Kamm Tail Vetter Helix

There's been some discussion about tail fairings in general, and Kamm tails in particular, on Facebook of late (December 2016) so I thought I'd put this new photo up, taken back in March 2011, of Craig Vetter's Helix with its original prototype fairing, but in 'Kamm tail mode'. The long tail was only held on with bungee straps and could literally be removed in seconds. I rode it in both versions, and couldn't really feel much difference, but it was more nimble and a damn sight easier to park with a short tail! I urged Craig to run a full tank test of the machine in Kamm tail mode to see whether the long tail actually made any difference to the fuel consumption, but he liked the long tail so much, he said he just preferred it that way, and so fixed it 'long' when he made the proper Mk2 silver aluminium fairing which can be seen in several other photos in this folder.
This one, for instance:
There is another photo of Craig sitting in the machine in this mode, which I'm pleased to see has been viewed more than 5,000 times since I put it up back in 2011. Go Here:
There is also a photo of Terry Hershner's 2012 Vetterised electric Zero with a Kamm Tail here:http://bikeweb.com/node/2713
Terry reports that the machine was about 15% more efficient when fitted with the full tail with which he did all his record-breaking.
Photo: © Paul Blezard

Kamm Tail Vetter Helix

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Kamm Can

Kamm tails now have quite a long history, from car racing. Herr Kam (Quite posibly Herr doctor Kam) originally cut off an extended tail to save weight and discovered it made no difference to drag. Bear in mind that this all predated CFD, much in the way of wind tunnels, or separation theory. Later on the serious sporty car racers, Porshce, Mclaren, Ferrari etc. concluded that kamm tails were the way to go up to about 150mph, so Le Mans cars tended towards long tails (200mph terminals) and short circuit cars tended to have Kam tails. I tend to believe that these guys know what they were doing.

Much later it was shown that one of the reason Kam tails worked at the lower speeds was that the tail vortices efficiently filled the void behind the tail, even producing a desirable high pressure area on the rear surface, but that this effect was lost at higher speeds.

Tails on FF's are a slightly different issue. The Banana showed that even a short 'tombstone' tail, more or less a rider profile 'plank' reduced drag and markedy improved cockpit environmant, by preventing re-circulation into the cockpit, and forcing all tail vortices to remain to the rear of the vehicle. These are both effects worth having. In addition, stability requirment have reliably shown the need for tail side area at least as great as frontal side area.

So regardless of drag considerations there's a need for a 'rider profile' box behind the rider, with a sharp upper edge to provide transverse separation and enough side area for stability. At this point you have aleady arrived at something rather like Craig's tail as shown, and of course a wide variety of other FF's

Reducing drag beyond this simple arrangment is more problamatical. A Kamm tail, as above but with a rear panel, is theoretically as good as a long tail at road FF speeds but the real issue is practicality. In order to maintain close airflow over a long tail the taper should not be greater than 2-3 degrees, and this produces a length that is totally impractical in real traffic. In addition it is highly susceptible to turbulance. It's even outside the 'box' alloed in E-bike racing. So for all practical purposes it is neccessary to use a Kam-style tail, with minimal taper to avoid excessive side separation, and let the air go it's own way after that.

The system can be modified, like FJ, by exiting the radiator air into the area behind the tail - A form of 'base bleed' as used in long range artillary shells, and air from the rear wheel can be directed up the tail panel, all attempts to 'fill' the space with turbulent air at higher pressure than ambient and 'disconnect' the vortices that would naturally form in the space.

Of course extreme fuel economy contests are again different and it may well be that a gain can be achieved from a long tail, at fairly low speed in still air. But essentially, low drag is about minimal frontal area, followed closely by early and efficient sepration to minimise 'wetted area' The trick is to combine these two features in a shape that is light, practical and non-injurious (to pedestrians), with lights, intakes and access. Study of production cars provides most of the solutions.

Herr Professor Wunibald KAMM 1893-1966

From Wikipedia: Wunibald Kamm (April 26, 1893 – October 11, 1966) was an automobile designer, engineer, and aerodynamicist. He is best known for his breakthrough in reducing car turbulence at high speeds; the style of car bodywork based on his research has come to be known as a Kammback or a Kamm-tail.
More here: https://en.wikipedia.org/wiki/Wunibald_Kamm