GB2259115A - Aircraft engine nacelle profile - Google Patents

Abstract

A nacelle has an inlet whose front view portrays an elliptical shape and whose side view (Fig 10) conforms to an arcuate profile 104. The profile 104 is determined by hilite points H1, H2, H2' H3 located at extreme top, side and bottom locations of the nacelle with each hilite location being set at an individually predetermined distance from a plane which is perpendicular to the inlet centerline. The arc may be circular (Figs. 10, 11) or elliptical (Figs. 12A, 12B) and hilite point H3 may be axially forward of hilite point H1 (Figs 11, 12A, 12B). <IMAGE>

Description

AIRCRAFT ENGINE NACELLE RAVING CIRCULAR ARC PROFILE CROSS-REFERENCE TO RELATED APPLICATION This application is related to British co-pending application Serial No.S z D 3 (13DV-10256) filed concurrently herewith.

BACKGROUND OF THE INVENTION The present invention relates to nacelles for aircraft gas turbine engines and, more particularly, to an elliptical nacelle which is constructed so that a side profile of the. nacelle inlet exhibits a circular arc profile (CAP).

Conventional subsonic transport aircraft typically include wing mounted gas turbine engines which are mounted below the wing by using conventional pylons and are surrounded by an annular conventional nacelle for providing an aerodynamically smooth envelope. With reference to FIG. 1, there is shown an exemplary subsonic commercial transport aircraft 10 powered by two turbofan engines with each engine being mounted to a wing on opposite sides of the plane.

Only one wing and engine are shown in FIG. 1.

Turbofan engine 12 is mounted to wing 14 by conventional pylon 16. Surrounding engine 12 is nacelle 18 which channels freestream airflow 20 into and about the engine 12. Engine 12 utilizes airflow 20 for combustion and the generation of thrust.

Illustrated in FIG. 2 is a vertical centerline sectional view of the nacelle 18 and engine 12 of FIG.

1. The engine 12 includes a conventional longitudinal engine centerline axis 22 which, during cruise operation of the aircraft 10, is disposed at an engine angle-of-attack a,, angle AE being formed by the direction of the airflow 20 and the engine centerline axis 22. The nacelle 18 includes a generally annular forward nacelle inlet portion 24, and a conventional annular aft nacelle portion 26. The aft nacelle portion is spaced from the engine 12 to define a conventional bypass duct 28 and extends downstream from a conventional fan 30 of the engine 12.

During conventional operation, the engine powers the fan 30 which bypasses a portion of the freestream airflow 20 through the bypass duct for generating thrust for powering the aircraft 10. A portion of the airflow 20 is conventionally channeled through the engine 12 where it is mixed with fuel and undergoes combustion for generating combustion gases which are discharged from the engine 12 after powering, among other things, the fan 30.

With further reference to FIG. 2, the forward nacelle portion 24 includes an annular leading edge, or hilite, 34 which defines an upstream facing of a generally annular inlet face 36 which receives the freestream airflow 20 for channeling to the fan 30.

The airflow 20 which enters the inlet face 36 is also referred to as the capture streamtube 38 which enters the forward nacelle 24 through the inlet face 36.

Spillage airflow 40 is that portion of the freestream airflow that enters the inlet face 36 but not the fan 30 and is deflected around the forward nacelle 24.

The forward nacelle includes a throat 42. The throat 42 is defined as a flow region of minimum area and is positioned downstream from the leading edge 34.

An annular diffuser 44 extends downstream from the throat 42 to the fan 30. The throat is sized for channeling a predetermined mass flow rate of the airflow 20 through the fan 30. The diffuser 44 is disposed in flow communication with the inlet face 36, the throat 42, and the engine 12 and is sized and configured for reducing velocity of the airflow while increasing its static pressure.

