Loss of Tail Rotor Effectiveness, recovery etc

[Flying Lawyer]

FODCOM 01/04: Latest information on loss of TRE

Click here: Loss of Tail Rotor Effectiveness (http://www.caa.co.uk/publications/publicationdetails.asp?id=1183)

[Helinut]

Certainly worth reading for all helicopter pilots.

It is a rather strange place to promulgate such information, since it is not really something peculiar to commercial operations, but it is good to see such a document - it should be circulated as widely as possible.

[RotorDompteur]

:ok: Thanks for the hint.

RD

[RDRickster]

It's a good read... short and to the point. Thanks for the post! I noticed that they didn't really describe the "onset" of LTE in any real detail. I suppose that's somewhat type specific, depending on tail rotor design.

I've never experienced LTE in the R22... even when flying in conditions that SHOULD produce LTE. That's definately a testimate to Frank's great TR design. I don't think anybody would disagree with that statement (even if you don't like Robbies).

As much as I enjoy the Brantly B2B, I have to say that the lack of TR authority is it's worst trait. As described in the article above, you are almost out of pedal in an IGE cross-wind hover (an indication of how susceptible the ship is to LTE). You definately have stay on top of LTE situations in a Brantly, and it is very easy to get into. LTE is easy to pick up in the B2B because you run out of pedal at a fast rate of decay (almost immediate, you have to react immediate).

The B47 isn't too bad; however, it seems fairly susceptible to get into LTE with an OGE cross-wind (no shock there). Actually practiced this scenario at higher altitudes a few times, and it can get dicey if you let it get too far. In an OGE hover, I noticed more and more left pedal over time, but it was much more gradual than the B2B's rapid decay. In the B47, you feel a gradual and progressive need to apply left pedal... until you pretty much run out. Then, a yaw begins to develop... sometimes with a slight vibration.

In both ships, as the article suggests, forward airspeed is the quick cure. In the B47, we let LTE get to the point where the tail started to come around... not good. I was with a CFI, and we dumped the collective and used autorotation to get out of it. I haven't experienced LTE in the 280FX, but I don't have enough time in the Enstrom to know anything.

What about other helicopters?

[Ascend Charlie]

In 35 years of flying, over 12,000 hours and many types of military and civilian aircraft, i have never experienced LTE.

Many times I have run out of pedal, but when it is on the stops, you have to expect that. Lower the lever, lower the nose, fly away using the margin of performance that you left for yourself.

But never had an uncommanded yaw when I still had plenty of pedal travel available. Crosswind hover, downwind hover, American / French / fenestron, never appeared. Lucky? Or careful? A bit of both perhaps.

[Hilico]

Funnily enough, the January AAIB reports have two mentions of this very thing, in differing circumstances.

This one (http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/page/dft_avsafety_026657.hcsp) describes a 206 on a photography detail losing it in a right-hand slip at fairly low speed.

And this one (http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/page/dft_avsafety_026659.hcsp) says what happened to a heavy-ish 500 departing a hotel lawn.

[NickLappos]

Ascend Charlie has is dead on. 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.

[SASless]

Nick,

I can recall experiencing "LTE"...but it was at 5800 RPM...red lights on...horn honking...right foot stuck clear out...collective under the armpit...and turning right...all the while wishing I was somewhere else. But that wasn't the tail rotor's fault....bet you been there too!;)

[Helinut]

FL has started another thread going "Pilot's death may have been avoided" about the sad BO 105 accident during external loads. The thread includes reference to the recent AAIB report which relates how the TR was impacted by the load during the accident. The report then goes on to talk about "loss of tail rotor effectiveness" and make recommendations that the CAA should publish information about LTE for helo pilots.

This may explain the FODCOM about LTE coming out just now - it is the CAA ticking the box on an AAIB recommendation.

This use of the term LTE adds yet another extreme definition - Is it LTE if you have lost tail rotor thrust (and/or your TR)? Surely loss of TR or TR thrust is a different animal from LTE?

[SASless]

Helinut...

So it would seem...afterall...assuming you encounter an uncommanded yawing...at some point if the tail rotor is still attached and working "normally" then LTE as conventionally defined will end at some point. That assumes the pilot has the ability to assess the situation and determine no major mechanical failure has occurred and continues to fly the aircraft until sufficient forward speed is attained by which "normal" control of yaw is regained.

[RDRickster]

In reading the previous links and a few other articles on LTE, I've noticed that almost every aircraft "decends" after experiencing LTE. Why is that? Why wouldn't the a/c simple spin, or I guess a better question is what factors rob the a/c of power?

Nick, being one of the newer generation of newbie pilots, I can tell you the "LTE" is found in most of the training curriculum out there. Just trying to understand your definitions of over pitching. Are you saying that when we brought the B47 into an OGE cross-wind hover (about 2500' MSL / 2000' AGL) that it was over pitching that caused the tail to come around like that? Please explain.

[helmet fire]

Ahhhhh terminology. It is the bane of all good technical discussions.

SASless, I think you will find that the thrust of Nick's post was that at 5800 RRPM and full left pedal in the Huey you most definately were NOT experiencing LTE.

RDR: the training manuals will not use the same terminology that allows for technical correctness: they often use a term to ensure that the basic concept is quickly inderstood by a pilot whom does not need to get involved in the pure technicality of the issue, see the threads on gyroscopic precession and centrifugal forces for example. On a previous thread I proposed a terminology "cleanup" for this topic in particular response to the fact that so many seem to confuse LTE with the overpitching example Nick gives.

Nick: I have chosen the term LTA in preference to overpitching because overpitching infers that the pitch has exceeded the ability of the blade thrust to overcome the drag induced by the pitch angle, in the same way as it is used to describe the main rotor situation. I believe it is possible to run out of pedal without overpitching - ie you have reached maximum tailrotor lift ability but the main rotor torque is producing a requirement exceeding this. LTA covers overpitching as a cause as well as torque requirement exceeding lift producing ability.

Here are my thoughts for you all to shoot down:

Loss of tail rotor control: You are not able to control the tail rotor pitch mechanism.

Loss of tail rotor thrust: Little spinning thing at the back stops spinning or falls off.

Loss of tail rotor effectiveness (LTE): "Newly" discovered and named in the 80's after many (in particular OH-58/B206) accidents. Although somewhat awkwardly named (as the tail rotor is still effectivly working and must be providing thrust) LTE refers to what is thought to be an injestion of main or tail rotor vorticey through the tail rotor which causes an onset of yaw in the direction induced by torque that cannot be overcome by the application of full "power pedal". The yaw rotation can build up quickly enough to fool most pilots into believing they have experienced a loss of tail rotor thrust. The concept has come under fire lately because of the early thoughts that the tail rotor enters vortex ring state being a little hard to proove. Oh - and then there is fenestron stall that possibly fits into this category too, although strongly denied as a possibility by the manufacturer whilst alledgedly being strongly experienced by the pilots!

Loss of Tail Rotor Authority(LTA): A term to make the old Huey war story of "..and then I ran out of bloody left pedal and..." sound a little more sophisticated and technical. In this situation, the tail rotor does not produce enough lift to counteract the torque/crosswind combination you require, your power pedal hits the stop, and around you go - though often quite gently when compared to LTE or loss of thrust. A lot of aircraft are susceptable to this, but the UH-1D/H Huey is famous for it - and many people have had the earth come up and smite them as a result. Someone mentioned the BK117 - it too is quite susceptible to LTA unless fitted with the C model tail rotor.


...be gentle:8

[SASless]

Helmet Fire....

I fully understood from whence Nick was speaking....and used the Standard Vietnam UH-1D takeoff profile as an example of asking more from the machine than it could provide. Good CRM in those days meant ensuring you could always turn to the right on takeoff and hopefully descend after clearing the obstacles...without the other pilot wondering just what you were up to.:D

[Shawn Coyle]

Excellent discussion. I'll try to work the terms and definitions into the next edition of the book....

[[email protected]]

Helmetfire - Nick was not referring to the TR overpitching, he is talking about the main rotor - the reason being that as you overpitch the MR the NR decays, therefore the TR slows down as well reducing its efficiency and thrust.

