Christopher Yeo/SACHON, SOUTH KOREA
Keen to apply aircraft development skills honed on licence manufacturing, South Korea's aerospace industry is facing a major test as it awaits an order for its first indigenously designed product - the KT-1 Woong Bee turboprop trainer.
The South Korean air force has completed operational evaluation of the KTX-1 prototype and is expected to order an initial 85 aircraft. Daewoo Heavy Industries (DHI) is now gearing up to begin production of the KT-1 in August 1999.
Award of the production contract will mark the beginning of a new stage of the country's aerospace industry. South Korea has already embarked on a far more ambitious programme to develop the KTX-2 supersonic advanced trainer and light combat aircraft, which will be a joint effort between Samsung Aerospace and Lockheed Martin.
The KTX-1 was a more modest starting point for the South Korean industry, and development has not been without its challenges. The design is now ready for production, however, and Flight International is the first magazine to flight test the aircraft.
The KTX-1 was designed to fulfil the basic trainer role with the South Korean air force, replacing the Cessna T-41 grading aircraft and T-37 basic trainer. The students will then progress to the British Aerospace Hawk. Flight International flew the fifth prototype shortly after the aircraft had completed air force operational evaluation testing, a major milestone for the programme.
The KTX-1 is the first aircraft to be designed and developed in South Korea, and it is no mean achievement for the programme to have reached this stage. The fifth prototype is representative of the production aircraft. It had flown 154h at the time of our test; the whole test programme from 1991 to date has accumulated 1,146 sorties and 1,444h.
BUSY AIRFIELD
The KTX-1 prototypes are based in a dedicated hangar on the north side of the busy joint civil and military airfield at Sachon in the south. The hangar and test facilities, which are well away from the bustle of the main military activities on the airfield, include a telemetry centre so that flights can be monitored in real time, using data transmitted from the aircraft.
Sachon is the South Korean air force's main training base and there was a constant stream of T-37s and Hawks in and around the circuit. The Korean Agency for Defence and Development and Daewoo use South Korean air force test pilots to fly their prototype and production aircraft. I was to fly with Maj Choong-Hwan Lee, a graduate of the National Test Pilots' School in Mojave, California.
The disciplines followed for this evaluation will be familiar to any test team: an initial review of the test plan, followed by a formal briefing of the objectives, the detailed test profile and flight safety. The flight was tracked by the telemetry team from the ground, each test point being announced before it began. Everybody in the team, therefore, knew in advance what was going to happen during the flight to maximise safety.
The KTX-1 is of conventional, all-metal, construction with a straight wing set low on the fuselage below a large single-hinged canopy. The flight controls are mechanical, but with electrically operated trim tabs. From the outside, the aircraft looks well made and solidly constructed.
The wing is free of stall strips and fences. The fourth prototype has a different wing section and leading-edge stall strips and there are split flaps under each wing trailing edge and a single airbrake under the fuselage.
The fuselage features several large access panels covering flight test equipment, as well as the UHF, VHF, interrogation friend or foe and TACAN boxes.
The walk-round checks easily and quickly accomplished, I boarded at the left wing-root trailing edge via an integral ladder, which is pulled down from the fuselage. The large canopy, which is fitted with linear detonating cord to fragment the transparency during an ejection, is hinged on the right and gives wide access to the Martin Baker Mk16 ejection seats. Strapping in is straightforward and familiar to any pilot who has used a Martin Baker seat. It is easy to get comfortable, a refreshing feature of modern ejection seats.
The cockpit has plenty of room, although I had to move the electrically adjustable rudder pedals fully forward to accommodate my leg length. The control column and throttle fell naturally to hand.
Most of the cockpit units are standalone instruments and control boxes, although the primary flight displays and engine instruments are electronic. The result is a pleasant instrument panel that blends the essential information in a natural way that is easy to assimilate. The engine instrument display is particularly efficient, making good use of analogue formats, including colour coding, so that values within an acceptable range are presented in green, while those approaching a cautionary limit are yellow, and those outside the normal range are red.
The exact numerical value of each parameter is presented digitally in a box under the analogue format.
After closing the canopy and completing a left-to-right check, it was time to start the engine. For my two flights in the KTX-1 (one from the front seat and one from the rear seat to get the instructor's perspective), a ground power unit was used. Starting a Pratt & Whitney Canada PT6 is a busy time for a pilot unfamiliar with the engine since several switches have to be operated in sequence, and engine temperature and oil pressure monitored throughout the start cycle.
