The Swept Wing
	The concept of sweeping an aircraft's wings is to delay the drag rise caused by the formation of shock waves. The swept-wing concept had been appreciated by German aerodynamicists since the mid-1930s, and by 1942 a considerable amount of research had gone into it. However, in the United States and Great Britain, the concept of the swept wing remained virtually unknown until the end of the war. Due to the early research in this area, this allowed Germany to successfully introduce the swept wing in the jet fighter Messerschmitt Me-262 as early as 1941.     Early British and American jet aircraft were therefore of conventional straight-wing design, with a high-speed performance that was consequently limited. Such aircraft included the UKGloster Meteor F.4 , the U.S. Lockheed F-80 Sooting Star and the experimental U.S. jet, the Bell XP-59A Airacomet.
	After the war German advanced aeronautical research data became available to the United States Army Air Force (USAAF) as well as Great Britain. This technology was then incorporated into their aircraft designs. Some early jets that took advantage of this technology were the North American F-86 Sabre, theHawker Hunter F.4 and the Supermarine Swift FR.5.     Not to be outdone, the Soviet Union introduced the swept wing in the Mikoyan Mig-15 in 1947. This aircraft was the great rival of the F-86 during the Korean War.

Relative Wind
	The relative wind is a relationship between the direction of airflow and the aircraft wing. In normal flight circumstances, the relative wind is the opposite direction of the aircraft flight path.         1. If the flight path is forward then the relative wind is backward.         2. If the flight path is forward and upward, then the relative wind is backward and downward.         3. If the flight path is forward and downward, then the relative wind is backward and upward.     Therefore, the relative wind is parallel to the flight path, and travels in an opposite direction.1     Also relative wind can created by a stationary object and the motion of the air around it, as when an aircraft is pointed down a runway for takeoff. This is why takeoffs are normally into the wind.     During takeoff, when the aircraft is stationary, the relative wind would be the motion and direction of the airflow around the aircraft created by the wind. As the airplane accelerates down the runway, the wind and motion of the aircraft are combined to create relative wind. Once the aircraft becomes airborne, only the motion of the aircraft produces relative wind, and the relative wind becomes opposite and parallel to the flight path of the aircraft.     Although the wind can affect the ground speed and aircraft drift, the relative wind will always remain opposite and parallel to the aircraft wing.     Refer to the page on angle of attack for comparison.

	Laminar Flow is the smooth, uninterrupted flow of air over the contour of the wings, fuselage, or other parts of an aircraft in flight. Laminar flow is most often found at the front of a streamlined body and is an important factor in flight. If the smooth flow of air is interrupted over a wing section, turbulence is created which results in a loss of lift and a high degree of drag. An airfoil designed for minimum drag and uninterrupted flow of the boundary layer is called a laminar airfoil.     The Laminar flow theory dealt with the development of a symmetrical airfoil section which had the same curvature on both the upper and lower surface. The design was relatively thin at the leading edge and progressively widened to a point of greatest thickness as far aft as possible. The theory in using an airfoil of this design was to maintain the adhesion of the boundary layers of airflow which are present in flight as far aft of the leading edge as possible. On normal airfoils, the boundary layer would be interrupted at high speeds and the resultant break would cause a turbulent flow over the remainder of the foil. This turbulence would be realized as drag up the point of maximum speed, at which time the control surfaces and aircraft flying characteristics would be affected. The formation of the boundary layer is a process of layers of air formed one next to the other, i.e.; the term laminar is derived from the lamination principle involved.

