T/R Loss of Effectiveness
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T/R Loss of Effectiveness
A penny for your thoughts: The B206B-3 that I fly, has a diagram in the flight manual showing a tail wind from the 3-7 o'clock position as being unfavourable w.r.t tail rotor control effectiveness (TRCE). Without getting bogged down in the interplay of main rotor induced flow and blade tip vortices on the tail rotor in certain cross wind conditions, looking at a basic 90 deg X wind condition.
Considering a cross wind from 3 o'clock, this will basically increase the induced flow the tail rotor generates when creating thrust to overcome MG torque. This increased induced flow can reach a point where the tail rotor is unable to overcome it, and therefore a loss of TRCE is felt.
However there is the other school of thought which goes with vortex ring state. Again simply put, a cross wind from the 9 o'clock position, will oppose the induced flow through the tail rotor. This could reach critical a point, where the induced flow through the tail rotor is matched by the crosswind, holding the tail rotor blade tip vortices and the disturbed airflow at the blade roots, on the tail rotor disc, and as with the main rotor, a vortex ring state condition exists, causing a loss of TRCE.
Should the flight manual address this? which one of the two conditions mentioned above is more common? It seems to me there is no clear cut answer? Fortunately in my experience, the effect of cross wind on the lateral weathercock stability of the 206, normally will alert you to a pending problem before you go into a full blown loss of TRCE, as you realise you are putting Fred Astaire to shame with your dancing feet on the pedals, followed by alarming fluctuations of the torque gauge!
Considering a cross wind from 3 o'clock, this will basically increase the induced flow the tail rotor generates when creating thrust to overcome MG torque. This increased induced flow can reach a point where the tail rotor is unable to overcome it, and therefore a loss of TRCE is felt.
However there is the other school of thought which goes with vortex ring state. Again simply put, a cross wind from the 9 o'clock position, will oppose the induced flow through the tail rotor. This could reach critical a point, where the induced flow through the tail rotor is matched by the crosswind, holding the tail rotor blade tip vortices and the disturbed airflow at the blade roots, on the tail rotor disc, and as with the main rotor, a vortex ring state condition exists, causing a loss of TRCE.
Should the flight manual address this? which one of the two conditions mentioned above is more common? It seems to me there is no clear cut answer? Fortunately in my experience, the effect of cross wind on the lateral weathercock stability of the 206, normally will alert you to a pending problem before you go into a full blown loss of TRCE, as you realise you are putting Fred Astaire to shame with your dancing feet on the pedals, followed by alarming fluctuations of the torque gauge!
It's called Tin Plating, covering the corporate backside so that any problem relating to a lack of T/R response can be blamed on anything other than the design of the tail rotor.
Let them blame LTE, or vortex ring state, or weathercocking, or anything but poor design.
There are very few wind directions which are acceptable, according to the book.
However, comma, in 7000+ hrs in a 206 and 206L I have NEVER run out of tail rotor, simply by keeping a bit up the sleeve and being aware of how fast I am feeding the pedal in and where the spot is where I poke the nose over and fly away.
Look up the many Nick Lappos posts about the under-designed tail rotor on the original machine. The new asymmetric blades have gone a long way towards fixing any problems.
Let them blame LTE, or vortex ring state, or weathercocking, or anything but poor design.
There are very few wind directions which are acceptable, according to the book.
However, comma, in 7000+ hrs in a 206 and 206L I have NEVER run out of tail rotor, simply by keeping a bit up the sleeve and being aware of how fast I am feeding the pedal in and where the spot is where I poke the nose over and fly away.
Look up the many Nick Lappos posts about the under-designed tail rotor on the original machine. The new asymmetric blades have gone a long way towards fixing any problems.
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a cross wind from the 9 o'clock position, will oppose the induced flow through the tail rotor.
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Had it, got away with it with about 10 inches to spare. Didn't like it.
206L fully loaded, wind was initially from the 9 o'clock with low pitch on TR and pedal turn to the right at low speed coming to the hover ending with wind at the 5 o'clock applying full left pedal. The turn continued through a further 120 degrees whilst descending from 10 feet until just above ground ending with the airframe pointing into wind. That's how I learned of the phenomenon, young, lucky to get away with it and wishing I had known beforehand......
