Abstract:
A fresh air inlet for an aircraft features a ram air inlet with a ram air inlet opening, a secondary air inlet opening separate from the ram air inlet and a movably mounted flap. The flap can be moved into a first or second position, and essentially covers the secondary air inlet opening in the first position and at least partially opens the secondary air inlet opening and extends away from the aircraft body in the second position to shield the ram air inlet opening from foreign matter in the air. The secondary air inlet reduces the pressure loss of the air inlet on the ground or during flight phases with relatively slow speed due to the enlarged cross-sectional surface. The ram air inlet can be optimized for cruising phases, and the energy expenditure for any downstream compressors and flow-induced noises can be considerably reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase entry under 35 U.S.C. §371 of International Application Ser. No. PCT/EP2009/056401, filed May 27, 2009, published in German, which claims the benefit of the filing date of United States Provisional Patent Application No. 61/130,451, filed May 30, 2008, and of German Patent application Ser. No. 10 2008 026 117.3, filed May 30, 2008, the entire disclosures of which are hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a fresh air inlet for an aircraft. 
     BACKGROUND OF THE INVENTION 
     Commercial aircraft require a fresh air supply. This air can be provided by various sources such as an engine compressor, an auxiliary power unit (“APU”) or other sources. It is common practice to supply environmental control systems in larger aircraft with bleed air from aircraft engines. Air-conditioning units of popular aircraft environmental control systems comprise one or more large-volume heat exchangers, through which ram air flows during the flight. The ram air is used for cooling the bleed air. The ram air is usually supplied through ram air ducts. Relatively modern commercial aircraft are provided with environmental control systems that can be operated without or with relatively little bleed air. In order to provide the relatively large air quantities required for pressurizing, ventilating and air-conditioning the cabin, it is practical to utilize relatively large fresh air inlets on the fuselage of the aircraft. Such fresh air inlets usually extend outward from an outer contour of the aircraft fuselage and have, for example, a curved, scoop-like shape that is also referred to as “ram air scoop” or “ram air inlet scoop.” 
     The shape of this type of fresh air inlets is designed for the cruising state that by far represents the longest flight phase, particularly for long-range commercial aircraft. In these flight phases, a relatively high speed of, for example, 0.8 Ma is reached such that a pronounced ram pressure is generated at the fresh air inlets despite the low air density at cruising altitude. The air flowing into an air duct arranged behind the fresh air inlet due to the ram pressure is preferably compressed further with the aid of a compressor in order to increase the air pressure to the value required for pressurizing the cabin and fed into a downstream network of ducts, from where it can be routed into the corresponding air treatment systems. In order to fulfill the specified acoustic requirements during ground operations—a certain air speed should, in particular, not be exceeded—and to minimize the pressure loss so as to save energy, it may be required to install one or more secondary air inlets that provide additional inflow cross sections on the ground in order to thusly reduce the flow resistance and to lower the average flow speed. However, the operation of these secondary air inlets requires mechanically actuated flaps or shutters that seal the secondary air inlets during the flight. 
     In the use of the above-described fresh air inlets, it is furthermore disadvantageous that these fresh air inlets need to be protected from dust and small objects being swirled around during operating phases near the ground, wherein this is usually realized with a deflector shield. This deflector shield is arranged, for example, in a pivoted fashion upstream of the fresh air inlet referred to the flow direction and able to absorb the kinetic energy of the foreign matter. This additional mechanical device increases the complexity of such a fresh air inlet system. 
     It is the object of the invention to propose a fresh air inlet for an aircraft that has the lowest complexity possible, a low weight and the fewest movable parts possible and ensures a sufficient supply of fresh air for an environmental control system of an aircraft that essentially operates without bleed air during flight and ground operations, namely with the lowest possible power demand of a downstream compressor. 
     This object is attained with a fresh air inlet for an aircraft that features at least one ram air inlet with at least one ram air inlet opening, at least one secondary air inlet opening that is spaced apart from the ram air inlet and at least one movably mounted flap, wherein the flap can be moved into a first position and into a second position, and wherein the flap essentially covers the secondary air inlet opening in the first position and at least partially opens the secondary air inlet opening and at least in certain areas extends into the air flow directed toward the ram air inlet opening in order to shield the ram air inlet opening from foreign matter in the second position. 
