Maximum angle for boat tail?

[bombloader]

Bad idea if your worried about crosswinds. A vertical fin would tend to turn the vehicle into the wind, like a weathervane. This is why airplanes have them, it keeps the nose pointed into the relative wind. You want a car or motorcycle to stay pointed down the road, not swerve into the wind. Besides, you would add drag.

[evolutionmovement]

I thought the fins on the Alfa BAT concepts increased stability. And I thought aircraft tail fins were used with a rudder to counter yaw effects (and help turn the aircraft). No reason a vertical fin couldn't be employed with a rudder, if necessary. Your small increase in drag to counter potentially dangerous destabilizing crosswinds would be nothing compared to a traditional high-drag shape.

[bombloader]

I'm not familiar with the Alfa BAT concepts. It may have been the least bad solution to the problem. Your basically right about how the vertical fin works on an aircraft, but without a rudder the fin would tend to increase the problem, not decrease it. Light aircraft do have this problem when taxiing in high winds. You have to use aileron into the wind to reduce this tendency(and also keep the wing from picking up.) As to adding a rudder, you've added a lot of complexity now for uncertain benefit. True, this website is full of people who've added complexity for small fuel economy benefits, but I still think its possible that a fin/rudder combination, would cost you most or all of your drag reduction. Keep in mind that if you can really aero the shape of your car, you should experience fewer problems with crosswinds in general.

[cfg83]

bombloader -

Here are some Alfa Bats from then :



And now :



The older ones look like they are ttying to "funnel" the air based on how much they are tapering inwards.

CarloSW2

[bombloader]

I wonder if those fins are supposed to reduce the wake somehow? I don't think they do what evolutionmovement was suggesting however.

[bombloader]

I wonder if those fins are supposed to reduce the wake somehow? I don't think they do what evolutionmovement was suggesting however.

[bombloader]

I wonder if those fins are supposed to reduce the wake somehow? I don't think they do what evolutionmovement was suggesting however.

[Bicycle Bob]

A major problem with stability is that a streamlined shape of constant density will have the center of pressure, or lift, ahead of the center of gravity, so it naturally wants to tumble. Aircraft have enough tail to balance the fuselage, plus a bit to reduce the workload in the cockpit, but not not so much so that you can't control a side-slip for a quick descent. Taxing in a crosswind stalls both shapes, changing the balance.

A car is usually analogous to an aircraft fuselage missing both tail and fin, so adding a tail should help bring the cg and cp together.

The BAT cars were highly experimental. The fins work as ducting to allow a more abrupt afterbody between them, and to increase surface area, thus adding energy to the boundary layer without using turbulence.

[bombloader]

Bicycle Bob-
On aircraft the center of pressure is generated on the wing. Cg is actually forward of Cp normally. The tail actually raises the nose constantly. Yes, this causes more drag, but otherwise the aircraft would be unstable in pitch(The Wright 1903 Flyer was not designed this way, and was consequently nearly impossible to fly. In the next few designs the Wrights figured this out.) This is why pilots usually calculate CG before every flight, because a too far aft CG could make the plane uncontrollable. Also, a technique to increase range on airplane is to load as close to the aft CG limit as possible to reduce the horizontal tails load, and thus the trim drag it produces. The original question was about a vertical fin, however, which along with the rudder acts in yaw to keep the plane pointed toward the relative wind and not slipping. It seems like adding one to a car would cause problems just for this reason. You don't really want a car that always wants to point itself into the relative wind.

[Bicycle Bob]

Did you think we disagreed? The best condition for a car is neutral steer in a crosswind in the cruising to top speed range. The VW bug was notorious for pointing away from the wind.