Electric flight

November 11, 2013 by Tim McAdams

In 2010, Sikorsky Aircraft introduced “Project Firefly,” an all electric helicopter technology demonstrator based on the S-300C airframe. The intent was to have it flying a year later and set a world record for the first all electric manned helicopter flight. Unfortunately, the company did not make the target date and last year the first all electric powered helicopter flight was achieved by pilot and designer Pascal Chretien in France.

Chretien took a different approach than Sikorsky, rather than use a heavy existing airframe he designed a new lightweight highly energy efficient aircraft. As an electrical engineer and commercial rated helicopter pilot he knew what areas to target. A tail rotor can consume 10 percent of available power, so to eliminate the tail rotor Chretien used a coaxial main rotor system. He employed a second generation asymmetrical rotor blade design which provided a 19% increase in lift over his initial blades. Powered by a lightweight (128 pounds) Lithium ion polymer pouch cell battery pack the two DC powered electric motors provided a total of 43 hp, enough power to lift the required 545 pounds. Each rotor system has its own motor and yaw is controlled by varying the electrical signal to each motor.

In July and August of 2011, the aircraft made 29 flights totaling 99.5 minutes with some flights lasting 6 minutes. Then on 12 August 2011, the world’s first un-tethered manned flight of a helicopter powered only by an electric motor took place at Venelles, France. Chretien hovered above the ground for 2 minutes 10 seconds entering the Guinness World Book of Records.

 

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Bleed air

November 1, 2013 by Tim McAdams

Turbine powered helicopters use bleed air for heating, demisting and other systems like sand filters. Bleed air is taken from the compressor section of the engine. For example, the Arriel 1 series engines use a two stage compressor section.

Arriel 1 compressor section

Arriel 1 compressor section

The first stage uses an axial compressor to increase the speed and pressure of the ambient air.

Capture2

The second stage uses a centrifugal compressor to further compress the air and raise the temperature. This is where bleed air is taken from the engine.

Capture3

Prior to entering the combustion chamber the air is extremely hot from compression alone. For cabin heating, the bleed air is mixed with outside air to cool it.

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Night vision

October 24, 2013 by Tim McAdams

On July 24, 2000 in Sumner, Georgia, an AS350B Astar collided with the ground in a heavily wooded area during a clear dark night. According to the NTSB, no pre-existing airframe or engine malfunctions were identified during the post-crash examination. They determined the probable cause as the pilot experiencing spatial disorientation, which resulted in a loss of control of the helicopter. A contributing factor was the dark night.

Threats, clearly visible during the day, are masked by the darkness. In fact, Controlled Flight into Terrain (CFIT) at night is a major problem for fixed and rotor wing operations. CFIT is defined as colliding with the earth or a man-made object under the command of a qualified flight crew with an airworthy aircraft.

Due to the unique low flying operation of helicopters CFIT at night during VMC has been troublesome, especially for the aeromedical industry. According to the Air Medical Physician Association, half of all EMS accidents happen at night.

For example, on November 14, 2001 the pilot of a BO 105 LS reported that while performing a normal take-off from a non-airport shortly after midnight on a dark moonless night, he lost visual references outside the helicopter, became disoriented, and collided with terrain. He further stated that no mechanical malfunction or failure had occurred during the accident flight. The NTSB determined the probable cause was the pilot’s failure to maintain adequate separation from terrain during the initial climb. Factors include spatial disorientation and a dark moonless night.

Obviously, the key to preventing these accidents is improving the pilot’s ability to see obstructions–the idea behind Night Vision Goggles (NVG). In the late 1990s, the idea of using NVGs in the civilian sector was starting to evolve. By early to mid 2000, EMS operators started using NVGs in their operations. Today, the vast majority of EMS operators and law enforcement units fly their helicopters under NVGs. With the proper training, use of NVGs increases the safety margin associated with flying helicopters at night.

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Full flight simulators

October 13, 2013 by Tim McAdams

Flight training in full flight simulators (FFS) has been the standard in large fixed-wing aircraft for years. In the past, there were only a couple of helicopter simulators. These were mainly large twin-engine IFR helicopters like the Sikorsky S76 and Bell 430. Recently, simulator manufacturers have been introducing more helicopter devices and seeking certification at higher FFS levels (levels B, C and D with full motion capabilities).