206L fully loaded, wind was initially from the 9 o'clock with low pitch on TR and pedal turn to the right at low speed coming to the hover ending with wind at the 5 o'clock applying full left pedal. The turn continued through a further 120 degrees whilst descending from 10 feet until just above ground ending with the airframe pointing into wind. That's how I learned of the phenomenon, young, lucky to get away with it and wishing I had known beforehand......
LTE is most noticeable when heading control is not maintained, and a yaw rate develops that the aircraft pedal margin can not stop. The left cross wind is interesting in the fact that it can offload the tail rotor, but in doing so, also reduces the amount of cross wind required to develop a VRS condition(40-75% of induced flew velocity) less TR thrust means less xwind needed. Once the yaw develops, it continues into the weathercock region, and a snowball effect occurs. With a right xwind a right yaw puts the nose into the winds. Beeping up the rotor, staying ontop of yaw rates, and making pedal turns to the left(power pedal) will keep you well ahead. Another interesting tidbit, in a 206 if a right yaw occurs, the aircraft will descend, left yaw results in a climb, all without touching the collective, this requires up collective when a right yaw occurs, which will again aggrevate the situation.
I agree with Ascend Charlie......there is NO problem with LTE in the 206. The problem is with the pilots not planning accordingly. Read this:
NTSB Report supposed LTE
Emphasis added by me
NTSB Report supposed LTE
Emphasis added by me
The pilot, who had landed at this helipad on numerous occasions, said the approach was normal until he got closer to the helipad. He said he felt fast "about 12-15 knots" and a "little high," so he decided to abort the approach. At this point, with about ¼ to ½ -inch of left anti-torque pedal applied, he added power, "tipped the nose over to get airspeed," and "pulled collective." The pilot said that as soon as he brought the collective up, the helicopter entered a rapid right turn. He described the turn as "violent" and that it was the fastest he had ever "spun" in a helicopter. The pilot told the crew to hold on and that he was "going to try and fly out of it." The pilot said he tried hard to get control of the helicopter by applying cyclic and initially "some" left anti-torque pedal "but nothing happened." The pilot said he added more, but not full left anti-torque pedal as the helicopter continued to spin and he was still unable to regain control.
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The flight manual shows flight parameters that if they are exceeded, we risk the chance of damaging the equipment or in this case losing control of the helicopter. The reason that most US designed helicopters have a critical wind azimuth area from the right side is because under certain conditions of DA, weight and wind, directional control WILL be lost no matter what the pilot does. It is a design limitation of the tail rotor.
LTE is a pilot technique issue, or should we say a bad technique. The manufacturer tested the machine and tells you how much x-wind it can handle. They cannot predict when control will be lost if the pilot doesn't keep the yaw rate under control when experiencing M/R vortex interference, T/R vortex ring or weathervane effects.
LTE is a pilot technique issue, or should we say a bad technique. The manufacturer tested the machine and tells you how much x-wind it can handle. They cannot predict when control will be lost if the pilot doesn't keep the yaw rate under control when experiencing M/R vortex interference, T/R vortex ring or weathervane effects.
LTE stands for Lack of Technique and Experience.
It is NOT something that "every helicopter gets".
It was, as Nick said many moons ago, a Spin applied by a certain manufacturer to explain why their too-small tail rotor was causing problems in the Army. It has subsequently woven itself into the myths and fabric of helicopter lore and a lot of people believe it to be Absolute Truth. Thus, every helicopter that is mishandled to the extent that it spins and crashes is attributed to the fabled LTE and is therefore not the pilot's fault.
It is NOT something that "every helicopter gets".
It was, as Nick said many moons ago, a Spin applied by a certain manufacturer to explain why their too-small tail rotor was causing problems in the Army. It has subsequently woven itself into the myths and fabric of helicopter lore and a lot of people believe it to be Absolute Truth. Thus, every helicopter that is mishandled to the extent that it spins and crashes is attributed to the fabled LTE and is therefore not the pilot's fault.
Ascend Charlie
I will correct you slightly, emphasis added:
"LTE stands for Lack of Technique AND / OR Experience."
LTE stands for Lack of Technique and Experience.
"LTE stands for Lack of Technique AND / OR Experience."
