Patent Application
20160252051
The subject matter disclosed herein relates to aircraft. More specifically, the subject matter disclosed herein relates to suction fuel feed systems for aircraft.
Aircraft, for example, helicopters, include fuel feed systems having fuel lines and plumbing components which carry fuel from the fuel tanks to the engines. In modern helicopters, the engines are typically mounted higher up than the fuel tanks. Suction type engine fuel feed systems utilize a suction pump mounted on the engine which draws fuel up from the fuel tank. Suction type fuel feed systems are used in many helicopters because of the safety they provide. In the event of a fuel feed line leak, such as a rupture due to ballistic damage, air is drawn into the fuel line rather than fuel spraying out as would occur in a pressurized fuel system.
In a suction type fuel feed system, however, the suction acting on the fuel can cause air and fuel vapor dissolved in the fuel to come out of solution and form bubbles in the fuel as it travels up the fuel line to the engine fuel pump. The engine fuel pump has a maximum allowable vapor-to-liquid volume fraction (V/L) and a maximum allowable bubble size at the pump's fuel inlet which the pump can tolerate, beyond which, the fuel pump will lose its priming and stop pumping fuel, resulting in engine flame-out.
Engine fuel feed lines are generally designed to limit the fuel line pressure drop such that the calculated V/L ratio at the engine fuel pump inlet, based on the worst case combinations of altitude, fuel type, fuel pressure, fuel temperature, and aircraft g loading, is limited to the level the engine fuel pump can tolerate. The calculated V/L ratio represents a time-averaged value of the ratio of air-vapor volume to liquid fuel volume entering the engine fuel pump. However, constraints on the fuel line sizing and routing imposed by the helicopter configuration can cause occasional or periodic local concentrations of bubbles in the fuel line which are larger than the maximum bubble size that the engine pump can tolerate even when the overall time-averaged V/L ratio determined based on altitude, fuel type, fuel pressure, and fuel temperature is within the engine fuel pump limit.
A possible cause of a periodic accumulation of a large amount of air and fuel vapor at the engine fuel pump's fuel inlet is operation of the helicopter at conditions in which the fuel velocity in the fuel feed line is lower than the natural velocity at which a bubble would rise upward in the fuel line due to buoyancy (the “bubble rise velocity”). This situation is generally the result of sizing the fuel feed line diameter to meet the calculated pressure drop limit associated with the fuel pump's V/L limit at the maximum fuel flow, altitude, and maneuvering g level. The diameter and flow area of the fuel line needed to meet the allowable fuel line pressure drop for a given fuel line geometry can result in fuel velocities below the bubble rise velocity, particularly at low fuel flows. Over time, the volumes of liquid fuel and air-vapor would be consistent with the allowable V/L at the fuel pump inlet, but instantaneously, the V/L would be above the average value part of the time and below average the rest of the time. The time in which the V/L is above average corresponds to an above-average bubble size, which, if it is large enough, would cause pump loss of prime and engine flame-out. When this problem occurs, the fuel feed system operating envelope in terms of altitude, fuel temperature, fuel system pressure drop, and aircraft g capability is reduced below the envelope that would be available based on the engine fuel pump's stated V/L capability.
Another possible cause of a momentary large amount of air-vapor at the engine fuel pump's fuel inlet is a natural bubble trap in the fuel line which can occur at some aircraft attitudes, but not at others. If the aircraft operates for a period of time at attitudes in which a large enough volume of air-vapor to cause fuel pump loss of priming is trapped at a local location in the fuel line and then the aircraft changes to an attitude at which the air-vapor is no longer trapped at that location, the large volume of air-vapor would suddenly travel all at once to the engine fuel pump inlet, possibly resulting in engine flame-out. This could occur even when the time-averaged V/L ratio is well within the allowable value for the engine fuel pump. Engine fuel feed lines with relatively long, near-horizontal sections can cause this situation.
For example, in a helicopter in which the fuel tank is located a significant distance forward of the engine it feeds, the fuel line section from the fuel tank to the cabin ceiling may be nearly vertical, followed by a long, near-horizontal fuel line section going aft to the engine. A natural fuel trap would occur in the corner between the vertical and horizontal fuel line segments when the aircraft is in a nose-up attitude, but not in a nose-down attitude. An air-vapor volume trapped in the corner while operating nose-up would quickly travel to the engine fuel pump inlet if the aircraft changes to a nose-down attitude.
