The Bell TH-67/206 is being offered in the UK's all-service military-helicopter training competition.The BELL TH-67/206 is one of the main contenders for the UK Ministry of Defence (MoD) requirement for ab initio and advanced training helicopters to train all services at a combined defence-training school. It was put through its paces by Flight International and proved to have good student pilot-handling qualities with wide margins of safety and outstanding auto-rotation characteristics.
The contract will be to provide 32,000 flying hours over ten years and the chosen contractor will be expected to supply everything. The three main bidders are Short Brothers of Belfast, Bond Helicopters (teamed with Hunting Aviation) and FBS, a conglomerate formed by FR Aviation, Bristow Helicopters and Serco-IAL. The bids are now in and a decision is expected in May. Two of the three main bidders are offering the TH-67/206 (a version of the US Army's Bell 206-based TH-67 Creek training helicopter).
I flew the instrument-trainer version, which has a large instrument panel giving each pilot his own set of flying instruments, three attitude indicators, two electrical systems, some stabilisation and a force-trim system which allows the pilot to select a neutral-stabilised cyclic-stick position. There is also a visual-meteorological-conditions (VMC) version with fewer aids and instruments. The MoD will probably produce a specification somewhere between the two.
The TH-67/206 is founded on the commercial Bell 2063B JetRanger. Bell has manufactured nearly 8,000 models which have amassed more than 32 million flying flying hours.
The main changes in the training version are the provision of three 30G-crashworthy seats (one for each pilot and one for the only other occupant, a student in the back seat who has his own basic instrument panel and who can view and hear what is happening in the cockpit between the instructor and his pupil). For instrument flights, the back-seat student can also serve as the look-out safety pilot.
The fuel system is crashworthy and the undercarriage skids and cross-tubes have been reinforced to accept the many landings which training aircraft carry out, including running landings with sideways drift - a critical and highly stressful manoeuvre on any aircraft, but very common when the student pilot is practising or demonstrating engine-off or hydraulics-off landings. Another major change is the provision of five-point harnesses for all occupants. Bell says that the aircraft has good roll-over protection. The IFR aircraft can be used for all VMC training.
Sam Boyer, one of Bell's senior test pilots, showed me round the aircraft. The weather was benign for test-flying purposes at 300ft (90m) pressure altitude (+6¡C), but there was a 17kt (30km/h) wind - right on the US Federal Aviation Administration certification limit. Weight was 1,270kg - 160kg below maximum. The centre of gravity was at the mid-point. I like to test-fly aircraft at maximum gross weight, but this was not possible for various reasons.
I noted the ease of access to all the vital components of the pilot's pre-flight inspection including the main rotor-head and blades. I recognised the Allison 250-C20JN engine and AlliedSignal Bendix fuel-control unit (the "N "signifies that there is an extra pad on the engine for providing a second generator). The 315kW (430shp) engine is flat-rated to 240kW, so allowing full power to be available at altitude and also giving the engine an easier time with longer life and fewer problems at lower altitudes.
The two-bladed tail rotor has been lengthened and increased in pitch over the years to cope with the high altitudes of which the engine is capable. One of the limitations which can quickly exhibit itself when trying to land heavy helicopters at high-density altitudes (and these can occur at close to sea level in hot deserts, for example) is the lack of sufficient yaw/pedal control. Helicopters have been known to run out of directional control in these conditions with trainee or other inexperienced pilots at the controls. I was to investigate the power of the tail rotor with fast sideways flight and rapid spot turns during the flight.
I noted the high-inertia metal main rotor-blades with weights embedded halfway along and at the tips. I looked forward to the subsequent autorotation qualities. The blades are attached right at the hub (ie, zero offset). This reduces gust sensitivity and increases stability, making a good instrument platform, especially for student pilots. Similar two-bladed tail rotors have been flown deep into blade stall with no adverse effect. I noted the engine exhaust stack on the roof of the aircraft - causing no safety problem with rotors-running refuelling or excessive heat for bystanders. The landing light is steerable, as it should be for night flying.
