Rotor-Wing Principles of Flight

Drafted — not technically validated

Module revision 1 · How a helicopter's rotor flies, written for military pilots of every service: the blade's own relative wind, the forces in a hover and a turn, the collective, cyclic and pedals, rotor systems and torque, hover power, forward flight and dissymmetry of lift, the rotor's hazards and autorotation. It follows the Navy's Aerodynamics for Naval Aviators and the Army's Fundamentals of Flight, with the FAA Helicopter Flying Handbook as the supplementary source for most hazards. Worked numbers use an invented utility helicopter.

Every figure is interactive: move a control and the drawing and readout follow. The wing itself (lift, drag, angle of attack and stall) is in the companion module, Fixed-Wing Principles of Flight. Tiltrotors, weight and balance, and type-specific procedures are outside this revision.

Purpose and learning objectives

Every US service flies helicopters. The Army's aviation runs from the AH-64 Apache to the tandem-rotor CH-47 Chinook; the Marine Corps took the Chinook's smaller predecessor as the CH-46 Sea Knight; and the UH-1 Huey served all the US armed forces, from Army air assault to Air Force special operations and Navy river gunships. Navy helicopters fly plane guard beside the carrier, which is why the Navy's flying-qualities specifications asked for sideways flight up to 30 knots. [1, p. 322]

The physics is the same as a wing's. The Army's Fundamentals of Flight works the same laws through to the rotor: lift varies as the square of the velocity, and past a critical angle of attack the flow over the blade is disrupted and it stalls. [2, ¶¶ 1-28, 1-29] Aerodynamics for Naval Aviators says the same of the helicopter: the rotor is subject to the same physical laws of aerodynamics and motion as the fixed wing, but in much more complicated flow. [1, p. 400] So read the fixed-wing module first for angle of attack, lift, drag and stall; this module starts where the wing starts to turn.

By the end you should be able to: explain how rotation and induced flow make each blade's relative wind and angle of attack; say what the collective, cyclic, pedals and throttle each do, and why a cyclic input takes effect 90° later; tell semirigid, rigid and fully articulated rotors apart and say what torque and translating tendency are; explain why a hover takes the most power and what density altitude, weight and wind do to it; describe translational lift, ETL, transverse flow and dissymmetry of lift; recognise retreating blade stall, compressibility, vortex ring state, dynamic rollover, ground resonance, low G, low rotor rpm and loss of tail rotor effectiveness, and give the first recovery action for each; and explain the regions of an autorotating rotor and why the height–velocity diagram has areas to avoid.

This is a conceptual reference, not a flight manual. The example utility helicopter (16,000 lb, a four-bladed 27 ft radius rotor at 260 rpm, 2,000 shp) is invented to put numbers on the ideas. Its rotor turns counterclockwise seen from above, like most American-built single-rotor helicopters, [2, ¶ 1-78] so the advancing blade is on the right; for a clockwise rotor, swap left and right. Your helicopter's speeds, limits and emergency procedures come from its flight manual: the operator's manual for the Army, NATOPS for the Navy and Marine Corps, the -1 for the Air Force.

The blade and its relative wind

A rotor blade is an airfoil, and most light helicopters use symmetrical airfoils for their main rotor blades. [3, p. 2-8] Its relative wind has two parts. The rotational relative wind comes from the blade's own turning: it flows parallel to the plane of rotation and is fastest at the tip, falling uniformly to zero at the mast. [3, p. 2-9] Aerodynamics for Naval Aviators divides the blade into elements and adds a second part, the induced flow or downwash the rotor itself drives down through the disk; the resultant of the two is the relative wind each element meets. [1, pp. 400–401] Because the tips move faster than the roots, blades are twisted, with more pitch at the root and less near the tip, to spread the lift more evenly. [3, p. 2-9]

Two angles follow. The angle of incidence, usually called blade pitch, is the mechanical angle between the chord line and the plane of rotation, set by the collective and cyclic. The angle of attack is the aerodynamic angle between the chord line and the resultant relative wind; induced flow, climb, descent and airspeed all change it with no change in pitch. [3, pp. 2-8, 2-12–2-13] Increase angle of attack past the critical angle and the blade stalls, as a wing does. [3, p. 2-13]

The rotor turns at a nearly constant rpm. So, unlike an airplane, which holds an efficient angle of attack by flying at different airspeeds, the helicopter changes blade angle to cope with altitude and weight: rotor thrust is proportional to air density, a thrust coefficient and the square of the tip speed. [1, p. 400]

Figure 1 — The blade's relative wind and angle of attack (hover)

Station along the blade Pitch at 75% radius Induced flow

Interactive figure unavailable. At 75% of the example rotor's radius the blade meets the air at 327 kt from its rotation. With 38 ft/s of induced flow the relative wind tilts 3.9° downward, so 8° of pitch gives about 4.1° angle of attack.

