By Rohan Williams
Scone, New South Wales, Australia - With the steady increase of operating costs, downward trend in trade, and the aging technology employed by aerial agriculture, the dilemma now faced by our industry world wide is to find a cost effective alternative to the purchase of new aircraft. The lack of our ability to justify the exorbitant cost of such new aircraft today is reflected by the current position of the major agricultural aircraft manufacturers. Most have slowed their production line to a crawl if still manufacturing at all.
While the downward trend in trade has contributed to the lack of demand for new aircraft, we are still trading nonetheless. We are still working an aging collective fleet of aircraft with many of those falling into the reciprocating piston engine class. Of course these aircraft will eventually be re-powered with a rejuvenated version of the same piston engine, preceding the same intensive maintenance regime at the expense of the operator.
Stallion Conversions Pty. Ltd. of Scone, New South Wales, Australia, has produced a conversion which has seen the robust FU24-954 Fletcher airframe reach previously unheard of levels of productivity along with a host of other welcome advantages resulting from the conversion. The concept was born of the need for long-time Fletcher operator, "Airpasture" of Scone, to increase the productivity of their existing fleet without losing the advantageous qualities of the Fletcher airframe and without committing to a major capital outlay in the process.
The FU24 Fletcher
Designed by John Thorp in the early 1950s, the FU24 Fletcher aircraft was initially produced in the U.S. by Fletcher Aviation until the sales and then manufacturing rights were sold to Air Parts New Zealand Ltd in 1957 and 1964 respectively. This company, after various mergers and takeovers, is today known as the Pacific Aerospace Corporation and still manufactures replacement components as well as having the ability to produce complete Fletcher Aircraft to order at their facility in Hamilton, New Zealand.
Today's Fletcher FU24-954 has been developed through stages from a 235 hp Continental power plant in the original FU24, through to a 400 hp Lycoming IO-720 A1A eight-cylinder engine. The airframe is a low-wing, all metal monoplane with the hopper located behind the two-place cockpit, affording the pilot unparalleled visibility for rough, short-field operations. The undercarriage is of a fixed tricycle configuration with heavy-duty oleo suspension. The tail plane features John Thorp's well-known "stabilator" which has been prominently featured on other Thorp designs such as the Piper Cherokee, P2V Neptune and the sporty Thorpe T18 home built.
The large continuous-chord wing creates an enviable lifting capacity with docile, but stable, handling characteristics offered by the pronounced dihedral on the outer wing section. The airframe performance remains stable right throughout the airspeed spectrum with the stall being well heralded and free from undesirable consequential outcomes. The FU24 Stallion
The main feature of the Stallion conversion, the first Australian designed turbine conversion of an agricultural aircraft, is its Allied Signal Aerospace (now Honeywell) TPE331-10AV "Garrret" turbine engine. The turbine, while capable of 715 shp, has been flat-rated to 600 shp to allow for extreme operations such as shorter strips and hot/high/overload situations. A McCauley four-blade propeller with Scimitar-shaped blades was chosen for its thrust, high level of efficiency and quiet operation. The need for reduced operating noise levels has been a major concern in the past with the piston powered Fletcher proving to be a hazard for livestock and an annoyance for people close to the treatment areas. Revisions of fuel and electrical systems, as well as instrument layout, have also been incorporated into the conversion. David Simons, Technical Director of Auto Avia Design Pty Ltd, said "This installation has been designed as a simple, no-nonsense conversion to provide safety and reliability in a hard working environment. All performance goals were achieved, climb rates and gradients are particularly impressive. A very comprehensive test and certification programme was conducted to provide data for the award of the STC (Supplementary Type Certificate)."
Auto Avia Design Pty Ltd of Bankstown in Sydney carried out the design and certification of the Stallion conversion. Scone Aircraft Maintenance built the first prototype, VH-EUO in 1996 and the conversion was endorsed by the then Allied Signal Aerospace who gave approval for full life engine warranty on any engine sourced through Allied Signal and installed on the Stallion.
