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Old 04-14-2010, 07:47 PM   #1 (permalink)
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Adding air to side mirrors.

Reading through the thread :Lowest drag side mirrors one though occurred to me.

Would adding air into the base of the mirror assist the aerodynamics of the shape?
I am thinking of adding air immediately behind the reflecting glass / plastic to "fill" the void created by the mirror as it moves through the air.

Anyone?

Cheers , Peter.

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Old 04-14-2010, 08:29 PM   #2 (permalink)
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Yep, that would reduce drag in the mirrors' wake.

The biggest problem with aerodynamic air injection systems is they consume an awful lot of air. Where are you going to get the air from? If you use an electric or belt-driven blower, you might not save enough fuel at the mirror to run the pump, and certainly not enough to justify the expense.

If you use a duct, make sure it doesn't create more drag at the inlet than it reduces at the outlet.

Hmm, my mirrors are mostly a hollow plastic shell, and there's a gap between the housing and the glass. If I drill a few large holes near the mirror's stagnation point, I wonder if it would reduce drag, or just whistle a lot.
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Old 04-15-2010, 06:21 PM   #3 (permalink)
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Prandtl

Quote:
Originally Posted by Peter7307 View Post
Reading through the thread :Lowest drag side mirrors one though occurred to me.

Would adding air into the base of the mirror assist the aerodynamics of the shape?
I am thinking of adding air immediately behind the reflecting glass / plastic to "fill" the void created by the mirror as it moves through the air.

Anyone?

Cheers , Peter.
Peter,I think Ludwig Prandtl's work with blown and suctioned wings might give some insight into your question.
Some modern mirrors are essentially 'invisible' to the air due to refinements made in the wind tunnels over last 3 1/2 decades.
I'm not sure what gains are out there to be had.
I would like Secretary Chu to have President Obama pressure the USDOT to allow camera/monitor systems in new cars.Then we could skip the mirror issue altogether.
The admin. appears to be interested in global climate change,increased CAFE standards,etc.. This would be one small part in that play.
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Old 04-16-2010, 02:56 PM   #4 (permalink)
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Why not get air to fill the mirror wake from a scoop mounted in the grille inlet? Should do the same thing as a grill block, but you could run tubing from the scoop to the mirror (inside of the body paneling, of course). Just thinking out loud.
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Old 04-16-2010, 03:01 PM   #5 (permalink)
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Quote:
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I would like Secretary Chu to have President Obama pressure the USDOT to allow camera/monitor systems in new cars.
All that complexity, cost, and expense for a potential .00000001 mpg gain? I think not.
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Old 04-16-2010, 03:13 PM   #6 (permalink)
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Using the 3%-6% mirror contribution to drag, from here: http://ecomodder.com/forum/showthrea...tified-95.html

And the calculator here: Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

I get a 2-4% improvement in mpg at 45 mph from eliminating the mirrors. Everything else on the calculator I left at default.

Also, later in that first link, Metro saw a 2.3% increase from removing one mirror and folding the other, at 55 mph.
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Old 04-16-2010, 03:19 PM   #7 (permalink)
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Seeing as modern vehicle mirrors have been shown to account for between 3 and 6% of the total drag on a car, I would think that deleting mirrors to hit CAFE standards would make sense. Not necessarily both mirrors, but even deleting one mirror would improve the aerodynamics of a vehicle (like the Civic VX and Metro XFi).
Plus, 3-6% is a pretty large gain. Adding variable valve timing is quoted as improving EPA fuel efficiency by 5%, and turbocharging is quoted as improving FE by 7.5% *. Redesigning an engine to add a turbo, or VVT, or direct injection is much more expensive than deleting mirrors and adding cameras in their place. Just my $0.02.

*FE statistics found at Fuel Economy
Engine Technologies
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Old 04-16-2010, 04:18 PM   #8 (permalink)
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the most simple solution imho would be to have a setup somewhat like this



a low drag shape to start with (this car had a 0.28 Cd ) but it could be made to acts as a duct from the engine bay to the mirrors, so you could vent engine bay air trough the mirrors. or you could have an intake at the base of the windshield. the first solution would have the added benefit of venting warm air alongside the mirrors and the side windows wich would be nice in winter, but it might also increase airflow throuh the radiator ever so slightly, allowing it to be ever so slightly smaller.

none of this might be worth the investement, but on the other hand i don't think there's any downside to this setup and all it needs is a simple duct, no high tec involved. plus the way i see it all these little "neglectable" benefits might add up with other clever tweaks and nock another 0.01 of the Cd
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Old 04-16-2010, 05:55 PM   #9 (permalink)
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Quote:
Originally Posted by eco86 View Post
Seeing as modern vehicle mirrors have been shown to account for between 3 and 6% of the total drag on a car, I would think that deleting mirrors to hit CAFE standards would make sense. Not necessarily both mirrors, but even deleting one mirror would improve the aerodynamics of a vehicle (like the Civic VX and Metro XFi).
Plus, 3-6% is a pretty large gain. Adding variable valve timing is quoted as improving EPA fuel efficiency by 5%, and turbocharging is quoted as improving FE by 7.5% *. Redesigning an engine to add a turbo, or VVT, or direct injection is much more expensive than deleting mirrors and adding cameras in their place. Just my $0.02.

*FE statistics found at Fuel Economy
Engine Technologies

3 to 6% drag reduction does not equal or lead to 3-6 more mpgs or even 3-6% more mpg.
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Old 04-16-2010, 07:26 PM   #10 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
3 to 6% drag reduction does not equal or lead to 3-6 more mpgs or even 3-6% more mpg.
Are you suggesting we add extra mirrors, since the result would be "only" a 2-4% increase in fuel consumption?

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