Full down autos

August 17, 2013 by Tim McAdams

When practicing autorotations, the maneuver is initiated by reducing the engine to idle causing the freewheeling clutch to open, which then disconnects power to the rotor system. As the helicopter glides toward the ground, there are two ways to terminate the maneuver. One is to add the engine power back in and bring the helicopter to a hover, this is known as a power recovery autorotation. The other is to leave the engine at idle so the freewheeling clutch stays open, keeping the engine disconnected from the rotor system. Known as a full touch down autorotation, the pilot will increase collective pitch at the right time to create a momentary burst of lift to cushion the touch down.  In the helicopter industry, there are differing opinions on the value of practicing autorotations to the ground. 

The touch down requires precise timing because as the pilot adds collective pitch, rotor rpm begins to decay. If this is done too early, the rotor rpm can get too low causing controllability issues, excessive blade coning and loss of ability to cushion the touch down. By avoiding ground contact with a power recovery autorotation the risk of damaging the helicopter from a hard landing is reduced considerably. Some instructors and companies believe the risk of damaging a helicopter during touch down is too high and the benefit of actually landing does not justify the risk. The thought being that if a pilot performs the proper entry, maintains rotor rpm, maintains appropriate airspeed and then flares at the correct altitude the autorotation will be survivable. In reality, accidents from practice autorotations rarely cause serious injury or death, however, there have been many helicopters damaged from practicing autorotations. In fact, the US Army stopped practicing autorotations to the ground because they were damaging too many helicopters. 

I understand the risk vs. benefit analysis that leads to the decision to only perform power recovery autorotations. However, I think it is beneficial to practice autorotations in the most realistic environment that can be safely done, including full touch down to the ground. The risk can be minimized by using an experienced instructor with proper and extensive training in autorotations. Factory schools like Bell and Eurocopter have been doing full touchdown autorotations for many years with a good safety record.

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Power limits

August 1, 2013 by Tim McAdams

Helicopters powered by normally aspirated piston engines use manifold pressure as an indicator of power levels. Typically, pilots calculate limit manifold pressure for each day which is the maximum power setting allowed by the helicopter’s manufacturer. It is not necessarily the maximum rated horsepower limit for the engine. In many cases, the helicopter manufacturer de-rates the engine to reduce internal stress levels and extend TBOs. However, the pilot can exceed the limit manifold pressure (depending on factors like air density etc…) and still have available power.

In a gas turbine engine, the pilot must monitor three different indicators. Turbine outlet temperature (TOT) which refers to the temperature of the gas as it is exiting the engine, when the ambient air temperature is high this can be a limiting factor. Another is torque, which refers to the amount of torque the engine is applying to the transmission and is normally shown as a percentage. The third one is gas producer rpm, referred to as Ng or N1. When the air density is low, this section of the engine can reach its maximum operating rpm because it needs to spin faster to move the same amount of air.  A pilot of a turbine helicopter must monitor all three of these gauges and stop adding power when the first one reaches its limit.

Eurocopter uses something called a first limit indicator (FLI) to simplify the monitoring of all three parameters. One large gauge with a fixed yellow arc (indicating take-off power range) monitors all three parameters. So when the pilot adds power and the needle enters the yellow arc, then one of the three parameters has exceeded its maximum continuous power limit. To the right of the gauge, are the three values shown digitally (TOT, torque, Ng) and whichever one is the limiting value will be underlined in yellow.

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Coaxial rotors

July 19, 2013 by Tim McAdams

Traditional helicopter designs use a main rotor for lift and thrust, and a tail rotor to counter the torque applied to the fuselage. Another design, known as coaxial rotors, uses a pair of helicopter rotors mounted one above the other to produce both lift and thrust. Sikorsky’s high speed technology demonstrator the X2 uses this design as well as many Russian helicopters.

