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.