Cd 0.21 Ford Fusion 999 hydrogen LSR spoiler

[Blacktree]

Originally Posted by ME_Andy

Fill me in, is rear lift good or bad for mpg?

The rear section of many vehicles is sloped downward. Aerodynamic force acting on that surface will have a force vector perpendicular to the surface. Since the surface is sloped to the rear, the force vector will be angled to the rear as well. In other words, that lift also creates drag. The image below should help to visualize it.



That said, when designing a drag spoiler you also need to consider the rear wake. For example, if you make the angle of attack too aggressive, you'll increase the size of the wake. And that will create more drag.

[aerohead]

Originally Posted by ME_Andy

Fill me in, is rear lift good or bad for mpg?

1) for mpg, lift implies induced drag.
2) the roofline ought to generate flow deceleration and pressure recovery for its entire length from the roof apex, with slowest, highest-pressure flow at the rear separation line.
3) if the Prius had 50/50 weight distribution, at its EPA test weight, then we'd be looking at 1,890- pounds static load on the rear axle.
4) at 70- mph that would drop to 1,873-pounds. A 0.899% difference.
5) if you were willing to spend 400-hours fabricating an OEM-quality rear spoiler to cut 17-pounds of lift, well then, there you go.
6) and that presumes that, your very first design will 'solve' the lift issue.
7) or, anyone that we might buy one from.
8) @ $4,000 / hour of wind tunnel time to verify the performance.

[aerohead]

Originally Posted by Blacktree

The rear section of many vehicles is sloped downward. Any lift acting on that surface will have a force vector perpendicular to the surface. Since the surface is sloped to the rear, the force vector will be angled to the rear as well. In other words, that lift also creates drag.

That said, when designing a drag spoiler you also need to consider the rear wake. For example, if you make the angle of attack too aggressive, you'll increase the size of the wake. And that will create more drag.

I would just like to interject the observation that, it's not necessarily the 'size' of the wake, but the base pressure within it that will determine the drag.
There's going to be a 'sweet spot' for every vehicle spoiler. Size and geometry.
The worst-case is a slope that will create a vortex-pair, which induces a downwash-induced small wake , but coincident vortex drag, creating enormous total drag.
In the 1st-gen VW Golf/ Rabbit, the Cd could vary from 0.34, to 0.437, just by altering the rear slope angle.

[Blacktree]

Aerohead makes a good point. In the case of the Prius, eliminating the rear lift is only going to reduce a few pounds of drag. And that's assuming you manage to eliminate all the lift, without incurring a drag penalty. So realistically, reducing rear lift should be a secondary consideration.

The main purpose of a drag spoiler would be to clean up the rear wake. It's the same basic principle behind kammbacks and boat-tails. And it also appears to be what the spoiler in the OP is designed for.

[aerohead]

* On the light-table, I overlaid the image of the 999 image, comparing it to ten of my contours.
* POPULAR MECHANICS Magazine had an X-Ray image of the Bonneville spoiler. It is a 'level', 10-inch extension off the decklid, parallel to the upper edge of its capping plates.
*As shown, the trailing edge of the spoiler is an exact fit to the 2013 AST, with an imaginary flow line exiting the spoiler at 14-degrees below the horizon.
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* This photo of 999 does not represent its 2-inch ground clearance, as raced.
* Nor does it indicate the MOON Equipment's convex wheel covers.
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* I finally got dimensions for the car this morning for the dimensional analysis.
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* Also, in their report on the 187.607-mph, 2013, Jetta Hybrid LSR car, Volkswagen AG provided enough information from which to reverse-engineer the air density at Bonneville, which will help estimate the 999's dynamics.

[aerohead]

Yes!
Also consider that, in January, 2008, they returned to the Salt for a 209.297-mph, two-way average.
At these velocities, when you trip the lights at the end of the course, you don't step on the brake pedal.
And when driver, Rick Byrnes pulled the handles for the dual-parachutes, the spoiler and capping plates helped with efficient pilot-chute inflation, pulling the main 'chutes' out, and into a more stable wake region for safe deceleration.

[aerohead]

* 536.4-horsepower from fuel cell ( published )
* @ 98% efficient inverter, downstream power = 525.672- hp
* @ 93% efficient motor, downstream power = 488.874- hp
* @ 94% efficient, non-overdrive transaxle, power = 459.542- hp
* Total Road Load Power = 459.542- hp.
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* @ estimated OEM Projected frontal area = 24.546-sq-ft
* @ estimated projected frontal area = 23.3187-sq-ft ( SCTA allows a 5.3% Af reduction )
* Cd 0.21 ( published )
* 209.297-mph ( 306.968-ft/second )
* rho ( air density )= 0.002183 ( reverse-engineered from 2013 VW Jetta hybrid LSR data )
* Aerodynamic road load power = 281.104-hp
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*Total Road Load power minus aerodynamic road load = 178.4374- hp rolling-resistance.
* @ estimated 6,895- pounds race weight, Cf r-r + 0.046368162 on the 'Salt.'
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* OEM Cd 0.34 ( published )
* Mirror delete = delta- Cd 0.015 ( from Ford )
* 100% grille-block @ est. delta- 0.025 ( Ford )= Cd 0.315
* 100% belly pan @ est. delta- 0.025 = Cd 0.275 ( Ford said delta Cd 0.01 )
* MOON wheel covers @ est. delta- 0.024 = Cd 0.251 ( Ford makes no mention of )
* Ground Clearance lowered to 2-inches @ est. delta 0.0211 = Cd 0.2298 ( Ford published delta- Cd 0.08 for lowering )
* 10-inch deck extension = Cd 0.21, allowing delta 0.0198***** ( Ford makes no mention of 10-inch extension )
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* 999 has a 4.769% length addition
* a 7.923% height reduction
* a 67.558% ground clearance reduction
* a 4.552% forebody reduction
* a 6.416% aft-body elongation
* a 11.15% Vejungungsverhaltnis increase
* a 5.3% Af reduction
* a 7.491% L / sq-root of Af increase
* a 41.32% Af reduction
* a 38.23% Cd reduction
* A backlight-to-boot departure angle relaxation, from 16-degrees, to 14-degrees