Quote:
Originally Posted by The Atomic Ass
Is the co-efficient of drag related to how much of the front profile actually catches air dead on instead of deflecting it in one direction or another? To be more specific, can it safely be assumed that a flat-faced RV has a cd of 1.0?
Also, are there any calculators available that could help me plot the energy consumption of a vehicle at various speeds, based on it's weight/cd/inclines/etc? Keeping in mind that the math portion of my brain is retarded and my eyes glaze over at mathematical formula?
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Atomic,I saw your post the other day and didn't want to chime in until I'd pulled my wagons in a circle.The members have tossed out a lot of good for you to chew on,and I thought I'd add to that.
With respect to your RV question,I know of no vehicles of any type with Cds as high as 1.0.The Volkswagen van of 1951 had Cd 0.75 and with rounding of the sides and roof leading edges,the drag dropped to Cd0.42.
Modern busses have Cds on the order of 0.20.With sculpting of the rear roof-line,and sides,that number can be reduced to Cd0.16.
18-wheelers of the 1970s,with Cd0.85 can and have been lowered to 0.24.
As far as formulas which would solve for the various forces as you would encounter on a real road ( curves,hills,wind spectra,temperature and weather variations,etc.),would require partial differential equations simultaneously solving for all those variables,something the big-boys won't even attempt.
If you want to get a "feel" for what's possible,use the force equation Trebuchet provided for the aero portion.Think straight and level roads,with no wind or very light wind and no cross-wind.That will get you out of a lot of calculus and trig.
The following will give you some extra things to consider:
Any testing is conducted only after 30-minutes of continuous driving
at 50-mph ( to get everything up to equilibrium temps).
70-degrees F is considered "COLD',and testing below this temp can be problematic.
Air drag varies directly as a function of frontal area ( reduce area 50% drag is reduced 50%).
Air drag varies directly with Cd ( cut your Cd in half and you've cut your drag in half).
Aero drag force varies as the square of the velocity.
The power to overcome aero drag varies as the cube of the velocity.
MPG can vary 14% depending on your rate of acceleration.
It takes 6X more fuel to accelerate a car from a dead stop than it does from 2-mph.
A 3% uphill grade can cost you 32% mpg.
A 7% grade can cost you 55% mpg.
Wet roads can cost a full mpg.-
Optimum mpg will occur at about 30-mph.-
At 40-mph you'll lose 11%.
At 50-mph you lose 20%.-
At 60-mph/ 31%.
70-mph/41%.-
80-mph/52%.
From 70-degreesF,driving 50-mph at 50-degrees F will cost you 5% mpg.
Driving at 20-degrees F will cost you 11% mpg.
Also,a stuck-open thermostat can cost you 7% mpg.-
Stop-and go driving can cost you 50% mpg.
Fluctuating between 40 and 45-mph can cost 1-mpg.
A 1-mile trip versus a 40-mile trip will cost you 75% mpg.
A 10% change in weight can effect mpg by 4.3%(urban),2.4%(@70-mph).
For an 18-mph tailwind,you gain 19% mpg HWY.For 18-mph crosswind you lose 2%,and for a 18-mph headwind,you lose 17%.
Think about these things,and if you want to go "deeper" we'll steer you into the big calculations. Happy noodling!