I'm not sure the hood is "capturing" air, nor that unzipping the rear window
will help that problem, at least not by "letting it out". I think Mr. Boyle
and our old friend the aerofoil can explain, but maybe I'm wrong. As the air
rushes over the outside of the hood, by definition faster than the air is
moving on the inside, it creates an area of lower pressure, just like on the
top of an airplane wing. You allude to this, by saying there is positive
pressure inside the car, but unless I misunderstood you, it isn't created
the way you think.
Unzipping the rear window might help slightly to equalize the pressure, but
not significantly, would be my guess. It might even create another hole
which creates even more drag. When you're zipping down the highway at 60mph,
you'd have to have the air moving through the cabin to be at 60mph, too, to
have equal pressure. And even then, the hood itself is shaped like an
aerofoil, so even if, say, there were no windscreen and no rear window,
there would still be lift from the basic shape of the hood.
My .02. I'm a restaurant manager, not a physicist. YMMV. :-)
-Mark
- Topless Smurf ('75 MGB):
http://www.bones.org/torch/photos/mgb/mark_mgb_1.html
>
> When I run my my '69 B roadster at 60+ mph with the top up, the
> ceiling fabric rises above the top bows. I presume that there is
> positive pressure inside the car and the top is balooning. Or, to put
> it another way, the top is capturing air that the car is trying to
> penetrate. Either way, it must take energy from the fuel to pressurize
> that top. When the top is down, there's no energy going to pressurize
> the top, so I figger the car is slipperier (is that a word?) when the
> top is down. Next top will have a zip-down rear window, so as to let
> the air out.
>
> Cheers,
>
> CR
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