TECHNICAL FIELD 
     The present invention relates generally to crash attenuation systems and specifically to crash attenuation systems for use in aircraft. 
     DESCRIPTION OF THE PRIOR ART 
     Currently internal airbags are used in the automotive industry within the occupied volume to mitigate occupant injuries. Similarly, external airbags have been used to attenuate decelerative loads to air and space vehicles, such as escape modules, upon contact with the ground or water. Examples include the NASA Mars Rovers and the crew module of the General Dynamics/Grumman F-111. 
     During impact, the gas in the airbag must be vented to prevent gas pressurization and subsequent re-expansion, which may cause the occupant to accelerate backward. This effect is commonly known as rebound. In addition, the gas may be vented to prevent over-pressurization, which can cause failure of the airbag. Venting may be accomplished, for example, through discrete vents or through a porous membrane that forms at least a portion of the skin of the airbag. 
     One shortcoming of prior external airbag systems is that they fail to prevent post-impact pitch-over, or “tumbling,” of an aircraft having a forward and/or lateral velocity at impact with a hard surface. For example, referring to  FIGS. 1   a - 1   e , an aircraft  10  that is equipped with a prior external airbag system  12  is shown at different points during a crash sequence from (a) to (e). The crash sequence involves the aircraft  10  having both forward and downward velocities at (a) and (b). The airbag system  12  properly deploys its airbags  14  at (b), but still incurs serious damage due to pitch-over of the aircraft  10  as shown at (d) and (e). Thus, improvements are still needed in external airbag systems, particularly improvements to the pitch-over stability of an aircraft equipped with an external airbag system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1   a - 1   e  show a crash sequence for a helicopter equipped with a prior external airbag system; 
         FIG. 2  is a perspective view of a helicopter equipped with an external airbag system; 
         FIG. 3  is a perspective view of an airbag used with the external airbag system shown in  FIG. 2 ; 
         FIGS. 4   a - 4   c  are a cross-sectional views of a vent valve in full-open, partially-open, and closed configurations; 
         FIG. 5  is a diagram of the vent plate shown in  FIGS. 4   a - 4   c;    
         FIG. 6  is block diagram of the helicopter shown in  FIG. 2 ; 
         FIG. 7  is a block diagram illustrating the operation of the crash attenuation system of the helicopter shown in  FIG. 2 ; 
         FIG. 8  shows a chart of exemplary data representative of a relationship between airspeed of the helicopter and open vent area; 
         FIGS. 9   a - 9   d  show a crash sequence for a helicopter equipped with an external airbag system according to the present disclosure; 
         FIG. 10  shows a cross-sectional view of an airbag of the external airbag system of the present disclosure; and 
         FIG. 11  shows a perspective view of a helicopter equipped with an alternative external airbag system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides for an inflatable crash attenuation system for aircraft. The system comprises an airbag that is inflated prior to impact and controllably vented during impact so as to prevent aircraft pitch-over. The present invention may be used on all models of aircraft, for example, helicopter, fixed wing aircraft, and other aircraft, and in particular those that are rotorcraft. The system of the invention improves on the prior art by providing automatic control of the venting valves based on sensed crash conditions, thereby effectively shifting the center of impact pressure and preventing aircraft pitch-over. 
       FIG. 2  shows a helicopter  100  incorporating the crash attenuation system according to the present invention. Helicopter  100  comprises a fuselage  102  and a tail boom  104 . A rotor  106  provides lift and propulsive forces for flight of helicopter  100 . A pilot sits in a cockpit  108  in a forward portion of fuselage  102 , and a landing skid  110  extends from a lower portion of fuselage  102  for supporting helicopter  100  on a rigid surface, such as the ground. 
     A problem with rotor  106  or the drive system for rotor  106  may necessitate a descent from altitude at a higher rate of speed than is desirable. If the rate is an excessively high value at impact with the ground or water, the occupants of helicopter  100  may be injured and helicopter  100  may be severely damaged by the decelerative forces exerted on helicopter  100 . To reduce these forces, an airbag assembly  111  comprising inflatable, non-porous airbags  112 ,  114  is installed under fuselage  102 . Though not shown in the drawings, airbags  112 ,  114  are stored in an uninflated condition and are inflated under the control of a crash attenuation control system (described below). 