FIG. 3 is a front perspective view of the nacelle of FIG. 2. With reference to FIG. 3, the forward nacelle inlet 24 further includes first and second transversely spaced apart sides 58 and 60, respectively, extending oppositely from the keel 52 to the crown 50 and radially outward from the inlet axis 46 which is discussed subsequently. The radial upper crown 50 and the radial lower keel 52 are transverse cross-sections of the forward nacelle 24 along a vertical plane extending through the centerline axis 22.

Conventional nacelle inlets are typically drooped wherein the nacelle diffuser has an inlet centerline or droop axis which is inclined relative to the engine centerline axis. This inlet centerline of a nacelle may be curved to correspond to the curvature of the airflow within the inlet. An example of such a curved inlet centerline can be found in U.S. Patent No.

4,722,357 to Wynosky which is herein incorporated by reference. This drooped axis arrangement allows the nacelle inlet face to be perpendicular to the freestream of airflow when the aircraft is in its design cruise mode of operation and results in minimizing installed drag over the nacelle. The acute angle formed by inlet centerline axis 46 and the engine centerline axis 22 is referred to as the droop angle, aD, and is a fixed geometric parameter. The angle formed by the engine centerline axis 22 and the airflow 20 is referred to as the engine angle-ofattack, aE, which varies with changing aircraft modes of operation.

FIG. 4 is a schematic, transverse sectional view of an exemplary nacelle 18 and illustrates the coincident relationship of inlet centerline axis 46 and airflow 20 during the design cruise mode of operation. Notice that inlet face 36 is perpendicular to inlet centerline 46. Also, the engine angle-ofattack, UE, when the aircraft is in the design cruise mode of operation is equal to the droop angle, aD.

However, the engine angle-of-attack aE varies depending on the operating mode of the aircraft, as illustrated in FIG. 5. FIG. 5 is an exemplary, transverse sectional view of nacelle 18 during a climb mode of operation. Comparing to FIG. 4, the engine angle-of-attack AE is greater during times when the aircraft is climbing than during the design cruise mode of operation. Thus, there is a range of AE over the various operating conditions.

Since weight and drag of an aircraft are important considerations, it is desirable that the nacelle be as small as possible and as light as possible for reducing weight and aerodynamic drag due to the freestream air flowing through and around the nacelle. The length, diameter, and thickness of the nacelle are parameters which directly relate to weight and drag.

Typical aerodynamic performance parameters for evaluating low speed operation of the nacelle include total pressure recovery, circumferential pressure distortion, angle-of-attack capability of the nacelle without flow separation, and crosswind effects acting on the nacelle. At cruise operation of the aircraft, performance considerations include the variation of drag along the external surface of the nacelle due to changes in the engine airflow, freestream Mach number, and the incidence angle of the freestream airflow relative to the nacelle. The Mach number indicates the ratio of the speed of the nacelle as it travels through the air to the speed of sound in an air medium.

Furthermore, increasing environmental concerns over noise pollution have resulted in Government regulations which typically limit the amount of acceptable noise which may be directed to the ground during low speed, takeoff operation. Conventional nacelle inlets require acoustic treatment within the nacelle for meeting noise regulations and require relatively thick nacelle lower lips for meeting low speed high angle-of-attack requirements for obtaining acceptable flow separation margin. Both of these requirements add weight to the nacelle and the relatively thick lower lip also increases drag.

One proposed method for reducing ground noise caused by aircraft turbine engines has been to employ nacelles having a scarfed or scooped inlet design. In the past, several types of scarfed or scooped inlet designs have been tested. These inlet designs are characterized by the bottom lip of the nacelle protruding in a forward manner relative to the upper lip of the nacelle, i.e. the lower lip extends forward of the conventional inlet plane. This is clearly appreciated by viewing FIGS. 6A, 6B, and 6C. FIG. 6A illustrates a side-view of deflector inlet 70. Inlet 70 receives airflow 20 and directs the airflow to engine 12. Deflector inlet 70 has a stair-shaped profile, with the forward boundaries of the inlet 70 being defined by upper leading edge 72 and lower leading edge 74.

