GB2179405A - Gas turbine engine with cowled prop-fan - Google Patents

Abstract

In order to give the prop fan, the best characteristics of a known ducted fan gas turbine engine, without increasing the weight and drag penalties of known duct constructions, an ultra thin, short cowl 20 is provided which is affixed to the or one of the stages of propellers 16, 18. The internal profile if the cowl corresponds to that portion of the core gas generator which is radially aligned therewith. The fixing may comprise pivots which will enable pivoting of each of the blades of the propellers 16 or 18 so as to facilitate reverse thrust. <IMAGE>

Description

SPECIFICATION Gas turbine engine with cowled propeller The present invention concerns a gas turbine engine which is suitable for powering an aircraft.

More particularly, the invention relates to a gas turbine engine which pressurises ambient air prior to passing it to combustion equipment and accelerates further ambient air along the exterior of the gas turbine engine.

The acceleration of the outer airflow is achieved by aerofoil blades which extend radially of the core gas generator, beyond the casing in which the core gas generator is enclosed.

One example of an engine which is embraced by the foregoing description, is known by those skilled in the art as a propeller turbine engine ie., the core gas generator drives one or more propellers, each of which may have six or more aerofoil blades.

Another example is known as a ducted fan gag turbine engine, wherein a relatively large number of fan blades are enclosed in a cowl which also surrounds at least a portion of the core gas generator and thus forms a duct therewith. In the latter example, the fan cowl is fixed via struts to the core gas generator and commonly includes thrust reversing apparatus.

A hybrid engine is now being considered, ie., an engine which has a core gas generator driving at least one stage of aerofoil blades which project beyond the core gas generator casing. The blades, in chordal proportions, are rather similar to aerofoil blades in a fan stage, but like a propeller turbine in that the blades are not enclosed by a cowl. Furthermore, they have increased tip diameter relative to that of a conventional cowled fan, or ducted fan, gas turbine engine. Such hybrid engines are known as propfans.

It is known that the propulsive efficiency of both a propeller and a fan is a fuction of the magnitude of the mass flow of air through the engine. It is further known that for a given stage diameter the flow efficiency and quietness can be improved in given operating conditions if the stage is enclosed by a cowl. It is recognised however, that cowl structures generate considerable drag and, in the case of prop fan engines in which the tip diameter of the prop fan stage is large relative to that of a ducted fan on a ducted fan gas turbine engine, the drag so generated if a cowl of conventional design is utilised would be greatly increased, as would weight. In use on an aircraft, such a combination, though probably showing an improvement in low forward speed performance, would definitely result in an unacceptably poor performance at cruise.

The present invention seeks to provide an improved prop fan engine.

According to the present invention, a prop fan gas turbine engine includes rotatable aero foil blade means projecting radially outwards therefrom, beyond a casing which surrounds a core gas generator thereof, a cowl surround ing the aerofoil blade means and spanning the tips of the aerofoil blade means in a direction chordally thereof and mounted for co-axial co rotation therewith, wherein the cowl provides interior and exterior flow surfaces which termi nate at a maximum of one aerofoil blade chor dal width upstream and a maximum of one and a half aerofoil blade chordal widths down stream of respective extreme upstream and downstream edges of the aerofoil blade means and the interior flow surface of the cowl is complimentary to that portion of the core gas generator which is radially aligned therewith so as to minimise drag.

The rotatable aerofoil blade means may comprise a single stage of aerofoil blades and the cowl is affixed to the tips thereof.

Alternatively the rotatable aerofoil blade means may comprise a plurality of coaxially mounted stages of aerofoil blades and the cowl is affixed to the tips of the aerofoil blades in one of the stages thereof.

Preferably the stages of aerofoil blades con sists of a pair of coaxial, contra-rotatable stages of aerofoil blades and the cowl is affixed to the tips of the downstream one of the pair of stages of aerofoil blades.

Preferably at least the aerofoil blades in one stage are pivotable about axes which are aligned radially of the engine so as to enable a driven reversed flow through the stage or stages.

The cowl may comprise two cowl portions, one of which is aifixed to the tips of the aerofoil blades in one stage thereof and the other portion of which is affixed to the tips of the aerofoil blades in the other stage thereof, such that the adjacent end faces of the two cowl portions are in close spaced relationship with each other.

The invention will now be described by way of example and with reference to the accom panying drawings in which: Figure 1 is a diagrammatic view of a prop fan gas turbine engine in accordance with the present invention.

Figures 2 and 3 are identical, enlarged part views of the prop fan gas turbine engine of figure 1.

Figure 4 is an alternative embodiment of the present invention incorporated in the prop fan engine of figure 1.

