Aeronautics and astronautics – Aircraft sustentation – Sustaining airfoils
Reexamination Certificate
1998-02-06
2001-01-30
Poon, Peter M. (Department: 3644)
Aeronautics and astronautics
Aircraft sustentation
Sustaining airfoils
C244S05300R, C244S05300R, C060S231000
Reexamination Certificate
active
06179251
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an air vehicle engine nacelle, and more particularly to an aerodynamic engine nacelle designed to eliminate intake airflow boundary layer separation.
BACKGROUND OF THE INVENTION
When a turbo engine air vehicle is at a relatively low speed and a significant engine power increase is required, the problem of intake airflow boundary layer separation at the engine inlet is presented.
Take for example, an aircraft performing take-off operations. The aircraft is initially stationary on the tarmac. The pilot, desiring maximum engine power output, throttles-up the engines. The velocity of the intake airflow prior to entering the engine inlet is relatively low in comparison to the airflow at the engine face within the engine housing or engine nacelle. The engine may be described as rapidly sucking in airflow in order to meet the high power output requirement. Under these conditions (relative low aircraft speed and high engine output requirements), unless adequately compensated for in the design of the engine inlet, boundary layer separation of the intake airflow at the engine inlet will likely occur. Another circumstance of when this may occur is where an aircraft is loitering at a relative low speed (for example, waiting its turn to land at an airport) and a sudden high power output is required (possibly, in order to perform an evasive maneuver).
Boundary layer separation of the intake airflow at the engine inlet results in a significant negative impact on the net power output of the engine. This is because the intake airflow is initially laminar with the airflow being efficiently sucked into and through the engine. Once boundary layer separation occurs, however, the airflow become vortical, having significant localized pressure and direction variations, resulting in a significant loss of engine face pressure. Engine face pressure directly impacts the engine power output performance. In the context of commercial airliners, for example, a loss of 1% of engine face pressure can result in a 1.2-1.5% loss of engine thrust. Moreover, in a worst case scenario, the vortical flow may result in an engine stall.
Previous efforts to address this engine inlet boundary layer separation problem focused at altering the surface contours of the inlet lip. It is known in the art that by smoothing or rounding the leading edge curvature of the inlet lip the boundary separation phenomenon can be altered. This smoothing or rounding of the inlet lip results in a “thick lip” inlet design.
Within the boundary layer, frictional inlet surface forces act to decrease the momentum of the airflow. Boundary layer separation of the airflow results when the momentum of the localized airflow is insufficient to overcome these frictional forces. The effect of rounding the inlet lip is to locally increase the momentum of the airflow within the boundary layer. Thus, the localized airflow is energized as it is swept around the curved thick inlet lip, thereby avoiding boundary layer separation.
The relative thickness of the inlet lip can be described in terms of the ratio of the circular area defined by the diameter defined by the leading edge of a engine inlet to the minimum circular area defined by the interior surface of the engine inlet (lip contraction ratio). In the context of commercial airliners, the industry standard for this lip contraction ratio is 1.33. “Thick” lip designs, however, increase the maximum diameter requirements of the engine nacelle or engine housing, thereby incurring weight, volume, and high speed aerodynamic/drag penalties.
Other previous efforts have employed variable inlet geometry designs. For example, these designs have employed translating engine cowls, where an aerodynamic thin lip inlet has a forward portion which slides forward (on tracks, for example) revealing a localized smooth or rounded inlet lip. Likewise, inlet designs have employed auxiliary inlets or ports, where a thin lip inlet has flaps or doors which reveals a localized smooth or rounded inlet lip. While these designs may address the boundary layer separation problem, they incur significant penalties in relation to weight, volume, airflow leakage, manufacturing costs and maintenance costs.
Accordingly, there is a need in the art for a turbo engine inlet design which addresses inlet boundary layer separation problem without attendant penalties in relation to aerodynamics, weight, volume, and cost.
SUMMARY OF THE INVENTION
In accordance with the present invention, an aerodynamic turbo engine nacelle, disposable about an engine having an engine face, for mitigating boundary layer separation of intake airflow comprises an engine inlet. The engine inlet is provided with an interior surface and a curved portion. The curved portion having a leading edge. The geometry of the engine inlet is such that the ratio of the circular area defined by the diameter defined by the leading edge of the curved portion to the minimum circular area defined by the interior surface of the engine inlet is less than 1.33. The engine nacelle is further provided with a pressurized fluid injecting device for injecting pressurized fluid at the engine inlet in a direction generally parallel to intake airflow in response to sensed airflow conditions. The sensed airflow conditions may comprise boundary layer separation at the engine inlet or airflow pressure at the engine face.
In addition, the present invention contemplates injecting the pressurized fluid at a pressure and velocity which are a function of the loss of momentum of the airflow due to friction at the interior surface of the engine inlet, the change in velocity and pressure of the airflow due to the curved portion of the engine inlet, the pressure and velocity of the intake airflow, and the pressure and velocity of the airflow at the engine face.
It is preferred that the pressurized fluid is injected at an airflow rate of greater than zero and less than 5% of the engine airflow rate. The injected pressurized fluid may be taken from the airflow downstream of the engine inlet. For example, the pressurized fluid may be bled from the turbo compressor of the engine. As a result, because the present invention contemplates using a relatively small amount of injected airflow, less than 5% of the engine airflow rate, the penalty on the engine output performance is minimal. Further, where the present invention is disposable in a nacelle having an anti-icing airflow, the injected pressurized fluid may be taken from the anti-icing airflow. This is especially advantageous, because where anti-icing airflow is bled from the engine the penalty in engine output performance due to the bled airflow has already been incurred. Additionally, the fluid injecting device may be located upstream or downstream of the point of where boundary layer separation occurs without injecting the pressurized fluid. The fluid injecting device may be provided with at least one jet. Furthermore, the above described invention may be disposed in a turbo engine air vehicle.
In another embodiment of the present invention, there is provided a method for mitigating intake airflow boundary layer separation in a turbo engine nacelle having an engine inlet having an interior surface and a curved portion having a leading edge, and a ratio of the circular area defined by the diameter defined by the leading edge of a engine inlet to the minimum circular area defined by the interior surface of the engine inlet less than 1.33. The method begins with an initial step of injecting a pressurized fluid at the engine inlet in a direction generally parallel to intake airflow. The method further provides for controlling the pressure and velocity of the injected pressurized fluid in response to sensed airflow conditions. The sensed airflow conditions may comprise boundary layer separation at the engine inlet or airflow pressure at the engine face. The pressurized fluid may be injected at a pressure and velocity which are a function of the loss of momentum of the airflow due to friction at the int
Davis Warren
Karanik James J.
Tindell Runyon H.
Anderson Terry J.
Dinh Tien
Hoch, Jr. Karl J.
Northrop Grumman Corporation
Poon Peter M.
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