       FIG. 3  is an enlarged view of airbag  112 , which has a non-porous bladder  116 , which is sealed to a housing  117  having a plurality of discrete vents  118 . Airbag  112  is shown in  FIG. 3 , but it should be noted that airbags  112  and  114  can have generally identical configurations. In a preferred embodiment, the bladder  116  is formed of a fabric that comprises Kevlar and/or Vectran. Vents  118  communicate with the interior of bladder  116 , allowing for gas to escape from within the airbag  112 . In the embodiment shown, vents  118  are open to the ambient air, though vents  118  may be connected to a closed volume, such as another airbag or an accumulator (not shown). Also, while a plurality of vents are shown in the embodiment illustrated in  FIG. 3 , alternative embodiments can include only a single vent  118 . 
     Referring to  FIGS. 4   a - 4   c , each vent  118  has a vent valve  120  for controlling the flow of gas through vent  118 . Vent  118  and vent valve  120  together form a vent passage  122  for channeling gas flowing out of airbag  112 . Each vent valve  120  is sealingly mounted in housing  117  (or bladder  116  in some embodiments) to prevent the leakage of gas around vent  118 , which forces venting gas to flow through passage  122 . A vent plate  124  is configured to be moveable between an open position, for example shown in  FIG. 4   a , at least one intermediate position, for example as shown in  FIG. 4   b , and a closed position, for example as shown in  FIG. 4   c .  FIG. 4   a  shows vent plate  124  in the open position, or open state, in which a maximum amount of gas is allowed to flow through passage  122  from within airbag  112 .  FIG. 4   b  shows vent plate  124  in an intermediate position, or intermediate state, in which a selected amount of gas less than the maximum is allowed to flow through passage  122  from within airbag  112 .  FIG. 4   c  shows vent plate  123  in the closed position, or closed state, in which gas is prevented from flowing out of airbag  112  through the passage  122 . Though only a single intermediate position is shown, it should be understood that various additional intermediate positions can be selected in order to control the amount of gas that is allowed to escape from within the airbag  112  through the vent  118 . Also, while the vent valve  120  is shown as a sliding valve, it will be understood by one skilled in the art that vent valve  120  may alternatively be other suitable types of valves. Control of vent valves  120  may be accomplished though any number of means, including, for example, electrorheological means. In some embodiments, the vents  118  can be sealed with an optional pop-off pressure release mechanism, preferably a pressure sensitive fabric  125 . In such embodiments, once the fabric  125  pops off, the vent valve  120  controls release of the pressurized air inside the airbag  112 ,  114 . 
     Referring next to  FIG. 5 , as will be discussed in greater detail below, each vent plate  124  can be selectively positioned to any position between a full open position and a full closed position. In the view shown in  FIG. 5 , the hatched area  127  represents the open vent area, through which gas can escape from within an airbag  112  or  114  through passage  122 . The vent plate can be moved a distance A according to a desired amount of open vent area  127 . The open vent area  127  will be a total open vent area “S” if there is only one vent  118 ; otherwise, the open vent area  127  of each vent  118  is summed to be a total vent area “S.” The total vent area S is a function of crash conditions:
 
 S=f ({dot over ( x )}, {dot over ( z )}, θ, φ, {dot over (θ)}, {dot over (φ)}, . . . )
 
where {dot over (x)} is represents forward velocity, ż represents downward or sink velocity, θ represents pitch angle, φ represents roll angle, {dot over (θ)} represents pitch rate, and {dot over (φ)} represents roll rate.
