Photography by Mark Wagner
BELL HELICOPTER TEXTRON introduced two new helicopters at Asian Aerospace '96 at Singapore - the intermediate-sized, twin-engined 430, with its advanced four-blade main rotor, and the manufacturer's latest light single-engined seven-seat machine, also with a four-blade rotor, the 407.
The 407 is an updated 206-L4 LongRanger IV, with the updated four-blade rotor used by the US Army since 1982 on its Bell OH-58D Kiowa Warrior armed scout-helicopter
Although the basic weight of the 407 has increased by about 135kg from that of the L4, its maximum take-off weight has increased by 250kg, to 2,270kg, and the permitted external load weight by 455kg, to 2,500kg. This is one of the many advantages of the four-bladed rotor and bigger engine - a 605kW (815shp) Allison 250-C47, derated to the new increased transmission limits of 305kW. This allows the aircraft to maintain its performance to higher temperatures and altitudes, and also allows increased service life and reliability.
Many other changes have been made - for example, the cabin is 178mm wider than that of the Bell 206 series; the passenger windows are bigger, and bubbled, allowing more shoulder room; the engine has full-authority digital engine-control (FADEC); the tail has been strengthened and fitted with a different tail-rotor design; and the helicopter is faster. There are many other changes. Bell is studying a shrouded tail rotor, which would be useful for the more "agricultural" use of the aircraft.
The weather was good for an evaluation flight - 28°C (ISA +13) at sea level, giving a density altitude of 1,500ft (450m) with 5kt (10km/h) of wind. We were unable to ballast up to maximum weight on the first flight, but achieved 2,180kg on a subsequent flight, which is just 90kg short of maximum weight.
John Williams, a Bell test pilot, showed me round the aircraft. As we approached the aircraft, I noticed its sleek appearance - the flush door handles and the smooth, faired, surfaces to reduce the aerodynamic drag. I expected some high forward speeds as a result.
I noted the use of composite materials for the main- and tail-rotor blades, cowlings, aft fuselage, sidewalls and doors; I also noted the upgraded transmission, engine and tail-rotor oil-sight gauges, new tailboom design, a hinged fuel cap (simple, but effective, it prevents damage to the fuselage) and new material for the belly and nose. There was, however, only one foothold to allow climbing up the side to inspect the rotor system. It is on the left-hand side and high up, and you have to kneel on the roof with the other leg.
The hub is composite and has fewer parts than those of older models, allowing less maintenance, greater reliability and life and less weight and corrosion. The blades and hub have no life limits - just on condition maintenance. Tip speed is comparatively low (45.000ft/ min/230m/s), reducing noise. This aircraft, however, can cruise at 130kt at maximum continuous power, thus increasing the tip speed of the advancing blade to 300m/s, very nearly Mach 1, which can generate noise and compressibility problems, although Bell seems to have overcome these. The rotor can be started safely in up to 60kt winds.
There is a 115kg, 0.56m3 (19.8ft3) aft baggage-compartment. Our aircraft also had the optional 76litre auxiliary fuel tank here, but there was still plenty of room.
All the doors except the baggage compartment have gas struts to hold them open - it has taken the industry a long time to achieve this essential modification. Propping open the door with one's foot while waiting for passengers, rotors running in high temperatures, or having a gust of wind slam open the door and cause damage, is no longer necessary or possible. These doors close with just a push. Safety features include rollover bulkheads and a rupture-resistant fuel system.
COCKPIT AND START-UP
As I opened the cockpit door, I noted the document-storage space behind the two front seats - another hitherto neglected requirement, but there were no door pockets. Bell tells me, however, that provision has been made for stowing an operations manual, maps, approach plates/airport directory, portable global-positioning system and the aircraft logbook.
Getting in is easy. The cyclic stick is attached to the floor just beneath the front edge of the seat cushion, but it is shaped like a semi-circle, so one can slip in easily between it and the seat without the usual contortions. There is no seat adjustment, only adjustable pedals, but I found a comfortable position where I could rest my right forearm on my right thigh for good cyclic control. The two front seats have a four-point harness and all the rear passenger seats are fitted with a three-point harness - now a mandatory Canadian Ministry of Transport requirement. I could see and reach everything.
I looked around and liked very much what I saw. Manufacturers are now producing pilot-friendly cockpits instead of a mess of instruments, warning lights and switches positioned haphazardly around the cockpit - almost, in some cases, as if the designer had gathered them all together, thrown them in and installed them where they landed. In the 407, all the caution and warning lights sit at the top of the instrument panel, directly in the pilot's line of sight. As well as temperature, pressure, doors-open warnings and the like, there is a new one, "cyclic centering", which warns the pilot that, when on the ground, the cyclic position is not central. Bell is keen that it should be, particularly before the start. There are three audio warnings - low rotor-RPM, engine out and one I had not heard of before, for FADEC failure - a very important event in a single-engined helicopter.