Rotational speed Ω r; inflow angle φ = arctan(induced flow ÷ Ω r); angle of attack = local pitch − φ, with pitch quoted at 75% radius and an invented 8° twist from root to tip. Induced flow is taken as even across the disk, the simplest momentum-theory assumption; 38 ft/s is the example helicopter's ideal hover value, √(T ÷ 2ρA). The critical angle is about 15°. [3, p. 11-15]

Forces, hover and turns

Once it leaves the ground a helicopter is acted on by the same four forces as an airplane: lift, weight, thrust and drag. [3, pp. 2-2–2-3] In powered flight the rotor's total lift-thrust force acts perpendicular to the rotor disk; tilt the disk and the force tilts with it. In a no-wind hover the forces balance and the helicopter stays put; tilt the disk forward and the force divides into lift acting upward and thrust acting in the direction of flight. [3, pp. 2-14, 2-17]

The rotor's drag comes in three kinds. Profile drag is the blades' own friction and form drag and changes little with angle of attack; induced drag comes from the lift itself and is the major cause of drag at low airspeed; parasite drag comes from the fuselage, mast, gear and other non-lifting parts and grows with the square of the speed. The bottom of the total drag curve is L/D_max. [3, pp. 2-6–2-7]

A banked turn tilts the disk sideways as well, so only part of the rotor's force holds the helicopter up; to hold altitude and airspeed in the turn, collective pitch must be increased. [3, pp. 2-24–2-25] The load on the rotor rises as it does on a wing: little up to 30° of bank, then fast, twice the weight at 60°. Each blade carries its share, so a three-bladed rotor's blades each carry a third of it. Turbulence adds sudden load too, because a vertical gust raises the blades' angle of attack. If the extra power is not there, the helicopter descends or slows. [3, pp. 2-5–2-6]

Level, coordinated turn: rotor load = weight ÷ cos(bank), shown for the example 16,000 lb helicopter
Bank	Load factor	Rotor must carry
15°	1.04 G	16,565 lb
30°	1.15 G	18,475 lb
45°	1.41 G	22,627 lb
60°	2.00 G	32,000 lb

Controls and gyroscopic precession

A helicopter pilot has three major controls, the collective, the cyclic and the antitorque pedals, plus a throttle, usually on the collective. [3, p. 3-1]

Collective, by the pilot's left side: raising it increases the pitch of all the main rotor blades at once and by the same amount, and so the rotor's thrust. More pitch means more drag and less rotor rpm unless power is added, which a governor or correlator does on most turbine helicopters. [3, p. 3-2]
Cyclic: it changes each blade's pitch as it goes round, so the disk tilts in the direction the stick moves, and the thrust with it. [3, pp. 3-3–3-4] The Navy text compares the cyclic to an airplane's control stick and the collective to its throttle. [1, p. 321]
Antitorque pedals: they set the pitch, and so the thrust, of the tail rotor. In a hover they control heading; above translational lift they trim the helicopter for coordinated flight while the cyclic turns it. In tandem, coaxial and intermeshing helicopters the pedals work through the main rotors instead. [3, pp. 3-4–3-5]

In a hover every control affects the others: the cyclic holds position, the collective holds height, the pedals hold heading and the throttle, if no governor does it, holds rpm, and a change to one needs a correction of the other two. [3, p. 2-14]

The spinning rotor behaves like a gyroscope. A force on it shows its result about 90° later in the direction of rotation: gyroscopic precession. So the cyclic linkage changes a blade's pitch about 90° before the point where the disk should tilt: forward cyclic, on a counterclockwise rotor, takes pitch off the blades on the right side and adds it on the left, and the disk tips forward. [3, pp. 2-17, 3-4] Aerodynamics for Naval Aviators describes the same rig and adds that the rotor's tilt, its thrust and the height of the hub above the centre of gravity set the control moment, so control power falls when rotor thrust is low. [1, pp. 320–321]

Figure 2 — Cyclic, blade pitch and the 90° lag

Cyclic Forward Aft Left Right

Interactive figure unavailable. Forward cyclic on a counterclockwise rotor gives the blades their least pitch on the right side and their most on the left. The blades fly lowest over the nose and highest over the tail, 90° later, so the disk tips forward.

Rotor turning counterclockwise seen from above. Pitch changes are drawn at the four quarter points; the flapping they cause peaks about 90° later in rotation. Disk tilt in the side view is exaggerated.

Rotor systems, torque and configurations

As the rotor turns, centrifugal force pulls the blades straight out from the hub and gives them their rigidity; lift bends them up, so in flight they form a shallow cone: coning. [2, ¶ 1-77] [3, p. 2-16] Excessive coning comes from low rotor rpm or high gross weight; it stresses the components and costs lift, [2, ¶ 1-77] and if rpm falls far enough the blades can fold up with no chance of recovery. [3, p. 2-16] As blades flap and cone their centres of mass move in and out, so, like a skater drawing in her arms, they speed up and slow down within each turn. Rotor systems are built around these motions. [3, pp. 2-16–2-17, 4-5]

Semirigid rotor: usually two blades rigidly fixed to a hub that teeters on the mast, so they flap as a unit, one up while the other goes down; they feather but have no lead-lag hinge. [3, p. 4-2]
Fully articulated rotor: each blade can flap, lead and lag, and feather on its own; found on helicopters with more than two blades. [3, pp. 4-4–4-5]
Rigid rotor: no flapping or lead-lag hinges; the blades bend to allow those motions and are feathered at the hub. It is very responsive and not usually susceptible to mast bumping, but passes more vibration into the cabin. Bearingless rotors are a variation that relies on the hub and blade structure to absorb the stresses. [3, pp. 4-3–4-4]

