Drone Patrol

You cannot sink a battleship with a UAV."* This remark, attributed to the operator of a Pioneer unmanned aerial vehicle (UAV) during the Persian Gulf War, relates to the rather haphazard method used to recover the ship-launched aircraft. Operators at sea tried to fly them into deck-rigged nets--and not always successfully.

Nevertheless, the unassuming UAV was something of an unsung hero. In a theater where laser-guided bomb hits vied with fiery Patriot missile launches for air-time on CNN, Pioneers droned dutifully behind enemy lines attracting little attention from press or foe. The plucky little (463 lb) robotic planes flew over 300 sorties performing reconnaissance, artillery spotting, damage assessment, and other missions collecting information people previously risked getting shot at for.

The exploits of Pioneers (and their shortfalls) were inspirational to Hugh Schmittle, chairman, CEO, and co-founder of Freewing Aerial Robotics Corp., College Station, TX. Then based in Maryland, Schmittle and his colleagues were devising the mathematical models behind a radically new kind of UAV, a plane whose wings would literally shift with the breeze while the body pivots to vary the direction of engine thrust. These characteristics promised to provide an aircraft with unparalleled stability and extremely short take-off and landing (ESTOL) performance.

Designers labored to turn these concepts into AutoCAD geometry. The result is the Scorpion, a revolutionary UAV now ready for production at Freewing's brand new Texas facility. The buzz proceeding the Scorpion was considerable: Design News gave Schmittle an Excellence in Design award for the "Tilt-Body" prototype in 1994. The current version has attracted a partnership agreement with Matra Defense to develop as the Marvel. Intended for the French navy, this UAV's ESTOL characteristics enable it to land on frigate helipads. No net required.

Tilting toward the future. According to Schmittle, the Tilt-Body concept as applied to UAVs imposes a number of considerable design challenges. Chief among them are the additional degrees of freedom, which necessitate a completely new autopilot and teleoperation system. Conventional fixed-wing aircraft have six degrees of freedom: roll, pitch, yaw, and acceleration on each axis. The Tilt-Body has eight degrees of freedom.

"We needed extensive computer modeling and wind-tunnel testing to come up with the dynamic equations to describe the plane," Schmittle says. A new UAV design requires engineers to tackle all the problems of an aircraft program plus all the problems of a robotics program. "The math models are needed to develop the flight software used by the operator control units."

In order to understand why Freewing seemingly complicates the issue with all that tilting and floating, one has only to look at the Scorpion's performance. The freewing, which pivots in the pitch axis, provides stability for the aircraft during transition phases of flight. These occur as the body is tilted to produce thrust vectoring for small-area launch and recovery and for low-speed flight.

"Just watch the hands of conventional UAV operators," says Matt Majernik, test-pilot at Freewing and a former U.S. Army UAV operator and flight-instructor himself. "When they're on autopilot, the hands are nice and steady. But when they're controlling the aircraft directly, the hands are really working. The freewing automatically compensates for wind gusts and can never stall the airplane. This makes the Scorpion a much smoother UAV to handle at all speeds."

The Scorpion's 50-hp Rotax engine provides a top speed of 173 mph. Much of the transit portions of a mission are handled by the autopilot, which has access to GPS navigation data. Thrust vectoring results in on-target loiter speeds of 63 mph and the ability to land with a very steep angle of attack typical of helicopters. Operators can land the Scorpion on a 60-meter strip without any special arrester gear. A good head wind--such as those encountered by ships at sea--enables near-vertical launch and recovery.

The mechanics of Tilt-Body operation in the Scorpion involve only three moving parts and are relatively straightforward. A single carry-through shaft passes through the center body, providing a hinge for the free-floating freewing. The landing gear and aft boom assembly, cross-connected by a torque tube, attach to the main body by a jack-screw. This component, powered by an electric motor from Motion Systems Corp., operates a control horn that moves the booms up and down via the torque tube. Dynamic pressure from airflow in flight forces the booms to remain horizontal and the carbon-graphite body is what tilts. The tractor-mounted engine tilts along with it, providing vectored thrust.

"Most of the components of the Scorpion are off-the-shelf," Schmittle says, adding that Freewing's policy is to tackle only one R&D problem at a time. "Modeling flight dynamics proved challenging enough."

The military is just one source of potential UAV customers. Freewing currently is exploring how its Tilt-Body aircraft might be used for powerline/pipeline inspection, agriculture support, disaster assessment, fire survey, security, and search and rescue.

Schmittle reports he was approached by the owner of a tuna fleet operating out of Guam. The captains currently employ MD-500 helicopters from their fishing boats to search for tuna. The fleet experiences an average of one fatal crash a year, and an alternative method of tuna spotting is being sought. One possible solution involves operating Scorpions from the fleet's helipads instead.

