GB2345113A - Energy absorbing shield - Google Patents

Abstract

An energy absorbing shield (2) is of sandwich construction and comprises a thick plate (4), covered on at least one side by a honeycomb layer (8). A thin skin (10) is provided, adjacent at least one side of the thick plate (4) and covering the honeycomb layer (8).

Description

ENERGY ABSORBING BARRIER OR SHIELD The invention relates to an energy absorbing barrier or shield. In particular the invention concerns a substantially planar barrier or shield capable of absorbing a high energy impact..

The field in which the present invention finds useful application is the protection of a critical part of an aircraft structure and/or its occupants from damage caused by uncontained high energy debris, such as a released fan blade. In the event of a compressor or fan blade failure it may be necessary to provide protection to critical areas of an aircraft for example the pilot or a second engine. Usually this is achieved by ensuring the compressor or fan section casing is thick enough to contain the blade.

For high blade energies this results in the casing being thicker and therefore heavier than it would otherwise have been. On single engine military aircraft extra weight is undesirable and, in order to minimise weight, high energy debris containment may be restricted to protecting smali critical areas of the aircraft. If only small areas are to be protected a purpose built shield may mean that the engine casing thickness can be reduced to a nominal value not influenced by containment requirements. For large diameter fans this can offer significant weight savings compared to providing a fully reinforced fan casing.

According to one aspect of the present invention there is provided an energy absorbing barrier having a sandwich construction comprising a thick substrate, a thin outer skin and secured therebetween an intermediate honeycomb layer.

Preferably the substrate layer, outer skin and honeycomb are formed of metal or metal alloy materials, but not necessarily the same materials. Furthermore the wall of the intermediate honeycomb layer are upstanding from and bonded to a surface of the substrate, and the outer skin is shaped to encapsulate the honeycomb and its margins are joined to the substrate along all sides so as to fully constrain the honeycomb.

The invention will now be described in greater detail with reference, by way of examp ! e only, to the embodiments illustrated in the accompanying drawings, in which: Figure 1 shows a section of an energy absorbing shield according to the invention, partly cutaway to reveal interna construction; Figure 2 is a sketch of a VTOL aircraft showing a shield of Figure 1 placed immediately behind the pilot's cockpit, and Figure 3 shows a section of a double-sided energy absorbing shield.

Referring firstly to Figure 1 the shield 2 comprises a stiff backing plate 4 in the form of a rectangular flat plate the stiffness of which is achieved by reason of its thickness. To one surface 6 of the backing plate 4 there is bonded a layer of honeycomb celles generally indicated at 8 which cover the entire surface 6 of the plate. The honeycomb layer 8 is encapsulated by a thin membrane 10 made in the form of a rectangular envelope the margins of which along all four edges are joined to the margins of the backing plate 4. The thin membrane fully encapsulates the honeycomb and prevents the structure disintegrating in the event of an impact. The honeycomb 8 is bonded to the backing plate 4 but not to the covering thin membrane 10 to avoid the bond joint being stressed if the two large metal surfaces, spaced apart by the honeycomb 8, move relative to one another during use for example due to thermal growth effects.

For the purpose of aeroengine fan blade containment metals or metal alloys were used throughout the shield construction. In a preferred embodiment, described further below with reference to Figure 2, the backing plate comprises a steel alloy of half inch thickness, the rectangular dimensions to suit the installation. The membrane 10 comprised a sheet of similar alloy. The honeycomb 8 comprises extruded aluminium having a cell width 0.125 inch and a wall thickness of 0.002 inch and a cell length 3 inch, thus the honeycomb layer is 3 inch thick and is upstanding from the surface 6 of the backing plate 4. In the preferred embodiment the aluminium honeycomb layer is bonded to the backing plate 4 using an epoxy adhesive.

The backing plate 4 is pierced by an aperture 12 by means of which the interna volume encapsulated by the thin membrane 10 is vented to ambient atmosphere.

