Helicopter engine filter system

A specially contoured barrier type filter disposed in front of a helicopter turbine inlet provides for maximized airflow during forward, as well as sideways flight. The contouring additionally reduces the tendency of the filter to clog. A bypass mechanism is provided should the pressure differential across the filter exceed a predetermined value. The bypass configuration achieves a limited particle separation function in order to provide residual protection. An air box attached to the inlet is formed to provide low distortion levels in the airflow entering the engine inlet.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY 
The present invention provides a barrier-type filtration system for 
helicopter turbines that overcomes the shortcomings of previous adaptation 
of barrier filters for such applications. The system's configuration 
serves to maximize airflow through the filter element for airflow 
approaching the filter from a variety of directions, including from along 
the aircraft's longitudinal axis, as well a from an angle thereto. 
Additionally, the filter system provides for the optimization of airflow 
into the engine after its filtration. This configuration additionally 
renders the filter element less prone to clogging, especially by the 
buildup of ice. Should bypass of the filter element nonetheless become 
necessary, a bypass mechanism is provided which is configured so as to 
achieve a limited particle separating function to thereby provide some 
residual protection. Finally, the construction and mounting of the filter 
element itself is substantially stronger than conventional barrier filters 
so that such element can withstand the sustained high vibrational loads 
encountered in the helicopter environment. 
The filtration system of the present invention achieves the above-set-forth 
advantages with a contoured filter element that includes filtering 
surfaces that are oriented both perpendicular to the longitudinal axis of 
the aircraft, as well as surfaces that curve toward an orientation 
parallel thereto. The filter itself consists of pleated oil impregnated 
multilayer cotton fabric held between layers of bonded wire mesh with a 
final safety screen to protect the engine. The overall curvature of the 
filter element serves to strengthen the structure, and the potting of the 
edges of the material in a thick border of polyurethane serves to insulate 
the filter element from vibration. 
The filter element is held in place by a rigid, sealed air box that is 
attached to the engine inlet. The box includes a bellmouth that provides a 
smooth transition into the turbine so as to maintain attached laminar flow 
pattern thereby minimizing distorted flow into the engine. Additionally, 
the air box includes a moveable flap that provides an alternate flow path 
to the engine. The flap is oriented such that incoming airflow must make a 
sharp change in direction of flow prior to induction into the air box, and 
a second sharp change in direction in order to enter the engine thereby 
achieving a particle separating effect. The pressure drop across the 
filter element is monitored at all times, and a warning signal is 
generated when the pressure change reaches a predetermined limit proven 
safe in FAA approved flight tests. 
These and other features and advantages of the present invention will 
become apparent from the following detailed description of a preferred 
embodiment which, taken in conjunction with the accompanying drawings, 
illustrates, by way of example, the principles of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The drawings illustrate the air filtration system of the present invention 
as adapted to a specific helicopter configuration. The system is attached 
directly to the turbine's inlet and is positioned completely within the 
existing cowling. Air entering the cowling, either through forward inlets 
or lateral inlets, must pass through the filter element to be inducted 
into the turbine. 
FIG. 1 is a perspective view of a helicopter 12 sans cowling showing the 
intake air filtration system 14 of the present invention as installed. The 
system is positioned directly in front of the inlet to the turbine 16. Its 
filter element 18 includes a filtering surface facing forwardly along the 
helicopter's longitudinal axis, as well as filtering surfaces facing 
laterally. 
FIG. 2 is an enlarged cross-sectional view of the intake air filtration 
system 14 of the present invention. The system includes a rigid air box 20 
that is attached to the turbine inlet and a filter element 18 that is, in 
turn, attached to the air box. The filter element is contoured such that 
the majority of its surface area 22 faces forwardly and additionally 
curves to form two laterally facing sections 24, 26. The forwardly facing 
area is bowed slightly inwardly as is clearly visible in the drawing. 
Additionally, the filter element 18, as well as the air box 20, is formed 
at 28 to add strength and accommodate output shaft 38 extending between 
turbine 16 and transmission 30. 
A bypass flap 32 is positioned along the top of air box 20. Upon deployment 
of activator 34, the flap is hinged upwardly to provide an alternate air 
path to the engine. The area uncovered by the flap is approximately twice 
that of the turbine inlet. 
The cross-sectional views of FIGS. 3 and 4 illustrate the positioning of 
the air filtration system relative the turbine inlet 36. The back side of 
air box 20 has a bellmouth 44 formed therein that provides a smooth second 
derivative curvature into the turbine inlet. A first annular mounting ring 
46 rigidly attached to the bellmouth, and a second annular mounting ring 
48 rigidly attached to fire wall 37 are joined by V-band clamp 42. FIG. 3 
additionally shows output shaft 38 passing directly below the filtration 
system. The one-piece construction of the air box prevents the seepage of 
water into the area of the engine inlet. 
