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1. Background of the Inventionxe2x80x94Field of Invention
The invention relates to air vehicles such as manned and unmanned aircraft, missiles, and bombs that fly in the air and, in particular, to air vehicles having scissors wings that improve their aerodynamic performance at a wide range of speed.
2. Background of the Inventionxe2x80x94Description of Prior Art
The contradictory requirement of wing aspect ratio at high speed and low speed flying is a major obstacle preventing existing air vehicles from efficiently and effectively flying at both low speed and high speed up to supersonic speed.
The aspect ratio of a wing is defined as the square of the wingspan divided by the area of the wing. At transonic and supersonic flying, wings of low aspect ratio, such as delta wings and swept-back wings, are preferred for air vehicles because they have much less drag than wings of high aspect ratio. However, wings of high aspect ratio are preferred for air vehicles in the condition of low speed flying, take-off, and landing because they have higher lift-to-drag (L/D) ratio than wings of low aspect ratio. This contradictory requirement of aspect ratio for flying at high speed and low speed makes existing supersonic aircraft with wings of low aspect ratio inefficient in the flying conditions of low speed flying, take-off, and landing.
Concorde, the supersonic passenger aircraft having a pair of delta wings, needs a long take-off run to reach its 400 kilometers/hour (250 miles/hour) take-off speed that is higher than the take-off speed of most subsonic aircraft like Boeing 747. In addition, the low lift-to-drag ratio (L/D) of its delta wings demands high engine thrust to take-off and fly at low speed, making Concord a noisy aircraft around airports. The requirement of high engine thrust at low speed also makes Concorde consume a lot of fuel to climb to its cruise altitude, contributing to the high operating cost of the aircraft.
Although modern fighter aircraft with wings of low aspect ratio have excellent take-off and landing performance, it is mainly achieved by high thrust-to-weight ratio and low wing loading, which are too costly and inefficient for non-fighter aircraft. In other words, at an expense of high fuel consumption, fighter aircraft mainly rely on high engine thrust and big wings to achieve short take-off and landing.
In addition, the xe2x80x9carea rulexe2x80x9d of transonic and supersonic flying adds another problem to aircraft with wings of low aspect ratio. In order to conform to the xe2x80x9carea rulexe2x80x9d to have low transonic and supersonic drag, aircraft with wings of low aspect ratio usually need to have a fuselage with an indentation, or xe2x80x9cwasp waistxe2x80x9d, around the place where wings are installed. This xe2x80x9cwasp waistxe2x80x9d increases the cost to manufacturer the aircraft and reduces the spaces for payload such as passengers and cargo.
One attempt to make aircraft efficiently flying at both low and high speed is variable sweep wings. The concepts of aircraft with variable sweep wings are shown in U.S. Pat. No. 3,053,484 issued to W. J. Alford, Jr. et al, U.S. Pat. No. 3,405,280 issued to F. G. Willox et al, U.S. Pat. No. 3,405,891 issued to R. Jacquart et al, and U.S. Pat. No. 3,447,761 issued to P. C. Whitener et al. Existing aircraft like Boeing B-1B bomber have adopted variable sweep wings. According to the configuration, at take-off, landing, and low speed flying, the sweepback angle of the wings is small, making the wings somewhat similar to a pair of straight wings. At high speed flying, the wings are swept back so that the sweepback angle is big, making the wings similar to a pair of swept-back wings.
Variable sweep wings have structure and control disadvantages. The pivotally mounted left and right wings generate huge bending moments on the left and right pivot points at wing roots. In order to transfer the bending moments as well as ensure the smooth swiveling of the wings, the pivot points must be structurally strong and thus structurally heavy. These heavy pivot points and the additional control system for swiveling the wings make an aircraft structurally inefficient. In addition, the swiveling of the wings changes the center of lift and center of gravity of the aircraft, requesting large horizontal stabilizers to balance the aircraft. Some aircraft even have a system that can quickly transfer fuel from one place to another place in the aircraft to facilitate the balancing of the aircraft when the wings are swiveling. The shift of center of lift and center of gravity make the aircraft difficult to control, and the large horizontal stabilizers and fuel transfer system also add weight to the aircraft.