FIG. 6B is a side view illustration of a scoop inlet 76 whose profile beginning at the upper leading edge 78 is a straight vertical line which curves to meet the lower extreme boundary defined by lower leading edge 80. FIG. 6C illustrates a side view of a scarf inlet 80 whose profile is characterized by a positively sloped straight line which connects the upper leading edge 82 with the lower leading edge 84.

The scarfed or scoop designs are further characterized by the inlet face not being perpendicular to the inlet centerline. It is apparent that the extended lower lip of the scarfed or scooped inlet design prevents noise from propagating toward the ground by reflecting noise in an upward direction.

However, any design specifications must not disregard the effects on engine performance.

During aerodynamic testing of nacelles having the scarfed or scooped inlet design it was demonstrated that the low-speed angle-of-attack (AOA) capability of such inlets was greatly improved. Unfortunately the aerodynamic performance of other parts of the nacelle, namely the top lip and sides, was compromised. The capability of airflow on the upper lip to remain attached was degraded at high flow, low AOA conditions (ground static conditions), for inlets of the type shown in FIG. 6C. The sides of the inlets tended to shed vortices and hence produce high fan face total pressure distortion when the profile shape was highly curved or discontinuous as in FIGS. 6A and 6B.

The reason these effects occur is that the air flow entering the inlet is altered by the forward or aft displacement of one lip relative to another. At mass flow ratios (MFR) greater than one, more mass flow of air is pulled into the inlet around the aft lips, and less around the forward lips, than would occur if the same lips were located in a plane normal to the inlet centerline as is the more usual convention. The reverse is true for MFR's less than one, where more mass flow of air spills out of the inlet around the aft lips and less around the forward lips. It is felt that mass flow ratio variation tends to occur more readily at the most aft lips, because they are in closer proximity to the fan face pressure field which is accelerating or decelerating the flow entering the inlet.In general, this mass flow variation improves the aerodynamic performance of the forward lips, allowing smaller lip thicknesses, and degrades the aerodynamic performance of the aft lips, requiring larger lip thicknesses to regain lost performance. Normally, one would be free to increase the thickness and length of these lips until performance requirements are met.

In some aircraft applications, the length of the inlet is constrained by mechanical considerations, and the vertical height is constrained by aircraft installation considerations. In these applications, an elliptical nacelle may be used. Such nacelles make it very difficult to maintain attached airflow at high flow, high AOA conditions (takeoff conditions) for the internal lower lip, and at windmill conditions for the external upper lip. Thus, for large, high-bypass engines, it is apparent that a need exists for an elliptical nacelle which can accommodate aircraft which have installation limitations resulting from a lack of available vertical height or horizontal width and which meets or exceeds existing performance requirements.

SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to provide an elliptical nacelle with improved airflow characteristics while reducing the vertical height to satisfy installation considerations.

The present invention provides a method for improving the performance characteristics of an elliptically-shaped nacelle, the nacelle having a nacelle inlet defined by upper, lower, and side lips, the method comprising the step of conforming the profile of the nacelle at the nacelle inlet to a generally circular arc passing through the most forward point of a side lip of the nacelle inlet and the most forward point of at least one of the upper and lower lips of the inlet.

In a further aspect, the invention provides a nacelle inlet having a forward axial configuration comprising an elliptically-shaped throat; upper lips, lower lips, and side lips which are connnected and formed in an elliptical shape which surrounds and defines said elliptically-shaped throat; an elliptically-shaped maximum diameter which defined an extreme outer bounder of said upper, lower, and side lips; and a circular arc profile.

A preferred embodiment of the present invention provides an elliptical inlet and nacelle for meeting vertical height limitations, and by forming the side lips so as to exhibit a circular arc profile (CAP) when the nacelle is viewed in profile to meet performance requirements. The profile is determined by hilite points which are located on the top, side, and bottom lips. The hilite points are the most forward points along the inlet centerline for each of the top, side, and bottom lips.

