Archive for June, 2013

Ground effect

Sunday, June 30th, 2013

Hovering in ground effect results in a condition of improved performance that comes from operating near a firm surface. A helicopter is normally considered to be in ground effect when it is hovering less than one-half of its rotor diameter from the ground. However, the amount of benefit varies as a function of height. A lower hover will generate more efficiency and as the helicopter climbs the advantage decreases reaching zero about one and one-quarter times the rotor diameter.

A helicopter requires less power to hover in ground effect for two reasons. The main reason is the reduced velocity of the induced airflow caused by the ground. (Induced flow is air flowing down through the rotor system and is also called downwash.) This reduced velocity results in less induced drag and a more vertical lift vector. As such, the lift needed to sustain a hover can now be generated with a lower angle of attack in rotor blades, which requires less power.

The second reason has to do with vortices generated at the rotor tips. The close proximity of the ground forces more air outward and restricts vortex generation. This reduces drag and increases the efficiency of the outer portion of the rotors.

The maximum benefit is achieved from hovering over a hard surface such as concrete. When a helicopter hovers over an area such as tall grass or water, energy is absorbed by displacing the surface, allowing the induced flow to increase, thus reducing the lift vector. This will require the pilot to add power to maintain that hover height.

When a helicopter is in a high hover, or out-of-ground-effect, it requires a lot more power because there is no obstruction to slow the induced flow or force it outward. This results in a more vertical downwash and also allows the formation of stronger rotor tip vortices, reducing efficiency.

The helicopter’s Pilot Operating Handbook (POH) has both In-Ground-Effect (IGE) and Out-of-Ground-Effect (OGE) hover charts. This allows the pilot to take the density altitude and gross weight into account to predict hover performance.

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Ground resonance

Sunday, June 16th, 2013

A helicopter’s rotor system, engine(s), and other dynamic components generate vibrations in the airframe. These components will vibrate at a natural frequency which in turn causes other parts like the landing gear, tail boom, cabin etc… to vibrate as well. Each part’s frequency will vary according to its weight, stiffness, shape, etc…. As such, a helicopter contains a complex set of vibrations that add up to a resulting airframe vibration. Engineers attempt to reduce the overall vibration level by tuning the natural frequency of all the components. 

When a helicopter is in flight, the airframe’s natural frequency (the sum of all the components’ frequencies) will vibrate without interference. However, when ground contact is made with the landing gear it can interfere with the airframe’s ability to vibrate at its natural frequency. Ground resonance happens when ground contact alters the natural frequency of the main rotor system. This unbalanced condition triggers vibrations that are augmented with every blade revolution causing a reflected impulse that increases in amplitude very quickly. The only rotor systems susceptible to ground resonance have three or more blades. This is due to each blade’s ability to lead and lag (speeding up and slowing down) independently. If something causes the blades to depart from their symmetry, the rotor system’s center of gravity shift causes it to become out of balance allowing divergent oscillations to rapidly become strong enough to cause serious damage to the helicopter. In some cases, complete destruction  can occur with many componets coming loose and being thrown from the helicopter. 

Engineers design damping systems for the main rotor and landing gear to absorb the energy and prevent the oscillations from accelerating. Still, a sudden shock to the airframe, like a hard landing, can unbalance the main rotor system beyond the damping system’s ability to absorb the energy and ground resonance can start. Improperly serviced or malfunctioning dampeners are usually the cause.  Ground resonance happens very fast, however, if the pilot recognizes it in time and there is enough power and rotor RPM available to lift the helicopter off the ground the divergent oscillations will stop. This is the quickest way to stop it and hopefully will result in little or no damage. If the situation is such that there is not enough power to lift into a hover, then a full power reduction is the only option. However, this approach will take time for the vibrations to fade and significant damage can occur.

Gemini ST

Monday, June 3rd, 2013

The problem with very light twin-engine helicopters is payload and range. The extra weight of the second engine, combining gearbox, extra fuel tanks and related systems severely limits the airframe’s payload.  Also, that smaller payload capacity will need to include the weight of more fuel. As such, carrying enough fuel for an acceptable range normally leaves little weight capacity for passengers. Operators of small helicopters who wanted the redundancy of a twin have to make some big trade-offs. 

In the late 1980s, Doug Daigle, a 14,000-hour helicopter pilot and owner of Tridair Helicopters, came up with an idea that tried to solve this problem.  He took a popular single-engine helicopter, the Bell LongRanger (206L3), and removed the 650-shaft-horsepower Allison 250-C30 engine (the 500-shp C28 for the older L1s) and added a pair of 450-shp Allison 250- C20R engines to create a twin with good single-engine performance. However, what was different about his design was it was the first twin to be certified for normal operation on one engine in all areas of flight. Certified by the FAA in 1994, he named it the Gemini ST conversion. 

When operating both engines, the Gemini consumes fuel at 45 gallons per hour, compared to the LongRanger at 38 gph. Because both helicopters have the same fuel capacity (he did not add extra fuel tanks), endurance drops from 2.9 hours for the LongRanger to 2.5 hours for the Gemini. However, the Gemini’s C20R engine burns only 28 gph in single-engine mode, and this increases the endurance to 3.9 hours. Daigle believed long trips will normally have some extended cruise flight, where the pilot could choose to fly on one engine. The other engine can be started for critical maneuvers that require the redundancy of a second engine. 

The extra weight of the engine and related systems does increase the empty weight, so there is some trade-off with the Gemini. It has a smaller useful load of 1,610 pounds, compared to 2,175 pounds for the LongRanger. The Gemini can carry 740 pounds of fuel (the same as the LongRanger), which leaves a payload capacity of 870 pounds. 

Bell Helicopter then entered into an agreement with the company that supported Tridair’s conversion of existing LongRangers while Bell would build a new production model, the 206LT TwinRanger, under Tridair’s STC. Bell only delivered 13 airframes and the concept of shutting one engine down in flight never caught on. This model was replaced by the Bell 427 which was under development for the EMS market. The 427’s cabin proved too small and Bell canceled the program and introduced the larger, single-pilot IFR certified 429 in 2009.