 
       FIG. 6  shows airbags  112  and  114  mounted to a lower portion of fuselage  102  and show additional components of the crash attenuation system according to the present disclosure. A computer-based control system  126 , which is shown mounted within fuselage  102 , is provided for controlling the operation of components associated with airbags  112 ,  114 . Each airbag  112 ,  114  has a gas source  128 , such as a gas generator, for inflation of the airbags  112 ,  114 . In some embodiments, a secondary gas source, such as compressed gas tank (not shown), can be provided for post-crash re-inflation of airbags  112 ,  114  so that the airbags  112 ,  114  can be used as floatation devices in the event of a water landing. The gas source  128  may be of various types, such as gas-generating chemical devices or compressed air, for providing gas for inflating airbags  112 ,  114 . In addition, the crash attenuation system has a sensor system  130  for detecting crash conditions used to determine the total vent area S, such as rate of descent and/or ground proximity. Airbags  112 ,  114  can also have a water-detection system (not shown), which may have sensors mounted on fuselage  102  for detecting a crash in water. Gas source  128 , vent valves  120 , and sensor system  130  are in communication with control system  126 , allowing control system  126  to communicate with, monitor, and control the operation of these attached components. In addition, control system  126  may be in communication with a flight computer or other system for allowing the pilot to control operation of the crash attenuation system. For example, the pilot may be provided means to override, disarm, or arm the crash attenuation system. 
     The sensor system  130  is shown in  FIG. 6  as a discrete component for the sake of convenience. However, it should be noted that actual implementations of the sensor system  130  can comprise a number of components that are located at various locations on the helicopter  100 . For example, the sensor system  130  can include, for example, sensors for detecting pitch and roll attitude, pitch and roll rate, airspeed, an altitude, and rate of descent. 
     Referring next to  FIG. 7 , an exemplary embodiment of the sensor system  130  is configured to detect various crash conditions, which can include, for example, one or more of the sink speed, forward speed, pitch and roll attitude, pitch and roll rate, and proximity to the ground of the helicopter  100 . The control system  126  receives data from the sensor system  130  representative of the detected crash conditions. In a preferred embodiment, the control system  126  is a microprocessor-based system configured to operate as a crash predictor. When excessive oncoming velocity of the ground within a certain altitude range is detected by the control system  126 , the gas source  128  is triggered to inflate the airbags  112 ,  114  (indicated at box  126 A) prior to impact of the helicopter  100  with the ground. At the same time, the control system  126  activates the vent valves  120  to adjust the open vent area based on an active vent valve algorithm as indicated at box  126 B. 
       FIG. 8  shows an example of a relationship that can be used by the control system  126  for adjusting the open vent areas at  126 B. In  FIG. 8 , a chart is shown that illustrates a relationship between open vent area and forward velocity of a helicopter for a given sink velocity of  36  feet per second. The line  134  maps open vent areas to forward velocities for the forward airbag  112 , while the line  136  maps open vent areas to forward velocities for the aft airbag  114 . It should be appreciated that the relationship will vary for different sink velocities. The relationship will also vary depending on a number of other factors, for example aircraft characteristics, such as aircraft weight and balance, and the number and characteristics of the airbags. The data can be determined using known flight simulation techniques, for example simulation software, for simulating crash results. Using such techniques, data can be collected based on simulation of crash results for various crash conditions and open vent areas. 
       FIGS. 9   a  through  9   d  illustrate operation of the crash attenuation system. In operation, if an impending crash is sensed by sensor system  130 , for example, by excessive oncoming rate of the ground within a certain attitude range, control system  126  triggers gas source  128  to inflate airbags  112 ,  114  at the appropriate time to allow inflation just as airbags  112 ,  114  contact the impact surface (ground or water). 
       FIG. 9   a  shows an impending crash onto ground  132 , which is sensed by the control system  126  based on data received from the sensor system  130 . At  FIG. 9   b , gas source  128  is triggered, causing airbags  112  and  114  to inflate just prior to contact with ground  132 . The control system  126  also calculates the open vent areas for each of the airbags  112 ,  114 . In this case, the control system  126  determines that the crash conditions correspond to the line  138  shown in  FIG. 8 , which requires the open vent area of aft airbag  114  be greater than the open vent area of forward airbag  112 . Accordingly, at  FIG. 9   c  the open vent area of aft airbag  114  is set to an area of about 0.0205 square meters and the open vent area of forward airbag  112  is set to an area of about 0.0145 square meters. Thus, as shown in  FIG. 9   c , the aft airbag  114  deflates faster than the forward airbag  112 . As a result, as shown at  FIG. 9   d , the helicopter  100  comes to a stop without experiencing a pitch-over. 
