Loss of Tail Rotor Effectiveness, recovery etc
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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.
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.
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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?
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?
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.
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.
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Funnily enough, the January AAIB reports have two mentions of this very thing, in differing circumstances.
This one describes a 206 on a photography detail losing it in a right-hand slip at fairly low speed.
And this one says what happened to a heavy-ish 500 departing a hotel lawn.
This one describes a 206 on a photography detail losing it in a right-hand slip at fairly low speed.
And this one says what happened to a heavy-ish 500 departing a hotel lawn.
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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.
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.
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!
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!
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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?
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?
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.
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.
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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.
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.
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
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
Last edited by helmet fire; 12th Jan 2004 at 05:24.
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.
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.
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.
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
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
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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
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
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).
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).
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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.
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.
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!
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!