THE PRINCIPLE behind tilt-rotor aircraft has been around almost since the Second World War. Bell developed the first experimental design in the mid-1950s, carried the idea forward with the XV-15 technology demonstrator, then, in association with Boeing Helicopters, began work on the V-22 Osprey military transport. Having proved the concept, the companies are now moving towards development of the D600 11-passenger civil tilt-rotor.
As a way of testing the flying characteristics of a tilt-rotor craft, the manufacturer gave me access to the V-22 development simulator, based at Fort Worth, Texas.
After being warned by Jon Tatro, Bell's crew systems engineer, to duck my head as I clambered into the left-hand seat of the $20 million V-22 Osprey simulator, I soon realised that this is the only minor discomfort in the whole cockpit.
For such a complex aircraft, the V-22 cockpit is surprisingly uncomplicated and well designed. Bell Boeing has incorporated pilot recommendations into the design and so I found it ergonomically pleasing, functionally efficient and easy to manage. The simulator has no motion system, but I was to find the "flight" realistic enough because of the wrap-around dome visual system.
FLIES LIKE THE AIRCRAFT
The simulator is designed to fly in exactly the same way as the actual aircraft. The controls and instruments are identical.
The seat and pedals will adjust as required. Instead of a fixed-wing control yoke, there is a helicopter-type cyclic-control stick with a helicopter-type "coolie hat" and trim-release button to re-align the stick and pedals datum. The left hand rests on a power lever, which is nicely contoured to fit the human hand. This control, is unique to the Osprey and is moved forwards and backwards to increase and decrease power, respectively.
Neatly positioned just where your thumb rests is a small wheel, which you can click forwards and backwards in prolonged bursts or single clicks to move the tilting engine nacelles between horizontal and vertical, from 0¡ to 97.5¡ in 12s. There is a small diagram of a right angle in the corner of one of the multi-function displays (MFDs), plus a digital read-out showing you where the nacelles are. You can, of course, look out of the window. This is also possible in the simulator, with its wrap-around visual system.
On the overhead panel are two engine-speed select levers, which are used to run up the engines to the RPM operating range, then left alone for the rest of the flight, and a few switches for lights and other systems.
The instrument panel in front of each pilot is dominated by two MFDs with their associated keys around the edges and a standby control display unit (CDU), in the middle of the panel. These show all the information that the pilots require for all stages of the mission. I was given all the flight parameters of airspeed, ground speed, altitude (both barometric and radar), attitude, compass, rate of climb and descent, power used and track on one screen, with a moving-map display showing the way points of our proposed cross-country flight on the other. Each pilot can monitor all the systems independently, such as flight instruments, navigation aids, communications and aircraft health on the displays.
Despite there being two independent computers to drive the MFDs, there are basic conventional mechanical "get-you-home" instruments on the panel - attitude indicator, compass, airspeed indicator and altimeter. I well remember having to revert to these in another modern aircraft when both sets of MFDs went down. There is a pilot-activated cursor on the MFDs to remind the pilot which function he last used. He can then scroll it to any position on the screen to activate another function - a useful device.
To minimise the time spent head down in the cockpit dialing up new radio frequencies, up to four frequencies can be pre-set into a display at the top of the instrument panel and selected as required.
The centre console is dominated by a CDU "scratch pad" with a keyboard beneath to create or modify data. There are other controls on the console, such as the undercarriage and flap selectors.
SIMULATED FLIGHT
Tatro simulated us rapidly up into a gentle cruise at 500ft (150m), where I took over control. I advanced the power lever, keeping a wary eye on our attitude on the MFD, and we quickly achieved 250kt (450km/h). We followed a preselected route on the navigation system, but I deviated at my leisure, flying around hills, diving down valleys, seeing cables while keeping low and fast. The daylight colour-visual system shows all aspects of the countryside - roads, rivers, trees, hills, valleys etc. There is not much texture when you are low and slow, as I was to find out when I tried to hover. I have not yet experienced a helicopter visual system, which does provide this. Stability is good, turns easy and balanced - just like a fixed-wing aircraft.
After such fun, I got back on track, following the information on the MFDs. As we approached our first target (still at 250kt), where we planned to land on a road, Tatro talked me through a deceleration to the approach and hover, known in the tilt-rotor world as a conversion.
TRANSITION TO HOVER
Having flown the aircraft just like a fixed-wing aircraft, it was now time to revert to helicopter techniques. Tatro talked me through the deceleration stages, starting about 7km (4nm) out. It was going to take all of this on my first transition from forward flight to the hover. Subsequent attempts took less time. I am told, that an experienced pilot, can transition from 250kt in about 3km. It is really quite straightforward - you match speed with distance-to-go with height and adjust accordingly, using the stick to control forward speed, the angle of the nacelles to control direction of thrust, and thus speed, and power to balance all and control the rate of descent.
We arrived too fast and high for my liking, so I reverted even more to a helicopter-type steep-approach technique, keeping my designated landing area in the same spot on the windscreen and using all the controls to drive the Osprey down until we arrived over it. Because I was concentrating hard on external visual cues, Tatro obligingly called out speeds, distance and height, while I kept an eye on rate of descent, which was clearly displayed on an MFD. On later approaches there was time to acquire all this information myself from the MFD. This and other information is easily readable.
