engineering

Loss of tail-rotor effectiveness

Loss of tail-rotor effectiveness is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Loss of tail-rotor effectiveness rather than just read about it. In short: "Loss of tail rotor effectiveness" shall not be taken literally. It has been used in the early 1980s by the US Army to designate unanticipated yaw of a helicopter and does not mean that at any time the efficiency of the anti-torque rotor is diminished.

Key takeaways

Loss of tail-rotor effectiveness belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Loss of tail-rotor effectiveness to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Loss of tail-rotor effectiveness from memory before moving on to harder problems.

Reference excerpt

"Loss of tail rotor effectiveness" shall not be taken literally. It has been used in the early 1980s by the US Army to designate unanticipated yaw of a helicopter and does not mean that at any time the efficiency of the anti-torque rotor is diminished. Such a behavior has never been demonstrated. This phenomenon remains poorly understood, and literature on the subject provides little light on how it occurs. It is true that flight mechanics struggles to explain a helicopter rotating rapidly to the right with the pedals fully extended to the left. Complex aerodynamic phenomena that cause rapid variations in tail rotor thrust have only been identified, and it has been recommended that the areas in which they occur be avoided. Accidents characterized by a loss of yaw control on a fully operative aircraft are numerous. An analysis by Airbus Helicopters has shown that they mostly occur close to the ground, where the maneuver usually recommended for recovery is not applicable. Airbus Helicopters explains the phenomenon based on the pedal curve. This explanation assumes that the pilot only counters the yaw start with a limited amplitude control and does not reach the control stop. Objective data (videos, recorders) collected during such accidents are beginning to become more numerous and confirm this limited amplitude. Airbus Helicopters' explanation covers not only the majority of events in the opposite direction to the rotation of the main rotor, but also the rarer cases in the direction of this rotation and even accidents occurring on NOTAR (NO Tail Rotor) aircraft for which it is difficult to incriminate the interactions between the main rotor and the tail rotor or the vortex regime of the tail rotor.

Historical

US Army and Bell At the end of the 1970s, the US Army records a series of similar accidents on its OH‑58 helicopters, a military version of the Bell 206. At low speed, during a right turn, the yaw rate (rotation around the vertical axis of the aircraft) increases without any action from the pilot and the tail rotor appears incapable of stopping it. A complex theory is proposed, involving an alternation of stalls and recoveries of the tail rotor assumed to be at its maximum pitch. The phenomenon is then called " tail rotor stall ", without this theory being experimentally validated. A new procedure is defined by the US Army which consists, when the yaw movement to the right is detected, to add foot to the right, in the direction of the turn (presumably to recover from the tail rotor stall) and cyclic stick to the front-right. A working group, bringing together different components of the US Army with the participation of Bell Helicopter Textron, is also set up to understand the problem and remedy it. Its findings are released in 1984. How unanticipated yaw (UY) occurs is not explained, showing that it is not well understood. A curve measured in a wind tunnel giving, as a function of the wind direction relative to the helicopter, the value of the yaw moment created by the tail rotor—at constant main rotor and tail rotor collective pitches—is used to identify the wind directions giving rise to the most significant changes, estimated to be most conducive to the occurrence of UY. Three areas are thus highlighted at low speeds, which pilots are recommended to avoid as much as possible: 1. wind in the sector between 280° and 330°, in an area corresponding to the passage of the main rotor horseshoe vortex to the tail rotor, 2. wind in the sector between 210° and 330°, zone where the tail rotor can enter into vortex regime (the amplitude of this zone seems exaggerated when we know that a small speed component in the plane of the disk is enough to bring the main rotor out of this regime), 3. Tailwind, between 120° and 240°, where the instability of the helicopter places a significant workload on the pilot. Further lessons are learned from these tests and a flight campaign.

the OH-58 is not the only helicopter affected and any single-rotor helicopter can experience unanticipated yaw. the hypothesis of a tail rotor stall is eliminated, explaining why the new procedure did not improve anything (and undoubtedly contributed to increasing the number of accidents) an increase in tail rotor pitch never has adverse consequences. the basic tail rotor of the OH-58 (a larger diameter rotor was also developed and tested) has always been able to stop yaw rates up to 115° per second by applying maximum pitch together with forward cyclic. It is not said whether this forward cyclic is mandatory or only accelerates the exit from the problem. the recovery procedure is modified with the application of full left pedal and, simultaneously, forward cyclic to increase speed. If the height allows it, lowering the collective can help.The forward cyclic in addition to the pedals, the natural control around the yaw axis, suggests that the tail rotor alone is not enough . Three improvements to the OH-58 are also proposed: (1) a power transient optimization kit, (2) a 3-axis automatic pilot (SAS) and finally (3) the increased diameter tail rotor. A further research program is finally recommended, of which no trace is found subsequently. LTE accidents reported in Flightfax, the US Army's mishap prevention magazine, are becoming fewer, although some still occur—and not always on OH-58s—as acknowledged on the occasion of Flightfax 's 20th anniversary. At about the same time, Bell publishes an Operations Safety Notice and then an Information Letter containing roughly the same information on the phenomenon. Bell uses " unanticipated right yaw " to describe it, but reports in the second letter that the US Army calls it " Loss of Tail Rotor Effectiveness (LTE)", a term Bell considers misleading.

AC 90-95 Such accidents also affect civil helicopters and in 1995 the FAA publishes AC 90-95. This is still considered today as the "bible" of unanticipated yaw. Other documents follow, all over the world, which clearly take up the information. The AC 90-95 is consistent with the results given by the US Army with one notable exception. While the US Army Information Digest gave the reassuring message that even the extreme yaw rates encountered during the tests could always be stopped, the AC 90-95 suggests on several occasions that this might not be the case.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Loss of tail-rotor effectiveness

Start with the simplest possible case. Write down what Loss of tail-rotor effectiveness claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Loss of tail-rotor effectiveness before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Loss of tail-rotor effectiveness ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Loss of tail-rotor effectiveness

In research: Loss of tail-rotor effectiveness appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Loss of tail-rotor effectiveness in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Loss of tail-rotor effectiveness is common in secondary-school and first-year university syllabi. It links to neighbouring topics Helicopter aerodynamics, so understanding it makes those chapters shorter.
In everyday life: Look for Loss of tail-rotor effectiveness outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Loss of tail-rotor effectiveness” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Loss of tail-rotor effectiveness in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Loss of tail-rotor effectiveness means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Loss of tail-rotor effectiveness out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Loss of tail-rotor effectiveness in simple terms?

Why does Loss of tail-rotor effectiveness matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Loss of tail-rotor effectiveness?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Loss of tail-rotor effectiveness.