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Old Jun 22, 2013, 06:41 PM
greg
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Originally Posted by BMatthews View Post
So the key would seem to be to experiment with some area and to set up a way to slightly angle the tip fin from slightly toed in to slightly toed out and see which setting seems to let you glide better or maintain altitude with less power.
while i assume any type of panel might help reduce the flow from the underside to the topside of the wing, which may improve the efficiency of the wing near the wing tip

i thought a well designed winglet is actually an airfoil facing the aft/horizontal-ish flow inward on the topside surface of the wing that develops lift in the direction the aircraft travels (i.e. forward and opposite the drag force) when you combine the effect from both winglets.

not sure how an inward facing winglet could accomplish this.
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Old Jun 23, 2013, 02:12 AM
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Greg,

Turns out that a traditional winglet (one that goes up from the wingtip) must "lift" inward for minimum drag. The paper here discusses optimal loading on non-planar lifting surfaces. Figure 18 shows the direction various surfaces must lift for minimum drag.

The basic idea is that drag is minimized when the vorticity shed to the wake is spread out. Although a vertical winglet contributes no lift, it allows you to reduce the concentration of vorticity in the wake. This is why a taller winglet is generally better. The circulation along the span of a wing with an "outward lifting" winglet changes sign at the transition from the wing to the winglet (if you took an outward lifting winglet and "bent it down" to align with the wing, it would be lifting downward). This leads to a large concentration of shed vorticity at the tip (an outward lifting "upward winglet" actually strengthens the tip vortex). It is not true that any vertical panel at a wingtip will improve efficiency.
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Old Jun 23, 2013, 05:25 AM
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Greg, here's a quick study of winglet toe-in / toe-out. An un-toed winglet will be lifting "inward". Best L/D for the configuration shown (at 6 deg AOA) was abut 0.5 degrees of toe-in. You can see that a outward lifting winglet really hurts induced drag.
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Old Jun 23, 2013, 10:11 AM
greg
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i assume there are different types of winglets.

the dryden website has web page on Winglets. I believe the 4th paragraph under benefits describes how winglet can produce forward thrust.
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Old Jun 23, 2013, 10:46 AM
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I think you meant "can reduce drag". If a winglet could provide thrust we'd have perpetual motion.
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Old Jun 23, 2013, 11:16 AM
greg
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Originally Posted by Brandano View Post
I think you meant "can reduce drag". If a winglet could provide thrust we'd have perpetual motion.
i believe the NASA web page describes a mechanism that actually generates a force opposite drag, just as a wing generates lift while aircraft is moving. The sailing Here's the quote from the NASA page
Winglets, which are airfoils operating just like a sailboat tacking upwind, produce a forward thrust inside the circulation field of the vortices and reduce their strength.
i think it is difficult to design such a winglet or may only do so under a specific set of conditions, otherwise it may just reduce drag.
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Old Jun 23, 2013, 01:19 PM
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It always just reduces drag. That thrust is a logic artifice to explain how drag is reduced, essentially trying to bleed more energy off the pressure differential between the upper and lower surface to do useful work. If it is possible to maintain the pressure differential while reducing the bleed across the wing tip, the tip vortexes will be less pronounced, and less energy will be wasted while producing lift. This means less induced drag. It's not an easy task, mind you: somewhere the two air masses will mix again to reach the same pressure. Finding how to do this efficiently is the challenge.
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Old Jun 23, 2013, 04:24 PM
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Originally Posted by ciurpita View Post
i assume there are different types of winglets.

the dryden website has web page on Winglets. I believe the 4th paragraph under benefits describes how winglet can produce forward thrust.
Greg,

Agreed. A reduction in induced drag is in many (if not most) cases indistinguishable from a local increase in thrust. The purpose of my post was just to isolate one variable (the toe-in angle of an untwisted and otherwise fixed-geometry winglet). I believe this specific example illustrates something that is generally true about winglets: that the specifics of their geometry can strongly influence their effectiveness in creating thrust or reducing drag.

I think there's a belief that any plate that disrupts the formation of the tip vortex will reduce induced drag. That is not the case. Unless a winglet provides a beneficial redistribution of the vorticity shed to the wake, it can have a neutral or adverse effect on induced drag.
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Old Jun 24, 2013, 07:11 PM
greg
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i've read some people describe the wing tip losses as induced drag. I don't understand this.

I do understand that the vortex flow around the wing tip reduces the efficiency of the wing near the tip. I believe VLM quantifies this loss of lift. I wouldn't know how to quantify this in terms of drag.

My understanding of induced drag is that it is due to the lift vector leaning aft because the freestream over the wing has a downward angle due to downwash. The horizontal component of the lift vector is induced drag.

What if the lift vector actually tilted forward? this would be termed induced thrust.

of course you know that the airflow over the wing is normally parallel to the direction of the aircraft (but there is often lateral flow depending on planform shape). placing a wing vertically in this (?) normal flow would create (non productive) lift perpendicular to the direction of the aircraft. (Or as Shoe suggested, you can configure an winglet to produce outward lift, that would balance the pressure difference and reduce the vortex flow )

Another approach (not necessarily the best approach) is to take advantage of the vortex flow resulting in the airflow near the wing tip being angled slightly inward. A different winglet configuration could produce a horizontal lift vector tilted slightly forward ...

here's a modeling article that better explains my understanding of this approach: RCSD September 2002, page 5.

this is all i was try to describe. Based on the other responses, this approach may not be as practical or efficient as other methods
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