Abstract:
A system and method for reducing aerodynamic drag is disclosed. A compression seal is attached to the inboard edges of the stabilizer and elevators of an airplane. The seal blocks airflow in a gap located between these inboard edges and a fuselage. The shape of the compression seal changes as the shape of the gap changes due to movement of the stabilizer and elevators during flight to effectively block airflow through the gap during flight. By blocking the airflow, the seal reduces the aerodynamic drag of the airplane.

Description:
FIELD 
     The disclosure is related to reducing aerodynamic drag of an airplane and, more particularly, to flight control surface seals that reduce aerodynamic drag. 
     BACKGROUND 
     An airplane includes flight control surfaces that a pilot can adjust to control the aircraft&#39;s flight attitude. Airplane design determines what flight control surfaces are available on a particular airplane. Typical flight control surfaces include the wing&#39;s slats, flaps, spoilers, and ailerons; vertical and horizontal stabilizers; rudders, and elevators. 
     A horizontal stabilizer is a horizontal wing attached to the aft end of the fuselage of an airplane to trim the airplane about the longitudinal axis by providing a stabilizing force to the aft end of the airplane. While some horizontal stabilizers are fixed, others can be moved during flight. These movable horizontal stabilizers, which may be referred to as variable incidence horizontal stabilizers, allow the pilot to adjust the angle of the horizontal stabilizer based on the aircraft&#39;s longitudinal stability parameters, such as center of gravity location. 
     Elevators are flight control surfaces that control the aircraft&#39;s longitudinal attitude by changing the vertical loads on the aft end of the fuselage. Elevators are usually hinged to the aft end of the horizontal stabilizer. 
     Since these movable horizontal stabilizers and elevators move relative to the fuselage, a gap exists between these flight control surfaces and the fuselage except at the point where the surface is attached to the fuselage (i.e., the pivot point of the surface). Since most aft fuselages are convex curved about the longitudinal axis of the airplane, the gap between the movable horizontal stabilizer inboard edge and the fuselage is not constant. This gap normally increases as the stabilizer is moved more from its neutral position. This is also true of the elevator. As the size of the gap increases, so too does the aerodynamic drag of the airplane, which impacts the performance of the airplane. 
     SUMMARY 
     A system and method for reducing aerodynamic drag of an airplane is disclosed. The system includes a flight control surface of an airplane and a seal connected to the flight control surface. The seal blocks airflow through a gap located between the flight control surface and a fixed structure of the airplane. In a preferred embodiment, the flight control surface is a horizontal stabilizer or an elevator, the fixed structure is a fuselage, and the seal is a bulb seal. 
     The method includes placing an exterior surface of a seal adjacent to an inboard edge of a flight control surface of an airplane, positioning a fastener adjacent to an opposite exterior surface of the seal, and attaching the seal to the flight control surface with the fastener. The seal fills a gap located between the flight control surface and a fixed structure of the airplane. The method further includes applying a low friction coating, such as Teflon® paint, on the fixed structure. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Presently preferred embodiments are described below in conjunction with the appended drawing figures, wherein like reference numerals refer to like elements in the various figures, and wherein: 
         FIG. 1  is an illustration of an empennage of an airplane, according to an example; 
         FIG. 2  is an illustration of an isometric view of a horizontal stabilizer and an elevator, according to an example; 
         FIG. 3  is an illustration of a cross-sectional view of a seal, according to an example; 
         FIG. 4  is an illustration of a cross-sectional view of a fastener for attaching the seal to the horizontal stabilizer and the elevator, according to an example; 
         FIG. 5  is an illustration of a view looking down on the stabilizer and elevator identifying a location for attaching the seal, according to an example; and 
         FIG. 6  is an illustration of a view looking up on the stabilizer and elevator identifying a location for attaching the seal, according to an example. 
     
    
    
     The drawings are for the purpose of illustrating example embodiments, but it is understood that the inventions are not limited to the arrangements and instrumentality shown in the drawings. 
     DETAILED DESCRIPTION 
       FIG. 1  is an illustration of an empennage  100  of an airplane. The empennage  100 , also known as the tail or tail assembly, contributes to the stability and the control of the airplane. The empennage  100  includes a horizontal stabilizer  102  with elevators  104 . The empennage  100  also includes a vertical stabilizer  106  with a rudder  108 . The horizontal stabilizer  102  and vertical stabilizer  104  are connected to a fuselage  110  of the airplane. 
