Peter Gray/ARLINGTON
The tilt-rotor concept has been around for many years, but only recently has the first military application (the Bell-Boeing V-22 Osprey) received production approval from the US Department of Defense. Even more recently, Bell and Boeing have launched the Model 609 civil tilt-rotor which is scheduled to fly in mid-1999, with deliveries starting in 2001.
Although it is an earlier-generation machine (the first of two demonstrators was flown in 1977), the Bell XV-15 is proving to be a useful research and development tool for the 609 project and, indeed, the sole flying XV-15 has been re-liveried to represent that aircraft. It is because the tilt-rotor looks like becoming a commercial, as well as a military, success that Flight International arranged with Bell to test this XV-15 and to try to gauge just what sort of transition a fixed-wing or helicopter pilot will have to make when flying this new breed of aircraft.
The outstanding impressions, as John Ball, Bell's senior experimental test pilot, and I approached the XV-15 are of the two large, vertical, engine nacelles housing four of the five transmissions on the ends of the wing, the large 7.6m (25ft) prop-rotors on top, and the 6.5í forward sweep of the wing. The sweep is to give the rotors plenty of clearance when they flap back when in the fixed-wing, horizontal-flight, mode. The rest of the aircraft looks much like any other nine-seat turboprop.
As Ball took me round the outside, I noted the large area - 2.9m sq (31ftsq) - of flaps and flaperons. This is to help offset the effects of the downwash in the wing from the rotors while in the hover. This peculiarity of tilt-rotors can destroy 8% of the lift. The flaps also help provide a more efficient wing at low speeds.
As we stepped inside among the banks of test equipment, I saw the complicated mixing unit in the roof between the two wings. This is entirely mechanical, a wonder for its age (it was designed over 20 years ago), and does everything necessary to convert the controls and control movements from helicopter to fixed-wing mode. This includes changing the rotors from the cyclic-pitch mode of vertical lift to the constant-pitch mode of fixed-wing flight.
Although most of the systems and even the materials in the 609 will be different from those of the XV-15, the handling, I am told, will be much the same. It will be about the same size. So, although I read the flight manual before my flight, received a comprehensive briefing, had a question-and-answer session and understood most of the systems (including the ejection seat) I concentrated on the handling and how the average pure- helicopter pilot and pure-fixed-wing pilot would manage the tilt-rotor. (I have the advantage of being dual-experienced.)
Although the XV-1 5 is old, Bell has updated it over the years to continue the research, especially in support of the development of the larger V-22 for the US Marines, Air Force and Navy. Flight International has flown the V-22 simulator (13-19 March, 1996), but this was to be my first flight in the real thing.
We had two Lycoming LTC1K-4Ks (T53 turboshafts modified to run vertically) capable of producing 1,155kW (1,550shp) each for two-engined take-offs and 1,340kW for single-engine contingency. The five gearboxes are not designed for this amount of single-engined power in the experimental XV-15 - in the 609 they will be.
START UP AND GO
The aircraft is the only experimental tilt-rotor which Bell Boeing has, so we had to be careful with it. The engine starts were slow and benign, as one would expect from T53s. The two rotors are interconnected by a shaft which runs along the span of the wing with a (slight) direction-changing gearbox in the middle, so that, as soon as the first engine was started, both prop-rotors turned. This same shaft will allow both rotors to be driven by one engine only in the event of a single engine failure.
In the event of a double-engine failure, both rotors will autorotate at the same speed. Equal speed is not as critical as in, say, a Boeing Chinook or Kaman K-Max, where the rotors intermesh. Out-of-synch rotors can produce problems, however, because of the inequality of the lift being produced. The XV-15 pilot has a left/right switch to fine-tune individual rotor-torques to overcome poor synchronisation.
It was a fine, clear day with only 6kt (11km/h) of wind. I would have preferred something stronger to test the out-of-wind characteristics. We were at 200ft (60m) pressure altitude, but with slightly less density. We had full fuel, which, with all the test equipment on board (there is no room in the cabin for a passenger) gave us a weight of 6,080kg, compared with a maximum permissible of 6,800kg. The 609 will have a further 500kg available.