Figure 5 is a further embodiment of the pre sent invention incorporated in the prop fan en gine of Figure 1.

Figure 6 is a further arrangement embodying the present invention.

Figure 7 is a still further arrangement embo dying the present invention.

Referring to figure 1. A prop fan gas turbine engine 10 includes a core gas generator 12 of generally known construction, enclosed by a cowl 14. The core gas generator 12 is connected via gearing (not shown) to drive a pair of stages of propeller blades 16 and 18 in contrarotating manner, about a common axis of rotation.

The propeller stages 16 and 18 are shown at the upstream end of the core gas generator 12, i.e., upstream having regard to the direction of flow of gases therethrough. The propeller stages 16 and 18 however, could be mounted at any position from upstream to downstream of the core gas generator 12, without adversely affecting the efficacy of the inventive combination.

A cowl 20 surrounds both stages of propellers 16 and 18 and terminates just upstream of the leading edges of the propeller stage 16 and just downstream of the trailing edges of the propeller stage 18. The cowl 20 is thus very short. The cowl 20 is supported from the tips of the propeller stage 18 for rotation therewith.

The cowl 20 is ultra thin in directions radially of itself, that is, ultra thin when compared with the corresponding dimensions of known cowls eg., on ducted fan gas turbine engines (not shown), but presently in use.

Further, the interior flow surface 22 of the cowl 20 has a profile which is designed so as to match the external flow surfaces 28, 30 of those portions of the bosses 24 and 26 to which the roots of the stages of propeller blades 16 and 18 are respectively fixed and which portions lie radially inwards of the cowl 20, so as to achieve the desired flow characteristics, i.e. minimum drag.

Referring now to Figure 2. The prop fan engine 10 is intended for use on an aircraft (not shown) and therefore, has to operate in widely varying conditions i.e., idling when the aircraft is static, take off of the associated aircraft (not shown), cruise thereof and under conditions of reverse thrust when the associated aircraft is landing.

In Figure 2, the prop fan gas turbine engine 10 is depicted operating in an idling mode as when its associated aircraft is stationary. This is indicated by the flow pattern of ambient air which is being sucked into the duct defined by the cowl 20 and the parts 28 and 30. The air flow is represented by the broken lines 32 and its direction by the arrow heads 34.

It is seen that a large space volume 36 of relatively stagnant air lies adjacent the inner flow surface 22 of the cowl 20. This is a characteristic of airflow across rotating propellers at zero speed or low forward speed of an associated aircraft.

Although not depicted in pictorial form, the airflow which would be generated if reverse thrust is selected, would be as shown in Figure 2 except that its direction would be reversed. In order to enable reverse thrust, each of the propeller blades in the stage 18 is attached to the cowl 20 and the part 30 by a respective pivotable stub shaft 38 and 39.

Appropriate known drives (not shown) may be provided to achieve pivoting of each propeller blade in the stage and such pivoting, combined with rotation of the stage 18 draws ambient air into what is normally the nozzle 40 of the duct defined by the cowl 20 and parts 28 and 30. The combined effect of reduced forward speed of the aircraft (not shown) and the reverse pumping of ambient air, quickly generates the flow conditions and consequent void but from a reverse direction.

The pivoting of the blades also enables matching of the air flow to other flight regimes i.e. take off and cruise.

Referring to Figure 3, in which it should be understood that the aircraft (not shown) which is powered by the prop fan gas turbine engine 10 of the present invention, is flying in its cruise configuration.

The forward movement of the engine 10, combined with the rotation of the propeller stages 16 and 18 within the cowl 20, causes the ambient air immediately in front of the propeller stages to act as if it is moving in a diffuser aligned with the duct defined by the cowl 20 and parts 28 and 30, when it reaches the intake plane thereof, as is indicated by the broken lines 32a. With virtually no change in direction, the air enters the duct and that part of the air which meets the lip of the cowl 20, divides over a very narrow angle, by virtue of the ultra thin construction of the cowl 20, and flows over the respective inner and outer flow surfaces thereof.The presence of the cowl gives the usual advantages of ducted flows i.e. the high entry velocities are reduced and maintained at the values most suited to the blading; less noise is generated and, the noise which is generated is kept largely inside the duct, instead of being transmitted to the aircraft and externally.

The short axial length of the cowl 20 greatly reduces the surface area over which the air must flow before leaving the trailing edge.

The enclosing of the propeller stages 16 and 18 in a cowl thus improves the efficiency and quietness of the flow of air through those stages for given operating conditions and therefore improves the efficiency and quietness of the engine 10. The use of a cowl for such purposes is known. The arrangement of the present invention however, achieves the known result without the usually attendant disadvantages, as follows hereafter.

