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I'm currently in the process of designing a winglet for my low speed aircraft in xflr.

While researching about winglets I read in a Boeing article that if the baseline outboard wing is relatively lightly loaded, as on the 747-200, the potential benefit of tip devices can be greatly reduced. I don't understand the logic behind this statement.

Also what would be the optimal airfoil selection for the winglet? How would the chord, thickness, camber, etc. affect the weight and drag reduction due to the winglet?

And would the geometric twist affect the performance of the winglet?

Any other tips for winglet design would be greatly appreciated.

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    $\begingroup$ A highly loaded wingtip makes the tip vortex stronger. The rotation of the vortex makes the apparent direction of the wind at the tip face inboard. The winglet toes out into this wind allowing its lift vector to rotate slightly forward which creates thrust or reduces drag, whichever you prefer. A lightly loaded wingtip makes a weak vortex, reducing the effect of the winglet. Designing one is as complex as designing a wing, so I'll let someone else take it from here. $\endgroup$ – Pilothead May 31 '18 at 19:46
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    $\begingroup$ @Pilothead You have it exactly right, with a minor quibble. Yes they make thrust the same as a sail boat's sail. Which is why they were originally called "tipsails". However while they are toed out to optimize AOA, the lift vector itself is a function of the inward angle of the tip flow, lift being at 90 degrees to the airflow, not AOA. Almost nobody actually mentions the "sail" aspect of a winglet, just that there is some mysterious drag reduction. $\endgroup$ – John K May 31 '18 at 21:30
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    $\begingroup$ @JohnK: You are right about the sail analogy, however, this is just a different manifestation of the effect a winglet has on the flow. It spreads out the vorticity gradient so the lift vector is less inclined backwards (which is almost equivalent to saying that the winglet contributes a forward-slanted component, right?). $\endgroup$ – Peter Kämpf Jun 1 '18 at 6:09
  • $\begingroup$ @Zaber - Your information is correct. In fact, some Japanese carriers experimenting with winglets found there where no fuel savings (even possible losses) on short haul routes within Japan. To be efficient, winglets need to be "tuned" for a specific speed and load. $\endgroup$ – jwzumwalt Jul 2 '18 at 4:24
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The comments explain the logic behind a winglet well, so I will focus on the remaining questions:

what would be the optimal airfoil selection for the winglet?

As a first rule of thumb, continue with the wing airfoil. A cambered winglet will be most efficient, because winglets are only beneficial if they produce lift. How much? This depends on the ratio of wing tip chord to winglet root chord. Ideally, there should be no break in the spanwise distribution of circulation, so the product of local wing chord and local lift coefficient should be the same at the transition and taper out towards the winglet tip.

would the geometric twist affect the performance of the winglet?

Yes: The incidence must be chosen such that the desired lift coefficient is trimmed at the operating point (which is the cruise condition in case of airliners). Twist will only be needed when the planform differs a lot from the elliptical distribution. When flying with a high wing loading (again, just as airliners do), be sure to include the effects of aeroelastic deformation: They can be quite severe around the wing tips in cruise.

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I'll provide a partial answer. What they are saying is related to the fact that the thrust generated by the winglet is only high enough to exceed its own drag penalty when the wing is producing strong vortices. The vortices are stronger as you get slower and AOA goes up. There is some crossover point as you speed up where the weakening vortices don't produce more thrust than the winglet's own drag and they no longer produce a net benefit.

The result is that they are only worthwhile on an airplane that cruises "slowly" from an indicated airspeed perspective. Generally, airplanes that cruise at low indicated airspeeds are those that cruise above 30000 ft, where they may only be flying at 2x stall or a little more. This is why you only see winglets on jet airliners as a rule (yes there are exceptions but not many, and the weird turned up wing tips on the draggy slug CL-415 water bomber aren't winglets at all).

I would only put a winglet on a low speed aircraft if its intended cruising speed is close to its best L/D, like a glider.

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