Sums

In the never ending quest to distract myself from revising for various
ecology and survey methodology exams, I've decided to take up aerodynamics.
The thing is, it's years since I did any proper maths. So how the **** do I
calculate maximum speed from frontal area, Cd and power? I can't even
rearrange mildly complicated equations any more...

 

Google it.

 

It's surprisingly ungoogleable. I suppose that's the nature of google
though - based on what people link to. And people are thick cunts who link
to lots of calculations that let you work out that your Civic will do 200
because it's gearing would let it, if it had 500 horses...

 

Try rec.auto.sports.f1 - there are a couple of aerodynamicists on
there. You might even find the equations have been posted before if you
Google the group.

--
Krusty
www.MuddyStuff.co.uk
Off-Road Classifieds

'02 MV Senna '03 Tigtona 955i '96 Tiger '79 Fantic Hiro 250

 

P is power, F is force, V is speed, S is area, rho is density:

P = FV, (1)

and force is equal to drag:

F = Cd 0.5 rho V^2. (2)

So, insert (2) in (1):

P = Cd 0.5 rho V^3, (3)

and

V = (2P/Cd/rho)^(1/3).

 

Strangely enough, one of the pedal-power groups appears to have come up
trumps:
http://groups.google.com/group/rec.bicycles.tech/msg/f4785fed806cbe0d

 

All SI units? Where's frontal area in that lot?

 

Paging TOG

 

[wrong stuff snipped]

Bugger. It should read:

P is power, F is force, V is speed, S is area, rho is density:

P = FV, (1)

and force is equal to drag:

F = S Cd 0.5 rho V^2. (2)

So, insert (2) in (1):

P = S Cd 0.5 rho V^3, (3)

and

V = (2P/S/Cd/rho)^(1/3).

 

<TOG>

S = CG ^ <MSOHP>
----------------
STEEP HILL

</TOG>


--
Krusty
www.MuddyStuff.co.uk
Off-Road Classifieds

'02 MV Senna '03 Tigtona 955i '96 Tiger '79 Fantic Hiro 250

 

Ta for that. I'm using a 15% fudge factor for transmission losses & rolling
resistance (looking at various versions of the Golf) and I get rather high
top speed figures for MK2s and 3s and 4s, but pretty much bang on for the
later MK5s. I suspect this could be down to gearing mind, as the MK2s are
geared to get to the red line pretty sharpish in fifth rather than to reach
all out top speed, and it's also perfectly possible that VW didn't want the
MK2 G60 to do 155mph given that it's probably generating considerable lift
at that speed.

 

It is the same as what you should all ready know for fluid dynamics
except the fluid isn't quite as much of a fluid but is still very
dynamic. Easy really.

 

Don't forget that if the drag is higher than the power output lower in
the rev range the car may never get to the top of it's powerband in
top gear.

You can cheat with:
<http://www.cartestsoftware.com/cartest4.5/downloads/CarTest.zip>

It has the 1991 16v in there already and there is nothing to stop you
adding any more you like if you have the right information.