When Australian manufacturer Gippsland Aeronautics (GA) sought a viable new project, the direction it took seemed an obvious one. The company's highly successful GA-200 two-seat agricultural aircraft was already surprising operators with its performance at 1,800kg gross weight, powered by only a 185kW (250hp) engine driving a fixed pitch propeller. That, says director/designer Peter Furlong, suggested that the GA-200's wing would neatly match the niche he believes exists worldwide for a piston-powered eight-seat utility single.
"We asked ourselves what was the most aeroplane we could afford to build, using that wing and the engine category we had already selected for economy; and offering eight seats because that was the major opportunity left after the decision to relaunch the Cessna 206. Our decision to build the GA-8 Airvan limited the amount of work we had to do aerodynamically with the wing, because we knew it would work, and that it would lift the weight," says Furlong.
The wing needed minor internal structural changes to carry more fuel in five wet-wing bays, instead of the agricultural aircraft's three, and to provide attachments for the single underwing strut which replaced the GA-200's two overwing struts. "We knew the flap and aileron performance, so the planning got down to how wide we could build the fuselage. The configuration was pretty much spoken for by the Caravan," says Furlong. The aircraft has apparent growth potential, including an obvious turbine option, particularly for military uses. GA recognises that some hot-and-high operators may require a turbocharged engine, and is retaining that option. The company expects to offer the Airvan at around the same price as Cessna's six-seat 206.
Second prototype
Flight International was offered the opportunity to fly the second production prototype about 140h into the certification programme, which GA hopes to complete by mid-1997, when it expects to have three airframes flying. This Airvan was powered by a Lycoming IO-540 driving a three-bladed McCauley propeller, providing about 210kW - 20kW short of the 230kW IO-580 destined to power production Airvrans. The new engine will deliver its rated power at 2,450rpm, complying with US noise rules. Both Hartzell and McCauley are now working on the development of propellers optimised for that lower-than-usual rpm.
The initial design goal of a 2.4 x 1.2m flat floor in a constant-section cabin has since been only slightly compromised by small 45 degree floor to sidewall insets to provide corner bracing, but still leaving about 5m2 (54ft2) of flat floor area.
Because the strut and undercarriage attachment points are not co-located, they are linked by a strengthened structure which enhances underfloor structural strength and stiffness.
The main undercarriage is based on two separate tapered and faired spring steel tubes. The firewall-mounted nosegear leg uses an oil-damped coil spring which, unlike conventional oleo nosegear, is designed to be almost fully extended at rest. This should help keep the deck level during loading, regardless of load distribution. With the front seats unoccupied, the weight of seven passengers in the cabin will not lift the nosewheel off the ground.
The wing is a modified US 35 aerofoil, similar to that on the Piper Aztec, but with noticeable washout and dihedral, and a tip shape which minimises vortices. A trimmable horizontal stabiliser provides a wide trim range and, unusually, directional control is provided by a deep-chord rudder occupying only about the lower 45% of the vertical-fin trailing edge. A small ventral fin underneath the tail adds directional stability, and will be strengthened to provide protection against tail-scrape and provide a tie-down capability. A 1.1m-square main door, which slides forward on external rails, can be opened in flight, and clears the flaps in all configurations. The door is wide enough to allow forklift loading of the cabin, making it easy to install enough lead weights to bring the Airvan up to maximum take-off weight for this test.
High tailplane
The tailplane is high enough to drive a pick-up truck underneath, reducing the risk of damage during loading. The front seats are accessed by separate doors on either side, and entry is by a fixed step and overhead handgrip. All door latches are simple, easy to use, and rugged. The front-seat windows are bubbled outwards by about 100mm, to provide visibility vertically downwards. GA may also bubble the 430 x 650mm passenger windows slightly.
