AW609 tiltrotor prototype crashes during test flight
Have you compared the 139 and 189 fin and tailplane? 189 has big chunks out of it too. I guess they figured the 139 had too much area on it so they reduced the size for the 189.
Now, didn't the 609 originally have a rudder??? Would that help with Dutch Roll? Or perhaps introduce some anhedral, but that would make it a nasty to run the cross shaft through the wing.
Now, didn't the 609 originally have a rudder??? Would that help with Dutch Roll? Or perhaps introduce some anhedral, but that would make it a nasty to run the cross shaft through the wing.
Unlike V-22 and XV-15, the 609 has never had a rudder. Like mentioned earlier, yaw control in AP mode is exclusively accomplished with differential collective power.
Regarding the wing, the 609 currently has a bit of dihedral, as the crossshaft coupling can accommodate a few degrees of misalignment.
Regarding the wing, the 609 currently has a bit of dihedral, as the crossshaft coupling can accommodate a few degrees of misalignment.
What is clear from reading both the accident report and that NASA history thing is that after 60+ years of development and research, the designers still really don't know everything about how a tiltrotor flies. The accident report confirms this: The computer modeling of the aircraft behavior and required pilot inputs wasn't even close in the high-speed regime. When it began to go pear-shaped, things happened FAST!
And so the challenges facing the computer guys are immense and daunting. It's not so much a matter of "Back to the drawing board!" but "What do we do if this happens again? And will simply making the fin larger do the trick for sure?" If it is true that the old fin design was just as fast and just as stable...well...that might work. But what if it doesn't?
You have to admit one thing: In airplane mode, if that thing were a normal fixed-wing you'd say to yourself, "Man, that thing has a TINY rudder!" But it's not a normal fixed-wing.
Hey, "stuff" happens in the development of any aircraft. Perhaps Agusta will go back to the (sexier!) XV-15/V-22 tail. Probably won't be as fast, but maybe won't lose control at high speed.
Either way, this accident is a sad commentary that after all these years...after all these flight hours of testing...after all these computer models...there are aspects of How A Tilt Rotor Flies that elude us. How far has this accident set the program back? I'm sure another ten...err, twenty years of development ought to do it. (I wonder if Agusta will ever tire of throwing money down that black hole?)
And so the challenges facing the computer guys are immense and daunting. It's not so much a matter of "Back to the drawing board!" but "What do we do if this happens again? And will simply making the fin larger do the trick for sure?" If it is true that the old fin design was just as fast and just as stable...well...that might work. But what if it doesn't?
You have to admit one thing: In airplane mode, if that thing were a normal fixed-wing you'd say to yourself, "Man, that thing has a TINY rudder!" But it's not a normal fixed-wing.
Hey, "stuff" happens in the development of any aircraft. Perhaps Agusta will go back to the (sexier!) XV-15/V-22 tail. Probably won't be as fast, but maybe won't lose control at high speed.
Either way, this accident is a sad commentary that after all these years...after all these flight hours of testing...after all these computer models...there are aspects of How A Tilt Rotor Flies that elude us. How far has this accident set the program back? I'm sure another ten...err, twenty years of development ought to do it. (I wonder if Agusta will ever tire of throwing money down that black hole?)
So the accident appears due to sudden onset of unanticipated pitch-yaw coupling at high speed?
From the photo of the crash site it looked like there might have been in-flight breakup - I assume due to the sudden air loads beyond design limits.
From the photo of the crash site it looked like there might have been in-flight breakup - I assume due to the sudden air loads beyond design limits.
What is clear from reading both the accident report and that NASA history thing is that after 60+ years of development and research, the designers still really don't know everything about how a tiltrotor flies. The accident report confirms this: The computer modeling of the aircraft behavior and required pilot inputs wasn't even close in the high-speed regime. When it began to go pear-shaped, things happened FAST!
and - unless I have missed something - this happened during "classic" flight mode, not transition. My uneducated guess would have been that this part of the flight envelope to be well understood and modeled.
from the photo of the crash site it looked like there might have been in-flight breakup - I assume due to the sudden air loads beyond design limits.
this happened during "classic" flight mode, not transition. My uneducated guess would have been that this part of the flight envelope to be well understood and modeled.
That said, it is all highly dependent on specific rotor tuning where it would be most sensitive.
Lonewolf seems to be on to something. Haven't we seen this kind of thing occurring elsewhere as we incorporate more computerization?:
AW609 flight control laws may have contributed to fatal accident - Vertical Magazine
https://www.flightglobal.com/news/ar...estiga-426696/
Rotor & Wing Magazine :: AW609 Probe Cites Flight Laws Mismatch
AW609 flight control laws may have contributed to fatal accident - Vertical Magazine
https://www.flightglobal.com/news/ar...estiga-426696/
Rotor & Wing Magazine :: AW609 Probe Cites Flight Laws Mismatch
ANSV says the tiltrotor’s aerodynamic behaviour at high speed was not accurately predicted by the manufacturer Leonardo Helicopters; during simulator tests it proved virtually impossible to replicate the accident sequence.