Oblique wing is another attempt to optimize both high speed and low speed performance by modifying the wing configuration of an air vehicle during flight. This concept is shown in U.S. Pat. Nos. 3,971,535 and 3,737,121, both issued to R. T. Jones, and U.S. Pat. No. 5,154,370 issued to J. W. Cox et al. The basic idea of oblique wing is to have a main wing pivotally installed on the fuselage or fuselages of an aircraft. The pivotal attachment allow the main wing to be yawed relative to the fuselage or fuselages for high speed flight, and to be positioned at right angles with respect to the fuselage or fuselages during take-off, landing, and low speed flight.
Oblique wing configuration has inherent stability and control disadvantages. One problem is the coupling of roll and pitch movement of the aircraft. For example, suppose the wing is yawed to an angle so that the right side of the wing becomes a swept-forward wing, the left side of the wing becomes a swept-back wing, and the aircraft uses ailerons or flaperons to achieve roll control. When the pilot of the aircraft wants to bank the aircraft to the left, the aircraft will make an unexpected nose-up movement while banking to the left. On the country, when the pilot wants to bank to the right, the aircraft will make an unexpected nose-down movement while banking to the right. The reason of this problem is that the ailerons or flaperons on the left and right wings are not located along the same transverse axis thus generate pitch movement moments when they are adjusted to roll the aircraft. Another problem is that the aerodynamic lift generated by the oblique wing is not evenly distributed along the long axis of the wing when the wing is at a high yaw angle from perpendicular to fuselage. This causes a roll moment on the aircraft. In order to solve this problem, both U.S. Pat. Nos. 3,971,535 and 3,737,121 issued to R. T. Jones invent the wing to be upwardly curved at both ends of the wing. The upward curved ends make the swept-forward part of the oblique wing has a higher angle-of-attack than the swept-back part of the wing. By constructing the wing to a specific upward curvedness, the aircraft can fly at a certain speed with the wing yawed at a certain angle without generating the unexpected roll moment. However, this fixed upward curvedness cannot eliminate the roll moment at a wide variety of flying conditions, limiting the flexibility of an oblique wing aircraft.
U.S. Pat. No. 4,998,689 issued to R. R. Woodcock and U.S. Pat. No. 3,155,344 issued to R. Vogt show a concept of xe2x80x9ctwo-position wingxe2x80x9d. The xe2x80x9ctwo-position wingxe2x80x9d is rotatably mounted on the fuselage of an aircraft and consists of two sets of wings, one set is a pair of supersonic wings and another is a pair of subsonic wings. At low speed flying, take-off, and landing, the xe2x80x9ctwo-position wingxe2x80x9d is at a position that the pair of subsonic wings is used and the pair of supersonic wings is attached to the fuselage. At supersonic flying, the xe2x80x9ctwo-position wingxe2x80x9d rotates 90 degrees to another position so that the pair of supersonic wings generates lift but the pair of subsonic wings is attached to the fuselage.
This concept has two disadvantages. First, both the supersonic and subsonic wings have to have their airfoils and shapes fit to both wing positions. This requires the compromise of key parameters of the wings for both wing positions. For example, in order to have small drag and good controllability when the pair of subsonic wings is attached onto the fuselage, the pair of subsonic wings is better to have a front-and-rear symmetry airfoil. But this airfoil is not the desired one to generate lift when the pair of subsonic wings is used at low speed flying, take-off, and landing. This kind of compromise prevents the wings from having the optimum shape and airfoils as well as the desired flaps and ailerons, or flaperons, to generate lift. Second, the aircraft is difficult to control and maintain stability when the xe2x80x9ctwo-position wingxe2x80x9d is shifting from one position to another position. During the shifting of positions, the aircraft is very easy to have an unexpected roll movement just as an oblique wing because the lift it generates is not distributed evenly along the wing when the wing is being rotated to have a big yaw angle from perpendicular to fuselage.