HOVER TAXI
One of the several differences that helicopter pilots will have to address is their use of the powerful lift forces generated by the rotors. Many of the manoeuvres usually performed by moving the cyclic stick are now done by tilting the nacelles, which can go from 0í (ie, horizontal) for fixed-wing mode, to 90í for helicopter mode or anywhere in between, and even to 95í to fly/taxi backwards, slow down quickly, or maintain a level attitude when hovering downwind. With 1,155kW available from each engine, the slightest tilt makes a significant difference. The 609 will have even more engine power (1,380kW from each Pratt & Whitney Canada PT6C-67A) and an extra 600mm of diameter on the prop-rotors.
I beeped the rotor RPM up to 98% for helicopter mode, applied just a bit more than 20% torque to enter the RPM-governed range and give good rotor control and, keeping the cyclic stick neutral, tilted the rotors 1¹ forward and we taxied towards Bell's flight-test runway at Arlington, Texas. A dab on the brakes revealed the usual characteristic of helicopter brakes: not so powerful as to tip the aircraft on to its nose, yet powerful enough when used differentially to turn the aircraft. This is required because the rudder power is weak at this stage. It is to be hoped that the 609 will have nosewheel steering. There is a light and audio warning to inform the pilots that the undercarriage is up and a light also to warn that the brakes are on.
The cabin door on the experimental model is in the same position as it will be on the civil version - on the right-hand side where the pilot can see the winch when on a rescue mission and far enough behind the engine prop-rotor plane to be out of harm's way. The experimental and commercial versions are both designed for single-pilot operation, even to instrument-flight-rules conditions.
The other overriding impression is that, although an extremely complex aircraft lurks under the skin, the management of the systems is straightforward and the handling benign. Rockwell-Collins will provide a glass cockpit for the 609. I had the preliminary, early version at my side: a large liquid-crystal display which gave me all the information I needed to fly - attitude, airspeed, barometric and radar altitude, rate of climb and descent, heading, flight-director information, flap and, most importantly, the positions of the nacelles and the flaps. The 609 will have other information and other screens.
The cockpit is comfortable from every aspect, although the civil pilot will not have to deal with the ejection-seat bits and pieces and blow-out window handle pins. The seat and pedals are adjustable for reach and height, and are comfortable. I recognised several Bell helicopter parts - the rudder pedals are from the UH-1 Huey; the temperature and pressure gauges and the triple tachometer from the 212.
Visibility is good. Although we had bright sunshine, I could easily read all the instruments. The 609 instruments will be totally different - circles with needles on a flat-panel electronic screen with digital readout repeaters alongside. The stick, too, is familiar. What would be the collective lever in a helicopter becomes the power lever in the tilt-rotor. It works in the same manner in the hover, but does not control blade pitch directly. Instead, it merely applies more or less power to the engines.
The RPM governor then adjusts collective pitch to keep the RPM constant. Movement of the power lever causes the throttles located at the bottom of the centre console to move. After engine-and-rotor start, they are latched to the flight position.
Should the governor malfunction, there is a manual system - a large wheel between the pilots' seats with a window showing the manual governor position. Movement of this wheel allows the pilot to control blade pitch manually. Thus, the important systems all talk to each other, but helicopter pilots should note that the engines do not.
The engines have a lot of work to do in a tilt-rotor - to drive big prop-rotors to sustain the aircraft in the hover at commercial all-up weight, transition it to forward flight and then propel it at high speed. Therefore, they are large and thirsty. The XV-15's performance is relatively poor on one engine. Unless the weight and temperature are low and there is some wind, a single-engine hover is unlikely. The flight manual calls for a run-on landing.
The 609 will be full Category A certificated. The engines will be more powerful and the rotor blades 300mm longer - a huge performance enhancer and, thus, the 609 will be more capable on one engine than is the XV-15.
Speed is controlled by tilting the nacelles. There is the luxury of having two switches on the research aircraft for this - the 609 will have only one. Both are conveniently available on the speed lever just where the pilot's thumb rests. One switch allows manual selection to any angle at a fairly brisk rate, as I was to find out (90 í to 0¹ in about 12s). The other moves between preselected angles (90¹, 75¹, 60¹ and 0¹) at a much more modest rate.
To relieve pilot workload and not let the acceleration/deceleration get out of hand, the 609 will have the automatic selection only, at the lower rate but, in the meantime, Ball and I could enjoy the fast conversions. "Conversion" is the term used by tilt-rotor pilots when going from helicopter to fixed-wing mode and back.