The engine accelerated rapidly, the inter-turbine temperature (ITT) peaking at 735°C, comfortably below the normal maximum of 800°C, and was idling with the propeller unfeathered after 20s.
The weather for the test flights was calm, warm and sunny, with an outside air temperature of 22°C, although naturally it felt much warmer once the canopy was closed. It was a pleasant relief, therefore, to start the environmental control system (ECS) and have a constant stream of cold air while the post-start checks were completed. This is a prototype and so the instrumentation was turned on and the telemetry team checked in before taxi clearance was requested.
Sachon is a busy airfield, so it was necessary to adhere to a slot time system to fit in with the training squadrons. These units were busily making up for lost time caused by typhoon Janni, which had crossed the airfield, bringing thick low cloud and heavy rain for a couple of days before my test flights.
Once the chocks were away, the aircraft needed a small amount of power to move off the ramp, thereafter taxiing comfortably at idle power. The wheelbrakes were smooth and progressive. While the aircraft could be steered using differential braking, this slowed it markedly and it was more efficient and comfortable to use the nosewheel steering system. The pedal forces were quite high with the steering engaged, but not excessive. The steering was direct and the aircraft could be accurately positioned.
Despite my unfamiliarity with the aircraft, I managed without difficulty to complete all the preflight procedures and taxi to the holding point within 30min.
After completing the "last chance" checks at the holding point, we were cleared to take off from runway 06R - Sachon has parallel 2,750m (9,000ft) runways. The technique used was to hold the aircraft on the wheelbrakes at full power (46% torque and 768°C ITT, ECS on), check the engine and then release the brakes. The second take-off was made with the ECS off to determine the performance advantage.
The aircraft accelerated rapidly to the rotation speed of 65kt (120km/h). Using the nosewheel steering, it was easy to keep straight on the runway centreline. At the time of these flights, the final take-off trim setting had still to be finalised and, during the initial climb-out, there were some out-of-trim forces in all three axes to be corrected. Once clear of the ground and climbing positively, the undercarriage was raised (taking 9.5s) and the flaps retracted (3s) at 110kt. There was a slight nose-down change of trim as the flaps were raised.
POWER REQUIRED
Turning left out of the pattern, the aircraft quickly settled into a climb at 140kt and 2,500ft/min (13m/s), with ECS on - 2,900-3,000ft/min, ECS off.
These figures illustrate how much engine power is required to drive an effective ECS. The KTX-1 is not unique in this respect. In my experience, other turboprop trainers are similarly affected to a considerable extent. The pilot has a choice: use the ECS and lose some performance, or turn off the ECS, gain performance and rely on ram air cooling.
I quickly settled in to flying the KTX-1. The controls during the climb were well harmonised and the aircraft could be trimmed accurately. The climb was continued to 12,000ft to put the aircraft above an air route north of Sachon. During the climb, and throughout the flight, I noted a mild random vibration through the stick, probably caused by wash from the propeller. This effect did not intrude to any extent and was soon forgotten.
Once above the air route, the aircraft was operated between 12,000ft and 15,000ft, climbing later in the flight to 19,000ft for spinning and maximum dive-speed tests, and descending to 5,000ft north of the airway for a level speed check. With the ECS on, it took 7.5min from brake release to 12,000ft. From start-up to top of climb, 50kg (110lb) of fuel was used.
The testing started, logically, with getting to know the aircraft in the middle of its speed envelope at 180 ±20kt. Test pilots can wax lyrical about stability and the testing to establish it; my assessment of the KTX-1 can be summed up by noting that the dutch roll was heavily damped, the pitch stability deadbeat back to the trim condition and the aircraft was just spirally unstable, although this latter feature could be masked by small variations in trim.
The controls at low and medium speeds were well harmonised and the control forces satisfactory. Both the ailerons and rudder were powerful. There was some adverse yaw associated with aileron inputs, but this was easily countered by leading aileron inputs with a small amount of rudder. I would argue that this is a reasonable feature to teach control co-ordination.
At 180kt and 13,000ft, the aircraft sustained 2.1g with the ECS on, and 2.6g ECS off, and achieved a maximum 4.2g. Rapid rolling was brisk, with excellent roll acceleration; 60° to 60° banked turns required 1.25s, while 360° rolls took 3s, similar to some jets.
