Archive for July, 2011

Torsion-tension straps

Monday, July 25th, 2011

The mass of a spinning rotor blade produces a strong force that pulls the blade away from the hub. It is known as centrifugal force and the greater the rpm or the higher the blade’s mass, the stronger the force. For helicopter engineers this presents a challenge because while the blade must be sturdily attached to the hub, it also needs the flexibility to change pitch.

Each manufacturer has designed its own method to address this issue. In the 1960s Bell helicopter designed something called a torsion-tension (TT) strap for use on the 206 series helicopter. The straps are made of a durable, elastic material and resemble a pair of elongated rubber bands with reinforced grommets on each end. One end is attached to the hub and the other to the blade. They are very strong in tension and at the same time can twist (torsion) to allow blade pitch change.

In the mid 1970s three 206B accidents were attributed to a TT strap failure where the component ripped apart under the centrifugal forces of the main rotor. In August of 1976 Bell responded by making the straps 1,200-hour life limited components, instead of on condition. Then in 1980, following the fourth failure of a TT strap in a Bell 212 the company added a 24 month retirement life.

Research by Bell showed that all four failures were the result of operations in a highly corrosive environment. Company engineers have been working on a solution to the TT strap problem by testing new materials and corrosion-resistant coatings. However, many customers have complained that the 24 month requirement is too short, increases their operating costs and is not necessary for operators who do not regularly fly near sand and salt water. Nevertheless, Bell Helicopter remains convinced that it is desirable to err on the side of caution, and that considering the factual history it is prudent to maintain the 24-month calendar life retirement until they can engineer a safe replacement.

Taxi operations

Wednesday, July 13th, 2011

Hover taxi, air taxi and surface taxi are the three basic ways a helicopter can move around an airport.

The most common of the three is hover taxi. By definition this is operating below 25 feet, however, most hover taxiing is done from the surface to about 10 feet. When deciding on how high to hover taxi there are two main considerations. One is hovering high enough to minimize the danger of catching a skid on a small sign or other low object. Conversely, the other is not hovering too high so that a hovering autorotation can be safely performed in the event of an engine failure. The type of helicopter makes a difference, a high main rotor inertia helicopter like a Bell 206B can safely hover higher than a Robinson R22. Personally, I hover taxi on the higher side because my feeling is that an engine failure is less likely than not seeing an object and catching the skid.

Sometimes you might hear a controller ask a helicopter pilot to air taxi. This is normally done to taxi a longer distance within an airport boundary. The pilot is expected to remain below 100 feet and to avoid overflying equipment and personnel. The pilot should choose an airspeed and altitude combination that is clear of the shaded area of the height-velocity diagram (a chart in the flight manual that depicts airspeed/altitude combinations that supports successful autorotations).

Finally, helicopters with wheels can surface taxi. Since the rotor system only has to produce enough thrust to allow the helicopter to move on wheels (a lot less than hovering) the pilot can taxi closer to objects and buildings without creating a lot of downwash. However, unlike hover and air taxiing, surface taxiing requires a hard surface. Of course, a wheeled helicopter can always hover or air taxi as well.

Simulator sickness

Monday, July 4th, 2011

Simulator sickness is a form of motion sickness caused by physical and/or visual motion in a simulator. Compared to motion sickness, the symptoms tend to include more visual disturbances than gastrointestinal issues. Symptoms include dizziness, nausea, eye strain, feelings of warmth, headache, disorientation, and fatigue. What causes this is a conflict between the vestibular, visual, and proprioreceptive senses and pilot’s expectations based on past flight experience. As such, pilots with high amounts of actual aircraft experience are more susceptible to simulator sickness than students with little flight time in the actual aircraft. Flight time aside, some pilots are able to adapt very quickly to simulator training and others have a more difficult time.

Any flight simulator has the potential to cause simulator sickness, however rotor wing simulators are known to cause higher rates of simulator sickness compared to fixed wing ones. Helicopters simulators generally have larger visual areas and this increases the potential for a conflict. Some maneuvers require hovering close to objects which amplifies any discrepancy between what the pilot sees and feels versus expectations. 

Several techniques and restrictions have been developed to help reduce this phenomenon in helicopter pilots:

  • Limit session duration to 2 hours daily
  • Having the pilot close their eyes before freezing the visuals or repositioning the aircraft
  • Limit hover and autorotation training until the pilot can adapt to the simulator
  • Careful maintenance to eliminate any optical distortion caused by misaligned or poorly calibrated optics
  • Maintain health and be well rested
  • Insure adequate ventilation and cooling in the simulator

Flight simulators are a safe and cost effective alternative to actual flight and are an invaluable tool for training. As helicopter pilots are exposed to simulators more, the propensity for simulator sickness decreases. As technology and computer processing power continue to evolve helicopter simulators will also improve.