Archive for July, 2013

Coaxial rotors

Friday, July 19th, 2013

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

Uncommanded yaw

Sunday, July 7th, 2013

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.