Steady heading sideslips to full rudder with the undercarriage and flaps up showed progressive build-up of sideslip, bank angle and control forces. The final sideslip angle was guessed at 12-15°. With the undercarriage and flaps down (120-130kt), the same general characteristics were present, but a mild lightening of the rudder forces was noted over the last few degrees of input. During all full-rudder sideslips there was a mild nose-down change of trim just before the rudder reached full travel.
I doubt that most service pilots would notice either of these effects.
Stalling tests were made in the clean and landing configurations in straight and turning flight at 14,000ft. The wings-level clean stall was made with the engine at ground idle. There was no discernible natural airframe buffet until the stall at 78kt, but adequate alerting was provided by a loud voice warning at 90kt and a vigorous right pedal shaker at 85kt. Even the most unthinking student would be hard put to ignore either of those warnings.
At the stall, the aircraft wandered slightly in yaw and roll and, if this were not corrected, would gently drop a wing. Provided that the pilot stayed actively in the control loop, and worked on the rudder to minimise sideslip, the aircraft could be flown until the stick was fully back, when the nose pitched gently down.
With the undercarriage selected down (taking 8s), the flaps at landing setting (8s) and the engine set to flight idle, the audio warning occurred at about 80kt, the pedal shake at 75kt and the stall, a moderate left wing drop, at 65kt. Recovery from any stall was immediate as soon as the angle of attack was reduced.
Turning stalls at 30° of bank in the landing configuration (simulating a student looking over his shoulder at the runway threshold and indifferent to decreasing airspeed) were essentially the same; audio warning at 82-85kt, pedal shaker at 80-81kt and a left wing drop through about 30° at the stall. Opening the throttle gave an immediate power response and quickly accelerated the aircraft.
The audio warning and pedal shake systems remained active in the clean configuration and so provided warning of high angle of attack at all speeds. The aircraft characteristics during dynamic turning stalls were benign. If the pilot ignored the warning and continued to increase the angle of attack, the aircraft simply started gently to pitch nod as the wing alternately stalled and unstalled. Given the good handling of the aircraft in this configuration, it would be practical to disable the stall warning with the undercarriage and flaps up.
HIGH SPEED TEST
Having explored the low speed handling, it was time to examine the other end of the speed envelope and the aircraft was dived to 5,000ft. In level flight, the aircraft achieved 250kt with ECS on and 258kt ECS off - comfortably above the South Korean air force specification of 250kt at sea level. Control forces remained satisfactory and the cockpit environment comfortable and quiet.
Zooming up at full throttle to 19,000ft, the aircraft was prepared for the maximum speed tests. The normal maximum operating speed in service will probably be 320kt. For the purpose of this test, the aircraft was dived to 340kt, 10kt below the maximum cleared speed. The test technique was to half-roll inverted, pitch down to 45° and accelerate at full power. The aircraft gathered speed quickly and the whole test from entry to level off was completed between 19,000ft and 12,000ft. At lower altitude, less height would be required.
While the noise in the cockpit increased somewhat, inter-cockpit and radio communication remained clear. There is an overspeed voice warning at 340kt, but this is rather quiet considering its importance and notably less strident than the stall warning. The control forces increased during the dive, but did not become excessive and the recovery from the dive was accomplished without difficulty.
The one problem that was noted was that the ailerons became over-sensitive above 320kt. My opinion is that this characteristic is probably acceptable. I came to this conclusion because the effect is not noticeable within the normal service envelope.
Furthermore, throughout the rest of the flight envelope roll control is very good, combining precise control, good roll acceleration and a high roll rate. It would be a pity to compromise these features to correct a phenomenon at the extreme of the speed range.
One unusual facility fitted to the fifth prototype KTX-1 is a rudder automatic trim system. This equipment uses a map of airspeed and torque, together with sideforce, to set the rudder trim. This reduces the need for the pilot to set trim constantly, as is usual in high-powered single propeller aircraft. I found the system to work well. With the rudder auto-trim engaged, the degree of yaw trim activity by the pilot was reduced, particularly during smooth throttle operations.
The system does not entirely eliminate the need for yaw trimming (accomplished by a rocker switch conveniently mounted on the forward face of the throttle), but then, if one subscribes to the opinion that a trainer should teach control co-ordination, perhaps it should not. During aggressive throttle slams, the system is limited in authority to the maximum rate of the yaw trim motor and the aircraft will yaw in response to the power change until the trim motor catches up.
