Anybody here know about aerodynamics?

[Guest]

OK, sounds a bit off topic, but not really.
I was chatting with another lister yesterday about roof racks, roof
tents and obviously apart from raising the centre of gravity a lloaded
roof rack / roof tent ruins the LC's svelt lines causing increased drag
and bumping up the fuel consumption.
Now to my question.......
Normally with a roof rack or a roof tent it is raised above the roof
line so that air can flow between the roof and the tent/roof rack.
Obvioussly due to the nature of the roof rails support a roof tent or
the supports under a roof rack there is a fair bit of turbulence.
Is there any merit in having some sort of aerofoil/cover running from
the top of the windscreen to push the air up and over the tent/rack - a
bit like the big aerofoils you see on the front of lorries?
--
Regards,
Julian Voelcker
Mobile: 07971 540362
Skype: julianvoelcker
Cirencester, United Kingdom
1994 HDJ80, 2.5" OME Lift

 

[Guest]

Have thought about that (yes really).
My thoughts;
On the negative side the deflector would add to the frontal area and
increase the resistance to the frontal air flow.
On the positive side there would only be one area of turbulence behind the
LC and that area would be "simpler", a join between the air around and under
the vehicle rather than lots of airflows joining.
Back to the negative the single turbulent area might well be larger and so
have a lower negative pressure and more drag.
I sometimes play with a flight simulator called xplane which allows you to
very quickly "make" "aircraft". The flight model is very good and if I
remember it gives you figures for drag and lift. I wonder if I could model
a simple LC and a box.......sure it won't fly but I could but a little jet
engine at the back to mimic the engine...
At a guess, little difference with just a front deflector, a cone type
addition on the back as well might smooth the joining of the airflows would
perhaps help!
Malcolm Bagley
Stafford, UK
1975 FJ45 Pickup (in work)
1976 FJ45 Pickup (next one)
_______________________________
-----Original Message-----
From: [Email address removed] [mailto:[Email address removed]] On
Behalf Of Julian Voelcker
Sent: 11 February 2007 23:31
To: [Email address removed]
Subject: [ELCO] Anybody here know about aerodynamics?
OK, sounds a bit off topic, but not really.
I was chatting with another lister yesterday about roof racks, roof
tents and obviously apart from raising the centre of gravity a lloaded
roof rack / roof tent ruins the LC's svelt lines causing increased drag
and bumping up the fuel consumption.
Now to my question.......
Normally with a roof rack or a roof tent it is raised above the roof
line so that air can flow between the roof and the tent/roof rack.
Obvioussly due to the nature of the roof rails support a roof tent or
the supports under a roof rack there is a fair bit of turbulence.
Is there any merit in having some sort of aerofoil/cover running from
the top of the windscreen to push the air up and over the tent/rack - a
bit like the big aerofoils you see on the front of lorries?
--
Regards,
Julian Voelcker
Mobile: 07971 540362
Skype: julianvoelcker
Cirencester, United Kingdom
1994 HDJ80, 2.5" OME Lift

--
No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.5.432 / Virus Database: 268.17.29/673 - Release Date: 06/02/2007
17:52
--
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.5.432 / Virus Database: 268.17.29/673 - Release Date: 06/02/2007
17:52

 

[Guest]

Julian,
Yes there is. The front of a roof tent is usually square-ish and
that is the worst shape you can have for air disturbance. With the
tent lifted some way off the vehicle you would be better making the
front, and sides, a half round shape. With the tent almost on the roof
you should go for the 90 degree option of the same shape (this is the
better option). This shape is the most aerodynamic that you will
achieve. Not only will turbulance effect you less but the wind will
not try to turn your vehicle over. The air slips over, instead of
fighting with, the shape.
Regards, Clive.

