Why do some aeroplanes have a bend at the end of the wing?

Answer 1

The small "winglet" at the tips of the wings increases the lift by forcing a vortex to remain at the wingtip.

An airplane wing forces air downward. At the same time, air from below the wing leaks around the wingtip and moves to the top, and a rotating "vortex" appears at the wingtip. Sometimes the location of this "leakage" process moves away from the wing tip, and the vortex migrates inboard along the wing toward the fuselage. The vortex then extends from the middle of the wing and not from the tip. When this occurs, negative lift is produced by the part of the wing outside the vortex location. The function of the small winglet is to increase lift by forcing the vortex back to the wingtip.

to improve lift at the tip of the wing by redirecting and smoothing airflow - otherwise some wings at the tip have practically no lift, or in other words are nearly stalled - although its easy to see the large upwards pointing winglets on larger aircraft, some smaller aircraft (usually ones with high wings) also have a smaller version that is a slightly down curved wing tip that does the same thing

Some aircraft have small winglets at the tip of the wings. This improves the lift of the wing. As the airflow passes over the wing, the flow on top of the wing has to travel a longer distance, so it speeds up and thus reducing pressure. This reduction in pressure on Top of the wing and the increase in pressure of the air flowing along Bottom of the wing produces the lift.

However, the airflow wants to travel the path of least resistance. So, it tends to move Outboard away from the fuselage and toward the tip of the wing. The airflow at the tip slides off the end of the wing and produces a Vortex. This vortex can sometimes be seen under certain atmospheric conditions as vapor trails or contrails. This vortex is a lost Lift.

The winglets reduce this vortex greatly and thus prevents lost lift.