The outstanding feature of the panel, however, is the design and presentation of the Litton instruments. They are still round in shape, but, instead of a needle, there is a liquid-crystal display with normal, transient and maximum operating ranges clearly marked around the dial. In addition, there is a digital readout to give exact readings. They are brilliantly lit for easy viewing in harsh sunshine, and at night, from both front seats. The engine instruments have a light on the face, warning and recording any exceedance. There are no maintenance requirements. There are no moving parts to wear out - the entire instrument is built around microprocessor technology, giving superior reliability compared with conventional mechanical instruments.
Part of the pre-start checks is a self-test operation of the instruments. The engine instruments contain a complete engine performance-monitoring system and reveal any previous exceedances, which cannot be erased without special equipment. As an additional option, the information gathered and stored can be downloaded, on to a laptop computer.
The other remarkable feature of the 407 is the Chandler-Evans FADEC. It gives an automatic, fully controlled and cool start using one finger, instead of the usual two hands, and controls the engine/rotor RPM to within tight limits. During the flight, I tried to make the RPM droop with rapid up and down lever movements, but did not succeed. Bell claims that the FADEC provides lower operating costs, better engine life and reliability. There is a manual throttle should it fail.
The centre console is well equipped with modern radios and navigation aids (optional extras), with plenty of space remaining for additional items as required by the operator. The overhead panel has the usual light and other electrical switches, circuit breakers and rotor brake (an optional extra), including an avionics master switch to speed up the start and shutdown and (206 helicopter pilots rejoice) a switch to turn off the fuel-boost pumps without having to pull circuit breakers.
A check for full and free movement of the cyclic stick and collective lever is possible, but difficult, because of the friction from the linkage to the four blades which makes them hard to move with the rotor stationary.
The start is pleasantly simple, well controlled (not by the pilot, but by the FADEC) and cool. Just open the throttle on the end of the lever to the idle stop, select start and sit back and watch. The starter disengages automatically at the appropriate point in the start cycle. Bell tells me that if you are stuck in a remote area and for some reason it will not start automatically, you can revert to the usual JetRanger technique, which works well.
I pulled up slowly into my first take-off and hover. Our hover attitude was slightly nose-up, and almost level, but I was able to feed in the appropriate amount of slight forward cyclic to compensate. We rose gently into a hover over our parking spot. Heading control was straightforward, very little footwork being required to keep a steady heading. I was able, with little effort, to maintain a very steady hover in the 5kt wind. I was pleased with the result - not the usual rocking-and-rolling performance after a period of abstinence from hovering the two-blade 206 JetRanger, thanks to the stability and other design features of the 407. A quick check of the easy-to-read instruments and warning lights showed that all was well and that we had plenty of torque, RPM and engine temperature in hand.
Subsequent landings and take-offs were straightforward - I was able to land exactly on designated spots on any heading. The significant features of the hover are the stability and lack of vibration - a very smooth ride.
The flight manual gives the sideways and backwards speed limits as 35kt, a much more positive statement than the usual US Federal Aviation Administration requirement of "...demonstrated up to 17kt, but not necessarily restrictive". So I went for it in all three directions to see if the new tail rotor could do it. It passed the test.
Some helicopters have a nasty habit of suddenly pitching nose-down when flying backwards quickly, leaving you looking down at the blades of grass with the cyclic already a long way aft. In the worst situation, you may not have enough rearwards cyclic available at this point to raise the nose and recover to the hover attitude. Bell has somehow alleviated the problem in the 407 - the stick was nearly central as we flew backwards at an estimated 35kt with the nose-down pitching suppressed and well controlled and controllable.
I took the aircraft to an estimated 35kt left and right. I used full-left pedal as we moved right, but still had some pedal available as we went to the left - adequate proof that the tail rotor can do the job. Heading control was accurate without the usual twitching that these manoeuvres can cause.
During the high-altitude trials, the 407 underwent take-offs and landings at 12,000ft density altitudes, with 45kt winds from all directions at maximum gross weight. No critical wind limitations were found, Bell says.
INTO FORWARD FLIGHT
Air-traffic control at the busy Seletar airfield restricted us to tight 600ft circuits, so we had to be slick - which the 407 is. I deliberately kept the speed up high - well in excess of 100kt - so had to use tight turns to keep within the small-circuit dimensions.