Getting into the right-hand pilot's seat is straightforward, as is installing the five-point harness, which can be locked, or selected, to allow the occupant free movement, relying on the inertia-harness reel to lock it in the event of a sudden deceleration. The seat is comfortable and I was able to adjust the pedals so that I could rest my right forearm on my right thigh to acquire the best cyclic-stick control for hover manoeuvres. Headroom is adequate - although the seats have no up/down, forward/back adjustments, the cockpit has been designed to accept all but the very tall or very short. Visibility, so important for a training aircraft being flown solo in busy airspace, is good; the door pillars are thin and do not impede vision excessively.
Communications with the occupants of the other two seats and the outside world is good. For an instrument-flight-rules aircraft, stowage of instrument plates, manuals, charts and the other paraphernalia of instrument flying is poor - just the two front-door pockets. The cockpit area is quite small for an instrument-flying aircraft, so it would have been refreshing to see stowage areas designed into the existing space. Telescopic clipboards to hold a letdown plate attached, say, to the door pillars, would be a useful addition.
All other aspects of the cockpit ergonomics are pleasing. The warning lights are arranged in line at the top of the instrument panel, everything can be reached and read by both pilots and the instrument panel is suitably shrouded from sunlight effects. The instruments are legible and their limits adequately placarded. There is a low-rotor-RPM warning system - an audio and a light at 90% RPM. The aircraft will still fly at 90% RPM.
There is enough space to install any extra avionics an operator might require. The overhead panel contains circuit breakers, the rotor-brake handle and some systems switches - all adequately labelled and accessible and all lit for night flying.
I tried the collective lever, cyclic stick, pedals and the throttle on the end of the lever for full and free movement and found the results satisfactory. Lever and cyclic frictions are nicely balanced so that the controls stay where you put them, but the pilot can over-ride them if necessary. They are particularly useful during engine/rotor start when the prudent pilot will use both hands - one on the throttle and the other on the starter button and idle-release button to allow the idle stop to be released and the throttle closed quickly should something go wrong.
The engine was still hot from a previous flight, so I concentrated particularly carefully on the turbine temperature during the start. The engine lit up immediately the throttle was opened, peaking quickly on turbine temperature. There is no need or requirement with this engine to adjust the throttle during the start. All the electrics, radios and other relevant systems are switched on and, after a few basic checks, the throttle wound fully open. With the RPM adjusted with the beeper trim switch on the lever, we were ready to go.
I had not flown a JetRanger for six years and recall that, even after a short break of just two weeks, there is a tendency to overcontrol in the hover. I was pleasantly surprised at my first attempt, which turned out to be very steady and accurate. This is probably because of the additional stabilisation components installed as part of the instrument flying package. The power instruments (torque, engine temperature and compressor speed), although small, are easy to read and interpret; maximum continuous take-off and full-power parameters are clearly shown. Because we were not heavy, and the day was cold, we had plenty of power in hand. Visibility was good, despite the large instrument panel. Noise levels were acceptable and Boyer and I could confer quite easily over the intercom. Radio calls were satisfactory, as they should be in a training aircraft.
Landings and take-offs are also trouble-free - there are no unexpected lurches of attitude as you feel the aircraft off the ground, trying to guess the actual hover attitude. It sits in a fairly level, comfortable, attitude, slightly left-skid low - all beneficial for a student pilot.
One would not normally expect an early student to do far-out-of-wind-hovering manoeuvres in a 17kt wind. Control difficulties can lead to loss of control and confusion. From an into-wind hover, a turn to the left through 90¡, hover, landing and take-off (another 90¡), we were now downwind - hover, landing and take-off, and so on, all the way round. The exercise was trouble-free. Control, aircraft attitude and visibility were adequate throughout, with plenty of pedal and power in reserve.