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My extensive research on accidents shows that "LTE" only occurs in a few types of helicopter. The sensitivity to wake ingestion and wind angle is an aspect of a marginal tail rotor, and therefore LTE is mostly due to marginal tail rotors with little residual "extra" thrust.
Besides the wind angle/reingestion relationships, the biggest single cause of LTE for those types is pulling excessive torque on landing or takeoff. Since anti-torque is used to smother MR torque, the higher you pull the engine torque, the more pedal you need. The classic LTE accident is when the pilot flares too much at the bottom of an approach, and pulls 10% more power than the steady hover will demand. In a marginal helicopter, that will make you hit the pedal stops, and cause a momentary loss of control. If the wind is right, the trips around the mast will be eye opening, as will your attempts to regain yaw control and sanity. I have 1000 hours of combat in a Cobra, and experienced LTE several times, so I am a pro at what not to do right.
As Ascend Charlie, Gordy and HueyLoach have said, technique is the key. Learn to baby your machine, think ahead and practive sneaking into a hover at the end of any critical approach (where you are heavy, or high or both). When you make an approach, beedy eye the torque and see how much extra you need to enter the hover at the bottom. A good pilot can sneak into the hover with almost no extra torque. An LTE prone pilot will see a torque hump in the bottom of the flare that is trying to consume your extra pedal.
I know it will open a can of worms, but many helicopters, most helicpters, have NEVER experienced LTE, even when moderately abused, while some helicopters get LTE even when reasonably babied. The available excess TR thrust that is designed in is the key to eliminating LTE, and so is the published crosswind limit of the machine. Show me a machine with barely 17 knots of demonstrated crosswind , or one with a published "wind off the nose only" hover chart and approach procedure, and I will show you a helicopter just waiting to embarrass its pilot.
I repeat: 1) Not all helos can get LTE, in fact, most cannot. 2) Some helo models love LTE and get it because they have marginal tail rotors. 3) All helicopters can be operated safely, even those more prone to LTE, if the pilot flys them wisely.
Besides the wind angle/reingestion relationships, the biggest single cause of LTE for those types is pulling excessive torque on landing or takeoff. Since anti-torque is used to smother MR torque, the higher you pull the engine torque, the more pedal you need. The classic LTE accident is when the pilot flares too much at the bottom of an approach, and pulls 10% more power than the steady hover will demand. In a marginal helicopter, that will make you hit the pedal stops, and cause a momentary loss of control. If the wind is right, the trips around the mast will be eye opening, as will your attempts to regain yaw control and sanity. I have 1000 hours of combat in a Cobra, and experienced LTE several times, so I am a pro at what not to do right.
As Ascend Charlie, Gordy and HueyLoach have said, technique is the key. Learn to baby your machine, think ahead and practive sneaking into a hover at the end of any critical approach (where you are heavy, or high or both). When you make an approach, beedy eye the torque and see how much extra you need to enter the hover at the bottom. A good pilot can sneak into the hover with almost no extra torque. An LTE prone pilot will see a torque hump in the bottom of the flare that is trying to consume your extra pedal.
I know it will open a can of worms, but many helicopters, most helicpters, have NEVER experienced LTE, even when moderately abused, while some helicopters get LTE even when reasonably babied. The available excess TR thrust that is designed in is the key to eliminating LTE, and so is the published crosswind limit of the machine. Show me a machine with barely 17 knots of demonstrated crosswind , or one with a published "wind off the nose only" hover chart and approach procedure, and I will show you a helicopter just waiting to embarrass its pilot.
I repeat: 1) Not all helos can get LTE, in fact, most cannot. 2) Some helo models love LTE and get it because they have marginal tail rotors. 3) All helicopters can be operated safely, even those more prone to LTE, if the pilot flys them wisely.
Last edited by NickLappos; 17th Nov 2014 at 16:31.
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LTE
Gordy/AC quite right ... or perhaps Lack of Tailrotor EDUCATION ?
