AOPA Hover Power

Flying in conditions conducive to ice formation is problematic for virtually all helicopters. Moreover, many twin engine IFR helicopters are not certified for flight in known icing conditions. As such, helicopter pilots should understand the problems an encounter with icing can create for the rotor system.

Ice buildup on rotor blades will change the shape of the airfoil and consequentially, its ability to produce lift while increasing drag. The increased drag will slow the main rotor requiring the pilot to add power – which in some cases might not be available. Ice accumulation on the airframe can increase the helicopter’s gross weight requiring more power as well. Ice buildup is rarely, if ever, symmetrical causing an imbalance that produces vibrations in the rotor system. These vibrations can cause shedding of the ice and if all the ice comes off, vibration levels, lift and drag will return to normal. Asymmetrical shedding, however, can make the vibrations worse. Hopefully, the increased vibration will shed the remaining ice before any damage can occur. Ice accumulation is less on the outboard section of the rotor blade which is helpful because this area produces a larger amount of lift. However, an autorotation could be more difficult as the driving region is closer to the blade’s center.

Deicing refers to removing ice that has accumulated, while anti-icing is the prevention of ice formation. The few helicopters that having ice protection on the main rotor system use a de-icing system as the power required to anti-ice a main rotor system is extremely high. One of these is the Sikorsky S92 and it uses heater mats in the rotor blades to melt a thin layer of ice in contact with the blade surface causing the remaining ice to shed from the blade. According to Sikorsky, heat is applied to the mats to melt the ice in specific zones at precisely the right time for controlled shedding. Opposite main rotor blades are deiced simultaneously in order to prevent rotor imbalance and small sections of the rotor blades are deiced alternately to reducing the amount of electrical power required at any given time. The tail rotor ice protection system can be set to de-icing mode, which applies power in a scheduled manner or anti-icing mode in which heat is continuously applied to tail rotor heating mats.

Not every surface a helicopter lands on is perfectly level. So a slope landing is a maneuver that helicopter pilots need to know how to perform. The first step is bringing the helicopter to a stabilized hover into the wind and insure the ground is stable (for example, no loose gravel). Care must be taken when making pedal turns to avoid getting the tail rotor too close to ground. In the case of the ground sloping laterally, the pilot should slowly lower the collective until the upslope skid contacts the ground. At this point, apply lateral cyclic to firmly seat the skid into the slope. Maintain heading control with the pedals to prevent the skid from pivoting. Holding the upslope skid against the slope with cyclic, continue slowly lowering the other skid a little at a time with the collective. As the pilot continues lowering the collective, more lateral cyclic is required to hold the upslope skid firmly against the ground. If the pilot runs out of lateral cyclic prior to the downslope skid becoming firmly seated on the ground, then the slope is too steep and the landing should be aborted. When performing slope landings pilots need to be aware of the increased risk of dynamic roll over and, with a semi-rigid rotor system, mast bumping.

Once both skids are securely down, some instructors recommend centering the cyclic after the collective reaches flat pitch in order to have more clearance with the rotor system on the upslope side, others recommend keeping it displaced into the slope for the duration of the landing to prevent any sliding.

Lifting off a slope is essentially the reverse procedure. Raising the downslope skid with collective while moving the cyclic back neutral. Once the helicopter is level, lift off the slope. The pilot should keep in mind if a lot of weight is off loaded the CG might have changed enough to shift the cyclic neutral point, which could compromise a safe lift off.

Since helicopters land in areas that have not been previously approved, the pilot must make some last minute decisions regarding the landing site. One of these is the slope of the land where the helicopter will be touching down. Depending on the model helicopter the flight manual might have published limits.

The Bell 206 Jetranger is one helicopter that does not have slope limits listed in the limitations section of the flight manual. Bell’s approach is that slope landings are a function of available cyclic margin. In other words, if the pilot determines that the limit of cyclic control (close to or at the physical stop) will be reached before the helicopter is completely seated on the slope, then the slope is too steep and the landing should be aborted. (The proper technique to execute a slope landing is another discussion coming up.)

However, in the case of Eurocopter’s AS350 AStar the helicopter’s flight manual contains limitations on the amount of slope (in degrees) depending on the direction the pilot wishes to land. This is due to stress placed on the mast when landing on a lateral slope greater than 8 degrees.

