To Strake or not to Strake

[SayItIsntSo]

Bell Helicopter has issued an Information Letter to B206A/B & L series operators with a warning about using the BLR Strakes recently issued with a FAA STC for these models. The warning states that “Operating the aircraft with these strakes in place could result in unknown and undefined maintenance and inspection requirements and, additionally, render the procedures, retirement lives, etc., contained in the Bell manuals inapplicable.”

Are there any such operators or pilots / maintenance personnel on this forum that can advise how this warning is being taken by the industry /companies in their area, and their regulators.

It would seem possible (and in some countries probable) that this Information Letter could carry implications regarding the validity of a C of A for an aircraft with this STC applied. What does your organisation / regulator say about this warning?

Say it....

[NickLappos]

It is amazing how folks believe the reports from small companies about cars that run on water, spring loaded cylinders that mount in your trunk and make the car handle better, and strakes that solve all manner of handling ills. I have chuckled at the strake issue for a few decades, frankly.

I would suggest that the company that designed, tested, and certified your helo is the best source of advice. When they say things as strongly as the quote you provide, I believe it is fairly obvious that they are warning you, in no uncertain terms.

[SayItIsntSo]

In general I wouldn’t deny that what you say isn’t so, but Bell Helicopter also say in the same notice that “Bell has no knowledge regarding the designing and testing of these strakes…”.

So is it a case of establishing a legal defense just in case? Or is it a case of “Not invented here”?

As to whether strakes are a con-job or not, as you indicate you feel they are, I am sure that you know that Eurocopter tried them on the B2 model of the AS 350, and that there are Governments that have had them fitted to their Bell 205 fleets. Didn’t I once hear that New Zealand was a risk sharing partner, or similar, in this venture?

But does it affect ‘airworthiness’?

Sayit…

[Brian Abraham]

Nick, I see the strake came about from NASA research ( http://www.nasa.gov/centers/langley/news/releases/1998/Dec98/98-097.html ) and the USCG are fitting the item to their HH-65. I can understand Bell saying what they are saying so they can divest themselves of any liability should proceedings arise in the future. Are you saying the proclaimed benifits are all smoke and mirrors?
Brian

PS Can any Oz or Kiwi defence guys report on their experiences with the fitment of the strakes on the Hueys?

[TukTuk BoomBoom]

Bell have said the BLR strakes cause cracks in 205 tailbooms
Bell are now coming up with a strake kit for the 205/212 tailbooms themselves.
Eurocopter had a strake on the B2 but the B3 doesnt have one.

its a bit like the aeroplane T-tail arguments.

If nothing else its a handy place to put yor tools when working on the driveshaft.

[spinwing]

Interesting .... my experience with both the 212 and the 412 without the BLR kits is ... that the 212 tends to behave itself MORE than the 412 which is VERY twitchy when exposed to a right (turbulent) crosswind ...... the way that aircraft kicks and bucks would lead me to wonder what sort of stresses are imparted to the t'boom/empanage in the standard a/c configuration ....haveing read the BLR blurb on their website I would have thought the stresses with their kit fitted would have been reduced not increased!

I doubt that the FAA would have issued an STC for something unsafe or without the appropriate flight test and stress data being made available.

I see that most new a/c (i.e. AW139) seem to have them now!

Guess we will have to wait and see ???????? ;)

Cheers :hmm:

[Deiceman]

I doubt that the FAA would have issued an STC for something unsafe or without the appropriate flight test and stress data being made available.


Not necessarily true on the data side - there was an STC air conditionning system certified for a light twin a few years back that used bleed air - the FAA essentially took a quick hop and signed it off. Never mind that the hover performance took a big hit at other conditions than the day they flew! :ugh: The FAA from one ACO did not talk to the FAA in another region.
:confused:

I see that most new a/c (i.e. AW139) seem to have them now!

yep - but the darned boom is DESIGNED for it! :=

[NickLappos]

The STC can be issued by a region where the long-term effects of the changes are not within the expertise of the approver, an unfortunate aspect of the STC system. Manufacturers and original certifying regions are often frustrated by the degree and scope of STC work that sometimes violates the original design intent. The latest FAR approvals require that documentation as to the critical aspects of the aircraft (for future STC purposes) must become part of the certification data package that the manufacturer files with the other data. The S-92 has such a data package, I know of no other helo that yet complies.