I then tried sideways and backwards flight, beginning with a modest 10kt sideways to the right, gradually increasing to an estimated 30kt. The helicopter is remarkably stable at this speed and I still had some left pedal available - the first limit normally reached in such a manoeuvre. A similar manoeuvre to the left showed similar stability, but I now needed full right pedal. Fast backwards speed, produced by applying a lot of rearward cyclic stick, can produce sudden, quite severe, nose-down pitching in most helicopters, requiring even more rearwards stick, but I found all these effects comparatively benign as I took the TH-67/206 backwards to about 30kt.
Boyer and I combined rapid spot turns with a demonstration of a tail-rotor drive failure causing the helicopter to spin rapidly to the right. Boyer applied a large "bootful" of right pedal to get it going round. We spun rapidly on the spot and, as we approached the into-wind heading, he chopped the throttle, doing nothing further with the pedals. As the spin stopped, he cushioned the aircraft gently down with the lever before it started spinning the other way.
Student pilots would not normally achieve such speeds in sideways, backwards and spot turns, so when solo, they are unlikely to approach any control limits.
I transitioned into forward flight, pulling take-off power. We shot up to 1,000ft (300m). Reducing power to maximum continuous, the indicated airspeed, which coincided with the true airspeed on this day, settled at 112kt. We were just over160kg below the maximum permissible all-up weight.
While we were still fairly heavy I dived down to achieve the Vne (never exceed speed) of 130kt. Vibration levels, which can be significant at Vne, were benign, handling crisp, as expected at this speed, noise levels low - I could communicate comfortably with Boyer - and turns left and right produced no ill effects or surprises.
We settled down to straight-and-level flight again before conducting steep turns left and right. There is no laid-down limit in the flight manual - just that at which you feel comfortable. I achieved 60¡ both ways. The all-important visibility in the turn is good. Throughout all this, vibration and noise levels are satisfactory. After settling into straight-and-level flight, I investigated rotor droop, with rapid lever-movements up and down - a typical student over-reaction when under pressure. This manoeuvre is particularly relevant in this helicopter since the power-on rotor/engine RPM limits are small. Despite fairly rapid lever movements, I could not get the RPM to move much outside the upper and lower limits - a comforting thought for instructor and student.
Balance control throughout all these manoeuvres is good - pedal is needed in all the turns, but the controls are nicely balanced.
CIRCUIT WORK AND AUTOROTATIONS
We returned to the circuit and I flew a steep approach to a designated spot. Despite the larger instrument panel, forward and downwards visibility is good.
An engine-off landing from the hover is undramatic - a little pedal, a little lever, nothing much with the cyclic, and we were down. In the event of a sudden engine failure in the hover and a solo student doing nothing, the aircraft would probably "land itself" with no damage.
To demonstrate main-rotor inertia, Boyer chopped the throttle, while sitting on the ground at 100% RPM, lifted the aircraft into a brief hover and put it down again.
We returned to circuit height and checked rotor droop (after a throttle chop to simulate a sudden engine failure), doing nothing for 3s, an average pilot-reaction time, before entering autorotation and doing an engine-off landing at the bottom. The aircraft remained stable in straight-and-level flight, with no detectable nose-pitch up or down. There was a slight yaw to the left. After 3s, rotor RPM had drooped from 100% to 90%, a satisfactory (and safe) conclusion.
We entered autorotation. A student would have to be particularly dull to get into low-rotor RPM-control difficulties in such an event.
The rate of descent was a modest 1,400ft/min (43m/s) at our best rate-of-descent speed of 55kt and rotor RPM settled at 100% - within the permissible range of 90-107%. The wind was still 17kt, so we needed to flare off only 38kt to achieve a zero-speed touchdown, a critical test for any helicopter. The flare was started fairly early and built up gradually to arrive in a hover attitude before cushioning the landing. I felt the flare "bite", giving us a temporary, but useful, small increase in rotor RPM - there is no danger of exceeding the 107% top limit - a satisfactory reduction in forward speed to zero and time to sort it out and get it right. There was no rush and, applying collective lever, we plopped gently on to the ground with the rotor RPM decreasing through 60% as we touched down.