Maximum continuous power was limited by engine-gas temperature, as expected on this hot (26¡C) day, but it gave us a healthy 140kt, which is also the never-exceed speed (Vne), increased from the usual 206 limit of 130kt. It is unusual in any helicopter to achieve Vne in straight-and-level flight - a dive is usually required, even in powerful combat helicopters. We were quite light at 400kg below maximum.
Vibration levels at this revealing speed were insignificant. Turns in both directions, putting even more stress on the rotor, did not induce any increase in the level of vibration - a very satisfactory ride.
Some manufacturers no longer lay down maximum bank angles for modern helicopters. The 407 is one such, so I rolled over to 60° to the right and straight into another to the left. Our speed was 100kt. Handling was benign and vibration levels insignificant. Visibility during the turns was satisfactory.
During the many circuits, jinks and other manoeuvres, which we performed, I established a good feel for the aircraft, feeding in the appropriate amount of pedal to keep the balance right. The several large vertical fins at the back help to achieve this.
ON TO THE UNUSUAL
On our next circuit, I asked Williams to switch off the single hydraulic system, which powers the cyclic and lever. Normal cruise flight manoeuvres were docile, with no unmanageable handling qualities such as unacceptable feedback. Stability remained about the same as for powered flight. The best technique is to let the aircraft fly itself with as little interference as possible from the pilot.
I elected to go for a low, flat, approach, decelerating slowly, gradually feeding in power and pedal, to arrive over my chosen hovering and landing spot. The lever and cyclic are stiffer to move than those of a conventional two-blade JetRanger, but not excessively so. We arrived at my spot, hovered briefly and landed.
You can ignore engine management with the FADEC - it appears to do everything. If it fails, a horn sounds an alert and you then revert to flying the aircraft like a Bell 47, not a 407 (ie, raise the lever and open the throttle, lower the lever and close the throttle). As well as the automatic start and RPM control, it provides protection from over-speed, surge and exceedances. It also attempts to relight the engine should it stop in flight.
To check forward and down visibility, I flew a very steep approach on to the airfield helipad. I was able to keep the "H" in sight all the way down until just before touchdown.
The aircraft I flew at Singapore had come straight from Bell's factory at Mirabel, Canada, where the outside air temperature was below freezing. There had been no time to re-adjust the RPM in auto-rotation, so we expected much higher values in the less dense air. The limits between maximum and minimum rotor RPM are enormous - 85% to 107%. We entered auto-rotation with little trepidation, but with care. When the throttle was eventually wound down far enough to allow the rotor to auto-rotate, we obtained 95%. Rotor RPM rose slowly to 100% and showed signs of increasing further, so the lever was raised slightly to hold them steady at this figure. We adjusted to the best rate-of-descent speed of 55kt. Rate of descent eventually settled at 1,900ft/min at our weight of 410kg below maximum. Rates of descent in auto-rotation are always a compromise for the designers, who have to take into account many factors, such as rotor power and size. Because we had only sodden grass available, we did not do an engine-off landing, but restored the engine on the way down, so I cannot comment on the effectiveness of the flare. Bell says, however, that the 407's engine-off performance is good.
I flew as a passenger in the back during a rotor-tracking flight. The aircraft had a full complement of passengers, giving us a weight 90kg below maximum. I sat in one of the rear seats. A 1m hover height required only 62% torque; a much higher hover, getting close to out-of-ground effect, required 70%, with 30% torque available, although we would probably have reached the engine-temperature limit first.
The headroom in all but the centre rear seat will accommodate a tall person ie, at least 1.83m (6ft). The centre seat is restricted to about 1.73m because of a main beam running down the middle of the cabin roof. In the cabin, three passengers are seated on the rear forward-facing seat and two more are opposite them, facing aft - a useful configuration if you want to have a conference. Legroom in the middle is "friendly", but adequate. The other two occupants are in the front. The extra cabin width gives satisfactory seat width and the bubble windows provide additional shoulder space. The cabin windows are large, giving a good view of the outside world. None of us wore ear defenders, but we could talk among ourselves quite easily. The seats are well designed, offering support in all the right places. All but the centre seat have elbow rests.
The wide door openings (right-hand 0.91m, left-hand 1.55m) allow a stretcher or bulky cargo to be carried inside, by folding the front left-hand seat if required.
The memorable features of the 407 are its easy-to-fly characteristics, its lack of power or control problems at sensible helicopter-density altitudes, the instrumentation, and its speed and comfort, with a lot of built-in safety features.
Source: Flight International