To neutralize the torque, the rotors spin in opposite directions creating equal and opposite torques that cancel each other and eliminate the need for a tail rotor. Yaw control is achieved by increasing the collective pitch of one rotor and decreasing the collective pitch on the other. Coaxial rotors also reduce the effects of dissymmetry of lift. Because they spin in opposite directions, both sides of the rotor disc have a retreating blade and an advancing blade.  

Another benefit of a coaxial design is a higher payload for the same engine power. A tail rotor consumes some of the available power produced by the engine. With a coaxial rotor design that extra power can be devoted to lift and thrust. Moreover, eliminating the tail rotor reduces noise, allows for a more compact design and increases safety on the ground.

The major disadvantage of the coaxial rotor design is the increased mechanical complexity of the rotor system. Two swash plates and their related linkages for both rotor systems need to be constructed on the same mast, which in itself is more complex because of the need to drive two rotors in opposite directions. This is offset somewhat by eliminating the intricacy of a tail rotor system. It would seem that the complexity of the rotor systems would increase the risk of a catastrophic failure. However, helicopters with this design have a good reliability record.

Sikorsky X2

Sikorsky X2

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Uncommanded yaw

July 7, 2013 by Tim McAdams

In cruise flight, should a helicopter experience an uncommanded yaw the cause in most cases would be an engine failure or tail rotor failure. The direction of the yaw indicates the type of failure.

In clockwise turning rotor system (like the Eurocopter AS350) the engine torque causes the fuselage wants to spin the opposite way (Newton’s third law – for every action, there is an equal and opposite reaction). In this case, from the pilot’s prospective, the nose of the helicopter wants to go to the left. The tail rotor applies a thrust that counters this reaction and pushes the nose back to the right. The pilot varies the amount of thrust with the pedals to control yaw. In powered flight, everything is in balance.

Should the engine fail, the engine torque that the tail rotor is opposing goes away. However, the tail rotor is still producing thrust that is trying to turn the nose right. In this case, the pilot will experience a right yaw and will use left pedal to neutralize it. With a tail rotor failure (loss of drive, producing a complete loss of thrust) the force opposing the engine torque ceases allowing the fuselage to spin the opposite of the rotor system. As such, the pilot will experience a yaw to the left. However, since the tail rotor is no longer effective applying opposite pedal does not work. Airflow over the vertical fin will prevent the helicopter from completely spinning and allow the pilot to fly the helicopter to a suitable area and perform an autorotation. Shutting the engine off eliminates the torque allowing the nose to come back to the right.

In a hover, it is a little different due to the reduced air flow over the vertical fin. Instead of just yawing, the fuselage will start spinning. In the case of an engine failure, opposite pedal will work. With a tail rotor failure, the pilot must immediately enter autorotation.

In a counter-clockwise turning rotor system (like the Bell 206) the theory is the same, but the nose moves in the opposite direction for each failure.

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Ground effect

June 30, 2013 by Tim McAdams

Hovering in ground effect results in a condition of improved performance that comes 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.

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.

hovering1

 

 

 

 

 

 

 

 

 

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Ground resonance

June 16, 2013 by Tim McAdams

A helicopter’s rotor system, engine(s), and other dynamic components generate vibrations in the airframe. These components will vibrate at a natural frequency which in turn causes other parts like the landing gear, tail boom, cabin etc… to vibrate as well. Each part’s frequency will vary according to its weight, stiffness, shape, etc…. As such, a helicopter contains a complex set of vibrations that add up to a resulting airframe vibration. Engineers attempt to reduce the overall vibration level by tuning the natural frequency of all the components. 

