AOPA Hover Power

The X3 is a hybrid aircraft that offers the speed of a turboprop-powered airplane and the full hover flight capabilities of a helicopter. It’s equipped with two Rolls-Royce Turbomeca RTM322 turboshaft engines, producing 2,309 hp each, powering a standard EC155 five-bladed main rotor system and two propellers on short-span fixed wings. The X3 first flew September 6^th, 2010 and then in May of 2011 achieved a speed of 232 knots while using only 80 percent of prop shaft torque limit. The X3 is a proof of concept aircraft intended to demonstrate advanced technologies that could be used on high-speed helicopters in the next ten years.

The X3’s rotor speed is slowed down to reduce the Mach number of the advancing blade tip. To prevent retreating blade stall at high airspeeds the load on the rotor is reduced while the short span wings provide up to 40% lift instead. Also, the X3 does not have a tail rotor. So to counter the torque of the main rotor at low speed prop differential thrust is used. At high speed, remaining anti-torque function is realized by vertical fin flaps.

In May of 2012 the American Helicopter Society awarded Eurocopter’s X3 development team the Howard Hughes award for an outstanding improvement in fundamental helicopter technology brought to fruition during the preceding year.

A helicopter’s tail rotor is necessary to counteract the torque of the main rotor. Without it, the fuselage would spin the opposite direction of the main rotor (Newton’s third law). However, it also creates issues from consuming power that could be used for lift to safety for ground personnel. Over the years engineers have developed different designs to address some of these concerns. One idea that was first used in the late 1960s was a ducted fan.

A conventional tail rotor typically has two or four blades, while a ducted fan design can have eight to thirteen blades. The blades are also much smaller, spin at higher speeds and are mounted within a shroud that forms part of the vertical tail fin of the helicopter. Called a fantail (or sometimes a fan-in-fin) the housing and vertical fin is integrated into the tail boom. Another term is Fenestron and is trademarked by French helicopter manufacture Eurocopter.

Some of the main advantages of a ducted fan design include good protection against ground obstacles and foreign object damage, increased safety for ground personnel working around the tail boom, and increased aerodynamic efficiency. Also, a ducted fan reduces noise and vibration levels. However, the system is more complex than a traditional tail rotor adding weight and cost. Moreover, a ducted fan needs to have sufficient width to be efficient which adds drag.  A large cambered vertical fin helps efficiency in forward flight, but can make crosswind hovering more challenging.  

Eurocopter’s Fenestron has been constantly evolving over the last 35 years and is currently used on the EC120, EC130, EC135, AS365 and EC155 helicopters. The Fenestron has features like stators and tuning weights to reduce the power requirement and pitch control loads. Also used in the design is an even number of unevenly spaced blades designed to reduce noise levels. Although currently Eurocopter is the predominate user of ducted fans in their tail rotor designs other manufactures have also built helicopters with this design. For example, the Boeing/Sikorsky RAH-66 Comanche, the Russian Kamov Ka-60, and the Japanese military helicopter, the Kawasaki OH-1 Ninja.

EC135 Fenestron

Eurocopter's Fenestron

On March 16, 2003, a Bell 430 helicopter landed at an accident site at night. After the medical crew exited the helicopter the pilot decided to reposition the aircraft to face west for departure. During this repositioning the tail rotor hit a roadway sign and the aircraft became airborne to around five or six feet. The pilot lowered the collective and rolled the throttles to idle to stop the aircraft rotation. The helicopter touched down on the left rear skid first and came to rest 180 degrees from its initial heading. A post-accident examination revealed the tail rotor and gearbox had departed the helicopter.

Darkness certainly makes objects harder to see. However, two years prior to this accident, during daylight conditions, a Bell 222UT was damaged when its tail rotor impacted 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 with loading the patient. 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.

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. 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.

On May 17, 1999, a Bell OH-58A, operated by a public service agency on a photo flight, was destroyed on impact with the terrain and the private pilot and passenger sustained fatal injuries.

A witness reported that he saw a helicopter flying southbound at an altitude of approximately 350-400 ft. He saw what was possibly a large bird hit the rear rotor of the helicopter, after which two objects approximately the size of grapefruits fell to the ground. He said that the objects were falling slowly as though they were light, not fast like something heavy. The helicopter made three to four rotations during its descent.

The NTSB examined the tail assembly and found 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 determined the probable cause of this accident was the tail rotor’s impact with the blue warm-up jacket and the subsequent overload of the tail rotor drive shaft. A contributing factor was the absence of the helicopter’s doors.

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 hit the tail rotor. The pilot and passenger were killed.

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 made recovery impossible. The importance of protecting the tail rotor cannot be emphasized enough.