Whatever the design, the Navy's text points out, the rotor corrects for forward flight the same way aerodynamically, so what is said about rotor aerodynamics holds for every rotor system: Juan de la Cierva's flapping hinges did it on the autogiro, the see-saw rotor does it by rocking the hub and the rigid rotor by cyclic pitch change. [1, p. 402]

Torque and the tail rotor

Driving the rotor twists the fuselage the other way. Torque must be counteracted, and in a single-rotor helicopter the antitorque (tail) rotor does it; give it less pedal than needed and the nose turns in the direction of torque. [2, ¶¶ 1-78, 1-79] The tail rotor is itself a rotor and takes engine power, and its greatest demands come when the engine is busiest: the most critical condition is a hover at maximum gross weight. [1, pp. 321–322] Its thrust also pushes a hovering helicopter sideways, in the direction of tail rotor thrust: translating tendency. Designers counter it by tilting the mast, rigging the controls so the disk sits slightly tilted, or accepting that the helicopter hangs one skid low. [3, p. 2-15] Hung from one point, the fuselage also swings like a pendulum, which over-controlling makes worse. [3, pp. 2-15–2-16]

Configurations

Most helicopters have a single main rotor and a tail rotor, the layout Igor Sikorsky settled on. [3, p. 1-4] A tandem rotor helicopter such as the Chinook has two main rotors turning in opposite directions, so their torques cancel, no tail rotor is needed and all the engines' power can go into lift. [3, pp. 1-4, 4-6] It yaws by tilting its rotors opposite ways with differential cyclic, and pitches by tilting both or by differential collective, more thrust on one rotor and less on the other. [1, pp. 321–323] A coaxial rotor stacks two counter-rotating rotors on one mast, like Kamov's Ka-50; an intermeshing rotor sets two masts at a slight angle so the blades mesh, as on the Air Force's HH-43 Huskie firefighting helicopter. [3, pp. 1-4–1-5, 4-6] Other antitorque designs are the Fenestron (a fan in the tail) and the NOTAR. [3, p. 1-3]

Hover, ground effect and power

To hover, the rotor drives a large mass of air downward. That induced flow is the primary cause of the high power a hover needs, and the tip vortices, part of the induced flow, add induced drag. [2, ¶¶ 1-88, 1-89] The Navy text puts it as momentum: the rotor makes lift by accelerating air downward, as a jet engine makes thrust by accelerating air out of its tailpipe. [1, p. 400] It is the air that counts, not the ground: hovering in a 20 knot wind is aerodynamically the same as flying at 20 knots in still air. [1, p. 400]

Near the surface, ground effect reduces the downward velocity of the induced flow, so the same lift takes less pitch and less power. It reaches to about one rotor diameter, and is strongest over smooth, hard surfaces and weaker over tall grass, rough terrain and water. Out of ground effect the induced flow is stronger, and hovering takes more pitch and more power. [3, pp. 2-12–2-13] More power is required in the hover than in any other flight regime, so if a helicopter can hover it can usually take off. As density altitude rises, more power is needed to hover until the power required equals the power available: the hover ceiling, which falls as weight rises. The in-ground-effect ceiling is usually higher than the out-of-ground-effect one, and the FAA advises planning an out-of-ground-effect hover wherever the landing area is uncertain. [3, pp. 7-4–7-5] Density altitude, weight and wind are the three major factors; a headwind gives translational lift and cuts the power needed, while crosswinds and tailwinds take tail rotor thrust, and so power, away from the main rotor. [3, p. 7-2]

Once the helicopter moves, translational lift reduces the power it needs; power required falls to a minimum at the best rate-of-climb speed and then rises again as parasite drag takes over, until it meets the power available at the top level speed. [3, pp. 2-17, 2-19] Figure 3 builds that curve for the example helicopter.

Figure 3 — Power required and power available

True airspeed Gross weight Density altitude

Interactive figure unavailable. At 16,000 lb at sea level the example helicopter needs 1,688 shp to hover out of ground effect, against 2,000 available. Power required falls to 957 shp at 72 kt and rises again to meet the power available at about 164 kt. At 8,000 ft density altitude the engine gives only 1,572 shp, less than the 1,765 shp the hover needs: the helicopter can fly but not hover out of ground effect.

Invented helicopter: 27 ft radius, 260 rpm, solidity 0.083, blade profile drag coefficient 0.010, 25 ft² of equivalent parasite area. Induced power = 1.15 T v_i, with v_i from momentum theory (√(T ÷ 2ρA) in the hover, falling with speed); profile power = (σ C_d0 ÷ 8) ρ A (Ω R)³ (1 + 4.65 μ²); parasite power = ½ ρ f V³. Power available falls in proportion to density (illustrative). Out of ground effect, still air; the speed limit set by retreating blade stall is not modelled.