Being so constructed, the inlet retains the low speed, AOA advantages of the scarf inlet while enhancing the performance of the top lip. The side lip performance is degraded during crosswind but can be overcome by thicker lips. The CAP profile eliminates the highly curved shape or discontinuities of the scoop inlet thus preventing the shedding of vortices. The nacelle can be constructed so that the hilite points form a circular arc when viewed from the side.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the drawings and many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:: FIG. 1 is an exemplary schematic representation of a subsonic transport aircraft having a wing mounted gas turbine engine; FIG. 2 is an exemplary transverse sectional view of the nacelle which is shown mounted to the wing of the aircraft in FIG. 1; FIG. 3 is an exemplary front view perspective illustration of the nacelle assembly shown in FIG. 2; FIG. 4 is an exemplary schematic transverse sectional view of a nacelle which illustrates the engine angle-of-attack, AEI during a cruise mode of operation; FIG. 5 is an exemplary schematic transverse sectional view of a nacelle which illustrates the engine angle-of-attack, aye, during a climb mode of operation; FIG. 6A is an exemplary schematic side view illustration of a nacelle having a deflector inlet; FIG. 6B is an exemplary schematic side view of a nacelle having a scoop inlet;; FIG. 6C is an exemplary schematic side view of a nacelle having a scarf inlet; FIG. 7A is an isometric view of an CAP/elliptical Nacelle according to one embodiment of the present invention; FIG. 7B is a side view of the nacelle of FIG. 7A and illustrates the circular arc profile; FIG. 7C is a front view of the ellipticallyshaped nacelle of FIG. 7A; FIG. 7D is a bottom view of the nacelle of FIG.

7A; FIG. 8A is a schematic illustration demonstrating an elliptically-shaped throat area of a nacelle according to one embodiment of the present invention having a vertical main axis; FIG. 8B is a schematic illustration demonstrating an elliptically-shaped throat area of a nacelle according to another embodiment of the present invention having a horizontal main axis; FIG. 9 is a schematic front view of the right half forward looking aft of a nacelle having an elliptically-shaped throat and axial projection of hilite points and maximum nacelle diameter points according to one embodiment of the present invention; FIG. 10 is a schematic side illustration of the nacelle of FIG. 9 and demonstrates a circular arc profile;; FIG. 11 is a schematic side illustration of a nacelle according to an embodiment of the present invention which illustrates how hilite points are positioned a predetermined distance from a reference plane; FIGS. 12A and 12B are side view schematic illustrations according to the present invention which illustrates how hilite points can be selected to determine the side profile of a nacelle; FIGS. 13 and 14 are graphs illustrating the performance improvements of CAP inlets during aircraft take-off and during engine-out climb, respectively; FIGS. 15A-15C are graphs illustrating performance of a conventional inlet at low speed, high angle-ofattack take-off; FIGS. 16A-16C are graphs illustrating performance of an elliptically-shaped nacelle at low speed, high angle-of-attack take-off; and FIGS. 17A-17C are graphs illustrating performance of an elliptical inlet incorporating the CAP of the present invention.

When referring to the drawings, it is understood that like reference numerals designate identical or corresponding parts throughout the respective figures.

THE DETAILED DESCRIPTION OF THE INVENTION FIG. 7A is an isometric view of a nacelle 90 in accordance with one embodiment of the present invention for housing a gas turbine engine (not shown). Nacelle 90 is an elliptically-shaped nacelle comprised of an air inlet 92 which is formed by the upper lip area 94, lower lip area 96, and side lip areas 98 and 100, respectively. The throat 102 represents a region of minimum flow area in the air inlet of the nacelle 90. At the most forward position on the upper lip is located the hilite point H1.

