AOPA Hover Power

Newton’s third law of motion says, for every action, there is an equal and opposite reaction. So, when a helicopter’s rotor system spins in one direction, the fuselage wants to spin in the opposite direction (since this is a rotational force it is called torque). To prevent this engineers put a small thrust-producing rotor on a moment arm (the tail boom) to create a rotational force (torque) that is equal, but opposite, to the force trying to spin the fuselage. Its technical name is an anti-torque rotor, however it is often referred to as a tail rotor.

A set of pedals in the cockpit change the pitch of the tail rotor to vary the amount of thrust produced. Although they control yaw, they function differently than rudder pedals in an airplane.

As long as the main rotor rpm stays constant, so will the tail rotor’s. In fact, if you turn the main rotor by hand the tail rotor will also turn. This is because a system of drive shafts and gearboxes directly connect it to the main rotor transmission. Depending on the helicopter’s design, the tail rotor will spin 3 to 6 times faster than the main rotor.

When viewed from above, most main rotor systems spin counterclockwise (CCW). Sometimes people refer to this as the American direction and clockwise (CW) as the European direction. This is not really accurate as some models built in Europe also turn counterclockwise. For example, Augusta (based in Italy) manufactures models that spin CCW and several Eurocopter models (EC135, EC145) do as well. However, the most popular helicopter with a CW turning rotor system is the Eurocopter Astar.

The rotor system’s rotational direction makes very little difference to gravity or air, but it does change things a little for the pilot. When a pilot increases power (raising the collective control) the torque applied to the fuselage increases. In a CCW turning rotor the pilot must add left pedal to increase the tail rotor’s pitch, and therefore thrust, to keep the nose straight. Likewise, decreasing power requires right pedal input. Right pedal reduces the pitch and thrust allowing excess engine torque to turn the fuselage. In a CW turning rotor just the opposite is true.

Pilots who routinely switch between airframes with different rotor directions, have to remember which one they are in as over time collective movement and the associated pedal movement become automatic. Even if they forget, it is not that big of a problem as it is fairly easy to just react to yaw direction with the necessary pedal movement. Spend enough time switching airframes and eventually it becomes an automatic response again for each airframe.

When an engine fails the torque goes away. As part of the entry into autorotation the pilot must neutralize the tail rotor thrust. With a CCW turning rotor this means pushing almost full right pedal and for a CW turning rotor it’s left pedal. In a hover this must be done quickly as the unnecessary tail rotor thrust will start spinning the helicopter. In forward flight, the pilot will experience a yaw to the left as airflow over the vertical fin helps hold the tail straight.

Coming next is more on tail rotor emergency maneuvers and different types of anti-torque designs.

There are several different main rotor system designs that are used on modern helicopters. The three basic designs that have traditionally been taught to students are semi-rigid, fully articulated, and rigid. Today there are versions that make extensive use of composite materials and are known as hinge less systems.

A fully articulated system normally has more than two blades. In this design each blade is attached to a hub with hinges that allow it to move independently of the others. A feathering hinge is used to change the pitch of each blade. A flapping hinge allows each blade to move up and down to compensate for dissymmetry of lift. Blades are able to move fore and aft or lead-lag, (called hunting) by use of a drag hinge. Normally a damper is attached to the blade and hub to restrict excessive movement. The drag hinge is used because, when a rotor blade flaps up, its center of mass moves closer to the axis of rotation. This causes the rotor system to spin faster, much like a spinning ice skater speeds up when pulling her arms in closer. Allowing the blades to lead-lag reduces this tendency.

A semi-rigid system refers to a two-blade system where each blade is mounted to a hub that has a center teetering hinge. In this configuration, when one blade flaps up the other one flaps down – like a see saw. As with the fully articulated system, each blade has a feathering hinge. The two blades are mounted in an under-slung position, that is where the teetering hinge is mounted above the plane of rotation. The geometry of this arrangement minimizes the change in distance between the center of mass and the axis of rotation during flapping. This allows a semi-rigid system to not need a drag hinge.

In a slight departure from the traditional semi-rigid design, Frank Robinson used a coning hinge on each blade (some refer to this as a flapping hinge, but it is used for blade coning). When rotor blades produce lift (especially under high load or low rotor rpm) they flex upward (coning). This places a high stress load at the blade’s root, so in order to relieve this stress Robinson’s design allows the blade root to cone about a hinge. This reduced the amount of reinforcing required at the blade root making for a lighter easier to manufacture rotor blade.