Forward flight and dissymmetry of lift

As the helicopter gains speed, or meets a headwind, the rotor flies into air it has not already disturbed. Induced flow falls and the same blade pitch gives more lift: translational lift. [2, ¶ 1-24] [3, p. 2-21] Between about 16 and 24 knots the disk completely outruns the recirculation of its old vortices: effective translational lift (ETL), felt as a transient vibration and a jump in performance. The tail rotor gains efficiency the same way, translational thrust, which yaws the nose left on a counterclockwise rotor until the pilot eases the left pedal. [3, pp. 2-22–2-23] On the way through, the front of the disk loses induced flow sooner than the back, and the difference, transverse flow effect, shows as vibration around 12 to 15 knots and a slight roll to the right through about 20 knots; the vibration is easily mistaken for ETL. [3, p. 2-23]

In forward flight the advancing blade meets the air at its rotational speed plus the helicopter's airspeed, the retreating blade at its rotational speed minus it. Left alone, the difference, dissymmetry of lift, would roll the helicopter toward the retreating side; the Navy text notes that it defeated many early helicopter and autogiro projects. [3, pp. 2-19–2-20] [1, p. 402] Flapping is the primary means of compensating for it: the advancing blade flaps up, which reduces its angle of attack, and the retreating blade flaps down, which increases it; the pitch change the cyclic makes around the disk also creates differential lift. [2, ¶¶ 1-32, 1-33] [3, p. 2-20] The same flapping tilts the disk back as speed builds, so the nose rises: blowback, which the pilot holds off with forward cyclic until the transition is complete. [3, p. 2-20]

Figure 4 — Dissymmetry of lift (rotor seen from above)

Forward airspeed

Interactive figure unavailable. The example rotor's tips meet the air at 436 kt in a hover. At 120 kt the advancing tip sees 556 kt, Mach 0.84 at sea level, and the retreating tip 316 kt; at the same blade angle the advancing tip would make about 3.1 times the lift. Reverse flow covers the retreating blade inboard of 7.4 ft.

Blade airspeed at 90° and 270° of azimuth is Ω r plus or minus the forward speed. The rotor is the example helicopter's (27 ft radius, 260 rpm). The lift ratio compares the tips at equal blade angle, before flapping evens them out; the Mach number uses the sea-level speed of sound, 661.5 kt.

Look along the blades at high speed and the angles of attack vary widely. The whole advancing blade makes positive lift. On the retreating side, next to the hub, the air comes from the trailing edge (reverse flow); further out is a region of negative stall, then negative lift, then positive lift, and near the tip the angle of attack climbs past the stall. Flapping and cyclic even out the lift on the two sides but not the drag, and the varying drag shakes the rotor and adds to the helicopter's vibration. [1, pp. 402–403]

The rotor also makes the helicopter hard to fly hands-off. In forward flight a rotor is unstable with angle of attack, because a nose-up disturbance raises the advancing blade's lift more than the retreating blade's and the gyroscopic rotor tips further back; it does have speed stability, which gives a hover some apparent stability. Most helicopters without artificial stabilization have an unstable long-period (phugoid) oscillation, which is why helicopters meant for instrument flight need stabilization. In the Navy text's words, flying qualities that would be unsatisfactory in a fixed-wing aircraft are normal for helicopters. [1, pp. 319, 323–324]

Retreating blade stall and compressibility

To make the same lift as the advancing blade, the retreating blade must work at a higher angle of attack, and the faster the helicopter flies the higher it goes. At high forward speed the tip of the retreating blade passes its stall angle, and the stall spreads inboard from the tip as speed or blade pitch rises. Retreating blade stall becomes severe, and control marginal, when the outer quarter of the retreating blade is stalled; with about 15 percent of the disk stalled, control is impossible. [1, pp. 402–404]

Signs: rotor roughness, erratic stick forces, and a vibration and stick shake at a frequency set by the number of blades (three beats per revolution on a three-bladed rotor), then a pitch-up and loss of control. [1, p. 404] The FAA lists a low-frequency vibration, the nose pitching up and a roll toward the retreating side. [3, p. 11-11]
What brings it on: high airspeed, low rotor rpm, high gross weight, high density altitude, high load factor, turbulence and abrupt control movements, all of which raise the retreating blade's angle of attack. Because a blade needs more angle at altitude, retreating blade stall comes at a lower airspeed the higher you fly, and manufacturers publish a never-exceed speed (V_NE) that falls with altitude. [1, p. 404] [3, p. 11-11]
Recovery: reduce the blade angle of attack below the stall, by lowering collective, slowing down, increasing rotor rpm and easing the maneuver. Forward cyclic to hold the nose down only makes it worse, because it raises the retreating blade's angle of attack. [1, p. 404] The FAA adds that aft cyclic worsens it too, that the first step is usually to reduce collective, and that the cure follows the cause: roll out of the steep turn that brought it on, or restore low rpm. [3, p. 11-11]

Warning is good when the stall is approached slowly; it is poor only when blade pitch is raised quickly, so a severe stall is most likely in abrupt control movements or rapid accelerated maneuvers. [1, p. 404]

The advancing tip has the opposite problem. It meets the highest relative speed, and when its Mach number passes the blade section's critical Mach number, drag rises sharply: more power is needed to hold rotor rpm, and the rotor runs rough, shakes the stick and twists its blades. High airspeed, high rotor rpm, high weight, high density altitude, low temperature and turbulence bring it on. Retreating blade stall and compressibility share most of their causes and their recovery, but compressibility comes at high rpm and retreating blade stall at low rpm, so the rpm correction is opposite. [1, pp. 404–405]

