The BA609 may combine the characteristics of both helicopter and aeroplane, but Bell will have to certificate not two, but effectively three flight modes. This is because of the tiltrotor's unique ability to vector the thrust of its proprotors.
Certificating the tiltrotor for safe civil operation is perhaps the programme's biggest challenge. "Every aircraft before had one flight envelope. We have two or a blended envelope," says Don Barbour, Bell executive director, civil tiltrotor programmes. "What makes this different is that portion of the envelope where the pilot can change the thrust vector at will."
The BA609's nacelles can tilt from 5º aft of vertical to fully forward. "We looked at all possible combinations of aircraft weight, altitude, wind, temperature and nacelle angle, and we came up with a huge matrix of possible test conditions, "Barbour says. "So we asked can we narrow the number of combinations and standardise training for the expected flight profiles?"
The BA609 pilot will be able to tilt the nacelles to any angle between 95º and the slow loiter position at 75º. But a single button push will move the nacelles smoothly to the loiter position at 60º and another push will move them fully forward. "We do not anticipate anyone wanting to fly with the nacelles 2-3º off fully forward, so we don't need to demonstrate the envelope for every position of the nacelle."
Even after limiting the possible nacelle angles to those of practical use to the pilot, the BA609's capability is much expanded over that of a conventional aircraft. "With the nacelles down, the flight envelope is similar to that for a fixed-wing aircraft. With the nacelles up it is a different envelope. So you have to manage where you are in the envelope, "Barbour says.
Although the BA609 is a fly-by-wire aircraft, the flight control system will not prevent the pilot going outside the flight envelope. Instead, the pilot is provided with "lots of cues" by the Rockwell Collins Pro Line 21 display system. "We have to allow the aircraft's flexibility to be communicated to the pilot," says Barbour. "In fixed-wing mode, stall speed is calculated and displayed in real time." Similarly, in helicopter mode the system calculates and displays hover performance.
In tiltrotor mode, the speed envelope varies with nacelle position. With the nacelles vertical, forward speed is limited to 60kt (110km/h) by loads on the proprotors. The allowable range is indicated to the pilot as a green arc from 0kt to 60kt on the airspeed display. "Move the nacelles to 75º and the arc moves to 130kt. Go to aeroplane mode and it's all green out to 275kt," says Barbour. All the pilot has to do is "stay in the arc".
With the nacelles tilted, a stickshaker will alert the pilot when the aircraft is approaching the limits of power available. "He can continue to pull power for single-engine capability, says Barbour. "He can cook an engine if he chooses, but with the vibration indicating power level he can't do it inadvertently while looking out."
The issue of vortex ring state highlighted by the V-22 crash will be tackled mainly by training pilots to stay within the proper flight envelope, says Barbour, and by teaching them how to recognise and escape the condition. A form of rotary-wing stall, vortex ring state occurs when the aircraft catches up with the wake ejected by the rotor. "That will require a very high rate of descent in this aircraft because of our disc loading, and this is desirable for us," he says.
The normal rate of descent in the BA609 will be 300-500ft/min. At 800ft/min the pilot will receive an indication that descent rate has reached the limit of normal operations. This is well below the 2,000-5,000ft/min rate required to enter vortex ring state, says Barbour. And if the pilot does encounter vortex ring, escape will be a matter of tilting the nacelles down and applying power to accelerate out of the proprotor wake, he says.
The certification basis agreed with the US Federal Aviation Administration for the world's first civil tiltrotor represents a melding of helicopter, transport category fixed-wing, and unique airworthiness requirements, says Barbour. The resulting tiltrotor type certification basis is a blending of the US Part 25 rules governing large aircraft and the Part 27 regulations for helicopters, plus new rules written to cover the unique tiltrotor aspects.
The BA609 is Bell's first transport category aircraft, requiring systems to meet a failure frequency threshold of 10-9. "This is a level higher that normal category aircraft," he says. "We are reaching for the highest airworthiness standard with this aircraft."
Source: Flight International
