Undaunted by aerodynamic reality, the design team at Pilatus/Britten-Norman has announced plans for the BN2-XL (Extra Loud), promising more noise, reduced payload, a lower cruise speed, and increased pilot workload.
We spoke to Mr. Fred Gribble, former British Rail boilermaker and now Chief Project Engineer. Fred was responsible for developing many original and creative design flaws in the service of his former employer, and assures he will be incorporating these in the new BN2-XL technology under a licensing agreement.
Fred reassured BN-2 pilots however that all fundamental design flaws of the original model had been retained. Further good news is that the XL version is available as a retrofit.
Among the new measures is that of locking the ailerons in the central position, following airborne and simulator tests which showed that whilst pilots of average strength were able to achieve up to 30° of control wheel deflection, this produced no appreciable variation in the net flight path of the aircraft.
Thus the removal of costly and unnecessary linkages has been possible, and the rudder has been nominated as the primary directional control. In keeping with this new philosophy, but to retain commonality for crews transitioning to the XL, additional resistance to foot pressure has been built into the rudder pedals to prevent overcontrolling in gusty conditions (defined as those in which wind velocity exceeds 3 knots).
An outstanding feature of Islander technology has always been the adaptation of the 0-540 engine, which mounted in any other aircraft in the free world (except the Trislander) is known for its low vibration levels, so as to cause it to shake and batter the airframe, gradually crystallise the main spar, desynchronise the accompanying engine, and simulate the sound of fifty skeletons fornicating in an aluminium dustbin.
Britten-Norman will not disclose the technology they applied in enhancing this effect in the XL, but Mr. Gribble assures us it will be perpetuated in later models and sees it as a strong selling point; "After all, the Concorde makes a lot of noise," he said, "and look how fast it goes."
However, design documents clandestinely recovered from the Britten-Norman shredder have solved a question that has puzzled aerodynamicists and pilots for many years, disclosing that it is actually noise which causes the BN-2 to fly. The vibration set up by the engines and amplified by the airframe, in turn causes the air molecules above the wing to oscillate at atomic frequency, reducing their density and causing lift. This can be demonstrated by sudden closure of the throttles, which causes the aircraft to fall from the sky. As a result, lift is proportional to noise rather than speed, explaining amongst other things the aircraft's remarkable takeoff performance. In the driver's cab (as Gribble describes it), ergonomic measures will ensure that long-term PBN pilots' deafness does not cause inflight dozing. Orthopaedic surgeons have designed a cockpit layout and seat to maximise backache, enroute insomnia, chronic irritability, and terminal (post-flight) lethargy. Redesigned 'bullworker' elastic aileron cables, now disconnected from the control surfaces, increase pilot workload and fitness.
Special noise retention cabin lining is an innovation on the XL, and it is hoped in later models to develop cabin noise to a level which will enable pilots to relate ear pain directly to engine power, eliminating the need for engine instruments altogether.
We were offered an opportunity to fly the XL at Britten-Normans' developmental facility, adjacent to the Britrail tea rooms at Little Chortling. (The flight was originally to have been conducted at the Pilatus plant, but aircraft of Britten-Norman design are now prohibited from operating in Swiss airspace during the avalanche season).
For our mission profile, the XL was loaded with fossil fuel for a standard 100 nm with Britrail reserves, carrying one pilot and nine passengers to maximise discomfort.
Passenger loading is unchanged, the normal under-wing protrusions inflicting serious lacerations on 71% of boarding passengers, and there was the usual entertaining confusion in selecting a door appropriate to the allocated seat.
The facility for the clothing of embarking passengers to remove oil slicks from engine cowls during loading has also been thoughtfully retained.
Startup is standard, and taxying, as in the BN-2, is accomplished by brute force. Takeoff calculations called for a 250 decibel power setting, and the rotation force for the (neutral) C of G was calculated as 180ft/lbs of back pressure.
Initial warning of an engine failure during takeoff is provided by a reduction in flight instrument panel vibration. Complete seizure of one engine is indicated by the momentary illusion that the engines have suddenly and inexplicably become synchronised. Otherwise, identification of the failed engine is achieved by comparing the vibration levels of the windows on either side of the cabin. (Relative passenger pallor has been found to be an unreliable guide on many BN-2 routes because of ethnic considerations).
Shortly after takeoff the XL's chief test pilot, Capt. "Muscles" Mulligan, demonstrated the extent to which modem aeronautical design has left the BN-2 untouched; he simulated pilot incapacitation by slumping forward onto the control column, simultaneously applying full right rudder and bleeding from the ears. The XL, like its predecessor, demonstrated total control rigidity and continued undisturbed.
Power was then reduced to 249 decibels for cruise, and we carried out some comparisons of actual flight performance with graph predictions.
At 5000' and ISA, we achieved a vibration amplitude of 500 CPS and 240 decibels, for a fuel flow of 210 lb/hr, making the BN-2 XL the most efficient converter of fuel to noise since the Titan rocket.
Exploring the constant noise-variable speed and constant speed-variable noise concepts, we found that in a VNE dive, vibration reached its design maximum at 1000 CPS, at which point the limiting factor is the emulsification of human tissue. The catatonic condition of long term BN-2 pilots is attributed to this syndrome, which commences in the cerebral cortex and spreads outwards.
We asked Capt. Mulligan what he considered the outstanding features of the XL. He cupped his hand behind his car and shouted. "Whazzat?"
We returned to Britten-Norman field convinced that the XL model retains the marque's most memorable features, while showing some significant and worthwhile regressions.
Pilatus/Britten-Norman are however not resting on their laurels. Plans are already advanced for the three-engined Trislander XL, and noise tunnel testing has commenced. The basis of preliminary design and performance specifications is that lift increases as the square of noise, and as the principle of acoustic lift is further developed, a later five-engined vertical takeoff model is another possibility.