Vortex ring state

In an out-of-ground-effect hover some air curls up around the blade tips and back in from above: tip vortices. Let the helicopter settle into its own downwash and the vortices grow, the inner blades meet air coming up through the disk, and much of the engine's power goes into stirring a doughnut of air: vortex ring state, formerly called settling with power. [3, p. 11-9] The Navy text explains it the same way: upward flow in the centre of the disk and downward flow outside it, the main lifting part of the rotor not stalled, and a net thrust that can fall toward zero. [1, pp. 408–410]

The FAA gives the combination of conditions likely to cause it in any helicopter: a vertical or nearly vertical descent of at least 300 feet per minute, the rotor using some of the available power (20 to 100 percent), and a horizontal speed slower than ETL. Steep approaches with a tailwind and out-of-ground-effect hovers without precise height control lead into it, and it can follow a quick stop or the recovery from an autorotation. A fully developed vortex ring state brings uncommanded pitch and roll, little or no collective authority, and a descent that may approach 6,000 feet per minute. [3, pp. 11-9–11-10]

Figure 5 — The three conditions for vortex ring state

Rate of descent Horizontal airspeed Power in use

Interactive figure unavailable. Descending at 800 fpm at 5 kt with 60 percent power, all three conditions hold: vortex ring state is likely. Faster than ETL, descending slower than 300 fpm or with no power in use, it is not.

Conditions from the FAA handbook; the actual critical descent rate depends on gross weight, rotor rpm and density altitude. ETL is taken as about 16 to 24 kt, the handbook's figure. Schematic, not to scale.

Recovery means leaving the vortex. The pilot's instinct, more collective, only enlarges the stalled area and increases the descent. The traditional recovery is to increase airspeed with forward cyclic and/or partially lower the collective, until the helicopter passes through ETL into a normal climb. The Vuichard recovery instead uses lateral cyclic with more power and antitorque thrust to stop the descent; in most helicopters it is the quickest exit. Training entries should allow recovery by at least 1,000 feet above the ground. [3, p. 11-10] The 1965 Navy text also recommended diving out as the recovery that loses least altitude, and noted that many cases called "power settling" are really a high sink rate with too little power to stop an approach, common when heavy or at high density altitude. [1, pp. 410–411] Follow your flight manual's procedure.

Dynamic rollover and ground resonance

A helicopter with one skid or wheel on the ground can roll over it. Dynamic rollover needs three things: a rolling moment, a pivot point other than the normal centre of gravity, and rotor thrust greater than weight. The pivot can be a skid caught on a tie-down, an object, ice, mud or soft asphalt, or a slope. Past the critical rollover angle, about 5 to 8°, the cyclic no longer has the range to stop the roll and thrust rolls the helicopter onto its side. [3, pp. 11-11–11-12] Quickly reducing collective is the most effective way to stop it, because it takes the thrust away; a smooth, moderate reduction, about full up to full down in two seconds, may be enough, while dumping the collective can let a blade strike the fuselage or bounce the helicopter over the other way. [3, pp. 11-12–11-13] With a counterclockwise rotor it is most critical with the right skid down, a crosswind from the left and a right lateral centre of gravity. [3, p. 11-12] Slopes are covered in the Airfield Operations module's helicopter material and your flight manual.

Ground resonance affects helicopters with articulated rotors on the ground. A hard landing on one wheel can shove the blades out of their even spacing on their drag hinges; the out-of-balance rotor then shakes the airframe at its natural frequency, and with the engine feeding it the vibration can destroy the helicopter in seconds. If rotor rpm is low, close the throttle and lower the collective fully; if rpm is in the normal range, lift off and let the blades rephase before landing again. Rigid and semirigid rotors, with no drag hinge, do not suffer it, and skid gear is less prone than wheels. [3, p. 11-11]

Low G, mast bumping and low rotor rpm

Helicopters rely on positive G for much or all of their control response, because the cyclic controls the fuselage by tilting the rotor's thrust. In low G, such as a pushover from moderate or high speed, thrust and control authority fall away. A rotor with three or more blades and offset hinges keeps some control; a two-bladed teetering rotor keeps none. If the fuselage then rolls and the pilot answers with opposite cyclic, the angle between disk and fuselage can exceed the hub's clearance and the hub strikes the mast: mast bumping, which can sever the mast. The FAA notes it was first met in military nap-of-the-earth flying, and has caused numerous military and civilian fatal accidents. [3, pp. 11-14–11-15] Recover by first gently applying aft cyclic to restore normal G, then correct the roll; in two-bladed helicopters avoid abrupt forward cyclic and descend with the collective instead, and slow down in turbulence. [3, pp. 11-14–11-15] Flapping angles, and so the risk, grow with high speed, low rotor rpm, high density altitude, high weight and turbulence. [3, p. 4-3]

A helicopter needs rotor rpm as an airplane needs airspeed. Let rpm fall below the safe range and the blades must fly at ever higher angles of attack; at the critical angle, about 15°, the whole rotor stalls. Rotor stall is not retreating blade stall: it can happen at any airspeed, the rotor stops carrying the helicopter, the blades can blow back into the tail boom, and it is not recoverable; above about 50 feet it is likely to be fatal. [3, pp. 11-15–11-16] Low rpm comes from not lowering the collective at once after an engine failure, or, more often, from demanding more power than the engine has, overpitching, at high density altitude or heavy weight. A 10 percent loss of rpm costs about 10 percent of the engine's power, so it feeds on itself. The signs are the rpm gauge and warning, a yaw, less engine noise and, at speed or in turns, more vibration. Recover at once: lower the collective, increase throttle if available, and use aft cyclic to hold a level attitude. [3, p. 11-16]