     In the fresh air inlet according to the invention, a deflector shield required for the ram air inlet is realized in the form of the flap that simultaneously serves for covering another secondary air inlet opening. The first position of the flap is assumed in flight phases, in which a relatively high flying speed is reached and the aircraft is located far above the ground. The secondary air inlet opening could be realized, for example, in the form of an opening that is located directly in an outer surface of the aircraft. This opening could be arranged, in particular, in an area on the underside of the aircraft fuselage, wherein it would be possible to realize, for example, an opening in the fairing of the wing-fuselage transition (“belly fairing”). In order to cover the secondary air inlet opening, the flap fits tightly against the edges of the secondary air inlet opening such that a largely smooth, continuous surface is created when the secondary air inlet opening is closed. In this case, the ram air inlet is not impaired by the flap such that the entire surface of the ram air inlet opening is available for taking in ram air. 
     In the second position of the flap, the secondary air inlet opening is not covered by the flap such that the entire secondary air inlet opening is available for taking an air from the surroundings. When the aircraft is on the ground and the environmental control system is switched on, air can be taken in from the surroundings through the secondary air inlet opening, as well as through the ram air inlet, and used by the environmental control system. In this context, it needs to be observed that the flap should be positioned at a sufficient distance from the ram air inlet such that no or only minimal fluidic influences occur between the secondary air inlet opening and the ram air inlet during ground operations. At slow speeds such as, for example, during takeoffs and landings of the aircraft, a considerably enlarged inlet opening for ambient air is also available for the ram air inlet due to the combination of a secondary air inlet flap and a deflector shield according to the invention. 
     The fresh air inlet according to the invention has a number of advantages in comparison with fresh air inlets known from the prior art. On the one hand, the pressure loss of the fresh air inlet can be reduced due to the increased cross-sectional surface on the ground or in flight phases with relatively slow speed. This results in a considerably lower energy expenditure for any downstream compressors or blowers. In addition, the intensity of flow-induced noises is reduced due to the more favorable flow characteristics. It is furthermore particularly preferred to essentially design the ram air inlet such that it is only used while cruising. Consequently, it is not necessary to make any compromises that would deteriorate the flow characteristics while cruising for the benefit of ground operations. 
     In one advantageous embodiment of the fresh air inlet according to the invention, the secondary air inlet opening is situated upstream of the ram air inlet. This favorably affects the simultaneous use of the flap as a cover for the secondary air inlet opening and as a deflector shield for the ram air inlet. This results from the fact that an upstream secondary air inlet opening is passed by the surrounding air flow first such that a flap for closing the secondary air inlet opening that is arranged in the vicinity of the secondary air inlet opening can be easily extended into the air flow flowing directly to the ram air inlet. 
     In an advantageous additional development of the invention, the flap is essentially located upstream of the ram air inlet and the secondary air inlet opening in its second position. This means that the entire air inlet surface between the flap and the ram air inlet is made available on the ground and the influence of the ram pressure on the secondary air inlet opening is minimized at slow speeds. This likewise makes it possible to realize a relatively large distance between the flap and the ram air inlet opening such that less fluidic interference effects occur at the ram air inlet. 
     In another advantageous additional development of the air inlet according to the invention, the flap is essentially located between the ram air inlet and the secondary air inlet opening in its second position. This makes it possible to reduce fluidic interferences between the air introduced into the secondary air inlet opening and the air flowing into the ram air inlet opening. In this case, it is particularly practical to adapt the flap to the shape of the ram air inlet because this could favorably affect a shorter distance of the flap from the ram air inlet and a reduced influence on the flow such that the secondary air inlet opening could also be positioned relatively close in front of the ram air inlet in order to reduce the flow resistances of downstream air ducts. Another favorable effect of this arrangement is described further below with reference to another advantageous additional development. 
     A preferred embodiment of the fresh air inlet according to the invention furthermore features a secondary air inlet duct that is connected to the secondary air inlet opening, as well as a ram air inlet duct that is connected to the ram air inlet opening and a fresh air duct, wherein the secondary air inlet duct and the ram air inlet duct can be connected to the fresh air duct. This makes it possible to combine the flows in the secondary air inlet opening and the ram air inlet such that the entire air taken in from the surroundings is available in a common fresh air duct and can be withdrawn, for example, by an environmental control system of the aircraft. 
