Intent behind Bell 205/212 WAT charts?

[heliman500]

Might seem like a pretty simple question, and i've always thought i had a pretty good understanding of it until someone asked why the 205/212/412 has it but some other bell products do not.

Understandably it provides for a take off/landing weight limitation based on factors of temp and altitude. The WAT chart is advertised in the limitations section, so clearly it becomes a limitation as opposed to a performance reference like that of the HIGE/HOGE charts. However, the HIGE charts mirror/inorportate the WAT chart so perhaps its intent is more of an engine performance indication.

The 212 WAT chart envelope was expanded with the upgraded 412 T/R which would perhaps imply the reasoning behind the WAT chart in the first place was that of tail rotor authority.

Perhaps it was for certification and to ensure compliance with H/V chart. I may be overthinking it, but any thoughts on the charts true intent would be much appreciated. Thanks!

[SASless]

Bell seemed to have a fondness for underpowered Tail Rotors.....for whatever reason and by doing so had to fabricate their concept of LTE when it was due to the design.

I imagine you may have opened up a rather good topic for discussion when you ask for a defining of "Intent".

[NickLappos]

In FAA Part 29 manuals, the maximum operating weight for the type of operation (Cat A or B) is expressed in the WAT curve, the weight, Altitude and Temperature that defines the max operating weight. For Cat B that is based on the basis of safe landing after engine failure, in conjunction with the H-V curve, and on the ability to demonstrate normal takeoff and landing. The Cat B WAT chart is many times identical to the HIGE hover chart, but many Bells have operating conditions where the tail rotor prevents crosswind ops and therefore becomes the limitation, instead of the main rotor performance. In those, the WAT chart shows a lower weight at some places than the HIGE chart. It can be seen as "bent lines" at perhaps 4000 feet where clearly something is reducing weight faster than normal atmospherics.

Some Bell manuals have a magic recovery of operating weight where pilots are allowed to ignore tail rotor weakness. These WAT curves for the 412 are specially marked "For operations with winds + 45 degrees of the nose". These special WAT curves allow operations where loss of yaw can occur when landing or hovering in cross winds. The normal FAR REQUIRES a cross wind capability of 17 knots for all helicopters, Bell applied for a special consideration as long as they were operating Cat B, with 9 passengers or less. The operator can attempt to work around the tail rotor's limitations by assuring themselves that they will not have a cross wind.

If you want my personal belief on this, PM me.

[megan]

SAS, Wonder how much of an LTE difference these make. Claim more thrust. Sorry if you think it advertising Mods, no involvement. Delete if necessary.

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[heliman500]

Thanks Nick; I was hoping you'd chime in. All makes sense now. We all want your personal belief ;-) Cheers

[SASless]

Megan,

One way to confirm "shortcomings" of an aircraft (in some cases) is by the kind and numbers of mod's that follow the fielding of an aircraft. Some stem from new technology but sometimes that derives from a perceived need.

Bell came out with some Long Ranger tail rotor mod's, they changed the 205 Tail Rotor's design, and other designs such as the Fast Fin have been developed.

Brother Lappos can tell the story as well as it can be told as he has a wealth of knowledge and experience at all levels.

[NickLappos]

It is bad form to make direct negative reference, otherwise I'd look like HC!

Seriously, the ability to have some excess yaw control is important, and should not be lightly tossed away. The crosswind is only one measure of yaw, it is easy to saturate your yaw control by raising the collective a bit too much at the bottom of an approach. After all, a 10% torque spike to arrest a slightly hairy flare needs 10% more tail rotor power, and if you PLANNED on having none, well don't be too surprised. Frankly, I wouldn't call it LTE if you planned the operation with no tail rotor margin!

[albatross]

The system that I never liked on a 212 was the torque limiter.
If set too low it could get you in a world of trouble.

[paco]

One thing I am sure to tell all my students about......