2
$\begingroup$

*Make sure to read the following as I cannot explain my question in one sentence

Winglets are a kind of wingtip device that is installed on an aircraft to reduce drag and improve fuel efficiency. There are two types of practical winglets that have already been installed on real aircraft, the canted winglet and blended winglet.

Winglets work by reducing wingtip vortices while producing an aerodynamic lift vector that points inboard and also slightly forward. The slightly forward component, as many author states, is due to the change in flow direction because of the wingtip vortices at a high angle of attack. With the increase in AoA, the main-wing part increases lift with a high pressure field below, than the winglet part, thus producing another vortex around the transition part of the main-wing and winglet. At higher AoA, the vortex also increases and strikes the leading edge of the specially placed winglet, forming this slightly forward-pointing lift.

My question rises when speaking of canted winglet and blended winglet. For canted winglet, the cant angle cannot be decreased too much, or

A sharp interior angle in this region can interact with the boundary layer flow causing a drag inducing vortex, negating some of the benefit of the winglet.

from https://en.wikipedia.org/wiki/Wingtip_device

In the case of a blended winglet, the transition part of the main-wing and the winglet is smoothed. It is being said that this feature decreases the lift and gives a better aerodynamic load. However, this smooth transition, at higher AoA, should also decrease the vortex formed at the transition, as this feature also provides a smooth lift force transition. Is this what they meant by

This feature sacrifices some of the potential induced drag reduction, in return for less viscous drag and less need for tailoring the sections locally.

according to Boeing?

It seems counter-intuitive that blended winglets reduce more drag than canted winglet, as canted winglet would use up more vortex formed to produce a thrusting lift force. Speaking of this, then, how is blended winglet better?

$\endgroup$

1 Answer 1

5
$\begingroup$

The blended winglet has two advantages:

  1. The smooth change of contours means that pressure gradients also are smooth and the flow remains attached through the operating range. The sharp corner of a canted winglet will provoke early transition in the corner, similarly to a poorly designed wing-fuselage intersection.
  2. The smooth winglet has less surface area for the same winglet height. Instead of two straight parts with the length of the smoothing radius each, the length of the transition is only $\pi$/2 times radius.
$\endgroup$
4
  • $\begingroup$ "pressure gradients also are smooth and the flow remains attached", Any chances that the canted winglet can use the more intense vortex formed around the transition part to create more "vortex induced flow (inboard tilted flow)", that consequently produces greater lift on the winglet, also greater forward-pointing vector of the lift? $\endgroup$
    – Frank
    Commented Dec 9, 2022 at 8:15
  • $\begingroup$ @Frank No, that vortex carries energy away. The winglet can only capture the incoming flow which comes at a sideslip near the wingtip. $\endgroup$ Commented Dec 9, 2022 at 11:22
  • $\begingroup$ What do you mean by "comes at a sideslip near the wingtip"? Isn't the sideslip flow triggered by the inboard circling vortices near the wingtip? $\endgroup$
    – Frank
    Commented Dec 9, 2022 at 11:41
  • $\begingroup$ @Frank No, the lateral speed component is the result of low pressure on the wing. This attracts more air from all sides and causes upwash and, near the tips, lateral flow. The vortex is only the result of this flow component. You are confusing cause and effect, but you are not alone. A lot of people do. But when you think about it, the suction explanation makes a lot more sense. $\endgroup$ Commented Dec 9, 2022 at 16:57

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .