question: why are the tips of the wings on an airplane curved up?
comment: There's a low pressure area on top of the wing, and a high pressure area bellow it. This is caused by the shape of the wing, and it's what allows an aircraft to fly. Because of this pressure difference, the air wants to flow from the bottom of the wing to the top, and at the wingtip it can do this. This leads to a loss of lift at the end of the wing, because the pressure can equalise. It also causes strong vortices called Wingtip vortices that are very hazardous to aircraft flying behind. Winglets/sharklets prevent/reduce this flow of air over the tips and thereby increase the lift provided by the wing
passages: Licensing the design became one of the company's major sources of income in the 1920s. The original designs were in the form of a fixed slot near the leading edge of the wing, a design that was used on a number of STOL aircraft.
   Early airplane engines had little power, and lightness was very important. Also, early airfoil sections were very thin, and could not have a strong frame installed within. So, until the 1930s, most wings were too lightweight to have enough strength and external bracing struts and wires were added. When the available engine power increased during the 1920s and 30s, wings could be made heavy and strong enough that bracing was not needed any more. This type of unbraced wing is called a cantilever wing.
  
  Section::::Characteristics.:Wings.:Wing configuration.
   Washout is commonly achieved by designing the wing with a slight twist, reducing the angle of incidence from root to tip, and therefore causing a lower angle of attack at the tips than at the roots. This feature is sometimes referred to as structural washout, to distinguish it from aerodynamic washout.
   Some much older reports gave some similar results. A 1932 NACA report about the effect of leading-edge slots of various lengths said, "this is an indication that the slotted portion on each tip of the wing operates to some extent as a separate wing".
   The vertical location of the CG changes the amount of dihedral effect. As the "vertical CG" moves lower, dihedral effect increases. This is caused by the center of lift and drag being further above the CG and having a longer moment arm. So, the same forces that change as sideslip changes (primarily sideforce, but also lift and drag) produce a larger moment about the CG of the aircraft. This is sometimes referred to as the pendulum effect.
   Section::::Anhedral and polyhedral.:Polyhedral.
  
  Most aircraft have been designed with planar wings with simple dihedral (or anhedral). Some older aircraft such as the Vought F4U Corsair and the Beriev Be-12 were designed with gull wings bent near the root. Modern polyhedral wing designs generally bend upwards near the wingtips (also known as "tip dihedral"), increasing dihedral effect without increasing the angle the wings meet at the root, which may be restricted to meet other design criteria.
   The Lockheed Martin F-22 Raptor has chines along its nose section that align with its engine air intakes. The small horizontal surfaces forming a fillet between the forward wing root and the air intake is more usually called a Leading Edge Root eXtension (LERX) or Leading Edge eXtension (LEX).
  
  Section::::Forward chines effects.
  
  Large chines along the forward fuselage can have a significant effect on aircraft lift, drag, longitudinal balance and directional stability.
  
  Section::::Forward chines effects.:Effect at high Mach numbers.
   Early airplane engines had little power and light weight was very important. Also, early aerofoil sections were very thin, and could not have strong frame installed within. So until the 1930s, most wings were too light weight to have enough strength and external bracing struts and wires were added. When the available engine power increased during the 1920s and 1930s, wings could be made heavy and strong enough that bracing was not needed anymore. This type of unbraced wing is called a cantilever wing.
  
  Section::::Characteristics.:Wings.:Wing configuration.
   On some supersonic aircraft a chine extends sideways for some distance, with a very sharp edge blending in with the main wing leading edge root. The rest of this article is concerned with this type of chine.
  
  Section::::Configuration.
  
  A chine can in aerodynamic terms act as a long extension of the wing root along the fuselage. Such chines first appeared on the Lockheed A-12 precursor of the SR-71 Blackbird, where they ran forward from the wing roots along the fuselage sides, into which they blended.
   A similar problem affected some models of the Supermarine Spitfire. At high speeds, the ailerons could apply more torque than the Spitfire's thin wings could handle, and the entire wing would twist in the opposite direction. This meant that the plane would roll in the direction opposite to that which the pilot intended, and led to a number of accidents. Earlier models weren't fast enough for this to be a problem, and so it wasn't noticed until later model Spitfires like the Mk.IX started to appear. This was mitigated by adding considerable torsional rigidity to the wings, and was wholly cured when the Mk.XIV was introduced.
   BULLET::::- To improve low-speed performance, a de Havilland DH4 was modified by Handley Page to a monoplane with high-lift devices: full-span leading-edge slats and trailing-edge flaps; in 1924, Fowler flaps that extend backward and downward were invented in the US, and used on the Lockheed Model 10 Electra while in 1943 forward-hinged leading-edge Krueger flaps were invented in Germany and later used on the Boeing 707.
  
