Abstract:
Lift produced by an airfoil of an aircraft is increased by suppressing fluid detachment from the surface of the airfoil. An engine cowling extends outwardly from the surface of the airfoil that has an exit plane configured for directing exhaust gases toward a rear of the aircraft. Fences extending outwardly from the surface and proximate to the exit plane of the engine cowling are configured to guide the exhaust gases along at least a portion of the airfoil surface, thereby restricting spanwise movement of the gases and increasing the Coanda Effect exhibited by the gases, thereby increasing the amount of lift produced along the surface of the airfoil. Such techniques may be used in short take-off and landing (STOL) aircraft.

Description:
TECHNICAL FIELD 
     The embodiments described herein generally relate to producing lift in an aircraft, and more particularly relate to methods and apparatus for increasing engine-powered lift produced along a surface of an aircraft. 
     BACKGROUND 
     Scientists and engineers continue to seek improvements in all areas of aircraft performance. Recent military campaigns, for example, have demonstrated an increased need for improved short takeoff and landing (STOL) performance to allow aircraft to operate in environments where modern airports and other landing facilities may not be available. In particular, it is desirable to create aircraft that are able to takeoff and/or land even on short runways. 
     One way to improve STOL performance is to increase the amount of lift produced along the airfoil surfaces of the aircraft. By increasing the lift capability of each wing, for example, the aircraft can become airborne at a lower airspeed, thereby reducing the length of runway needed for takeoff. Various aircraft designs have attempted to maximize the amount of lift produced along an airfoil surface through exploitation of the well-known Coanda Effect. In many of these designs, air moving over the wing can be “bent down” towards the ground using flaps and a jet blowing over a curved surface to increase the amount of lift produced. Aircraft that have successfully exploited the Coanda Effect for STOL purposes include the Boeing YC-14 and C-17 Globemaster III, as well as various types of unmanned aerial vehicles (UAVs) and the like. Nevertheless, there remains a desire for aircraft designs with even better STOL performance. 
     BRIEF SUMMARY 
     Lift produced by an airfoil of an aircraft is increased by reducing fluid movement away from the surface of the airfoil. The fluid movement away from the surface is often referred to as flow separation, which is a measure of flow inefficiency. In one embodiment, an engine cowling extends outwardly from the surface of the airfoil that has an exit plane configured for directing exhaust gases toward a rear of the aircraft. One or more fences extending outwardly from the surface and proximate to the exit plane of the engine cowling are configured to guide the exhaust gases along at least a portion of the airfoil surface, thereby restricting span-wise movement of the gases and reducing flow separation. This results in augmentation of the Coanda Effect exhibited by the gases, thereby increasing the amount of lift produced along the surface of the airfoil. 
     In another embodiment, an aircraft is provided with a first and a second airfoil each having a cowling extending outwardly from a surface of the airfoil. A first engine and a second engine are associated with the cowlings of the first and second airfoils, respectively, wherein the first and second engines are configured to produce exhaust gases that are directed toward an aft end of the aircraft by the cowlings. Fences disposed on the first and/or the second airfoils proximate to the cowlings guide the exhaust gases along the surfaces of the first and second airfoils. 
     Another exemplary embodiment provides a method of increasing the lift produced along a surface of an airfoil of an aircraft. Exhaust gases are generated and directed toward a rear end of the aircraft. The exhaust gases are then guided along the surface of the airfoil with one or more fences extending outwardly from the surface of the airfoil to thereby restrict three-dimensional movement of the exhaust gases away from the surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a top view of an exemplary aircraft with improved lift capabilities; 
         FIGS. 2 ,  2 A and  3  are perspective views of exemplary airfoils with improved lift-generating capabilities; 
         FIG. 4  is a plot showing exemplary differences in lift that may be produced with an exemplary embodiment; and 
         FIG. 5  is a plot showing the improved load distribution resulting from one exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Generally speaking, the amount of lift produced along a surface of an aircraft can be increased by improving the flow of air over the surface. Especially in the case of STOL aircraft designed to exploit the Coanda Effect, lift can be reduced by three-dimensional effects whereby air flowing across an airfoil detaches from the surface of the aircraft. By restricting the three-dimensional effects of airflow and instead encouraging two-dimensional flow across the surface, the Coanda Effect along the surface is suitably increased, thereby resulting in increased lift. 
     One way to reduce three-dimensional airflow along a surface of an aircraft is to provide one or more fences that can restrict the flow of air across a flap or other surface. Such fences can be deployed at least during takeoff, approach and/or landing to provide augmented lift. In various embodiments, the fences can be retracted or otherwise stowed while the aircraft is in cruise. By reducing three-dimensional effects of airflow while the fences are in use, the Coanda Effect produced on the surface of an airfoil can be improved. The flow becomes streamlined to the surface, thereby resulting in increased lift and improved STOL performance for the aircraft. 
