G2 down in Switzerland - 15 June 2022
LTE would be when the TR just can't produce the thrust required to maintain or correct the desired heading. Caused by poor design or exceeding design limits.
Undemanded yaw is when the pilot hasn't applied sufficient pedal to maintain or correct the desired heading. Caused by lack of anticipation, awareness or skill.
Ref the 2001 article - all good except for the term Yaw Divergence - if it was used in the military, it was only by the RN, not the RAF or AAC.
From what I understand, the term divergence is usually paired with another axis implying cause and effect - ie yaw/roll divergence (Puma).
Undemanded yaw is when the pilot hasn't applied sufficient pedal to maintain or correct the desired heading. Caused by lack of anticipation, awareness or skill.
Ref the 2001 article - all good except for the term Yaw Divergence - if it was used in the military, it was only by the RN, not the RAF or AAC.
From what I understand, the term divergence is usually paired with another axis implying cause and effect - ie yaw/roll divergence (Puma).
From Webster:
1a : a drawing apart (as of lines extending from a common center)
b : difference, disagreement
c evolutionary biology : the development of dissimilar traits or features (as of body structure or behaviour) in closely related populations, species, or lineages of common ancestry that typically occupy dissimilar environments or ecological niches : divergent evolution
2 : a deviation from a course or standard
3 : the condition of being mathematically divergent
I would say 2 applies here. Yaw divergence would be that the tail rotor doesn't do, what it is supposed to do.
Definition of divergence
1a : a drawing apart (as of lines extending from a common center)
b : difference, disagreement
c evolutionary biology : the development of dissimilar traits or features (as of body structure or behaviour) in closely related populations, species, or lineages of common ancestry that typically occupy dissimilar environments or ecological niches : divergent evolution
2 : a deviation from a course or standard
3 : the condition of being mathematically divergent
Originally Posted by [email protected]
LTE would be when the TR just can't produce the thrust required to maintain or correct the desired heading. Caused by poor design or exceeding design limits.
Undemanded yaw is when the pilot hasn't applied sufficient pedal to maintain or correct the desired heading. Caused by lack of anticipation, awareness or skill.
Ref the 2001 article - all good except for the term Yaw Divergence - if it was used in the military, it was only by the RN, not the RAF or AAC.
From what I understand, the term divergence is usually paired with another axis implying cause and effect - ie yaw/roll divergence (Puma).
Undemanded yaw is when the pilot hasn't applied sufficient pedal to maintain or correct the desired heading. Caused by lack of anticipation, awareness or skill.
Ref the 2001 article - all good except for the term Yaw Divergence - if it was used in the military, it was only by the RN, not the RAF or AAC.
From what I understand, the term divergence is usually paired with another axis implying cause and effect - ie yaw/roll divergence (Puma).
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LTE
FAA used the LTE wording in AC 90-95 as a synonym of "unanticipated yaw"
Do not mind what Loss, Tail Rotor and Effectiveness mean, LTE is only a synonym of "unanticipated yaw". I cannot imagine that FAA, as a Certification Authority, would consider certifying a rotor that could lose efficiency and, to my knowledge, nobody has ever demonstrated such a phenomenon.
Before LTE, "Tail Rotor Stall" was used... untill they demonstrated that no stall occurred. But they replaced a myth by another one, because nobody remember the origin of LTE. This is why many think, erroneously that it designates a loss of effectiveness of the tail rotor that does not exist.
LTE was used by US Army to designate unanticipated yaw, when they had the problem in the 80's, mainly on OH-58. Bell already noticed in a Service Letter dated July 1984 that the term was misleading.
FAA used the LTE wording in AC 90-95 as a synonym of "unanticipated yaw"
Do not mind what Loss, Tail Rotor and Effectiveness mean, LTE is only a synonym of "unanticipated yaw". I cannot imagine that FAA, as a Certification Authority, would consider certifying a rotor that could lose efficiency and, to my knowledge, nobody has ever demonstrated such a phenomenon.
Before LTE, "Tail Rotor Stall" was used... untill they demonstrated that no stall occurred. But they replaced a myth by another one, because nobody remember the origin of LTE. This is why many think, erroneously that it designates a loss of effectiveness of the tail rotor that does not exist.
