A straight sided fairing ?

[Xist]

Originally Posted by sendler

All the Vetter tails have straight sides.

That is not problematic?

[PeterS]

Originally Posted by sendler

I see now that you mean to taper the sides so they are narrower in the back. By "straight sided" I had assumed that you meant parallel. The Vetter tails are tapered at 16 degrees and flat sided just like your truncated tail that you are showing.

A correction, because of the required geometry with a flat sided taper the angle isn't 16 degrees , probably between 5 and 7 but as it starts further foreward the max width and the end width are the same as the Vetter..... no curve .

[aerohead]

Originally Posted by PeterS

Here are few photos of a rough model I made today. The sides are flat and the maximum cross section is around mid way but there is no foil. The rear could be narrowed more or even tapered a little without generating lift.

It is quite similar to the Vetter fairing and would have the same frontal area . Could i have some comments on this idea ?

*you're streamlining to reduce pressure drag.
*you must reduce or eliminate separation to do this.
*historically,gentle,progessive plan taper is the only way to accomplish this.
*and this requires a curvilinear pathway.
*a straight curve has too much pressure gain to respect a turbulent boundary layer's needs to mitigate counterflow,eddy initiation,then full-blown turbulence.
*to complicate things,the body upstream of the taper is mutilated due to the cavity for the rider,so the onset flow to the tail is adulterated already.
*boat-tailing requires clean,turbulence-free flow to perform properly.
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*Since your fairing is a crude,upright wing section it cannot tolerate a Kamm truncation.Chopping it off creates an exponential drag increase.
*for drag reduction the tail needs to be curved to a point,even if it's not to an ideal length.
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*if you're concerned about crosswind and gust dynamics,it's my opinion that you need to ride with extreme vigilance,and use the body cavity to lean out,using body English to address your immediate riding environment.
here is the minimum drag section (center)

Here you can see how drag is compromised as you leave the ideal contour

Here is the numerical relationship with which you can predict your drag penalty for truncating the tail

Here are some examples of proper streamlining

[PeterS]

Originally Posted by aerohead

*you're streamlining to reduce pressure drag.
*you must reduce or eliminate separation to do this.
*historically,gentle,progessive plan taper is the only way to accomplish this.
*and this requires a curvilinear pathway.
*a straight curve has too much pressure gain to respect a turbulent boundary layer's needs to mitigate counterflow,eddy initiation,then full-blown turbulence.
*to complicate things,the body upstream of the taper is mutilated due to the cavity for the rider,so the onset flow to the tail is adulterated already.
*boat-tailing requires clean,turbulence-free flow to perform properly.
--------------------------------------------------------------------------
*Since your fairing is a crude,upright wing section it cannot tolerate a Kamm truncation.Chopping it off creates an exponential drag increase.
*for drag reduction the tail needs to be curved to a point,even if it's not to an ideal length.
--------------------------------------------------------------------------
*if you're concerned about crosswind and gust dynamics,it's my opinion that you need to ride with extreme vigilance,and use the body cavity to lean out,using body English to address your immediate riding environment.

Here are some examples of proper streamlining
]

Thank you but I am well aware of what ideal streamlining looks like , the problem is that it isn't practicable to use on a busy road . I'll end up dead .Lovely pictures though.

''*a straight curve has too much pressure gain to respect a turbulent boundary layer's needs to mitigate counterflow,eddy initiation,then full-blown turbulence.''

Could you explain that in other words ? I'm finding your meaning a little hard to derive .
I've modified my rough model , improving the nose area shape and smoothing it more back to the rider's position, the rear remains flat but tapered.

My understanding of this is that it is the angle at which the body leaves the air that is most important. I appreciate that my design will generate some flow separation forward .... but then that is the whole point if the side lift is to be avoided and that seems the primary gain, it could keep me alive.

[sendler]

Basically what he is saying is that your design won't be very low in drag.

[PeterS]

I guess I'll have to build it and see .

[Xist]

Tuft test, if you can.

[sendler]

But you will gain some nice storage capacity and weather protection so even if it is only a slight improvement in drag, it is still nice to have a fairing and trucated tail. My tail and a new chain have given me an extra 10% in fuel economy to now average 102 mpgUS and now I can go shopping for more items or lock up my stuff when I am gone camping.
.
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[PeterS]

I hope for a fair bit more than 10% !

I guess I'll see.

[veloman]

Sendler, I'm a little disappointed your well made tail is only improving your mpg by 10%. From my studying on the subject I would have guessed around 20% improvement.

The utility aspect is surely a nice gain though. I need to build a tailbox. Though I have been hesitant since I carry my mountain bike on my rear rack once a week. I'd have to make the tailbox quickly removable.