04-06-2011, 05:16 PM
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#51 (permalink)
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Fighter jets have pretty upright canopies for the sake of visibility, and long "hoods" to house radar... so we should look at what they do.
The F22 has a gradual transition from hood to windshield
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04-06-2011, 05:21 PM
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#52 (permalink)
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04-06-2011, 05:38 PM
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#53 (permalink)
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Do more with less
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I remember a post by aerohead where he quotes an 1986,SAE Paper 860211 showing that a 15deg hood slope can reduce cd by .0475. That is pretty huge! It is from the technology of cars of the era that my car comes from. When I made the hood splitter I shot for that angle, 15 deg.
So if my car has a cd of .36 stock that and the paper says 16% less wind drag, that makes it about .31. If half of my drag is from aero that means that I might have 8% less overall drag. My previous tank was 47.7 mpg. An 8% increase says 51.5 mpg My car got 50.1 over 500 miles.
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04-06-2011, 06:25 PM
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#54 (permalink)
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Quote:
Originally Posted by winkosmosis
It always takes less energy to change direction with a larger radius... compare coasting your car through a 90 degree corner vs a wide sweeping curve.
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Not if a bubble forms. Apples n oranges.
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04-06-2011, 06:26 PM
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#55 (permalink)
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Quote:
Originally Posted by winkosmosis
Fighter jets have pretty upright canopies for the sake of visibility, and long "hoods" to house radar... so we should look at what they do.
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Supersonic = different set of rules.
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04-06-2011, 06:30 PM
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#56 (permalink)
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Quote:
Originally Posted by Frank Lee
Not if a bubble forms. Apples n oranges.
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You're assuming a bubble is just as aerodynamic as metal. Following that reasoning, why put a boattail on a car when a bubble forms back there in the shape of a boattail anyway?
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04-06-2011, 06:32 PM
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#57 (permalink)
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Quote:
Originally Posted by Varn
I remember a post by aerohead where he quotes an 1986,SAE Paper 860211 showing that a 15deg hood slope can reduce cd by .0475. That is pretty huge! It is from the technology of cars of the era that my car comes from. When I made the hood splitter I shot for that angle, 15 deg.
So if my car has a cd of .36 stock that and the paper says 16% less wind drag, that makes it about .31. If half of my drag is from aero that means that I might have 8% less overall drag. My previous tank was 47.7 mpg. An 8% increase says 51.5 mpg My car got 50.1 over 500 miles.
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Yes if the hood is too horizontal and the front end too blunt and sharply radiussed then the hood experiences non-laminar flow so it stands to reason if changes are made such that flow over the hood straightens out then drag goes down.
I'm sorry, don't take it personally, but to use that to leap to your second paragraph makes no sense. Look at any Rabbit in a wind tunnel, or Hucho's book, and you'll see that VW did angle the hood and radius the leading edge of the hood properly so too bad, I'd wager you are stuck with .36, blister or not.
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04-06-2011, 06:33 PM
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#58 (permalink)
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Quote:
Originally Posted by winkosmosis
You're assuming a bubble is just as aerodynamic as metal. Following that reasoning, why put a boattail on a car when a bubble forms back there in the shape of a boattail anyway?
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A bubble is not the same as turbulent wake.
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04-06-2011, 08:30 PM
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#59 (permalink)
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Moderate your Moderation.
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A bubble in this sense is a high pressure area which will resist inflow. Since the pressure at the base of the windshield remains fairly constant, we can identify that it's working to this effect, meaning that, in some respects, it is acting as though a dam of solid construction were actually in place. Pressure, however, cannot be maintained outside of a container without some transfer of energy. Because of this, there should be a gain to be had at the base of the windshield, as long as there was a bubble there in the first place. The size and pressure of the bubble determine the actual scale of improvement.
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04-07-2011, 05:28 PM
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#60 (permalink)
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Quote:
Originally Posted by winkosmosis
It always takes less energy to change direction with a larger radius... compare coasting your car through a 90 degree corner vs a wide sweeping curve.
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For the largest part, that's the tyres' drag increasing in order to make the sharper turn.
Or indeed, any turn. My fuel consumption always goes up in a constant speed turn - say on the motorway.
Then there's aero drag increasing from going through the air in a skewed way rather than straight as the car was mostly designed to do.
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