GB2034267A - Device for Towing by an Aircraft - Google Patents

Abstract

An aircraft towed target sleeve (Fig. 3) has a tubular-sleeve part (2) at the rear end of which is attached a spoiler (11) in the form of an annulus having a cylindrical part (13) attached to the sleeve part (2) and a radially extending flange part (14) which extends into the airflow (S) and acts as a spoiler edge, to prevent flutter of the target sleeve at high towing speeds. A modification, (Fig. 6) also has an internal spoiler (23'). In another form, the spoiler is a frusto- conical member (Fig. 5, not shown). An aircraft towed target flag (Fig. 7, not shown) has a T-section spoiler secured to the flag trailing edge. <IMAGE>

Description

SPECIFICATION Device for Towing by an Aircraft This invention relates to a device for towing by an aircraft, for example a towed target sleeve or flag, particularly for aerial target representation.

One characteristic of such a device when made of flexible material is its tendency to flutter when towed. This substantially increases its air resistance. At relatively low flight speeds, say up to 200 knots, this fluttering can be counteracted by providing in a sleeve one or more openings at the end of its tubular portion, near its end wall, to enable air to escape from the interior of the sleeve. It is also known to reduce the diameter of a sleeve gradually towards its end wall, and such means may be used in combination with the opening or openings. A strong internal pressure develops during flight, thus keeping the sleeve inflated and substantially rigid.

At higher flight speeds however the air resistance increases to such an extent that an uneconomically powerful aircraft is required for towing. It is therefore customary at higher speeds to use a towed sleeve in which the air inlet crosssection is only slightly greater than the air outlet cross-section in the region of the end wall.

The fluttering, which occurs mainly at the rear end of the towed sleeve and in the same way also in towed flags, due to the flow leaving the surface of the towed body, reaches such an intensity when the dynamic pressure becomes high that it is often too much for the strength of the fabric so that tears are formed which can destroy the towed device quite quickly.

It is an object of the invention to overcome or reduce the destructive effect of fluttering in flexible towed device such as a towed sleeve or flag, without adversely affecting its air resistance.

It is another object of the invention to make towed devices suitable for high flight speeds than has hitherto been the case.

According to this invention a device for towing by an aircraft, for example a towed sleeve or flag, has one or more than one spoiler arranged in the region of the rear end of the device so as in use to extend into the airflow over the surfaces of the wall of the device.

With the construction of the invention, increased pressure (accumulation of flow) is produced on the front side of the spoiler, viewed in the direction of flight, so that the tension in the material of the towed device is increased. The cause of the buffeting flutter, namely the alternating change in the direction of bulging of the fabric due to localized pressure changes as the air streams over the devices, is thereby prevented or substantially reduced. The spoilers may be made of metal or plastics or, if suitably shaped, they may also be made of an elastic material.

Conventional towed target sleeves and towed target sieves in accordance with the invention are shown side by side in the drawings and are described below.

Figure 1 ia a perspective view of a known towed sleeve target with an end wall and flow openings, as used for low flight speeds; Figure 2 is a perspective view of another known towed target sleeve without an end wall, as used for higher flight speeds; Figure 3 is a perspective view of an exemplary embodiment of a towed target sleeve in accordance with the invention; Figure 4 is an enlarged longitudinal section of part of Figure 3, showing flow round the rear end of the sleeve; Figure 5 is a similar longitudinal section through another embodiment; Figure 6 is a similar longitudinal section through another embodiment; and Figure 7 is a perspective view of an embodiment of towed target flag in accordance with the invention.

Referring to Figure 1, a known towed sleeve target 1 has a tubular or slightly conical sleeve part 2 of flexible material open to the air-stream through an inlet 3, and closed at the rear end by an end wall 4. In the region of the end wall the sleeve has outlet apertures 5 distributed around its circumference to enable the air flowing into the tubular part 2 to be discharged in a particular manner so that the sleeve is kept inflated. A towing harness is indicated at 7, a towing cable (attached to the aircraft (not shown) at 8, and flow round the towed sleeve at S.

Figure 2 shows a sleeve that does not have the end wall, but instead, a rear outlet 3'. Such a sleeve would be used for towing at higher speeds than that of Figure 1.

For comparison with the known sleeves, Figure 3 shows a towed target sleeve 1 in accordance with the invention, in which a spoiler 11 in the form of an annulus extending into the flow S beyond the external contour of the sleeve part 2 is fixed to the rear end of the sleeve part. This annulus is Lshaped in section (Figure 4), and comprises an annular cylindrical connecting part 13 which is fixed to the sleeve part 2 by for example riveting or gluing, and an annular flange part 14 which forms a spoiler edge extending outwards into the airflow.