During most of the flight, rudder auto-trim is transparent to the pilot. The system is fitted to give the KTX-1 more "jet-like" handling. With the auto-trim turned off, the aircraft simply needs a bit more care and attention from the pilot in order to maintain balanced flight, something which I found was easy and natural to accomplish.
AEROBATIC MANOEUVRES
The aircraft is cleared for most of the basic aerobatic manoeuvres. Performing aerobatics at a higher altitude than we would have liked (12,000-19,000ft) - to be above the air route - the aircraft performed well and completed loops, barrel rolls, slow rolls, inverted flight (30s maximum) and Cuban and horizontal eights. The aircraft did not need more than maximum continuous power to maintain height and could be flown with precision. Given the powerful rudder, slow rolls were easy to accomoplish - even for a pilot who has not performed aerobatics for some time.
The aerobatic phase was completed by a nose-high, controls-central recovery to simulate a student who has become disorientated during manoeuvring flight. The aircraft was climbed at 60° at full power and the speed allowed to decay. As the nose yawed gently left under the influence of the torque, the throttle was closed to idle and the controls centralised. As the speed decreased further, the aircraft pitched smoothly down into a dive with a slight residual roll to the left.
The general handling and aerobatic testing showed the engine and propeller to be well matched. Constant-speed propellers provide quick response to throttle inputs while the propeller is at its governed RPM (2,000RPM in the KTX-1). If, at low power, the propeller slows below the governed RPM, then a subsequent throttle opening will not produce useable torque until the propeller reaches the governed speed. As this happens, there can be a rapid increase in longitudinal acceleration as the engine and propeller settle to a steady state condition. This characteristic is well masked in the KTX-1 and the acceleration was smooth even after a period at low power.
The final high-altitude test was to spin the aircraft erect and inverted. The South Korean air force minimum height for spin entry is 18,000ft, so we added a 1,000ft comfort margin and started at 19,000ft. Before initiating a full spin, the aircraft's ability to recover at the incipient stage with the controls centralised was tested. After a conventional spin entry 5kt above the stall (85kt), pro-spin control was held for half a turn, and the controls were then centralised. The aircraft smoothly recovered to controlled flight after three-quarters of a turn, losing only a few hundred feet.
The KTX-1 is cleared for six-turn spins, but, because of the limited height block available in the spinning area, we elected to recover after four turns. From entry to recovery these spins used 4,000-4,500 ft. Spins to the right were a little more oscillatory than those to the left, but both directions were comfortable. The first turn was in a fairly flat attitude; thereafter, the pitch attitude dropped progressively. Each turn took about 3-4s.
The pitch attitude varied between about 45° and 60° nose down, and the roll and yaw rates varied steadily during each turn. The spins became more oscillatory during the fourth turn. Applying one-quarter out-spin aileron stabilised the spin motion, remarkably so to the left, when the aircraft adopted a steady nose-down pitch attitude of 60° with constant yaw and roll rates.
The aircraft was first recovered using the conventional technique: opposite rudder and stick smoothly forward to neutral. The KTX-1 regained controlled flight in one and a quarter turns. From the next spin, the controls were released at the end of the fourth turn. The aircraft recovered smoothly to a steep dive after a turn and a half, an excellent feature for a basic training aircraft.
Lee then demonstrated the inverted spinning characteristics. The technique used was to put the aircraft into a climb, roll inverted, apply pro-spin controls and retard the throttle to ground idle at the same time. The spin was oscillatory during the initial stages, but progressively settled towards a more steady, recognisably inverted, attitude by the fourth turn. At this point, the spin rate was about 2s/turn at -1.6g. The aircraft took one and a half turns to recover ,using opposite rudder and stick neutral.
Overall, should the air force clear inverted spinning for service use it would be an excellent teaching tool for an instructor to demonstrate to a student, but it is just a bit too disorientating and oscillatory for a solo student. The erect spin is benign and comfortable and should give a student confidence that he can recognise and correct an out of control condition.
BACK TO SACHON
At the end of the spinning phase, we recovered to the pattern at Sachon to meet our slot time. The descent was made at 200kt, with 20% torque set. This condition gave a comfortable descent rate of 2,000ft/min . When a more rapid descent was required, the speedbrake increased the rate of descent to 4,500ft/min at the same torque setting. The speedbrake did not affect trim, but caused a degree of airframe rumble.