     It is furthermore preferred that the flap can be moved into a third position, in which the flap at least partially extends into the air flow in order to route ram air into the secondary air inlet opening from a surface of the aircraft and at the same time essentially not impair the air flow that is directed toward the ram air inlet opening. Due to this constellation that is favorably affected, in particular, by a flap arranged between the secondary air inlet opening and the ram air inlet, an additional mass flow can be generated in certain flight phases and routed into the secondary air inlet opening in flight by means of the ram pressure. In this operating mode of the air inlet according to the invention, the flap is opened just so far that it does not yet impair the air flow directed toward the ram air inlet opening, but already protrudes into the air flow by such a distance that a clearly noticeable mass flow induced by the ram pressure is introduced into the secondary air inlet opening. This operating mode is referred to as “boost mode” below. 
     It is furthermore advantageous if the fresh air inlet according to the invention features a check valve that is arranged in the secondary air inlet duct. This makes it possible to prevent the air flow taken in through the ram air inlet from flowing back into the surroundings of the aircraft through the secondary air inlet opening due to the relatively high ram pressure at the ram air inlet, particularly when the secondary air inlet opening is closed. Since the secondary air inlet opening is merely subjected to the static ambient pressure but the ram air inlet opening is subjected to the dynamic ram pressure, a concise pressure differential would form between the two openings and could only be compensated with a very strong suction effect in the fresh air duct if no check valve would be provided. 
     It is furthermore preferred that the cross-sectional surface of the fresh air duct is at least identical to the sum of the cross-sectional surfaces of the secondary air inlet duct and the ram air inlet duct. This prevents the mass flow flowing into the fresh air duct from being subjected to an increased flow resistance and pressure peaks or vibration effects from occurring at the inlet of the fresh air duct. 
     In one particularly preferred embodiment of the air inlet according to the invention, an emergency ventilation duct is provided and can be connected to the secondary air inlet duct. In this way, it is possible to create or boost an emergency ventilation system that allows an emergency ventilation of the cabin on the basis of ram air being taken in. This leads to an additional reduction of the weight. The combination of secondary air inlet opening, ram air inlet and emergency ram air inlet is particularly advantageous when the flap is in its third position and additional ram air from the surroundings is also available through the secondary air inlet opening 
     It is furthermore particularly advantageous if a movably mounted emergency ventilation inlet flap is arranged in the emergency ventilation duct and deactivates the emergency ventilation through the secondary air inlet opening and the ram air inlet during the normal operation of the aircraft, wherein this emergency ventilation can, however, be reactivated on demand by opening the emergency ventilation inlet flap. 
     It is also preferred that the air inlet according to the invention furthermore features an actuator for moving the emergency ventilation inlet flap in order to open or close the connection between the emergency ventilation duct and the secondary air inlet duct. This would make it possible to automatically open the emergency ventilation inlet flap at the push of a button or automatically with a suitable controller or control unit. 
     In an advantageous embodiment of the fresh air inlet according to the invention, a compressor for conveying air is located in the fresh air duct. The compressor is primarily required for compressing the fresh air due to the differential pressure of about 0.5 bar or more between the surroundings of the aircraft and the cabin while cruising, as well as the ducts and air treatment systems that create an additional flow resistance. Since the entire inlet opening surface of the secondary air inlet opening and the ram air inlet opening is available, the compressor can have a relatively low power demand in comparison with the prior art because the flow resistance is clearly reduced due to the relatively large inlet opening surface. 
     It is furthermore advantageous if the flap is pivotably mounted on a hinge. This is particularly simple with respect to mechanical considerations and can be realized with numerous commercially available, perfected and low-maintenance components. 
     It is also particularly advantageous if the fresh air inlet according to the invention features an actuator for moving the flap that may be realized in the form of an electric, hydraulic or pneumatic actuator. This actuator could be positioned in the immediate vicinity of the flap and reduces the expenditures for mechanical actuating elements. 