FAA used the LTE wording in AC 90-95 as a synonym of "unanticipated yaw"
Do not mind what Loss, Tail Rotor and Effectiveness mean, LTE is only a synonym of "unanticipated yaw". I cannot imagine that FAA, as a Certification Authority, would consider certifying a rotor that could lose efficiency and, to my knowledge, nobody has ever demonstrated such a phenomenon.
Before LTE, "Tail Rotor Stall" was used... untill they demonstrated that no stall occurred. But they replaced a myth by another one, because nobody remember the origin of LTE. This is why many think, erroneously that it designates a loss of effectiveness of the tail rotor that does not exist.
In other words, if I understand that getting below ETL in a left quartering headwind will cause the nose to want to yaw right, then I will be prepared for it, with left pedal ready to go.
Anyway, that's how I look at it in FAA land.
,...but yeah, the words themselves don't represent the condition very well.
Right ... err ... about that Rotorcraft Flying Handbook. Don't get me wrong, it meets the purpose, but ... well ... it has a bit too many "lies to children" in it.
It isn't worse than other basic books about helicopters - after all they all copy from each other - but I would not take it too serious. It is alright for the average pilot to learn the basics, but too many subjects are oversimplified. Some things are plain wrong. For example, the last time I looked there was still gyroscopic precession mentioned for the rotor as an explanation, why the swashplate moves the blades before the intended place where the action actually happens. Well that is (almost) plain wrong, that would only happen in a vacuum (Prouty). But it is good enough for the average Joe pilot, because the real reason is quite a bit more complicated. Still the book teaches all the right manoeuvres one should know. It isn't bad in that regard, just the explanations are ... well ... sometimes...
Some years ago Shawn Coyle asked here on Pprune, if somebody would help to find all the incorrections in that book so I started to read it again. We did not get very far in the project, but I found quite a few things that are questionable in the first 50 pages or so. But it is getting better, I think.
Having said that, put yourself in the shoes of the poor guy who has to write that book. There is no doubt, that many FAA experts (and CAA experts and so on) know exactly how reality looks, but you can't put that in a book for the average student pilot with a very basic graps on physics.
For me, Prouty's books were my starting point and quite the eye opener and quite entertaining sometimes. Worth the read. Shawns books are also very helpful to get a better understanding.
It isn't worse than other basic books about helicopters - after all they all copy from each other - but I would not take it too serious. It is alright for the average pilot to learn the basics, but too many subjects are oversimplified. Some things are plain wrong. For example, the last time I looked there was still gyroscopic precession mentioned for the rotor as an explanation, why the swashplate moves the blades before the intended place where the action actually happens. Well that is (almost) plain wrong, that would only happen in a vacuum (Prouty). But it is good enough for the average Joe pilot, because the real reason is quite a bit more complicated. Still the book teaches all the right manoeuvres one should know. It isn't bad in that regard, just the explanations are ... well ... sometimes...
Some years ago Shawn Coyle asked here on Pprune, if somebody would help to find all the incorrections in that book so I started to read it again. We did not get very far in the project, but I found quite a few things that are questionable in the first 50 pages or so. But it is getting better, I think.
Having said that, put yourself in the shoes of the poor guy who has to write that book. There is no doubt, that many FAA experts (and CAA experts and so on) know exactly how reality looks, but you can't put that in a book for the average student pilot with a very basic graps on physics.
For me, Prouty's books were my starting point and quite the eye opener and quite entertaining sometimes. Worth the read. Shawns books are also very helpful to get a better understanding.
Right ... err ... about that Rotorcraft Flying Handbook. Don't get me wrong, it meets the purpose, but ... well ... it has a bit too many "lies to children" in it.
It isn't worse than other basic books about helicopters - after all they all copy from each other - but I would not take it too serious. It is alright for the average pilot to learn the basics, but too many subjects are oversimplified. Some things are plain wrong. For example, the last time I looked there was still gyroscopic precession mentioned for the rotor as an explanation, why the swashplate moves the blades before the intended place where the action actually happens. Well that is (almost) plain wrong, that would only happen in a vacuum (Prouty). But it is good enough for the average Joe pilot, because the real reason is quite a bit more complicated. Still the book teaches all the right manoeuvres one should know. It isn't bad in that regard, just the explanations are ... well ... sometimes...
