The Science Behind an EFFICIENT Winglet Design

Many of you probably already know what a WINGLET is. If you don't, you probably have seen them on airliners or RC Flying Wings. You may even know what they do; but do you know how to MAXIMIZE their potential?

To get a general idea of winglets and finding an efficient design, watch this video first. Then read on below for some more detail and thoughts regarding RC aircraft and winglets.

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UPDATE 10-31-18: In the video I  said something about "Winglets move the tip vortex back behind the wing" and this isn't entirely true. The key takeaway is that they capture the vortex in such a way that they can put a small amount of the energy into a forward vector, actually producing thrust from the tip vortex instead of drag.

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So now you know a bit about winglets, their function, and their dual purpose for RC Flying Wings; but did you know that MOST 'winglet' designs on our RC flying wings aren't actually increasing efficiency at all? What is commonly referred to as 'winglets' on RC wings are actually wingtip fences. Although similar to winglets in function, wingtip fences are much less efficient due to the excess increase in surface area on the wing (causing drag). This is okay, because wingtip devices are put on RC flying wings mostly for yaw stability, as most foamie models are not created for maximum efficiency anyway (for a typical basher wing, a proper winglet would probably make a negligible difference in efficiency). However, when you start creating long-duration RC aircraft, you may want to pay more attention to your wingtip design. The most important key is to not oversize the winglet. Only make the winglets as big as you need for yaw stability. If possible, take care to blend the winglet into the wing. A good example of this is is on the old Skywalker X8.

A quick quote from "The Design of Winglets for Low Speed Aircraft"

“It has been known for over a century that an endplate at the tip of a finite wing can reduce the spanwise flow and thereby reduce the induced drag. Unfortunately, to be effective at this, the endplate must be so large that the increase in skin friction drag far outweighs any induced drag reduction. A winglet, rather than being a simple fence that limits the spanwise flow, carries an aerodynamic load producing a flowfield that actively interacts with that of the main wing to reduce the amount of spanwise flow. That is, the downwash (sidewash) produced by the winglet opposes the spanwise flow on the main wing. This effect has been measured experimentally and is shown in Fig. 5, where it can be seen that the spanwise flow has been largely eliminated by the presence of the winglet. In essence, the winglet diffuses or spreads out the influence of the tip vortex such that the downwash, and consequently the induced drag, is reduced. In this way, the winglet acts like an endplate in reducing the spanwise flow but, by carrying the proper aerodynamic loading, it accomplishes this with much less wetted area. Nevertheless, recalling the penalty of profile drag with increasing airspeeds, the designer’s goal is that of gaining the largest reduction in induced drag for the smallest increase in profile drag."      -Mark D. Maughmer [http://www.mandhsoaring.com/why%20winglets/wl-it.pdf]

Now, should you be worried that your FT Versa wing is totally inefficient and terribly designed? No. Technically speaking, most RC aircraft are extremely inefficient. But that wasn't what they were designed for; slap a well-designed winglet on your foamie instead of that wingtip fence, and you'll get an increase of maybe 5% or so... about 30 seconds extra flight time on your 10 minute flight.

So no, you shouldn't be too worried about your next winglet design, unless you are doing multi-hour flights. However, I find the aerodynamics aspect intriguing so I will be building a test platform (simple flying wing) to test some 3d printed winglet designs. If you are interested in seeing that, subscribe to me on YouTube and check back here often because I will be posting supplementary articles along with the weekly videos.

https://www.youtube.com/user/corsair2014