     It is ultimately also advantageous if the fresh air duct, the ram air inlet duct and/or the secondary air inlet duct is designed for accommodating at least one heat exchanger. In this way, the need for additional structural space and the expenditures for integrating a heat exchanger of the environmental control system or other systems can be eliminated. This measure is primarily suitable for relatively small systems that do not impair the main function of the fresh air inlet and for which the air flow provided in the respective duct suffices. 
     The objective is ultimately also attained with the use of a fresh air inlet according to the above-described characteristics in an aircraft. The objective is furthermore attained with an aircraft with at least one above-described fresh air inlet according to the invention. 
    
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
       Other characteristics, advantages and possible applications of the present invention result from the following description of exemplary embodiments and the figures. In this respect, all described and/or graphically illustrated characteristics also form the object of the invention individually and in arbitrary combination regardless of their composition in the individual claims or their references to other claims. In the figures, identical or similar objects are furthermore identified by the same reference symbols. The figures show: 
         FIG. 1 : a schematic representation of a fresh air inlet according to the prior art with extended deflector shield; 
         FIG. 2 : a schematic representation of a fresh air inlet according to the prior art with retracted deflector shield; 
         FIG. 3 : a first exemplary embodiment of the fresh air inlet according to the invention with extended flap; 
         FIG. 4 : a second exemplary embodiment of the fresh air inlet according to the invention with extended flap; 
         FIG. 5 : the second exemplary embodiment of the fresh air inlet according to the invention with retracted flap; 
         FIG. 6 : the second exemplary embodiment with the flap in the boost mode; 
         FIG. 7 : a schematic representation of the effective ram air surface of the second exemplary embodiment in the boost mode; 
         FIG. 8 : a diagram for lowering the compressor power demand in the boost mode; 
         FIG. 9 : a third exemplary embodiment of the fresh air inlet according to the invention; 
         FIG. 10 : another schematic representation of the third exemplary embodiment with closed emergency ventilation inlet flap; and 
         FIGS. 11   a  to c: an overview of different realizable ram air inlet shapes. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  shows a fresh air inlet  2  according to the prior art that is located, for example, on the underside of an aircraft fuselage  4 . The fresh air inlet  2  features a ram air inlet opening  6  that points in the direction of flight  8 . A deflector shield  10  is arranged upstream of the ram air inlet opening  6  and mounted on the aircraft fuselage  4  such that it can be pivoted about a hinge  12 . An actuator  14  is connected to the deflector shield  10  in order to realize the pivoting movement thereof. In the illustration shown, the deflector shield  10  is extended and protrudes outward from the surface of the aircraft fuselage  4 . Objects that move toward the ram air inlet  2  opposite to the direction of flight  8  are held back by the deflector shield  10 . These objects may consist, for example, of foreign matter in the form of the particles, dust or the like. A ram air duct  16  is connected to the fresh air inlet  2  in order to convey ram air introduced into the ram air inlet  2  through the ram air inlet opening  6 . A compressor  18  located within this ram air duct  16  serves for taking in air from the surroundings during ground operations and for compressing the ram air being taken in and conveying this ram air to an environmental control system or the like during flight operations. 
       FIG. 2  once again shows the arrangement according to  FIG. 1 , but the deflector shield  10  is retracted in  FIG. 2 . Due to this measure, the ram air inlet opening  6  is able to take in ram air from the surroundings without being impaired by the deflector shield  10 . This is the case while cruising, namely when objects are not expected to fly around and damages to the ram air inlet  2  essentially can be precluded. 
       FIG. 3  shows a first exemplary embodiment of the fresh air inlet according to the invention. This figure shows a ram air inlet  20  that is connected to a ram air inlet duct  22 . A secondary air inlet opening  26  that is connected to a secondary air inlet duct  28  is arranged adjacently in the direction of flight  24 , i.e., upstream. A flap  30  is arranged upstream of the secondary air inlet opening  26  and pivotably mounted on an outer surface  34  of the aircraft by means of a hinge  32 . In the example shown, the flap  30  extends outward from the outer surface  34  of the aircraft and therefore is located in the air flow directed toward a ram air inlet opening  36 . The secondary air inlet duct  28  and the ram air inlet duct  22  are connected to a fresh air duct  38 , wherein the cross-sectional surface (A 3 ) of the fresh air duct  38  at least corresponds to the sum of the surfaces of the secondary air inlet duct  28  (A 1 ) and the ram air duct  22  (A 2 ). In this way, a largely optimal continuity of the flow from the two different sources shown is realized. 