History of Laminar Flow

The P-51 Mustang was the first aircraft intentionally designed to use laminar flow airfoils. However, wartime NACA research data shows that Mustangs were not manufactured with a sufficient degree of surface quality to maintain much laminar flow on the wing. The RAF found that the Bell P-63, despite being designed with laminar airfoils, also was not manufactured with sufficient surface quality to have much laminar flow.     The Mustang a mathematically designed airplane and the wing foil that was to be classified as a "semi-empirical venture" by the British was cleared for adoption on the new design. The wing section would be the only part of the fighter which would be tested in a wind tunnel prior to the first test flight. Due to the speculation of the success of the radical foil, the engineering department was committed to adopt a more conventional airfoil within thirty days of the tests in the event the wing did not come up to specifications. A one quarter scale model of the wing was designed and constructed for tests in the wing tunnel at the California Institute of Technology.     The use of this airfoil on the Mustang would greatly add to the drag reducing concept that was paramount in all design phases of the airplane. The few applications of this foil, prior to this time, had been handbuilt structures which were finished to exacting tolerances. An absolutely smooth surface was necessary due to the fact that any surface break or rough protrusion would interrupt the airflow and detract from the laminar flow theory. Because of the exactness required, the foil had been shelved by other manufacturers due to the clearances and tolerances which are used in mass production. The engineers at NAA approached this problem with a plan to fill and paint the wing surface to provide the necessary smoothness. The foil which was used for the Mustang had a thickness ratio of 15.1 percent at the wing root at 39 percent of the chord. The tip ratio was 11.4 percent at the 50 percent chord line. These figures provided the maximum thickness area at 40 percent from the leading edge of the wing and resulted in a small negative pressure gradient over the leading 50-60 percent of the wing surface.     The B-24 bomber's "Davis" airfoil was also a laminar flow airfoil, which predates the Mustang's. However, the designers of the B-24 only knew that their airfoil had very low drag in the wind tunnel. They did not know that it was a laminar flow airfoil.     There were several aircraft modified by NACA, in the late 1930s, to have laminar flow test sections on their wings. Hence, such aircraft as a modified B-18 were some of the first aircraft to fly with laminar flow airfoils.     The boundary layer concept is credited to the great German aerodynamicist, Ludwig Prandtl. Prandtl hypothesized and proved the existence of the boundary layer long before the Mustang was a gleam in anyone's eye.       Example: First, lets get more specific about what laminar flow is. The flow next to any surface forms a boundary layer, as the flow has zero velocity right at the surface and some distance out from the surface it flows at the same velocity as the local outside flow. If this boundary layer flows in parallel layers, with no energy transfer between layers, it is laminar. If there is energy transfer, it is turbulent.     All boundary layers start off as laminar. Many influences can act to destabilize a laminar boundary layer, causing it to transition to turbulent. Adverse pressure gradients, surface roughness, heat and acoustic energy all examples of destabilizing influences. Once the boundary layer transitions, the skin friction goes up. This is the primary result of a turbulent boundary layer. The old lift loss myth is just that—a myth.     A favorable pressure gradient is required to maintain laminar flow. Laminar flow airfoils are designed to have long favorable pressure gradients. All airfoils must have adverse pressure gradients on their aft end. The usual definition of a laminar flow airfoil is that the favorable pressure gradient ends somewhere between 30 and 75% of chord.     Now Consider the finish on your car in non-rainy conditions. Dust and leaves have settled on the hood's paint. We go for a drive. At once the leaves blow off. But the dust remains. We speed up. Even if we go very fast, the dust remains because of the thin layer of air that moves with the car. If you drive with dew on your car, the dew will not so quickly be blown dry where the air flow has this thin laminar layer. Downstream, where the laminar flow has become turbulent, the air flow quickly dries the dew.     In the fifties this was dramatically shown in a photograph of the top of a sailplane wing (in-flight) that had dew on it. A few tiny seeds had landed on forward area the wing while on the ground. In flight these seeds, tiny though they were, reached through the laminar layer and caused micro-turbulence causing the dew to be blown dried in an expanding vee shaped area down stream of each tiny seed.