Many operators and airframe manufacturers have already acquired low level non-motion FTDs (Flight Training Devices) to supplement flight training in the aircraft. With computer power and visual systems getting better and cheaper, higher level simulators are becoming more popular. Even for light single-engine helicopters operators are starting to embrace simulator training. Especially for FAA Part 135 operators who can complete required annual check-rides in a FFS.

During the next 5 to 10 years, more helicopter training will be done in simulators. Hopefully, this will lead to more frequent and comprehensive training that will help reduce the accident rate, especially for EMS operators.

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Minimum autorotation rpm

September 28, 2013 by Tim McAdams

When a helicopter is in autorotation (that is, gliding without the benefit of engine power) rotor rpm must be maintained. This is done when entering the autorotation by lowering the collective control. If the rotor rpm approaches an upper limit, the collective is raised to add pitch. This increases drag and slows the rotor rpm. A low rotor rpm situation is just the opposite, lower the collective pitch to reduce the drag and allow the rotor rpm to speed up.

Rotor rpm in autorotation is a function of several factors like density altitude, gross weight and airspeed. An over speeding rotor is easy to manage since the collective control was lowered on entry there is plenty of movement upward to add drag. However, if a pilot enters autorotation and lowers the collective control all the way down and the rotor rpm is still too low this could be a problem. Typically, in this case, the main rotor pitch is set incorrectly. The helicopter’s maintenance manual has a procedure to adjust this by either lowering the collective control’s down stop or adjusting the main rotor blades’ pitch links. To accomplish this, a mechanic will note the helicopter’s weight and the density altitude and then reference a chart to get the correct rotor rpm. A flight test will then be performed at those conditions and the actual rotor rpm will be noted with the collective all the way down. If it is not at the correct rotor rpm stated in the chart, the mechanic will make an adjustment.

This is done to insure that in the worst case scenario (light helicopter, high density altitude) the pilot will be able to lower the collective control far enough to guarantee an acceptable rotor rpm in autorotation.

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Berkut VL

September 14, 2013 by Tim McAdams

Typically, coaxial rotor systems (one rotor system stacked on top of another that spin in opposite directions) are used on larger helicopters. The advantages are higher speed and more lifting power as a tail rotor is not needed. An aerospace start-up company in India (DASYS), a manufacturer of unmanned aerial vehicles, has designed a light two-seat helicopter with a coaxial rotor system.

Called the Berkut VL, the company plans to certify the helicopter in compliance with US FAR Part 27 standards. Currently there are two prototypes, one for testing and the other for demonstrations. These two airframes are equipped with a Russian ConverVAZ engine, but production models will have the option of a 150 hp Lycoming O-320 engine. Helicopters with American engines will get the designation Berkut VL M. The planned take-off weight is 1,830 lbs with a maximum speed of 108 mph and a range of 527 miles.

The helicopter will be produced at a plant in central Russia. Although no price has been released, the company has stated it will be affordable and plans are for it to compete with the Robinson R-22. As such, the company has announced a four-seat version will follow. First deliveries are scheduled for mid 2014.

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Cobra attack helicopter

September 2, 2013 by Tim McAdams

The first military helicopters were used for medical evacuation and supply missions. Although, their primary mission was utility, handheld weapons and some side-mounted guns were used for offensive tasks. At that time, the U.S. Army did not have a dedicated attack helicopter in its fleet. Bell Helicopter recognized the advantages of using a helicopter for offensive missions and began developing a new model designed specifically for these kinds of operations.

The first design was a modified Bell 47. It first flew in 1963 and used a two man crew arranged in tandem with the gunner in front and the pilot seated directly to the rear. The gunner operated a nose mounted machine gun with an assembly that resembled the pilot’s cyclic control. Since the gunner was also trained to fly, a small cyclic control was installed on the right side as a side arm controller. Yaw was controlled by twisting the grip on the side arm control. This model was called the Sioux Scout and was used as a test bed for the design of an advanced attack helicopter.

Bell took these concepts and applied them to the more powerful turbine-powered UH-1B, known as the Huey. The design work started in early 1965 and the prototype was flying a year and a half later. The newly designed attack helicopter carried the designation AH-1G and was named the Cobra or HueyCobra. The U.S. Army ordered 529 of these and by 1967 they were in action in the Vietnam conflict.

AH-1G Huey Cobra

AH-1G Huey Cobra

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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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