One current approach to the bubble problem is to use a boost pump to pressurize the fuel line at conditions within the aircraft operating envelope where bubbles may be formed which are large enough to cause suction pump failure. Using a boost pump to pressurize the fuel line, however, negates the safety benefit of the suction system when the boost pump is operating.
In one embodiment, a fuel feed system for a rotary-winged aircraft includes a fuel feed line extending from a fuel source to an engine and an engine fuel suction pump located at the fuel feed line to urge a flow of fuel through the fuel feed line to the engine. A collector is located along the fuel feed line between the fuel source and the engine to collect air-vapor which forms as bubbles in the flow of fuel. An air-vapor line extends from the collector and merges into the fuel line section between the collector and the engine fuel suction pump to flow the air-vapor captured in the collector to the engine fuel suction pump. The air-vapor line is sized and configured to limit the flow of air-vapor to the engine fuel suction pump to a flow rate that the engine fuel suction pump can tolerate.
In another embodiment, a rotary-winged aircraft includes an airframe and an engine located at the airframe, the engine operably connected to and driving a rotor assembly. A fuel feed system is operably connected to the engine and includes a fuel feed line extending from a fuel source to the engine. An engine fuel suction pump is located at the fuel feed line to urge a flow of fuel through the fuel feed line to the engine. A collector is located along the fuel feed line between the fuel source and the engine to collect air-vapor which forms as bubbles in the flow of fuel, and an air-vapor line extends from the collector to flow the air-vapor captured in the collector overboard or back to the fuel source such that the air-vapor does not enter the fuel line downstream of the collector and does not enter the engine fuel suction pump. An air pump in the air-vapor line urges the air-vapor captured in the collector to flow overboard or back to the fuel source via the air-vapor line.
These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.
The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
Shown in
Referring now to
A chamber, bubble collector 40, is located along the fuel line 36, between a lower fuel line 36a and an upper fuel line 36b. The lower fuel line 36a extends from the fuel tank 34 to the bubble collector 40, while the upper fuel line 36b extends from the bubble collector 40 to the engine fuel pump 38. The bubble collector 40 collects air-vapor 42 that separates from the flow of fuel 32 due to operation of the engine fuel pump 38. The air-vapor 42 collects in an upper portion 44 of the bubble collector 40, while the flow of fuel 32 proceeds through a lower portion 46 of the bubble collector 40 toward the engine 24 via the upper fuel line 36b. The bubble collector 40 includes a collector inlet 48 from the lower fuel line 36a and a collector outlet 50 to the upper fuel line 36b, with the collector outlet 50 located below the collector inlet 48 to facilitate separation of the air-vapor 42 from the flow of fuel 32.
The bubble collector 40 can be sized to capture the largest single local accumulation of air-vapor 42 which can occur in the lower fuel line 36a. The air-vapor 42 collected from one large accumulation can then be removed from the bubble collector 40 to make room for the next one.
In some embodiments, as shown in
Further, in some embodiments, the lower fuel line 36a includes a smooth venturi 56, or a smooth narrowing 56 in the lower fuel line 36a, to locally reduce the static pressure in the lower fuel line 36a, thereby encouraging additional air-vapor 42 formation without significantly increasing the overall total pressure drop versus fuel flow of the fuel feed system between the fuel tank 34 and the engine fuel pump 38. The venturi/reduced flow area 56 is included in the lower fuel line 36a to extract additional air-vapor 42 from the flow of fuel 32 upstream of the bubble collector 40, thereby increasing an amount of air-vapor 42 removed from the flow of fuel 32 at the bubble collector 40. The venturi/reduced flow area 56 is positioned close enough to the bubble collector 40 such that the additional air-vapor 42 formed by the venturi/reduced flow area 56 does not return to solution in the flow of fuel 32 prior to entering the bubble collector 40. The purpose of extracting additional air from the fuel upstream of the bubble collector 40 is to provide additional flexibility in the location of the bubble collector 40 between the fuel tank 34 and the engine pump 38 in a bubble collector 40 configuration in which the air-vapor line 52 merges with the upper fuel line 36b. Removing additional air-vapor from the fuel line 36a allows the bubble collector 40 to be located closer to the fuel tank 34 and at a lower height than would be needed without the venturi/area reduction 56.
In another embodiment, as shown in
Referring now to
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
This invention was made with government support under contract number N00019-06-C-0081 awarded by the United States Navy. The government has certain rights in the invention.
