Remarkably, Skyfox has been awarded the first certification for not one, but two aircraft under European/ Australian joint airworthiness regulations for very light aircraft (JAR/VLA). As a result of the certification of the tailwheel CA25 Impala, and its newer derivative, the nosewheel-equipped Skyfox Gazelle, the Queensland-based manufacturer is now promoting the Gazelle as filling a perceived void in the light-trainer market created by the high purchase price and operating costs of new or used conventional trainers.
The CA25 has undergone a complete rework, including redesigned and strengthened undercarriage, fuselage, tailplane and wings, to qualify for Australia's new "ultralight" standard, which carries a 450kg maximum take-off-weight (MTOW) limit. The airframe has been adapted to house the Rotax 912 engine, which develops 60kW (80hp) at 5,600RPM, reduced through a 1:2.27 gearbox to produce a propeller speed of 2,470RPM.
The basic empty weight of the Gazelle ranges from 300-315kg, according to installed equipment, and, although the aircraft can already carry two standard occupants, full fuel, and its full (10kg) baggage allowance, the company is developing a longer-range fuel tank which could slightly limit payload.
The structure is a welded chrome molybdenum 4130 steel frame of .058 gauge, drilled and injected with anti-corrosion chemicals, sandblasted, powder-enamelled, and covered in heat-shrunk dacron-based Stits Aircraft fabric. The structure provides a "crash cage" able to withstand pressures of up to 12g, a feature which business development manager David Anning points out is non-existent in most conventional monocoque light aircraft. Aileron and elevator actuators are 4130 steel push-pull rods, eliminating the need to inspect cables. The only flight-control cable goes to the rudder.
The main undercarriage suspension is based on steel shafts molecular-bonded into rubber, so that landing pressures place the rubber in tension. In drop tests well beyond certification standards, it withstood pressures of 8.8g, at which point the aircraft's belly almost contacted the ground, without breaking the suspension system. The nose leg uses nine rubber "biscuits" in compression so that there is no oil or gas in the system, and it is replaceable in a few minutes. The nose gear bolts directly on to the airframe rather than to the firewall, and is fully steerable, aided by independent hydraulic disc brakes. Low stall speeds mean that braking is barely necessary except in ground manouevring.
Dual ignition is provided for the Rotax 912 four-cylinder, four-stroke engine through two independent capacitor-discharge ignition systems, which weigh a few grams each and replace magnetos typically weighing 6kg each. The cylinder barrels are air-cooled without any complex baffling, while the cylinder heads are cooled by an 80/20 Glycol/water mixture from a 2.5litre tank, eliminating the thermal shock caused by sudden power increases and reductions.
Removal of top and bottom cowls takes 3min and provides access to all engine components, ancillaries, and even to the back of the instrument panel, reducing maintenance hours. In another 3min operation, the removal of two split pins, two locking pins and an overwing panel allows the wings to be folded for trailer transport, or to fit three aircraft in hangar space normally required by one.
The oil tank on the dry-sump engine has a 2.8litre capacity. The carburettors are altitude and octane adjusting, so there is no need or provision for a mixture control, and no adjustment needs to be made for fuel type. Skyfox advises that its customers regularly mix aviation gas and low-lead motor fuels, or operate solely on either one, without variations to performance or engine handling.
Anning's brother Fraser, Skyfox's marketing manager, believes that parts, labour and operating cost comparisons with other aircraft are valid, and are the key to a substantial potential for market expansion. "There are two main reasons why people drop out of flying early in the process. A large number find the cost too high, and up to 50% wash out around the stalling stage, when they get a little frightened or at least confused, and decide flying isn't as much fun as they expected. The combination of those two factors means that far too many customers are lost to the training industry and to flying generally. This machine does not have alarming stall characteristics, and it won't frighten them away from flying. If they want to do violent stalls later in something like a Beech Bonanza, they can do them when they have more confidence," he says.
Skyfox, unlike many VLA manufacturers, has recognised the demands of the training market for a degree of "usualness" to avoid confusing low-time students, and has provided space for a conventional six-flight instrument layout, with avionics in the central panel and engine instruments on the right. Because Australian standards require basic instrument flight training in the Private Pilot License syllabus, most schools are electing to fit an artificial horizon, directional gyro and transponder. Flight controls are also conventional, with the left hand operating the stick and the right hand on the throttle. Because of the Skyfox's 42kt (77km/h) stalling speed, flaps are not fitted.