Rigid rotor systems do not use hinges and limited movement is absorbed through the hub and rotor blades. Many of the modern composite rotor systems also do not use traditional hinges, but have elastomeric and specially designed composites structures (flextures) that allow the blades to flap, feather, and hunt. Manufactures do not use the term rigid rotor system, opting instead to describe these systems as a fully articulated hinge less rotor system. These systems do not require lubrication and are less maintenance intensive. The extensive use of composite materials also increases reliability and helps absorb vibration.

How exactly does flapping change a rotor blade’s angle of attack? That was a great question with many good explanations provided by readers. I think to fully understand it is important to know the difference between pitch angle and angle-of-attack. Pitch angle is the angle between the rotor blade’s chord line (a straight line intersecting the leading and trailing edges of an airfoil) and a reference plane of rotation. Angle-of-attack is the angle between the rotor blade’s chord line and the relative wind (the airflow that results from, and is opposite of, the velocity of an airfoil. Velocity is used here as a vector to mean speed and direction.)

When the rotor blades stay in the reference plane of rotation the pitch angle and angle-of-attack are the same. The pilot controls the pitch angle with the collective control and thus the angle-of-attack as well. However, when a rotor blade leaves the plane of rotation (flapping causes this to happen) the direction component of its velocity changes. Since relative wind is a function of velocity, it changes as well. In the case of a blade that flaps up the relative wind moves opposite the blade’s new direction. This change in relative wind direction reduces the blade’s angle-of-attack. The opposite is true for the blade that flaps down on the retreating side.

As the helicopter’s forward speed continues to increase, the retreating, or down flapping, side encounters higher angles of attack. Eventually, the rotor system encounters retreating blade stall.

From the pilot’s perspective, when this happens an abnormal vibration will be felt, the nose can pitch up, and the helicopter can have a tendency to roll in the direction of the stalled side. The amount and severity of pitch and roll will vary depending on the rotor system design.

The tendency for the nose to pitch up is because the spinning rotor system acts like a gyroscope and therefore experiences gyroscopic precession (a physical property that states when an external force is applied to a rotating body the effect will happen approximately 90 degrees later in the direction of rotation). As such, when the retreating blade stalls and stops producing lift, the effect of this happens toward the rear of the rotor disc. This causes the disc to tilt back, and the nose to pitch up.

Conditions like high density altitude, steep or abrupt turns, high blade loading (caused by high gross weight), turbulent air and low rotor rpm will increase the likelihood of encountering retreating blade stall when operating close to a helicopter’s Vne (never exceed speed). Helicopter flight manuals contain a chart or textual description in the limitations section that reduce the helicopter’s Vne at higher altitudes and temperatures. This is the airspeed limitation chart from a Bell 407.

Should a pilot encounter retreating blade stall, lower the collective and reduce airspeed. Other actions that will help are increasing rotor rpm and decreasing the severity of any roll or pitch maneuvers. Taking immediate action at the first sign will normally result in a quick recovery. However, if a pilot attempts to increase speed a severe stall would develop with possible loss of control.

Over the past year 28 people have died in EMS (emergency medical services) aircraft crashes. The industry is experiencing one of the worst accident rates in its history. Solving this problem is a complicated issue for sure, however I have some very basic thoughts on how this problem can be fixed.

Flying an EMS helicopter was some of the most demanding flying I have done. Flying at night and landing on streets or other confined areas, having to make quick weather decisions sometimes with little information available, and having to block out the pressure to fly. Yet many EMS helicopter pilots receive the minimum amount of required training.

Conversely, when I flew a corporate helicopter it was normally airport to airport or heliport. The occasional off-airport landing was performed, however it was planned and I had plenty of time to assess the area. This was far less demanding and risky than flying an EMS helicopter. Yet, it was also where I received the best and most consistent training. We had the time and resources available to practice our skills and FlightSafety training every six months.

Corporate helicopters are not expected to make money and the person who has the authority to cut training expenses normally rides in the back. That’s a strong motivator to ensure that the pilots know what they’re doing. EMS helicopter operations by contrast need to make money and that means keeping a close eye on costs. Also, because of the competitive bid process hospitals use when selecting vendors, margins are thin. Training costs come right off the bottom line. If a vendor increases its training costs and the others do not, then that vendor is at a competitive disadvantage. Hospital-owned programs are also in business to get patients to their hospital and make money.

