Static vent units

Abstract: A static vent unit (11) for use e.g. on the side of an aircraft (A) and which in operation is subjected to airflow from any direction within a wide airflow incidence range comprises a generally flat plate (12) having an outer surface for fitment generally to flush to an outer surface (14) of the aircraft (A), a vent port (15) through the plate (12) for connection during operation to a static vent line (16) in the aircraft (A), and a bulge (19) on the plate outer surface surrounding at least part of a circumference of the vent port (15) and having in cross section a convex curved upper surface extending radially inwardly from an outer edge (22) on the plate (17) to an inner edger (23) on the plate (17) adjacent the circumference of the vent port (15) and adapted during operation to promote attachment of an airflow from within at least part of the airflow incidence range to its surface and direct the airflow into the vicinity of the vent port (15) to provide a compensating increase in ambient pressure at the vent port.

Description: Description of Invention This invention is concerned with static vent units and particularly but not exclusively with static vent units for use on rotary wing aircraft such as helicopters. Static vent units are used on both fixed and rotary wing aircraft to sense the ambient pressure of the air through which the aircraft is flying. This provides an indication of the altitude at which the aircraft is flying and, in combination with a pressure signal sensed by a forward facing pitot tube, an indication of the airspeed and, for fixed wing aircraft, the Mach number. For this reason, static vent units have ideally to be located in a position where the local ambient pressure varies only with altitude and not with other features such as airspeed and relative wind direction, and are therefore usually located in the sides of an aircraft fuselage. However, it is extremely difficult to find a suitable location where the local pressure remains unaffected by disturbing factors throughout the entire operating range so that the local ambient pressure is often in error by a significant amount, resulting in inaccurate information to the pilot. US-A-3612439 discloses a static vent unit for a high performance fixed wing aircraft designed to operate over a wide Mach number range extending to supersonic values. The unit includes compensation means in the form of an exterior shallow bulge having a thickness which decreases smoothly from a central region to the edges and includes a static vent formed by an aperture located in the surface of the bulge itself and offset in respect of a major axis of the vent unit. The prior device operates to modify the airflow locally in operation so as to compensate a position error arising from variation in Mach number, and in respect of airflow from one generally uniform direction. The problems concerned with location and operation of static vent units on a rotary wing aircraft, particularly a helicopter, are exacerbated both because of the downwash created by the main sustaining rotor and the wide range of operating conditions of which the helicopter is capable. This means that the direction of relative airflow changes significantly with changes in operating conditions and can in some circumstances be completely reversed resulting in a very wide airflow incidence angle in which the static vent unit is required to operate. An objective of this invention therefore is to provide a static vent unit with improved accuracy throughout a wide airflow incidence range. Accordingly in one aspect this invention provides a static vent unit for use on a vehicle and which in operation is subjected to airflow from any direction within an airflow incidence range of ± 80°, comprising a generally planar mounting member adapted for fitment generally flush to an outer surface part of the vehicle, a vent port through the member for connection during operation to a static vent line in the vehicle and a bulge extending outwardly from the member and surrounding at least part of the vent port, the bulge comprising portions, in operation upstream of the vent port, having in cross section, a convex curved upper surface extending radially inwardly towards the vent port from an outer edge on the member to an inner edge the member adjacent the vent port, said portions of the bulge being adapted during operation to promote attachment of an airflow from within at least part of the airflow incidence range to its surface and to direct the airflow into the vicinity of the vent port to provide a compensating increase in ambient pressure at the vent port. According to a second aspect of the invention we provide a vehicle having a static vent unit according to the first aspect of the invention. The invention will now be described by way of example only and with reference to the accompanying drawings in which, Figure 1 is a graph plotting airflow incidence (a) against a pressure coefficient (C PL ) at an intended location for a static vent unit on the side of a helicopter, Figure 2 is a graph plotting airflow incidence (a) against a pressure coefficient (C PD ) showing a calculated required compensation and an actual compensation achieved by a static vent unit according to this invention, Figure 3 is a plan view of a static vent unit constructed according to one embodiment of the invention, Figure 4 is a sectioned view taken on lines A-A of Figure 3, Figure 5 is a sectioned view taken on lines B-B of Figure 3, and Figure 6 is a sectioned view similar to Figure 4 showing a modification of the unit of Figure 3. Figure 7 is an illustrative view of an aircraft embodying the invention. In the design of a new helicopter a desired location for a static vent unit on the side of the fuselage was determined. Wind tunnel tests to indicate the ambient pressure characteristics at the desired location were conducted through a wide range of airflow incidence angles expected to be encountered during operation of the helicopter. The results are shown in Figure 1 which plots a local pressure coefficient (C PL ) against a range of airflow incidence angles (a) between -80 degrees and +80 degrees. In explanation, an airflow incidence angle a of 0° relative the static vent unit location is a generally horizontal airflow from the front