The Frontier Systems A160 unmanned helicopter is intended to demonstrate an endurance of well over 30hrs - some associated with the program are talking 48hrs or more - a service ceiling of 55,000ft (16,760m) [Gale says 35-40,000 ft] and an unrefueled range of 3,700-5,500km.

These numbers are so far beyond current helicopter records (by a factor of two or more, in many cases) that they would strain credulity, were it not for the source. The designer of the A160 is Abraham Karem. The Leading Systems Amber, which Karem designed for DARPA in the 1980s, demonstrated 28hrs+ of endurance in a small unmanned aerial vehicle (UAV), together with unparalleled reliability. Amber is the direct ancestor of today's Predator. Frontier Systems' W570 contender for the Tier 2 Plus requirement (which led to the Global Hawk), designed for Loral, was arguably more advanced in concept than the contest winner.

The A160 is based on a reappraisal of the basics of helicopter design. Conventional helicopters operate within a very narrow rotor revolutions per minute (RPM) range. Their rotors are articulated to provide control authority and have flexible blades to save weight. These features lead to a complex and dynamic pattern of vibration; traditionally, rotors are designed for smooth and safe operation at a single speed point. They operate around 100% RPM whenever the helicopter is airborne.

The operating RPM is usually the highest speed possible, because this reduces the difference between the speed of the advancing blade (which is moving forward into the airstream) and the retreating blade, when the helicopter is in forward flight. The upper limit (450-500rpm on a small helicopter) is set to keep the tip speed of the advancing blade in the subsonic realm - about Mach 0.6 - at the helicopter's design cruising speed. Particularly at less-than-maximum speeds and weights, however, the helicopter rotor operates much faster than necessary. This reduces the lift/drag ratio of the blades and requires more power to turn the rotor.

The A160 rotor can be slowed down to as little as 40% of its maximum RPM, operating between 150-350rpm with tip speeds as low as Mach 0.25. The blades are tapered and change in thickness/chord ratio from root to tip to improve their lift/drag ratio. To avoid vibration problems, the rotor blades are light and stiff, and their stiffness in flap, lag and torsion is progressively reduced from root to tip, so that the tips are more flexible than the root. This is made possible by the use of tailored carbon fiber construction. The A160 rotor is hingeless and rigid, and has a larger diameter and lower disc loading than a conventional helicopter rotor with the same maximum lift.

The result is a dramatic improvement in aerodynamic efficiency at low speeds and weights. This can be combined with a fuel-efficient engine; a key to the performance of Karem's Amber vehicles was their use of high-performance reciprocating engines, specially developed by Leading Systems. (The demonstrator has a 300kW piston engine.) The result is a helicopter with very long range and endurance, and a respectable maximum speed of 140kt. Since noise is closely linked to rotor speed, the A160 will also be exceptionally quiet.

The A160 project has been under way since early 1998. Frontier Systems started by modifying a used Robinson R22 light helicopter to test the A160's autonomous flight control system software and hardware. This is designed to allow the A160 to be operated safely under the control of a non-pilot. The demonstrator was lost in an accident in February, but had flown successfully under autonomous control for 215 hrs  an enviable achievement for a VTOL (vertical take-off and landing) UAV of any description.

The unique A160 rotor system is now being tested on a ground rig at Victorville. The development team is anticipating some vibration problems, but modeling shows that vibration should be confined to discrete bands, leaving a well-defined operating regime which is large enough to provide good performance.

The next decade should see the testing of other prototypes which break what were once considered iron-clad limits on aircraft design and performance. These include stealthy, agile aircraft with no conventional flight controls; and efficient supersonic-cruise aircraft with no sonic boom signature.

Computing is the most important technology behind this renaissance in the flight sciences. Fly-by-wire (FBW) flight control systems using artificial stability were an early application of compact digital and analogue computers in the 1970s, and made it possible for the designers of the F-16 to relax the standards of natural stability that had governed aircraft design. Today, computer technology has reached the point where aircraft may be highly unstable about all axes, and the FBW system will integrate aerodynamic and propulsion control to fly the aircraft.

Frontier Systems Incorporated

Founded               1993   [Gale says 1991]

Employees           30   [Gale says 40+]

Annual revenues   $7-10M

Flight facility         George AFB, CA

Owner                  Private (shareholders)

President/CEO     Abraham E. Karem

Business dev       Gale Kerem 

Phone/fax            949 623 2222

Website                www.recce.com - very sparse website

Markets                USG: N/A, Commercial: N/A, Foreign: N/A

Products              Aerodynamic pods, UAVs

Spoke with Gale Kerem on 7/20/00 and 7/31/00  Slight corrections are enclosed in [  ]:  
They are producing this for DARPA, but both are looking for commercial uses.  
They have an export license for it.
Blades are probably foldable for transportation