     The constellation shown with the extended flap  30 , the opened secondary air inlet opening  26  and the ram air inlet  20  arranged downstream of the flap  30  applies, in particular, to the situation on the ground or during takeoffs and landings of the respective aircraft. In this case, the flap  30  acts as a deflector shield as described above with reference to the prior art in  FIGS. 1 and 2 . Since the ram air inlet opening  36  is subjected to no ram pressure or only a low ram pressure on the ground or when the aircraft flies near the ground at slow speeds, the available inlet opening surface for taking in pressure from outside can be considerably enlarged due to the extended flap  30 . This lowers the flow resistance at the air inlet point into the aircraft such that the power demand of a not-shown compressor can be reduced. 
       FIG. 3  furthermore shows a check valve  40  that serves for preventing ram air flowing into the ram air inlet opening  36  from once again flowing back into the surroundings through the secondary air inlet duct  28  and the secondary air inlet opening  26 . This backflow could occur when the aircraft is in motion and the ram air inlet opening  36  is subjected to a noticeable ram pressure such that the overall pressure at the ram air inlet opening  36  exceeds the static ambient pressure at the secondary air inlet opening  26 . 
       FIG. 4  shows a second exemplary embodiment, in which the flap  30  and the hinge  32  and rotary actuator  33  are located between the secondary air inlet opening  26  and the ram air inlet opening  36 . This figure also shows a constellation that is used while the aircraft is on the ground or during takeoffs and landings. The flap  30  serves as a deflector shield for the ram air inlet  20  and prevents objects that are swirled around and could destroy the ram air inlet  20  from being taken in. 
       FIG. 4  furthermore shows a blower or compressor  42  that conveys the air from the two openings  26  and  36  to an environmental control system or another consumer via the fresh air duct  38 . Possible heat exchanger positions are labelled  41 . 
       FIG. 5  shows the second exemplary embodiment with a closed flap  30  as it is used during normal cruising. The outer surface  34  is as smooth as possible in the region of the secondary air intake opening  26  such that no additional flow resistances are created in this region and the ram air inlet  20  can take in ram air from the surroundings in an unimpaired fashion. 
     Another variation of the operation of the second exemplary embodiment is illustrated in  FIG. 6 . In this case, the flap  30  is in a third position that lies between a completely closed position (the “first position”) and a completely extended position (the “second position”). The flap  30  is extended just so far that the ram air inlet  20  is able to take in ram air from the surroundings in a largely or essentially unimpaired fashion, wherein the flap  30  extends, however, into the air flow such that an additional ram air effect is exerted upon the secondary air inlet opening  26 . The overall effective ram air surface of the air inlet according to the invention therefore is larger than merely the surface of the ram air inlet opening  36 . 
     The increase of the effective ram air surface is elucidated with reference to the schematic illustration in  FIG. 7 . In a section perpendicular to a longitudinal axis of the aircraft, it becomes clear that the original ram air surface formed by the ram air inlet opening  36  can be additionally enlarged with a flap  30  that is in the third position. In a top view of the ram air inlet  20 , the edge of the flap  30  that faces away from the outer surface  34  of the aircraft is located on the upper edge of the ram air inlet opening  36  and forms another ram air surface  44  that extends up to the outer surface  34  of the aircraft. In this way, the ram pressure upstream of the compressor  42  can be increased such that the compressors can be designed for a smaller range of pressure ratios than variations known from the prior art. A cabin compressor frequently reaches its performance limits, particularly at a maximum differential pressure between the passenger cabin and the surroundings. The fresh air inlet according to the invention can reduce the required power and pressure peaks by utilizing this so-called “boost mode.” This applies, in particular, to single-stage compressors with fixed blade geometry that are preferred for use in aircraft due to their simplicity and robustness. 