Taxiing was straightforward. I managed some tight turning circles with fairly hefty pushes on the rudder pedals and brakes. With engines and rotors outboard, there is a lot of weight on the ends of the wings, so the aircraft tends to rock more than a helicopter or fixed-wing aircraft does. Tilting the rotors aft brings a taxiing XV-15 to a stop very quickly.
I now put the rotors at 90í and pulled up into my first hover. This can often be a tense moment in an aircraft which the pilot has not flown before, with tendencies for it to hover with left/right-wing-low attitudes and simultaneously nose-high/low pitch. The result can be an unexpected wander from the desired position over the ground. This is not so with the tilt-rotor. We came to a neat hover and I was easily able to hold it exactly over the spot, the attitude remaining level.
The attitude can, in fact, be varied by using a combination of rotor-tilt with cyclic. I tried this by tilting the rotors forward and simultaneously beeping the cyclic aft. We finished up in a nose-down attitude with improved visibility over the nose. Steady hover taxiing forwards and backwards was achieved just by tilting the rotors (there was no need to move the cyclic), with the result that the fuselage remained level throughout. I went backwards to the 35kt flight-manual limit.
There was none of the usual helicopter nose- down pitching - we stayed remarkably level. Sideways-flying is controlled conventionally by the pilot by moving the cyclic, but the mechanism is different. Different amounts of collective pitch (and thus lift) are applied to each rotor, which tilts the aircraft in the desired direction, and off it goes left or right - we went up to about 35kt, still with no pedal application or twitching into wind.
The fuselage still remains level. Spot turns were also easy to perform, with no tendency to accelerate/decelerate while in the turn, though there was little wind. Helicopter pilots will find these manoeuvres a delight. Fixed-wing pilots have to work harder and need a little help.
Landings were equally problem-free, especially with the level attitude. This aircraft does not suffer from the handling problems which some helicopters have when hovering with the wind in certain sectors. The test pilots are not aware of any settling with power/vortex-ring characteristics. They think that a very high rate of descent would be needed to achieve the entry conditions for the vortex ring to establish itself.
FIRST CONVERSION
Now came the transition into forward flight. As the nacelles were tilted to 60 degrees, I felt the thrust through my body and off we went - unlike the sensation in the V-22 simulator. All of the vertical take-off and landing aircraft I have flown suffer from a loss of lift at this point because they lose the benefits of ground cushion and vertical thrust from the rotor/s before picking up the benefits of forward flight such as translational lift in the case of the pure helicopter (or wing lift, in the case of the tilt-rotor). The XV-15 is no exception, so the pilot has to be ready with the power lever to prevent an undignified sink. This caught me out in the simulator for the much larger V-22, where the nose went down rather hard and we lost some height.
I anticipated nicely with the XV-15, however, and we climbed up. Ball brought up the undercarriage and reduced the flaps. I beeped the rotor rpm down to 86% and soon saw more than 120kt appear on my display. The climb was continued, and the nacelles tilted to 0í and were locked. We accelerated rapidly.
This research aircraft has two big torque meters on top of the instrument panel which clearly show maximum continuous and maximum power. I held maximum continuous and we achieved 222kt at 5,000ft. Bell's chief pilot said later that this was a bit low, but, in fairness, I did not hold the power on very long and I was still searching for the perfect level attitude. Helicopter pilots will be impressed, and even pilots of fixed-wing aircraft of comparable size and weight may be surprised. After some climbs, descents, turns, some steep, all showing benign characteristics, I lowered the nose to achieve the never-exceed speed (Vne) of 250kt.
VIBRATION LEVELS
Vibration levels remained much the same - impressively low. The V-22, the XV-15's big brother, has a higher Vne of 340kt. The XV-15 experimental aircraft was not built for speed, but for overall assessment of all tilt-rotor systems. Even so, for a 20-year-old aircraft, I was impressed with its docility at this speed. Helicopter pilots can forget about retreating-blade stall, reversed airflow and Mach-number drag on the advancing-blade tips in this aircraft. They must remember, however, that, at what is for them a fast speed (l20kt), this aircraft will stall.
The aircraft has a stabilisation system, more for research reasons than for handling necessity, so it is extremely stable in all axes. Hands-off flight produced no deviations from the straight and level. We did not have time to explore handling with the stabilisation off, but I am told that there are hardly any differences.