Located to the aft of the forward lip location on the side lip area 100 is hilite point H2 at the most forward point of the side lip. A corresponding hilite point H2' is located on the side lip area 98. A line drawn between hilite point H2 and corresponding hilite point H2' would be perpendicular to the inlet axis of nacelle 90. At the most forward location of the lower lip is located hilite point H3. Hilite points H1 and H3 are both located forward of side hilite points H2 and Hz'.

FIG. 7B is a side view of nacelle 90 and demonstrates how the contours of the nacelle form a circular arc profile (CAP) 104 which is defined by hilite points H1, H2, and H3. FIG. 7C is a front view of an elliptically-shaped throat area 102 and FIG. 7D is a bottom view of nacelle 90 which gives the reader a further appreciation of the spatial relationships between bottom hilite point H3 and side hilite point H2 and H2'.

With reference to FIG. 8A, a schematic illustration depicting an elliptically-shaped throat 102 according to one embodiment of the present invention has a major axis Y and a minor axis X which gives the ellipse an elongated appearance in the vertical direction. The corresponding Y coordinates of the ellipse are designated by the letter "b" and- the corresponding X coordinates of the ellipse are designated by the "a". The origin designated by the coordinates (0,0) represents the X and Y coordinates of the inlet centerline axis 106 of the throat 102.

The inlet centerline axis is perpendicular to the Y axis and the X axis.

With reference to FIG. 8B, a schematic illustration of the elliptically-shaped throat 102 of nacelle 90 according to another embodiment of the present invention is depicted. In FIG. 8B the major axis is the X axis with the result being that the throat is elongated in the horizontal direction. The elliptical throats of FIGS. 8A and 8B are defined by the following equation: X2 y2 a2 b2 The numeral 102S depicted in FIG. 8B identifies the shape of the throat area 102 of yet another embodiment of the present invention. In this embodiment the throat is given a super-elliptical shape compared to the elliptical shape 102.

In FIG. 9, a front view of the right half of the nacelle 90 is presented. The axial projection of the throat 102A and axial projection of the hilite points 108H give the reader a further appreciation of the present invention. These axial projections are elliptical in shape as is the axial projection representing the maximum diameter 110 of the nacelle 90. The forebody or outer lips to the nacelle 90 are designated by numeral 112 and the inner lips are designated by numeral 114. It is realized that the hilite points 108H project in front of the adjacent and sloped sides of the inner lips 114 and outer lips 112.

In FIG. 10, a side view of the nacelle 90 is shown with the engine centerline 118 defining the center of the engine 12. The inlet axis or centerline 106 defines the center of the throat area 102. The inlet profile is indicated by CAP 104 connecting hilite points H1, Hz, and H3 located along the inlet centerline axis 106.

The CAP inlet of the present invention has a profile which is determined by specifying the lip extensions AX relative to some reference plane that is perpendicular to the inlet centerline axis 106. In FIG. 11, a distance dX1 separates hilite point H1, a distance Ah separates hilite point H2/H2', and a distance AX3 separates hilite point H3 from the reference plane 122. Note that AX1 does not have to equal AF so that the engine inlet may have an asymmetrical configuration. A value of AX is specified for each hilite point with the three values of AX determining the circular arc profile of the nacelle.The circular arc profile gives a smooth distribution of AX from the top of the nacelle to the bottom of the nacelle unlike the discontinuous distributions inherent in prior art designs.

How far aft the hilite H2 is located will affect the side profile of the nacelle 90 and its performance. For example, FIG. 12A is a side of the nacelle 90 and shows H1, Hu, and H3. CAP 104B connects hilite points H1, H29, and H3 and CAP 104A connects hilite points H1, Ha, and H3. The performance of the inlet with CAP 104B profile will realize improvements in inlet low speed, high angle-of-attack performance compared to an inlet with a CAP 104A profile. Additionally, the profile does not have to be limited to a circular arc 104 as illustrated in FIG. 12B. FIG. 12B compares a super-elliptical profile 104C to a circular arc profile 104B, each profile passing through the same three points.The profiles 104A, 104 B, and 104C can be used to bring about different aerodynamic results for the nacelle 90.