[Genghis the Engineer]

Daft question from a plank pilot here if I may.

As I understand it LTE is primarily due to the horseshoe vortex intercepting the tailrotor disk, probably during slow crabbed flight. This causes an inevitable loss of yaw authority, and the aircraft yaws right (American helicopter) Left (Russian helicopter) or some indeterminate direction (European Helicopter).

Why, I ask myself, does the aircraft not yaw an alarming but not necessarily life-threatening 45° or so until the tailrotor disk is out of the horseshoe vortex, restoring normal yaw authority and allowing the pilot to bring the yaw rate back to zero and restore normal control over the helicopter? Is it a case that most pilots, following their training act before that point is reached as if they'd suffered a tailrotor transmission failure?

G

[LGNYC]

Hello,

It seems to me that the nice and logical distinction that Helmet Fire made between LTA and LTE parallels the one between SWP and VRS:
- Settling with power = you are asking for power you don't have, in a way equivalent to LTA, and
- Vortex ring state = the aerodynamic doughnut can happen to the main rotor, the "usual" VRS, or to the tail rotor, and then it is LTE.

Now, from what I understand, there is another effect, which is due to the main rotor/tail rotor interaction (with vortices being blown from the main to the tail), which Helmet Fire puts in with LTE. For this one I don't see a main rotor equivalent.

Bon vent

[[email protected]]

Two of the 3 incidents cited at the beginning of the Fodcom occurred during mountain flying approaches in breezy conditions, one an approach to a ridge line and the other an overfly of a peak. These are both situations where the wind strength and direction can change rapidly due to mechanical turbulence from the topography (and not just the immediate feature but others well upwind of it).

As the Fodcom states, a critical stage of flight is around the translational lift boundary - normally you accelerate or decelerate through this speed range in a progressive manner, adjusting the collective and pedals as the rotors gain or lose the benefits of ETL. In the mountains or anywhere in gusty wind conditions, the rotors can go from ETL to no ETL in the blink of an eye as the wind can change in both direction and speed.

The worst situation would be a sudden loss of ETL as the aircraft would descend and yaw right (for american rotation) so the pilot would instinctively raise the lever (especially if close to mountainous terrain. The TR was only producing enough thrust for the 'above ETL' condition and now suddenly there needs to be enough pedal input to compensate for both the loss of ETL and the increase in MR pitch and Tq. If it is available and then applied then no problem - if it is not available then you have run out of TR authority and only forward airspeed will save you. If it is available and you don't use enough of it quickly enough the the yaw will continue and possibly accelerate - the response to this increased yaw rate is probably one reason why the aircraft descends - the pilot instinctively lowers the lever.

The other type of scenario is when the aircraft, as Nick points out, is built with insufficient yaw control margins to cope with the effects of the MR vortex from the retreating side of the disc entering the TR. This phenomenon is what they really seem to mean by LTE.

As for the TR experiencing Vortex ring -Boll8cks, which is the same answer to the Fenstron Stall argument. The Aerospatiale video of the test pilot getting the Gazelle up to 120 degrees per sec rotation and then stopping it dead using right pedal (not sure what it did to the transmission as that much pedal in a Gaz will produce a huge Tq spike) should poo poo anyone's theories about Fenestron Stall (it's just mishandling).

[NickLappos]

Ghengis,

The dreaded horseshoe vortex and the other artiifacts of LTE are almost as fake as crop circles.

The main wake spilling into the tail rotor causes about 5% reduction in thrust, easily seen and measured on most helos. The pilot sees it as a bit more pedal needed at that condition. Try it, just move forward at about 10 to 15 knots in the 1 to 2 oclock direction. You can hear a different noise from the tail, and the pedals will vibrate a bit. Usually, the left pedal will have to go forward a bit to hold the nose, or else a right yaw will develop. This is what most helos experience, not the dreaded LTE. The only tail rotors that cannot withstand the loss of this 5% are those that are so marginal that they cross the line to negative anti-torque margin when the interaction occurs.

Some facts:

Most single rotor helicopters with normal (powerful) tail rotors have never experienced LTE. Never in their service lives, with thousands of pilots. Never.

Most LTE events are experienced by one type/model of helicopter. About 95% of the LTE accidents worldwide are on this type, I know I did a survey of three years of worldwide accident data back when the maker of that one helo tried to get the FAA to insert that LTE drivel into every single rotor helo flight manual. If they had succeeded, then lawyers for that company would have been able to claim that the cause is common to all helos, instead of explainimnmg that the real reason is the poor residual yaw authority, which leaves little margin to cover minor variations in tail thrust.

That blurb in the first post of this thread is so pervasive that many incidents now being mislabled as LTE when they are actually due to over pitching - demanding too much power, causing rpm reduction and then loss of yaw control. Calling overpitching "LTE" is like saying that an airplane hit a cliff and stall was to blame because the wing was doing 0 mph right after impact.

Let me take you through the typical overpitching accident:

Pilot pulls main torque and hits engine power limits.

RPM reduces and main torque continues to increase (constant power is shown by increasing torque as rpm reduces, since power is torque times rpm.

Tail thrust is being reduced as rpm drops, since the tip speed is dropping, and max thrust is reduced by the square of the rpm (it is a v squared function).

As torque goes up, moore anti-torque is needed, even though power is constant. This eats more left pedal.

The pilot hits the pedal stops, rpm reduces more, torque goes up more and the right yaw develops.

Uh-oh....

Just dont call it LTE.

Good post Crab, BTW.

[Ascend Charlie]

Good posts, Crab and Nick.

Question for Nick: You said that Power is Torque times RRPM - I can't say I've ever heard that stated anywhere, and it is a good equation to throw in front of a student.

But I tried to do a dimensional analysis on the equation and got bogged down. Probably the rum-riddled brain, but how is this:
Dimensions used are Mass=M, distance=L, time=T

Power = work done per unit time
Work = force x distance
Force = mass x acceleration
Acceleration is distance per second per second, or LT(-2) (Sorry, can't put up a squared sign)
Thus Force is MLT(-2)
Work is MLT(-2) times L, or ML(2)T(-2)
and Power isML(2)T(-2) divided by T, or ML(2)T(-3)



But RRPM - what dimensions does it use? radians/sec? And Torque is a twisting force, so it must have MLT(-2) in it, so is RRPM simply distance per second, or LT(-1)?
Help!:sad: :confused: :(

[Genghis the Engineer]

Nick and Crab, thanks very much I consider myself educated.

However Nick, living where I do (a few miles from Middle Wallop) I have to tell you that crop circles are entirely real and provide much summer amusement for local FW pilots, as well as a valuable extra source of revenue for a couple of helicopter operators. However, like LTE, the cause is sometimes subject to debate ;)


Ascend Charlie Being absolutely correct power is torque (N.m or lb.ft) times rotational velocity (radians/s) measured at the same point. It's important that these are measured at the same point, not either side of the gearbox which puts your sums out by the gearbox ratio².

So if you multiply torque by rotational speed you get dimensions LM/T. Energy (or work done) = force times distance (LM). Divide by time and you get power (=work done per unit time).

I think you confused yourself by bringing acceleration into the equation, which isn't necessary.

G


N.B. For a squared² sign on a normal keyboard, hold down the left-hand "Alt" key, type 253 on the small keypad, and release the alt key.

[Shawn Coyle]

A couple of points

The UK military Gazelle had several instances of loss of fenestron effectiveness (for lack of a better word). High yaw rates that could not be stopped with pedal. So it can happen in other types than the one alluded to by Nick...

One of the little understood parts of the problem is that a high yaw rate will have a substantial effect on the governor- believe it or not. For the dreaded LTE, yawing to the right in a North American direction of rotation main rotor, the governor thinks the main rotor is turning much faster than the datum speed (yawing counter to the rotation of the main rotor), so it reduces fuel flow to the maximum extent it can. This of course reduces the tail rotor RPM as well, with all the effects that Nick elucidated earlier. This effect is not well understood, but is there and a major factor.

