02GF74
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| posted on 27/1/06 at 08:57 AM |
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quote:
Originally posted by Hellfire
quote:
Originally posted by locost_bryan
quote:
Originally posted by Spyderman
What a load of ballony gets talked about air pressures and aerodynamics!
First off, no amounts of increasing the underbonnet air pressure will cause lifting of car/bonnet of any sort. It is only the high speed low pressure
air that flows across the top of the bonnet that will cause it to lift. So fit as many vents as you like, it won't change! If you don't
believe me try cellotaping, across front edge, a piece of paper to the table and then blowing across it. The paper will lift even though air can not
get under the front.
Experiment 2. Get a show box with a lid. Put a hole in the end of the box. Poke the hose from your vacuum cleaner in the hole. Turn the vacuum
cleaner on to blow. Now, where's the lid gone? Blown right off, it has!  (unless you used Dzeuss fasteners )
Sorry but I can't see how that has anything to do with aerodynamics and bonnet lift on a seven style car. Maybe if you were to repeat that
experiment but instead, take the bottom out of the box, cut a few small holes in the sides and then a few small holes in the lid and then hold the
vacuum cleaner nozzle a foot or so away from the box, I doubt very much that the lid would blow off.
Phil
I would go with wot phil says. another thinkg to remember is that the area for air coming in via the nose cones becomes larger so that should cause a
reduction in pressure - never that simple since the underside is open so air get in from there.
now if the underside of the engine bay was sealed, then that may be more the case.
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Rob Palin
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| posted on 27/1/06 at 05:46 PM |
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Pressure builds up in front of the radiator due to the very high resistance of any of the exit paths on offer to it (especially if it is fully ducted
to the radiator matrix, which is particularly unpleasant path for the air to take, from its point of view).
The pressure in front of the radiator will be significantly above the atmospheric ambient pressure. The maximum amount this pressure can rise is
fixed. If we call that +100% relative to an ambient of 0% then the pressure in front of the radiator will be around +70-90%.
Whether the rest of the volume behind the radiator will be quite a lot higher or ever so slightly lower than ambient depends on how well ducted the
radiator is, how big the various exit vents/holes are and what the pressure is at each one.
In almost all cases there is a 'positive' pressure in the engine bay because there is a substantial flow of air mass into the volume and a
combination of leakage around the radiator, some resistance to it getting out and the pressure at the outlets not being especially low (only about
-25% at most; that is 25% below the ambient). This tends to leave a pressure of up to a maximum of about +30% inside the engine bay.
This may not be very big but the bonnet area is very large and force = pressure x area. As Bryan says it's essentially how hovercraft work,
only they're better sealed so the effect is exaggerated.
Many times at work we've been faced with this on both road and race cars and the trick is to prevent high-pressure air from the front getting
into places you don't want it and making sure that any air that does go in (for cooling etc) also gets out very easily. On some cars the effect
can be dramatic. On 7-style cars it is less so unless you start panelling the underfloor or otherwise restricting air getting out.
It's worth stating that getting the air into the car is easy, getting it out is harder but every bit as important.
Rob
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