Loss of tail rotor effectiveness

Loss of tail rotor effectiveness (LTE) is an uncommanded, rapid yaw toward the advancing blade, to the right on a counterclockwise rotor, that does not stop by itself. It comes from the airflow around the tail rotor, not a mechanical failure, and can affect any single-rotor helicopter with a tail rotor. [3, p. 11-18] High power at low speed, out of ground effect, in the wrong wind, asks more of the tail rotor than it may be able to give. The FAA names three wind regions for a counterclockwise rotor: 285 to 315° (left front), where 10 to 30 knot winds blow the main rotor vortex into the tail rotor; 120 to 240° (tailwinds), where the helicopter weathervanes and a right yaw can accelerate; and 210 to 330°, where a tail rotor vortex ring state makes the pedals work hard. [3, pp. 11-19–11-21] Avoid low, slow flight out of ground effect, tailwinds below 30 knots, large power changes at low speed and fast right turns; keep maximum power-on rotor rpm. [3, pp. 11-20–11-21] If an unanticipated right yaw starts: full left pedal, forward cyclic to gain speed and, if altitude permits, reduce power; if the rotation cannot be stopped near the ground, an autorotation may be the best course. [3, p. 11-21]

Autorotation

An engine failure does not stop the rotor. A freewheeling unit lets the rotor turn without the engine, and by lowering the collective, which must be done immediately, the pilot cuts lift and drag and starts a descent, so air flows up through the disk and keeps the rotor turning. [3, pp. 2-25, 11-2] The tail rotor is still driven by the main rotor, so the pedals still control heading. [3, p. 11-2] An autorotation is also the answer to a complete tail rotor failure, since there is virtually no torque in one. [3, p. 11-2]

Look along an autorotating blade, the Navy text explains, and the forces differ with radius. Near the tip, at low angle of attack and high speed, the blade's lift and drag leave a net drag in the plane of rotation that slows the rotor; at about half span, the inflow tilts the lift forward enough to give a net forward pull that drives it. [1, pp. 405–406] The FAA names three regions in a vertical autorotation: the driven region, the outer 30 percent or so; the driving region, normally between 25 and 70 percent of the radius; and the stall region, the inner 25 percent, which is above its stall angle and adds drag. The collective sets their balance and so the rpm. [3, p. 2-26] Autorotation is essentially stable: if the rotor slows, the driving region grows and speeds it up again; if it speeds up, the driven region grows. But let rpm fall too far and the rotor becomes unstable and keeps slowing unless the pilot acts at once. [1, p. 407]

Figure 6 — Blade regions in a vertical autorotation

Collective

Interactive figure unavailable. Balanced, the inner 25 percent of the blade is stalled, the driving region runs from 25 to 70 percent of the radius and the outer 30 percent is driven, so rotor rpm holds. Raising the collective shrinks the driving region and rpm falls; lowering it does the opposite.

Balanced sizes are the FAA handbook's. How far the boundaries move with each step of collective is illustrative; the handbook gives the direction, not the amount. In forward flight the regions shift outboard on the retreating side. [3, p. 2-26]

Airspeed and rotor rpm control the rate of descent. It is high at zero airspeed, least at about 50 to 60 knots in a typical helicopter, and rises again beyond that; each type's flight manual gives its own autorotation airspeed and rpm. [3, pp. 11-2–11-3] The Navy text distinguishes the speed for the minimum rate of descent from the faster speed for the greatest glide distance, found where a line from the origin touches the curve of descent rate against airspeed, just as for a gliding airplane. [1, pp. 407–408] The FAA puts the maximum-range speed at usually about 25 percent faster than the minimum-descent speed. [3, p. 7-4] Raising the collective lowers rpm and lowering it raises rpm; aft cyclic, in trim, also sends more air up through the disk and raises rpm. Accident investigations found pilots facing a real failure at cruise speed slow to apply down collective, aft cyclic and pedal together. [3, p. 11-3]

Near the ground the pilot flares with aft cyclic to slow down, which also raises the rpm, and the collective keeps rpm in limits. [3, p. 2-26] The last cushion comes from the energy stored in the spinning rotor: raising the collective converts it into a brief burst of lift. [1, pp. 408–409] [3, p. 11-2] A rotor with little stored energy loses rpm fast after the failure, which leaves the pilot less time to lower the collective. [1, p. 408]

The height–velocity diagram

Some combinations of height and airspeed leave too little time, height or speed to enter a steady autorotation. The flight manual's height–velocity diagram shades them. Low and slow, high above a survivable drop but well below autorotation speed, there is not enough height to accelerate; the 1965 Navy text put the minimum at about 300 feet for most small helicopters and 500 to 600 feet for larger ones. Fast and close to the ground, there is too little time to flare before hitting the ground or striking the tail. [3, pp. 7-2–7-3] [1, p. 409] The diagram applies only within its gross weight and density altitude, and multiengine helicopters that can fly on one engine do not show the fast, low area. [3, pp. 7-3–7-4] The usual safe takeoff stays out of the shaded areas by accelerating close to the ground and climbing as speed approaches a safe autorotation speed. [3, p. 7-3]

The entry, descent, flare and touchdown are taken step by step in Autorotation Practice, which ends in a standalone engine failure exercise.