Visibility throughout the cruise and turn manoeuvres was above the average, given the large overhead windows and side panels.
We went from the fast to the slow, Ball allowing me to stall the aircraft clean (ie, with flaps and wheels up). Pure-helicopter pilots will, of course, need to remember at this point that they are flying a conventional wing and not a rotor, and not try to fly too slow and low. Even this early experimental aircraft is slick and it took a while before I could get the speed down from more than 200kt to a little over 100kt.
With lots of aft stick and nose-high attitude, the aircraft eventually vibrated slightly and continuously and started to sink This occurred as we decelerated through 110kt. I watched the rate-of-descent indicator on my display go from zero to ever-increasing numbers. I could feel the sink rate through my body, too.
There was no wing drop or other effect, and the recovery was benign and instantaneous. I relaxed the large amount of back pressure I was holding on the stick, raised the power lever to about 60 % torque, and out she came and went straight into a climb .The aircraft has not been known to spin. The XV-15 is stressed to +3 to -0.5G, although aerobatics are not allowed. The V-22 limits are +3/ -2, a much wider range.
CIRCUIT WORK
We were allowed to do a straight-in approach on our return to the airfield, so Ball gave me the challenge of going for a marked spot on the crosswind taxiway. This is when both fixed- and rotary-wing pilots need to reprogramme their brains and use the tilting rotors to maximum effect to control indicated airspeed combined with rate-of-descent and rate-of-closure indications. Ball dissuaded me from using the stick and got me to rely on the tilt capability. The technique is different, but works well. On all approaches to the hover that I made onto the spot on the taxiway, I arrived each time over the target.
If going into a confined area where maximum visibility was required, a pilot could pitch the fuselage nose-down to increase the forward visibility and compensate by tilting the rotors back - impossible in any other aircraft.
I taxied in and Ball shut down the aircraft. Although there are safety systems in place to prevent the nacelles falling toward and doing damage, Ball took the additional precaution of pulling the already-identified circuit-breakers.
The aircraft will accept fore and aft landing slopes of up to 15 degrees quite comfortably, which is beyond the range of normal helicopters. Helicopter pilots need not worry about banging the tail rotor when landing down-slope. Start-ups, shut-downs and operations in strong winds are said to be safe.Commercial operators will want to make sure to exploit the tilt-rotor's unique capabilities and to operate city-to-city from roof tops and other similar areas within built-up areas. Excessive noise, quite rightly, would inhibit or even prevent this, so Bell Boeing has conducted extensive tests to determine the major noise-sources.
Rotor tip-speed is one of the major producers, with the usual helicopter-phenomenon of each rotor passing through its own wake and making that unmistakable and loud slapping sound. Most of the noise is caused during final approach. So, the test pilots have been busy finding approach profiles to reduce it. Tip speeds in the 609 will be lower and the tip design modified from that of the XV-15.
PERFORMANCE
All pilots, both rotary and fixed-wing, will have to learn new criteria, such as conversion corridors - those areas of maximum and minimum speeds, heights, weights and configuration between which conversions should take place. Helicopter pilots will have to respect stall speeds, flap-setting limitations and the like. There are two sets of limitations to learn, one for helicopter mode, the other for that of fixed-wing configuration.
Helicopter pilots will recognise the power- required curve for straight-and-level flight. It has the usual shape and bottoms out at about 60kt, depending on the weight and altitude. Fixed-wing pilots will recognise the same curve for aircraft mode: the bottom of the curve moves to the right, giving a higher speed as the aircraft converts.
The best rate of descent in helicopter autorotation is about 75kt. The graph in the manual gives a rate of 2,150ft/min at 5,900kg, although the test pilots quote 4,000ft/min using the better flare-effect speed of 90kt.
Man's achievements in aviation have been highlighted by the very first flight, the first helicopter flight, the first supersonic flight, and man landing on the Moon. To be able to hover, take off vertically, then fly at turboprop aircraft speeds and heights, has fired the imagination of man for decades. The XV-15 does all of this, so I would now add its first flight to my list as another unique achievement, using all man's ingenuity, imagination and expertise.
Deposits for firm orders have already been received by Bell from more than 40 organisations, including private owners, two offshore operators, air-taxi companies and corporate bodies. On the strength of this evaluation of the XV-15 prototype, it is time for these organisations to covert those orders.
Source: Flight International