And the formula of Power equals torque times RPM must include some constant - if all you have is a torque meter in percent, and RPM in percent, then the dimensions are a moot point. For the Bell 206BIII, using 310 shp as the transmission limit of 100% torque at 100% rotor RPM, the torque constant is 0.0310. So, assuming you're at 100% rotor RPM and 85% torque, the horsepower you are using is 85x100x.0310 = 263.5 shp
Useful for flight testing when you have to measure power required to hover and compare it to power available.

[NickLappos]

Shawn is right, others can get LTE, but the early model Gazelles that experience the LTE had less tail fan thrust. Later models that had the increased thrust are much less likely to experience LTE - I have heard of none for them. My premise is simple - LTE is not a pervasive single rotor helo problem, it is only a problem on helos with very marginal tail thrust.
In short, LTE is an excuse to accept marginal systems. The term must not be allowed to enter our usage as a common helicopter problem, or we will lose the ability to hold for reasonable capabilities. An example is that several model helos have been approved by the FAA with NO yaw authority (their hover weight charts are marked "For Operations with the wind within + 45 degrees of the nose") . These are LTE events waiting to happen.

Regarding the rotational speed of the helo, this is a contributor, but a very small one, and only important if the aircraft has very low yaw margins. A 30 degree per second turn is a moderately sporty one, and yields about 5 RPM. For a helo with 300 RPM main rotor, this is a reduction of 1.66% RPM, or about 3% thrust. In smaller helos, the reduction is even less as a percent, of course. This reduction is in the noise, unless you are in a marginal aircraft to begin with.

Ascend, try this:

Torque is Ft-Lb, and rpm is simply per second, so the product is ft-lb per second, the unit of power. Another way to see it is to understand that torque is simply the twisting force, similar to a pure force, and the revolution is the distance through which it passes. Thus the torque taken through a revolution is work, and the number of revolutions per unit time is the work per unit time, or power.

[212man]

On a related note; what is it that pitches the nose down at high rats of yaw? On the EC-155 the yaw rate to the right is limited by the torque limit, but at lower weights when reaching this limit the nose down pitch is dramatic. I've seen 25 degrees, which feels like you are going to slide into the ground. Not seen it so much in left turns, but then I'm more circumspect about yaw rates to the left as stopping the turn could be awkward! Either way, there's no danger of lack of authority in the machine, without a doubt!

[PPRUNE FAN#1]

I'm hesitant to jump in here, because Nick and Shawn and others have addressed the canard of "LTE" so well. But out of curiosity I read that FODCOM thing and was quite amused. To wit:The pilot reported that he was on approach to a ridgeline landing zone about 70 ft above ground level decelerating through about 20 kt. Suddenly a gust of wind induced a loss of directional control. The helicopter began to rotate rapidly about the mast and impacted the ground.

The pilot reported that he made a low pass over a mountain peak into a 40 kt headwind before losing tail rotor effectiveness. He then lost directional control and struck the ground.

The pilot was manoeuvring the helicopter at about 300 ft AGL at slow speed when the aircraft entered an uncontrolled descending turn. Unable to regain control the pilot closed the throttle and attempted an emergency landing.So...we have three accidents here. With seemingly little other justification, we are asked to take the word of these three aviators that it was LOSS OF TAIL ROTOR EFFECTIVENESS! that caused their crashes.

Hmm. I'm suspicious.

I remember reading something years ago (was it put out by Bell or some other agency...perhaps an FAA publication?) on LTE. In it, there was some sage advice given to pilots who encountered "LTE." In the first place, pilots were urged to apply FULL pedal, if necessary to counter the spin. The authors of the publication evidently believed that pilots come to a conclusion that their tail rotors are "stalled" like the wing of an aeroplane, and that a reduction of angle-of-attack ("power-pedal") is called for when in reality, the tail rotor is not stalled - is never stalled - and is still producing thrust!

I was going to make some pompous, definitive statement like "I have NEVER gotten into LTE with a quartering wind from the front! Never!" But then again, I've never flown a helicopter at really high altitudes and gross weights. Down where I've flown for most of my life (sea level), I've always had plenty of anti-torque pedal available. So maybe this mysterious interaction between the main rotor and tail rotor vortices could happen.

But I wonder... Of the three hapless pilots cited in the FODCOM, how many actually pushed and held full power-pedal as their helicopters corkscrewed themselves down to the ground?

Finally, an aside to 212man. I'm no Shawn Coyle, but I do know this: When the anti-torque device is lower than the main rotor, there will be a roll/yaw coupling. The 205/212 does not exhibit this trait, naturally, as the tail rotor is up nice and high near the hub. But on other types (like the 206), if you look at the ship in a hover you'll see that the t/r is well below the hub level. Just because it's way "back there" doesn't mean that it can't have an effect up where we're sitting. In these ships, there will be a noticeable rolling moment when yawing in the direction of MR rotation. The harder the tail rotor/fenestron "pushes," the stronger the rolling tendency will be. That roll translates (pun intended) into the nose pitching down.

[NickLappos]

PPrune Fan#1 said:

".........but I do know this: When the anti-torque device is lower than the main rotor, there will be a roll/yaw coupling. The 205/212 does not exhibit this trait, naturally, as the tail rotor is up nice and high near the hub."

Nick sez:

Sorry Fan, but the tail rotor is a worse actor when it is high. The concept that the rotor is the center of action for the aircraft is simply incorrect. All forces and moments can be considered to act on the center of gravity of the aircraft, not the rotor hub. The height of the tail rotor above the roll centroid of the aircraft determines the amount of cross coupling you get from a yaw input. If the tail rotor his high above the tail cone, the aircraft rolls sharply when you put in pedal. If the tail rotor is centered on the tail cone, there is little roll for a yaw input. I have flown aircraft where we moved the tail rotor down (away from the rotor but toward the tail cone - recall the Fantail S-76?) and the whole stability of the aircraft improved!

Somehow, we pilots tend to see the aircraft as hung from the rotor hub, so we think of that place as the center of the aircraft's little universe. Not so, the CG of the aircraft is where the forces act, the higher above the CG, the more roll we get for a yaw force.

Most helos have a vertical CG that is about in the mid point of the cabin, height-wise. Think of the principle masses as the fuel tanks, the cabin cargo/pax and the transmission. Those three chunks kind of average as half way up the cabin from the floor, above the place where the longitudinal CG is.

[Shawn Coyle]

...some days I'm no Shawn Coyle either....


The three instances of supposed LTE are pretty thin on important details.
If they all happened to Bell 206 series machines then a common, but unreported (and hence unknown) thread might be that they were all turning right from into wind to downwind at an out of ground effect height above the terrain when they 'lost tail rotor effectiveness'. But as with a lot of the Army accidents that prompted this whole thing, there is no indication of having sufficient power to hover out of ground effect in the first place - also turning right from into wind to downwind and trying to maintain a constant ground speed will probably put you into a good position for vortex ring state (or since power available may have been less than power required is it 'settling with power?).
Couple the whole thing up with the effect of the governor, and you get a descending right turn where adding collective doesn't seem to stop the rate of descent, and adding right pedal doesn't stop the rotation, so it must therefore be the dreaded LTE.
Unfortunately in my experience, accident investigators don't often know what information they should be looking for in some of these accidents, and we're left with less than perfect knowledge of what could have caused the whole thing.

Just one more thing - the position of the tail rotor with respect to the vertical CG is important as previously mentioned - put the tail high (as in a nose down situation) and it will have a big effect, which may just get worse as the rate of rotation increases.

[PPRUNE FAN#1]

Nick sez:Somehow, we pilots tend to see the aircraft as hung from the rotor hub, so we think of that place as the center of the aircraft's little universe. Not so, the CG of the aircraft is where the forces act, the higher above the CG, the more roll we get for a yaw force.Sorry, Nick ol' buddy, ol' pal, but you are wrong. Quite wrong.

What you describe may occur in forward flight, but in a hover the helicopter surely does "hang" from the hub, especially ships with teetering systems.

Simple question: Why does the UH1/205 (and the Sk76, and the Kaman H-2, and the FH1100) do nice, level pedal turns while the 206 has such a strong yaw/roll couple?

