Archive for November, 2013

Main rotor rotation

Sunday, November 24th, 2013

In basic terms, a helicopter’s main rotor system is what provides lift and thrust. The rotational speed (rpm) is held constant and the pitch angle of the blades is varied to change the amount of lift and thrust. Engineers look at many different variables when designing a rotor system and one is the direction the rotor spins, clockwise or counter clockwise (when viewed from above). Unlike the tail rotor, from an aerodynamic efficiency standpoint there is no difference between the two directions. As it turns out, in most cases the direction of rotation can be associated with the country of origin. For example, as a general rule, helicopters manufactured in the United States (and some other countries like Germany) turn counter clockwise, while French and Russian designed helicopters spin clockwise.

However, from a piloting viewpoint there are some differences. The biggest one comes from Newton’s third law that states for every action, there is an equal and opposite reaction. As such, the torque applied to the rotor system causes the helicopter’s fuselage to spin the opposite direction. This is the primary reason for the tail rotor, or more accurately called the anti-torque rotor. It applies a force opposing the main rotor torque to stop the fuselage from spinning. The more power the pilot demands, the higher the torque and the more thrust the anti-torque rotor must produce to control yaw. In the cockpit, it is the pedals that control the anti-torque rotor’s pitch and therefore its thrust. What this means to the pilot is in a clockwise turning main rotor, right pedal must be added as power is increased and left pedal when power is reduced. For a counter clockwise spinning rotor system it is just the opposite.

Some of the other differences that are not as noticeable to the pilot are translating tendency and dissymmetry of lift. Translating tendency is the tendency of the helicopter to drift in the direction of tail rotor thrust. A clockwise turning rotor will cause the helicopter to drift to the left. Dissymmetry of lift refers to a difference in lift across the rotor system as one blade advances into the wind (headwind) and the other side retreats (tailwind). Again, depending on which way the rotor spins the advancing side will be on the right or left side of the rotor disc.

Electric flight

Monday, November 11th, 2013

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.

 

Bleed air

Friday, November 1st, 2013

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.