Problems of scale. Many UAVs are designed as working craft to perform specified tasks. These guided vehicles are finished products, and aeronautical engineers developing them have many strategies at their disposal to make them air-worthy. The UAV's geometry can be maximized for performance.

However, one breed of UAV designer does not have so many options. Before full-scale aircraft are built, it is common practice to design, construct, and fly sub-scale models. Model-makers are constrained by the requirement to make their UAV act like the full-scale version it represents.

According to Bob Parks, principle at Intuitive Solutions, a firm specializing in contract-built test aircraft, designers of sub-scale models are faced with two important scaling laws: Reynolds numbers and dynamic scaling.

"Butterflies and 747s react to air very differently," Parks says, indicating that differences in size, weight, and speed can render spurious any conclusions drawn from flights of sub-scale models. "Designers have to be careful about taking test flights at face value."

A Reynolds number is a nondimensional parameter representing the ratio of the momentum forces to the viscous forces in fluid flow. According to Parks, it is more important for a model to have a similar Reynolds number to the full-scale aircraft under development than it is to look like the finished product. This is because a Reynolds number is better indicator of eventual performance than appearance.

Dynamic scaling refers to how the model appears to behave if observed at a speed appropriate to its size. In other words, Park says, if a video of the test flight of a 1/2 scale model is played back at half-speed it should appear to fly like the full-scale version.

"Reconciling Reynolds numbers and dynamic scaling is very challenging," Parks said. "Sometimes all you get from a test flight is a general feeling that you're on the right track."

Parks adds this general lightness of being is more valuable than it sounds: It's often harder to get a scale UAV flying properly than the real thing. This comes back to dynamic scaling. Servos actuating control surfaces on full-size aircraft have a more leisurely time frame in which to operate compared to their smaller cousins.

"There are aircraft you can fly from the cockpit that you can't fly from the ground," Parks says. "If you can make something work in the model it's a pretty good indication it will work in the real world."

Occasionally, however, the stars converge just so and it is possible to make direct comparisons between sub-scale and full-scale aircraft. A case in point is the so-called Mars Airplane, an aerial robot intended to fly through the air of the Red Planet on a future mission. Parks designed the scale model of the Mars Airplane, and it happens that the Reynolds number of the full-scale version in the thin carbon-dioxide-dominant atmosphere of Mars and the Reynolds number for the sub-scale model flying in the nitrogen-prevalent air here at home were about the same.

Parks designed his model using Ashlar Vellum. He had to work out a series of accordion folds that would enable the model to fit inside a can measuring 2 feet by 1 foot. This craft would then have to unfold and start flying on deployment. "We tried to accomplish this unfolding process aerodynamically, but we couldn't quite get it," Parks reports. "In the end, we had to resort to using little springs."

What is perhaps the world's biggest and most expensive UAV is now under development at Lockheed Martin's Skunk Works in Palmdale, CA. The X-33 VentureStar is a half-scale prototype of a possible follow-on to the Space Shuttle. The X-33, powered by a linear aerospike propulsion system, is designed to reach speeds up to Mach 15 and altitudes up to 50 miles. It also will fly autonomously, lifting off like a rocket and landing like an airplane.

Flight tests are scheduled to commence in 1999. A significant portion of the current development work involves creating the guidance and control hardware and software that will see the prototype off and back down again. The plan to go with autonomous control must come as something as a relief to would-be operators: The X-33's $941 million pricetag suggests there would be a sweaty palm on the joystick.

*Actually, it's been done

..In September 1943, Admiral Carlo Bergamini of the Royal Italian Navy led a fleet of three battleships, six cruisers, and eight destroyers toward Malta to surrender. A special unit of German maritime bombers, alerted to the move, sortied from Southern France to intercept. The unit was equipped with a new type of weapon: radio-controlled flying bombs. UAVs, if you will.

The attackers released their payloads between 12,000 and 19,000 feet. Crews directed the winged, armor-piercing bombs via control units on the loitering bombers. One of the UAVs struck the flagship, Roma, penetrating deep into the interior before the warhead detonated. The battleship went down with nearly all hands, including Bergamini himself.

Source: Engage the Enemy More Closely; The Royal Navy in the Second World War by Correlli Barnett (W.W. Norton & Company, NY, 1991)

UAV pioneers

The Pioneer was the first UAV deployed by the United States in numbers. Developed by AAI Corp. in cooperation with Israel Aircraft Industries, Ltd., the Pioneer can be equipped with high-quality video, a variety of sensors, and GPS navigation equipment.

In addition to the Gulf War, Pioneers have supported U.S. forces in Africa, the Pacific Rim, and the Mediterranean. The developers have formed a jointly owned company, Pioneer UAV Inc., Hunt Valley, MD, to market the Pioneer to other customers, including industry.