[Ascend Charlie]

A "couple" of reasons there:

1. In the 76, you are only yawing at less than 30 degrees per second (1 turn per 12 seconds) which is a Flight Manual limitation.

2. It has SAS on, stabilising the beast

3. Other machines will hang left skid low in the hover (cg and tail rotor thrust couple), so when it starts to turn rapidly, you already have a roll apparent. When Bloggs sees this and tries to get the picture "flat" he has in fact induced a roll, and away he goes.

[NickLappos]

pprune fan#1,

The idea that one hangs from the rotor like an object hangs from a string has legs, and seems to be unshakable. Stick with it if you'd like.

Oh, btw, how come the left skid hangs low in a hover when that good old rotor should just straighten things out with that hanging force?

[maintranschip]

"Fenestron stall" always was a good topic of conversation with plenty of theories, but unlike LTE, or whatever you wish to call it, noone has ever really seemingly come up with a decent explanation. Crab is correct in one thing, it wasn't a stall, although all the early teaching for dealing with it was based on that theory. (It's amazing what you can produce when talking about helicopter aerodynamics!) So the teaching was centralise the pedals don't put in any more right pedal(that would exacerbate the stall), a strange feeling when rotating rapidly left. Crab then contends that it was gross mishandling. I'm presuming he hasn't been there, and if you read the accounts of it you'll see that few of the instances appeared to be so. (I'll stand to be corrected). I'm sure if it had been the case then Boscomb Down, who carried out a number of trials, would have been able to replicate it, they couldn't. It was intruiging though that the manufacturers managed to prove you could stop it, but gave no insight into the onset. It obviously wasn't their problem, "look it's not a stall"(sorry I don't know the sign for a french accent) . As I remember it took something like a turn and a half for the applied pedal to take effect and the spin to come to a stop. Easy to do in retrospect of time and knowledge but seemingly an eternity if the applied pedal doesn't seem to be doing anything. "Now how long shall I hold this?" as the ground rapidly approaches.
The manufacturers continued to point the finger at "le rost beouf" (sp?), quietly overlooking the fact that there were some documented cases of the french military and some tail rotor problems or something or other that appeared to have caused the aircraft to hit the ground quite hard!!
Have there been any or many in the last few years? I don't know. Was it mishandling or was it a case of requiring a number of circumstances, wt, power, wind, degree of pitch applied etc, all to be applied coincidently. What I do know is that it happened with little control input to induce the problem. Over the years the Gazelle has been grossly mishandled by students and experienced alike but is incredibly forgiving. If it was simply mishandling then surely there would be far more occurences........ or is it a figment of imagination.
Any ideas?

[LGNYC]

Maintranschip: the right spelling is "les rostbeefs" - a loose translation would be the limeys

[The Nr Fairy]

LGNYC:

Sorry to be picky, but isn't it "les rosbifs" ?

[LGNYC]

Nr - you're right that I put a "t" where it should not have been, but I think there are two spellings: rosbifs and rosbeefs. Difficult to check for sure anyway. Sorry for the confusion.

[cpt]

Having flown the gazelle in the forces on long (much too long) hovers in all possible wind directions, I remember having experienced some kind of "tail rotor effectivness" mainly on right wind conditions....it always had been very short and easily recoverable in hover.
In my mind, this would have been caused by the position of the tail rotor in it's shroud causing a combined turbulent and uneven airstream with an angle of attack of relative wind.
I also have seen this on a 365C when lifting from an offshore platform, the wind was from my 2 o'clock, I wasn't very heavy but on lifting, the nose went left (torquewise) with not enough right pedal to stop it. I landed again quickly , on a different heading and following attempt was OK.
Apart from this, despite thousands of hours(sling,long line, off shore...) on AS 365 I have never met this again but I know it's waiting !

[[email protected]]

Maintranschip - the AAC who fly non SAS equipped Gazelles have never had a Fenestron Stall event - the RN and the RAF both flew SAS equipped aircraft and all the incidents occured on these aircraft.

Why? well in the hover the SAS yaw channel does some of the work for you, meaning that the pilot never gets a real feel (unless he flys SAS out for a lot of the time) for the idiosyncrasies of the Gazelle. Therefore it is easy for a disturbance in yaw to the left to be initially masked by the SAS which means the the real required pedal position is not selected as the SAS actuator has started to adjust the fenstron pitch.
Then, if the yaw deviation persists, eventually the yaw channel saturates (only 10% authority) and gives up the job. Now the actual pedal position is way behind that required to contain the disturbance and now the pilot has to do some work. He makes a sensible input and it doesn't work because it actaully needed twice what he put in to prevent further yaw so the yaw rate increases.
If the pilot now pushes full right pedal he will arrest the rate of yaw (the French test pilot proved it from 120 deg/sec) but more often than not pilots are reluctant to use that much pedal -they have never had to use it before so it can't be right so they lower the lever to reduce the rotation and usually land heavily.
An Army Gazelle pilot who has flown SAS out all his life will have used full right pedal before, especially operating with a left crosswind when heavy and will have no qualms about putting it to the stop which will arrest the rate of yaw.
As I said before it is not a stall, despite all the cobblers spouted at Shawbury about the suction over the lip of the duct providing a large percentage of the anti torque force and other great theories.
The gazelle was built for cruise flight when the tail could produce all the anti tq thrust at 120+ kts, it wasn't designed to spend all it's life in the hover and the fenstron is not s super efficient tail rotor - but it doesn't stall.

[Shawn Coyle]

crab:
I had a couple of AAC pilots relate to me that they'd had the machine swap ends unexpectedly, but didn't report it, as they didn't think it was unusual (mind you that was back in the '80s - 1980's that is).
And after the first incident / accident at Shawbury, one of the engineers from Performance Division at Boscombe Down went to France and got a pretty good briefing from the French about the fenestron and what would cause the problem.
At a subsequent meeting in town, a French test pilot privately admited to a problem, but in the meeting their side denied there was a problem. Typical....
So I'm not sure it was just a RAF/RN thing. But that was a long time ago, and far away. And memory is one of the first things to er, ah, what were we talking about again???

[maintranschip]

Crab. I hear what you are saying and like the bit about the SAS masking it initially, but how come the spin goes so far so quickly? Surely if we're only talking 10% then you're going to get some reaction after movement of the pedal. In these cases the "effectiveness" of the fenestron seems to be dissappearing pretty rapidly. In normal conditions the pedals are responsive in fairly high rate turns but for some reason that has gone? Yes the french proved it does happen eventually but it wasn't immediate by any means.

[[email protected]]

Maintranschip - I think that the SAS allows the yaw rate to get out of control because the natural reaction of the pilot is not to input a large yaw pedal movement. Most of us (even Sea King pilots) are taught to be gentle and progressive with the pedals so it is unnatural to be otherwise. Once you let it start to go it does seem to accelerate quicker than you might expect but maybe the boaundary between sufficient thrust and insufficient thrust is more critical in such a small TR.
The Gaz with SAS out can be rather twitchy, especially in a crosswind and I have had the little red Tq light on and almost full right pedal applied on quite a few occasions. I flew 3 years in the RAF/RN SAS equipped ones and then 7 years in the AAC non SAS aircraft - it took a while to develop the required sensitivity of the toes on the AAC ones.
Shawn, I know the AAC have had some swap ends but it is usually as a result if hovering with strong cross/downwind components trying to do observation work over Belfast.
There may well have been a similar problem of limited TR control margins (as Nick highlights on the 206) on the early Gazelles but Aerospatiale addressed this with the optimised Fenestron - although I can't remember how they optimised it.

[maintranschip]

As I remember it they basically enlarged the cover in the center of the fenestron to cover the roots of the fenestron blades and thereby improve the airflow.