The ultra thin structure of the cowl 20, plus the complementary duct wall surfaces, maintains frontal area, and the drag associated therewith, to a minimum.

The small angle of convergance of the downstream portion of the cowl 20 ensures the avoidance of breakaway of the airflow and base drag which is consequent thereon.

The short length of the cowl 20 reduces the surface area over which the air flows and thus reduces drag.

The thin cowl structure is light in weight relative to known cowl structures.

The fastening of the cowl to the or each stage of propellers obviates the usual necessity for fixed struts with which to support the cowl 20 from the casing 14 of the core gas generator 12. A main source of weight, drag and noise generation is thus obviated.

Referring now to Figure 4. The cowl 20 is divided peripherally into fore and aft portions 20a and 20b and each is fastened to a respective propeller stage. In each case the fastening is such as to enable pivoting of the propeller stages 16 and 18. However, either or neither propeller stage 16 and 18 may be pivotable.

The peripheral gas 46 is sealed by sealing features of any suitable known type (not shown).

In Figure 5, only a single stage of propellers 48 is provided and a cowl 50 of ultra thin, short construction is affixed thereto.

Referring now to Figure 6. Improved prediffusion of the intake air may be achieved by making the length of the intake portion 20c of the cowl portion 20a at most equal to the magnitude of the mean chord of the blades in the propeller stage 16. Similarly, a more efficient nozzle may be achieved by making the length of the nozzle portion 20d of the cowl portion 20b at most equal to one and one half times the magnitude of the mean chord of the blades in the propeller stage 18.

In Figure 7, the spaced apart edges of the cowl portions 20a, 20b are so shaped and arranged with respect to each other, as to define an annular passage 48 which extends in a generally downstream direction, but at the same time is inclined radially outwardly of the propeller duct. That portion of air which is pumped in a downstream direction (i.e. downstream with respect to the direction of flow of fluids through the engine) along the inner surface of the cowl portion 20a, is diverted through the annular passage 48 and over the outer surface of the cowl portion 20b. Such an arrangement improves the laminar flow thereover.

Claims (12)

1. A prop fan gas turbine engine including rotatable aerofoil blade means projecting radially outwards therefrom, beyond a casing which surrounds a core gas generator thereof, a cowl surrounding the aerofoil blade means and spanning the tips of the aerofoil blade means in a direction chordally thereof and mounted for coaxial co-rotation therewith, wherein the cowl provides interior and exterior flow surfaces which terminate at a maximum of one aerofoil blade chordal width upstream and at a maximum of one and one half aerofoil blade chordal width downstream of the respective extreme upstream and downstream edges of the aerofoil blade means and the interior flow surface of the cowl is complimentary to that portion of the core gas generator which is radially aligned therewith so as to minimise drag.

2. A prop fan gas turbine engine as claimed in claim 1 wherein the rotatable aerofoil blade means comprises a single stage of aerofoil blades and the cowl is affixed to the tips thereof.

3. A prop fan gas turbine engine as claimed in claim 1 wherein the rotatable aerofoil blade means comprises a plurality of coaxially mounted stages of aerofoil blades and the cowl is affixed to the tips of the aerofoil blades comprising one of said stages.

4. A prop fan gas turbine engine as claimed in claim 3 wherein the plurality of coaxially mounted stages of aerofoil blades comprises a pair of contra-rotating stages thereof.

5. A prop fan gas turbine engine as claimed in any previous claim wherein the aerofoil blades in the or at least one stage are pivotable so as to enable a driven reversed flow from the stage or stages.

6. A prop fan gas turbine engine as claimed in any of claims 3 to 5 wherein the cowl comprises a plurality of cowl portions, each of which surrounds and is affixed to the tips of a respective stage of aerofoil blades such that adjacent end faces of the cowl portions are in spaced relationship with each other.

7. A prop fan gas turbine engine as claimed in claim 6 wherein said spaced between the cowl portions comprises an annular slot which defines a generally downstream directed, radially outwardly inclined flow path which in operation ejects air from within the cowl onto the outer surface of the downstream cowl portion so as to improve laminer flow thereover.

8. A prop fan gas turbine engine substantially as described in this specification and with reference to Figures 1 to 3 of the drawings.

9. A prop fan gas turbine engine substantially as described in this specification and with reference to Figure 4 of the drawings.

10. A prop fan gas turbine engine substantially as described in this specification and with reference to Figure 5 of the drawings.

11. A prop fan gas turbine engine substantially as described in this specification and with reference to Figure 6 of the drawings.

12. A prop fan gas turbine engine substantially as described in this specification and with reference to Figure 7 of the drawings.