A break into the circuit was made at 200kt from 1,000ft above ground level. A brief selection of airbrake brought the speed down to 150kt, the maximum speed for undercarriage and flap selection. Downwind, the undercarriage and half flap were lowered to achieve 100kt at the start of the final turn. Full flap was then lowered in the base turn to achieve a touchdown at 80kt.
Even allowing for the still wind conditions, the aircraft was easy and straightforward to fly and position accurately. A similar circuit flown from the rear cockpit demonstrated that the instructor has a good view over the student's ejection seat, the touchdown point remaining in view until shortly before the flare.
The KTX-1 has two engine idle settings - ground and flight. Both are cleared for use in flight, but it is recommended that the throttle is not reduced below flight idle during the approach. The propeller drag is high and the aircraft can develop a high rate of descent. Flight idle and maximum continuous power are marked by detents in the throttle box and can easily be overridden when required.
Flapless circuits are equally as easy to fly as normal circuits. The pilot has only to add 10kt, fly a slightly longer final approach and monitor the incidence indicator rather more closely.
Glide patterns, to practise an engine failure condition, were made with 4% torque to simulate a feathered propeller. High key, over the centre of the runway, was at 3,000ft, where the undercarriage was lowered (using an emergency nitrogen bottle in the event of a real failure). Low key, opposite the touchdown point, was at 2,000ft.
The South Korean air force does not recommend the use of flap during the forced landing pattern, although it is available once the undercarriage is down, using the same nitrogen bottle. This is probably a sensible compromise when using Sachon's 2,750m runways but, given a shorter landing area, or a less than ideal situation, I would use the flap to refine the touchdown point and reduce the landing roll. In the event, I compromised and sideslipped off the extra height.
STRAIGHTFORWARD RETURN
After landing, using ground idle (once the nosewheel was on the ground) and gentle braking, the aircraft was turned off the runway at the first available exit, at the 3,000ft marker board. As with take-off, keeping straight was not difficult using the nosewheel steering. Thereafter, there was just the formality of the after-landing checks to complete before returning the aircraft to the ground personnel.
Each flight lasted 1h 15min and used about 215kg of fuel, or just under half of the total capacity. Much of both flights was made at or near to maximum continuous power. It was not necessary to balance fuel or to manage other systems at any time during the flight.
In summary, the KTX-1 is an impressive achievement for the South Korean Agency for Defence and Development, DHI and its subcontractors. The aircraft is well thought out and, if it is ordered into production, it will make an excellent basic trainer.
No aircraft is perfect, but the minor faults that the KTX-1 has do not detract from this assessment. I enjoyed flying the aircraft and quickly felt at home with it.
The KTX-1 could be flown accurately and it will tolerate the inevitable insensitive handling by student pilots. It has a good performance which students will initially find demanding. It will be interesting to see whether, in future, the South Korean air force returns to the idea of a lower-powered aircraft for grading before basic training on the KT-1.
The Korean script on the background of these pages is the name of the aircraft. Freely translated, it means "good flying". This seems entirely appropriate for the KTX-1.
PHILOSOPHY
Soon after jet fighters entered service, a debate began about the type of trainer that should be used to prepare pilots for high-performance aircraft. It quickly became accepted that advanced training was best carried out using jet aircraft, often two-seat versions of front-line aircraft.
As the performance of operational aircraft increased, the use of two-seat versions for training became impractical, both for reasons of expense and because the performance jump from the basic trainers in use was more than the average student could handle. A range of advanced jet trainers was therefore developed and put into service.
The arguments about the desirable requirements for a basic trainer were less clear cut. There were those who advocated all-through jet training and argued that only those flying skills associated with jet aircraft should be taught. The alternative to all-through jet training was to make use of powerful propeller-driven aircraft, both because of the economy of operation and because the torque effects of the propeller promoted control co-ordination skills.
There was also a fierce debate about the relative merits of tandem versus side-by-side seating. The argument for the latter was (and is) that the instructor can see exactly what the student is doing and encourage good practice. The proponents of tandem seating pointed out the performance gained by reducing the frontal area of the aircraft, and the fact that the student pilot was placed into something akin to an operational environment from the start of training.
The debate has largely been settled in favour of a tandem-seat basic trainer with a turboprop engine in the region of 500-800kW (670-1,100hp) output. Usually, a simple piston-engined type is retained as a grading aircraft. The graduates from basic training who are selected to fly front-line fighters then continue to advanced training on a jet aircraft.
Source: Flight International