We tried various different types of entry and autorotations to the ground, all with the same result - benign handling, lack of concern to controlling-rotor RPM within the generous upper and lower limits and good flare effects. We did not explore the more esoteric techniques available when operating an aircraft with a high-inertia main rotor over hostile terrain such as jungle with small emergency landing-pads in equally small clearings.
You can steer the aircraft down in autorotation with the landing platform displayed between your feet, ignoring airspeed, if there is any, taking a first bite of the lever as the platform looms up, washing off any residual ground speed, levelling and using the rest of the lever to cushion the landing. Even if the landing is heavy, it is a better technique than trying to land among trees or misjudging the approach on to the platform while trying to maintain textbook/flight-manual recommended airspeeds. We did not explore the time-honoured technique, used at night or during poor visibility, of a constant 30kt-attitude autorotation to the ground with a last second pull up on the lever to cushion the run-on landing. In this aircraft, if the terrain is rough, the forward speed can be flared off to zero before the aircraft starts to accelerate downwards.
When students progress to advanced operational-training exercises, they may find themselves downwind at treetop height or even lower, wondering what to do in the event of a sudden loss of power. The high-inertia disc of this aircraft will allow a 3s delay, followed by a steep cyclic climb while the lever is gradually lowered, allowing the helicopter to yaw to the left, indeed helping it round 180¡ with left pedal and landing off the resultant into-wind, nose-down, fully controllable attitude.
The powerful tail rotor, even when in autorotation or during engine-off landings with rotor RPM as low as 60%, has plenty of power to cope with all of these manoeuvres.
An ab initio student gets it wrong when approaching the ground in autorotation, the instructor may need to restore engine power to recover the situation. A last-minute acceleration of the engine from idle to full power is instantaneous with a piston engine, with no danger of overtemperature, or, unless there is a supercharger, overtorqueing. This manoeuvre is much more critical with a gas-turbine engine.
We tried, therefore, an autoration to the ground, but restoring the power at the last moment before touchdown. I found that the engine could be brought back during the flare, winding open the throttle fairly quickly, but keeping an eye on engine temperature and torque. There are generous transient overtorque and temperature limits for such occasions.
The aircraft has only one hydraulic system, which powers all the controls, so a loss of pressure requires the pilot to fly it manually. There is an on/off switch for the system. At my request, Boyer switched off the hydraulics as we took off. Control forces are non-existent until you move them, then you get some feedback and also require some force to move them - but neither is excessive.
A student pilot is advised to choose an area where he can carry out a run-on landing, which is much easier. I chose the worst condition - that of coming to the hover first. I carried out a low, flat, approach using as few control inputs as possible, gradually bringing in the power, feeding in left pedal. It worked out well. We settled in the hover without too much yawing, pitching and rolling, and landed.
Finally, we moved to the slope for landings. Again, there are no limits laid down in the flight manual - they are those with which you are comfortable. With 10¡ nose up (according to the attitude indicator) and 10¡ right-skid on first, the aircraft settled quite comfortably. The worst case of left-skid on first was problematical - we ran out of lateral cyclic before we could get the aircraft to settle on the ground, so we opted for a lesser slope. The military requires parameters of a maximum of 5¡ nose up and down and 8¡ laterally, which the Bell can cope with easily.
A training helicopter does not need to be "easy", but neither must it be too difficult to fly, with generous limits and adequate performance buffers between normal operating conditions and limits of control. Good all-round visibility is essential, as is a strong undercarriage, good internal and external communications. The helicopter must be economic to operate, easy to maintain and reliable.
Finally, should the worst happen and an accident is inevitable, some protection should be given to the occupants. This aircraft has all of these qualities.
Source: Flight International