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Old 07-22-2012, 09:55 PM   #1 (permalink)
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Boat tail, side angles, cross winds

I was looking at boat tails and such and thinking about optimum trailing angles. Most of the designs I have seen typically have no consideration for cross winds. I was wondering how much cross wind is typical and what the effects are. I've been a long time lurker here and I've never seen this covered.

I pulled the last year of wind speed data from weather underground for my area. I get 6.5mph average daily average wind speed, stdev of 3.4mph. So I take that and randomize the direction and calculate the angle of attack of my car (Lets just call it yaw angle like a plane) going 70mph. Absolute the results so that my values are a deviation from 0. It makes a neat distribution, and it shows an average yaw angle at 3.4deg, with the worst condition being a wind at around 85deg from the direction of travel resulting in at 5.3deg yaw.

So on an average day my car driving down the freeway sees a 3.4deg yaw angle on the wind. I'm thinking perhaps boat tails and general side of vehicle angles should be 3.4deg less than optimal taper to compensate?

But that's only average, so half of the days of the year should be better, half worse.

+1stdev on the average wind speed which should be around 60 days a year will be at or worse than 5.3deg yaw average, 8.3deg max yaw. +2stdev on average wind should be around 8 days a year at or worse than 7.1deg yaw average, 11.2deg max yaw.

Now I realize 70mph is probably far above my average speed, and the yaw numbers are larger for a vehicle going slower. So In the end to get myself to a comfortable level, I'm thinking a side taper for a boat tail should be ~5deg less than theoretical optimal on each side to deal with typical cross winds. For anyone who made it through this rambling, does that sound right? Part of me is starting to think that it may be best just to keep the sides parallel like a typical sports racer body.

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Old 07-22-2012, 10:17 PM   #2 (permalink)
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My two bits gleaned from stream lining motorcycles is that minimizing side wind effects is best achieved by rounding the sides at the top and bottom, as much as practical to reduce the Cd at 90* to the direction of travel, so that the net force is minimized. This will help to reduce the yawing moment to a minimum.
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Old 07-23-2012, 06:35 AM   #3 (permalink)
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In my experience, there is little adverse affect with rounding the tail( as Red said). I don't experience any scary steering issues, it even seems to handle better in cross/side winds. I usually only get a little shudder even in 25-30 mph winds.
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Old 07-24-2012, 05:33 PM   #4 (permalink)
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side angles

I'm given to understand that road vehicles are designed for zero crosswind condition.Their drag coefficients may be crosswind averaged up to some degree of yaw.Class-8 truck Cds are required to include crosswind conditions I believe.
I don't know of any vehicle which was ever designed for a particular yaw angle.
It's understood that on most days a motorist will experience crosswind.

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