Nick is right - there is almost no case of a Tail Rotor which is insufficiently powerful to overcome the torque produced, except perhaps at low RRPM (or EC135 at altitude?). Mostly it is poor pilot technique, and for those pilots it can appear that the tailrotor is inEffective. (However: A tail rotor may easily not have sufficient Authority to be able to Yaw (or sideslip) against an airspeed. Sometimes called Lack of Tailrotor AUTHORITY. but do they need to be flying sideways through the air at a speed that the tailrotor cannot overcome? (rarely) It'll default to straight anyway. Most pilots are caught out at some stage by unexpected yaw, normally by mis-estimating their path throught the air (wind direction))
An example, touching on the subject, here from the thread "ETL backwards": "You could just turn downwind and do a downwind transition but you might not have enough TR control in 30 to 40 knots for that."
L T 'Ed'?
Nick is right - there is almost no case of a Tail Rotor which is insufficiently powerful to overcome the torque produced, except perhaps at low RRPM (or EC135 at altitude?). Mostly it is poor pilot technique, and for those pilots it can appear that the tailrotor is inEffective. (However: A tail rotor may easily not have sufficient Authority to be able to Yaw (or sideslip) against an airspeed. Sometimes called Lack of Tailrotor AUTHORITY. but do they need to be flying sideways through the air at a speed that the tailrotor cannot overcome? (rarely) It'll default to straight anyway. Most pilots are caught out at some stage by unexpected yaw, normally by mis-estimating their path throught the air (wind direction))
An example, touching on the subject, here from the thread "ETL backwards": "You could just turn downwind and do a downwind transition but you might not have enough TR control in 30 to 40 knots for that."
L T 'Ed'?
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I believe a lot of "LTE" incidents are the result of pilot carelessness, pulling lots of power without pushing enough pedal. That lets the spin start, and once it starts, inertia becomes a factor, and it takes more pedal to stop the spin than it would have taken to prevent it in the first place, probably much more. If tail rotor control is at all marginal in the first place, overcoming the inertia of the spin may not be possible. One can opine at length about wind influence on the tail rotor, but IME that has little to do with it. The 206 has enough tail rotor authority to deal with all that, if the pilot is smooth and careful, but once the spin starts, it's hell to get it stopped, because of inertia. Of course other factors enter into it, and exacerbate the condition, but I think that as long as you keep the nose in one place, you should have no problems. I have something over 10k hours in Bells, and have never once experienced LTE. But I make sure the nose never moves from where I want it to be. You need to be smooth on the controls of any machine heavily loaded, but especially so in a Bell.
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An interesting discussion, duude, 10K on the 206!! Can someone live that long! Makes my paltry 1400 hrs like a mere whiff in the nostrils! A good point made, on the fact that for T/R VRS to occur, a window must be achieved where thrust is being generated (thus causing an induced flow) and an external RAF that will EQUAL the induced flow, causing the VRS to develop, once developed it has the nasty characteristic of locking onto the rotor disc until recovery is carried out, normally by reducing power (induced flow) and increasing fwd speed to blow the whole shambles of the disc. This is just like main rotor VRS and as some have said, it can occur at small torque pedal input values and minor cross winds due to the tail rotor entering the window of opportunity.
Fortunately, I think many tail rotor systems will either pass through what is a small window and or will power through VRS conditions. In the Alo III for example, demo'ing this to students, it was not easy to demonstrate vortex ring state on the main rotor due to the narrow entry window which had to be carefully flown. If the VRS is not allowed to fully develop, the Alo III would just power out of it (the power available causing the induced flow to rapidly overcome the VRS development). But then the Alo III was/is, a powerful little beast! Once you managed to carefully match the ROD airflow, to gently applied power (induced flow), she would get into it, with a lot of unhappy vibration and some pitching and rolling. Once developed fully on the rotor disc, you could pull the CP lever out the floor, and watch aghast as the ROD actually increased. Lower the CP and lower the nose you felt the seat press against your butt, as the rotor started to fly again, followed by an increase of morale in the cockpit!
So, it seems LTE due VRS is not common however to be aware of it with a wind from the 9 o'clock is good, the other condition of increased induced flow will cause LTE, but you should re-gain control once weathercocked into wind, but hopefully with just a red face, but worse case, causing a crash
As for the 206 climbing turning left and descending turning right, secondary effects of pedal in the hover, is it not?