with negative angles running clockwise therefrom and positive angles running counterclockwise (refer to Figure 3). As will be clear from Figure 1, the tests showed that the intended location was in a low pressure (i.e. below ambient pressure) area throughout the entire airflow incidence range as indicated by C PL which varies from about -0.18 at zero incidence to about -0.54 at -80 degrees and -0.66 at +80 degrees. This situation was entirely unsatisfactory since a static vent unit sited at the desired location would clearly register an under pressure throughout the entire airflow incidence range which would result in inaccurate information being provided to the pilot from the various instruments requiring a sensed static pressure. Such inaccuracies could severely endanger the safety ofthe aircraft and its occupants. In order to consider ways of overcoming this problem the inventor decided to calculate the compensation required from a static vent unit to provide a resultant pressure coefficient (C PR ) = 0 throughout the airflow incidence range. Such a compensation, if achievable, would of course provide an entirely accurate indication of ambient pressure throughout the airflow incidence range. Thus, starting from, C PR = [C PL + C PD ] - [C PL x C PD ] where C PR = Resultant pressure coefficient C PL = Local pressure coefficient C PD = Device pressure coefficient Substituting C PR = 0 then, C PL x C PD = C PL + C PD Which resolves into, C PD = C PL C PL -1 Figure 2 plots C PD against airflow incidence a and shows in full line the calculated compensation using equation (3) required to make C PR = 0 throughout the airflow incidence range. The requirement is for a positive C PD ranging from about +0.14 at α= 0° to about +0.34 at α= -80° and about +0.4 at α= +80°. Thus it was clear that a static vent unit to meet this requirement would have to be capable of providing a compensating overpressure for an under pressure throughout the entire airflow incidence range. Clearly the device of US-A-3612439 was incapable of providing such compensation since that unit was designed to compensate overpressure for under pressure with airflow in one direction relative its longitudinal axis and under pressure for overpressure with the airflow in the opposite direction. For this reason also, the inventor decided that the raised location of the mouth of the aperture in the surface of the bulge of the prior art device was not suitable in a device for solving his problem, and that the mouth of the aperture should preferably be flush with the external surface of the aircraft as in conventional un-compensated static vent units. The problem faced by the inventor then was how to artificially increase the pressure at the vent aperture by a variable amount throughout a wide airflow incidence range. He decided that what was needed was a combination of a ramp-type device behind the vent aperture to increase the pressure at low airflow incidence angles with some means for guiding the airflow into the area of the vent aperture and by varying amounts at the larger airflow incidence angles at both ends of the operating range. Eventually he decided to try to obtain such airflow guidance using attached flow characteristics, similar to that used on an upper surface of an aerofoil to create a differential pressure to produce lift, by shaping an area of the upper surface of the static vent unit such as to promote attachment of an airflow from radially outward directions at least towards the ends of the required airflow incidence range, and to guide the attached airflow into the area of the vent aperture. One embodiment of a novel static vent unit embodying such characteristics will now be described with reference to Figures 3, 4 and 5 of the accompanying drawings. The static vent unit, generally indicated at 11, includes a generally planar mounting member being in this example, a flat circular support plate 12 located in a recess 13 in the side of a helicopter fuselage 14 so that an outer surface of the support plate 12 is generally flush with an outer surface of the fuselage 14. A circular static vent port 15 extends through the support plate 12 and connects to a static vent line 16 in the fuselage 14 for transmitting, during operation, a signal representative of ambient pressure to helicopter instrumentation in known manner. For descriptive purposes it is useful to align the static vent unit 11 as in a typical operational environment and to that end it is to be noted that the vent port 15 is located on a horizontal centreline 17 of the support plate 12 and a little to one side of a vertical centreline 18. In operation, the horizontal centreline 17 is aligned with zero airflow incidence as indicated at 0° in Figure 3 in respect of airflow approaching in the direction of arrow C. In operation of the helicopter, the airflow incidence varies over a wide range and relative incidence angles up to -80° running clockwise from the 0° incidence and up to + 80° running counterclockwise from the 0° incidence are indicated in Figure 3. A bulge 19 generally semi-circular in plan is located on the plate 12 downstream of the vent port 15 and is centred parallel to the longitudinal centreline 17 and, therefore, the airflow in the direction of arrow C at 0° airflow incidence angle, so as to surround part of the circumference of the vent port 15. It will be noted that upper and lower curved lobe portions 20 and 21 of the bulge 19 extend forwardly beyond the extremities of both the positive and negative airflow incidence range. As indicated best in the sectioned view of Figure 5 a curved upper surface of the bulge 19 extends radially inwardly from an outer edge 22 on the surface of the plate 12 with an increasing radius of curvature to an inner edge 23 adjacent the circumference of the vent port 15. In this respect it is to be noted that the convex curved surface of the bulge 19 can be compared with an upper surface of an aerofoil in which edge 22 is representative of the leading edge and edge 23 is representative of the trailing edge. In the illustrated embodiment, the static vent unit 11 includes a dam portion 24 located downstream of the vent port 15. The dam portion 24 includes a forward facing surface 25 extending generally perpendicular to the surface of support plate 12 and perpendicular to