Quote:
Originally Posted by The Atomic Ass
Wow... I really don't understand any of this. 
But yeah, I don't want to get too complicated in my measurements... Mostly what I'm trying to achieve is a chart showcasing power requirements at increasing speeds and increasing, but otherwise simplistic inclines, to show hill-climbing requirements. I'll just add a fudge factor to over-calculate for real world conditions. Wind, temperature and other errata are off my checklist as this is basically designed to scale the battery pack (did I mention this is a pipe dream of mine, an all electric RV?), to a reasonable usefulness.
Now how does a modern bus manage a Cd of around 0.20? Being that it's for all intents and purposes a brick wall, I find that a bit difficult to swallow. Even the newer "chrome dome" style buses seem like they would have massive frontal drag.  |
Atomic,for the incline part of your quest,use the following rule: (and this is US Standard units),
1-horsepower is defined as lifting 550-pounds,one foot,in one second.
Weigh your RV at a truck scale to get within 20-pounds of its actual weight.
Pick any grade your interested in,and calculate the vertical rise over ,say a mile.
Pick a speed you'll be driving up that grade.
Converting your speed into feet-per-second,and knowing the rise over a mile,you can calculate the weight you lifted in one second.
Dividing by 550-lb-ft per second,gives you your horsepower necessary to lift the RV up that grade.
If you stay with that particular velocity,by plugging in other grades (rise over run),you can calculate the lifting horsepower for any grade to construct your graph.
Add in the aerodynamic drag horsepower requirements along with rolling resistance power requirements at the same velocity,and you'll have a pretty good picture of your electric motor requirements,given their particular efficiency ratings.
Your chosen driving range criteria will help you select a battery pack,etc. based on its allowable depth of discharge.
As to the modern bus with Cd 0.20,that's out of Hucho's book and other SAE Papers.The bus is long enough such that flow velocities and pressures playing over the body of the bus can yield these numbers.The fact that they are also rear-engine,with no openings in the front of the body doesn't hurt either.
And from the Jaray/Klemperer research of 1922,as I mentioned above,with aft-body taper (boat-tailling) the Cd can easily be dropped to 0.16.