Archive for August, 2009

Loss of tail rotor thrust

Thursday, August 27th, 2009

In a conventional tail rotor system, a complete loss of tail rotor thrust can happen from an internal drive system failure or if an object contacts the tail rotor and damages the blades or gearbox.

A complete loss of thrust from a drive failure is the easier of the two emergencies for the pilot to handle. In flight (airspeed at least about 60 knots) the pilot will experience a yaw to the left or right (it depends of which direction the rotor turns) that is not correctable with pedal input. The airflow passing over the vertical fin will prevent the helicopter from spinning and in this situation the helicopter can most likely be flown to a suitable landing area. Landing without a tail rotor thrust requires an autorotation. When the throttle is closed and the engine stops applying torque, the need for tail rotor thrust goes away. It’s important to keep the helicopter into the wind to prevent sideways movement during touchdown. Collective pitch should be added carefully because friction in the transmission can have a tendency to turn the fuselage. If the helicopter starts sliding sideways it could easily roll over.

In a hover or with low airspeed a tail rotor drive failure requires quick action. The helicopter will immediately begin spinning and the pilot will need to close the throttle and perform a hovering autorotation. A failure low to the ground is normally recoverable; however, for pilots performing high hovers (utility helicopters doing lift work for example) it is much more dangerous. In cases where this has happened some pilots have survived some have not.

Loss of tail rotor thrust resulting from an object striking the tail rotor is very serious. Many times the damage causes such an imbalance that the tail rotor assembly and gearbox will break free from the tail boom. The loss of weight at that long of a moment arm will cause the CG to shift too far forward. In addition to issues resulting from the loss of tail rotor thrust, the helicopter will pitch down and the pilot most likely will not have sufficient aft cyclic movement to recover. When this happens in cruise flight or a high hover the results are normally severe aircraft damage with a high potential for serious or fatal injuries. When pilots in a low hover (EMS accident scenes for example) hit something with the tail rotor the damage to the helicopter can be severe as well, but the potential for human injury is low.

 

Thoughts on the Hudson River midair

Thursday, August 20th, 2009

I flew a corporate Bell 430 in and out of New York for 7 years and prior to that I worked for Liberty Helicopters flying tours. That was 14 years ago, so I didn’t know the pilot or anyone else involved in the recent accident in the Hudson River corridor. However, this accident brought back memories about the airspace congestion in New York.

When I was flying tours we were all concerned about the possibility of midair collisions, especially on nice days. The airspace is highly congested and the areas that are excluded from the Class B are small and extend from the surface of the rivers to only 1,100 feet. Many pilots considered the level of attention required in this airspace comparable to flying in combat. Although, I have no military experience I can only imagine the level of vigilance necessary when someone is trying to shoot you down. From my prospective, flying around New York safely demands a high level of alertness.

There is a sequence of reporting points up and down both the Hudson River and the East River. The pilots I worked with in New York were extremely good at stating their positions regularly. Occasionally, a pilot would fly up and down the rivers without ever talking on the radio. Technically, it’s not required as the airspace is uncontrolled, however, the self-announce frequency is published on the charts. I often wondered if the non-local pilots who did monitor the frequency actually knew where the reporting points were located as most referenced a local landmark or bridge.

Some of the news reporters commenting on this accident seemed shocked that there was no requirement to talk to ATC. I don’t think making the airspace over the rivers part of the Class B is a good idea. New York controllers are already very busy and if everyone approaching or departing a New York heliport needed a clearance it would overwhelm ATC.

When I flew the Bell 430 around New York it had a Skywatch traffic system. It was a big help in identifying aircraft close to us. Good visual scanning skills and this type of technology might be the answer to making this level of congested airspace safer.

Anti-torque

Monday, August 10th, 2009

Newton’s third law of motion says, for every action, there is an equal and opposite reaction. So, when a helicopter’s rotor system spins in one direction, the fuselage wants to spin in the opposite direction (since this is a rotational force it is called torque). To prevent this engineers put a small thrust-producing rotor on a moment arm (the tail boom) to create a rotational force (torque) that is equal, but opposite, to the force trying to spin the fuselage. Its technical name is an anti-torque rotor, however it is often referred to as a tail rotor.

A set of pedals in the cockpit change the pitch of the tail rotor to vary the amount of thrust produced. Although they control yaw, they function differently than rudder pedals in an airplane.

As long as the main rotor rpm stays constant, so will the tail rotor’s. In fact, if you turn the main rotor by hand the tail rotor will also turn. This is because a system of drive shafts and gearboxes directly connect it to the main rotor transmission. Depending on the helicopter’s design, the tail rotor will spin 3 to 6 times faster than the main rotor.

When viewed from above, most main rotor systems spin counterclockwise (CCW). Sometimes people refer to this as the American direction and clockwise (CW) as the European direction. This is not really accurate as some models built in Europe also turn counterclockwise. For example, Augusta (based in Italy) manufactures models that spin CCW and several Eurocopter models (EC135, EC145) do as well. However, the most popular helicopter with a CW turning rotor system is the Eurocopter Astar.

The rotor system’s rotational direction makes very little difference to gravity or air, but it does change things a little for the pilot. When a pilot increases power (raising the collective control) the torque applied to the fuselage increases. In a CCW turning rotor the pilot must add left pedal to increase the tail rotor’s pitch, and therefore thrust, to keep the nose straight. Likewise, decreasing power requires right pedal input. Right pedal reduces the pitch and thrust allowing excess engine torque to turn the fuselage. In a CW turning rotor just the opposite is true.

Pilots who routinely switch between airframes with different rotor directions, have to remember which one they are in as over time collective movement and the associated pedal movement become automatic. Even if they forget, it is not that big of a problem as it is fairly easy to just react to yaw direction with the necessary pedal movement. Spend enough time switching airframes and eventually it becomes an automatic response again for each airframe.

When an engine fails the torque goes away. As part of the entry into autorotation the pilot must neutralize the tail rotor thrust. With a CCW turning rotor this means pushing almost full right pedal and for a CW turning rotor it’s left pedal. In a hover this must be done quickly as the unnecessary tail rotor thrust will start spinning the helicopter. In forward flight, the pilot will experience a yaw to the left as airflow over the vertical fin helps hold the tail straight.

Coming next is more on tail rotor emergency maneuvers and different types of anti-torque designs.