1. Field of the Invention
The present invention relates to systems for filtering air inducted into a turbine engine and, more particularly, pertains to barrier-type filtration systems for helicopter turbines.
2. Description of Related Art
A generally accepted approach toward filtering air supplied to a helicopter turbine engine comprises the use of aerodynamic particle separation principles. Such devices remove particulates by inducing vortex flow into the incoming air. Particulates contained in the air are thereby thrown outwardly leaving the core of the flow pattern relatively clean for induction into the engine. The periphery of the air flow, laden with the ejected particulates, is directed away from the intake, and discharged from the craft. Such systems have been favored because no scheduled replacement of filtering elements is necessary, although daily inspection is required. Particle separators do, however, suffer from a number of disadvantages, including a severely limited filtering capability. Such separators are able to achieve only about a 92% separation efficiency (A.C. Coarse Test Dust) which results in significant turbine, fan, and compressor erosion, especially when the craft is operated under severe conditions.
A further disadvantage associated with vortex-type particle separators is a direct result of the vortexed air flow. A significant amount of engine suction and resulting pumping loss is required to induce the incoming air flow to form a vortex and engine bleed air is used to purge the system. Additionally, the vortex itself is not conducive to an efficient flow of air into the engine by virtue of its distorted flow patterns, especially near the engine's intake ducting. These factors combine to significantly reduce the amount of power that would otherwise be available for powering the craft. A further limitation of prior art vortex type separator assemblies is that they are not sealed against water seepage which has been shown to allow ice to accumulate near the engine inlet with a resultant risk to flight safety in certain conditions.
Barrier-type filters offer a number of advantages over vortex-type particle separator which can reduce operating costs and increase safety. Particularly advantageous is the fact that air filtration efficiencies of greater than 99% are attainable. This has the immediate effect of substantially reducing engine wear thereby extending overhaul intervals, reducing unscheduled maintenance, and providing the ability to operate in adverse environments without engine damage concerns. Additionally, because no particulate laden air needs to be removed, no power robbing bleed air is required, while the absence of a vortex provides for a smoother air flow into the engine.
However, prior art barrier type filters fitted to helicopter turbines do suffer from a number of shortcomings. Adapting a flat filter element to the confines of a helicopter cowling presents significant packaging problems, while the resulting configuration yields less than optimum airflow and may be subject to icing. A problem that is inherent in barrier-type filters, and one that has not adequately been addressed in previously known adaptations thereof to helicopter applications, is the fact that the flow capacity of a barrier filter is a function of the direction of flow through the filter. A flat filter element oriented so as to maximize air flow when the craft is flown in a forward direction has considerably less flow capacity when the helicopter is flown sideways. Consequently, despite the fact that side openings may be provided in the helicopter's cowlings to provide airflow to the engine for this type of operation, the orientation of the filter is critical in optimizing the airflow that actually enters the engine.
Another disadvantage of barrier type filters results from the fact that filtrant necessarily accumulates and thereby gradually reduces airflow capacity. While this requires that the filter element be periodically cleaned or replaced, a more urgent concern is that such disposition to clog is especially problematic when the craft is operated under icing conditions. An impervious layer of built-up ice can quickly form as super cooled droplets suspended in the atmosphere freeze and cling to the filter element upon impact. Bypass mechanisms have been provided in the past whereby the pilot is able to completely bypass the filter when a clogged condition is indicated. However, the lack of any filtration of the air that results when air is inducted into the engine in such a manner is of concern. Additionally, previously know barrier filters are susceptible to failure as a result of the vibration to which they are subjected directly in front of a helicopter turbine inlet. Detachment of a sufficiently large portion of the filter element from its supporting frame and ingestion by the turbine could have catastrophic consequences.
A system is needed that allows the superior filtration capabilities of a barrier-type filter to be exploited in helicopter applications without the disadvantages attendant in previous configurations. More particularly, it would be most desirable for the filtration system to provide adequate airflow in all flight attitudes, to be less prone to clogging, especially as a result of icing, and to provide for a bypass capability that provides some residual protection. Finally, such system must be able to withstand the rigors of a high-vibration environment. The present invention meets all of these requirements.