Fortunately, I think many tail rotor systems will either pass through what is a small window and or will power through VRS conditions. In the Alo III for example, demo'ing this to students, it was not easy to demonstrate vortex ring state on the main rotor due to the narrow entry window which had to be carefully flown. If the VRS is not allowed to fully develop, the Alo III would just power out of it (the power available causing the induced flow to rapidly overcome the VRS development). But then the Alo III was/is, a powerful little beast! Once you managed to carefully match the ROD airflow, to gently applied power (induced flow), she would get into it, with a lot of unhappy vibration and some pitching and rolling. Once developed fully on the rotor disc, you could pull the CP lever out the floor, and watch aghast as the ROD actually increased.
Lower the CP and lower the nose you felt the seat press against your butt, as the rotor started to fly again, followed by an increase of morale in the cockpit!So, it seems LTE due VRS is not common however to be aware of it with a wind from the 9 o'clock is good, the other condition of increased induced flow will cause LTE, but you should re-gain control once weathercocked into wind, but hopefully with just a red face, but worse case, causing a crash
As for the 206 climbing turning left and descending turning right, secondary effects of pedal in the hover, is it not?
The aircraft climbs in the left pedal turn, and descends in a right pedal turn due to the applicable governor. when you make a right pedal turn, the reference point(we shall call it the nose) for RRPM is now moving against the rotation of the main rotor. This would seem to INDICATE that the rotor was trying to overspeed(when in fact the tip speed hasn't changed). the governor will adjust tip speed down to maintain an INDICATED 100% NR. This results in an actual decrease in the tip speed at the rotor. The faster the turn, the faster the descent(or climb in a left pedal turn). so when a right yaw is encountered during LTE the yaw will also have the secondary effect of lower the tip speed NR(although indicated still says 100%) and this results in a lower TR tip speed as well. Not something you need when a right yaw develops. for those that like math, one "normal" pedal turn generally takes about 15 seconds to complete a full revolution. that means the rate of turn is approximately 4 revolutions per minute. the main rotor is around 375RPM IIRC. so a "normal" turn results in a loss of just over 1% NR. a rapid yaw will cause a tip speed change proportional to the yaw rate.
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Rather than tip speed change, as stated above, think of rotor RPM with respect to the fuselage (or rate of blades passing the tail boom). the governor only sees that - so a rapid right yaw will make the governor think the rotor speed is faster than it actually is (for N. American direction of rotation of main rotor).
And the Gazelle had loss of fenestron effectiveness, a slightly different version of LTE.
And the Gazelle had loss of fenestron effectiveness, a slightly different version of LTE.
Finally, a great, productive discussion about LTE and the 206! Glory be.
I think the main problem with this issue is that there has developed a whole industry full of people who've never flown a 206 but think it has a "defective" tail rotor, and that a 206B parked on the ramp with the engine shut off and the blade tied down is just moments away from snapping into an uncontrolled/uncontrollable right spin.
Look, take a gander at a side profile shot of a 206. See that big vertical fin? Looks dead sexy, but it extends above and below the tail rotor, fully beyond the t/r diameter in both directions. Hmm, do we know of any other helicopters that have this configuration? None that I can think of. [LATE EDIT: What about the BELL 222 and the AS350!]Can we imagine what that big block-off plate does to tail rotor inflow? I sure can!
You DO NOT WANT the wind from four to five o'clock in a 206; you're just asking for trouble. The wind is going to want to push the tail, giving you a right-yaw. As Gomer points out, ANY TIME you let an uncommanded right yaw rate start without stopping it, you already f*-ed up. STOP IT! STOP IT NOW! With FULL pedal if you have to. Then hold the damn pedal in and get the hell out of there - it's a bad place!
But a lot of pilots are under the mistaken impression that you can also get into "LTE" with a crosswind from the left, of all places. They seem to forget that all helicopters exhibit strong weathercock stability. If the wind is off your left, the requirement for left-pedal is diminished. If you get into LTE with a left-crosswind, you're a pretty f*-ed up pilot who should probably turn in his certs and go drive a truck. Seriously.
I mean, look at the video of the 505's first flight. See how the test pilot did that sideways hover towards the end? See which way he was hovering? Right...I mean correct, to the left! Which gave him the dreaded left-crosswind that everyone is so paranoid about. OH MY GOD!!!