The only other compound curves in the entire structure are in the Kevlar-and-carbonfibre composite engine cowl, spinner, and aerofoil-tip fairings. The constant-section cabin is 1.27m wide, giving room for an aisle and uniform seat pitch. An optional nine-seat layout has two rows of two seats, and a row of three forward passenger seats facing aft in a "club" configuration. A rail down each sidewall provides for seat-belt attachments to be positioned according to seat configuration. Each lightweight seat, to GA's own design, weighs 5kg. For configuration changes away from base, GA plans to provide a stowage for the seats in the aft cabin behind the luggage compartment.
Engine access is by removable panels on either side of the cowl, each secured by Dzus fasteners, but hinged panels are under development. A NACA intake under the cowl directs air into a plenum area containing two oil coolers, cabin-air intake and fuel-injector intake. The engine warms quickly, but operates at relatively low cylinder-head temperatures.
The cabin would be wide enough to provide three-abreast front seating, but the space is used by a console for the engine controls and elevator trim. Panel layout is conventional, with a 2 x 8 layout, with the navigation instruments to the right of the six primary flight instruments. The engine instruments and radios are on the centre panel, which is canted towards the left seat. There is ample space for a dual right-hand-seat flight-instrument panel. The small overhead panel contains all the electrics - circuit breakers, master switch and lighting controls - which aids in reducing main-panel clutter.
The fuel system is simple, with only ON and OFF positions. Both tanks drain into a header tank, and are connected in such a way that they cannot crossfeed, even if the aircraft is parked on a transverse slope. Needle valves maintain the level of fuel in the header tank, with a fuel-levelling capability which uses the fuller tank first. The engine cannot draw air through an empty tank : when both transfer-warning lights are on, indicating that there is no more fuel in either tank, the header tank still contains 9litres. With the fuel switch at "off", the key cannot be reached to start the engine, and because it is a firewall-mounted cock, the engine will run for only a few seconds in the off position.
Electrical system
GA's electrical system is simple, says Morgan: "Certification standards have recently been changed, so that no light aircraft is now compliant. That's because of conventional layouts with one wire from the battery to the master switch, which closes the circuit by earthing the wire. A rational argument says that, if the aeroplane crashes and cuts that wire anywhere, it may turn on all the electrics automatically. So we needed another solution. It suited us to put the battery under the pilot seat, with the starter solenoid and other main solenoids in the same compartment, with resettable breakers protecting the leads to the master switch. With two runs right from the start, avionics and electrics can be split so that you don't lose everything at once and can reset a failed circuit. The layout appears to be compliant, and also sensible."
Maximum take-off weight is now frozen at 1,815kg. GA set out to attain an 815kg disposable load but ,early in the certification, it agreed to advance that to 915kg, and is now confident that, eventually, another 90kg could be justified structurally and on performance. Part of this gain comes from the unexpected aerodynamic performance, explains Furlong: "We attribute it to fuselage lift. We're seeing figures on the amount of load the strut should take, based on allowing for the wing going through the centre section, but the struts are pulling 5-10% less according to the strain gauges. This means the wing has a lower load than forecast, with less tip loss and other drag, and a better coefficient of lift. Fuselage lift usually comes with the baggage of extra drag, but the fuselage drag is quite low in this aeroplane, and the windscreen shape and windscreen contribute to lift."
GA test pilot David Wheatland assured me the Airvan is so unusually easy to handle that even extremely low-time pilots find it comparable with smaller aircraft such as the four-cylinder Piper Cherokee, and quickly become fully confident. Conditions were close to ISA, with outside air temperature at 17¹, and about 10kt (18km/h) of crosswind on our chosen runway.
Taxiing visibility is good from the pilot position, which is well ahead of the leading edge, with the wingtips in the field of view and the surface visible about 6m ahead of the aircraft. The angle of upward visibility is about 45¹, and turn visibility in flight is also good, again because of the pilot's eyeline being well ahead of the leading edge. Overhead transparent panels provide improved visibility in steep turns, and the bubbled window enabled me to check for other aircraft behind, before run-up.
Although the test aircraft was not lined or soundproofed, noise levels were quite acceptable with headphones on, and vibration was low. Nosewheel steering is relatively light and positive, and steering advantage has been adjusted so that the aircraft turns around the inside wingtip. Wheatland commented that the test aircraft was "slightly under-braked" at gross weight, and that the final braking configuration is still to be set.