     As the horizontal stabilizer  102  and the elevators  104  move relative to the fuselage  110 , a gap between the fuselage  110  and either the horizontal stabilizer  102  or elevators  104  changes size. To reduce aerodynamic drag, a seal is attached to inboard edges  112  of the horizontal stabilizer  102  and inboard edges  114  of the elevators  104 . The seal expands and compresses as the gap changes size to block airflow between these flight control surfaces  102 ,  104  and the fuselage  110 . 
       FIG. 2  is an isometric view  200  of the horizontal stabilizer  102  and the elevator  104 . The view  200  depicts trailing edge panels  206   a  and  206   b  of the horizontal stabilizer  102  and an elevator panel  208  of the elevator  104 . Seals  210   a  and  210   b  are attached to each of the panels  206   a  and  206   b , respectively. A seal  210   c  is also attached to the elevator panel  208 . 
       FIG. 3  is a cross-sectional view of a seal  300  that may be used for the seals  210   a ,  210   b , and  210   c . The seal  300  is a compression seal and is depicted in  FIG. 3  as a bulb seal and, in particular, a P-bulb seal. The bulb seal is flexible and changes shape as pressures are exerted on the exterior of the seal  300 . The flexible nature of the seal  300  allows it to expand and contract to fill the variability of the gap throughout the normal range of the horizontal stabilizer  102  and elevator  104 . Other flexible seal types may also be used. 
     The dimensions of the seal  300  depend on the design of the airplane and, more specifically, the size of the gap between the flight control surfaces  102 ,  104  and the fuselage  110  as the flight control surfaces  102 ,  104  move. As the elevator  104  typically has a greater range of motion than the horizontal stabilizer  102 , different seal dimensions may be used for the different panels  206   a ,  206   b , and  208 . For example, the diameter of the bulb may be larger for the seal  210   c  attached to the elevator panel  208  than the seals  210   a  and  210   b  attached to the trailing edge panels  206   a  and  206   b.    
     In one example, the diameter (d) of the bulb from the exterior edges of the bulb may be approximately 1.8″ and the thickness of the bulb wall (t) may be approximately 0.08″ when not subjected to external forces. In other examples, the diameter (d) may be between 1″ and 3″ and the bulb wall thickness (t) may be between 0.5″ and 1.5″. In other examples, the diameter (d) may be between 0.5″ and 5″ and the bulb thickness (t) may be between 0.1″ and 2″. 
     The P-bulb seal includes an attachment surface  302 , sometimes referred to as a handle or lip. The attachment surface  302  facilitates attachment of the seal  300  to the panels  206   a ,  206   b , and  208 . While other mechanisms and surfaces may be used to attach the seal  300  to the panels  206   a ,  206   b , and  208 , P-bulb seals are readily available and convenient to use. 
     The seal  300  is composed of a non-metallic material, preferably, silicone. In a preferred embodiment, the seal is composed of BMS 1-57 Type 2 silicone. Other non-metallic materials, such as rubber, may also be used. 
     The seal  300  may also be covered with an external covering  304 , such as a polyester fabric or other protective material. For example, the external covering  304  may include one or more layers of Mohawk D2000 Dacron® fabric or HT 2002 Nomex® fabric. Preferably, the external covering  304  has two reinforced plies of one of these two fabrics. In this example, the thickness of the external covering  304  is approximately 0.12″. In other examples, the thickness of the external covering  304  may be between 0.05″ and 0.25″. 
     The bulb seal  300  is attached to the panels  206 ,  208  with a row of fasteners. In one example, the fasteners are spaced 1.875″ apart. In other examples, the fasteners are spaced between 1.5″ and 2″ apart. In other examples, the fasteners are spaced between 1″ and 3″ apart. 
       FIG. 4  is a cross-sectional view of a fastener  400 . The fastener  400  includes a seal retainer  402 , a nut plate retainer strip  404 , a nut plate  406 , and a bolt  408 . A slotted hole  410  is located in the seal  300  and the seal retainer  402 . While  FIG. 4  depicts a typical slotted hole, other dimensions are suitable. 
     The seal retainer  402  provides support to the seal  300  as external pressures from the fuselage  110  deform the seal  300 . In one example, the seal retainer  402  is formed using one or more layers of carbon or carbon composite fabric. Preferably, the seal retainer  402  is formed from four plies of carbon composite fabric (e.g., BMS 8-256) having a thickness of approximately 0.034″. In other examples, the thickness of the seal retainer  402  may be between 0.02″ and 0.05″ or between 0.01″ and 0.1″. Additionally, in other examples, the seal retainer  402  may be formed using one or more layers of fiberglass fabric, such as 4-ply 181 fiberglass fabric, or other suitable materials. 