To level the playing field, I think two requirements are needed. The first is more frequent and comprehensive training. Not just training in maneuvers but scenario-based training that addresses issues such as crew coordination, judgment, and accident chains to name a few. Additionally, more IFR and inadvertent IMC training, even for VFR-only programs, is needed. Pilots need to be very comfortable initiating a climb and not descending if they get caught in bad weather.

This type of training can be done in simulators. Simulators are not only good for showing pilots how to do things correctly, but can also show how quickly a bad decision can degenerate into a serious problem. That’s a powerful learning tool.

Second is better equipment, such as terrain avoidance and warning systems and night vision goggles. In addition, important in adding new equipment is providing the appropriate level of training on how to use it effectively.

Another issue that should be addressed by the industry is pilot salaries. I have known many very good pilots that have left EMS for better paying jobs. This has made EMS a steppingstone for pilots to get to something better. EMS flying requires a very specific skill set and experience level. It should be the job that pilots aspire to get. Higher salaries will keep turnover down and keep experienced pilots in the industry.

I realize that all of my solutions cost money and that some operators will claim they cannot afford these programs. That is why training and equipment should be mandated for everyone who wants to operate an EMS helicopter. The difficult part is figuring out how the industry will get there.

The FAA has tried the quick and inexpensive solutions and they do not work. Case in point is the risk assessment matrix. Three years ago EMS pilots began filling out a questionnaire before each flight to determine a score that related to a risk level. The accident rate has gotten worse in the last three years.

As with most things in life, to get the best results one needs to spend the effort and money required. Cheap solutions are just that.

Not visible from the cockpit, a helicopter’s tail rotor is perhaps the most vulnerable component to striking objects in a hover. EMS pilots are especially at risk, as their job involves routinely landing in obstacle rich environments.

In 2003, a Bell 430 was substantially damaged when its tail rotor hit a roadway sign during an off-airport landing at night. Prior to touchdown, the pilot said he rotated the aircraft and landed on an easterly heading, at which point the medical crew departed the helicopter. Then, the pilot decided to reposition the aircraft to face west for departure. During the hovering turn the tail rotor hit a steel reflector post. The aircraft touched down on the left rear skid first and came to rest 180 degrees from its initial heading. The tail rotor and gearbox assembly had come apart and departed the helicopter.

Darkness certainly makes objects harder to see. However, two years prior to this accident, during daylight conditions, a Bell 222UT was substantially damaged when its tail rotor hit a barrel while landing on a paved traffic turn-around area. The pilot said that while hovering, he decided to reorient the aircraft to help load the patient easier. During the right pedal turn, the tail rotor struck a 55-gallon trash barrel. The helicopter yawed to the right and the pilot brought the throttles to flight idle and landed the helicopter. The tail boom was twisted, the tail rotor blades were damaged, and the tail rotor gearbox was nearly separated from the airframe.

Although a tail rotor strike in a hover can cause serious damage, the potential for personal injury is low compared to what can happen in flight.

In 1999, a Bell OH-58A, on a photo flight with doors removed, was destroyed on impact with the terrain and the private pilot and passenger sustained fatal injuries. A witness reported that he saw the helicopter flying at an altitude of approximately 350 to 400 feet. He saw what was possibly a large bird hit the rear rotor of the helicopter. The helicopter made three to four rotations during its descent.

Examination of the tail assembly revealed an elastic material with navy blue yarns wrapped around the tail rotor. The material, along with a sample of a navy blue warm-up jacket found along the reported flight path, was sent to the NTSB’s Materials Laboratory for examination. The color, size, and texture of the navy-blue yarns in the elastic material were consistent with those found in the navy blue warm-up jacket. The NTSB concluded that the jacket exited the helicopter and became entangled in the tail rotor.

Removing a helicopter’s doors places the tail rotor at increased risk. In 1993, an R22 helicopter flying with its left door removed crashed after an aluminum kneeboard exited the helicopter and struck the tail rotor.

There have been numerous cases where objects have come out of the cabin or an unsecured baggage compartment and struck the tail rotor. In some cases the pilots have been able to enter autorotation or otherwise land with minor damage or injury. However, as with the two preceding accidents, the tail strike inflicted enough damage to cause the tail rotor assembly to come apart. In these cases, the resulting center of gravity shift will make recovery nearly impossible. The importance of protecting the tail rotor cannot be emphasized enough.

