Comments on: Torsion-tension straps

By: sean c

sean c — Thu, 04 Aug 2011 08:31:21 +0000

The original article gives the impression that TT straps are made solely out of some type of elastomeric or rubber-like material, and that isn’t true. They look like rubber when you pick one up, but the actual working material inside the component is steel wire. If you take a look inside one that’s been cut open (and I have) what you see is thousands of hair-thin stainless steel wires wrapped around steel mandrells that the bolts go through, all inside the rubber coating. There is no way that a rubber strap could safely take that kind of load in tension, it’s literally tons of force pulling the blades outward. The same basic system (rubber coated multi-wire strap assemblies) also takes the centrifugal force load (not centripetal, BTW) inside the supposedly “rigid” rotor head of the BO-105 and BK-117 as well, hidden away inside the titanium hub, bathed in oil. On the larger 2-bladed Bells such as the 205 and 212 the TT straps are surrounded by grease, which leaks less than the older oil-filled hubs of the UH-1 series. Incidentally, elastomeric materials do a fine job of taking up the centrifugal force load in several common designs such as the AS-350 series, the Dauphin, the UH-60 Blackhawk, and several others, but always in compression, not tension.

By: John Montana

John Montana — Wed, 03 Aug 2011 20:43:16 +0000

Airwolf Aerospace LLC is worlds only manufacture of FAA-PMA TT Straps for the Bell 206 JetRanger & Long Ranger helicopters. We have spent lots of years and dollars developing a TT Strap that is superior to the OEMS. While our current part has the same life limits as the OEM part, we are well on our way of justifying a life extension to our TT Straps. We are helicopter operators ourselves and are doing everything we can to lower Direct Operating Costs for Bell Helicopters. We are not a one trick pony and you will see many new PMA parts coming down the line. Right now TT straps are our main focus as they are at the top of the list as far as we are concerned as they are replaced the most often.

For all you Robinson operators out there, you will just have to wait and see what we have planned for you.

By: Jim Borger

Jim Borger — Tue, 02 Aug 2011 21:44:43 +0000

I can’t remember the name of the chemicals involved but the EPA said they were cancer causing. After the problem was solved as to why the TT straps were failing the procedures were changed and no more 206s were lost due to that particular failure. I can only surmise that Bell didn’t want to accept the liability that would incur if they raised the replacement time and another failure occured.

One TT strap failure resulted in no injuries. Tony, on an offshore platform, loaded his pax and cranked up. He lifted to a hover and decided that the weather was dropping so he landed and shut down. Later the weather lifted and he loaded up again. In a hover, prior to taking off, the TT straps failed and the main rotor departed. The aircraft landed hard but stayed upright. Others weren’t so lucky.

By: Alex Kovnat

Alex Kovnat — Tue, 02 Aug 2011 13:49:33 +0000

@Jim Borger:

What coating material or materials did the EPA object to?

From what you tell us, better quality control over the curing process ought to solve the corrosion problem and thus hopefully, increase TBO times on the TT strap. Why hasn’t Bell done this?

By: Tim McAdams

Tim McAdams — Tue, 02 Aug 2011 02:01:18 +0000

Thanks for the comments regarding centrifugal force and centripetal force. Keeping it simple, centripetal force pulls toward the center and centrifugal force pulls away from the center (a centtifuge). The TT straps can fail when a force pulling away from the center becomes too strong. In this case, I believe centrifugal is correct.

Tim

By: Dave Alexander

Dave Alexander — Sat, 30 Jul 2011 00:02:57 +0000

I wish such articles by smart people wouldn’t contribute to the perpetuation of the fallacy of centrifugal force. The hub has to provide enough centripetal force to pull the rotor blades into the center of the circle, instead of continuing in a straight line that inertia otherwise imposes.

By: Jim Borger

Jim Borger — Fri, 29 Jul 2011 22:32:36 +0000

I saw the remains of a failed TT strap from a 206 crash. The 206 TT straps were a bundle of wires wrapped in a plastic like material. The EPA outlawed the chemicals used in the coating so the manufacturer had to change materials. The old material would continue to cure on the shelf even if it wasn’t ‘cooked’ properly but the new stuff stopped curing when the preperation process stopped. This allowed corrison to set in when the coating wasn’t properly cured and even a new set of straps sitting on the shelf could be well on the way to failure before even being installed, thus the calendar retirement.

By: Russ Kinne

Russ Kinne — Fri, 29 Jul 2011 21:09:11 +0000

There is a complicated relationship between centrifugal and centripetal forces, and calling one ‘pseudo’ only complicates things further.
They are both at work in rotary-wing aircraft, and it takes some work to understand them.

By: Jacques Beaudouin

Jacques Beaudouin — Fri, 29 Jul 2011 15:14:40 +0000

Dear Sir,
The force you mention is called Centripetal and not Centrifugal…… A Centrifugal force is a pseudo force……
Respectully yours.
J. Beaudouin

By: Avi Weiss

Avi Weiss — Wed, 27 Jul 2011 14:59:37 +0000

Mark;

Unfortunately, the stress of aging contributes to wear on the aircraft and its hard to know exactly how much time to allow, given the unbounded nature of environmental exposure during aging. I would agree that 12 years is likely ultra-conservative, but Frank has always been that way, most likely to protect his company. It doesn’t hurt for generating revenue either.

Bell discontinuing of the 206 was due in large part to that market’s transition to the R44, which meets most of the mission profile capability of a 206 for a fraction of the cost, as Bell has pretty much admitted to. What is sad is that Bell has not come up with an offering that would challenge the R44, but given the certification process and the market strength, I guess Bell has chosen to focus on the larger-mission profile market.