The invention relates to a device installed at the rear in an air-cushion vehicle for generating thrust and lift po¬ wer, comprising an engine, a propeller driven by the engine and installed on top of the vehicle, an axial compressor likewise driven by the engine and installed in the vehicle, an intake opening, an intake channel connecting the intake opening to the compressor, and an exhaust duct widening in the direction of flow, which duct connects the outflow opening of the compressor to a plenum chamber present bene- ath the vehicle and bounded on all sides by a skirt.

It is known in principle to use a single power source in an air-cushion vehicle (hovercraft) for both the forward propulsion and for generating an air cushion whereby the vehicle is carried. On account of the weight and complexity of such an installation it is of great importance that it is given a compact form and that the transmission of forces over large distances is avoided. The compact embodying of such a device has the additional advantage that the device can take the form of a simply exchangeable module, whereby the performing of maintenance tasks is greatly simplified.

A problem preventing compact construction of a device for generating thrust and bearing power is that for noise rea¬ sons a propulsion installation is preferably placed as far to the rear of a vehicle as possible (in the attempt to "sail ahead of the noise") , while or. the other hand, from considerations of flow technique, there is little point in installing the compressor used for generating the air cushi¬ on at the rear of the vehicle. It is not a simple matter, making use of a compressor placed at the rear of a vehicle, to draw in the outside air flowing along the vehicle, to

compress and distribute it uniformly in a plenum chamber present under the whole of the vehicle without this entai¬ ling great losses.

A _. possible way to solve this problem is to use a centri- fugal compressor for generating the air cushion. In a cen¬ trifugal compressor the flow direction of the air for com¬ pressing is deflected through an approximate right angle whereby the air sucked in at the rear of the vehicle can be guided forward into the plenum chamber. A drawback to the use of a centrifugal compressor is however that such a compressor is large and heavy, whereby space and lift power for transporting useful cargo is lost. The efficiency of a centrifugal compressor is in addition very low and such a compressor is expensive. An axial compressor may therefore often be employed for generating the air cushion. Axial compressors are lighter, smaller, more efficient and cheaper than centrifugal com¬ pressors. The use of an axial compressor requires however that both the direction and the section of the channel through which the air is guided over the compressor into the plenum chamber has a fluent progression in order to limit flow losses as far as possible. In an air-cushion vehicle according to the prior art wherein this approach has been adopted, the Hovercraft Development Ltd. HD2, the two axial compressors used are oriented with their suction side to the rear of the vehicle. A great drawback to this arrangement is that the compressors draw in the highly turbulent air from the wake of the air-cushion vehicle and its propulsion installation whereby it is practically impossible to main- tain a constant pressure in the plenum chamber present under the vehicle.

The present invention has for its object to provide a device installed at the rear in an air-cushion vehicle for generating thrust and lift, wherein the above stated draw- backs are obviated.

This is achieved according to the invention in that the intake opening is situated in front of the compressor seen

in the direction of forward movement and the exhaust duct comprises a bend portion running behind the vehicle wherein the course of the duct undergoes a change of direction of "roughly 180° . In a preferably applied embodiment of the device accord¬ ing to the invention the intake opening is placed so far to the front of the vehicle that the air flow sucked in by ^' the compressor is substantially laminar. The bend portion of the exhaust duct can be provided with guide vanes following the line of the bend.

In a preferred embodiment of the device the propeller and compressor are installed in a self-supporting frame, which frame is releasably attached to the vehicle.

The engine preferably drives a single output shaft, this shaft driving both the propeller and the compressor over a transmission. The shaft can have a length such that a main¬ tenance space is formed between the engine and the transmis¬ sion.

Mentioned and other features of the device according to the invention are elucidated hereinafter on the basis of an embodiment, while reference is made to the annexed drawing, wherein corresponding reference numerals designate corres¬ ponding parts, and: fig. 1 shows a view of an air-cushion vehicle provided with a device according to the invention; fig. 2 is a partly cross sectional side view of the device according to the invention; fig. 3 is a partly cross sectional view along line III- III in fig. 2; and fig. 4 is a view of the self-supporting frame.

In fig. 1 can be seen how in the rear of an air-cushion vehicle 1, the direction of forward ^' ovement of which is indicated with an arrow "V", is arranged a device 2 for generating thrust and lift consisting of an engine (not shown here) , a propeller 4 placed on the top of the vehicle 1 and an axial compressor 5 installed in the bottom of the vehicle. The air required for generating the air cushion

bearing the vehicle 1 is drawn in through an intake opening 6 placed in front of the axial compressor 5, compressed in the axial compressor 5 and guided through a widening exhaust duct 8 having a bend portion 11 therein to a plenum chamber 5 10 present under the vehicle 1.

