I’m going to break this up into a two part-er of why I think these aren’t all they’re cracked up to be. So here goes:
First off: I am a HUGE fan of biologically inspired design. Nature generally does everything better than we can. Case in point: wings. High efficiency gliders and recently developed composite passenger aircraft.
The albatross is (arguably) the most fascinating bird out there, it can fly literally 1000’s of miles without flapping it’s wings once. Half of the the equation explaining how it can do this are its wings:
It’s almost like a 747 and a sailplane had a baby: the 787. The reason why the 787 has such a funky wing shape is because the name of the aircraft design game is: efficiency. Now take a look at this wing you’d find on mondern-ish to old aircraft:
You should notice two things: first, are the small nubs halfway down the wing, these are called ‘vortex generators’ and second, the wing sticking straight up at the end of the wing (insert Yo Dawg joke). These two devices try to accomplish the same thing: reduction of drag caused by lift (lift causes drag, which is a whole Aero chapter onto itself).
However these are not elegant solutions. The vortex generators are literally creating turbulent drag in order prevent stalls in the wing during take-off and landing, but generate a not insignificant and unwanted amount of drag during 99.999% of the rest of the flight.
The vertical wing-tip also reduces drag caused by blocking the swirling wing tip vorticies, HOWEVER the price you pay is you have to drag two of these vertical wings that don’t contribute to lift of the aircraft. Both of these devices reduce overall drag by creating more drag in a particular way, but the benefits outweigh the drag penalties of these devices.
Now comes Boeing, and more importantly the new carbon manufacturing techniques of the 21st century that allows ANY shape to be crafted with the structural integrity required for a wing. With exhaustive design and testing they came to the shape seen above which, lo and behold, looks an awfully like one of the most efficient wing designs crafted by eon’s of evolution. The wing essentially accomplishes the same goal as the wing tip and vortex generators, but without the massive drag penalty they incur.
Now, I would not say off hand that all the dimples and bumps put on a Zipp wheel make it less aerodynamic since I do not have any data to back that claim up. However, dimples and and bumps represent a trade-off: their mechanism has to benefit the design more than hurt it. Generally the trade off works like this: increased skin friction drag for lower pressure drag.
The golf ball is a great example, the dimples increase the surface roughness of the ball (increasing the skin drag), but reduces pressure drag. The reason why this works so well on a golf ball is because….it’s a ball….it has to be round. An aerodynamic rim on the other hand, has a lot more play room with regards to it’s shape, if you were to design the optimal golf ball, it’d look a lot more like a rain drop (it just wouldn’t be very hit-able).
This is why I think that things like these dimples or hump bumps (I’ve so named since the design seems to come from humpback whales), are somewhat gimmicky. Like Boeing showed, solutions can be found for complex flow problems without using these flow devices to ‘force’ the flow to do what you want it to do. I’m sure the micro-vorticies do help in the way that Zipp claims, but I’m skeptical if the improvements they generate are offset by their disturbance.
Great design in nature is elegant, smooth, and most importantly simple (because physics). We should all try to mimic it.
Stay tuned for part 2!