Additional information

Profile drag

This comprises two components: surface friction drag and normal pressure drag (form drag).     Surface friction drag: This arises from the tangential stresses due to the viscosity or stickinessof the air. When air flows over any part of an aircraft there exists, immediately adjacent to the surface, a thin layer of air called the boundary layer, within which the air slows from its high velocity at the edge of the layer to a standstill at the surface itself. Surface friction drag depends upon the rate of change of velocity through the boundary layer, i.e. the velocity gradient. There are two types of boundary layer, laminar and turbulent. Although all combat aircraft surfaces develop a laminar boundary layer to start with, this rapidly becomes turbulent within a few per cent of the length of the surface. This leaves most of the aircraft immersed in a turbulent boundary layer, the thickness of which increases with length along the surface. The velocity and hence pressure variations along the length of any surface can have adverse effects on the behavior of the boundary layer, as will be discussed later.     Surface friction drag can amount to more than 30% of the total drag under cruise conditions.     Normal pressure drag (form drag): This also depends upon the viscosity of the air and is related to flow separation. It is best explained by considering a typical pressure distribution over a wing section, first at low AOA and then at high AOA.     At low AOA the high pressures near the leading edge produce a component of force in the rearward (i.e. drag) direction, while the low pressures ahead of the maximum thickness point tend to suck the wing section forward, giving a thrust effect. The low pressures aft of the maximum thickness point tend to suck the wing rearwards, since they act on rearward-facing surfaces. Without the influence of the boundary layer, the normal pressure forces due to the above drag and thrust components would exactly cancel.     There is a favorable pressure gradient up to the minimum pressure point, with the pressure falling in the direction of flow. This helps to stabilize the boundary layer. Downstream of the minimum pressure point, however, the thickening boundary layer has to flow against an adverse pressure gradient. Viscous effects reduce momentum within the boundary layer, and the thickness of the layer further increases so that the external flow sees a body which does not appear to close to a point at the trailing edge. A narrow wake is formed as the boundary layer streams off the section. This prevents the pressures on the aft-facing surface of the wing section from recovering to the high value obtaining near the stagnation point on the leading edge, as they would have done if a boundary layer had not formed. There is thus a lower than expected pressure acting on the aft facing surface, giving rise to normal pressure drag. In the low-AOA case this component is small, most of the profile drag being made up of surface friction drag.     As the AOA of the wing section is increased, the point of minimum pressure moves towards the leading edge, with increasingly high suction being achieved. This means that the pressure then has to rise by a greater extent downstream of the minimum pressure point and that the length of wing surface exposed to the rising pressure is increased. The resulting adverse pressure gradient becomes more severe as AOA is increased. This has serious implications for the boundary layer, which is always likely to separate from the wing surface under such conditions.

Inherent Stability
	A slight wing dihedral of an aircraft wing's will creates inherent stability.
	Inherent stability is the tendency of an aircraft to return to straight and level flight, when the controls are released by the pilot. Most aircraft are designed with this in mind and are said to be "inherently stable."     High-performance aircraft, such as fighter planes and aerobatic aircraft, often have little or no inherent stability and when the pilot releases the controls, the aircraft may bank or pitch in one direction or another. These aircraft take much more skill and concentration to fly safely, while the most sophisticated aircraft are computer controlled. Most civilian aircraft are designed to provide a high amount of inherent stability.    Inherent stability was first discovered by Sir George Cayley, but not fully understood until it was later theorized by Alphonse Pénaud.1
The Grumman X-29 has no Inherent Stability and must be computer controlled.