The limitations on simulators and simulations is that you still have to find out what the aircraft actually does when you are establishing the performance envelope that your customers can use.
(For an example I am familiar with (my brother in law's dad worked for Beach ages ago) go back 40 years and check out what happened to a T-34C as they were working their test program up toward the 300 knot spec. The tail failed in flight. When the Navy finally put it into service, the red line was 280.)
From Aviation International News
AW609 Tiltrotor Flight Testing Resumes
AW609 tiltrotor prototype AC1 arrived at the Leonardo-Finmeccanica Philadelphia plant yesterday after recently resuming flight testing in Arlington, Texas. The AW609 flight-test program had been voluntarily halted following the fatal October 2015 crash of AC2 in Italy.
Plans call for AC1 to be be based out of Philadelphia, before being shipped to Italy for updates and modifications. The AW609, slated to be certified by the FAA, will be built in Philadelphia.
In May, Italian prosecutors impounded AC3 before it could make its first flight as part of their manslaughter probe into the AC2 crash. That aircraft was released by prosecutors last month and is expected to be shipped to the U.S. to join the flight-test program in Philadelphia, where AC4 is currently being assembled and readied for first flight in 2017.
Despite the 10-month delay in the flight-test program, as well as calls for wind tunnel retesting and redesign of the fly-by-wire flight control system by Italian ANSV aviation investigators, the company insists that the AW609 remains on track for certification in 2018.
AW609 Tiltrotor Flight Testing Resumes
AW609 tiltrotor prototype AC1 arrived at the Leonardo-Finmeccanica Philadelphia plant yesterday after recently resuming flight testing in Arlington, Texas. The AW609 flight-test program had been voluntarily halted following the fatal October 2015 crash of AC2 in Italy.
Plans call for AC1 to be be based out of Philadelphia, before being shipped to Italy for updates and modifications. The AW609, slated to be certified by the FAA, will be built in Philadelphia.
In May, Italian prosecutors impounded AC3 before it could make its first flight as part of their manslaughter probe into the AC2 crash. That aircraft was released by prosecutors last month and is expected to be shipped to the U.S. to join the flight-test program in Philadelphia, where AC4 is currently being assembled and readied for first flight in 2017.
Despite the 10-month delay in the flight-test program, as well as calls for wind tunnel retesting and redesign of the fly-by-wire flight control system by Italian ANSV aviation investigators, the company insists that the AW609 remains on track for certification in 2018.
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The fatal crash of the second Leonardo Helicopters (formerly AgustaWestland) civil tiltrotor prototype (AC2), N609AG, on Oct. 30, 2015 at Tronzano Vercellese, Italy, is ascribable basically to the “combination of three factors”: the development of latero-directional oscillations; the inability of the fly-by-wire flight control system (FCS) control laws to maintain controlled flight; and the failure of the engineering flight simulator (SIMRX) to “foresee the event in any way,” according to the final report from Italy's National Agency for Flight Safety (ANSV—Agenzia Nazionale per la Sicurrezza del Volo). The accident aircraft had accumulated 567 hours since first flying in 2006. It took off from the company's production facility at Cascina Costa and crashed at 10:42 a.m. local time while executing a third planned high-speed descent as part of test flight T664. During the descent the aircraft entered uncontrolled flight in a series of lateral-directional oscillations, broke up and caught fire in flight before striking the ground, killing both test pilots.
Difficulty of Recovery
The ANSV said that a combination of ground debris mapping and telemetry data led it to “hypothesize with reasonable certainty” that the aircraft broke up in flight as a result of multiple prop-rotor strikes from excessive blade flapping on the wings as a consequence of excessive yaw angles reached during the fatal dive. This damaged the hydraulic and fuel lines that are positioned along the wing leading edges, precipitating the in-flight fire. The aircraft was equipped with flapping stops, but they were not designed to “contain the effects of the extreme aerodynamic forces generated during the event.” Because of the aerodynamic characteristics of the aircraft and the specific conditions created by the dive, the flying pilot's attempt to counteract the oscillations with a roll-tracking maneuver to level the wings was ineffective, partly because the FCS was designed to “couple” on more axes than the command inputs given on the single axis by the pilot.
Specifically, “Total lateral control resulting from the summation of pilot input and automatic FCS input has an effect on the yaw axis through aerodynamic coupling and feedforward and feedback turn coordination automatically provided by the FCS. Consequently, giving a command in counterphase on the roll axis to dampen the relative oscillations creates an effect on the yaw axis that can be in phase with the yaw oscillations. This occurred during the accident: the correction of the roll oscillation induced, by the control laws of the FCS, a manuever in phase with the oscillations of the yaw axis, generating a divergence of the oscillations.” The ANSV said that the “low frequency and low amplitude nature of the oscillations” made them difficult for the pilots or ground crew to perceive until the roll and yaw “reached excessive levels only a few seconds before loss of control.”
