Quote:
Originally Posted by Cd
I skimmed over the many pages here, and just wanted to ask if using window screen on the sides of a fairing / tail would allow sidewinds to have less effect on stability while still providing a surface for air to slip over from straight onward.
|
It's worth mentioning that driving north at 60 mph with a 15 mph crosswind from the east, the apparent wind angle is 14 degrees off the nose at 62 mph. If the shape of the fairing is like that of an airfoil (when viewed from above) the fairing could be generating significant lift at this angle, and the shape would be close to the stall angle of attack. The lift direction would be perpendicular to the apparent wind direction, so 14 degrees forward of abeam (an angle that is pretty effective in pushing the bike around).
On the leeward side of the bike, much of the flow on the back half of the bike will be detached, and the center of lift will be at a point about 25% back from the nose. On the windward side (the east side) the flow will be attached all the way back to the tail.
The above applies to a lesser or greater extent, depending on how much the fairing is like a symmetrical airfoil section standing on end, and the degree to which flow from the high pressure side can travel over to the low pressure side, over the top of the bike or under it.
So... to reduce side force, the screen would need to be fairly far forward on the sides. Most laminar flow from that point rearward would probably be disrupted. Transition grit is used in wind tunnel studies as a boundary layer trip. (
You can read bout it here.)
Window screen is probably of a roughness that would work as a trip.
You can search for info an "porous wing surfaces" -- there is quite a bit available, with most of it having to do with
reducing drag or increasing lift on airfoil sections.
I think it would be an interesting area to pursue, although a difficult one. To get meaningful results, you'd need a wind tunnel -- although you could probably learn a lot from scale models in the small tunnels available at a lot of universities -- which can often be accessed for free if you can make a student project out of it.
My gut feel is that window screen is about the right size, although perhaps something thinner would be better.
Of course, you'd want to first build a streamliner and get a feel for sidewind sensitivity. With the right distribution of lift (which in this case is acting horizontally, rather than vertically as in an airplane wing) you may find that the bike is self correcting, and that there is no need for porosity. There are many bikes like Sendler's, in which it feels like the wheels are getting pushed out from under the bike, causing the bike to bank into the wind.
If you found the need, then a pressure distribution map across the area of the fairing could be produced (best in a wind tunnel, because you would want to look at constant angles of attack of 10 - 15 degrees) and the venting areas would be best positioned at the highest pressure areas, and communicated to the lowest pressure areas on the opposite side. (The reverse of this is done in some porous wings studies.)