     The nut plate retainer strip  404  is located between the seal retainer  402  and the nut plate  406 . A bolt  408  attaches the seal retainer  402  to the panels  206 ,  208 . The size of the bolt depends on the type of nut plate  406  selected. Preferably, the bolt is a 3/16″ bolt, but other bolt types may also be used. In one example, a 3/16″ titanium BACB30VF bolt is used in a BACN11G nut plate. The slotted holes  410  in the seal  300  and the seal retainer  402  allow the bolt  408  to slide left and right as the bolt  408  is installed. While a slotted hole is not necessary, it is easier to install the bolt  408  with this ability to adjust the location of the bolt  408  within the slotted holes  410 . 
     To attach the seal  300  to the panels  206 ,  208 , an installer places an exterior surface of the attachment surface  302  adjacent to the inboard edges  112 ,  114  of the panels  206 ,  208  such that the seal  300  extends from the panels  206 ,  208  and contacts the fuselage  110 . During installation, the seal  300  is compressed against the fuselage  210 . The amount of compression is based on the range of motion of the flight control surfaces  102 ,  104  and the maximum width of the gap expected. 
     The installer positions the fastener  400  adjacent to an opposite side of the exterior of the attachment surface  302  aligning the slotted holes  410  in the seal  300  and the seal retainer  402 . The installer then positions the nut plate strip  404  and the nut plate  406  on the seal retainer  402 . The installer then installs bolts  408  through the nut plate  406 , the nut plate strip  404 , the seal retainer  402 , and the panels  206 ,  208 . 
     While  FIG. 4  depicts a particular fastener design, it is understood that other attachment mechanisms may be used. It is also understood that the fastener  400  may be modified to include more or less components. The fastener  400  may also use different materials and dimensions than described herein. 
       FIG. 4  also depicts how the seal  300  changes shape based on external pressures. As the seal  300  is pressed against the side of the fuselage  110  when the panels  206 ,  208  move closer to the fuselage  110 , the seal  300  deforms as shown by the dotted deformation line  412 . For example, the diameter (d′) of the bulb from the exterior edges of the bulb may be reduced from 1.8″ to 1.5″. While this is only one example, it shows how the seal  300  is able to block the airflow between the fuselage  110  and the panels  206 ,  208  as the gap size changes. 
     In addition to the contact pressure from the fuselage  110 , the seal  300  is also subjected to friction as it moves along the fuselage  110 . To reduce friction, a low friction coating may be applied to the fuselage  110 . For example, a polytetrafluoroethylene (PTFE) (i.e., Teflon®) coating or paint may be applied to the fuselage. 
     The seal  300  was flight tested on an on a Boeing 787-9 airplane.  FIG. 5  depicts where the seal  210   a  was attached to the trailing edge panel  206   a  of the horizontal stabilizer  102  and the seal  210   c  was attached to the elevator panel  208  of the elevator  104 .  FIG. 6  depicts where the seal  210   b  was attached to the trailing edge panel  206   b  of the horizontal stabilizer  102 . Flight test data confirms that the seal  300  reduces aerodynamic drag. Test results showed that the seal  300  improved drag by an equivalent of 600 pounds of airplane weight. This improvement results in a more fuel efficient operation of the airplane. 
     While the seal was tested on a Boeing 787-9 airplane, the use of the seal  300  is not limited to any particular type of airplane. For example, the seal  300  may be used on private airplanes and military airplanes, e.g., tanker aircraft, in addition to commercial airplanes. Moreover, the seal  300  can be retrofitted onto older airplanes that are currently operating without the seal  300 . 
     While the seal  300  was described with respect to the horizontal stabilizer  102  and the elevators  104 , the seal  300  may be useful for reducing drag between a fixed structure of the airplane (e.g., the fuselage  110 , fixed wing portions) and other control surfaces. For example, the seal  300  may be attached to flight control surfaces associated with the wing (e.g., slats, flaps, spoilers, and ailerons) or the vertical stabilizer  106  (e.g., the rudder  108 ). As another example, the seal  300  may be useful for reducing drag between two control surfaces, such as between the horizontal stabilizer  102  and the elevators  104 . 
     By reducing aerodynamic drag through the use of the seal  300 , the airplane becomes more fuel efficient. Moreover, the fuel savings obtained from use of the seal  300  are much greater than the cost of adding the seal  300  to the airplane. As a result, the cost of operating the airplane and the impact to the environment is reduced. 
     It is intended that the foregoing detailed description be regarded as illustrative rather than limiting and that it is understood that the following claims including all equivalents are intended to define the scope of the invention. The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.