The aircraft was test flown by Ken Hazelwood, an experienced Fletcher pilot who also boasts experience with Garret turbines installed in the Ayres Turbo Thrush. Airpasture Chief Pilot, Kevin Towse, then took over to carry out an intense program of development operations that included on-the-job spreading work.
According to the Managing Director of Stallion Conversions, David Reid, "The FU24- Stallion has consistently shown better profitability than the piston-powered FU24". "Reliability has been excellent. The improved power/weight ratio increases safety greatly and hopper loads do not need to be reduced in adverse conditions. The reduced noise level has attracted favorable comment from farmers and the public."
To give testament to the value of our older airframes and in this case, to the robust nature of the FU24 Fletcher, David, who is also the Managing Director of Airpasture, stated that the second Stallion conversion to be built, VH-EOW, carries a serial number which makes it identifiable as one of the original U.S.-built Fletchers of the 1950s. Of course many of the original components have been replaced numerous times. However, with the appropriate records, this one airframe could be attributable to an unimaginable amount of return over its many years of service.
As can be expected, the greatest advantage offered by the Stallion conversion is the drastically improved power to weight ratio. With a slightly lower empty weight and at least 50% more horsepower available 100% of the time, The Stallion conversion easily outperforms the conventional Fletcher. With the capacity to operate from shorter, higher, warmer airstrips without reducing the payload, the Stallion repeatedly outclasses its predecessor in terms of productivity.
Apart from the obvious advantages of turbine power over that of the piston engine such as increased reliability, service life and reduced maintenance, the decision to install the TPE331-10AV was a calculated one. There are many aspects to the design of this turbine engine, which lend well to agricultural operations and in particular, operations amongst steep terrain.
The single shaft engine allows the pilot an instantaneous power response on demand without the lag experienced with dual shaft engines. This is a particular advantage to agricultural operations with spreading aircraft performing up to 100 take-offs and landings each day while operating within the grasp of the mechanical turbulence associated with mountain flying. Steep, controlled descents may be executed with greater accuracy and safety using the precision of single-lever power manipulation. This, coupled with the advantage of reverse thrust, leads to the ability of the Stallion to land with a greatly reduced landing roll which allows the aircraft to be operated on much shorter airstrips than the piston Fletcher. The TPE331-10AV uses an effective particle separator system that allows solid particles of gravel or sand to pass through the running engine without causing damage internally. The engine operates without the use of air filters utilizing an efficient straight-through airflow through rugged centrifugal compressors without bleed valves. This configuration makes this an ideal engine for remote bush operations due to a reduced need for service consumables and also allows for a lower cost of operation.
The flat rating of the engine in the Stallion allows for minimized stress and wear on the engine and gearbox components under normal operating conditions. This provides an improved possibility that those components will realize the full term of their published service lives, as well as reducing the operational maintenance costs within that period.
This engine currently has a 5,000-hour published TBO, which is enviable for any piston engine operator and also has a considerably lower fuel consumption rate than its equivalent rival turbine manufacturers' engines. David Reid was very happy to remark that during an investigation at the 4,000-hour mark VH-EUO's engine was found to be in excellent condition with ample potential to reach TBO.
The Stallion conversion represents a brilliant combination of a tried and tested airframe and a modern, advanced power unit. The aircraft maintains all of the advantages offered by the Fletcher airframe while affording a much more productive unit being enhanced by the advantages of the turbine conversion. Since the completion of Airpasture's second conversion it has been found that two Stallions are quite easily covering the work that once took four piston-powered Fletchers to complete.
At this stage the FU24 Stallion is essentially engaged in agricultural operations with two being operated out of Scone by Airpasture and another, ZK-TPE being utilized by the South Cantebury Co-op of Fairlie near Timaru on the South Island of New Zealand. However the potential for freight and geophysical survey operations has also been further enhanced by the Stallion conversion as well as many other applications such as parachute dropping or even an air ambulance role.
Airpasture have effectively replaced their old piston-engined aircraft with a "new" turbine engine aircraft, which has more than twice the productive capacity and has a service life that will carry the company well into the future; at a fraction of the cost of a new ag-aircraft.