The position of the bypass flap 32 disposed along the top of air box 20 is 
controlled by linear actuator 34. Energization of a solenoid overcomes the 
force of a mechanical spring that normally holds the flap open (to provide 
fail safe operation). Electrical interconnection 35 provides for both the 
energization of the circuit, as well as an indication of its position. 
Pressure sensor 40 measure the pressure drop across the filter element 18. 
A pressure differential greater than a predetermined value causes a 
warning signal to be sent to the cockpit via conduit 41. 
FIG. 4 additionally shows the construction of the filter element 18. 
Multiple layers of cotton grid fabric 17 are pleated between two layers of 
bonded wire mesh 19. In addition to enhancing flow, the turned back side 
sections, as well as the bowed form of the center section, serve to 
increase the filter element's mechanical strength. The edges of such 
structure are subsequently potted in a thick layer of polyurethane to damp 
vibrations. Prior to use, the filter element is impregnated with an oil 
that has a bright yellow dye incorporated therein. 
The air filtration system of the present invention is easily fitted or 
retrofitted to a rotorcraft. In addition to the removal of the components 
of any previously used filtration system, installation merely requires 
attachment of mounting ring 48 to fire wall 37. The air box 20 is then 
fitted thereto with the tightening of V-band clamp 42. Electrical 
interconnections to a power source, as well as the appropriate switches 
and warning lights within the cockpit, completes the installation. The 
filter element is impregnated with oil, its bright color giving a clean 
indication of coverage. 
In use, the turbine draws air through the filter element, the oil 
impregnated cotton grid fabric being capable of a 99% separation 
efficiency (standard A.C. Coarse Test Dust). In normal forward flight, air 
flows to the engine through forward facing inlets in the cowling and 
passes through the forward facing surface 22 of the filter element 18, as 
shown in FIG. 5. A minimal amount of resistance is encountered in such an 
air path 60, and the smooth shape of the bellmouth 44 promotes a clean 
laminar flow into the engine. As the forward facing area of filter element 
traps more and more particulates, an increasing amount of airflow enters 
through the filter's lateral surfaces 24, 26. Such flow path 62 does 
encounter more resistance by virtue of the change in direction the 
inducted air must undergo to enter and pass through the filter, but it is 
precisely the change in direction that prevents or delays the complete 
clogging of the filter. The abrupt change in direction serves to separate 
particles from the airflow causing them to continue in backward direction. 
Super cooled water droplets therefore tend to cling and freeze to 
none-essential surfaces, rather than icing up the filter element pleats to 
impede and ultimately restrict air flow. 
The lateral facing filtering surfaces 24, 26 of the filter element 18 
provide an additional benefit, as shown in FIG. 6. The contoured surface 
of the filtering element provides a straight flow path therethrough and to 
the bellmouth regardless of the direction in which airflow approaches the 
filter. If, for example, the rotorcraft is flown sideways, wherein more 
and more air enters the cowling through lateral intake ducts, as opposed 
to through the forward intake ducts, the lateral filtering surfaces 
provide a straight flow path 64 into the bellmouth to thereby minimize 
suction required by the engine. 
The heavy potting 50 of the edges of the filter element 18 not only serves 
to more positively join the edges of the wire mesh and cotton grid fabric 
but insulates the filter element from the high vibrations it would 
otherwise by subject to. Additionally, the inward curvature of the face of 
the filter element adds strength to resist deformation due to any pressure 
differential it may be subject to. 
In the event the filter element does eventually become clogged to the point 
where the pressure drop across the filter, as measured by sensor 40, 
exceeds a preselected value, a warning signal is sent to the cockpit. This 
gives the pilot the option of opening bypass flap 32 to provide an 
alternate flow path for induction air thereby increasing safety of 
operation. The orientation of the flap relative to the inlet 18 forces 
airflow inducted into the engine to undergo two abrupt changes in 
direction thereby, as shown in FIG. 7 serving to separate particles from 
the airflow. Super cooled droplets separated from the flow of air will 
impact adjacent surfaces to which they freeze and cling, and will thereby 
avoid being inducted into the engine where they could cause damage. 
Any particulate trapped by the filter will cause the appearance of the 
bright yellow color of the impregnating oil to darken. During maintenance 
of the aircraft, the technician can therefore immediately gauge the 
condition of the filter element by visual inspection. The filter element 
is easily removed by detachment of the fasteners, after which a cleaning 
solvent is applied, it is backflushed with water, dried, reimpregnated, 
and reinstalled. 
While a particular form of the invention has been illustrated and 
described, it will also be apparent to those skilled in the art that 
various modifications can be made without departing from the spirit and 
scope of the invention. Accordingly, it is not intended that the invention 
be limited except by the appended claims.