     Referring next to  FIG. 10 , a cross-section of a preferred embodiment of an airbag  112 ,  114  is shown. The hatched area  140  represents the portion of the airbag  112 ,  114  that is adjacent to the underside of the fuselage  102 . The arrow  142  points towards the forward end of the helicopter  100 . The broken line  144  is the widest portion of the airbag  112 ,  114  between the top (hatched area  140 ) and bottom  146  of the airbag  112 ,  114 . As shown in  FIG. 10 , for a width W of the airbag at line  144 , the distance D 1 , which is the distance between the top  140  and the line  144 , and the distance D 2 , which is the distance between the bottom  146  and the line  144 , are equal and determined based on the following relationship: 
               D   ⁢           ⁢   1     ,       D   ⁢           ⁢   2     =     W     2   ⁢     3                 
This geometry maximizes crush distance for optimal energy absorption management. Also, the curved region  148  provides anti-plow, anti-scooping geometry to assist in preventing pitch-over of the helicopter  100 .
 
     Referring next to  FIG. 11 , an alternative embodiment of the helicopter  200  is shown. As mentioned above, while the present crash attenuation system has been discussed primarily in connection with two airbags  112 ,  114 , alternative embodiments can have additional airbags. For example, the helicopter  200  shown in  FIG. 11  has an airbag assembly  211  comprising four airbags  212 ,  213 ,  214 , and  215 . Like the helicopter  100 , the helicopter  200  comprises a fuselage  202  and a tail boom  204 . A rotor  206  provides lift and propulsive forces for flight of helicopter  200 . A pilot sits in a cockpit  208  in a forward portion of fuselage  202 , and a landing skid  210  extends from a lower portion of fuselage  202  for supporting helicopter  200  on a rigid surface, such as the ground. 
     A problem with rotor  206  or the drive system for rotor  206  may necessitate a descent from altitude at a higher rate of speed than is desirable. If the rate is an excessively high value at impact with the ground or water, the occupants of helicopter  200  may be injured and helicopter  200  may be severely damaged by the decelerative forces exerted on helicopter  200 . To reduce these forces, inflatable, non-porous airbags  212 ,  213 ,  214 , and  215  are installed under fuselage  202 . Though not shown in the drawings, airbags  212 ,  213 ,  214 , and  215  are stored in an uninflated condition and are inflated under the control of a crash attenuation control system. 
     The crash attenuation system of the helicopter  200  can operate as discussed above in connection with the helicopter  100 . In addition, compared to the helicopter  100 , the helicopter  200  provides additional lateral roll-over prevention capabilities. Each of the airbags  212 ,  213 ,  214 , and  215  is independently actively vented during a crash sequence. Thus, if the helicopter  200  is approaching the ground with a lateral velocity, the airbags  212  and  214 , which are located along one side of the helicopter  200 , can be vented more or less than the airbags  213  and  215 , which are located along the other side of the helicopter  200 , as necessary based on detected crash conditions in order to prevent the helicopter  200  from rolling over after impact with the ground. 
     The above disclosure describes a system and method for actively controlling the venting of external airbags based on sensed crash conditions, such as airspeed, sick speed, pitch attitude, roll attitude, pitch rate, and roll rate. This active venting of the external airbags causes different airbags located at different locations of an aircraft exterior to deflate at different rates upon impact, thereby shifting an aircraft&#39;s center of impact pressure. 
     While this invention has been described with reference to at least one illustrative embodiment, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description.
A crash attenuation system for an aircraft, the system having an airbag carried by the aircraft and inflatable generally adjacent an exterior of the aircraft. The airbag has at least one vent for releasing gas from the interior of the airbag. A gas source is in fluid communication with the interior of the airbag for inflating the airbag with gas generated provided by the first gas source. A vent valve is provided for controlling a flow of gas through each vent, each vent valve being selectively configurable between an open state, in which gas can pass through the associated vent from the interior of the airbag, any number of intermediate states, in which the vent is partially open, and a closed state, in which gas is retained within the interior of the airbag.