Why don't all planes have winglets?

Answer 1

Wings generate lift by having a difference in air pressure between the top and bottom of the wing. Without a winglet, the higher pressure from the bottom of the wing will spill over to the top of the wing, reducing the overall lift generated. Prior to installing winglets, the designer had to account for this lost lift, and would create longer wings, with the tip extending the spill over out as far as possible. By installing a winglet, the spill over is blocked from the top of the wing, ensuring that the top of the wing generates lift all along its length. This allows for a shorter, lighter wing, reducing the so-called 'empty weight' of the vehicle, thereby improving fuel efficiency.

Smaller planes are generally designed to have much lower air speeds, so have a wider wing without much (if any) taper as it approaches the tip. While the spill over still occurs, the different wing shape makes the loss of lift at the tip less critical. The wider wing at the tip would require a larger winglet, adding additional weight without allowing a shorter wing.

As the wing becomes tapered more, the spill over at the tip becomes more important. This is why the single person BD-5 jet or glider planes sometimes have winglets, while the multi-passenger Cessna doesn't.