Inspecting an Airplane Before Flight - 1918
	Examination of the powerplant should be thorough.
	Inspection of Propeller -- Power-Plant -- Gasoline and Oil System -- Cooling System Parts -- Landing Gear -- Fuselage Nose -- Wing Fittings -- Brace Wires -- Struts -- Ailerons -- Rudder -- Fuselage Interior -- Stabilizers
	Control Wires--Tail Skid.
	IT is important that all parts of an airplane should be inspected thoroughly before the machine is allowed to leave the ground, and this inspection must be carried on periodically while the machine is in service. The inspection should follow a certain well-devised and logical sequence of events, and should not be done in a haphazard manner. Unless the inspection processes follow logically and in a regular order, the inspectors are very likely to omit some important part that may result in faulty action while in flight. A series of special illustrations which accompany this chapter have been posed by a practical aviator, and are intended to bring out the important points that should receive periodical inspection.
Inspection of Propeller. The first point that should receive attention is the propeller. It should be carefully examined to determine that the blades are in good condition. This means that they should be clean and well polished, and if provided with copper or cloth tips, these should be securely in place. Any splinters or cracks in the blade may result disastrously; and the propeller should be removed unless both blades are absolutely sound. The hub-assembly and the propeller should be inspected with a view to locating any looseness in the propeller hub bolts, or the nuts and cotter pins. After a propeller has been in use for a time the hub flanges may compress the wood and the propeller be loose in the hub. This condition is easily remedied by screwing down the propeller hub flange retention knots until the propeller is securely clamped. Another point that should be looked at is the method of holding the propeller to the engine shaft. This may be determined by grasping the propeller firmly and shaking it to see if there is any lost motion between the hub and the shaft. If the hub retention nuts have not been properly applied some looseness is apt to develop after the machine has been in flight. A propeller should fit the engine shaft absolutely tight, because any looseness will result in injurious vibration. Examining landing gear brace wires.     Inspection of Power-Plant. The power-plant is the next point which should be thoroughly checked over, and as previously emphasized, the pilot should not accept anybody's opinion that the power-plant is in good condition. He should satisfy himself of this before the machine leaves the ground. The radiator and all water connections should be checked over to see that there are no serious water leaks. It is also important that the radiator be full of water. The oil indicator on the side of the crank case, in some engines, will show the amount of oil there is present in the sump. The external oil lines, particularly those leading to the oil pressure gauge, should be absolutely tight, and all piping that conveys oil must also be examined to see that the joints are securely fastened and that there is no opportunity for loss of lubricant. The fuel system demands a more rigid inspection than either the cooling or oiling systems because a gasoline leak is apt to be the cause of fire and, of course, should be guarded against.     The points that should be inspected most carefully are the joints in the pipe line at both fuel tank and carburetor. If a gravity feed system is installed, the inspector should make sure that the vent in the tank filler cap is free and clear so that it will admit air to the tank. If a pressure feed system is fitted it is important that the tank cap and piping conveying air pressure be absolutely tight. The relief check valve should be tested to see if the pressure releases at the proper point. Excessive pressure is apt to result in excessive fuel consumption. of course, it is important that the tank be full of gasoline. The hand pump should be tested to make sure that it is in proper working condition. If a strainer or filtering device is included in the fuel pipe line this should be emptied from time to time to clean out any water or sediment that may be trapped therein.     The engine should be run slowly to make sure that it is firing on all cylinders and then speeded up to be sure that it develops good power. The clearance between the valve operating mechanism and the stems of the intake and exhaust valves should be checked over. All wiring must be clean and the insulation whole. It is important that all connections be tight. The grounding switch for cutting out the magneto should be tested to make sure that it functions properly. The rod or wire connection going from the hand throttle lever to the throttle of the carburetor should be inspected as, if it should become loose in flight, the throttle might jar closed and seriously impair the power production of the engine. Both magneto and carburetor should be firmly attached, the former to the bracket of the engine base, the latter to the induction manifold. The oil pressure should be carefully watched to make sure that it is sufficient for the engine in question. oil pressures will vary from twenty to sixty pounds, depending upon the design and type of the engine. Examining wing fitting and landing and flying wires.     When examining the power-plant, especial attention must be directed to the parts of the magneto that have to do with the timing and distribution of the ignition current. This means that the distance between the breaker points should be checked to make sure that it is adequate and it is well to remove the distributor board to examine the contact brushes and the current distributing segments if there is any tendency for the engine to misfire slightly.     Landing Gear Inspection. While at the front end of the airplane the next logical point to inspect will be the landing gear. The point that should receive attention first is the tension of the bracing wires that run from the fuselage longerons to the landing gear strut fittings. Next, the attachment of the wiring to the eyebolts in the landing gear and the security wiring on the turnbuckles. All the nuts and bolts on the strut sockets should be examined to make sure that none of the nuts have loosened up, and that all the cotter pins are in place. Examine the wheels to see that there are no loose or broken spokes and that the wheels run true. See that the tires are properly inflated and make sure that they have no weak spots or cuts in the casing that might result in a blow-out when landing.     The wheels should be tested to make sure that they run freely on the axle and the lock member holding the wheel in place on the axle should be inspected to make sure that it is securely in place. The shock-absorber rubber should be wound evenly and have the proper tension and should be clean. In some types of airplanes, the oil will drip from the engine compartment and flood over the rubber shock absorbers, which produces the rotting effect on the cable, thereby weakening it and resulting in premature depreciation. The wooden fairing on the axle should be inspected to make sure that it is not cracked or split and that there are no splintered pieces projecting from it.     Fuselage Nose Parts. While at the front end of the machine, examine carefully the front end of the fuselage to make sure that the radiator is properly secured to the carrier plate and that the carrier or nose plate is properly secured to the front end of the fuselage longerons. The engine bed and engine retaining bolts should be examined to make sure that all parts are held tight. The wire braces in the fuselage should be examined with special care in the front compartment, as considerable strength is imparted to the engine carrying portion of the fuselage by these wires. They should be tight and the turnbuckles should be well safety wired. Another point at the fuselage nose is the anchorage of the wind drag bracing, or the drift wires as they are called. Two of these are found on each side of some types of airplanes, one leading to the lower wing, the other to the upper wing. The soldered ends of these wires should be examined to see that the retention fittings are in the proper tension. Another point that demands inspection is the fastening of the motor compartment cowls and the motor hood cover. These must be secured and all screws that hold them to the fuselage should be in place. Special care is needed in examining any inspection doors in the motor compartments, as these are apt to be left unsecurely fastened and on some types of machines may open up and shake around when the machine is flying. Looking over top control horn on aileron. Inspecting lower control horn on aileron.     