Online tool: shows aerodynamic drag, rolling resistance, power required & est. MPG

[msd1107]

There is a down loadable spreadsheet at http://www.lotuselan.net/forums/elan...ts-t18445.html that allows the user to model virtually any vehicle including IC engined vehicles, PHEVs, and EVs.

The spreadsheet is unlocked so if you do not like something, change it.

[todayican]

Just felt the need to say thank you for such an incredibly valuable tool!

[Frank Lee]

Testify!

[minAirForce]

Hello,

You seem to be the expert on all this stuff. Any aero engineers on this site? What is the optimum streamline tear drop shape? I've seen mentions of 2.3:1, 3:1 in an aero book, and 2.7:1 calculated from diagrams? I would think the optimum shape would depend on speed, but have not found any reference to that.

[aerohead]

Originally Posted by minAirForce

Hello,

You seem to be the expert on all this stuff. Any aero engineers on this site? What is the optimum streamline tear drop shape? I've seen mentions of 2.3:1, 3:1 in an aero book, and 2.7:1 calculated from diagrams? I would think the optimum shape would depend on speed, but have not found any reference to that.

mini,I settled on 2.5:1 as the 'ideal.' Hucho demonstrates it at least 4-times in his book.It is the 'shortest' structure ( body-of-revolution ) with Cd 0.04.
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In free-air,2.1 has the same Cd,but in ground-effect its aft-body down-slope exceeds 22-degrees,which was shown By W.A.Mair to be the absolute maximum body camber angle which can support attached flow for a bluff body in ground effect.
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If the body is lengthened beyond 2.5:1,surface area grows,increasing skin-friction,so overall drag begins to rise.
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When the 2.5:1 teardrop is lowered into ground-reflection it creates a vehicle 'Template' which produces a vehicle of Cd 0.08 body drag ( w/o wheels ) which has a length,5X its height,including its ground clearance.
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For road vehicles,the drag coefficient is constant above 20-mph,so the Cd is basically fixed and is not affected by speed.

[minAirForce]

Thanks! Not quite following that. Can you post the references you mention - Hucho and Mair. I haven't been able to find any of Hucho or Mair's papers online.

My background has all been aerodynamics on airplanes. I'm trying to design the best aeroshell for a pu-truck. I'm thinking the 2.5:1 teardrop would not apply since the windshield has already deflected the air upward by the time it hits the shell and the 12-15 degree back sloping shell might be too steep if the airflow has already started to separate.

[ausias]

very useful tools

[oldbeaver]

Originally Posted by MetroMPG

Aerodynamic & rolling resistance calculator - EcoModder.com

Back in the day, krousdb made a spreadsheet to calculate aero & rolling resistance forces against his del Sol, their respective power requirements, and effect on fuel consumption at various speeds.

I've ported/expanded the spreadsheet into an online tool which asks you to enter or accept default values for...

- vehicle weight
- Crr (coefficient of rolling resistance - .008 represents a low rolling resistance tire on a smooth surface - see Wikipedia for other sample values)
- Cd
- Frontal Area
- Fuel energy density (in Watt hours / US gallon)
- Engine efficiency
- Air density (rho)

... and it spits out a table showing resistance force values, estimated power requirements and fuel consumption across a range of speeds.

It's online at EM here: Aerodynamic & rolling resistance calculator - EcoModder.com

It's not perfect (e.g. engine efficiency changes with load in real life, but remains fixed at the value you supply for all speeds in the calculations for fuel consumption).

Discussion, questions, suggestions welcome.

Excellent work! However, who knows what is the frontal area of a car? As it is important, car makers should give that measure to the owners. Maybe someone knows a link where to look for this variable?

[Ladogaboy]

You usually have to just calculate it yourself with a measuring tape.

[oldbeaver]

Yes, one can write a sketch, take measures and make an estimation.

Anyway, there is some links to tables at the bottom of the page, where there is the table. I tried it up with some precise measures and some very approximated others, and the results are VERY illustrative.

At 120 miles/h my car spend 47 HP of which about 40 are to overcome air resistance and only 7 correspond to rolling resistance. The table shows progression over all speeds.

Now I understand why people devote so much effort to aero mods. Even the manufacturers spend a lot on design to producing better aero cars.

Again, one of the most interesting and valuable contribution to the subject.

Thank you very much !