From: gary@ke4zv.uucp (Gary Coffman)
Newsgroups: sci.environment,sci.physics,sci.energy
Subject: Re: Is car pooling for real? (was Re: Are bikes really less pollutin
Date: 23 Jul 92 15:13:59 GMT
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group

In article <1992Jul21.220152.19182@aio.jsc.nasa.gov> mancus@sweetpea.jsc.nasa.gov (Keith Mancus) writes:
>In article <1992Jul21.140032.23459@ke4zv.uucp>, gary@ke4zv.uucp (Gary Coffman) writes:
[munch]
>>Yes, a motorcycle offers some advantages over a car. It also suffers
>>several disadvantages in bad weather. It becomes markedly less safe
>>and comfortable on wet roads. Also, few motorcycles really achieve
>>high MPG ratings because of poor aerodynamics. Enclosed, streamliner
>>bikes are possible, and have been built for specialized uses, but they
>>aren't mass market material yet. The primary demand for power in a
>>vehicle is to overcome air resistance at speeds over 30 MPH.
>
>	Sorry, but I think this is wrong. Aircraft are more efficient than cars
>precisely because the car loses energy due to causes other than air resistance.
>	For example, a Mooney 201, which is pretty crude technology today,
>can cruise at >180 mph and tops out around 200 mph on 200 HP.  This is
>a four place airplane (thought I'd be the first to admit the back seats
>are pretty small).  Can anyone point me to a 200 mph car that only has
>200 HP?  No fair pointing to exotic racing machines.  There are other,
>smaller, more modern aircraft that put the Mooney to shame.  For example,
>the Rutan-designed Q2 cruises >150 mph with a VW engine, carrying two people.
>Where is the equivalent car?
>	In fairness, I suspect that most cars fall far short of their potential
>minimum air drag due to insufficient use of laminar flow.  Just look at
>all the crummy antennas, door gaps, sharp angles, etc.

Here are the appropriate equations for automobile power requirements:

(1) P=Fw*v      where P is in kilowatts, Fw is shown below, and v is in km/hr

(2) Fw=Fro+Fl+Fst      all in Newtons where Fw is total driving resistance,
                  Fro is rolling resistance, Fl is aero drag, and Fst is 
                  climbing resistance

(3) Fro=f*m*g     where f is the coeficient of rolling resistance (0.015 for
                  concrete or asphalt), m is vehicle mass in kilograms, and
                  g is the acceleration due to gravity (9.81 m/s^2)

(4)Fl=0.5*Q*Cw*A*v^2  where Q is air density (1.202 kg/m^3), Cw is the drag
                      coefficient (about .3 to .4  for normal cars), A is
                      frontal area in meters^2, and v is speed in km/hr

(5) Fst=sin(Phi)*m*g  where Phi is the angle of incline, m is vehicle mass,
                      and g is acceleration due to gravity

If you will work some examples, you'll find that Fl dominates at speeds over
50 km/hr. Auto drag is bad mainly because of the large frontal area (about
4 sq meters), the aerodynamically dirty *underside* of the vehicle, the 
coupling to the ground via the Bernoulli effect, and vortices off the rear of 
the vehicle. In airplanes, much attention is paid to maintaining a laminar flow
over the entire structure and minimizing frontal area. In a car, that isn't 
possible because of other constraints on design. However, Mercedes ran a 
special *diesel* car on a closed course to 280 km/hr with a 75 hp engine. 
The streamlining was extreme, the car was only 24 inches tall.

Gary
Newsgroups: sci.environment,sci.physics,sci.energy