Some years ago Shawn Coyle asked here on Pprune, if somebody would help to find all the incorrections in that book so I started to read it again. We did not get very far in the project, but I found quite a few things that are questionable in the first 50 pages or so. But it is getting better, I think.
Having said that, put yourself in the shoes of the poor guy who has to write that book. There is no doubt, that many FAA experts (and CAA experts and so on) know exactly how reality looks, but you can't put that in a book for the average student pilot with a very basic graps on physics.
For me, Prouty's books were my starting point and quite the eye opener and quite entertaining sometimes. Worth the read. Shawns books are also very helpful to get a better understanding.
It isn't worse than other basic books about helicopters - after all they all copy from each other - but I would not take it too serious. It is alright for the average pilot to learn the basics, but too many subjects are oversimplified. Some things are plain wrong. For example, the last time I looked there was still gyroscopic precession mentioned for the rotor as an explanation, why the swashplate moves the blades before the intended place where the action actually happens. Well that is (almost) plain wrong, that would only happen in a vacuum (Prouty). But it is good enough for the average Joe pilot, because the real reason is quite a bit more complicated. Still the book teaches all the right manoeuvres one should know. It isn't bad in that regard, just the explanations are ... well ... sometimes...
Some years ago Shawn Coyle asked here on Pprune, if somebody would help to find all the incorrections in that book so I started to read it again. We did not get very far in the project, but I found quite a few things that are questionable in the first 50 pages or so. But it is getting better, I think.
Having said that, put yourself in the shoes of the poor guy who has to write that book. There is no doubt, that many FAA experts (and CAA experts and so on) know exactly how reality looks, but you can't put that in a book for the average student pilot with a very basic graps on physics.
For me, Prouty's books were my starting point and quite the eye opener and quite entertaining sometimes. Worth the read. Shawns books are also very helpful to get a better understanding.
You may not agree with what it says about things like LTE, but that knowledge has kept my nose straight in all sorts of wind.
As for gyroscopic precession? Right or wrong, what I learned about it from that book allowed me to pass the test. After that, I have no use for that knowledge.
LTE was used by US Army to designate unanticipated yaw, when they had the problem in the 80's, mainly on OH-58. Bell already noticed in a Service Letter dated July 1984 that the term was misleading.
Rotorbee - I was talking about using the term divergence in an aviation context not a literal dictionary one.
The FAA handbook has a lot of people still believing that "flapping to equality" is happening all the time, and that the advancing blade is flapping up in forward flight. Never mind the reality of looking at the disc tilted forward, that is just a distraction. Sure, flapping does happen as soon as you move forward, but the pilot puts in forward cyclic to stop it, and continue moving forward. Go faster, need more and more forward stick to keep that disc tilted forwards.
Search for "helicopter urban myths" posted on this forum many years ago by Nick Lappos. It clears up a lot of misunderstood things.
Search for "helicopter urban myths" posted on this forum many years ago by Nick Lappos. It clears up a lot of misunderstood things.
In other words, if I understand that getting below ETL in a left quartering headwind will cause the nose to want to yaw right, then I will be prepared for it, with left pedal ready to go.
Especially with fenestrons.
a left quartering headwind will cause the nose to want to yaw right,
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Originally Posted by [email protected]
read what Ascend Charlie posted above, #20 I think, LTE was made up by Bell to cover the fact they put an inadequate TR on the 206.
Rotorbee - I was talking about using the term divergence in an aviation context not a literal dictionary one.
Rotorbee - I was talking about using the term divergence in an aviation context not a literal dictionary one.
Bell seems to have been very reluctant to use the LTE wording.
LTE is not the explanation of unanticipated yaw, it is unanticipated yaw.
If a gust of wind hits you and the heading changes, the TR hasn't lost effectiveness.
If it was LTE you wouldn't be able to correct it.
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Originally Posted by [email protected]
No, you can get unanticipated yaw in a helicopter without loss of effectiveness, it just means you don't have enough pedal applied.
If a gust of wind hits you and the heading changes, the TR hasn't lost effectiveness.
If it was LTE you wouldn't be able to correct it.
If a gust of wind hits you and the heading changes, the TR hasn't lost effectiveness.
If it was LTE you wouldn't be able to correct it.
extract from AC 90-95
I am not an english speaker but I understand from that extract of AC 90-95 that "unaticipated yaw" and "LTE" are two ways to designate the same phenomenon.