the horizontal axis 17. The surface 25 has a sharp upper edge 26 which blends at its ends into the upper surface of the bulge 19, and a rear sloping surface 27 which again blends with the upper surface of bulge 19. In a modification illustrated in Figure 6 the vertical surface 25 of dam 24 is replaced by a convex curved portion 25 a again blended rearwardly with the curved surface of the bulge 19. In operation of the static vent unit of this invention, airflow approaching at high airflow incidence angles within the indicated airflow incidence range on Figure 3 (greater than about -30° or + 30° in the illustrated embodiment), contacts the surface of the bulge 19 and is encouraged by the convex curved shape to attach to the surface and flow along the increasing radius of curvature into the area of the mouth of the circular vent port 15 as indicated by the flow arrows 19 a in Figure 5, to provide a compensating increase in pressure. At mid-range airflow incidence angles (i.e. between about -30° and + 30° in the illustrated embodiment), the desired compensating increase in pressure at the mouth of the vent port 15 is ensured by the airflow contacting the forward facing surface 25 or 25 a of the dam portion 24. Reverting now to Figure 2, the broken line indicates the compensation actually achieved in tests of the static vent unit 11 of Figures 3, 4 and 5 and can be compared with the calculated compensation shown in full line required to provide C PR = 0 throughout the airflow incidence range. It will be seen that at low airflow incidence angles almost all of the required compensation has been achieved. At the larger airflow incidence angles, the achieved compensation with the illustrated embodiment, whilst not sufficient to match the calculated required compensation, nevertheless represents a significant improvement in performance that will greatly enhance the accuracy of the aircraft instrumentation. Furthermore, it is confidently envisaged that further improvements in these areas will be achievable by fine tuning of the curved surface of the bulge 19 at the requisite locations. Thus, the static vent unit 11 of this invention when located in a low pressure (below ambient) area on the side of a helicopter fuselage operates throughout a wide airflow incidence range to increase a sensed ambient pressure to compensate for the prevailing low pressure. Figure 7 illustrates a typical rotary wing aircraft (A) to which the invention may be applied. Whilst several embodiments have been described and illustrated it will be understood that many modifications may be made without departing from the scope of the invention. For example, the bulge 19 may be formed of separated portions each surrounding part of the circumference of the vent port 15 so as to operate within selected parts of the overall airflow incidence range. The vent port 15 may comprise a plurality of smaller diameter holes which may be arranged in concentric circles, and may be located at any desired position in the support plate 12. Although the invention has been particularly described with reference to a rotary wing aircraft, the invention may be applied to a fixed wing aircraft, or even to any other vehicle where the problems outlined in the introduction to this specification, may arise.

Claims: A static vent unit (11) for use on a vehicle (A) which in operation is subjected to airflow from any direction within an airflow incidence range of ± 80°, comprising a generally planar mounting member (12) adapted for fitment generally flush to an outer surface part (14) of the vehicle, a vent port (15) through the member (12) for connection during operation to a static vent line (16) in the vehicle and a bulge (19) extending outwardly from the member and surrounding at least part of the vent port (15), the bulge comprising portions, in operation upstream ofthe vent port (15), having in cross section, a convex curved upper surface extending radially inwardly towards the vent port from an outer edge (22) on the member (12) to an inner edge (23) on the member (12) adjacent the vent port (15) said portions of the bulge (19) being adapted during operation to promote attachment of an airflow from within at least part of the airflow incidence range to its surface and to direct the airflow into the vicinity of the vent port (15) to provide a compensating increase in ambient pressure at the vent port (15). A unit (11) according to claim 1 characterised in that the upper surface of said portions ofthe bulge (19) has a radius of curvature that increases from the outer (22) to the inner (23) edge. A unit (11) according to claim 1 or claim 2 characterised in that the bulge (19) is generally semi-circular in plan and centred parallel to a mid-range airflow incidence angle (17) said portions of the bulge being curved lobe portions (20,21) extending forwardly beyond the extremities of both positive and negative airflow incidence angles defining the airflow incidence range. A unit (11) according to any one of claims 1 to 3 characterised in that the unit (11) includes a dam portion (24) in operation downstream of the vent port (15), having a forward facing surface (11) extending generally perpendicular to the mid range airflow incidence angle (17). A unit (11) according to claim 4 characterised in that the forward facing surface (25) is generally perpendicular to the upper surface ofthe mounting member (12). A unit (11) according to claim 5 characterised in that the forward facing surface (25) has a sharp upper edge (26) blended at its ends with the upper surface of the bulge (19). A unit (11) according to any one of claims 1 to 6 characterised in that the generally planar mounting member (12) comprises a flat plate received in a recess (13) of the outer surface part (14) of the vehicle (A). A unit (11) according to any one of the preceding claims characterised in that the vehicle (A) is an aircraft. A unit (11) according to any one of the preceding claims characterised in that the aircraft (A) is a rotary wing aircraft. A vehicle (A) having a static vent unit (11) according to any one of the preceding claims.

Inventors: PERKINS TERENCE LEYDON, PERKINS, TERENCE LEYDON
Applicants: GKN WESTLAND HELICOPTERS LTD, GKN WESTLAND HELICOPTERS LIMITED
Application year: 1995
Main classification: G01P5
IPC codes: B64D43, G01P5