U.S. Pat. No. 5,671,898 issued to B. B. Brown and U.S. Pat. No. 4,913,378 issued to G. E. Calvert are inventions using a secondary wing in addition to the high speed wing to help the aircraft achieve short take-off and landing and efficient low speed flying. The secondary wing of both inventions is rotatably mounted on the fuselage of the aircraft. During take-off, landing, and low speed flying, the secondary wing is generally perpendicular to the longitudinal axis of the fuselage to generate lift. At high speed flying, the secondary wing is rotated at 90 degrees so that it forms a part of the fuselage. This concept still has the problem of generating unexpected roll movements similar to that of oblique wing and xe2x80x9ctwo-position wingxe2x80x9d when the secondary wing is being rotated to big yaw angles from perpendicular to fuselage. In addition, the secondary wing adds weight to the aircraft without generating lift at high speed flying, making the aircraft have low structural efficiency and consumes more fuel to carry this weight.
Coplanar joined wings shown in U.S. Pat. No. 5,899,410 issued to T. M. Garrett is a concept having a pair of forward wings extending from the fuselage of an air vehicle laterally outward and backward, and a pair of aft wings extending from the fuselage laterally outward and forward. Each forward wing is connected with the respective aft wing at a common wingtip to form a joined wing. The forward and aft wings are on the same plane to be mutually coplanar and the coplanar joined wings can be either fixed wings or variable wings. As variable wings, coplanar joined wings have two ways to change sweep angles. The first way of changing sweep angles is that either the wing root of the forward wings or the wing root of the aft wings or the wing roots of both can be moved along the longitudinal axis of the fuselage. Another way of changing sweep angles is that a pair of outer wings are rotatably mounted on the two joints of the forward and aft joined wings so that the sweep angle of the outer wings can be adjusted similar to variable sweep wings.
Both as fixed and variable wings, the coplanar joined wings have a disadvantage of unfavorable aerodynamic interference. Positioned at the downstream of the forward wings, the aft wings are inefficient in generating lift because their incoming airflow is the distorted airflow just passes through the forward wings. As variable joined wings using the above-mentioned first way to change sweep angles, the fuselage requires a track structure along its longitudinal axis. This track structure not only guides the movement of the wings but also should be strong enough to transfer load from the wings to the fuselage. Such a track structure increases the structural weight of the air vehicle. When using the above-mentioned second way to change sweep angles, the configuration has disadvantages similar to those of variable sweep wings. One of these disadvantages is the low structural efficiency caused by the pivotal points that not only have to sustain high bending moments but also should ensure the smooth swiveling of the outer wings. Another one is the stability and control problems caused by the shifting of center of lift and center of gravity when the outer wings change sweep angle.
Patent JP 404317891 A xe2x80x9cAircraft for Vertical Take-Off and Landingxe2x80x9d issued to H. Hatano is an invention utilizing two sets of rotor/wings that can convert between rotors and fixed wings. The two sets of rotor/wings are either mounted above the fuselage of an aircraft in tandem with one rotor/wing in front of another, or co-axially installed above the fuselage with one rotor/wing above another. During take-off, landing, and low speed flying, the rotor/wings rotate in opposite directions to make the aircraft flying just as a helicopter. At high speed, both of the rotor/wings stop rotating to become fixed wings and the yaw angles of both fixed wings can be changed to fit different speeds.
The rotor/wings of the invention have to compromise their aerodynamic performance and structures to fit both the helicopter and fixed-wing modes. For example, the leading edge of one side of a rotor/wing at helicopter mode becomes the trailing edge of the rotor/wing at fixed-wing mode. This makes the airfoils of the rotor/wing difficult to have high efficiency at both helicopter and fixed-wing modes. This shift between leading edge and trailing edge also makes it difficult to arrange flaps, ailerons, or flaperons on the rotor/wings. In addition, when both of the rotor/wings are co-axially mounted above the fuselage, the hub of the rotor/wings are very difficult to be built in small size because it has to contain control mechanisms to control the cyclic pitch, collective pitch, and sweep angles of two counter-rotating rotor/wings. This makes the hub very easy to generate big drag at high speed flying, especially at supersonic flying.