     With reference now to  FIG. 1 , an exemplary aircraft  100  suitably includes one or more airfoils  101 A-B that produce lift in response to fluid flow across the surface of the airfoil. In various embodiments, aircraft  100  exploits Coanda Effects produced along the airfoils  101 A-B for improved STOL performance. To that end, aircraft  100  may include any number of engines that produce exhaust gases expelled along the surfaces of airfoils  101 A-B toward the rear end of aircraft  100 . One technique for producing lift with engine exhaust passing over a flap or other airfoil surface is the so-called upper surface blowing (USB) design used in the YC-14, among others. In such embodiments, engine exhaust emanates from engine cowlings  102 A-D that extend outwardly from the surfaces of airfoils  101 A-B. In such embodiments, exhaust gases are directed along a surface of a flap  104 A-B or other control surface associated with airfoil  101 A-B as appropriate. In various embodiments, flaps  104 A-B are extended during takeoffs and/or landings that demand STOL performance, and engine exhaust from cowlings  102 A-D suitably flows across the surfaces of flaps  104 A-B to provide increased lift in response to Coanda Effects occurring as exhaust gases flow along the curved surface of the airfoil formed by the wing  101  and/or flap  104  of aircraft  100 . 
     Engine exhaust may alternately be applied to flaps  104  and/or to other surfaces from sources other than cowlings  102 A-D. A nozzle and/or other structure attached to the engine, for example, could be used to direct exhaust gases in any desired manner. One example of an aircraft design that exploits USB effects to increase lift produced across a wing/flap structure is described in U.S. Pat. No. 6,926,229, although many other designs could be equivalently applied in other embodiments. To that end, aircraft  100  is any type of manned or unmanned vehicle capable of producing lift though the direction of engine exhaust across the surface of any wing, flap, control surface and/or other airfoil  10 A-B. Aircraft  100  may therefore be designed according to conventional USB principles, and/or may incorporate other designs and lift-generating techniques in a wide array of equivalent embodiments. 
     As noted above, the Coanda Effects of engine exhaust flowing across the surface of airfoils  101 A-B can significantly increase the amount of lift applied to aircraft  100 , particularly when the flaps  104 A-B are deployed. Coanda Effects, in turn, are increased when the exhaust plume flows across the airfoil surface in a predominantly two-dimensional manner; that is, when the exhaust flow “sticks” to the surface of the airfoil instead of breaking away from the surface in a more three-dimensional manner. 
     This two-dimensional movement can be encouraged by providing a lateral “bound” for the exhaust flow in the form of one or more mechanical “fences”  106 ,  108 . These fences  106 ,  108  are any mechanical structures capable of limiting airflow “spreading” across the surface of airfoil  101 A-B and/or flap  104 A-B that could otherwise result in vortices or other three-dimensional effects that could reduce Coanda Effects produced by the exhaust flow. Fences  106 ,  108  may be formed of metal, carbon composite and/or any other appropriate material, and may be shaped as desired and placed on the surface of airfoils  101 A-B in any location. Fence patterns used in various embodiments can exhibit widely varying forms; fences may project from the surface  101 A-B at any shape or angle, for example, to create any desired profile. Fences  106 ,  108  may be designed to exhibit predominantly smooth, serrated or wavy edges, or to exhibit regular or irregular shapes and other features. Further, fences  106 ,  108  may be formed with any uniform or non-uniform length, thickness and/or density. 
     Fences  106 ,  108  may be deployed in any quantities, and at any location(s) on airfoil  101 . In various embodiments, fences  106 ,  108  are provided to prevent lateral spreading of exhaust across an airfoil surface; to that end, fences  106 ,  108  may be located on the surface over which Coanda Effects are produced. That is, since fences  106 ,  108  are intended to limit lateral spreading of airflows across a surface of a wing, flap or other airfoil  101 , the fences may be deployed on the actual surface where lift is generated (e.g. on the flap  104 ). Further, in various embodiments, fences may be aligned approximately in parallel with the primary direction of exhaust flow (e.g. along the long axis of the aircraft). These aspects are in contrast to conventional vortex generator structures that generally strive to re-energize the viscous boundary layers of airfoils by creating three-dimensional vortices within the boundary layer being re-energized, and which are generally displaced at an angle that disrupts the predominant airflow across the surface of the airfoil. Notwithstanding these distinctions, various embodiments of fences  106 ,  108  may be displaced in a manner that is at least somewhat angled with respect to the direction of airflow (e.g. to direct exhaust flow outwardly from the axial direction of exhaust flow), or that are located and/or otherwise arranged in any manner that is appropriate for the particular aircraft design and performance parameters desired. 
