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.

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.

Prandtl
 

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.

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.

All that complexity, cost, and expense for a potential .00000001 mpg gain? I think not.

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.

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

the most simple solution imho would be to have a setup somewhat like this

http://upload.wikimedia.org/wikipedi...point8.arp.jpg

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

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?

i think what frank is saying is that aerodynamic drag is not the only factor influencing fuel consumption, there's rolling resistance, vehicle weight, the gear ratios. also aerodynamic drag increases with speed, so vehicles driven at slower speeds will see less benefit from aero improvements that vehicles driving a lot at highway speeds.

thus mathematically a 3% reductions in aero drag equals not a 3% improvement in FE.

True, but a 2-4% improvement in FE is HUGE. Find two or three of those that your competitors miss, and you've got a car that people will think of as more efficient than the rest.

Note also that a typical car burns 7000 gal during its lifetime. Each 1% improvement is 70 gal, or $150-300 worth of fuel.

I figure a dual camera replacement mirror system might cost $200 over the life of the car, while saving $1200 in gas and another $1200 worth of gas-fueled wars, resource depletion, and climate change.

From preliminary coast down numbers earlier this year, a 3 to 6% drag decrease would equate to a 2 to 4% decrease in fuel consumption.

At 55 mph, 66% of the overall drag was associated with air drag...

Some math...

(3 * 0.66)% - (6 * 0.66)% mpg decrease

Every little bit helps, Jim.

Hi All,

Its commonly said that Aero Drag reduction in % will improve highway mileage by half that %. For example, improve aerodyamic drag by 20 % will give a 10 % reduction in highway fuel consumption.

On the blown mirrors idea, the issue is air friction. A scoop system has air friction too. Probably more then just letting the air flow over the top of the car. To get it less, the tubing that routes the air to the mirrors, would need to very large. For example, for dust collector systems, the difference between 4 and 6 inch diameter is quite significant to how easily the low pressure air will flow down the tubing.

The way one might get an adavantage is route air that is already coming through, to the mirrors. And the exhaust it uniformly around the whole circumfrence of the rear of the mirror. Exhausting only partially around the circumference will result in lift or sideways thrust, which will generate more drag.

So, now, how does one get a 6 inch hole from the engine compartment to the backside of the mirror?

Nope, not suggesting that.

Just sayin', I don't want complicated, expensive, relatively unreliable systems to replace simple, cheap, proven ones for a fractional mpg gain (ex. 33 mpg x 1.02 = 33.66. And as I noted somewhere here on EM re: sidewinds I'm not so sure these mirror drag effects could be proven at all anyway).

http://ecomodder.com/forum/showthrea...s-11580-2.html

http://i146.photobucket.com/albums/r...0212009001.jpg

I suspect I drive through x-wind yaw more severe than depicted most of the time. How much "frontal" area and drag are these mirrors adding? I bet the downwind mirror adds 0 to drag; while the upwind one may add zero to frontal area but may add some form drag...

gain
 

Frank,I hear what you're saying.
I'm seeing more and more side mirrors with built-in turn indicators,or electric mirrors.My CRX was pre-wired for accessories I never had in the car.'guess my thought is,that auto makers are okay with the wiring issue.
The electronic speedometers are a cost saver over gear-driven,cable-operated units.They save weight and can't break as in my T-100.
I would suppose that the camera system could save weight,couldn't be broken off the car,could easily be demisted/de-fogged/wiped,and I suspect at a cost 'savings' over conventional mirrors.
Electronics costs, do not necessarily represent their actual 'cost.' And 'economies of scale' with mass-production should push cost through the floor.
As far as drag reduction,I'm probably thinking more of 350-class pickups with mirrors of the frontal area of Sam Whittingham's Varna racing cycle.
And with the speeds people drive in Texas,the cameras might compensate for the added resistance.
Just some churnings.

blown mirror
 

I looked at Hermann Schlichting's Boundary Later Theory.He addressed this issue,and from the tone of his writing,this is not a project you would attempt without a wind tunnel.
If the air jet is not perfectly sized,sited,and flow calibrated,it will increase drag due to induced vorticity.
If I had to rate the project on a "Buy or Bust" basis,I'd have to rate it a "bust."

how 'bout mirrors that flip out when you need them. i already have 3 mirrors inside the rabbit.

If the housing around the mirror fits tight enough to reduce frontal area, then there would be no room for air jets and such.

The trailing edge of the housing should be closing in shape to reduce the wake behind the mirror.

I do favor the idea of a flip-out mirror however. I think Metro was the first to mention this, in the postings that I have seen.

Jim.

Frank - thanks for that picture, and the thought it provokes.

But then again, we do have several empirical tests showing an improvement. I'd guess in a crosswind the effect would be less, but still nonzero.

Ahh , some interesting insights there.
Thanks to all posters.

Peter.

That's exactly what Volvo have done on their S40/V50 range.

At least on the earliest (2004 thru 2007) and smallest versions of the mirrors, the black plastic aft part of the housing tapers in slightly.


The mirrors have since been increased in size twice, because they were rather small.

I do not think that the tube would have to be very big. If a tube say the size of a garden hose or slightly larger were ran from the back of the engine bay to the side mirror and is terminated just before center with the outlet facing the front of the car (for diffusion) it could do a few things. First since there is lower pressure behind the mirror it would create a vacuum pulling air from the engine bay, along with this it could help reduce high pressure in the engine bay especially if it has a full belly pan. Once the car is warm it would work as a passive defroster. Also with the smaller tube it will not be able to move a lot of air but should have decent velocity.

1. Just how big does a side mirror need to be?

2. How much base or strut does it really need?

3. If the RAF Spitfire fighter plane of WWII got by with a faired ~4" round mirror on a ~4" thin stalk and flew at ~400mph (apparently with no ill effects), why do we ~70 years later need anything bigger or less streamlined?

Thoughts?


I ask because my old Porsche has "flag" mirrors of clunky angular shape, flat, fat stalks, and housings at least 25% bigger than the mirrors. My Ford van has mirrors copied from Dumbo the elephant. This from companies old enough to know better.

Seems to me a thin stalk of teardrop profile and oriented into the relative wind, blended smoothly into an elliptical shell no bigger than the mirror itself, would be much better. Wouldn't be rocket science to include the tiny remote control motor gizmo and a turn signal.

I'd like to see both the ground and the vehicles aft and to the side of me.
On the earlier S40/V50 that wasn't the case: adjusted to see traffic, they're rather useless for parking, and vice-versa. So they were increased in size on later models.

The old Spit lost a few mph in top speed because of the external mirror, despite having well over 1000 HP
Probably lost a couple mpg as well, but no-one was keeping track of that back then.

Spitfires didn't need to shuffled out of underground shopping centre car parks either!

Mirror sizes and locations are dictated by legislators and not by common sense.

Cars may be driven by the many and legislated by the few but this was NOT their finest hour.

Peter.