X-wing configuration, as shown in U.S. Pat. No. 4,711,415 issued to J. A. Binden, consists of a rotor/wing that can work as a rotor at helicopter mode and a fixed xe2x80x9cXxe2x80x9d shape wing at fixed-wing mode. The airfoils of X-wing are also difficult to have high efficiency at both helicopter mode and fixed-wing mode because the leading edges of some blades of the rotor/wing at helicopter mode become trailing edges at fixed mode. For the same reason, it is also very difficult to arrange flaps, ailerons, or flaperons on X-wing. Furthermore, the sweep angles of the blades of a X-wing at fixed-wing mode are affected by the requirement of helicopter mode, normally with two blades at 45 degrees sweepback angle and two blades at 45 degrees sweep forward angle, making the rotor/wings difficult to have high efficiency at a wide range of speed when flying at fixed-wing mode.
The fundamental object and advantage of my invention is to build an air vehicle that can efficiently and effectively fly at a wide range of speed. Specifically, the objects and advantages of an air vehicle based on my invention are:
1. Have low take-off and landing speeds and can take-off and land over a short distance;
2. Have low engine noise and low fuel consumption during take-off, landing, and flying;
3. Can efficiently fly at transonic speed and low supersonic speed without generating sonic boom;
4. Is easy to control and have good stability;
5. Have high structural efficiency and is easy to manufacturer;
6. Can be stored or carried with compact lateral dimension without adding significant weight and complexity.
A scissors wings configuration has been invented to achieve the above-mentioned objects and advantages. The invention has a fuselage and two continuous main wings. Both of the main wings are rotatably mounted on the fuselage and they can be yawed in opposite directions relative to the fuselage. One way to install the main wings is to have one main wing mounted above or at the upper part of the fuselage and another mounted beneath or at the lower part of the fuselage. The second way to install the main wings is to mount both of the wings above or at the upper part of the fuselage with one main wing over another. Similarly, the third way to install the main wings is to mount both of the wings beneath or at the lower part of the fuselage with one main wing over another. The fourth way to install the main wings is to mount both of the main wings at around the middle part of the fuselage with one main wing over another.
The following paragraphs explain how can the invention achieve the above-mentioned six objects and advantages.
First, the invention can make an air vehicle have low take-off and landing speeds and can take-off and land over a short distance. During take-off and landing, both of the main wings can be rotated to a position so that their long axes are either generally perpendicular to the longitudinal axis of the fuselage or only have a small yaw angle from perpendicular to the fuselage. By this way, the air vehicle has two high aspect ratio wings that have high lift-to-drag ratio (L/D) at low speed. This arrangement can make the air vehicle have low take-off and landing speeds and have short take-off and landing distances.
Second, the invention can make an air vehicle have low engine noise and low fuel consumption during take-off, landing, and flying. When the axes of the main wings are either generally perpendicular to the longitudinal axis of the fuselage or have a small yaw angle from perpendicular to the fuselage, they are of high aspect ratio thus when generating the same lift, they have much less drag than wings of low aspect ratio. The much less drag makes relatively low engine thrust enough to power the air vehicle to take-off, land, and fly at low speeds, resulting in low engine noise and low fuel consumption. In addition, the rotatable scissors wings can be yawed to different angles from perpendicular to the fuselage so that the air vehicle can have the optimum lift-to-drag ratio (L/D) virtually at any speed within its flight envelop, making the air vehicle only need relatively low engine thrust throughout its flying at different speeds. Furthermore, when the air vehicle is flying at a supersonic speed and its scissors wings are yawed at a big angle from perpendicular to the fuselage, the scissors wings can have a very smooth cross-sectional area distribution along its longitudinal axis. This means the air vehicle is very easy to be designed to conform to the xe2x80x9carea rulexe2x80x9d, making it only need relatively low engine thrust to penetrate the sonic barrier and fly at supersonic speeds. All the above-mentioned characteristics make an air vehicle with scissors wings capable of achieving low engine noise and low fuel consumption at every stage of its flights.
Third, the invention can make an air vehicle efficiently fly at transonic and low supersonic speed without generating sonic boom. When yawed at a big angle from perpendicular to fuselage, the supersonic drag characteristics of an air vehicle with scissors wings is similar to that of an air vehicle with oblique wing because both have similar cross-sectional area distribution along the longitudinal axis of fuselage, and both are composed of generally straight, continuous wing or wings yawing at an angle. According to U.S. Pat. No. 3,971,535 issued to R. T. Jones, oblique wing configuration can make an air vehicle fly up to Mach 1.3 without generating sonic boom. As the scissors wings configuration has the similar supersonic drag characteristics as oblique wing configuration, air vehicle having scissors wings is also able to fly at low supersonic speeds without generating sonic boom. This feature makes an air vehicle with scissors wings able to fly over population centers at a supersonic speed without disturbing people on the ground with sonic boom.