When a helicopter is in flight, the airframe’s natural frequency (the sum of all the components’ frequencies) will vibrate without interference. However, when ground contact is made with the landing gear it can interfere with the airframe’s ability to vibrate at its natural frequency. Ground resonance happens when ground contact alters the natural frequency of the main rotor system. This unbalanced condition triggers vibrations that are augmented with every blade revolution causing a reflected impulse that increases in amplitude very quickly. The only rotor systems susceptible to ground resonance have three or more blades. This is due to each blade’s ability to lead and lag (speeding up and slowing down) independently. If something causes the blades to depart from their symmetry, the rotor system’s center of gravity shift causes it to become out of balance allowing divergent oscillations to rapidly become strong enough to cause serious damage to the helicopter. In some cases, complete destruction  can occur with many componets coming loose and being thrown from the helicopter. 

Engineers design damping systems for the main rotor and landing gear to absorb the energy and prevent the oscillations from accelerating. Still, a sudden shock to the airframe, like a hard landing, can unbalance the main rotor system beyond the damping system’s ability to absorb the energy and ground resonance can start. Improperly serviced or malfunctioning dampeners are usually the cause.  Ground resonance happens very fast, however, if the pilot recognizes it in time and there is enough power and rotor RPM available to lift the helicopter off the ground the divergent oscillations will stop. This is the quickest way to stop it and hopefully will result in little or no damage. If the situation is such that there is not enough power to lift into a hover, then a full power reduction is the only option. However, this approach will take time for the vibrations to fade and significant damage can occur.

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Gemini ST

June 3, 2013 by Tim McAdams

The problem with very light twin-engine helicopters is payload and range. The extra weight of the second engine, combining gearbox, extra fuel tanks and related systems severely limits the airframe’s payload.  Also, that smaller payload capacity will need to include the weight of more fuel. As such, carrying enough fuel for an acceptable range normally leaves little weight capacity for passengers. Operators of small helicopters who wanted the redundancy of a twin have to make some big trade-offs. 

In the late 1980s, Doug Daigle, a 14,000-hour helicopter pilot and owner of Tridair Helicopters, came up with an idea that tried to solve this problem.  He took a popular single-engine helicopter, the Bell LongRanger (206L3), and removed the 650-shaft-horsepower Allison 250-C30 engine (the 500-shp C28 for the older L1s) and added a pair of 450-shp Allison 250- C20R engines to create a twin with good single-engine performance. However, what was different about his design was it was the first twin to be certified for normal operation on one engine in all areas of flight. Certified by the FAA in 1994, he named it the Gemini ST conversion. 

When operating both engines, the Gemini consumes fuel at 45 gallons per hour, compared to the LongRanger at 38 gph. Because both helicopters have the same fuel capacity (he did not add extra fuel tanks), endurance drops from 2.9 hours for the LongRanger to 2.5 hours for the Gemini. However, the Gemini’s C20R engine burns only 28 gph in single-engine mode, and this increases the endurance to 3.9 hours. Daigle believed long trips will normally have some extended cruise flight, where the pilot could choose to fly on one engine. The other engine can be started for critical maneuvers that require the redundancy of a second engine. 

The extra weight of the engine and related systems does increase the empty weight, so there is some trade-off with the Gemini. It has a smaller useful load of 1,610 pounds, compared to 2,175 pounds for the LongRanger. The Gemini can carry 740 pounds of fuel (the same as the LongRanger), which leaves a payload capacity of 870 pounds. 

Bell Helicopter then entered into an agreement with the company that supported Tridair’s conversion of existing LongRangers while Bell would build a new production model, the 206LT TwinRanger, under Tridair’s STC. Bell only delivered 13 airframes and the concept of shutting one engine down in flight never caught on. This model was replaced by the Bell 427 which was under development for the EMS market. The 427’s cabin proved too small and Bell canceled the program and introduced the larger, single-pilot IFR certified 429 in 2009.

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Fit for flight

May 19, 2013 by Tim McAdams

Every so often I come across an accident that really makes me stop and think. Many of these can be a learning experience and some are just hard to understand. 

According to the NTSB, on July 22, 2010 a Eurocopter AS 350 B2 helicopter impacted trees near Kingfisher, Oklahoma. The commercial pilot and one flight nurse were fatally injured and one paramedic flight nurse was seriously injured. 

