Re: ihc-digest V5 #230 HP vs speed

[Steve Stegmann <stegmann@domain.elided>]

Scouters,

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.

I can't say where this point is, it varies for each case, but in a Scout
it won't be 150 mph.  If we are into aero drag as dominant at 100 mph then
an increase to 110 mph will take (110/100)^3 = 1.33 times the hp.  To go 
from 100 to 125 would be (125/100)^3 = 1.95 times the power.  This is why 
nobody ever gets much over 400 mph in a speed record car with normal gas 
engines.  Only a jet or helicopter gas turbine packs enough power into a 
small enough volume to keep the drag to hp ratio  under control.

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. 

If our 100 -> 110 change is below the aero drap point it will take 
(110/100)^2 = 1.21 times the hp.  (125/100)^2=1.56 vs 1.95.

Anyway, sorry I don't have time to properly edit.

Steve
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Steven A. Stegmann
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