[Shawn Coyle]

My previous post in this thread on the effect of the governor on the whole issue of LFE / LTE is more magnified than usual in the Gazelle, thanks to the wonderful governor.
My experience has been that the tighter the RPM control that the governor has, the more pronounced the effect that yawing has on rotor RPM (and height control in a hover) - a governor that keeps the rotor RPM very close to the datum will react more strongly than one that sort of keeps the rotor RPM more or less perhaps maybe constant.
One manufacturer had to get a modification to their FADEC to overcome the problem of tight rotor (or N2 if you're really anal) control when they went through translational lift and the sudden change in drag on the rotor due to the change inflow made the governor decrease fuel flow quite markedly. Change in governor gain stopped them from plummeting to the ground.
The Gazelle has a very good governor- uses engine oil pressure as the 'medium' to sense changes, and reacts quickly to changes in power to keep the single-spool engine at the datum speed. If you start yawing quickly to the left, the governor senses the rotor speed is too high, and cuts back on the fuel. 60 degrees per second is not unusual, and that relates to 10RPM. Can't remember the Gazelles rotor RPM in actual RPM, but 10RPM is significant enough that the governor would sense it. So, without doing anything to the collective, you'd descend.
But one of the problems is that few people own up to having had a loss of tail rotor effectiveness and fewer bother to report it through official channels (in the civilian world, you'd not be looked on with any favor in your company if you started doing this, for example).
And as previously stated, accident investigators often don't think of this as a possible cause of accidents.
I know that the H-500 early series are notoriously lacking in the T/R department (we have one...)

[lotsahueys]

After watching the 2 LTE videos posted recently, I pondered on how to recover form such incidents.
The only definite I know is that at the onset of LTE - (Being High Power setting and Full pedal), that you need to lower collective and forwrd cyclic out of it.

I would imagine the Skycrane would kind of cyclic out of the situation it found itself in.
But the Jet / Longranger incident...has me pondering.



Regards

Lotsahueys

[Ascend Charlie]

First of all, you gotta get your definitions right.
LTE= loss of tail rotor effectiveness
Running out of pedal= demanding more from the system than it can produce.

There is some doubt about whether LTE really exists or is a figment of the B206's imagination.

If you are just running out of pedal, reduce power, reduce weight (pickle the load) and gain forward airspeed.

Know your aircraft, get a feel for it, and when you know your left leg is nearly straight out, poke the nose over and get some speed - try to keep some reserve up your sleeve - or trouser leg.

[Woolf]

Paper on LTE published by the UK CAA (http://www.caa.co.uk/docs/33/FOD200401.pdf)

[rjsquirrel]

Ascend Charlie,

Thank you! You are so very spot on. The problem we have is that the folks who make one helicopter that gets itself into NETR (Not Enough Tail Rotor) calls the condition LTE, as if a very adequate tail rotor suddenly enters the Bermuda Triangle and quits helping. This is to obscure the fact that it has too little tail rotor authority to begin with, so some slightly abusive flying causes it to get the spins.

Some facts:

95% of all LTE mishaps are suffered in ONE helicopter model (guess which one!) and 100% occur with one Brand of helicopter.

The term LTE was devised to help that company protect itself from suits from pilots around the world, and also to be sure people think all single rotor helis can get LTE.

If the pilot droops the main rotor rpm by a lot and hits the pedal stops and turns around, it is NOT LTE.

If a pilot pulls 120% main rotor torque and spins around it is NOT LTE.

Very few heli models have ever experienced LTE, most models will never experience it, some models get it way too often. That is because the lucky winners of this smallest tail rotor contest are blessed with too little tail rotor thrust, and actually suffer from NETR.

The real cure for LTE is to avoid those models that have experienced 95% of all true LTE events.

I would also say if your left leg is getting too much unbent, lower the power somehow, as the fastest way to get some antitorque back.

[Canadian Rotorhead]

Like they say...

"An ounce of prevention is worth a pound of cure"

RH

[lotsahueys]

Thanks guys for the good info so far. As a fledgling I am all ears.
Keep it coming!

[PPRUNE FAN#1]

rjsquirrel:Some facts:

95% of all LTE mishaps are suffered in ONE helicopter model (guess which one!) and 100% occur with one Brand of helicopter....And which brand would that be, Sikorsky Skycrane?

If the tail rotor is not supplying enough thrust to counteract the MR torque, thus allowing an uncommanded yaw, then I would accept that condition as one in which the tail rotor is not being effective (i.e. has lost it's effect). If we want to strictly and narrowly define LTE as...well, I'm not sure what the strict definition of LTE is, so I cannot finish that sentence.

We know and even Bell admits that during textbook LTE the tail rotor never stops producing thrust - that it is never "stalled" as an airplane's wing can stall. So for whatever reason, sometimes a yaw rate builds and the maximum thrust provided by the t/r is not sufficient to stop it. Sounds like LTE to me, why quibble?

Uncommanded yaw rates are bad. Pilots must be conditioned if it happens to stick in and hold *full* countertorque pedal for as long as it takes and to reduce the MR torque if and when possible. Neutralizing the controls or putting in anything less than full pedal is not going to help and maybe only going to make things worse.

[Ascend Charlie]

OK, Fan, so if a pilot allows his RRPM to bleed by pulling too much power, and the aircraft settles or even crashes, is that a case of LEE (Loss of Engine Effectiveness)?

Of course not.

It is a pilot error, by demanding more than the system can produce. Mind you, under some conditions it is pretty hard to avoid going for that extra pull, but if it is outside the aircraft abilities, it is not LTE.

LTE would be defined as a situation where the tail rotor should be producing sufficient thrust, with plenty of pedal travel available, but suddenly decides not to.

A bit like a car going around a corner - it should be within its capabilities, but in the case of one car (I think it was a rear-engined Ford) it decided instead to roll over. Did Ford commission an investigation into bad roads? No, they removed the car after Ralph Nader showed the public that it was unsafe at any speed.

Even Mercedes Benz redesigned their baby car after it failed the moose swerve test.

I have over 6000 hrs on the 206 and ran out of pedal, knowingly, many times. Mostly on powerline inspections, or else film jobs. I always knew when i was approaching the limit, and just flew out of it. Whether the relative wind was from 330, or 080, or 160 or straight up the choof, it never behaved in an unpredictable way.

But I cursed it soundly for not being able to hang in there.:{

[[email protected]]

lotsahueys - going back to your original question - in the skycrane incident the sudden change in wind strength and direction was the culprit taking the aircraft from ETR (enough TR) to NETR quite quickly. In situations like this, if height and obstacles permit, allowing the ac to yaw into wind can often sort the problem, accompanied with a slight lowering of the lever. If all that doesn't work then you are into the process described above - lose weight (pickle load) and try to get forward speed -ETL over the TR.

I have suffered NETR in the Wessex (Sikorsky) on many occasions and the old Mk 1 Lynx (Westlands) was renowned for it - manufacturers who don't put a big/powerful enough TR on the back of their machines shouldn't hide behind the label of LTE.

[rotaryman]

...Ascend Charlie:

LTE would be defined as a situation where the tail rotor should be producing sufficient thrust, with plenty of pedal travel available, but suddenly decides not to. ???Umm I beg to Differ!

I have never yet flown any machine that has a mind of its own and can decide to not produce sufficient T/R Thrust to over come Torqe!! :hmm: I always thought it was a series of Conditions that the pilot has found himself in...:ok: :E

[Ascend Charlie]

Unfortunately there is no avatar smilie for showing tongue-in-cheek irony.

Of course no helicopter ever does it.

That's why LTE is horsefeathers. Simply, Not Enuf Tail Rotor.:yuk:

Sorry, that tongue is right outside the mouth, but you get my drift.

[the coyote]

lotsahueys,

Back to your original question of how to recover.

If for whatever reason or definition you don't have enough anti torque (tail rotor thrust) and you have lost directional control (ie spinning), the only solution is to reduce torque. Whether it be LTE, or low RRPM, or no more pedal left, or a control failure or a severed drive shaft, the Torque is winning over the anti torque.

Hindsight is always easy, especially when you are sitting on the ground and not in the saddle at the time. But when I watch that video of the 206 I personally think the pilot was quite slow to take corrective action, ie it spun a few times prior to recovery.