AMDEC - that is the same Bell myth used to disguise a poor design and unfortunately reproduced by the FAA.
If you put a TR on a helicopter that can't cope with certain wind conditions then you either impose handling/wind limits on it or put a bigger TR on it.
Bell sales would have been affected by a wind limit that competitors did not have so they invented LTE as an explanation.
If your TR can't produce enough thrust to maintain or correct a heading then you have a loss of TR effectiveness. That is not the same as not putting enough pedal in.
Plenty of people have been spun round by weathercocking in helicopters but it is often lack of anticipation rather than poor TR thrust.
Losing ETL on the MR also affects the TR and you can suddenly experience an increase in Power required from both rotors - you raise the lever to compensate and also require more power pedal - both to compensate for the loss of ETL on the TR and to counter the increased torque reaction caused by you pulling more collective.
If you put a TR on a helicopter that can't cope with certain wind conditions then you either impose handling/wind limits on it or put a bigger TR on it.
Bell sales would have been affected by a wind limit that competitors did not have so they invented LTE as an explanation.
If your TR can't produce enough thrust to maintain or correct a heading then you have a loss of TR effectiveness. That is not the same as not putting enough pedal in.
Plenty of people have been spun round by weathercocking in helicopters but it is often lack of anticipation rather than poor TR thrust.
Losing ETL on the MR also affects the TR and you can suddenly experience an increase in Power required from both rotors - you raise the lever to compensate and also require more power pedal - both to compensate for the loss of ETL on the TR and to counter the increased torque reaction caused by you pulling more collective.
LTE was also associated with helicopters that had the TR rotation going the wrong way ie not upwards into the MR downwash - the Lynx is a typical example, it had issues with running out of yaw control until they changed the TR direction and it suddenly had awesome TR capability
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In the certification process you have to claim for a flight envelope (in our case wind envelope) and you have to demonstrate the maneuverability (pedal margin) in that wind envelope. If your tail rotor is limited, it will make a limitation on your hover performance and it will be clearly visible.
If your TR is limited and you are unable to maintain the critical heading, you will stabilize at the heading corresponding to your pedal stop but in no case you will start spinning.
Yawing right with full left pedal is nonsense. It could happen in pure hover without wind if you exceeded the MR torque that you are able to counterbalance with the full TR thrust. Certification does not allow that. The TR thrust is designed to cope with at least 17 kt in the critical azimuth and hover performance is limited to that point. You have little pedal margin at maximum hover performance in the critical azimuth but you recover a lot when the wind comes from a different direction.
If your TR is limited and you are unable to maintain the critical heading, you will stabilize at the heading corresponding to your pedal stop but in no case you will start spinning.
Yawing right with full left pedal is nonsense. It could happen in pure hover without wind if you exceeded the MR torque that you are able to counterbalance with the full TR thrust. Certification does not allow that. The TR thrust is designed to cope with at least 17 kt in the critical azimuth and hover performance is limited to that point. You have little pedal margin at maximum hover performance in the critical azimuth but you recover a lot when the wind comes from a different direction.
Sikorsky had it right, we just messed it up after that
On a good windy day regularly do some 360 slow rotations on the taxiway for a good 15 minutes to get your feet moving and you will be fine.
Control authority is there, plenty of it, if you push early and strongly enough.
AMDEC - I agree with what you say, the ability to cope with at least 17 kts at various test points must be demonstrated.
But what happens in dynamic manoeuvres or when the wind is gusting in excess of 17 kts? That is where a poorly designed TR causes problems..
You said yourself you have little pedal margin when it comes to critical azimuth. Which operator is going to limit themselves to 17 Kts cross or downwind?
But what happens in dynamic manoeuvres or when the wind is gusting in excess of 17 kts? That is where a poorly designed TR causes problems..
You said yourself you have little pedal margin when it comes to critical azimuth. Which operator is going to limit themselves to 17 Kts cross or downwind?
Just out of curiosity, since you guys don't agree that in certain wind angles the tail rotor becomes "less effective" thus requiring more left pedal, do you also disagree when we (FAA land) say that when the tail rotor passes through ETL on takeoff it becomes "more efficient" thus requiring less left pedal?