We intercepted a steep slow approach, to an area on a road, along side some trees by reducing power, holding height to slow us down (the aircraft is slick and takes time to slow down) and then started to bring up the nacelles with bursts of the wheel at my left thumb. It is not yet known if the Osprey suffers from vortex-ring settling with power, the aerodynamic condition where, with a rate of descent of more than 500ft/min (2.5m/s), airspeed below about 20kt and in powered flight, loss of control and an uncontrollable sudden increase in the rate of descent can occur.
With some encouragement from Tatro, I gave the V-22 a big handful of power by moving the power lever forward as we approached hover height, and we came to an unsteady hover over the road. The nacelles were now vertical, the RPM of the big engines and rotors having been increased automatically, to 100%, during our transition to the hover, with control transferred from the flaperons, elevators and rudders to the swash plates under each proprotors - just like a helicopter.
The remarkable characteristic of the approach was the aircraft's level attitude throughout. The deceleration is carried out, by tilting up the nacelles, not by raising the nose as in a helicopter or fixed-wing aircraft. The nacelles can be tilted slightly aft (7.5¡) to allow a level attitude, even when hovering downwind or flying backwards.
My hover was poor - we drifted slowly sideways, backwards and then forwards. This was because my control inputs were incorrectly timed, and the lack of a lot of close-up detail in the visual scene made it difficult to grasp immediately one's exact position in space to take the necessary corrective action. I was also over-controlling a little. Tatro said that it was a comparatively good hover for the first try in such a unique aircraft simulator. In the actual aircraft, the visual cues from the outside world would probably produce a better hover.
I relaxed and settled down. The aircraft is actually very stable and hovers best when left alone. Having finally controlled the hover to my satisfaction, I drew back the power lever and we landed.
TRANSITION TO FORWARD FLIGHT
Now came the challenge of coming to the hover and transitioning into forward flight. Tatro briefed me on how to do it. I pushed the power lever gently forward, adjusted the stick to achieve a vertical climb and did what came naturally with my feet on the pedals to hold the heading. We came up into the hover. Power was applied and the nacelles tilted down slowly, giving a level acceleration. Like the deceleration, the acceleration is achieved by cranking the prop-rotors down and not by lowering the nose. The nacelles can be "beeped" fully down to aircraft mode or stopped at some intermediate setting.
Tatro had warned me about the abrupt pitch-down during the transition and acceleration. I was too late, but Tatro picked up the nose with the stick and we scraped over the hill in front. The nacelles were cranked fully down with my left thumb. During the conversion we heard the engines slow down to the normal fixed-wing RPM of 84%, and control reverted to the flaperons.
While in the cruise, I asked to experience a reversion to single-engined flight. Tatro immediately snapped back, one of the speed selects. Both prop-rotors were now being driven by one powerful Allison T406, through an interconnecting drive shaft. A double engine failure is dealt with by tilting the nacelles up and descending in a 110kt glide to a 30kt run-on landing. I looked around the cockpit after our single engine failure and saw a warning that we were exceeding some limits on the remaining power plant. The power lever was brought back until the warning was extinguished. The engines have a 10min 5,090kW (6,830shp) emergency rating. The aircraft will easily stay airborne on one engine.
I chose a flat strip of grass beside a lake on which to carry out a single-engined landing and, on Tatro's advice, went for a flat approach, gradually washing off speed, cranking up the nacelles and increasing the power. The power lever was adjusted to hold the rate of descent to about 500ft/min, controlling attitude with stick and heading with pedals. It worked well and we performed a running landing at about l0kt on to the grass.
To explore the navigation systems further and to practise another conversion to a fast cruise, we left the area and progressed to our next way-point, a road. A much quicker transition to the hover was attempted. I was getting a feel for the aircraft and, from about 5km, intercepted an approach which was steeper than I had intended it to be. It did not matter; the nacelles were cranked up and power and height were adjusted to go down the now steep approach. I was able to keep an eye on the MFD to monitor speed, distance and rate of descent. We arrived over the chosen spot in the hover. I heard the engines accelerate to hover mode.
SATISFACTORY HOVER
My hover was satisfactory so, feeling confident, I tried some sideways and backwards flight and turns on the spot. The lack of wind probably helped a lot, and I was able to achieve all these manoeuvres with remarkable ease. Even a pure fixed-wing pilot should not have too much difficulty. I did not take the aircraft to the higher sideways speed of 30kt and backwards rate of 20kt of which, it is capable. The aircraft stayed level throughout.
Tatro invited me to hover-taxi up the road, turn the comer and then perform a running take-off along the road. My hover-taxi was excellent, right down the middle with no rolling, pitching or yawing. When I remarked on this to Tatro, however, he mentioned that we had actually landed about a minute earlier and were taxiing along the road.
For the running take-off, power was applied and the nacelles brought down from the vertical to 65¡ of tilt. We broke ground quickly, I applied a lot of power, full horizontal nacelles and, this time, caught the nose-down pitch. We accelerated idly to 250kt.
Tatro ran through an engine fire drill. The warning systems gave lucid notice of the problem, but no information on emergency operating procedures - these have to be done from memory and the checklist.
BENIGN HANDLING
After only 50min in the simulator I felt that, although the Osprey is a complicated and sophisticated aircraft, the ease of management of the systems, the information available at the appropriate times on the MFDs and the benign handling qualities (apart from the nose-down pitch), I would be capable of taking the actual aircraft safely through the same routine. Bell tells me that helicopter pilots have no problems with the fixed-wing mode and that fixed-wing pilots with no helicopter experience have few problems once they accustom themselves to hovering manoeuvres.
Source: Flight International