 

[Guest]

I've got a part attempt of a "slant" on the front of my roof-rack.
It doesn't go down all the way to the top of the truck and is 'based' and the bottom of the roof-rack.
I enclose a pic so that it makes more sense (note its at full height - normally I only have at at 1/2 height and
'aerofoil' is adjustable for angle)
the only way 2 really gauge it would be to have a certain amount of luggage etc. on top. Ensure no 'slant' object on front.
Fill up truck and drive one way maybe 100 miles or so.
Fill up and check mpg.
Drive back. Fill up again.
Put 'slant' effect on front and drive same route.
Fill up and get mpg.
Compare the two (presuming same conditions, if flat and no wind then maybe return journey would b enough?)
I've not done it!
Lal
Julian Voelcker wrote:
> OK, sounds a bit off topic, but not really.
>
> I was chatting with another lister yesterday about roof racks, roof
> tents and obviously apart from raising the centre of gravity a lloaded
> roof rack / roof tent ruins the LC's svelt lines causing increased drag
> and bumping up the fuel consumption.
>
> Now to my question.......
> Normally with a roof rack or a roof tent it is raised above the roof
> line so that air can flow between the roof and the tent/roof rack.
> Obvioussly due to the nature of the roof rails support a roof tent or
> the supports under a roof rack there is a fair bit of turbulence.
>
> Is there any merit in having some sort of aerofoil/cover running from
> the top of the windscreen to push the air up and over the tent/rack - a
> bit like the big aerofoils you see on the front of lorries?
> --....

 

[Guest]

Hi Malcolm,
Glad I'mm not the only one ;-)
True, but you have that already with a loaded roof rack or roof tents.
Unless you filled the gap in between the rack/tent and the roof all the way
around so it acted more like just a raised roof.
With all these things I guess you need to look at the cost benefit - if there
was a noticeable difference in consumption and it didn't cost much to do then
might be worth it.
At the simplest level I was thinking of some sort of rigid plastic strip that
could be wrapped around the base of the tent/rack sitting in the gutter on the
sides and angled forward at the front.
Anybody got access to a wind tunnel?
--
Regards,
Julian Voelcker
Mobile: 07971 540362
Skype: julianvoelcker
Cirencester, United Kingdom
1994 HDJ80, 2.5" OME Lift

 

[Guest]

Julian
Crudely air resistance is made up of drag and turbulence, which rise as
speed squared and are more or less function of "roughness"; and an
"inertia" term (pushing the air aside and back again) that rises with
speed and is a function of volume and shape.
I suspect that for a "small" (eg 4 inch) gap between underside of
roof-rack and roof you are better off closing the gap (front and sides),
as the reduction in turbulence and drag probably outweighs the inertia
terms. If you can't close the gap making the underneath and sides of
the roof-rack as smooth as possible will help.
Making the top and sides of the roof-rack and contents smooth, ie
rounded shape will also definitely help - less drag.
Making the tail end of the roof-rack taper down to a point (like an
aerofoil wing shape) will also help a lot to reduce vortices behind the
vehicle. Those roof-top boxes you can buy are a pretty good indication
of the shape to aim for.
A triangle facing the / is actually more "draggy" than one
Wind point first / | facing the wind flat side first.
Wind =3D=3D> / |
| This is because the tapered tail
reduces vortices in the wake.
You could have quite a lot of fun making a model out of plasticene on
top of a toy car, restraining it via a piece of of string and a spring
balance and then blowing air at it with a fan or hair-drier to see how
shape effects wind resistance. Perhaps a hair-drier wouldn't be enough
and you'd need a more powerful fan. Do I feel an ELCO wind-tunnel in
the offing...
Christopher Bell
|Now to my question.......
|Normally with a roof rack or a roof tent it is raised above the roof
|line so that air can flow between the roof and the tent/roof rack.
|Obvioussly due to the nature of the roof rails support a roof tent or
|the supports under a roof rack there is a fair bit of turbulence.
|
|Is there any merit in having some sort of aerofoil/cover running from
|the top of the windscreen to push the air up and over the
|tent/rack - a
|bit like the big aerofoils you see on the front of lorries?
____________________________________________________________
Electronic mail messages entering and leaving Arup business
systems are scanned for acceptability of content and viruses

 

[Guest]

Thanks Chris, you have confirmed what I was thinking.
I'll let you know if I ever get around to building the wind tunnel !
--
Regards,
Julian Voelcker
Mobile: 07971 540362
Skype: julianvoelcker
Cirencester, United Kingdom
1994 HDJ80, 2.5" OME Lift