A Global Positioning System (GPS) device recovered from the accident scene revealed the helicopter was cruising at approximately 130 knots and about 200 to 300 feet above ground level. Seconds before impact, the helicopter descended at 385 feet per minute, followed by a descent rate of 1,890 feet per minute two seconds later. The location and altitude of the helicopter, as recorded by the GPS corresponded to the location rotor impact marks with the trees. 

In an interview with the surviving paramedic flight nurse, he recalled that during the flight, the left side door had come unlatched and was slightly ajar. The paramedic informed the pilot that he was getting out of his seat to close the door and secure the handle. The pilot acknowledged the paramedic. After securing the handle, the paramedic stated that he had sat back down and begun to gather his seatbelt when a conversation began about another pilot flying on a coyote hunt. The paramedic reported that the pilot made a statement similar to “like this… (with some laughter)” and made a nose down control input. He reported that the pilot pulled up on the collective and the helicopter struck a tree. During the ground impact, the paramedic, who was not secured in his seat, was thrown through the windscreen; the paramedic crawled away from the wreckage and dialed 911 on his cell phone. 

The pilot, age 56, held a commercial pilot certificate for airplane single-engine land, instrument airplane, rotorcraft-helicopter, and instrument helicopter. He held a second class medical certificate issued February 8, 2010. On the pilot’s last application for a medical certificate he reported having accumulated 12,241 hours, with 119 hours logged with the preceding six months. Of note, the pilot reported that he was not currently using any medications. 

An autopsy was performed on the pilot and toxicology noted the following: 

  • 39.31 (ug/ml, ug/g) Acetaminophen detected in Urine
  • Chlorpheniramine detected in Blood
  • Chlorpheniramine detected in Urine
  • 0.198 (ug/ml, ug/g) Diazepam detected in Blood
  • 0.026 (ug/mL, ug/g) Dihydrocodeine detected in Blood
  • 1.026 (ug/mL, ug/g) Dihydrocodeine detected in Urine
  • 0.15 (ug/ml, ug/g) Hydrocodone detected in Blood
  • 4.112 (ug/ml, ug/g) Hydrocodone detected in Urine
  • 0.302 (ug/mL, ug/g) Hydromorphone detected in Urine
  • 0.322 (ug/ml, ug/g) Nordiazepam detected in Blood
  • 0.629 (ug/ml, ug/g) Nordiazepam detected in Urine
  • 0.011 (ug/ml, ug/g) Oxazepam detected in Blood
  • 2.169 (ug/ml, ug/g) Oxazepam detected in Urine
  • 1.569 (ug/ml, ug/g) Temazepam detected in Urine 

A review of the pilot’s medical history found that the pilot was being treated for several medical conditions and had been prescribed multiple medications since at least 2007. In April 23, 2007, the pilot reported to his personal physician that he had bronchitis, hypertension, and sleep apnea, and after his visit, he was prescribed the following medications: Nexium (for gastroesophageal reflux), Caduet (for hypertension), Flexeril (sedating muscle relaxant), Lortab (hydrocodone and acetaminophen; narcotic pain medication), Lunesta (for sleep disturbance), and Requip (for restless leg syndrome). The pilot continued to report to his personal physician that he experienced increased pain and was prescribed stronger pain medications, to include prescription narcotics and benzodiazepines. In addition, steroid joint injections were applied to his right knee and shoulder to treat persistent pain. The last documented visit, February 25, 2010, the pilot was prescribed the following: Caduet (for hypertension), omeprazole (for gastroesophageal reflux); Meloxicam (a non-steroidal anti-inflammatory); Lunesta (sleep aid); Norco (10/325 hydrocodone/acetaminophen combination two tablets three times a day); baclofen (a muscle relaxant, 10 mg three times a day) and Valium (diazepam, a benzodiazepine, 10 mg three times a day). In addition to his prescribed medications, chlorpheniramine, an over-the-counter sedating antihistamine medication was also detected in the toxicology. There was no evidence that the pilot’s sleep apnea had been treated prior to the accident. In addition, the pilot did not report any of his conditions and prescription medications to the FAA, to the certificate holder, or to the operator.