In that particular situation I would have immediately lowered the collective fully and if that wasn't enough to stop the spin also rolled the throttle off. (ie removed ALL torque if I had to), then regained speed and brought the power back in. It looked like he had sufficient height to be able to do that.

None the less, full marks and well done to the pilot for recovering safely, it would have got the blood pumping no doubt!

[IHL]

Check out this link:


http://tcinfo/civilaviation/SystemSafety/Newsletters/tp202/1-02/V007.htm

[Devil 49]

Emergencies are seldom identical to training scenarios, so it really doesn't matter what you call'em. What counts, is not putting yourself into an unsurvivable situation. In other words have a realistic plan, safe limits, and situational awareness.
Yaw is primarily related to torque at a hover. You can induce yaw by increasing TQ, and reduce it by decreasing TQ. If you hit the pedal stop, and the pedal applied isn't effective, and power variation isn't the extra minute bit of authority, you must make big changes- first power, then airspeed to get the vertical stab effective, and perhaps to execute an autorotational landing if nothing works.

Sometimes, you have to say "no" to a request.

[Shawn Coyle]

To revisit history - the LTE incidents typically happen when turning right at no airspeed, or low airspeed, when out of ground effect.
part of the problem is the engine governor will reduce rotor RPM with respect to the earth (not necessarily with respect to the airframe) and this will affect the tail rotor RPM, and hence tail rotor thrust. The faster the rate of rotation, the more the governor will back off the main rotor RPM (with respect to the earth, not the airframe- the governor is directly connected to the N2 turbine, and is basically thus counting 'blades passing tailboom' - and since you're rotating in the opposite direction to the rotor, the governor thinks the rotor is going too fast)
The tail rotor is still producing thrust, but just not enough to overcome the rate of yaw.
The tested and proven solution is to add full left pedal and forward cyclic and get some airspeed over the vertical stabilizer to get some directional stability.
But prevention is the better solution.

[rjsquirrel]

Pprune fan,
Notwithstanding the video (which could have been experienced by any helicopter anywhere if the rotor rpm is pulled low enought) the vast vast vast majority of "LTE" accidents are in Bell 206, because that heli has a tail rotor that is just too small. Bell invented the term LTE as a clever way to make the problem universal, and therefore not theirs! It is far easer to take out the rubber stamp that says "Pilot Error" than it is to fix the helicopter.

Someone in pprune once posted an accident summary that showed that 95% of all LTE occurred in Jetrangers.

Note Shawn Coyle's comment that the Bell 206 hover weight is actually limited not by power, but by its tiny tail rotor!

LTE = NETR

[Spunk]

D:mad: it, this subject keeps coming back at me all the time. And everytime I think that I finally understand what you wise guys are talking about and most important what I'm doing up there in the air you get me all confused again.:{

Could someone please clearly point out the difference between LTE, LTA, tail rotor stall etc. And please if it doesn't cause any inconvenience use plain English:}

Cheers,

A youngster listening to the old wise guys

[NickLappos]

Plain English:

Any single rotor helicopter can run out of pedal if the main torque is driven high enough and the rpm is drooped low enough, but almost every helicopter has a hard time doing this within normal flight maneuvers because most tail rotors have enough margin.

Some helos with very marginal tail rotors are more prone to finding insufficient anti-torque within normal maneuvers, and for them, extra care is needed, and the term LTE was invented. LTE stands for "Loss of Tailrotor Effectiveness" which actually never happens, because the tail rotor never loses effectiveness, and actually produces its required thrust under all conditions within the normal envelope. When LTE rears its head for those helos that are prone to it, what really happens is that the small thrust margin is not enough to overcome the wind, or the extra main rotor torque that the pilot is pulling. In those cases, the pilot hits the pedal stops, the aircraft keeps spinning, and its Katy Bar the Door.

In the old days we thought the tail rotor was stalling or some such myth, but we now know that the paltry tail thrust margin of some helos is just not enough, and that the other forces (wind and main rotor torque, chiefly) can swamp the tail rotor. For those marginal helos, care must be taken with crosswinds and power applications at low speed to prevent this swamping of the tail thrust (LTE).

For most helos, the tail rotor is bigger, and has more pitch available so that it can overcome moderatly abusive flight conditions, and not make the helo swap ends. These helos do not ever experience true LTE.

Recovery? Reduce main torque, and also reduce the crosswind. Not easy when doing 100 degrees per second to the right! The old AH-1G cobra gunships that I flew were so prone to this (due to pitifully small tail rotor margins) that we used to practice recovery techniques in checkout.

[[email protected]]

Spunk - if the helicopter yaws when you don't want it to you apply pedal to stop the yaw. If you reach the pedal stop and the aircraft is still yawing you have reached the limit of your tail rotor authority (max pitch). In this condition the torque produced by the engine driving the rotor cannot be balanced by the anti-torque force from the TR.

One of 2 things can happen now - either a. the rate of yaw will gradually decrease and stop, possibly because you have yawed back into wind and weathercock effect on the fuselage/vertical satbiliser has produced sufficient anti-torque force to balance MR torque, or b, the rate of yaw will increase and your only course of action is to reduce the MR torque (lower the lever) and/or gain speed (the TR benefits massively from translational lift).

The question you have to ask now is why?

Have you put the aircraft in a position (high AUM, high DA) where you are at the limits of the RFM and been caught out by slightly different conditions ie wind direction and strength than you expected?

Have you flown the aircraft badly and failed to anticipate the requirement for extra pedal as you lose translational lift (requiring both more torque to hold height and more TR thrust to maintain heading)?

Have you allowed a rate of yaw to build up because you weren't paying attention and the available pedal isn't enough to stop it?

Have you pulled so much lever that the Nr is drooping so that the TR is slowing down as well and reducing the thrust it can produce?

All of these could be described as LTE because it isn't doing what you want it to and there is not enough TR thrust. BUT they are all equally a case of NETR for the same reasons.

Ideally a helicopter will always have a surplus of TR thrust available to cope with all situations but if the TR is too small then it is much easier to encounter uncorrectable yaw in all the above situations - THIS is what critics claim is wrong with the 206, it just doesn't have sufficient surplus TR thrust to make the pilot's job easier.

It is thoeretically possible to put a TR into vortex ring state but I think the rates of yaw/crosswind would be phenomenal unless the aircraft was hovering at very low power and subsequently TR pitch (therefore the flow through the TR would be minimal and small rates of yaw could give an opposing airflow) ie in a strong updraught.

Sorry Nick you must have posted while I was writing!

[NickLappos]

No sweat, Crab, your post was right on I think. The point is that some residual thrust is needed to assure control.

I do know that the official FAA reading at the time the 206 was certified was that full yaw control inputs were permitted during trimmed maneuvers as long as the aircraft was inside the envelope and the control was developing a positive yaw rate while against the stop! If you were hard against the pedal stop, but developing 1 degree per second of yaw rate, you were ok.

Regarding VRS in tail rotors, the resulting sideward speed is surprisingly small. On an S-76, it is perhaps 25 knots of left side flight, on an Agusta 109 I flew it was about 22 or so. You can tell if you simply fly sideward at ever increasing speeds to the left. VRS is where the pedals dance like crazy and suddenly shift by about 25% as the flow reverses through the tail rotor.

[Spunk]

Thanks Nick, thanks Crab,

glad to see I had the right idea but was just confused by the various amounts of "terms techniques";)

[Shawn Coyle]

Not trying to defend the indefensibly small tail rotor on the 206, but the H-500 early models aren't exactly sparkling in the tail rotor department.
And I'm sure there are others that have had LTE as well - just not as prevelant as the 206 / OH-58.

[SkyMaster19]

From the training side, I as well as others are confused by the FAA's Practical Test Standards (ATP Helo) that require pilots to have adequate knowledge of normal and abnormal procedures i.e.powerplant, electrical, hydraulics, but wait....there towards the bottom is "Loss of Tail Rotor Effectiveness" What's the normal procedure? How do you query a pilot on an aircraft that has NO history of LTE i.e. SK-76?

Keith

[NickLappos]

Skymaster19,
It might seem confusing, but it is true that some helos get LTE and some don't, even when an FAA publication makes you have to teach it to everyone.

