Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph

[Christ]

Originally Posted by aerohead

Neil,sorry,I'm trying to catch up.
I agree that angles can be steeper.Frank had just mentioned it and I used the example of Chrysler's Daytona 500 with the C-Pillar buttresses feeding air at the 22-degree backlight.
22-degrees is the angle Mair arrived at to be the absolute max for attached flow we could use,if everything upstream was clean and energetic.

So in short, once the front/sides are cleaned up, the rear can be even more dramatically altered, allowing for steeper trailing angles which create less skin drag while maintaining attached flow.

Yes?

[aerohead]

Originally Posted by Christ

So in short, once the front/sides are cleaned up, the rear can be even more dramatically altered, allowing for steeper trailing angles which create less skin drag while maintaining attached flow.

Yes?

A wind tunnel is helpful here.And if you play the Daytona 500 mental movie in your head,while they say they could get to 22-degrees,well that was the backlight,no mention is made of the trunklid.
You could lay a straightedge across the trunkid up the back of the roof and read that angle.This would be more representative of the car's actual aftbody.There may have been an enormous locked-vortex residing there between the spoiler uprights.
Really,what you can say scientifically,is that for a Daytona 500,you can do what Chrysler did and get what Chyrsler got.
The case-specific approach seems the best and most fair for assessing.
Kamm's roof fails at the last foot.
Impact/EV-1 is dirty.
Ultralite is dirty.
I've not seen the Precept under smoke,but I would suspect they have some aftbody separation on that one.
I keep going back to driver outward visibility as the issue.

[Tygen1]

Aero Warriors - The G-Series Wind Tunnel Test Report

Good read about the Daytona development...

[Christ]

Originally Posted by aerohead

A wind tunnel is helpful here.And if you play the Daytona 500 mental movie in your head,while they say they could get to 22-degrees,well that was the backlight,no mention is made of the trunklid.
You could lay a straightedge across the trunkid up the back of the roof and read that angle.This would be more representative of the car's actual aftbody.There may have been an enormous locked-vortex residing there between the spoiler uprights.
Really,what you can say scientifically,is that for a Daytona 500,you can do what Chrysler did and get what Chyrsler got.
The case-specific approach seems the best and most fair for assessing.
Kamm's roof fails at the last foot.
Impact/EV-1 is dirty.
Ultralite is dirty.
I've not seen the Precept under smoke,but I would suspect they have some aftbody separation on that one.
I keep going back to driver outward visibility as the issue.

Safety first, aero second.

Don't tell car companies that, though... they'll just find a reason to call aero efficiency "unsafe" so they don't have to do it.

LOL.

As usual, thanks for the great insight, Phil.

[aerohead]

I took a protractor to a side shot of the Dodge last night.
Measuring from the rear tip of the trunklid up over the backlight to where it contacts the rear of the roof yields an angle of 13.5-degrees vs 22-degrees for the backlight itself.

[Christ]

Originally Posted by aerohead

I took a protractor to a side shot of the Dodge last night.
Measuring from the rear tip of the trunklid up over the backlight to where it contacts the rear of the roof yields an angle of 13.5-degrees vs 22-degrees for the backlight itself.

Phil, I haven't searched for the posts particularly, but I have suggested on several occasions that spoilers could be helpful on vehicles specifically when they're used to decrease the "virtual" angle of the roof/tail transition.

[3-Wheeler]

Originally Posted by Nerys

...I don't even think a 10mph change in wind would have more than a few % impact on the results....

I've been mulling this over for the last two days, and have to say that a 10 mph difference is a big deal.

For example, we know that air drag is affected by (velocity)^2, so if Metro were driving at 56 mph as he indicated and going into a 10 mph head wind, the effective wind forces would be:

(66)^2 / (56)^2 = 1.38 or a 38% increase in drag.

Likewise if Metro were going with the wind, then it work out to:

(56)^2 / (46)^2 = 1.48 or a 48% reduction in drag.

Both numbers are nothing to sneeze at...

Metro was very wise to maintain his speed through the test within 1 mph or so. A 1 mph change amounts to...

(56)^2 / (55)^2 = 3.7%

If his speed varied by ± 1 mph then our measurement uncertainty for the gas mileage difference for the boat tail addition would be ± (3.7%/2) according to AeroHead, where the air drag forces affects the ultimate gas mileage by half.

Just my .02.

Jim.

[Nerys]

My point was the difference that 10mph would make WOULD NOT ACCOUNT for the 15% improvement in fuel economy (note NOT a 15% improvement in aero but in MPG)

ie inability to maintain speed would not be a valid reason to disregard his improvements since even a speed delta much greater that what he had (I chose 10mph) would not account for a 15% improvement in mpg

in my metro slowing down from say 45mph to 35mph WOULD NOT net me 15% improvement in mpg. 2 or 3% maybe.

[Christ]

The overall difference, if scaled, is nearly 100% more energy necessary to maintain speed based on forces applied.

It's alot different to say that you're going 35 vs 45, etc... wind loading is a totally different game.

[gregmid61]

In case it hasn't been mentioned, your driving behavior can also dramatically reduce your mpg rating. No jack rabbit starts, being miserly with acceleration, keeping speed down on highway etc.

I love the tail you built out of cardboard and duct tape, very nice. I'm impressed, really! I'm inspired, being a duct tape afficionado myself.

On a side note, your end result with the tail on the hatchback closely mimmicks what some sedans look like.