Forth, the invention can make an air vehicle easy to control and have good stability. An air vehicle of scissors wings configuration can generate symmetric moments along its longitudinal axis at any speed within its flying envelope. These symmetrically distributed moments prevent scissors wings configuration from having the two control and stability problems of oblique wing configuration. The two problems of oblique wing are the coupling of roll and pitch movement, and the roll moment generated by the uneven distribution of lift along the oblique wing when the wing is yawed at a big angle from perpendicular to fuselage. In addition, scissors wings configuration does not have the control and stability problems generated by the shifting of center of lift and center of gravity that bothers variable sweep wing configuration. The yawing of the scissors wings makes little, if any, shift of center of lift and center of gravity. Even the shift of center of lift from subsonic flying to supersonic flying is small because the scissors wings are of slender shape with narrow wing chord, making the center of lift shifting from one-forth of the wings"" average wing chord during subsonic flying to one-half of average wing chord during supersonic flying only a small distance. The small change of center of lift and center of gravity during the yawing of wings and the small change of center of lift during the shifting from subsonic flying to supersonic flying make scissors wings configuration easy to control and have good stability.
Scissors wings configuration also does not have the control and stability problems caused by the shifting between the big-wing mode and small-wing mode of the inventions by Superala""s Patent 505188 and Tammeo""s Patent 510430. As the scissors wings are the only wings to generate lift, they do not need to be shifted with other wings in flight thus do not have the control and stability problems of Superala and Tammeo""s inventions.
Fifth, the invention can make an air vehicle have high structural efficiency and is easy to manufacturer. Both main wings making up the scissors wings configuration are continuous and generally straight, making the wing structure have direct and short routes to transfer load. The main wings can also have relatively high relative thickness comparing with wings of low aspect ratio because big yaw angles from perpendicular to fuselage can ensure that they have subsonic normal component of incoming airflow when the air vehicle is flying at supersonic speed. For example, if an aircraft with scissors wings is flying at Mach 1.8 with its scissors wings yawed to be 60 degrees from perpendicular to fuselage, the normal component of incoming airflow is Mach 1.8 times cos(60xc2x0), which is March 0.9. The high relative thickness makes the main wing structure have high structural height to transfer bending moments. All these features help the main wing structure to be light and efficient. In addition, the pivots or hollow turrets that connect main wings to the fuselage can also be of lightweight. This is because each main wing has a structure that is continuous through out its pivot or turret thus neutralizes most of the bending moments it generates, making the pivot or turret of the wing sustains little, if any, bending moments. In comparison, the two pivot points of variable sweep wings configuration should be stronger thus heavier because they have to sustain the bending moments generated by the variable wings.
An air vehicle of scissors wings configuration is also easy to manufacturer. As the configuration conforms well to the xe2x80x9carea rulexe2x80x9d, it does not need to have xe2x80x9cwasp waistxe2x80x9d on its fuselage. The main wings are also continuous and have a generally straight shape. In addition, the air vehicle""s high lift-to-drag ratio (L/D) at low speed makes it able to take-off and land with low angle-of-attack, eliminating the need to have a movable nose like the one of Concorde. All these features make the air vehicle able to have simple shape and simple structure that are both easy and cost-effective to produce.
Sixth, the invention can make an air vehicle, especially an unmanned air vehicle, missile, or aerial bomb, be stored or carried with compact lateral dimension without adding significant weight and complexity. Both main wings of an air vehicle with scissors wings can be yawed so that the long axes of the main wings are generally parallel with the longitudinal axis of the fuselage of the air vehicle, making both main wings attached to the fuselage. This makes an air vehicle have compact lateral dimension without a folding device for wings. Comparing with the long track structure that is necessary to fold coplanar joined wings as shown in U.S. Pat. No. 5,899,410 issued to T. M. Garrett, scissors wings configuration is a simpler and lighter way to make air vehicles being stored or carried with compact lateral dimension.
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