Wing Fittings and Struts. The next points to examine are the wing panels and the points of attachment to the fuselage.     The best method of doing this is to examine completely the wing panels on one part of the machine before taking those on the other side. There are four points of attachment for the wings on each side of the fuselage, two for the upper wing and two for the lower. The wing fitting pins should be in place and properly cottered and safety wired. When this point has been checked off, the flying wires should be examined, one after the other. on those types of machines where double flying wires are used, it is imperative that equal attention be paid to each wire. The wires should not only have the required tension, but should not be so tight that the struts between the wings are bowed. The struts should be good, clear wood and have no knots or curly grains. After the flying wires have been checked over, the landing wires which are the single cables should be inspected. While these are not as important as the flying wires, at the same time they should have the proper attention and all fittings should be secured. All wires and turnbuckles should be cleaned and greased with graphite and hard grease to prevent all chance of rusting. The wing fittings at the base of all the struts should show no signs of distortion, and any extending tongues to which bracing wiring is attached should not be bent in such a way that the wire cannot exert a straight pull. The bolts going through the sockets at the base of the struts and through the wing fittings should be properly tightened, and the nut on each bolt should be retained with a cotter pin. The struts should not be loose in the wing fittings. This can be ascertained by hitting the side of the strut a sharp blow with the open hand at a point near the fitting. Any lost motion or looseness will be made evident by a clicking noise at the fitting. The incidence wires should be tight, as well as the landing and flying wires. These are the wires that go from the top of a pair to the bottom of the other of the same pair and are clearly shown in Fig. 101 in preceding chapter. Examining control wires.     Inspecting Ailerons. An important member of the control system that should be inspected as part of the wing panel is the aileron or balancing flap. This should be easily operated and should not be distorted or bent in any way. The various points of the hinge assembly should be gone over to make sure that the pins are not unduly worn and that they are securely fastened. A few drops of oil should be applied to the hinges periodically and if the aileron is removed for any reason, oil and graphite should be introduced between the hinge pin and its bearing. The control wire connections at the control wire, or pylon, should be checked over one by one to insure that all clevise pins are properly fitted and that the wire ends leading to the clevises have secure joints. Special attention should be paid to control wires as if these are frayed at any point they should be replaced at once. The pulleys over which control wires run should be inspected for cracks and should be greased to make sure that they will be free running. All ailerons are checked in turn. on some types of machines but two ailerons are used, one on each top wing, while on others four are provided, one on each wing tip.     Fuselage Interior. Before working down to the empennage, or tail of the machine, the cover should be taken off of the fuselage and the various wires used for bracing or control purposes should be checked over to see that they are at the required degrees of tautness, that none of the fittings are cracked or broken, and that all turnbuckles are properly safety wired throughout the fuselage. The inspection of the fuselage is an especially important matter in event of the machine having made a rough landing, or having been in use on service that required frequent "taking-offs" and landings as instructions at an aviation school. A rough landing is very apt to loosen up the brace wires in the fuselage, especially if a tail-low landing is made and the strain is taken by the tail skid before the wheels touch the ground. Control pylon of elevator showing wire control cable and hard wire bracing. Testing stabilizer attachment to fuselage. Control horn of rudder showing double control cable, clevises, and hard wire bracing. Testing elevators and attachment to stabilizer.     Stabilizers and Control Wires. In examining the horizontal stabilizer, the only points that demand special attention are the bolts that hold it in place on the fuselage and also the braces that extend from each side of the rudder posts to the under side of the stabilizer. In examining the elevators, the hinge assembly by which they are attached to the rear end of the horizontal stabilizer and the control horn should be gone over carefully. The same applies to the rudder, only in this case the hinge assembly is attached to the rudder post at the rear end of the fuselage. What has been said in regard to the bearing points and control wires of the other control surfaces apply just as well to those of the rudder. Testing rudder post and landing gear.     Just ahead of the rudder a vertical stabilizer fin is installed. The only points about this that demand attention are the bracing wires and the bolts and nuts by which these are fastened to the horizontal stabilizer. While at the rear end of the machine the tail skid should be looked over with special reference to the supporting hinge or swivel which is attached to the tail post of the fuselage, also to make sure that the wood is not cracked or splintered. The tail skids of most airplanes are provided with a removable shoe of steel which forms a rubbing surface when the tail skid tracks on the ground, as in flying or "taxi-ing." As soon as this shoe show signs of wear it should be removed and replaced with a new one, as this will save the tail skid and is much easier to do than replacing an entire tail skid member. Special attention should be paid to the shock absorber rubber of the tail skid.     The wing skids at the end of each wing on a machine of considerable spread should be looked at to make sure that these are properly secured and not cracked. The control system parts should be checked over periodically and operated to make sure that all the control surfaces operate as they should. In the Dep. control, the cable passes over a drum having a series of grooves cut into it to form a continuous spiral around which the control wire is wrapped. The drum around which the wire is coiled is not always of large diameter, and if wire of exceptional stiffness is used, or one that is not exactly the proper size, it is apt to fray, due to the sharp turn it is forced to make whenever the control is operated.     If the machine is provided with a stick control, special attention should be given to the universal joints which make it possible to move the stick forward and the control bar sideways at the same time. Naturally, every one of the multiplicity of connections at the control horns must be examined in con- nection with checking over the control system. Points that are apt to be neglected, such as where the wire runs inside the fuselage, are those which really demand inspection oftenest. By checking over the points enumerated carefully to ascertain if the machine is in proper flying condition before it leaves the ground, all danger of accident when in the air is minimized.