The aerodynamic advantage of a CAP inlet are illustrated in FIGS. 13 and 14. The data in these figures is representative of test results for the CAP concept. FIG. 13 shows the results at the low speed, high angle-of-attack, power-on condition for three freestream Mach numbers X1, , and %. The increase in angle-of-attack capability as a function of CAP angle is a result of forward displacement of the bottom lip which sees less mass flow being sucked into the inlet. FIG. 14 shows the results at the low speed, moderate AOA, windmilling engine condition.

Here again, the increased AOA capability is a result of the forward displacement of the top lip in which there is less mass flow spillage out of the inlet.

Depending on the three determinant values of the AX's, (FIG. 11) the CAP inlet of the present invention may accommodate a scarf inlet or a scoop inlet.

Furthermore, the definition of the present invention may be used to institute a circular arc profile for nacelles having an upper scoop inlet (where only the upper lip protrudes in front of the side lip) or any combination of inlet designs. The advantages of the CAP inlet concept of the present invention are improvements in performance and design flexibility.

Design flexibility arises from the CAP and its determination from the designation of the three AX1s at the top, bottom, and side hilite positions of the nacelle. Also, the circular arc profile of the present invention allows an inlet to be designed for specialized improvements. For example, the upper scoop inlet improves upper lip windmill capability.

The elliptical/CAP inlet provides performance improvement at all relevant off-design flow conditions. These flow conditions are characterized by the high angle-of-attack, high flow takeoff condition, by the moderate angle-of-attack, low flow windmill condition, and by the low angle-of-attack, low flow, high Mach number condition.

FIGS. 15-17 illustrate how the CAP/E (elliptical cap) concept evolved using the local Mach number distribution along the top, side, and bottom internal lips at a low speed, high AOA, power on condition.

FIGS. 15A, 15B, and 15C show the Mach number distribution at the top, side, and bottom lips, respectively, for a conventional nacelle, the critical lip being the bottom lip for this flight condition.

The performance of an elliptic inlet, which has the major axis from side-to-side in order to reduce the vertical dimension, is shown in FIGS. 16A, 16B, and 16C. The local Mach number on the bottom lip, FIG.

16B, just ahead of the shock wave (as indicated by the sudden drop in the Mach number), is a contributor to the inlet performance. The higher this Mach number, the stronger the shock becomes, the greater the likelihood of flow separation, and the less AOA capability of the bottom lip. Comparing FIG. 15B to FIG. 16B shows that the elliptical inlet would have a slightly stronger shock strength and therefore less AOA capability than the conventional inlet. By incorporating a CAP inlet with the elliptical nacelle, the AOA capability of the combined CAP/ellipse inlet is improved significantly as shown by the lower Mach number ahead of the shock wave in FIG. 17B. The performance exceeds that of the conventional inlet (illustrated in FIGS. 13 and 14), so that a thinner bottom internal lip can be used to obtain further reductions in the vertical dimension. The Mach number distribution for the side lip of the CAP/ellipse inlet is overall higher than either of the side lips from the conventional or elliptical inlets (see FIGS. 15C, 16C, and 17C). This demonstrates how the CAP concept re-distributes the lip loading to reduce the loading on the critical lip.

The elliptical shape of the nacelle inlet allows for a smaller vertical height for ground clearance but has a performance loss compared to a conventional nacelle. However, when a CAP inlet is incorporated into the elliptical nacelle, the performance of the nacelle is actually improved over conventional designs. By adjusting the three lip extensions AX, of the circular arc profile, and the total eccentricity of the elliptical cross section, it is possible to distribute aerodynamic loading circumferentially around the inlet rather than concentrating the loading at the top and bottom lips as occurs with conventional inlets. Thus, performance of the conventional inlet is improved while reducing the vertical diameter of the nacelle.