     As shown in the exemplary embodiment of  FIG. 1 , each engine cowling  102  has an associated set of fences  106 ,  108  that guide exhaust gases flowing across flaps  104 A-B to restrict exhaust flow in a lateral direction. These fences  106 A-D,  108 A-D are implemented as any sort of moveable or stationary outcroppings that project outwardly from the surfaces of airfoils  101 A-B to restrict three-dimensional airflow. In the embodiment of  FIG. 1 , each engine cowling  102 A-D emits engine exhaust that is at least partially contained by a left side fence  106 A-D and a right side fence  108 A-D (respectively) to restrict spreading of exhaust gas across the surfaces of flaps  104 A-B that could otherwise result in vortices or other three-dimensional effects that could reduce the amount of lift produced. Additional fences, although not shown in  FIG. 1 , could be provided as well, and/or the fences shown could be differently located as appropriate and desired. 
       FIGS. 2 and 3  show more detailed views of exemplary airfoils  101  that include fences  106  and  108 . As shown in these figures, an engine cowling  102  extends outwardly from the surface of a wing or other airfoil  101 , or some other source of exhaust gases is provided. Engine exhaust is directed toward flap  104  by exit plane  202  or some other structure (e.g. a nozzle or the like) associated with the engine and/or cowling  102 . The exhaust plume flowing across flap  104  is then contained by fences  106 ,  108  ( FIG. 2 ) or by fences  106 - 109  ( FIG. 3 ) as appropriate. Any number of fences  106 - 109  may be deployed in a wide array of equivalent embodiments. Similarly, the fences  106 - 109  may be located in any position on airfoil  101  and/or flap  104  to provide the performance desired. Two fences  106 ,  108  may be provided in association with the left and right sides of engine cowling  102 , for example, as shown in  FIGS. 2-3 , and/or any other fences may be placed at any point on airfoil  101  and/or flap  104  where restriction of three-dimensional airflow is desired. As noted above, the fences  106 - 109  serve to restrict airflow to the surface of flap  104 , thereby reducing vortices and other three-dimensional effects, which in turn results in increased Coanda Effects and increased lift. Other embodiments different from those shown in the drawing figures may also be formulated. In aircraft with one or more fuselage-mounted engines, for example, a single fence (or multiple fences) could be located near an engine cowling or nozzle at the edge opposite of the fuselage to contain exhaust flow along the body of the aircraft. The concept of containing exhaust flow to prevent three-dimensional effects can be applied in any manner, using any type of fences or other structures at any location on the aircraft surface. 
     In various embodiments, fences  106 - 109  may be extended during takeoffs, approaches to landings and/or landings when improved STOL performance is desired, and otherwise retracted. For example,  FIG. 2A  shows fences  106 - 108  in a retracted position within the flap. Extension/retraction of fences e.g.,  106 - 109  (or  106 - 108 ) may be performed in any manner. Fences e.g.,  106 - 109  may be hinged and hydraulically actuated, for example, or otherwise actuated using any sort of electromechanical actuation device. In some embodiments, fences e.g.,  106 - 109  are extended in response to pilot commands; alternately, fences e.q.,  106 - 109  are extended automatically as flaps  104  are extended from airfoil  101 . Again, fences e.g.,  106 - 109  may be designed to be moveable in any manner, or may be stationary as appropriate for the particular aircraft design and performance desired. 
     The improvements in lift produced when fences  106 - 109  are deployed have been discovered to be significant in many embodiments.  FIG. 4 , for example, shows the improvements in lift observed in one exemplary implementation. Plot  400  is a conventional lift coefficient curve wherein the dimensionless lift coefficient (C L ) is plotted with respect to angle of attack (α) for an airfoil. Three traces  402 ,  404 ,  406  are shown, with trace  402  showing exemplary performance with the engine off, trace  404  showing exemplary performance with the engine on, and trace  406  showing performance with the engine on and fences  106 ,  108  deployed. As can be seen from plot  400 , about 15% more lift is generated when fences  106 ,  108  are deployed, in comparison to lift produced when the fences are not deployed. This improvement is relatively consistent for all values of airfoil angle of attack. 
     Moreover, improved load distribution has been observed when fences  106 ,  108  are deployed.  FIG. 5  shows an exemplary plot  500  of dimensionless sectional lift coefficient (C L ) plotted across the physical span of airfoil  101 . In the exemplary embodiment used to generate plot  500 , fences  106  and  108  are placed at the left and right edges, respectively, of an engine cowling  102  that lies between the fences. In this example the height of fence  106  is a little shorter than the height of fence  108 . Plot  500  shows three traces  502 ,  504  and  506  wherein trace  502  shows lift produced with the engine off, trace  504  shows lift produced with the engine on but no fences deployed, and trace  506  reflects lift produced with the engine on and fences deployed. By comparing traces  504  and  506  in plot  500 , it can be seen that trace  506  is more level than trace  504  in the space between fences  106  and  108 , reflecting that lift is generated more evenly across the span of airfoil  101  when the fences are deployed. Hence, it has been shown in practice that fences  106 ,  108  (as well as any additional fences  107 ,  109 , etc.) can both improve load distribution across the surface of an airfoil  101 , as well as increasing the overall lift produced by the airfoil itself. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the described embodiments in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope as set forth in the appended claims and the legal equivalents thereof.