Cessna is well known for building a complete line of airplanes, from two-seat trainers to business jets. However, in the late 50s and early 60s the company also built a helicopter with a two-bladed main rotor and a reciprocating engine. The official model was the CH-1; however, it was also called the Cessna Skyhook.

Designed with a four place cabin the CH-1 was first certified in June, 1955 as a two-seat helicopter. After solving some longitudinal stability problems, Cessna sought and received four-seat certification in February, 1956. The helicopter had a supercharged Continental FSO-470 engine mounted in the nose and a tail boom that looked like the small airplanes they were building at the time. The forward engine design made for easy maintenance, but had other difficulties like where to route the exhaust. Initial designs ran the exhaust under the cabin, however high noise levels created a problem that Cessna was never able to completely fix.

The CH-1 had a fast never-exceed-speed of 122 mph, a maximum gross weight of 3,000 pounds and was considered a good performing helicopter. In fact, just three months after certification the CH-1 landed, hovered and took off from Pike’s Peak, Colorado, at an elevation of over 14,000 feet.  The flight was done as a demonstration for the US Army who eventually ordered 10 upgraded CH-1s for evaluations. The upgraded model used a 270 hp Continental FSO-526 engine and was designated the CH-1B by Cessna. A CH-1C model followed with a gross weight increase to 3,100 pounds and the US Government bought 15 for its Military Assistance Program. Cessna competed for additional military helicopter contracts, but ultimately lost.

In October, 1960 Cessna announced that the CH-1C would be put into production for civilian sales at a price of $79,960 with initial deliveries beginning in July 1961. Just three months before deliveries were to begin, a suspected tail rotor malfunction caused a crash in Texas killing Cessna’s marketing pilot. There were rumors of additional crashes including one in the Gulf of Mexico that resulted in four fatalities.

In January, 1962 Cessna announced they were terminating the helicopter program entirely citing poor sales and lost military opportunities. Cessna bought back all existing civil CH-1s in the field. It is estimated that a total of about 50 helicopters were built. Finally, 1989 Cessna cancelled the CH-1 FAA Type Certificate.

One of the more unique helicopter designs was built in 1940 by Austrian, Bruno Nagler. In an effort to reduce weight, he built the first helicopter with a single rotor blade. Named the NR 55 it was powered by a 40 hp engine that was mounted opposite the rotor blade and acted as a counter weight. Surrounded by an aerodynamic housing, it was located about four feet from the rotor hub.  A drive shaft from the engine passed through the hub and powered two small counter-rotating propellers (mounted on the leading and trailing edges) and located just over midway along the 18 foot blade. The single seat helicopter weighed 418 pounds and had a tripod style landing gear. Nagler was able to demonstrate hovering flight (inside with no wind) with a payload as high as 243 pounds.

However, the design had several problems. Gyroscopic precession from the propellers mounted on the rotor blade interfered with the blade’s ability to flap (a rotor system function needed for forward flight). Also, the rotor could spin up to 135 rpm producing large centrifugal forces acting on the engine causing ignition problems and fuel flow issues. Finally, high vibration levels proved difficult to reduce.

Nagler eventually retired the design in favor of a smaller design called the NR 54 V1. This version weighed 176 pounds, had a 13 foot rotor blade and a 24 hp engine. Plagued with many of the same problems, Nagler never got the NR 54 V1 to work and abandoned the single rotor blade concept in favor of a traditional two bladed design. Although, he kept the rotor blade mounted engine design because he felt it offered better weight and anti-torque advantages. Known as the NR 54 V2, it had a small 8 hp engine on each blade. However, it never flew due to centrifugal force issues with the engines.

After World War II, the British took the NR 54 V2 prototype into custody and it is now on display at the National Air and Space Museum. The original NR 55 was put in storage at the Nazi glider club in Vienna until a bombing raid in 1944 destroyed it.

On April 10, 2003 in Auburn, California, a Hughes 269B helicopter was destroyed when it collided with terrain while on an instructional flight. Prior to the accident, a witness on a farm about 3 miles away heard and observed the helicopter performing maneuvers consistent with multiple practice autorotations to a power recovery. The helicopter then flew off in the direction of the airport. Witnesses near the accident site observed the helicopter in level controlled flight about 500 feet above ground level flying in the direction of the airport. They observed the helicopter’s nose drop and then it dove toward the ground and disappeared behind a tree line. Ground scars and the wreckage exhibited evidence consistent with the helicopter colliding with the ground at a high rate of descent in a level upright attitude and rolling on its right side.

The National Transportation Safety Board determined the probable cause of the accident as the misjudged flare maneuver by an unknown crewmember during a likely practice autorotation that resulted in an in-flight collision with terrain.