The maximum slope when the ground is sloping down is 6 degrees. The shallower slope limitation in this direction is due to a 2 degree forward tilt that is built into the rotor mast. 

Also, the 2 degree tilt allows the maximum slope when the ground is sloping upwards to be 10 degrees 

Trying to determine the exact angle of a slope while hovering is difficult at best, however, with enough experience in a making off airport landings in a specific helicopter a pilot can become fairly good at judging the safety of landing on sloped terrain.

When a helicopter starts to move forward from a hover another aerodynamic condition (in addition to effective translational lift that was discussed previously) that occurs is transverse flow effect. This condition involves a differential airflow between the front and rear parts of the rotor system.

Moving forward from a hover, with no wind, the edge of the rotor system over the nose moves into clean air while the rear portion moves into air that has already been accelerated downward. This causes the angle-of-attack of the blades passing over the nose to increase, producing more lift. Because of gyroscopic precession, the maximum reaction occurs on the left side of the helicopter causing the rotor disc to tilt to the right. To continue moving straight the pilot must compensate with left cyclic.

Transverse flow effect can be recognized by an increased vibration of the helicopter at airspeeds around 12 to 15 knots and can be produced by forward flight or from the wind while in a hover. This vibration happens at an airspeed slightly lower than effective translational lift (ETL). The vibration happens close to the same airspeed as ETL because that’s when the greatest lift differential exists between the front and rear portions of the rotor system. As such, some pilots confuse the vibration felt by transverse flow effect with passing through ETL.

A hovering helicopter can require a lot of power. However, as it moves forward the horizontal flow of air across the rotor system improves the efficiency by changing the induced flow, and therefore the relative wind, which increases the blades’ angle of attack. This added efficiency is called translational lift. The forward motion also causes other aerodynamic issues with the rotor system, like dissymmetry of lift and transverse flow effect (a later discussion).

Wind can also create translational lift. Trying to hover at a constant altitude in gusty winds requires the pilot to constantly add or reduce power to compensate. Gusty winds can affect the tail rotor and power changes require pedal input as well. Holding a precise hover in these conditions is challenging.

With no wind, translational lift starts with any amount of airspeed and continues to develop as the helicopter’s speed increases. However, somewhere around 50 knots (it varies between different helicopters) induced drag increases to the point where it overtakes the gain in efficiency from translational lift.

Effective translational lift (commonly referred to as ETL) is a term used to describe the airspeed at which the entire rotor system realizes the benefit of the horizontal air flow. This happens when the helicopter’s rotor disc moves completely out of its own downwash and into undisturbed air. Depending on the helicopter this occurs between 12 and 18 knots of airspeed. The pilot will recognize effective translational lift on departure when the helicopter begins to have a noticeable tendency to climb and on approach when the helicopter starts to sink as the airspeed drops below ETL.

Some of the advantages of helicopters are the ability to fly very slow and land in small unapproved areas. As such, they perform many jobs that increase the risk of hitting a wire. Wire strikes have happened in just about all segments of the helicopter industry.

EMS pilots landing on roads and fields have to be extremely careful, especially at night or during the day when bright sunlight produces glare. Small power lines crossing roads and fields can be very difficult to see and several accidents have occurred when a departing helicopter contacts a wire. If a helicopter is heavily loaded and has little power available the pilot needs to gain airspeed to increase lift for climb out. This raises the risk of hitting an unseen wire. Just such an accident happened earlier this year in Tennessee (NTSB Identification: ERA11IA436). If power is available, a max performance or straight-up climb can mitigate the risk of an accident. I believe NTSB data shows the probability of an engine failure is smaller than the probability of hitting a wire or obstruction.

Even pilots who operate around power lines routinely must be alert. In July of 2011 a pilot flying an aerial application flight contacted a power line that ran perpendicular to the direction of the spray run. The pilot told the NTSB he was aware of the power line, but became distracted by horses that were located near the field. Moreover, during an aerial power line observation flight, the pilot hit the static wire for the power line he was patrolling. The pilot reported to the NTSB that he never lost control of the helicopter, but landed as soon as he could in a parking lot close to where the wire strike occurred. In this case, the helicopter was equipped with wire strike protection and a 12 inch piece of the 7-strand wire was found in the wire cutter located below the main rotor mast.