The CAP is the smoothest, lowest curvature shape that can be passed through the hilite points located at the top, side, and bottom of the nacelle, thereby avoiding the tendency to shed vortices from highly curved or discontinuous profile shapes. Furthermore, the smaller vertical height of the elliptically-shaped nacelle allows installation of larger engines in under-wing aircraft positions than previous designs.

The upper and lower lips of the elliptical/CAP nacelle inlet can be made relatively thin to minimize vertical height and the lip thickness of the sides of the nacelle 90 made thicker to maintain performance requirements for low speed, high flow crosswind conditions. An increase in side forebody thickness may be necessary to attach air flow under low flow, low AOA, high Mach number conditions (i.e., engine-out cruise or EROPS conditions) for CAP inlets. The flow redistribution effect caused by axial displacement of the lips, present at low speeds is also present at conditions where the Mach number may be in the 0.50.75 range. Thus, there may be a need to increase the side forebody thickness.

If the circumferential aerodynamic loading of the elliptical nacelle with CAP inlet is carefully controlled, by means of small variations of the profile shape from a pure circular arc, and by means of circumferential variation of the lip thickness and forebody thickness, it is possible to create a nacelle which has equivalent or improved performance at all relevant flow conditions.

A further advantage of the circular arc profile of the present invention for elliptical or conventional nacelles results from the interaction of the nacelle flow field and the ground. Aircraft engines mounted near the ground tend to create ground vortices which can cause debris to be kicked up and ingested by the engine. Since the CAP inlet of the present invention tends to pull a greater portion of the entering air flow around the side lips during static conditions, the CAP inlet nacelle will produce a weaker ground vortex than a conventional inlet.

Thus, the foreign object damage (FOD) potential is diminished by the present invention when the engine is mounted at the same ground-to-engine centerline distance.

The foregoing detailed description of the preferred embodiments of the present invention is intended to be illustrative and non-limiting. Many changes and modifications are possible in light of the above teachings. Thus, it is understood that the invention may be practiced otherwise than as specifically described herein and still be within the scope of the appended claims.

Claims (8)

1. A method for improving the performance characteristics of an elliptically-shaped nacelle, the nacelle having a nacelle inlet defined by upper, lower, and side lips, the method comprising the step of: conforming the profile of the nacelle at the nacelle inlet to a generally 'circular arc passing through the most forward point of a side lip of the nacelle inlet and the most forward point of at least one of the upper and lower lips of the inlet.

2. The method of claim 1 and including the of: conforming the profile of the nacelle at the nacelle inlet to a generally circular arc passing through the most forward point on each of the upper, lower, and side lips of the inlet.

3. An elliptical nacelle for a gas turbine engine having a generally circular arc profile at an inlet of the nacelle.

4. The elliptical nacelle of claim 3 wherein said inlet is defined by an upper lip, a lower lip, and a pair of opposed side lips, and including a throat for directing air flow into the nacelle and wherein a front view of said inlet reveals an elliptically-shaped axial projection of hilite points which includes hilite points H1, H2 H2,, and H3, said axial projection of hilite points being located radially outward from said throat.

5. An inlet according to claim 4 wherein said elliptically-shaped throat has a horizontal main axis and the nacelle vertical height is less than the nacelle horizontal width.

6. A nacelle inlet having a forward axial configuration comprising: an elliptically-shaped throat; upper lips, lower lips, and side lips which are connected and formed in an elliptical shape which surrounds and defines said elliptically-shaped throat; an elliptically-shaped maximum diameter which defines an extreme outer boundary of said upper, lower, and side lips; and a circular arc profile.

7. A method as claimed in Claim 1 and substantially as described with reference to the accompanying drawings.

8. An inlet as claimed in Claim 6 and substantially as described with reference to the accompanying drawings.