This letter from Bell is literally a washing of hands of the home-grown changes that they are seeing. Those who want to read NASA reports or STC providers web sites might write those other folks if their tail cone falls off or the tail rotor system develops cracks or fails, Bell is now on record as telling you that the continued safety of the installation after such unauthorized modifications is not their responsibility.

[Brian Abraham]

Bell have said the BLR strakes cause cracks in 205 tailbooms
Bit difficult for Bell to come to that conclusion seeing as when we operated 205/212/412 there was always a boom in the shop being reskinned due cracks, some you could drive the proverbial truck through.

[andTompkins]

Don't forget that you are talking about different tailboom construction techniques between the 205/212/412 and the 206-series. I don't know how well a monocoque tailboom might hold up to additional stresses imparted by the strake, but that sounds like what Bell is going after.

[heli1]

Actually I believe the first strake mod was on the Westland Sea King during the Falklands Conflict in 1982when the Mk 4 Commando version was rush-fitted with them.Subsequently all the Sea King fleet had strakes fitted and the idea was also the idea was adopted for the new build EH101 and more recently the A109 and AW139 .The introduction of strakes on US aircraft post dates these earlier examples.
Whether NASA did the original work I don't know but it did patent thr design in the US and Boundary Layer Research won the civil sales agreement .Vis-a-vis Bell My belief is that the company is just covering its back.

[Graviman]

...first strake mod was on the Westland Sea King during the Falklands Conflict in 1982 when the Mk 4 Commando version was rush-fitted with them.

Considering that the skin in a SeaKing tail boom is about 1mm thick, and Bell are concerned about stresses in their tailbooms, i'm amazed this didn't cause any problems. If strakes are so effective, then why not just design an aerofoil section tailboom? My limited understanding is that the flow is so chaotic (particularly entering translational flight) that any benefit would be at best unpredictable, and at worst uncontrolable.

... 412 is VERY twitchy when exposed to a right (turbulent) crosswind ... haveing read the BLR blurb on their website I would have thought the stresses with their kit fitted would have been reduced not increased!

Haven't seen the website, but it depends on how strake is fitted. The global stresses may go down, but there could be local stress peaks which exceed the recommended fatigue life stress. The strake might introduce a stiffness discontinuity into an otherwise constant stiffness region. I know from experience how quickly local stress peaks lead to cracks.

Mart

[Shawn Coyle]

The NASA tests were with a two strake configuration, and Westlands Sea King strake was a single strake.
The issue for the Sea King was to provide extra tail rotor authority in a very specific wind condition (right side wind of about 10 knots). It worked quite well.
The NASA strakes also worked for side wind conditions, but didn't appear to provide much benefit in a zero wind condition - which is the condition that hover performance is tested for.
It may make a difference for yaw control activity in many conditions. I can understand Bell's concern for the strake being solidly attached to the boom as it is.
If there is a performance improvement it would be slightly difficult to prove it - a lot of hover performance testing needed.
And I've also see some STC stuff that left me wondering about the qualifications of the persons approving...(say no more)

[Deiceman]

If there is a performance improvement it would be slightly difficult to prove it - a lot of hover performance testing needed.

From what I remember (84 seems so far away) the Commando improvement was driven by the inability to carry reasonable sling loads between the ships headed towards the Falklands. In the demo in Yeovil, back to back tetsing showed a SIGNIFICANT increase in payload capability!:ok:

interesting side note. The RN had no way to send drawings to the fleet (or at least that's what I heard), so the Westland guys designed a set of telexes to describe the installation -- sent them to Yeovilton for a crew to try out, then drove over there to see if the installation matched their intent - a couple of iterations later the mod was telexed to the fleet! :D

Or at least that's what I heard :O