Ironically, more accidents happen each year from practice autorotations than from actual engine failures. Surprise throttle cuts are especially critical because they can startle students and cause them to make sudden incorrect control movements. Inadvertently raising collective, pressing the wrong pedal or lowering the nose can drop the rotor rpm and risk stalling the rotor system. A student who simultaneously performs two or more of these movements could quickly stall the rotor system.

Before introducing forced landings, it is important that the CFI and student establish a strong understanding of what is expected and what can happen. Then, the CFI should introduce simulated power failures slowly by telling the student in advance of rolling off the throttle. At first, this should be practiced at very low power settings to allow extra reaction time. Only after the student’s reactions are correct and predictable should the difficulty level be increased. Even then, the CFI should always plan to initiate the autorotation and completely guard all the flight controls.

As the following accident demonstrates, guarding the flight controls is even more important for pilot examiners as they are probably not very familiar with the applicant’s background and habits. According to the NTSB, on June 28, 2003 a R22 helicopter impacted the ground and rolled over during a practice autorotation during a private pilot check ride. After completing a series of maneuvers, an autorotation was initiated. According to the designated pilot examiner, the entry and flare were uneventful, and as the pilot applied power to recover the engine and rotor rpm needles were in the green. The pilot then began to cushion the descent with collective and the low rotor rpm horn activated. The check pilot expected the pilot to lower the collective slightly and roll on additional throttle; instead, the pilot lowered the collective almost all the way down and rolled some throttle off. At this point the helicopter was very close to the ground. The check pilot told the pilot “I have the controls.” The helicopter entered a right turn, then the check pilot felt the right pedal move against his foot, and the helicopter settled to the ground at an angle. After bouncing once, it touched down on the left skid and rolled over.

Neither the pilot or the examiner were injured and after the accident, the DPE asked the pilot if he had been rolling the throttle off as he was pulling the collective up during the cushion portion of the autorotation. The pilot reported that in the past he had been over-speeding the engine, so he would slightly reduce the throttle to compensate. The density altitude at the time of the accident was 8,393 ft., leaving little margin for errors.

According to the NTSB, on the morning of August 7, 1998, after flying a Hughes 269B helicopter about 30 minutes herding cattle, the pilot located three cattle that were in a gated adjoining pasture. He landed the helicopter in a large mesquite flat near the gate so his passenger could get out and open it. After positioning the helicopter, he attempted a confined area takeoff. He stated that he had to lift straight up to clear trees. Upon reaching a hover just above the treetops, the pilot felt power was bleeding off so he lowered the nose trying to get airspeed. Unable to reach effective translational lift he turned toward a narrow clearing using right pedal and reduced collective to make a run-on landing. Upon ground contact, the right skid dug into the rain soaked ground, and the helicopter rolled onto its side. The commercial pilot and passenger were not injured.

After the accident, the pilot reported to an FAA inspector that it had been raining for a day and a half prior to the accident and that the weather was hot and muggy. He estimated the temperature to be about 95 degrees with high humidity and no wind. He also stated that he did not believe he had any type of mechanical failure and that the engine seemed to be performing normally. He felt that the density altitude, gross weight and out-of-ground effect operation all contributed to the accident.

Helicopter performance is a function of the density of the surrounding air. Density altitude is the reference standard used to measure performance and is determined by correcting pressure altitude for temperature. What is normally not factored into performance charts is the amount of water vapor present. Known as relative humidity, it is the amount of water vapor present (expressed as a percentage) versus the amount of water vapor the air can hold for a given temperature. Water is comprised of hydrogen and oxygen, which is less dense than the oxygen and nitrogen that make up dry air. As the humidity rises, the water vapor displaces the air molecules and lowers the density. Cooler air cannot hold a significant volume of water vapor, however hot air can hold a large amount, so as temperature and humidity rise aircraft performance will decrease.

Charts in the flight manual can be used to predict aircraft performance for a given density altitude. Since they are typically for dry air conditions, when temperature and humidity are high it becomes important to reduce expected performance levels. It is not just airfoils that are affected by humidity, but engine performance as well. A combustion engine can lose as much as 12 percent of its power on hot and humid days versus around 3 percent for a turbine.

At Baltimore-Washington International Airport a helicopter approached a construction site on the airport property with a quartering left tailwind. The helicopter turned right, and slowed to a stationary hover at about 250 ft with a direct tailwind. Once in a hover, it made a right rapid 180 degree pedal turn. Stopped momentarily and then initiated another rapid pedal turn to the right. The helicopter turned at a faster rate than the initial turn and continued into a spinning vertical descent and collided with Alpha taxiway abeam Runway 15 Right. The FAA’s examination of the helicopter found no mechanical anomalies.

The NTSB determined the probable cause was the pilot’s improper decision to maneuver in an environment conducive to a loss of tail rotor effectiveness and his inadequate recovery from the resulting unanticipated right yaw.