Robinson Helicopter has identified wire strikes as the number one cause of fatal accidents in helicopters. The company has published a safety notice (SN-16) that provides advice like crossing power lines at the support towers, being aware of the smaller grounding wires and flying at least 500 feet AGL whenever possible.

Although some helicopters have wheels, most have skid type landing gear. One of the biggest problems with skids is how to easily move the helicopter around on the ground. Attaching ground handling wheels to the skids is an option that works well for a small helicopter like the Robinson R22. However, for larger turbine helicopters the wheels are bigger and not very convenient to carry with the helicopter. Moreover, it normally requires more than one person to maneuver a heavy helicopter on wheels. As such, the helicopter dolly is a common option.

A helicopter dolly is a wooden, sometimes metal, platform with wheels that a helicopter can land on. Once the helicopter is on the dolly it can be towed with a tractor or tug. Landing on a dolly can be hazardous and there are some pilots that do not think it’s worth the risk. The danger comes from the difficulty seeing the skid gear while having to precisely set the helicopter on the platform. Some dollies do not have a lot of extra room so even a little drift at the last minute can cause one skid to miss the platform and the helicopter to roll over. Even if the pilot realizes this and attempts to abort there is the possibility that the skid will get caught on the edge, also causing a roll over. These types of accidents have all happened. There was even a case where the pilot did a nice dolly landing, rolled the engine to idle and then realized the dolly wheels were not chocked. The dolly started rolling and stopped when the helicopter’s nose hit a parked tug.

The pilots that support dolly landings say that with the proper mindset and approach, dolly landings are safe. For example, taking your time with the set down, not being nervous and getting instruction. Additionally, the dolly should be into the wind and large enough to accommodate the helicopter while allowing room for error.

On March 5, 2009, an Agusta A109E helicopter hit a bird on a medical evacuation flight while approaching Gainesville, Florida. The pilot received minor injuries, while the crew and trauma patient were not injured. It was a night VFR flight from an automobile accident site in Trenton, Florida, to a hospital helipad.

According to the pilot, the incident occurred when the helicopter was about 3 minutes from landing at the hospital’s rooftop helipad. The helicopter was descending at 145 knots through 800 feet, when the windshield exploded and the pilot was pelted with Plexiglas and other debris. The master caution warning light started flashing, but the pilot had difficulty reading the caution warning panel as the left lens to his eyeglasses was missing. The pilot was eventually able to determine that SAS number 1 had been disengaged, and after resetting the switches the master caution light extinguished. The pilot also noted that the instrument panel lights were off on the pilot’s side, so he reached up to the overhead panel and turned the lights back on. He then noticed that several circuit breakers and switches were broken off, and that several other switches had been moved aft, to the off position. The entire overhead panel was covered in blood. The pilot said that despite the wind noise, the helicopter was still operating normally and he then landed at its home base without any further problems.

Examination of the helicopter revealed that a 2 to 3 pound duck hit the helicopter and came to rest inside the cabin at the feet of one of the medical crewmembers. The pilot also stated that aside from electrical control switches, the power control levers were also located on the overhead panel and that if they had been hit and moved aft there would have been a reduction of engine power.

Just two months prior to this incident, that is exactly what happened to a Sikorsky S-76C++ that was en route to an offshore oil platform with two pilots and seven passengers. Data from the helicopter’s flight data recorder indicated that the helicopter was established in cruise flight at 850 feet and 135 knots. About 7 minutes after departure, the cockpit voice recorder recorded a loud bang  followed by sounds consistent with rushing wind, a power reduction on both engines and a decay of main rotor rpm. Due to the sudden power loss, the helicopter departed controlled flight and descended rapidly into marshy terrain – only one person survived.

Examination of the wreckage revealed that both the left and right sections of the cast acrylic windshield were shattered. Feathers and other bird remains were collected from the canopy and windshield at the initial point of impact and from other locations on the exterior of the helicopter. Laboratory analysis identified the remains as coming from a female red-tailed hawk; the females of that species have an average weight of 2.4 pounds. Based on main rotor speed decay information provided by Sikorsky, the accident flight crew had, at most, about 6 seconds to react to the decaying rotor speed condition. Had they quickly recognized the cause of the power reduction and reacted very rapidly, they would likely have had enough time to restore power to the engines by moving the overhead engine control levers back into position. However, the flight crewmembers were likely disoriented from the bird strike and the rush of air through the fractured windshield; thus, they did not have time to identify the cause of the power reduction and take action.