The pilot flying also made rudder-pedal inputs. As explained above, the inputs exacerbated the situation, taking sideslip to maximum values. The tiltrotor entered a dive at the 293-knot design dive speed. AC2 was fitted with a new tapered rear fuselage and redesigned vertical fin with less surface area. During the dive, the aircraft reached 306 knots.
Investigators attempted to recreate the accident flight in the AW609 SIMRX in Philadelphia using the same software and flight conditions, but could not; they came close by inserting algorithms that changed the aerodynamic configuration of the aircraft, but even then the lateral-directional oscillations developed were in a different phase. They did, however, use the exercise to verify the “great difficulty” of recovery to controlled flight under the conditions. The ANSV found the inability to replicate the accident flight in the simulator unremarkable, given “the lack of experimental data obtained previously in the wind tunnel and in-flight evaluations with those speed conditions and relating to the recent modified geometry of the tail fin; this last change was considered conservatively by entering a reduction in the tail fin area into the database and then implementing the computational fluid dynamics.”
The ANSV made several safety recommendations after the accident: more high-speed and complex-flight-condition modeling, verification and wind tunnel testing as part of the AW609 certification process; and verification of the flight control laws in extreme flight conditions, in particular reviewing their effectiveness with regard to pilot inputs and uncommanded coupling effects.
AW609 flight-testing resumed in August last year. AC3 is flying from the company's Philadelphia facility and recently completed testing for flight into known icing. AC4 is under assembly in Philadelphia and is expected to fly next year. AC1 is in Italy undergoing modifications before return to the flight-test program. Leonardo Helicopters expects FAA certification next year.
Difficulty of Recovery
The ANSV said that a combination of ground debris mapping and telemetry data led it to “hypothesize with reasonable certainty” that the aircraft broke up in flight as a result of multiple prop-rotor strikes from excessive blade flapping on the wings as a consequence of excessive yaw angles reached during the fatal dive. This damaged the hydraulic and fuel lines that are positioned along the wing leading edges, precipitating the in-flight fire. The aircraft was equipped with flapping stops, but they were not designed to “contain the effects of the extreme aerodynamic forces generated during the event.” Because of the aerodynamic characteristics of the aircraft and the specific conditions created by the dive, the flying pilot's attempt to counteract the oscillations with a roll-tracking maneuver to level the wings was ineffective, partly because the FCS was designed to “couple” on more axes than the command inputs given on the single axis by the pilot.
Specifically, “Total lateral control resulting from the summation of pilot input and automatic FCS input has an effect on the yaw axis through aerodynamic coupling and feedforward and feedback turn coordination automatically provided by the FCS. Consequently, giving a command in counterphase on the roll axis to dampen the relative oscillations creates an effect on the yaw axis that can be in phase with the yaw oscillations. This occurred during the accident: the correction of the roll oscillation induced, by the control laws of the FCS, a manuever in phase with the oscillations of the yaw axis, generating a divergence of the oscillations.” The ANSV said that the “low frequency and low amplitude nature of the oscillations” made them difficult for the pilots or ground crew to perceive until the roll and yaw “reached excessive levels only a few seconds before loss of control.”
The pilot flying also made rudder-pedal inputs. As explained above, the inputs exacerbated the situation, taking sideslip to maximum values. The tiltrotor entered a dive at the 293-knot design dive speed. AC2 was fitted with a new tapered rear fuselage and redesigned vertical fin with less surface area. During the dive, the aircraft reached 306 knots.
Investigators attempted to recreate the accident flight in the AW609 SIMRX in Philadelphia using the same software and flight conditions, but could not; they came close by inserting algorithms that changed the aerodynamic configuration of the aircraft, but even then the lateral-directional oscillations developed were in a different phase. They did, however, use the exercise to verify the “great difficulty” of recovery to controlled flight under the conditions. The ANSV found the inability to replicate the accident flight in the simulator unremarkable, given “the lack of experimental data obtained previously in the wind tunnel and in-flight evaluations with those speed conditions and relating to the recent modified geometry of the tail fin; this last change was considered conservatively by entering a reduction in the tail fin area into the database and then implementing the computational fluid dynamics.”
The ANSV made several safety recommendations after the accident: more high-speed and complex-flight-condition modeling, verification and wind tunnel testing as part of the AW609 certification process; and verification of the flight control laws in extreme flight conditions, in particular reviewing their effectiveness with regard to pilot inputs and uncommanded coupling effects.
AW609 flight-testing resumed in August last year. AC3 is flying from the company's Philadelphia facility and recently completed testing for flight into known icing. AC4 is under assembly in Philadelphia and is expected to fly next year. AC1 is in Italy undergoing modifications before return to the flight-test program. Leonardo Helicopters expects FAA certification next year.