According to FAA Advisory Circular AC90-95, any maneuver which requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur. It also advises of greater susceptibility for loss of tail-rotor effectiveness in right turns and states the phenomena may occur in varying degrees in all single main-rotor helicopters at airspeeds less than 30 kt.

Bell’s Operations Safety Notice OSN 206-83-10, regarding loss of tail-rotor effectiveness in the 206B and similar airframes describes the phenomenon as an unanticipated right yaw. It contains the following warnings when maneuvering between a hover and 30 mph:

“Be aware that a tail wind will reduce relative wind speed if a downwind translation occurs. If loss of translational lift occurs, it can result in a high power demand and an additional anti-torque requirement. Be alert during hover (especially OGE) and high-power-demand situations. Be alert during hover in winds of about 8-12 kt (especially OGE), since there are no strong indications to the pilot [of] the possibility of a reduction of translational lift… Be aware that if a considerable amount of left pedal is being maintained, that a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw.”

The pilot at BWI was performing an aerial photography mission, the nature of which requires maneuvering at low altitudes and slow speeds. Add to that the distraction of trying to work with a photographer to line up the desired shot and the mission can become very demanding. Any pilot performing a similar mission needs to understand the aerodynamics and limitations of maneuvering at slow speeds.

Using a simulator, researchers at the University of Illinois conducted a study with 20 pilots who had no instrument training to see the survivability of an encounter with IMC conditions. All of them lost control, and the only variable was how long it took. The range was as short as 20 seconds to as long as 480 seconds with the average being 178 seconds. This was done in a fixed-wing simulator and I do not know of a formal study like this done with helicopters. My guess is a helicopter pilot with little or no instrument training would lose control in a much shorter time. Continued VFR flight into IMC conditions has caused many helicopter accidents.

For example, according to the National Transportation Safety Board a Bell Jetranger and an R44 helicopter were in route to Astoria, Washington when they encountered an overcast layer. A passenger in the Jet Ranger reported that the pilots of both helicopters were in continuous contact during the flight and as the weather conditions deteriorated, the pilot in the accident helicopter asked the pilot in the Jet Ranger what they should do. The witness reported the pilot in the Jet Ranger stated, “I’m going to go through it, stay right behind me.” The pilot in the accident helicopter agreed. The witness reported that when the Jet Ranger entered the fog, the accident helicopter was behind and above us. Approximately 30 seconds later; the pilot of the Jet Ranger stated, “Go back up… it’s too low. It’s much lower than we thought. Go back up right now.”

The witness stated that as the Jet Ranger ascended, the pilot attempted to contact the accident helicopter, however the attempts were unsuccessful. The Jet Ranger departed the area and was eventually able to land in Astoria. A search for the other helicopter was initiated and two orange life vests and miscellaneous debris were located floating in the water. The bodies of both pilots and passenger were recovered in the general area later that day. Numerous smaller pieces of helicopter wreckage were recovered from the water; however the majority of the wreckage was not located.

The pilot of the R44 held a commercial pilot certificate with rotorcraft-helicopter and instrument ratings. The pilot also held a flight instructor certificate with helicopter and instrument helicopter privileges. As an instrument instructor I am sure the pilot had practiced instrument flying with a vision restriction device. However, there is a significant difference between practicing with a hood and losing all visual references while under stress.

Most people remember the TV show MASH. Set during the Korean War, it featured patients being flown into a Mobile Army Surgical Hospital (MASH) with Bell 47 helicopters. In the beginning of the conflict the helicopters would occasionally pick up wounded soldiers when not busy with other missions. When doctors started noting that the survival rate increased when patients were transported by air, the Army took notice. The military started dedicating helicopters to medevac missions and when the Korean War ended over 22,000 wounded troops were transported by helicopters resulting in a lower mortality rate than previous wars. The Army further developed this concept with the Vietnam War by adding more sophistication, like in-flight medical care. Mortality rates continued falling and during the course of this conflict more than 800,000 wounded soldiers were transported.

Many returning military personnel understand the advantages of transporting trauma patients by helicopter first hand and starting applying these concepts to the civilian world. In 1970, the Maryland State Police Air Unit transported the first trauma patient by helicopter. Two years later the first hospital based helicopter program was launched at St. Anthony’s Hospital in Denver, Colorado.  By 1980, there were roughly 32 hospital based programs flying about 17,000 patients a year.

At this point the HEMS (Helicopter Emergency Medical Service) was a proven concept and the industry began to really organize. Medical interiors started getting more sophisticated and standards, guidelines and training programs for crew members were developed. By 2000, the number of programs had grown to 231 operating over 400 aircraft. Today, there are over 300 air medical programs operating about 900 helicopters.

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