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A couple questions


Chewie
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My instructor and I were shooting questions back and forth last week regarding rigid rotor systems and Fenestrons, and decided to see if we could get some other opinions.

 

Regarding rigid rotor systems: Are there other helicopters (operating today) besides the BOs that use a rigid roter system? We couldn't think of any. Also, apart from increased responsiveness what are some pros and cons of that type of system. We have a few but are interested in your opinions.

 

Regarding Fenestrons: Are they equally as susceptible to all forms of LTE as standard tailrotors? Example, I think they would be less susceptible to T/R Vortex Ring State because of the small space between the fan and the shroud. I think they would be more susceptible to Weathervaning based on the smaller fan surface area (less authority?) and larger tail surface area. Main-Rotor Vortex Interference is a toss up, we came up with ideas that could both decrease, or not effect a ducted fan's behavior.

 

Thanks in advance for any thoughts!

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Eurocopter uses "hingeless" rotor systems, which are essentially rigid rotors.

 

A hingeless rotor system is more an advanced fully articulated system than a rigid system. Hingeless systems have composite structures in the head that perform the same function as the hinges they replace but require less maintenance. Hingeless systems still allow blade movement relative to the head through the composite structure, hence they still flap, feather and lead-lag whereas a rigid head only allows for feathering.

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Don't believe ALL the spin put out by Bell - you will find that the ONLY aircraft ever to suffer LTE was the B206 with the small tail rotor. Bell published the LTE story and now everybody, including the authorities, believe that all helicopters are susceptible to it.

Google "Nick Lappos" (Bell test pilot, previously at Sikorsky) or visit the PPrune website to find out more.

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Don't believe ALL the spin put out by Bell - you will find that the ONLY aircraft ever to suffer LTE was the B206 with the small tail rotor. Bell published the LTE story and now everybody, including the authorities, believe that all helicopters are susceptible to it.

Google "Nick Lappos" (Bell test pilot, previously at Sikorsky) or visit the PPrune website to find out more.

I'm curious to know what you mean by the 206 with small tail rotor is the "ONLY aircraft ever to suffer LTE?" When just this morning I was doing an OGE hover check in a H300 with a student, the wind conditions were just right enough for us to be full left pedal, all the way to the mechanical stop, and still our nose was moving right. Wouldn't that mean that our tail rotor wasn't effective enough to power through that wind angle? Granted it wasn't a sudden and fast loss of effectiveness, but nothing I've ever read or heard about LTE said it had to happen fast.

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There's a difference between running out of pedal (demanding more of the aircraft than it can provide) and LTE, which happens in a snap when you are nowhere near the pedal stops at the time.

 

Because the spin is now folklore, you believed you were in LTE, when all that was going on was that you were in a particular combination of weight, power demand, wind direction and pedal turn that made you reach the pedal stops and still didn't have enough tail rotor power to stop the turn. Anybody who has a run-out-of-pedal accident blames LTE and people accept it, instead of kicking the pilot's rear end for poor technique.

 

If you had LTE, the nose would have turned until the vortex was in a different spot, or the wind was no longer from the rear, and then it should stop.

 

Note that the recovery technique for LTE includes opposing the turn with full pedal, which means you wouldn't already be at full pedal when it happened.

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Don't believe ALL the spin put out by Bell - you will find that the ONLY aircraft ever to suffer LTE was the B206 with the small tail rotor. Bell published the LTE story and now everybody, including the authorities, believe that all helicopters are susceptible to it.

Google "Nick Lappos" (Bell test pilot, previously at Sikorsky) or visit the PPrune website to find out more.

 

I don't quite get this either. In fact just the other night, as I was about to set down an R22, I felt the nose get a little squirly. From what I've read, that was LTE due to a slight tail wind, (even though I didn't lose it completely).

 

As far as reaching the full stop, I've also done that in an S300, so I don't think any of us are mistaking that for LTE! <_<

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Regarding Fenestrons: Are they equally as susceptible to all forms of LTE as standard tailrotors? Example, I think they would be less susceptible to T/R Vortex Ring State because of the small space between the fan and the shroud. I think they would be more susceptible to Weathervaning based on the smaller fan surface area (less authority?) and larger tail surface area. Main-Rotor Vortex Interference is a toss up, we came up with ideas that could both decrease, or not effect a ducted fan's behavior.

 

Thanks in advance for any thoughts!

 

The Fenestron has about as much authority as a standard tail rotor. You are correct that it does tend to want to weathervane more because of the larger tail surface. As far as main rotor vortex interference goes, it is just as susceptible as any other tail rotor design. We have actually induced it in training to bring on LTE and then correct it.

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In fact just the other night, as I was about to set down an R22, I felt the nose get a little squirly. From what I've read, that was LTE due to a slight tail wind, (even though I didn't lose it completely).

 

That doesn't sound like LTE to me. Landing with a tailwind in most aircraft will make the nose a bit squirly. The aircraft doesn't like wind up the ass. Just because it gets twitchy doesn't mean that you have any loss of tail rotor effectiveness.

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Of course the tail will get twitchy with a tailwind!! It is called directional instability, and it is like trying to make a dart fly backwards.

 

Get into your aerodynamics books, read up on stability.