  BULLET::::- The 1927 large Propeller Research Tunnel at NACA Langley confirmed that the landing gear was a major source of drag, in 1930 the Boeing Monomail featured a retractable gear.
   Section::::Developments in aviation.
   Twist that decreases the local chord's incidence from root to tip is sometimes referred to as "washout". Twist that increases the local incidence from root to tip is less common and is called "wash-in". The X-29 had strong wash-in to compensate for the additional root-first stalling promoted by the forward sweep.
   Polar curve (aerodynamics)
  
  A polar curve is a graph which contrasts the sink rate of an aircraft (typically a glider) with its horizontal speed.
  
  Section::::Measuring a glider's performance.
   Section::::Development and use.:Limitations and abandonment.
  
  Only later was the reason for the Davis wing's performance properly understood. Largely through accident, the shape maintained laminar flow further back from its leading edge, to about 20 or 30% of chord compared to the 5 to 20% managed by most airfoil sections of the era. Although other designs greatly improved on this, with some designs maintaining laminar flow to upwards of 60% of chord, the Davis wing represented a great improvement at the time. 
   Octave Chanute's 1893 book, "Progress in Flying Machines", outlined all of the known research conducted around the world up to that point. Chanute's book provided a great service to those interested in aerodynamics and flying machines.
   Forward chines also act as leading edge root extensions (LERX) at low speeds and high angles of attack, generating a vortex flow over the inboard wing to stabilise the airflow and increase its speed locally, thus delaying the stall and also providing additional lift.
  
  Section::::Forward chines effects.:Directional effect.
   In real-world designs, the wing root, where the wing meets the fuselage, is thicker than the wing tip. This is because the wing spar has to support the forces from the entire wing outboard, meaning there is very little force on the spar at the tip, and the lift force of the entire wing at the root. Spars generally get much larger as they approach the root to account for these forces, and streamlining the wing profile around such designs generally requires the wing to be much thicker and be more heavily curved at the root than the tip.
   BULLET::::- The Fokker Eindecker (1915), the first German fighter aircraft.
  
  BULLET::::- The Fokker D.III (1916), one of the last operational fighters with wing warping.
  
  BULLET::::- The Christmas Bullet (1919), whose wing warping and unbraced sprung steel spars contributed to its notoriety.
  
  Section::::Modern re-assessment.
   At high speeds, both subsonic and supersonic, the wing would be pivoted at up to 60 degrees to the aircraft's fuselage for better high-speed performance. The studies showed these angles would decrease aerodynamic drag, permitting increased speed and longer range with the same fuel expenditure.
   The center of mass, usually called the center of gravity or "CG", is the balance point of an aircraft. If suspended at this point and allowed to rotate, a body (aircraft) will be balanced. The front-to-back location of the CG is of primary importance for the general stability of the aircraft, but the vertical location has important effects as well.
   The purpose of dihedral effect is to contribute to stability in the roll axis. It is an important factor in the stability of the "spiral mode" which is sometimes called "roll stability". The dihedral effect does not contribute directly to the restoring of "wings level", but it indirectly helps restore "wings level" through its effect on the spiral mode of motion described below.
  
  Section::::Uses of dihedral angle and dihedral effect.:Wing clearance.
   Early examples provided a solution to the problem of supersonic flight when engine power was limited. They were made so thin that they had to be machined from a thick, solid sheet of metal. Even with this low-drag wing the Douglas X-3 Stiletto was too underpowered to reach its design flight speed of Mach 2, but the design of its simple hexagonal-airfoil wing was developed for various other X-planes and for Lockheed's widely produced F-104 Starfighter Mach 2.2 high-altitude interceptor.
   BULLET::::- Canard — is an aeronautical arrangement wherein a small forewing or foreplane is placed forward of the main wing of a fixed-wing aircraft. The term "canard" may be used to describe the aircraft itself, the wing configuration or the foreplane.
  
  BULLET::::- Centennial challenges —
   Section::::History.
  
  Pitch-up problems were first noticed on high-speed test aircraft with swept wings. It was a common problem on the Douglas Skyrocket, which was used extensively to test the problem.
  