As can be seen with these two accidents, bird strikes are disorienting and can require quick action to recover. One of the reasons helicopter pilots wear helmets is to protect their face and vision in case of a bird penetrating the windscreen.

According to the NTSB a certificated flight instructor and student pilot were conducting a hover taxi in a Schweizer 269C helicopter from the hangar area to a fuel pump. The student was initially at the controls. The flight instructor took the controls from the student upon reaching the fuel pump, after the student stated he was uncomfortable landing on a raised platform in the confined area. The flight instructor landed the helicopter on the platform, where it then entered into ground resonance. The flight instructor rolled off the throttle immediately, but the ground resonance intensified, resulting in substantial damage to the helicopter.

Ground resonance happens in helicopters with fully-articulated rotor systems (rotor systems with three or more blades), or more specifically rotor systems with lead-lag hinges. These hinges permit the blades to independently move slightly forward and aft in the plane-of-rotation allowing them to speed up and slow down at different points as they spin around the mast. Known as a drag hinge they are necessary to relieve the stress that might otherwise damage the blades from the acceleration and deceleration of the rotor system. To prevent this back-and-forth movement from creating a serious vibration, hydraulic dampers are used to slow down the movement. Ground resonance cannot occur in a two bladed semi-rigid rotor system because the blades do not lead and lag.

Ground resonance typically occurs during a hard landing when the pilot sets the helicopter down on one corner of a skid or on one tire of a wheel equipped helicopter. The jolt transmits a shock through the fuselage to the main rotor system causing the blades to move out-of-phase with each other. In this condition the weight of the rotor system becomes concentrated on one side of the rotor disk causing the rotor system to become unbalanced. As long as the helicopter stays in contact with the ground the out-of-balance condition in the rotor system rapidly increases in frequency until the helicopter shakes itself apart.

If ground resonance starts, the best option is to lift the helicopter into the air allowing the blades to realign. If flight is not achievable then some improvement might be possible by reducing blade pitch and shutting down the engine. However, since the out-of-phase condition can cause major damage in a matter of seconds this approach is only sometimes successful. Helicopters with fully-articulated rotor systems can have shock-absorbing landing gear that will absorb the energy that feeds ground resonance. When ground resonance happens in these helicopters, it is usually because dampeners or shock absorbers have been improperly serviced.

In 1980 the American Helicopter Society offered a $250,000 prize for the first human powered helicopter. Known as the Sikorsky Prize, it was named in honor of the late helicopter pioneer Igor Sikorsky. To win the prize, the aircraft must reach a height of three meters and remain airborne for 60 seconds while staying in a 10 meter square.

 Although some attempts have been made, so far 32 years later the prize has not been won. The first helicopter to try to win the contest was the Da Vinci III in 1989, designed and built by students at Cal Poly San Luis Obispo in California. It flew for 7.1 seconds and reached a height of 8 inches (20 cm). The second was the Yuri I in 1994, designed and built by students at Nihon University in Japan. It flew for 19.46 seconds and reached an altitude of 20 cm.

On June 21, 2012 a team of engineering students from the University of Maryland’s A. James Clark School of Engineering have gotten the closest to wining the prize and achieved an unofficial world record of 50 seconds with their Gamera II human-powered helicopter, far surpassing any previous records. It will not become official until validated by the National Aeronautic Association. The pilot was Kyle Gluesenkamp, a Ph.D candidate at the school’s mechanical engineering department.

The first version (the Gamera I) stayed airborne for 11.4 seconds. The aircraft was re-engineered with improved airfoils and a new structural design that reduced weight by 39 percent. As with the previous version, the pilot produced power by pedaling. However, the Gamera II has hand cranks that can increase power output by as much as 20 percent. However, due to additional exertion required by the pilot, the output gains drop off after about 60 seconds. The Gamera II is about 105 feet from tip to tip and weighs about 75 pounds without a pilot.