The airflow in the embodiment of Figure 3 is shown in Figure 4. 1 5 and 16 indicate lines of flow close to the wall and the signs +, ++ and represent pressures above and below the static pressure of the undisturbed flow. The height to which the annular flange part 14 extends into the flow is represented by h, where h > the local boundary layer at maximum towing velocity. 16' denotes the circumferential line of stagnation points. Air entering the sleeve 1 through the inlet 3 can flow unhindered from the rear outlet 3'. The flow close to the wall (flow lines 15, 16) on the external surface of the sleeve part 2 must flow round the obstruction formed by the flange part 14, so that a zone of increased pressure builds up at ++ on the inside of the sleeve and a zone of excess pressure + on the outside.A low pressure zone - is formed in known manner behind the flange part 14. Due to this distribution of pressure, fluttering at the end of the sleeve is prevented even at high towing speed. Such a towed sleeve may be used for highter towing speeds than was hitherto possible.

In the embodiment of Figure 5, a spoiler 1 8 has a frusto-conical, funnel-like portion 1 9 which widens out in the direction of the flow S and acts on the flow near the wall, instead of the spoiler 11 of Figure 3 which is L-shaped in section. This spoiler 18 is attached to the tubular part 2 by a cylindrical part 20 fixed to the sleeve e.g. by riveting or gluing. The flow lines indicated in Figure 4 are also applicable in this case. When the target is towed, an area of increased pressure ++ builds up near the wall on the inside of the tubular part 2, an area of excess pressure + builds up on the outside and a reduced pressure zone -- builds up behind the spoiler 18.The height h of the funnel-like portion 1 9 is again 5 the local boundary layer at a maximum towing speed. The line of stagnation points extending round the circumference is again indicated at 1 6'.

Figure 6 shows another embodiment. A spoiler 21 is in this case of T section, with a cylindrical portion 22 connecting the spoiler 21 to the tubular part 2 of the sleeve 1. The annular flange parts 23, 23' project radially outwardly and inwardly form the cylindrical part 22 into the flow S, the heights h and h' each again conforming at least approximately to the condition that it should be greater than or equal to the local boundary layer at maximum towing speed.

Lastly, Figure 7 shows an embodiment of flag 25 to be towed by an aircraft. The spoiler 26 which is of T section is in this case situated at the rear edge of the flag 25 and is fixed to it by a stem portion 27 while the parts 28 and 28' project by equal amounts h, h' into the air-stream near each respective surface of the flag 25, where hand h' > the local boundary layer at maximum towing speed. The lines of pressure and flow produced as the air flows round the towed target in flight are as indicated in Figure 4. In addition, in the embodiment of Figure 7, the parts 28, 28' which are transverse to the airflow have a series of parallel slots 30 enabling the flow to pass through them. Perforations could be provided instead of slots.

Although in the embodiments disclosed above only a single spoiler has been described, the invention includes the provision of more than one spoiler. The or each spoiler may be made of an elastic material.

Claims (9)

Claims

1. Device for towing by an aircraft, for example a towed sleeve or flag target, the device having one or more than one spoiler arranged in the region of the rear end of the device so as in use to extend into the airflow over the surfaces of the wall of the device.

2. Device according to claim 1, wherein the height to which the or each spoiler extends from the device is equal to or greater than the local boundary layer at maximum towing speed.

3. Device according to claim 1 or claim 2, wherein the or each spoiler is of an elastic material.

4. Device according to any of claims 1 to 3 having a tubular sleeve part, wherein a spoiler is formed by an annulus which is L-shaped in section and comprises a connecting piece and an annular flange which forms the spoiler edge extending into the external airflow.

5. Device according to any of claims 1 to 3, having a spoiler which is T-shaped in section, having annular flange portions forming spoiler edges extending into the external and internal airflows respectively.

6. Device according to any of claims 1 to 3, having a tubular sleeve part, wherein the spoiler is frusto-conical and widens out in the form of a funnel in the direction of airflow.

7. Device according to any of claims 1 to 3 having a flag part, wherein a spoiler is T-shaped in section with the stem of the T attached to the end of the flag part, the parts forming the crossbar of the T extending into the airflows on both sides of the flag part.

8. Device according to claim 7, wherein the parts forming the crossbar have perforations for air tob pass through.

9. Device for towing by an aircraft, constructed and arranged substantially as herein described and shown in Figures 3 to 7 of the drawings.