The Apache, S-92, S-76 and Black Hawk are aircraft that have tail rotors designed with enough thrust margin that LTE is not at all probable, but you still have to teach LTE. How do you query a pilot on an S-76? Ask him/her "What do you do for LTE?"

The test standard is a testiment to the LTE crowd who made it part of the lingo, for reasons stated in posts above. I personally pushed back hard at an FAA wish to put those recovery words in every helicopter flight manual, because I did not want the typical pilot to believe that every helicopter could get LTE.

[PPRUNE FAN#1]

Yes, perhaps only 206 and Skycrane pilots should be queried on LTE if other helicopter types are immune. Everybody else? Oh, you can just skip that question.

[Mars]

Could Nick, Shawn or some other airworthiness guru explain to all of us why the issue of ineffective tail rotors have not been addressed in the Continuing Airworthiness process?

This thread appears to indicate that a basic design flaw is being ignored and substituted by a requirement for LTE training; or am I being naïve?

[NickLappos]

Mars,

Very astute question. The answer is somewhat mired in the politics of how these changes come about. I was on the FAR/JAR 27/29 harmonization team for the HQ area, and found one company that absolutely stonewalled any consideration of sharpening the regs about TR margins, notwithstanding that they had the vast majority of LTE accidents.

At that same time (1997?) that company was actually certifying several of their aircraft with NO tail rotor thrust beyond zero wind hover. Look at the Bell 412/430 performance data for those peculiar charts that describe hover with wind within 45 degrees of the nose. The reduction in TR capacity that reduced crosswind capability from 17 knots to 0 was cashed in as increased hover performance! The people who were doing that had no intention of allowing TR requirements to be increased.

The problem with PPruneFan#1's point of view (all cats are gray in the dark) is that when you call all TR control symptoms LTE, you obscure the ability to solve the problem. With a marginal TR, a pilot comes to a routine hover under conditions that are inside the envelope and finds himself spinning around - that is LTE. With any helo, even one with high TR margins, when he pulls the rotor rpm down below his ankles, it is not LTE. If we blur the issue we learn nothing from it.

[PPRUNE FAN#1]

Nick sez:The problem with PPruneFan#1's point of view (all cats are gray in the dark) is that when you call all TR control symptoms LTE, you obscure the ability to solve the problem. With a marginal TR, a pilot comes to a routine hover under conditions that are inside the envelope and finds himself spinning around - that is LTE. With any helo, even one with high TR margins, when he pulls the rotor rpm down below his ankles, it is not LTE. If we blur the issue we learn nothing from it.No, I just can't stand pedantic people. Whether you call it LTE or not, the result and the escape are the same.

It absolutely does not matter to the pilot whether he is inside or outside the envelope, or whether he is in an aircraft with a hugely strong tail rotor or a wimpy little one. The only problem that needs to be solved is if a helicopter pilot comes to a hover and cannot maintain directional control with full anti-torque pedal stuck in. At that point, as the water-bomber Skycrane pilot found out, he *is* in LTE. Period. The corrective action is the same for a Skycrane as it is for a 206: maintain full pedal, reduce torque, attempt to gain airspeed.

We know that the 206 has a weak tail rotor. Constantly bitching about it does not change that fact. It is a limitation that is not the sole proprietorship of the 206. Other helicopters also have weak tail rotors; early Hughes 500's and Enstroms for two, and the FH1100 was no monster either in that department. ISTR that the SA-341G fit in that group too and could quite readily get into LFE (you work it out). However it is only the 206 that seems to have acquired the dreaded appellation "LTE."

I have flown plenty of helicopters into situations where my left American boot was firmly pressed against the pedal stop, ready to input the corrective action if a yaw rate developed. Whether that yaw rate was the result of a too-small tail rotor or other factors is really moot. The issue of whether the tail rotor is being effective back there is certainly not blurred for me.

But if you chaps want to split hairs and get all technical just to denigrate one particular aircraft to the exclusion of all others, well, have at it.

[NickLappos]

PPRUNE FAN#1,

You must think people who don't agree with your simplistic view of flying as pedantic.

You foolishly blurted:
"It absolutely does not matter to the pilot whether he is inside or outside the envelope, or whether he is in an aircraft with a hugely strong tail rotor or a wimpy little one. The only problem that needs to be solved is if a helicopter pilot comes to a hover and cannot maintain directional control with full anti-torque pedal stuck in. "

As a matter of simple fact, it does matter if his aircraft bites him when he is inside the envelope. We sign an agreement with our aircraft for ourselves and our passengers. We agree to obey the rules, and we ask our aircraft to have adequate control and reliability to keep its end of the bargain. The statistics show that 95% of all "LTE" accidents are suffered in ONE type. This type is notoriously weak in tail rotor control, in any case. We will never improve our industry, and our safety records if we insist on trusting fate and relying on special skills instead of demanding of our machines some honest predictable characteristics. And we will get no changes if we allow those pesky rubber stampers to put "Pilot Error" on accidents that are caused by inadequate control margin.

You say that it does not matter to you when an aircraft loses control within its envelope. With that attitude, when you take out your next insurance policy, please list me a benificiary. I will pay the premiums! ;)

[delta3]

Nick

Interesting discussions and ideas sofar (except for some false tunes...)

Coming back to your TR-VRS figures : 22-25 Kts, I tried to do impuls theory on the tail rotor (assuming MR-polar efficientcy between 10-15) and came with the following figures (for instance for a R44)
Induced speed MR : 6.6 - 7.5 m/s, Induced speed TR: 13-16 m/s
Assuming the max danger zone at 0,8 induced speed this would give the TR VRS- danger area in the range of 10-12 m/s which is exactly the figures you give. This is of course a very rough calculation to determine orders of magnitude. Is it correct ?

As far as practical tests you referred to is concerned, what is a safe set up ?


d3

[NickLappos]

Delta3,

I used the S-76 TR which is highly loaded, and assumed 600 lbs of thrust on an 8 foot disk for a hover (about half the thrust of an R-22 main rotor?!!) to derive 29 knots as the downwash speed. Your estimate of .8 for the onset of VRS is good, I think.

To fly this, one must do what we call sideward flight. From a hover, carefully accelerate to the side, using a slight bank angle. Maintian height with collective, and carefully maintain heading at 90 degrees to the flight path, using pedals. The aircraft will naturally stop accelerating at a speed for the bank angle, and requires more bank to go past that speed. We generally use a pace car that has a calibrated spedometer, and we fix our flight path on that car, so it is frozen relative to the aircraft, thus the aircraft has the car's speed. On a calm morning, this gives us very good data on the crosswind/sideward handling qualities.

For many helos, the tail rotor is good for about 17 knots sideward (regulations wisely call for that as a minimum, except for those somewhat flakey "9 passenger or less" approvals had for some machines). At the maximum speed, the pedal will be close to the stops, and the aircraft cannot go any faster sideward, because the pedal cannot maintain the perpendicular. The aircraft slips out of sideward flight and the nose starts to swing toward the direction of flight.

For those helos which have powerful enough tail rotors, the max sideward flight speed can be as high as 50 knots (the S-76 family has 51 knots to the right and 57 to the left at MGW!). I have often demonstrated Black Hawks at 120 KM/HR, as shown on the doppler navigator. This is why I feel as I do about the LTE label being applied to all helos!

As the S-76 is accelerated beyond about 25 knots to the left, the nose twiches a bit, and the pilot must dance on the pedals to make it stay at 90 degrees. By about 25 knots, the pedals make a 2 inch shift in position, which is where the tail rotor shifts to windmill brake state and the downwash shifts from blown to the lefy to blown to the right.

Notwithstanding the type of test, a helicopter bombing about at 25 knots to the side is in a precarious place if an engine quits, so the pilot must carefully weigh the safety for himself. Also, if one gets diverted and allows the skid height to drift too low, a roll-over is a real concern, with disasterous results.

[Thomas coupling]

PPrune - I'd have assessed that response of yours as pedantic - no?

Don't get in a spin about it :D

They're only torqueing about it.