Uhh, but nothing happened.
Don't give me this, "The tail rotor gets into horizontal VRS!" bullcrap. It does not. And don't give me this, "The main rotor vortices interact and interfere with the tail rotor thrust!" bullcrap either. Those are all just girly, whiny excuses instead of just admitting that your piloting skills are so poor. KEEP THE DAMN NOSE STRAIGHT WITH THE PEDALS, ALRIGHT? I know you're already preoccupied making your two inept hands try to do the right things on the controls - I know that. But don't forget about your feet! They're in the fight as well. Use 'em or lose 'em.
The 206 tail rotor is fine, especially the one on the B-III. Don't fly the ship OGE, below ETL with the wind off your rear and you'll be fine too. In other words, don't do that U.S. Army NOE crap and you *might* not crash.
I think the main problem with this issue is that there has developed a whole industry full of people who've never flown a 206 but think it has a "defective" tail rotor, and that a 206B parked on the ramp with the engine shut off and the blade tied down is just moments away from snapping into an uncontrolled/uncontrollable right spin.
Look, take a gander at a side profile shot of a 206. See that big vertical fin? Looks dead sexy, but it extends above and below the tail rotor, fully beyond the t/r diameter in both directions. Hmm, do we know of any other helicopters that have this configuration? None that I can think of. [LATE EDIT: What about the BELL 222 and the AS350!]Can we imagine what that big block-off plate does to tail rotor inflow? I sure can!
You DO NOT WANT the wind from four to five o'clock in a 206; you're just asking for trouble. The wind is going to want to push the tail, giving you a right-yaw. As Gomer points out, ANY TIME you let an uncommanded right yaw rate start without stopping it, you already f*-ed up. STOP IT! STOP IT NOW! With FULL pedal if you have to. Then hold the damn pedal in and get the hell out of there - it's a bad place!
But a lot of pilots are under the mistaken impression that you can also get into "LTE" with a crosswind from the left, of all places. They seem to forget that all helicopters exhibit strong weathercock stability. If the wind is off your left, the requirement for left-pedal is diminished. If you get into LTE with a left-crosswind, you're a pretty f*-ed up pilot who should probably turn in his certs and go drive a truck. Seriously.
I mean, look at the video of the 505's first flight. See how the test pilot did that sideways hover towards the end? See which way he was hovering? Right...I mean correct, to the left! Which gave him the dreaded left-crosswind that everyone is so paranoid about. OH MY GOD!!!
Uhh, but nothing happened.
Don't give me this, "The tail rotor gets into horizontal VRS!" bullcrap. It does not. And don't give me this, "The main rotor vortices interact and interfere with the tail rotor thrust!" bullcrap either. Those are all just girly, whiny excuses instead of just admitting that your piloting skills are so poor. KEEP THE DAMN NOSE STRAIGHT WITH THE PEDALS, ALRIGHT? I know you're already preoccupied making your two inept hands try to do the right things on the controls - I know that. But don't forget about your feet! They're in the fight as well. Use 'em or lose 'em.
The 206 tail rotor is fine, especially the one on the B-III. Don't fly the ship OGE, below ETL with the wind off your rear and you'll be fine too. In other words, don't do that U.S. Army NOE crap and you *might* not crash.
Last edited by FH1100 Pilot; 19th Nov 2014 at 01:21.
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You DO NOT WANT the wind from four to five o'clock in a 206; you're just asking for trouble. The wind is going to want to push the tail, giving you a right-yaw. As Gomer points out, ANY TIME you let an uncommanded right yaw rate start without stopping it, you already f*-ed up. STOP IT! STOP IT NOW! With FULL pedal if you have to. Then hold the damn pedal in and get the hell out of there - it's a bad place!
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If you are needing Full Left Pedal to counteract Hover Torque and you reduce the Throttle...along with reducing Torque you also reduce the efficiency of the Tail Rotor as the RPM drops. Question for you is which reduces quicker....the Torque or the counter torque force?
If the tail rotor thrust decreases quicker than the Torque decreases....you would have a bigger problem than before you went rolling off the Throttle.
If the tail rotor thrust decreases quicker than the Torque decreases....you would have a bigger problem than before you went rolling off the Throttle.