The simple slotted flaps are positioned at 16¹ for take-off and 38¹ for landing. Setting the elevator trim to a new centre-of-gravity (CG) position initially requires considerable trim wheel movement, but, once trim is set, in-flight movement is small, and any out-of trim condition can be corrected with very modest stick forces.
Despite a fully forward CG, the Airvan lifts itself off comfortably at about 55kt with about 4kg of back pressure and no subsequent sink, and with brisk acceleration to the flap-retraction speed of 80kt. Recognising that a multi-role aircraft like the Airvan is likely to be operated under instrument-flight rules, GA has provided for autopilot operation with high directional stability. Because of the wing and tailplane are almost in-line, the down-flow off the wing with flap almost exactly counteracts the flap trim change once the Airvan is trimmed for the weight position. The pilot really needs to make little trim adjustment for the whole flight. Neither aileron nor rudder trim is provided, but with light control forces, neither was missed.
During climb at 80kt (150km/h) trim speed, the nose is only about 150mm above the horizon, and flight visibility is well above average. We set an initial climb rate in mild turbulence of 750-800ft/min (5m/s). Using a modest power setting and 2,500rpm /75% power, we reached 6,000ft in just over 8.5min. Speed stabilised in cruise at 120kt indicated airspeed (128kt true airspeed) with power at 2,400rpm (70%). At only slightly less power, fuel flow was 1litre/min for 120kt (ie. 0.5l/nm). Trim change with speed is modest, and controls are well-balanced.
Phugoid oscillations are damped in all configurations, with damping at its least with 15¹ flap and power on. Typically, in the cruise, the amplitude is halved within three cycles. A roll from 45¹ left to 45¹ right took about 3s, and rudder-induced yaw was overcome almost immediately by the aircraft's quite remarkable directional stability. Even in steep turns, stick forces remain relatively light, and the overhead-vision panels are appreciated. I found the aircraft comparable in every way with the better-handling US light singles, and particularly docile at low speeds
Control response
With the engine throttled back to 2,200rpm, the speed settled at 100kt for 45litres/h fuel consumption - very little more than for a Cessna 172 at the same speed, while carrying more than twice the payload. Control response in all three axes was still firm and positive, directional stability exceptional and a 45-45¹ roll took about 4s. Apart from the greater inertia, the Airvan's handling closely resemble that of a typical small four-seater. Wheatland says that control forces were initially even lighter, but had to be increased to meet certification specifications.
In all configurations there is ample elevator resistance and moderate aerodynamic warning of the onset of the stall, which is extremely mild with no tendency to drop a wing. The aircraft remains controllable by both rudder and aileron well beyond the stall, which can be arrested immediately with power alone. In a power-off clean stall with deceleration at 1kt/s, G-break occurred at 58kt. With landing flap, the Airvan stalled at 54kt and a height loss of about 100ft.
In a power-off glide at 65kt with landing flap, sink rate is around 1,000ft/min, and recovery from a gliding descent is accomplished with only about 20ft of further height loss. In a clean glide at 80kt, rate of descent is under 800ft/min.
In the circuit, the aircraft feels comfortable through the range of speeds from 65-120kt and, again, trim changes are minor and infrequent. The light controls and ease of handling, however, can lead a pilot to forget that the aircraft is relatively large and heavy, and to underestimate the time needed to slow to the flap-extension speed of 100kt. Circuit visibility is excellent, controllability close to the ground is superior, and the aircraft can easily be landed gently, even with a forward CG. Crosswind handling is good, and Wheatland expects the Airvan to be certificated for a crosswind above 20 kt.
The Airvan is a rugged, economical and versatile utility aircraft with honest and impressive performance, attractive features lacking in competing types, superior and uncomplicated handling and an expected price tag which would seem to guarantee it a notable position in the marketplace. It would be surprising if the type did not also ultimately develop into a tough competitor in the market dominated by Cessna's Caravan, and as an alternative to Pilatus Britten-Norman's Islander.
Source: Flight International