Terms

Induced flow: The downward flow of air through a rotor disk. In a hover it is the main cause of the high power a helicopter needs.
Angle of incidence: The angle between the chord line of a rotor blade and the rotor hub, usually called blade pitch angle. For a fixed airfoil such as a fin, the angle between its chord line and a reference plane of the aircraft.
Collective: The helicopter control, at the pilot's left side, that changes the pitch of all the main rotor blades at once and by the same amount, and so the rotor's thrust.
Cyclic: The helicopter control that changes each blade's pitch as it goes round, tilting the rotor disk, and its thrust, in the direction the stick moves.
Antitorque pedals: The helicopter pilot's foot controls for the tail rotor. They change the pitch, and so the thrust, of the tail rotor blades, which controls heading in a hover.
Gyroscopic precession: The deflection of a spinning object when a force is applied to it, appearing about 90° later in the direction of rotation. It is why rotor controls act 90° ahead of the response.
Coning: The upward bend of rotor blades as they carry the helicopter's weight, balanced by centrifugal force. Low rotor rpm or high weight increases it; excessive coning stresses the rotor and reduces lift.
Semirigid rotor: A rotor, usually two-bladed, whose blades are rigidly fixed to a hub that teeters on the mast, so they flap together as a unit. The blades feather but have no lead-lag hinge.
Fully articulated rotor: A rotor in which each blade can flap, lead and lag, and feather independently of the others. It is found on helicopters with more than two main rotor blades.
Rigid rotor: A rotor with no flapping or lead-lag hinges; the blades bend to allow those motions. It is very responsive and usually not susceptible to mast bumping.
Translating tendency: The sideways drift of a hovering single-rotor helicopter in the direction of its tail rotor thrust.
Tandem rotor: A helicopter layout with two large main rotors, one ahead of the other, turning in opposite directions instead of a single main rotor and a tail rotor.
Coaxial rotor: A pair of rotors on the same shaft turning in opposite directions. Their torques cancel, so no tail rotor is needed.
Ground effect: The surface's interference with the airflow around a wing flying within about one wingspan of it, which reduces induced drag. For a helicopter, the increase in rotor efficiency within about one rotor diameter of the surface, so a hover in ground effect takes less power.
Translational lift: The improved rotor efficiency that comes with forward speed or wind, as more fresh air enters the disk and induced flow falls; the same pitch then gives more lift.
Effective translational lift (ETL): The point, between about 16 and 24 knots of forward speed, where the rotor disk completely outruns the recirculation of its own vortices and the same blade pitch gives more lift.
Transverse flow effect: The difference in lift between the front and rear of a rotor disk in forward flight, as induced flow falls at the front and rises at the rear. It shows as vibration around 12 to 15 knots and a roll to the right.
Dissymmetry of lift: The difference in lift between the advancing and retreating halves of a rotor disk in forward flight. Blade flapping is the primary means of compensating for it.
Flapping: The up and down movement of rotor blades, about a hinge in a fully articulated rotor or as a unit in a semirigid one. It is the primary means of compensating for dissymmetry of lift.
Blowback: The nose-up pitch, and tendency to roll right, as a helicopter accelerates through translational lift, caused by dissymmetry of lift and transverse flow. Forward cyclic holds the rotor disk attitude.
Retreating blade stall: Stall of a helicopter's retreating blade at high forward speed, from its high angle of attack and slow relative wind. The first step in recovery is usually to reduce collective.
Vortex ring state: Also called settling with power: a vertical or nearly vertical descent of at least 300 feet per minute, with the rotor using some engine power and horizontal speed below ETL, in which the rotor settles into its own downwash. Adding power makes it worse.
Dynamic rollover: A roll about a skid or wheel pinned to the ground that carries a helicopter past the point where cyclic can stop it, about 5 to 8 degrees. Quickly reducing collective is the most effective way to stop it.
Ground resonance: A destructive vibration of a helicopter on the ground, at certain rotor speeds, when an out-of-balance articulated rotor drives the airframe at its natural frequency. It can destroy the helicopter in seconds.
Mast bumping: The rotor hub striking the mast when flapping exceeds its design limit, a hazard of semirigid (teetering) rotors that can shear the mast. Low G makes it likely.
Rotor stall: The stall of the whole rotor when rotor rpm falls too low, unlike retreating blade stall, which affects only part of the retreating side at high speed.
Loss of tail rotor effectiveness (LTE): An uncommanded, rapid yaw towards the advancing blade that does not stop of its own accord. It comes from aerodynamic interaction between the main and tail rotors, not a mechanical failure.
Autorotation: Descent with the rotor turned by the upward flow of air instead of the engine. After an engine failure the pilot lowers the collective at once to keep the rotor turning.
Height/velocity diagram: The chart of combinations of height above the ground and airspeed from which a helicopter can complete a landing after an engine failure. Its shaded areas are to be avoided.

Check your understanding

You raise the collective in a hover. What happens to the blades, and what must happen to the power?

All the blades take more pitch at once, so thrust rises; so does their drag, so power must rise to hold rotor rpm (a governor or correlator does it on most turbine helicopters).

Why does forward cyclic change blade pitch on the sides of the disk, not at the front and back?