One of the more confusing subjects for helicopter students to fully understand is known as the vortex ring state, also (correctly or incorrectly – depends on who you ask) referred to as settling with power or power settling. Much of the confusion comes from the terminology as these terms are used interchangeably in many textbooks. Yet, there are instructors who teach that settling with power is very different from the vortex ring state. This is true when settling with power is defined as simply not having sufficient power to hover, thus causing the helicopter to descend or settle when power required exceeds power available. However, many textbooks do not use that definition – hence the confusion.

Regardless of what labels you use, the important point to know is that there are two very different situations that can affect a helicopter in an out of ground effect hover or a steep approach. One is simply running out of power as described above, the other is the vortex ring state, which is an aerodynamic condition that forms when a helicopter is allowed to descend into its own downwash. It can happen even when a helicopter has more power available than needed.

To understand the vortex ring state imagine a helicopter hovering at 1,000 feet. The rotor system is drawing air from above and accelerating a large amount of air downward in a column underneath the helicopter. If the pilot allows the helicopter to descend vertically at too high a rate of descent into the column of downward moving air, the air that is now above the rotor system will still be moving downward. As the rotor system tries to draw air that is moving downward a re-circulation of air forms, causing the rotor tip vortices to become much bigger. (Normal rotor tip vortices are small and only cause a small loss of rotor efficiency.) Additionally, a secondary vortex ring will form near roots of the rotor blades.

In a well-developed vortex ring state, most of the engine power is consumed by accelerating air in a circular pattern around the rotor system. The vortex ring state causes turbulent rotational flow across the blades with increasing roughness and possible loss of control. The helicopter continues to descend and a natural reaction is to increase power by raising the collective control. This merely increases the strength of the vortex ring and the helicopter will settle even faster.

To recover from this situation, the pilot needs to remove the helicopter from the column of air. This can be done in any direction, however, it is best to accelerate forward and reduce the collective pitch slightly. This will normally result is a minimum altitude loss. In the early stages of development, a large application of power (if available) might be sufficient to overcome the upward moving air and initiate a recovery. Theoretically, entering autorotation would change the airflow and result in a recovery, although it would also produce a large loss of altitude.

The best plan is to understand what type of conditions can cause the vortex ring state and avoid them. The three basic conditions are: a vertical descent rate greater than 300 fpm (the actual descent rate required may be higher depending on density altitude and aircraft weight); the rotor system must be consuming 20 to 100 percent of available power; and airspeed less than about 15 knots.

Some of the maneuvers that are susceptible to encountering the vortex ring state are steep approaches (especially downwind) or hovering out of ground effect at high density altitude. To stay out of trouble, keep approach angles shallower than 30 degrees, and when performing steeper approaches keep your rate of descent no more than 300 fpm and don’t let airspeed get too slow.

Many times you will hear helicopter pilots refer to hovering in ground effect as resting on a cushion of air. Technically speaking, what they are referring to is the extra performance that hovering in-ground-effect (HIGE) provides versus hovering out-of-ground-effect (HOGE).

Ground effect is defined as a condition of improved performance that results from operating near a firm surface. A helicopter is normally considered to be in ground effect when it is hovering less than one-half of its rotor diameter from the ground. However, the amount of benefit varies as a function of height. A lower hover will generate more efficiency and as the helicopter climbs the advantage decreases reaching zero about one and one-quarter times the rotor diameter.

A helicopter requires less power to hover in ground effect for two reasons. The main reason is the reduced velocity of the induced airflow caused by the ground. (Induced flow is air flowing down through the rotor system and is also called downwash.) This reduced velocity results in less induced drag and a more vertical lift vector. As such, the lift needed to sustain a hover can now be generated with a lower angle of attack in rotor blades, which requires less power.

The second reason has to do with vortices generated at the rotor tips. The close proximity of the ground forces more air outward and restricts vortex generation. This reduces drag and increases the efficiency of the outer portion of the rotors.

The maximum benefit is achieved from hovering over a hard surface such as concrete. When a helicopter hovers over an area such as tall grass or water, energy is absorbed by displacing the surface, allowing the induced flow to increase, thus reducing the lift vector. This will require the pilot to add power to maintain that hover height.