 

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Delta-3

May 6, 2013 by Tim McAdams

Dissymmetry of lift occurs when a rotor system is flown edge-wise through the air. With helicopters, many times these discussions center on the main rotor system. However, this aerodynamic condition also affects the tail rotor.

Just like the main rotor, a tail rotor will equalize lift by flapping. However, most tail rotor flapping takes advantage of the Delta-3 effect. Also known as pitch-flap coupling or K-Link (French term). This effect is achieved by having the pitch horn on a different plane than the flapping hinge, which mechanically changes the pitch angle of the blade as it flaps. The amount of the delta-3 offset is measured in degrees and determined by design engineers after considering many factors. This offset can be also be accomplished by using a Delta-3 hinge (setting the hinge at an angle to the chord of the blade). In either case, when the advancing blade (the blade that experiences a higher relative wind) starts to flap the offset lengthens the distance between the blade’s pitch horn and the pitch link’s attach point. This forces the pitch link to pull the blade’s pitch horn closer, thereby reducing its pitch angle. On the retreating side, the distance is shortened and the pitch link forces the pitch horn further away, increasing the blade’s pitch angle. This effect minimizes flapping in order to control dissymmetry of lift on the tail rotor.

This can be demonstrated by moving a tail rotor blade with a delta-3 hinge through its flapping range and observing the pitch angle changes as you manually flap the blade.

 

AS350 tail rotor

AS350 tail rotor

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Eurocopter’s AStar

April 25, 2013 by Tim McAdams

In the late 1970s, Aerospatiale introduced the AS350B as a replacement for the company’s Alouette II helicopter. Named Ecureuil (Squirrel) it used a Turbomeca Arriel 1B engine rated at 641 shp. For the U.S. market Aerospatiale gave it the name AStar and the model number AS350C. Both models had a maximum gross weight of 4,300 pounds, however, the C model used the Lycoming LTS 101-600A engine rated at 592 shaft horsepower (shp) for takeoff. Six months later the helicopter was upgraded to the D model with the installation of the Lycoming LTS 101-600A-2 engine, boosting its takeoff rating to 615 shp.

In 1987, Aerospatiale discontinued the D model and upgraded the B to a B1. This new version had a more powerful Arriel 1D (684 shp) engine and a higher gross weight of 4,850 pounds. The rotor system was upgraded with larger (more inertia) asymmetrical rotor blades and rotor rpm was increased slightly. This resulted in increased performance and better autorotation characteristics. The company also produced the BA model that had the larger blades, but retained the B model’s Arriel 1B engine. Three years later the B1 was replaced with the B2, using a more powerful Arriel 1D1 (712 shp) engine and gross weight jumped to 4,961 pounds. The B2’s cruise speed at MCP (maximum continuous power) is 133 knots. In 1992, Aerospatiale merged with MBB to form Eurocopter.

In the late 1990s Eurocopter introduced the B3, a high altitude version.  It was powered by an Arriel 2B engine equipped with a single channel DECU (Digital Engine Control Unit) with a mechanical backup system. Although the gross weight remained the same, take off power increased to 747 shp. This was followed by a variant using the 2B1 engine with a dual channel FADEC (Full Authority Digital Engine Control). This version had a dual hydraulic system available as an option which when combined with high skid gear allows a gross weight increase to 5,225 pounds. The latest version, B3e, was introduced in late 2011 and is outfitted with a dual channel FADEC equipped Arriel 2D engine. Take off power rating jumped to 847 shp boosting its MCP cruise speed up to 137 knots and adding extra lifting capability.

CDPH0591-086

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