Quadruple fuel redundancy
In the fuel system, the capacity of the two wing fuel tanks is 51.75litres useable. The tanks are slightly ram-pressurised and fitted with flapper valves to prevent fuel escaping if a pilot omits to replace a filler cap. Anning points out that the fuel system has quadruple redundancy, with gravity feed, pressurised tank, and electric and mechanical fuel pumps. All fuel feeds through a collector tank system consisting of one tank inside the other behind the seat, externally vented. Because the engine draws fuel from the collector tanks, a dry main tank will not cause fuel starvation; and with both tanks empty, a 20min reserve remains in the collector tank.
The Gazelle's cabin is 20mm wider at the shoulder and hip than that of the Cessna 150, partly because of the outwards-bulging (transparent) door panels. Cabin entry is at least as easy, and once seated the comfort level is adequate if not luxurious. For pilots over 1.9m (6ft 3in) in height, Skyfox is developing a "deep" seat to ensure adequate legroom as no rudder pedal adjustment is provided.
The engine is started with the push of a button, and its unusual quietness, especially externally, comes as a surprise. Differential braking achieves extremely tight turns with the inside wheel turn radius at about 2m. Wheels and struts are well behind the eyeline of the pilot, who must remain alert to the relatively large wingspan when taxiing.
The Gazelle took off in little more than half the approved distance of 492m still-air run with a slight headwind component, entering a 650ft/min (3.03m/s) climb at 60kt without post-liftoff sink. Best rate of climb speed is 55 kt, which produces about 750ft/min in the same conditions.
Once airborne, the first impression is of extremely positive control response in all three axes, and of comfortable control force balance. Elevator trim, provided by a fore-and-aft lever which loads a spring bias on to the elevator control push rod, is barely necessary except for stabilising level flight. An intercom is standard equipment, as cabin noise is slightly higher than average without headphones. A tinted overhead panel covering almost the whole cockpit ceiling, enhances visibility in turns. In climb at 60kt, the nose is barely above the horizon. Ample cockpit ventilation is provided by rotatable scoops inset into the forward windows.
We reached 4,000ft in a little over 6min, and stabilised in cruise at 80kt true airspeed with power set at 5,000RPM which, according to the manual, burns 13.7litres/h, providing a total endurance of 3h 46 min without reserves. Inflight visibility in cruise is excellent, and the aircraft is stable in all three axes, with stick forces remaining equally light at the higher speeds, making light work of 360í steep turns, wingovers, and all other permitted manoeuvres. The airframe is low-drag, and care needs to be exercised during recovery from unusual attitudes, to avoid the never-exceed speed (Vne) of 93kt. Once trimmed, the aircraft can be manoeuvred into medium turns with rudder alone.
Taking crossed controls to the extreme, with cruise power applied, I was able to complete a wings-level 360í skidding turn in about 35s, with some sink as the resulting drag reduced indicated airspeed to 45kt; an interesting but admittedly relatively useless manoeuvre other than to demonstrate the Skyfox's tolerance to control inputs which would produce a variously alarming reaction in most other light-aircraft types. In general flight handling, it is difficult to see how a student could induce a dangerous and unrecoverable situation without the grossest of mishandling.
Atypically for a VLA category aircraft, drag is so low that speed reductions in the circuit need to be made early to achieve desired glideslope. Recommended glide speed at 55kt produces a sink rate of 600ft/min with wings level. The aircraft's remarkable sideslip qualities provide ample capability to lose height quickly, however. At 75kt, a 15í sideslip can produce a sink rate of 2,000ft/min, reducing to 1,000ft/min at 55kt. Normal into-wind approach at 60kt produces a brief float, while a "bush" approach at 50kt allows the aircraft to be placed on an aiming point with precision, for a maximum-energy stop in a very short distance. The rubber-in-tension suspension still provided an agreeably soft landing at each attempt.
The Gazelle will be an excellent trainer for schools which wish to offer a competitive private-licence package to students, some of whom may never wish to graduate to more sophisticated types. It will also provide commercial schools with the useful option of cutting course costs by using it as a primary trainer for at least part of more comprehensive courses.
Source: Flight International