     Due to the additional ram pressure that is generated by means of the increased ram air surface realized with the partially opened flap  30 , a permissible pressure ratio of the cabin compressor is achieved. This is furthermore illustrated in  FIG. 8  with the aid of a diagram that shows the mass flow as a function of the pressure ratio of the compressor. A surge limit  46  defines the performance limits of a compressor and depends on the mechanical design thereof. This diagram shows, for example, that the increase of the effective ram air surface achieved with the flap  30  makes it possible to lower the pressure ratio from the operating point  48  to a new operating point  50  that lies within the realizable range below the surge limit  46  of the compressor used. 
       FIG. 9  shows a third exemplary embodiment of the air inlet according to the invention. In this case, an additional emergency ventilation duct  52  is integrated and connected to the secondary air inlet duct  28 . An emergency ventilation inlet flap  54  is located between the emergency ventilation duct  52  and the secondary air inlet duct  28  and movably mounted on a hinge  56  with rotary actuator 57  within the emergency ventilation duct  52 . Due to the arrangement shown, air can also be introduced into the emergency ventilation duct  52  through the secondary air inlet opening  26 , particularly when the flap  30  is in the “boost mode.” In this arrangement, it should be particularly emphasized that an additional check valve  58  is arranged within the ram air inlet duct  22 . In this way, ram air taken in through the secondary air inlet opening  26  is prevented from flowing back through the ram air inlet opening  36 . 
     The emergency ventilation inlet flap  54  within the emergency ventilation duct  52  may be realized in the form of a three-way valve such that a total of three different operating modes are available. On the one hand, the emergency ventilation inlet flap  54  may be in its position illustrated in  FIG. 9  such that ram air is introduced into the fresh air duct  38  and into the emergency ventilation duct  52  through the secondary air inlet opening  26 . 
       FIG. 10  shows another position of the emergency ventilation inlet flap  54  of the emergency ventilation duct  52 , in which the ram air flowing into the secondary air inlet opening  26  cannot be introduced into the emergency ventilation duct  52 , but rather is routed to the fresh air duct  38  only. It would further more be possible to realize an embodiment, in which the emergency ventilation inlet flap  54  of the emergency ventilation duct  52  is moved into another position that blocks the connection between the secondary air inlet opening  26  and the fresh air duct  38  such that ram air from the secondary air inlet opening  26  can only be introduced into the emergency ventilation duct  52 . 
     The arrangement according to  FIGS. 9 and 10  has the advantage that additional fuselage openings and ram air flaps for the emergency ventilation inlet can be eliminated. This reduces the weight, the installation space and the direct costs. In addition, duct or pipe sections can be eliminated because they can be used for several functions such as, for example, secondary air supply and emergency ventilation. 
     In conclusion,  FIGS. 11   a  to  11   c  show possible shapes of ram air inlets that are intended to elucidate that the present invention is not limited to the pod-shaped ram air inlet shown. On the contrary, it would be conceivable to use all types of ram air inlets that in some way make it possible to take in and route ram air into a pipe or a duct. 
     As a supplement, it should be noted that “comprising” does not exclude any other elements or steps, and that “a” or “an” does not exclude a plurality. It should furthermore be noted that characteristics that were described with reference to one of the above exemplary embodiments can also be used in combination with other characteristics of other above-described exemplary embodiments. Reference symbols in the claims should not be interpreted in a restrictive sense. 
     REFERENCE SYMBOLS 
     
         
           2  Fresh air inlet 
           4  Aircraft fuselage 
           6  Ram air inlet opening 
           8  Direction of flight 
           10  Deflector shield 
           12  Hinge 
           14  Actuator 
           16  Ram air duct 
           18  Compressor 
           20  Ram air inlet 
           22  Ram air inlet duct 
           24  Direction of flight 
           26  Secondary air inlet opening 
           28  Secondary air inlet duct 
           30  Flap 
           32  Hinge 
           33  Rotary actuator 
           34  Outer surface of aircraft 
           36  Ram air inlet opening 
           38  Fresh air duct 
           40  Check valve 
           41  Possible heat exchanger position 
           42  Compressor 
           44  Ram air surface 
           46  Surge limit 
           48  Compressor operating point 
           50  New compressor operating point 
           52  Emergency ventilation duct 
           54  Emergency ventilation inlet flap 
           56  Hinge 
           57  Rotary actuator 
           58  Check valve