Convertible Musings

Was giving my father a ride out to look at a car in my Miata and he asked a question that got us both thinking. Figured I'd come here and ask the question and see what you guys say:

If you were driving a convertible with the top down, and you had a tail wind of the exact same direction and speed, would there be any turbulence in the cabin?

Our thinking was, no, there would not be. Just curious what everyone else thinks.

I agree, no. (hugely conditional if there, but still). It'd be like going nowhere with no wind.

If it were a Miata convertible, you would not experience any turbulence anyways...The designers nailed that one pretty well.

In reality, I think you would need a tailwind that was higher than the indicated speedometer. The windshield builds up a high pressure area and in order to get it around the car it would need to speed-up around the edges. You might even make it more turbulent

for some reason this makes me think of when parked planes take off in high winds like this.

NOHOME wrote: If it were a Miata convertible, you would not experience any turbulence anyways...The designers nailed that one pretty well. In reality, I think you would need a tailwind that was higher than the indicated speedometer. The windshield builds up a high pressure area and in order to get it around the car it would need to speed-up around the edges. You might even make it more turbulent

Uh, if the tail wind = road speed then there is no high pressure area in front of the windshield... am I missing something?

Remember the sailboat flag? No matter how fast the sail boat is moving, the flag always points forward.

edizzle89 wrote: for some reason this makes me think of when parked planes take off in high winds like this.

Dang, that cut off too soon. I bet the "landing" didn't go too well.

The only variable I see is the moving ground under the car. If you could insure a steady non-turbulent tailwind it would be calm all around the car. Might even be calmer as the car would help buffer against gusts and such.

I once caught a good tailwind while biking through Oklahoma and had about 30 minutes of slight downhill with a steady tailwind and I could reach down and feel a small turbulence from the spokes. I also became sweaty as hell so the car might show some cooling problems!

Bruce

Robbie wrote:

NOHOME wrote: If it were a Miata convertible, you would not experience any turbulence anyways...The designers nailed that one pretty well. In reality, I think you would need a tailwind that was higher than the indicated speedometer. The windshield builds up a high pressure area and in order to get it around the car it would need to speed-up around the edges. You might even make it more turbulent

Uh, if the tail wind = road speed then there is no high pressure area in front of the windshield... am I missing something? Remember the sailboat flag? No matter how fast the sail boat is moving, the flag always points forward.

You are right...got the wrong mental image in my head (happens a lot)

I think what you are describing is much like a hot air balloon ride where you move along at the same speed as the wind. I seem to recall reading that it is very quiet.

We run this condition in the wind tunnel on a regular basis - sort of...
We run the road under the car at full speed with the wind at zero speed, to get the offsets for wheel drag. There is a reasonable amount of air movement near the ground due to the road and wheels/tires dragging the air along, but only a little air movement near the upper half of the car.
The numbers are roughly 2-3mph indicated airspeed with the road running 175mph, after a few minutes to stabilize.

My thinking was that there would be some movement. Most road cars generate lift as the air going over the top of the cars from road speed speeds up, causing a lower pressure. So the air over the cockpit will be moving faster than road speed so still faster than the wind from behind. that would be compounded and complicated by the turbulence at the top of the windshield and the cockpit and the lower pressure above your head. All in all the air would be moving but I'd hate to say exactly what you feel.

Another factor to consider which is partially addressed in the road movement comments. The car is moving forward creating some turbulence at the corners as mentioned. Would need a tail wind a bit faster than forward movement to counteract that. Would most likely be calm in the cabin except at the top of the windshield.

Maybe you need a car that is identically shaped front and back so the air is being affected the same way from both ends!!

Yes this is joke don't worry, even if you made a push me pull me you'd still have weird turbulence to contend with.

As Stafford noted, the road (relatively) is the only thing moving at that point. Cooling should not be an issue either since the motor will only need to put out enough power to over come the bearing and tire drag (and some tire aerodynamics), maybe couple HP?