Ground Effect in Aircraft
	Ground effect is caused by ground interference with airflow patterns around an aircraft when the aircraft is within one wingspan of the surface. If the approach airspeed is too fast, the aircraft will tend to float down the runway, delaying touchdown of the aircraft. This can create a dangerous condition where the aircraft may actually run out of runway space, creating an unsafe landing condition. If this situation were to occur, a prudent pilot would perform a go-around and land again. Ground effectapplies to all fixed-wing aircraft, including sailplanes and microlights.
	The wingtip vortices streaming behind an aircraft follow a downward inclined path for some distance behind the aircraft and then gradually level out to follow a path at a lower level than the aircraft and also drift apart. The pressure pattern about an aircraft flying outside ground effect becomes almost cylindrical, with positive pressure below the wings and negative pressure above the wings. These pressure differentials are felt quite a distance from the airframe, and the cylinder of affected air has a diameter close to the wingspan of the aircraft.
	When the aircraft is close to the surface, the almost cylindrical vortex-induced circulation around the wing outside ground effect is modified by coming into contact with the surface. This flattens the cylindrical circulation pattern as well as reducing the downflow angle of the airflow behind the wing. The flattening on the circulation pattern spreads the pattern outwards below the wing, thus increasing both the effective span of the wing and its aerodynamic aspect ratio.
Ground Effect Vehicles
	During the 1980s, the Soviets continued testing various ground effect vehicles for use in coastal defense and amphibious operations. The ORLAN-Class takes advantage of the increased aerodynamic lift that occurs when a wing operates near the surface. This greatly increases the craft's ability to carry heavy loads over long distances, especially over water, making it well-suited for amphibious warfare.     In 1966 the Central Hydrofoil Design Bureau under Rostislav Alekseev produced a gargantuan "ekranoplan" ("surface plane") combining the smooth hull form of a ship with stub wings, a large vertical fin and horizontal tail. The craft featured ten engines: eight mounted in two clusters of four directly behind the cockpit to provide augmented lift, and two on the vertical fin to provide cruise power. This machine, which American intelligence organizations dubbed the Caspian Sea Monster, could lift 540 tons and cruise at over 300 mph at an altitude of over 10 feet.

	The three primary flight controls are the ailerons, elevator and rudder. Move mouse over the flight controls to see text below.