Since the membrane 10 is attached to the substrate or backing plate 4 along its entire perimeter its intemal volume is sealed other than through the vent aperture 12. In the absence of special manufacturing arrangements air at atmospheric pressure will, thus, be trapped inside the honeycomb. Without a venting arrangement this pressure could cause the membrane 10 to inflate and ultimately rupture when extemal pressure is reduced, such as would occur within the unpressurised spaces of an aircraft fuselage at high altitude. Such an occurrence would severely diminish the structural integrity of the shield, as well as be potentially dangerous itself. It is considered sufficient to provide a vent aperture 12 in the backing plate 4 of size which provides an evacuation path via a few neighbouring cells in the honeycomb layer. The membrane 10 is not itself attached to the honeycomb layer which should ensure that there exist adequate leakage paths between cells. In order to avoid the ingress of dirt, debris or liquids a fine grid mesh or gauge 14 is provided to cover the vent aperture 12. A mesh cover composed of thin gauge metal wire could be welded to the backing plate around the margin of the vent apertures.

\ In use the shield is positioned in front of a critical area and has overall dimensions such that a likely debris impact will have a high angle of incidence relative to the exposed shield surface. Tests have shown that debris is caught and drawn into the structure, the majority of the impact energy being absorbed by the honeycomb, thereby lowering the likelihood of high energy debris being deflected and potentially damaging neighbouring areas. The backing plate also suffered minimum damage. A shield of the dimensions quoted above successfully contained debris having an energy of 220,000 ft. lbf. In circumstances where debris will have lower energy levels the thickness of the backing plate and the depth of the honeycomb layer may be reduced and/or alternative materials such as titanium and possibly aluminium may be used in place of the steel alloy.

Referring now to Figure 2 a typical installation of the shield is illustrated in a VTOL aircraft. In the envisaged installation the vertical lift capability of the aircraft is provided by a lift engine or main engine driven lift fan generally indicated at 20 which is located in the aircraft fuselage 22 a short distance behind the cockpit 24. The lift fan 20 comprises two large diameter fan stages 26,28 which are disposed on a common rotational axis which is"normally"vertical. As a consequence of this vertical axis disposition of the lift fan the two rotary fan stages 26,28 occupy parallel planes which intersect the volume of cockpit 24. Thus making it desirable to protect the pilot, and other potential occupant, in the event of a failure occurring which results in the release of one or more of the fan blades. A shield 2 is therefore located between the lift fan or engine 20 and the cockpit 24 to absorb the energy of an incident fan blade.

Another embodiment of the invention is illustrated in Figure 3 comprising a doublelayered construction suitable for installation where potential impact could occur from either side of the backing plate 4. In the same manner as the example illustrated in Figure 1 a honeycomb layer 8a, 8b is attached to opposite faces 6a, 6b respectively of the backing plate 4. Each honeycomb layer is then encapsulated by a thin outer membrane 10a, 10b. Such an embodiment may find useful application where it is desirable for example to create a barrier between two engines mounted side-by-side.

Thus, in the event of one of the engines suffering an incident in which, say, a fan blade is shed on a trajectory encroaching on the other engine a shield of the kind shown in Figure 3 located between the engines may intercept the blade and prevent is impact on said other engine.

Claims (12)

1. CLAIMS 1 An energy absorbing barrier having a sandwich construction comprising a thick substrate, a thin outer skin adjacent at least one face of the substrate and secured therebetween an intermediate honeycomb layer.
2. 2 An energy absorbing barrier as claimed in claim 1 wherein the walls of the honeycomb are upstanding from and bonded to the face of the substrate.
3. 3 An energy absorbing barrier as claimed in either claim 1 or claim 2 wherein the substrate comprises a flat plate.
4. 4 An energy absorbing barrier as claimed in any preceding claim wherein the substrate layer, outer skin and honeycomb are formed of metal or metal alloy materials.
5. 5 An energy absorbing barrier as claimed in any preceding claim wherein the substrate comprises a thick plate of a steel alloy.
6. 6 An energy absorbing barrier as claimed in claim 5 wherein the thin outer skin , comprises a membrane of a steel alloy.
7. 7 An energy absorbing barrier as claimed in any preceding claim wherein the honeycomb comprises an aiuminium extrusion.
8. 8 An energy absorbing barrier as claimed in any preceding claim wherein the outer skin is shaped to encapsulate the honeycomb against the substrate.
9. 9 An energy absorbing barrier as claimed in claim 8 wherein the margin of the outer skin is joined to the substrates along all sides so as to fully constrain the honeycomb.
10. 10 An energy absorbing barrier as claimed in any preceding claim further including means to vent the interior of the honeycomb to atmosphere.
11. 11 An energy absorbing barrier as claimed in claim 10 wherein the vent means contains a fine mesh to avoid ingress of liquid.
12. 12 An energy absorbing barrier substantial as hereinbefore described with references to the accompanying drawings.