When a helicopter is in a high hover, or out-of-ground-effect, it requires a lot more power because there is no obstruction to slow the induced flow or force it outward. This results in a more vertical downwash and also allows the formation of stronger rotor tip vortices, reducing efficiency.

Helicopter pilots need to consider this when making very steep approaches as it has caused accidents. Typically, what happens is while a pilot is attempting to land, they allow their airspeed to get too slow and their approach too steep. They then realize they do not have enough power to slow the descent rate. In this case, the helicopter begins settling from a lack of available power. This is not the same as an aerodynamic condition called “settling with power,” which involves the generation of a vortex ring state (subject of future blog).

The helicopter’s Pilot Operating Handbook (POH) has both In-Ground-Effect (IGE) and Out-of-Ground-Effect (OGE) hover charts. This allows the pilot to take the density altitude and gross weight into account to predict hover performance. It is very important for pilots performing some missions such as ENG (Electronic News Gathering) or external lift operations to know if their helicopter can hover out of ground effect. Safe helicopter operations depend on good performance planning.

One important method for determining a helicopter’s health is vibration analysis. All helicopters have an inherent vibration. The type and intensity varies as a function of rotor design and isolation systems. Understanding basic vibration levels and being alert to changes is an important safety tool for helicopter pilots.

This is exemplified by the crash of a Bell 212 helicopter equipped with a cockpit voice recorder. About 18 minutes before the accident, the passenger (who was also a mechanic) stated to the pilot, “Boy, those catfish are going crazy down there, aren’t they?”

“Yep,” the pilot responded, “must have been the vibrations from the helicopter.”

About one minute 30 seconds before the accident, the pilot asked the passenger, “Has this vertical just gotten in here or has it been here for a while?”

“We haven’t had any verticals at all,” the passenger replied.

“We do now,” the pilot said.

“Yeah, well it started right after we left back there,” the passenger said. About 20 seconds later, the passenger stated that another person had tracked the helicopter’s blades before they left and that he was commenting on how smooth it was. Forty seconds after that, the pilot said, “This stuff is getting worse.”

The recording then ended.

According to the National Transportation Safety Board, the helicopter was allowed to continue in service with a loose pitch-change horn, which led to separation of the pitch-change horn from the blade grip and the in-flight breakup of the helicopter after the main rotor struck the tail boom. Contributing to the accident, the safety board said, was the pilot’s failure to respond to increased vibration in the main rotor system and land immediately.

Many components on a helicopter can fail and still allow the pilot to make a safe landing. The main rotor system is not one of them. Thus, any abnormal low-frequency vibration felt in the airframe or through the flight controls should be treated with extreme caution and investigated on the ground until the source is found and corrected.

Another telltale sign of a potential problem is difficulty with tracking and balancing the rotor system. Two accidents involving Robinson R22 helicopters, one in Israel and one in Australia, should never had happened. In both cases, investigations revealed that corrosion from water penetration initiated a fatigue crack in the main rotor blades. Both helicopters experienced an increase in main rotor vibration prior to final blade failure. In both aircraft, the vibrations were corrected with track and balance, only to reappear a short time later.

 It’s normal for a helicopter to require periodic tracking and balancing as paint and bearings wear over time. However, when a vibration reappears or abruptly changes, helicopter pilots need to take notice.

 Fortunately, failure of a critical component in the main rotor system is rare–it would be like a wing spar failure in an airplane. The good news is that a lot of times the helicopter will try to tell you. Many sharp pilots and maintainers have been alert to this and corrected an issue before anything happened. I think vibration awareness and analysis should be an important part of the helicopter private pilot curriculum. If everyone understood the importance of vibrations, we could virtually eliminate these kinds of accidents. 

In a recent blog (“Standing on a Basketball”) when discussing hovering, I stated that a helicopter is dynamically unstable. A reader commented, “Helicopters have neutral dynamic stability. They are not unstable.” This made me think that perhaps I should dig a little deeper into the subject. Sometimes a mathematical model or an engineer’s definition can be perceived differently when used in a practical application.

I’ll start with a basic definition of stability. An object is unstable if, displaced from its position, it continues to oscillate with increasing amplitude. It would be considered stable if it oscillated with decreasing amplitude, eventually returning to its original position. In the case of neutral stability, it does neither of these. That is, the amplitude does not increase nor does it decrease.

