Torsion-tension straps

July 25, 2011 by Tim McAdams

The mass of a spinning rotor blade produces a strong force that pulls the blade away from the hub. It is known as centrifugal force and the greater the rpm or the higher the blade’s mass, the stronger the force. For helicopter engineers this presents a challenge because while the blade must be sturdily attached to the hub, it also needs the flexibility to change pitch.

Each manufacturer has designed its own method to address this issue. In the 1960s Bell helicopter designed something called a torsion-tension (TT) strap for use on the 206 series helicopter. The straps are made of a durable, elastic material and resemble a pair of elongated rubber bands with reinforced grommets on each end. One end is attached to the hub and the other to the blade. They are very strong in tension and at the same time can twist (torsion) to allow blade pitch change.

In the mid 1970s three 206B accidents were attributed to a TT strap failure where the component ripped apart under the centrifugal forces of the main rotor. In August of 1976 Bell responded by making the straps 1,200-hour life limited components, instead of on condition. Then in 1980, following the fourth failure of a TT strap in a Bell 212 the company added a 24 month retirement life.

Research by Bell showed that all four failures were the result of operations in a highly corrosive environment. Company engineers have been working on a solution to the TT strap problem by testing new materials and corrosion-resistant coatings. However, many customers have complained that the 24 month requirement is too short, increases their operating costs and is not necessary for operators who do not regularly fly near sand and salt water. Nevertheless, Bell Helicopter remains convinced that it is desirable to err on the side of caution, and that considering the factual history it is prudent to maintain the 24-month calendar life retirement until they can engineer a safe replacement.

12 Responses to “Torsion-tension straps”

  1. Mark Says:

    The only thing more ridiculous is Robinson Helicopter’s
    mandate that their entire R44 helicopter must be rebuilt
    after twelve calendar years, even if the total hours flown
    have not exceeded their 2200 hour limits.
    I notice Bell has ended their production of the 206 Jet Ranger,
    due to (in some folks’ opinions) the vast number of R44′s that are
    purchased by former Jet Ranger owners.
    I wonder who really wins here?

  2. Pranesh Dey Says:

    Hi Tim,
    Helicopter engineering/design is surely more complex than fixed-wing aircraft, it seems. But Tim, are choppers more difficult to fly than choppers? I mean how do you sense the approach of a stall? If a chopper stalls or enters an unusual attitude, is recovery possible? In planes at least all portions (wing/nose) are at fixed angle of attack. But what about choppers? The bubble inside which the pilot sits may be at a different angle of attack than the angle of attack of rotor blades.
    Pranesh

  3. Avi Weiss Says:

    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.

  4. Jacques Beaudouin Says:

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

  5. Russ Kinne Says:

    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.

  6. Jim Borger Says:

    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.

  7. Dave Alexander Says:

    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.

  8. Tim McAdams Says:

    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

  9. Alex Kovnat Says:

    @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?

  10. Jim Borger Says:

    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.

  11. John Montana Says:

    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.

  12. sean c Says:

    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.

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