To wit:

SlowFin:

http://i.imgur.com/KONK1xs.jpg

FastFin®:

http://i.imgur.com/X9xj1BS.jpg

I/C

I'm sure BLR wants no association with this incident

Puzzling to think the engineers at Agusta/Finmeccannica/Leonardo would make a modification like this (CG/weight?) knowing the lateral flapping capability on the 609 is (currently) locked out by design.

That certainly looks to be a not-insignificant areal reduction.

Ok quick read but two things stand out of my quick reading:

1. Flight recorders where severely damaged and significant information was lost. Surprising.
2. They don't really know what happened but seems to be related to unusually complex aerodynamics at the edge of the flight envelope, which was being tested at the time of the crash. Again surprising given the state of CFD in this day and age.

comments obviously most welcome :)

:confused:
Can somebody please explain me what the lateral flapping has to do with it?
It was in airplane mode at the time. Lateral flapping should not play a role here. Where am I wrong?

ATA,

The ED-112 compliant recorder (hardware) survived with no issues. The missing parameters had simply not been implemented yet. The recorders that were destroyed were flight test insturmentation. As stated in the report the TM data had the information. Loss of data was not an issue here.

The Sultan

Where did you see that mentioned? The report says 'lateral-directional oscillation'.

Here:

What would it help in that case, if it wasn't locked out? In airplane mode?

An interesting article about it here, with a sad premonition of what (may have) happened: http://history.nasa.gov/monograph17.pdf

(page 41 onwards)

Lateral flapping itself isn't locked out (then the rotor would not be gimballed), however I believe there is no lateral cyclic actuator to actually control flapping in the lateral axis (yaw in AP mode and roll in helo mode is accomplished by DCP). Its my understanding that there are provisions for an actuator but it has never been installed. V-22, for example, has a flapping controller and cyclic actuator setup that can mitigate lateral flapping.

Notice the map photo and the red triangles marked "LH HIT" and "RH HIT", I suspect that the yaw oscillation caused by reduced yaw stability from a resized tail created enough unmitigated lateral rotor flapping for a set of wing strikes.

Your are quite correct that the flight recorder survived - I misunderstood that part (and am still surprised that they were actually not required to be fitted in the first place...).

On a bigger picture it would seem that the aerodynamics of this aircraft are simply not fully understood, which, again, surprises me given the sophistication of CFD simulations in this day and age. What is the sticking point here ?

ATA

The original fin was sized for a reason so Bell-Boeing understood the aero very well. Note: Ship one with the original tail went faster with older software.

The Sultan

Well you seem to know a lot about this beast - care to elaborate (within reasons, of course).

From what I read in the interim report things don't seem so clear cut aero-wise...

ATA

Ship 1 with original fin exceeded the accident speeds and did not crash. Pretty clear cut.

The Sultan

How does that go fast fin actually work?
Looking at the BLR page, they say for conventional rotorcraft it improves hot and high performance, as well as max speed.
Is that because in the hover or slow flight a wider traditional fin acts as a sail counteracting tail-rotor power in port or starboard vector?
Does a larger fin cross section inhibit higher cruise speeds?

The BLR fastfin system includes an area alteration which reduces inflow blockage on conventional helicopter tailrotors, plus strakes along the tailboom to enhance coanda effect.

Neither of these are relevant on a tiltrotor. Presumably the tail change by AgustaWestland once they took over the program from Bell was for weight and aesthetics.

And remember, this is not a fast fin and it is not BLR.

Sans, just because it went faster when it was a Bell-Agusta doesn't mean that it was any better. Faster and heavier just means less people less distance. Nobody would buy a 609 if it could carry a poodle and owner to the next town.

The entire program has been gone through and when people hear AW609, they really need to remember that many of the people from Texas are still involved with the program and that the program is still headquartered in the USA (now Philadelphia).

Weight saving measures were already being looked at by Bell to increase payload and versatility before they ran out of money (again) and had to be bailed out of the program, so let's wait and see what AW does to rectify the issues that have been identified. They have committed a serious amount of money to this program over the coming few years, so they're going to sort this out.

To be fair, I never suggested either configuration was faster (better?) per-se. Only pointed out the airframe changes made by AW which appear to be a potential causal factor of these yaw events.

Ah... so a normal fin might add stability in yaw vector, but acts as a barrier to sucking air through the tail rotor, decreasing tail rotor power.
Sorry for the thread drift... have watched their video, but can't quite understand how it works.
With the Coanda effect - essentially the downdraft from the main rotor sticking to, and curving around the boom performs an anti-torque function - is that right?
How does a strake make that happen?
I understand the MD-500N blowing air out a slot to make it work - but a horizontal strake just disrupts laminar airflow?
Confused...

Would the discussion of the BLR fin not be worthy of its own thread?

Probably! But its an interesting aside as the AW 609 fin mod truly does look like the BLR fin profile.

The longitudinal strake on a round-ish tailboom causes flow disturbance and boundary layer adjustment that virtualizes an airfoil shape in profile (imagine the tailboom was shaped like a wing with its nose up, quite expensive to manufacture and structurally inefficient). However, if you take a round profile and add a (simple and inexpensive) protrusion to one side near the stagnation point, you get a very similar aerodynamic effect, believe it or not. Think of it as a sort of vortex generator.

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.

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.

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?)

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.

Indeed.

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.

As I mentioned earlier, large yaw oscillations could laterally flap the rotor (particularly on the 609) beyond its design limits of around 10 degrees and cause a wing strike which is why we see markers for "LH HIT" and "RH HIT" in the map, I presume.

Transition is arguably not the real tricky area as it happens at lower speeds - its high speed high dynamic load flight with flapping rotors where you typically get to the fringes.

That said, it is all highly dependent on specific rotor tuning where it would be most sensitive.

It did indeed break up in flight. I've seen the photos of the pieces falling from the sky on fire. It isn't pretty.

The way I read the language was yaw-roll coupling ("augmented dutch roll") but there may be more to it than that due to FBW flight control laws being involved.

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

From your second linked article. (Commando Cody)
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.

Accident report

http://www.ansv.it/cgi-bin/ita/Final...t%20N609AG.pdf

Very tragic, but test pilots know the risks

Is there any news on how the AW609 is faring in more recent tests?

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.