To get an engineering perspective on helicopter stability, I reviewed what Ray Prouty has written. Prouty has contributed to the helicopter industry for more than 50 years. He has done work ranging from preliminary design to performance and flight testing. He has also been honored for his contributions to the industry by the prestigious American Helicopter Society (a group that emphasizes engineering excellence in rotorcraft design www.vtol.org ), which named him an Honorary Fellow in 1983. He has written three books on helicopter aerodynamics and his writing is some of the best in terms of taking a complex engineering concept and explaining it in easy to understand language.

Chapter 8 of his book titled Helicopter Aerodynamics addresses dynamic stability. Here he provides a great explanation of what happens when a hovering helicopter is displaced by a gust of wind. He states, “A typical helicopter will go back and forth across its starting point with an ever increasing swinging motion until the pilot (or someone else) stops it.” According to Prouty, the rate of growth from one cycle to the next is a measure of the degree of instability. Finally he concludes that a hovering helicopter is unstable. Based on my experience teaching students to hover, I agree.

However, in a quest to produce a more stable helicopter, engineers designing early rotor systems developed devices that acted like gyros. While this made hovering much more stable, it reduced controllability. It was later determined that with practice a pilot could learn to hover without these stability enhancing devices. Today, systems like electronic forced trim and Stability Augmentation Systems (SAS) provide increased stability without sacrificing controllability. Helicopters equipped with these systems would behave more dynamically stable or neutral in a hover.

More information about SAS and other topics is available in Prouty’s books and I would recommend them to anyone who is interested in learning more about helicopter aerodynamics. He is a long time columnist for Rotor & Wing magazine and still writes for them occasionally. For more information on how to obtain his books or read his past columns, visit their website at www.aviationtoday.com/rw/

When I tell people that I fly helicopters the comment that I hear a lot is, “Isn’t that dangerous? If something happens to the engine you can’t glide like an airplane.” Well, I explain that is not true, helicopters do glide, it’s called autorotation. Without going into too much detail about the aerodynamics, I describe how it works with the concept of stored energy.

For discussion purposes, a helicopter on the ramp switched Off contains zero energy. However, when a pilot starts the engine, the fuel is converted to energy that is used to start spinning the rotor system. The rotor rpm is brought up to 100 percent; the pilot then lifts off and begins accelerating and climbing. Once established at cruise altitude and airspeed, the helicopter has two kinds of stored energy—Potential energy (energy because of position) in altitude, and kinetic energy (energy do to motion) in airspeed and rotor rpm. Essentially, this is money in the bank to be used in an emergency. It is the successful manipulation of this energy that will bring the helicopter and its occupants to a safe landing during a loss of power.

When a helicopter’s engine stops in flight, a freewheeling unit disconnects the engine from the rotor system to prevent the engine drag from slowing the rotor rpm. In addition, the pilot must immediately lower the collective pitch allowing the helicopter to start descending and forcing the airflow up through the rotor system. Basically, the helicopter begins consuming altitude energy to maintain rotor rpm. This is a very important step because waiting too long to lower the collective will allow drag to slow the rotor system and stall the blades. If this happens we destroy the helicopter’s ability to manipulate energy and it will simply fall out of the sky with fatal results.

Once established in autorotation the descent rate is normally 1,200 to 1,500 feet per minute and the pilot should maintain about 60 knots and maneuver the helicopter to the best landing area available. Approaching 50 to 75 feet agl, the pilot begins to rapidly decrease airspeed with a flare. Airspeed energy is used to arrest the descent rate. If timed correctly, the helicopter should momentarily end up about five feet above the ground with little to no airspeed. With all the altitude and airspeed energy gone, the only energy left is in the rotor system. The helicopter will start descending and the pilot should then raise the collective pitch control and use the rotor rpm energy for a gentle touchdown. As the rotor system slows to a stop the helicopter returns to a state of zero energy.

Of course all this assumes ideal conditions. We all know that in the real world it doesn’t always work that way. There are certain combinations of airspeed and altitude that simply do not have enough stored energy to make a safe landing. For example, hovering at 150 feet the pilot must rely mainly on rotor rpm to cushion the landing. The vast majority of helicopters do not have enough energy stored in the rotor system to completely stop the descent rate. Most likely the landing will damage the helicopter and injure its occupants. If a pilot is hovering higher, say 500 feet, there is enough altitude energy to trade for airspeed and complete a successful autorotation.

This explains why helicopter pilots prefer to take off by moving forward to gain airspeed first, instead of going straight up.