Popular Mechanics - November 2006

Aerial Pirouette - Text by Sean Woods / Photographs by Colin Bennett.

THERE'S something strangely compelling about aerobatic aircraft. Pushing the limits of design and engineering, showcasing the formidable skill and courage of their pilots, they provide us with a mixture of excitement and dread: what if something goes wrong?

Giving the figurative finger to gravity, soaring and pirouetting with the effortless grace of a prima ballerina, the Slick 360 is the newest contender in the aerobatics arena. If ever an aircraft took control of the sky, rattled the cages of its adversaries and celebrated the art of derring-do, it's this one. And it was built right here in South Africa.

Tough and agile, with a power-to-weight ratio that would make an automotive engineer weep, it's an aerobatic mean machine with that rarest of qualities, a delicate touch. Put an experienced pilot behind the joystick, and that old expression, the sky's the limit, becomes null and void.

Local aerobatics ace Glen Dell thinks it's an amazing aircraft - and he should know. Dell won the Advanced World Aerobatics Championships (AWAC) in Sweden two years ago, and is an eight-times winner of the SA National Aerobatic Championships. Rated to fly over 250 different aircraft types, he's also a Boeing 737 training captain. This man has strapped on his parachute, tightened his five-point Hooker harness and out-flown the best more times than some of us have had birthdays. In short, he knows what he's doing.

So how did he come to develop the Slick 360?

Dell reckons the time was ripe for a powerful, responsive, safe and pilot-friendly aerobatics aircraft. "In competition aerobatics, the aircraft must make the flying as easy as possible for the pilot while he or she concentrates on the sequence and positioning. That's exactly what this aircraft does."

Another reason was the fact that most of the aircraft competing in the advanced aerobatic class are decidedly long in the tooth. For example, the Extra 230 (based on the Laser), a great performer that won two AWACs, ceased production 15 years ago. Dell joined the dots, concluding that the aerobatics world was ripe for a new aircraft based on the Extra's successful design.

There was one critical proviso: it would have to be constructed from more modern and more durable composite materials, and kitted out with as many upgrades as possible.

Dell teamed up with Aero-Cam, a trusted Wonderboom-based composite aircraft manufacturer responsible for designing, developing and producing the successful Celstar GA-1 competition aerobatic glider. Together with associate Francois Jordaan, an aeronautical structural engineer, he opted for the Laser's wing profile, elevator and rudder but decided to retain the Extra's control system. That's where the visual similarities ended - and where the benefits of carbon fibre and Kevlar became readily apparent.

Load factor explained

The load factor is simply the number of times the load experienced by the structure during a manoeuvre is more than that experienced during un-accelerated level flight, expressed as g's. Straight level flight is 1 g, and a co-ordinated turn with a 60-degree bank angle requires 2 g's. The US Federal Aviation Regulations Part 23

(FAR-23) - the established world airworthiness standard - requires a minimum positive load factor of +6 g and a negative load factor of -3 g for aerobatic category aircraft. - Francois Jordaan.

The Laser, like the Extra 230, had a wooden wing and a load limit of 6 g's. The Extra improved the load limit to 10 g's, but this could be achieved only by constructing a thicker wing. The Slick, on the other hand, although designed to the same competitive load limit specifications as the Extra (+10 g and -10 g), had a safety factor of 2.0 factored into its design (rather than the 1,5 required by FAR-23).

In theory, this meant that no part of the Slick, with its tough composite structure, should fail before it reached +20 or -20 g's. Thanks to its carbon fibre wing spar, this design also required a significantly thinner wing, enabling it to fly faster - and when you're performing your routines inside a 1 000 m³ competition "box" in the sky, speed rules.

In fact, the formidable strength of the Slick created a number of headaches for the engineering team. While testing the structural integrity of the airframe's major components on a static test rig, they broke it. The test rig, that is. The airframe was fastened to a cradle and a hydraulic jack applied pressure at the wing attachment points to simulate the stresses the fuselage would undergo during flight.

Critical areas included the pilot seat position, hanging gear position, wing attachment points and engine mounts. They managed to simulate 15 g's before the test rig's main steel girder started to buckle.

The wings, constructed from moulded glass fibre composite sandwich panels and joined together by the carbon fibre wing spar to form a single structure, also passed the load tests with flying colours. Explains Jordaan: "The aerodynamic loads are simulated through a system of beams and links connected to a single hydraulic actuator for each wing. It needs to sustain the test load for at least three seconds, and the aileron movements must be shown to be free and easy at the same time. After the test, no signs of residual deflection or structural failure should be evident."

This time, the test rig failed twice before they reached a load limit of 15 g's.

Some time in the future, say Aero-Cam, they'll test a wing until it fails - and they expect that point to be reached at around 20 g's. First, however, they'll have to build a stronger test rig! Jordaan reveals that all wings will be individually tested for structural integrity beyond its limit load (11 g) before being attached to the fuselage.

An aircraft's roll rate is also critical if you want to impress the judges; that's where the ailerons come in. Says Dell: "In an older aircraft, you might get in two rolls while going vertical, but with the Slick you could get in six." To achieve the Slick's impressive roll rate of a little over 400 degrees a second, the design team went through three complete designs before Dell was satisfied.

It wasn't easy: the ailerons needed to be relatively large, but at the same time they had to be aerodynamically light to give the pilot precise control. Refining the angles and dimensions involved experimenting with profiles and shifting the hinge line. Finally, spades were fitted under the wings to further lighten the aileron loads.

Carbon fibre longerons and ribs added to the overall strength of the predominantly carbon fibre fuselage, removing the necessity of installing the internal tubular frame usually found in aerobatic aircraft. This approach also freed up a significant amount of internal space, allowing for a wider, more comfortable cockpit.

Kevlar, with its high impact resistance, fully encases the cockpit to protect the pilot in the event of a crash. The tail section incorporates a variety of composite materials, doing away with the necessity for flying wires that add drag.

Powered is provided by a 179 kW four-cylinder AEIO 360 Lycoming engine specifically prepared for aerobatics and equipped with a three-blade MTV 9 lightweight propeller. The Slick, weighing in at 450 kg, has an exceptionally high power-to-weight ratio, giving it a maximum speed of 400 km/h.

But that's just one of its many advantages.

The engine's deep sump holds more oil than conventional powerplants, preventing the loss of oil pressure during manoeuvres - a phenomenon so often associated with four-cylinder engines. The crankshaft is tough enough to handle the extreme gyroscopic forces generated by the propeller when executing tumble manoeuvres, and electronic ignition allows for ideal timing over a far higher rpm, altitude and temperature range than would be possible with a magneto. Other features include cold air induction and a lightweight starter and alternator.

New though it is, the Slick 360 has already made an impact on the aerobatics world. So confident was Dell in his design that he flew a prototype at the 2004 South African Aerobatic Championships - and won, making it an instant hit in the local aerobatic fraternity. Insiders predict that it's destined to take the aerobatics world by storm.

Five Slicks are in production and another six are on order, all of them destined for the local market. Current plans are to build 12 a year, but as Jordaan puts it, "as soon as we start attracting an overseas market, we'll probably have to put our production line on skids!"

Popular Mechanics - November 2006

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