Think Flight: Using Simple Aerodynamics to Beat High Gas Prices

[aerohead]

I ran an energy balance for all the Subaru configurations.
The conditions were:
65-mph ( 95.333 feet/second )
estimated Af= 23.8153 sq-ft OEM, estimated 24.4394 sq-ft with airdam
rho = approx. 0.0023256823
Cd 0.29 ( CarsDirect.com )
tires- 205mm width
REGULAR Unleaded, reformulated gasoline @ 111,836-Btu/gallon
6.138-pounds/ gallon
CVT transmission of 87.71% mechanical efficiency
40% thermal efficiency for engine
2546 Btu/ brake-horsepower
2.2% engine accessory losses
Rolling resistance power absorption is carried as a constant
Brake Specific Fuel Consumption ( BSFC) is carried as a constant
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configuration-1: Cd 0.29, CdA= 6.9064 sq-ft, 35.644-mpg
configuration-2: Cd 0.214, CdA= 5.0981 sq-ft, 40.528-mpg
configuration-3: Cd 0.2671, CdA= 6.5289 sq-ft, 36.5637-mpg
configuration-4: Cd 0.1764, CdA= 4.3114 sq-ft, 43.0971-mpg
configuration-5: Cd 0.2291, CdA= 5.5467 sq-ft, 39.4562-mpg
configuration-'6'* Cd 0.1721, CdA= 4.2060 sq-ft, 46.5376-mpg
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'6' represents a 17.3% radiator blockage to compensate for a now, 'oversized' radiator area.
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The 'source' flow for the underbody and diffuser originates at approx. 16.11-inches above ground level before displacement. The separation edge for the slanted diffuser should not be any higher than 16.11-inches, as, at this elevation, the displacement kinetic energy nulls, and constitutes the 'reversal point.'
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Additional 'fruit' which lay outside the scope of the Impreza project, if added, could easily bring the Subaru into Cd 0.145 range

[Phase]

I wish he could build my Ioniq a rear boat tail like that. Wonder how stable it is at 100 mph with “ duct tape” though lol

[Bicycle Bob]

Aircraft mechanics often refer to "hundred mile per hour tape," but it even gets used on jets. A case of tape was used to temporarily replace much of the fabric on the fuselage and tail of a bush plane that smelled of fish in bear country. Workmanship matters. When sealing a glider canopy for a record run along a windy ridge, someone didn't press hard and consistently, and a small section turned into a screaming reed for over ten hours.

[Piotrsko]

Tape for aircraft comes in flavors, 100, 300, 500 mph mostly dependent on who you are talking to and the construction materials... safe to say 500 is really structural, not used much and super expensive. Doesn't come off either.

[Bicycle Bob]

Not a duct tape, but 3M has a double-sided tape that is popular for replacing lines of rivets on box-truck bodies. I have a sample, and I'm a firm believer.

[Phase]

Originally Posted by Bicycle Bob

Not a duct tape, but 3M has a double-sided tape that is popular for replacing lines of rivets on box-truck bodies. I have a sample, and I'm a firm believer.

I was thinking of using 3m double sided tape to attach my extended box cavity versus a full boat tail versus drilling the box cavity in and hurting the value of my car more

[freebeard]

Best would be a tape that can be removed with some solvent that doesn't affect the paint.

[aerohead]

With the original modifications, the upper contour of the boat tail best matched the W.A.Mair, and FKFS (Kamm) 1/2-body silhouettes.
Maximum down-slope angle is 19-degrees. Diffuser upsweep is 20.5-degrees.
The 'highest' streamline filament traveling under the Impreza begins about 16.1-inches above ground level.
Along with the other mods, the tail gets the car to approximately Cd 0.214.
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For the second-generation, #4 configuration modifications, the upper contour of the boat tail is almost an exact match to W.A.Mair's and Buchheim et al.'s 1981 VW Flow-body tails.
Maximum down-slope angle reaches 23-degrees. The diffuser angle is 5-degrees upsweep.
All mods contribute to an approximate Cd 0.1764, and achieves an almost 21% improvement in mpg.
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Averaging for all configurations, the delta-drag/delta-mpg relationship works out at:
delta-10% drag = delta-5.15% mpg ( assuming constant BSFC )