The rotor acts like a gyroscope: the flapping a pitch change causes peaks about 90° later in rotation. Least pitch on the right of a counterclockwise rotor makes the blades lowest over the nose.

The same helicopter can hover at sea level but not at 8,000 ft density altitude. Why?

Hover takes the most power of any flight regime, it takes more as the air thins, and the engine gives less. Where power required meets power available is the hover ceiling, lower at higher weight.

Accelerating through about 12 to 15 knots you feel a vibration and the helicopter rolls slightly right. Is that ETL?

Probably not yet: that is transverse flow effect. ETL comes between about 16 and 24 knots, with its own transient vibration and a jump in performance.

In a helicopter with a counterclockwise rotor, which side of the disk is advancing in forward flight?

The right side. Its blades add the forward speed to their own; the left side's blades subtract it.

Flying fast and heavy at altitude, you feel the stick shake, the nose pitches up and the helicopter rolls left. What is it, and what do you do?

Retreating blade stall. Reduce blade angle of attack: usually lower the collective first, slow down, restore rotor rpm, ease the maneuver. Don't push the nose down with forward cyclic.

Compressibility and retreating blade stall have similar causes. What is the difference in recovery?

Rotor rpm: compressibility comes at high rpm and retreating blade stall at low rpm, so the rpm correction is opposite.

You're descending almost vertically at 800 fpm, below ETL, with power applied. Why is more collective the wrong fix?

Those are the conditions for vortex ring state: more collective enlarges the stalled area and the descent. Gain airspeed and/or partially lower collective, or use the Vuichard recovery.

Lifting off, the helicopter starts to roll over a skid stuck in the mud. What stops it?

A smooth, prompt reduction of collective, which removes the thrust that is rolling it. Opposite cyclic alone cannot stop dynamic rollover.

In a two-bladed helicopter you push over and feel weightless; the fuselage starts rolling right. What first?

Gently apply aft cyclic to restore positive G, then correct the roll. Correcting the roll in low G risks mast bumping.

Hovering low and slow with a left-front wind, the nose starts a fast right yaw that won't stop. What do you do?

That is loss of tail rotor effectiveness: full left pedal, forward cyclic to gain speed, and reduce power if altitude permits.

The engine quits in cruise. What is the first control movement, and why so fast?

Lower the collective immediately: the high-pitch blades are losing rpm, and low rotor rpm can end in an unrecoverable rotor stall.

In an autorotation the rotor rpm is too high. Which way do you move the collective?

Up: raising it adds pitch, grows the driven region and slows the rotor. Lower it again as rpm returns to the normal range.

Sources and applicability

Military sources

[1] H. H. Hurt, Jr., NAVWEPS 00-80T-80, Aerodynamics for Naval Aviators, revised January 1965. Issued by the Office of the Chief of Naval Operations, Aviation Training Division; hosted by the FAA: read the book (faa.gov). Inspected 26 October 2026 from the full PDF: pp. 319–324 (helicopter stability and control) and 399–411 (helicopter problems: rotor aerodynamics, retreating blade stall, compressibility, autorotation, power settling). A US Government work.

[2] Headquarters, Department of the Army, FM 3-04.203, Fundamentals of Flight, 7 May 2007. Approved for public release; distribution is unlimited. Read the manual (irp.fas.org). Inspected 25 October 2026 through a web text-extraction tool that reached ¶1-89 of chapter 1; cited by paragraph: ¶¶1-24, 1-28–1-33, 1-77–1-79, 1-88–1-89. It has since been superseded: the Army Publishing Directorate lists TC 3-04.1-4, Fundamentals of Flight (18 March 2026), which replaced TC 3-04.4 of July 2022. No public download of the current circular was available, so this revision cites the 2007 manual for the paragraphs it read and the FAA handbook for the rest.

Supplementary: FAA handbook

[3] Federal Aviation Administration, FAA-H-8083-21B, Helicopter Flying Handbook (2019). Chapter PDFs on faa.gov, read in full on 26 October 2026: chapter 1, Introduction to the Helicopter; chapter 2, Aerodynamics of Flight; chapter 3, Helicopter Flight Controls; chapter 4, Helicopter Components, Sections, and Systems; chapter 7, Helicopter Performance; chapter 11, Helicopter Emergencies and Hazards. A US Government work, public domain.

Page-specific excerpts are recorded in the repository review evidence: Rotor-Wing Principles of Flight evidence. The FAA handbook is cited where the military texts read for this revision do not cover a point: most of the hazards (dynamic rollover, ground resonance, low G, rotor stall, loss of tail rotor effectiveness), the controls and rotor systems, and the vortex ring state conditions and Vuichard recovery, which postdate the 1965 Navy text. Aircraft examples link to the site's encyclopedia, which carries their sources.

The blade-element, power and rotor-speed figures are standard rotor mechanics (momentum and blade-element theory in their simplest forms) applied to an invented helicopter; the tests check them against hand-worked values. Figures 1–6 are original line drawings, not reproductions of any publication's figure, and are not to scale except where a caption gives the scale.

Applicability: rotor-wing principles for military helicopter pilots of every service. The example helicopter's numbers are invented and apply to no real type. Your helicopter's operator's manual, NATOPS or -1 governs its limits and emergency procedures. This revision is Drafted pending independent source-backed review.