Why do we not see winglets, tiplets, whatever you want to call them?

I see them on flying wings - apparently the primary purpose here is yaw stability. I have read the page on Martin Hepperle's site about winglets and it would appear to me that they would be useful to stretch the effective span on span limited classes, like HLG, MHLG and maybe even 2m. Like all things here, I am sure that it is not easy to design to get the most out of them, but why does nobody except the flying wing crowd use them?

From : Don Stackhouse

Winglets work well for single-operating-point designs, particularly ones that operate at high lift coefficients.

When lift is positive, the air on top of the wing is at a lower pressure than the air on the bottom. One of the side effects of this is that the air on the bottom spills around the wingtips to try to fill in the low pressure on top. The result is a pair of horizontal tornados trailing from the wingtips, which we call "tip vortices". Any time you put air in motion, it takes energy to do it, and the energy in those tip vortices represents the sum total of the energy consumed by the wing's induced drag.

When lift is high and speed is low, induced drag is high, and the energy in the tip vortices is high. Winglets work by recovering some of that energy in the tip vortices and putting it back into the aircraft. In effect they reduce the induced drag. A well designed winglet has about the same effect on induced drag as a horizontal wingtip extension about 2/3 the span of the winglet. It also increases the wing root bending moment (one of the biggest factors in the required size and weight of the wing spar) by an amount similar to a wingtip extension about equal to 1/3 of the span of the winglet. For a weight-critical wing or a span limited wing they can have significant benefits in the right cases, assuming the aircraft is properly designed. If your open class wing is overstrong to begin with, then adding winglets will not be as beneficial as simply reducing weight by optimizing the spar design. In any case, they will not help unless they are specifically designed to match the needs of that particular wing.

So why don't we see them on all well-designed wings? Remember, winglets work by recovering some of the energy from the wing's induced drag. The down side is that winglets have both parasite and induced drag of their own. Compounding this is the fact that the winglet's lift is sideways, not upward, so its lift is not adding to the wing's lift. As you increase airspeed and reduce lift coefficient, the induced drag of the wing decreases. At some point, the induced drag is so low that the energy recovered by the winglet is less than the energy consumed by the winglet's own parasite and induced drag. The airspeed where this occurs is called the "cross-over velocity", and at all speeds higher than this the winglets actually reduce performance. For models that have to penetrate, and for hlg's that have to launch at near zero lift coefficients, the winglets generally cost more than they save. This is why you don't see them on more applications.

They generally do well on flying wing foamies because these models usually need some vertical surfaces for yaw stability anyway, and the wingtips are the best place for them. In this case the winglets add not much more drag than a conventional fin, so their own energy consumption is already paid for by the energy they recover from the tip vortices.

There is another alternative, the "sheared tip" or "crescent planform". The airflow behind the wing is not flat, and a properly designed trailing edge extension behind the wingtip will look like a winglet to this 3-D airflow. The key difference is that since the lift developed by this extension is upward instead of inward, it adds to the total lift of the wing. Because of this key difference, this type of tip does not have a cross-over velocity; it improves performance at all airspeeds. Just like a winglet, it must be properly designed if it is to provide a benefit, and you must be careful to avoid creating other problems in the process. For example, one very popular HLG uses a plastic sheet gusset taped to the trailing edge at the tip to create the effect of a large sheared tip. Unfortunately it also butchers the airfoil shape in that region. I don't have enough data about this specific case to say what (if any) performance benefits that particular sheared tip might provide, but I do know that in some recent head-to-head performance tests with our Monarch 'D-lite', this model's performance although good, was was inferior to the Monarch's to varying degrees in virtually all measured areas.

Properly done, both computational performance predictions and wind-tunnel test results show that a good sheared tip design can improve L/D at all speeds, but the gains (as with any tip treatment) are small. If the tip design isn't carefully optimized, or if too many short cuts are taken in the construction, the net result can be a loss.

Don Stackhouse
DJ Aerotech