 

In over 13,000 hrs, of which 6,000 have been in the B206, I have never seen LTE. This is despite lurking in the supposed places where it lives, like photography, film work, police work, powerline work, bushfires, external loads and so on. I have come close to running out of pedal many times, but by simply gaining forward speed and/or reducing power, it has never gotten to the stage of losing directional control.

 

Blaming a "squirly" tail on LTE just goes to show how well you all have been brainwashed. :blink:

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I don't quite get this either. In fact just the other night, as I was about to set down an R22, I felt the nose get a little squirly. From what I've read, that was LTE due to a slight tail wind, (even though I didn't lose it completely).

 

As far as reaching the full stop, I've also done that in an S300, so I don't think any of us are mistaking that for LTE! <_<

 

The R22/44 have some of the best tail authority out there. They get squirrely at times, like the gent said, they dont like wind up their ass....but that was most likely not LTE.

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Blaming a "squirly" tail on LTE just goes to show how well you all have been brainwashed.

 

Common parlance at our school, when we'd get a little bobble from TR VRS or weather cocking tendency, would be "Feels like a little LTE there." Sounds like there's a bit of confusion in the way things are being taught: MR vortex interference, TR VRS, and weather cocking are occurring when you feel those bobbles (right?), but the confusion is that these are being equated to LTE.

 

Does that go toward the deprogramming, or did I make it worse?

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MR vortex interference, TR VRS, and weather cocking are occurring when you feel those bobbles (right?), but the confusion is that these are being equated to LTE.

 

 

The thing is that the Rotorcraft Flying Handbook from the FAA is what most schools teach and therefore most recent students (I'm guessing the last 5 years or more, myself included) learn from explains just that. It says those 3 wind directions are indications of the onset of LTE and if not kept in check then one gets the spin Eric is talking about.

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Sorry if this is a bit long, but the following is from Another Forum, and posted by a very experienced test pilot, Nick Lappos:

 

" Most helo pilot can't experience LTE becausae most helos cant get LTE.

 

The term LTE makes me wince. The concept of the tail rotor somehow losing effectiveness is a convenient one for folks to use, because it allows the people who make small tail rotors to blame a mysterious force of nature instead of fixing their problem.

 

There are two possibilities for an LTE event to be triggered. They are both the result of you having entered a region where the tail thrust is not enough to counter the main torque because the main torque rose by itself.

 

They are neither because the tail rotor suddenly experienced massive reduced thrust.

 

LTE is almost always because the tail rotor has too little thrust BY DESIGN to account for small normal reductions in its thrust. Typical thrust variations of 5% are easily handled by tail rotors with that much margin above the thrust needed to do their jobs. When a tail rotor has no margin, by design, these 5% variations are too much, and the main rotor torque dominates, causeing loss of yaw control.

 

The two cases cited by hilico show how the term has now been so badly abused as to have entered the lexicon for any pedal stop event. An overloaded helo that runs out of yaw control does so as its tail rotor is producing thrust well in excess of its design capability. The tail rotr is not the cause.

 

LTE is a term invented by the team from one manufacturer who has to quickly train a bunch of pilots to compensate for a marginal yaw control. the worldwide data base shows that about 95% of legitimate LTE events is experienced by one type of helo (the 206).

 

Look at the hover curves of several helos to note that the hover weight is not determined by the power, it is determined by the tail rotor design thrust. These are prime candidates for "LTE" because the have "Tailo Rotors Too Small".

 

Please, to be precise and to teach proper procedure for recovery, do not call overpitching and loss of yaw control LTE, call it overpitching."

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The Westland Lynx and related designs use a rigid rotor.

Like fixed wings, the design is supposed to be simpler and more durable. If the BO is an accurate indicator, rigid rotors ride hard, especially slowing through translational- it was startling the first time I experienced it, even with a brief to expect it. The BO also seems to cruise nose-low.

 

Never flown a fenestron. The 130 guys I talked to noticed the lack of T/R control boost more than anything else. Aerospats have pretty good yaw authority, never heard any different about the fenestron ships- except that it took more time and force for desired input in the 130.

My guess is that LTE is such an infrequent occurrence in anything except a 206 that fenestrons probably haven't a big history.

One of proposed designs by Bell to minimize the potential T/R vortex ring state was the 'ring fin', a ductless ring around a conventional tail rotor. Looked as funky as it sounds, but I can't find an image. Look here-

 

Gov. Pilot Evaluation

 

I've hit the mechanical pedal stops in a few helicopters, including the 206. I've never had an 'LTE event' in any of them, in spite of a couple thousand hours in the 206, usually heavy, and occasionally at a high hover. I was able to approach the limits gradually, predictably, with available alternates (power, primarily, for yaw)- or exit strategies when things trended towards departure from control.

I agree the 206 doesn't have the yaw control that a lot of other 'copters have. The 206 is a compromise (as all aircraft are) 'light observation/utility helicopter' with more emphasis on the 'utility helicopter' than the 'light observation'. And that, I think, gets to the point- the 206 is a small, efficient transport by design. More tail rotor would mean more structure, less payload, less efficiency. Put it where the known yaw limit can be a sudden surprise due to gust or airflow change from the main rotor, or whatever- and you'll be unhappy with it, as you would any of the other airframes that make up the 5% of LTE that aren't the 95% of LTE events in a 206...

It's a great little truck even if it's not really a Loach, and certainly not a Sikorsky.

Edited by Wally
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