Re: HP vs speed

[Daniel Youngquist <dany@domain.elided>]

On Fri, 6 Mar 1998, Steve Stegmann wrote:

> As a general rule the need for horsepower to increase speed proceeds
> as the square of the speed increase *until* aero drag becomes the
> dominant factor.  At this point the required horsepower increases as
> the cube of the speed increase. 

My initial reaction is that this doesn't stand up to the laws of
physics.  But not being a physicia.. physicis.. Having dropped out of
college physics 'cause I was too busy playing to do the work, I can't
tell you authoritatively why.  All I remember is, when I was in Jr. High
playing around with model rockets and doing calculations aimed at giving
a small rodent the ride of his life, the rule was resistance increases
as the square of speed.  In this case of course it was purely air
resistance.  Not that it really matters one way or the other, though, as
brick-shaped objects simply aren't intended to go very fast, and
_precisely_ what keeps them from going over any speed that's described
with 3 digits is probably irrelevant. :)  My dad has a degree in
physics, though, so if anyone _really_ wants to know, I can ask him.

> As you can see this switchover doesn't happen at normal highway
> speeds. Up to that point it is driveline losses, tire heating, stuff
> like that which is dominant.  Probably over 100 mph for a Scout.  I've
> had mine to 90 (still pulling strong but I chickened based on my
> tires).  You can tell the point if you have a big enough downhill. 
> (Mandera maybe) If you are flying along at "top" speed and go down a
> big hill and nothing happens, that's where aero drag takes over.

I've never had that happen, but I would've marked it down to being on
the top side of the engine's power curve, way up where more RPM = less
HP.  Go a couple MPH faster, and engine power drops off enough that you
can't accelerate any more despite the downhill. 

Daniel Youngquist / dany@domain.elided / 541-688-9263 // VISA & MasterCard
*HOMESTEAD PRODUCTS                    http://www.teleport.com/~dany/mill
 Grain mills, water filters, quality simple-lifestyle & outdoors products
*INTERNET MEMORY EXCHANGE  Computer Memory  http://www.teleport.com/~dany
-------------------------------------------------------------------------
"'Necessity' is the plea for every infringement of human freedom.  It is
 the argument of tyrants; it is the creed of slaves." -William Pitt, 1783