[delta3]

Nick,

thx, I did similar flights using large taxi ways on an airfield nearby practising the change over from forward to sideways. I was not precisely aware of the exact speeds since all my attention went to controlling the MR-blows (cfr your comment of steering the cyclic in the wind in case of lte pirouettes in on other discussion). So I never really figured out what the speed exactly was (15-20-25?). I could have watched GPS of course, but all my attention was fixed on IAS -to anticipate transitions- and attitude.

Your set up is indeed much more controled.

For S-76-C I get around 17,4 m/s at full load and assuming a ratio of 15 on the polar (which is approx 700 lbs) giving 35knts*0,8=28 knts

Doing these calculations makes me think now that those early exercices were dangerous, since I now understand there is a big difference between right and left for other reasons that I first assumed, namely TR-power versus TR-VRS and I do now remember left slips produced extra vibrations...

d3

[NickLappos]

Delta3,

Except for the possibility of ground contact, there is little danger as long as the flight maintains the sideward/crosswind limits of the flight manual. Power actually goes down with sideward speed, since the rotor really does not care about wind azimuth, only about velocity.

Your estimate of TR thrust is quite close! You can calculate the required anti-torque for any helo, if you know the total power needed, since the MR torque is a function of the total HP and the rpm. Then you can calculate the TR thrust by taking the anti-torque moment, and dividing by the distance from the main rotor to the tail rotor.

[PPRUNE FAN#1]

Dr. Nick wrote:As a matter of simple fact, it does matter if his aircraft bites him when he is inside the envelope. We sign an agreement with our aircraft for ourselves and our passengers. We agree to obey the rules, and we ask our aircraft to have adequate control and reliability to keep its end of the bargain. The statistics show that 95% of all "LTE" accidents are suffered in ONE type. This type is notoriously weak in tail rotor control, in any case. We will never improve our industry, and our safety records if we insist on trusting fate and relying on special skills instead of demanding of our machines some honest predictable characteristics. And we will get no changes if we allow those pesky rubber stampers to put "Pilot Error" on accidents that are caused by inadequate control margin.

You say that it does not matter to you when an aircraft loses control within its envelope. With that attitude, when you take out your next insurance policy, please list me a benificiary. I will pay the premiums!Nick, I'm going to cut you some slack, because I know you don't have all that much flight time (compared to some of us) and you have ZERO commercial/civilian time...you know, real-world flying.

I can assure you that real, working helicopter pilots sometimes run up against performance limits. In my long and varied career (10,000+ hours) which includes scores of survey/photo flights, I have hit collective (power) limits, run out of pedal and even touched a cyclic stop or two in different kinds of helos. Never did I feel like I was teetering on the precipice of disaster. Never did the aircraft depart controlled flight. So put your pompous, holier-than-thou attitude away, okay? Not all of us are know-nothing R-22 students bowing at your feet.

Your prejudice against and dislike of the 206 is well-known and documented. But let me ask you a simple question: What would you prefer, the complete grounding of the 206 fleet? You act like every airborne 206 is an LTE accident waiting to happen- that at any time the pilot of such a deathtrap terminating his approach to a hover might discover to his horror that his pedals have somehow become ineffective even though he is right in the middle of the flight envelope! Eek!

Some of you guys act like the 206 is the only aircraft to experience LTE, and moreover that this mysterious "LTE" is strictly a function of a poorly-designed helicopter. The fact that 95% of all LTE accidents happen to one particular type of helicopter and the fact that until recently this particular helicopter just happened to be the most popular helicopter in general use seems to elude people. I'd wager that if OH-6's were being used like the OH-58 scouts were (heavy, OGE hovering over trees with little respect for wind) you'd all be shrieking about the miserable tail rotor on the OH-6 and how it ought to be legislated out of existence.

In all of my flying in the 206 (around 8,000 hours- just about your total time, eh Nick?) , I got into "LTE" exactly once. I was messing around up high in a downwind hover, trying to get an accurate power check in an L or L-1 (forget which...whichever one needed zero airspeed and pull to the TOT limit). When it snapped around, it caught me by surprise. But by keeping full pedal in, lowering the collective and banking slightly into the spin, we flew out of it easily. No big deal. In fact, one need not be a test pilot to realize that the 206's big vertical fin blanks off a lot of t/r inflow air during certain flight conditions.

As one who's flown at relatively low altitudes, I have never been "surprised" and run out of pedal. Similarly, I've never had the nose yaw and not been able to stop it with a bootful of left pedal. Then again, I'm pretty diligent about keeping the wind on the nose or off my left. If I ever did get into a condition in which my left leg was sticking straight out, I didn't bother to look at a chart to see whether I was within the flight envelope or not. The fact that I was approaching a control limit was enough to tell me that I was in a corner of it. (Actually, I'd be worrying about overtorquing the ship.)

Luckily, that Skycrane fire pilot who experienced LTE knew what to do. Maybe he'd flown 206's at some point in his career, eh? I wonder if he checked to see whether he was within his flight envelope or not? Good thing he didn't go, "Blimey! Can't be LTE, not in this bird! Must be a bleedin' tail rotor failure!"

[NickLappos]

pprune fan #1,

NOW we know what pedantic is.......

[Nigel Osborn]

It's a pity some interesting threads get a bit personal.

I had my first helicopter ride 49 years ago in the Malaysian hills with an army friend. Naturally I was excited and asked numerous questions about the controls and what the footrests were for. Even that long ago he was able to point out the problems of LTE which of course didn't mean a thing to me. Later when I was a student, the problems of LTE were driven home to me and demonstrated.
In those days most of the helicopters were underpowered and depending on the wind, OAT and altitude, it was impressed on us to lead with rudder pedal just slightly before you applied power which seemed to prevent LTE. Years later I had to land a 206A with a C18 engine at 12000 ft and using that technique, had no LTE problems.

I'm sure a lot of problems, such as LTE & others, occured because there were more civilian students going through the system than in my youth. This IS NOT a bash against civilian students, I've had plenty of very good ones. It is just a question of the AMOUNT of training received as well as the quality. In other words a doctor who gets 6 years training should be better qualified than one who gets 3 years. It doesn't mean that one is smarter that the other, just had access to more training. That is the real difference between military and civilian training, the quantity.

As already suggested by others, some helicopters get LTE easier than others but if the pilot knows how to prevent that happening or takes the correct remedial action if he does get LTE, then the chances of it becoming a disaster should be reduced.:cool:

[NickLappos]

Nigel,

You are quite correct, (as is pprune fan, although please don't tell him) in that we must train to care for the yaw control, and LTE concepts help that training.

What we must not do, I believe, is accept the case that any helo can get "LTE" within its envelope. That is simply not true. But more importantly, it lets off the hook those helos that are underdesigned, and therefore hopefully obsolescent.

I believe with clear facts, our purchase choices will force the manufacturers to make good helicopters. I also believe that with macho bull about how we all should be able to fly crappy helos, and even the boxes they came in, we will not improve our lot.

BTW, pprune fan#1, with 25 years of experimental helicopter test, I have more time against the pedal stops than you probably have on the crapper. So let me buy you a beer at the next meeting, and have you tell me some of the good stories you have to tell! Where we differ is that I think you should place some emphasis on making better helos, so your son doesn't have to risk what you did getting those 8,000 hours in 206's.

[helmet fire]

...we seem to have run out of directional control........

ppruneF: I think it is fair to say that you just want one label to describe many situations because the recovery is the same - but others prefer to call a spade a spade in the hope that correct identification of the problem can lead to efforts to correctly address them. As we know, the BEST solution to any problem is to engineer them out, ie create a system that cannot suffer that error. The next best is to define the problem, recognise onset conditions and avoid them. The third best is to rote learn techniques to recover after onset. And that is where we are at: we have designed new helicopters that are not susceptible. We have defined the onset conditions in older susceptible machines to help avoid it. Finally, we have devised a recovery technique for those who suffer it. If you just want to focus on the recovery - great. But let everyone else gain an understanding at a deeper level if they want to.

Unfortunately, all the above requires a level of pedanticism. You get that.