The intake opening 6 is preferably placed so far forward that the indrawn air flow, which comprises the boundary layer flow of the superstructure 16 of vehicle 1, is sub¬ stantially laminar. The optimal position of the intake

10 opening 6 can be determined by calculation or wind tunnel testing. Even if it is not possible to place the intake opening 6 so far forward that laminar air flow is drawn in, placing of the intake opening 6 in front of the compressor 5 nevertheless offers advantages compared to placing in the

15 rear part of the vehicle 1, without other aspects of the design thereby being adversely affected, because, as a result of the air required by the device 2 being drawn in, the boundary layer flow of the vehicle 1 is sucked off whereby the resistance of the vehicle 1 is decreased.

20 Shown in more detail in fig. 2 is the combined thrust and lift generating device 2. This figure shows clearly how the air flow (indicated here with arrows 17) is sucked in through the intake opening 6 in front of the compressor 5 and undergoes only a small change in direction in an intake

•25 channel 7 prior to being compressed in the compressor 5. ^' Because the intake channel 7 is formed by the engine room, the air ^' drawn in in this manner is also used advantageously as cooling air for the engine 3. The compressed air flow leaving the compressor 5 (indicated with the arrows 18) is

30 guided into an exhaust duct 8 which widens in the flow direction and wherein a bend portion 11 is arranged running at the rear of the vehicle 1, whereby the flow direction of the air is deflected through an angle of approximately 180° and the air flow 18 is guided into the plenum chamber 10

35 located beneath the vehicle 1. Guide vanes 12 arranged in the bend portion 11 reduce flow losses in the bend portion 11. Owing to the continually enlarging passage opening of

the widening exhaust duct 8 the air flow 18 is slowed down and the static pressure in the duct 8 increases until the static pressure required to carry the vehicle 1 on an air cushion is finally reached in the plenum .chamber 10. The engine 3 has a single output shaft 14 which via a transmission 15 drives both the propeller 4 and the compres¬ sor 5. The length of the shaft 14 is such that between the engine 3 and the frame 13 there is sufficient space for performing maintenance activities. Also shown in fig. 2 is the self-supporting frame 13 which is releasably attached to the vehicle 1 and wherein the propeller and the compressor 5 are installed. The frame 13 comprises a horizontally running box 19 to which is fixed a vertically positioned body 20 in which the drive belt 21 of the propeller 4 is arranged and to which bearing blocks 22 and 23 are fixed. The tensile force of the drive belt 21 is wholly absorbed into the body 20 due to this constructi¬ on. The frame 13 further comprises a duct 24 around the propeller 4, a housing 25 for the compressor 5 and panels 27, 28, 31 running in lengthwise and transverse directions for transferring the loads into the hull 30 of the vehicle 1. The transverse panels 27, 28 moreover form the connection between the compressor housing 25 and the rest of the frame. The vertical transverse panels 27, 28 are fixed with bolts 34, 35 to supporting beams 29 for the engine 3.

Fig. 3 shows the rear side of vehicle 1. Here can be seen how the rear transverse panel 28 of frame 13 is fixed with bolts 35 to the engine supporting beams 29. The lengthwise panels 31 are fixed together with bollards 26 to stringers 38 by means of bolts 36, 37. If this should be required for performing maintenance work, the frame 13 can therefore be simply removed from the vehicle 1 by releasing the drive shaft 14 shown in fig. 2 and the bolts 34, 35, 36, 37, whereafter frame 13 can be lifted in its entirety out of - vehicle 1. The transverse panels 27, 28 are provided with horizontal part seams 32, 33. The upper and lower portions of the panels 27, 28 are mutually connected by strips 39

fixed onto both portions. By releasing the strips 39 the transverse panels 27, 28 can be separated over the seams 32, 33 whereby the compressor housing 25 can be taken out of the panels 27, 28. In fig. 4 can be clearly seen how the self-supporting frame 13 is constructed round a stiff, closed box 19 on which the propeller duct 24 is fixed and through which the body 20 is guided. Fixed to the box 19 are the lengthwise panels 31 and the transverse panels 27, 28, in which trans- verse panels 27, 28 recesses are arranged for receiving the compressor housing 25. When they are built on the stringers 38 the horizontal portions of the lengthwise surfaces 31, on which the bollards 26 are also arranged, transmit to the hull 30 of vehicle 1 the lengthwise and transverse forces of the device 2 for generating thrust and lift, while the transverse panels 27, 28 transfer the moments generated by the device 2 into the engine supporting beams 29.

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