Aerodynamics

[angib]

I may take a model and mount it on a spring loaded arm and drive down the road at 30 mph and see what the spring tension is, then change the model with a different front (flat) and see that the spring tension is....

If the results are going to be at all accurate, you would have to tow the two trailer models behind a minivan model.

At least, I'm assuming you don't let the full-size trailer go down the road on its own.......

One thing that has always intrigued me is that a boat with a bulbous bow has less resistance than one with a knife sharp bow.....

Yeah, but it's not at all comparable to a road vehicle - a ship is operating on the water-air boundary so it can make waves and they absorb a lot of power. The bulbous bow is there to create a 'negative' wave that cancels out with the 'positive' wave from the bow, so the overall wavemaking resistance is reduced. The bulbous bow only works right at one speed when the two opposing waves are in synch.

For a road vehicle, unless you are going to go fast enough to get near the speed of sound, a knife-sharp front will never be as good as a well-rounded front - all pointy fronts are there for styling, not low drag*.

Andrew

* There's bound to be one exception, that I can't think of right now, but someone will be sure to tell me.......

[Arne]

Coop, I got real excited about that trailer till I thought about trying to insulate and panel the interior.... that popped my balloon... With those compound curves, I can not think of any easy way to do it...

[goldcoop]

Coop, I got real excited about that trailer till I thought about trying to insulate and panel the interior.... that popped my balloon... With those compound curves, I can not think of any easy way to do it...

Arne-

Maybe a spray on expadable foam done by a pro?

Or flexible foil type insulation like Dynamat?

I dunno, BUT I did think you'd like that one!

Cheers,

Coop

[Arne]

Coop, that is a pretty wide trailer.. one thing that could be done is use the front for storage, and put up a wall, thereby dealing only with the more conventially shaped rear area...... I'm still thinking.

I just took a look, it is pretty weighty to start with... I figure with mattress, tv, a/c, etc. it would get up to a beefy 1,500#... in addition, it is noticably wider than my van, meaning extended mirrors, etc..... think I'll have to pass.

[Chip]

any body out there have access to a wind tunnel?? it would be interesting to see what the flow pattern of the smoke is over a car and trailer at different road speeds,,,

just my opinion, but a heavy trailer with a heavy tongue weight changes the attitude the pulling vehicle is pushing thru the air basically making it less aerodynamic,, then you add a box thats in clean air also pushing air and weight all will effect how well a tow truck and trailer work together,,,

lots of factors at work here

chipper

[vairman]

Dose anyone know what the drag coefficient there tow vehicle is??

Mine is about .43 (with the top up) GM data


I think SUV's and trucks will be higher... Just a thought...

Greg

[Chip]

Greg,,, good question,,, I never even thought to look mine up,,, Its a .33,

wounder what the combined is though,,,

chipper

[goldcoop]

For what its worth!

I have noticed when towing the Ctear (which is 6'w x 10'l x 5'h, guessing 2200lbs) at higher highway speeds that the whole trailer seems to crouch down probably 6" at the tongue!

It tows great! No sway, bounce, etc.

I guess thats "down force"!

But from this description does anyone have any idea of whats happening a good or bad thing?

Cheers,

Coop

[Sonetpro]

Greg, shouldn't be hard to estimate the drag coefficient of my Jeep, It has the same aerodynamics of a brick.

[angib]

I never even thought to look mine up,,, Its a .33,
wounder what the combined is though,,,

That Volkswagen data I quoted in the first page of this thread gave a drag coefficient Cd of 0.53 for both car/trailer combinations (without the roof spoiler). In this case the Cd is calculated from the trailer's cross section area, which is maybe twice that of the car. So with twice the area and a Cd 1.5 times the car's solo Cd, the aero drag of the car/trailer combo is about 3 times the solo car's!

All these numbers only apply to this sort of ratio of car and trailer size - not to teardrops.

Andrew

On edit: Perhaps I should explain that drag coefficients on their own are only half the story - the other part is the cross section area (roughly, height x width). To get the aero drag for a vehicle, the drag coefficient is multiplied by the cross section area. So a low Cd doesn't necessarily mean 'low drag', just 'low drag for its size'.

[critter]

hey all,
so what now i have to put a wing on the weekender kinda like the 70 superbird?

[Chip]

Andrew,, the cd I have of .33 is for a Caddy Catera,, not a beast but still a full size car, If I am interpreting this info,, a cd of .33 may be good for the size and frontal area of one car but be poor for another,, so you cant compare cd's except for like size objects,, I couldnt compare the cd" of a brick to an F-15 plane,, so is there a reference point or scale that a person could compare un-like and size objects to get and idea how efficient they move thru the air, or am I just missing the point here,,,

ya got to be gental with the information,, :Caution, old brains at work"

chipper

[angib]

Chip,

Sorry about that - I do tend to assume everyone knows the equations!!

Drag (the force holding you back) is made up of two important things, multiplied together:
- how slippery a shape the vehicle is, measured by the drag coefficient, Cd
- how big the vehicle is, measured by its cross-section area, A

Here are some good, OK and awful examples:

Honda Insight:
Cd 0.25 (the best production Cd, as far as I know)
Cross-sectional area = 19.7 sq ft
CdA = 4.9

Cadillac Catera (or Opel Omega, as I know it):
Cd 0.33
Cross-sectional area = 22.1 sq ft
CdA = 7.3

Hummer H2:
Cd 0.57
Cross-sectional area = 37.1 sq ft
CdA = 21.1

So the Insight has 2/3rds the drag of your Caddy with most of the difference being its lower drag coefficient - it's only got 11% less cross-section area than the Caddy, though it looks smaller to me.

It's no surprise that the Hummer is much worse for drag, but I'm surprised it's as much as 3 times the drag of your Caddy. No wonder it ain't fuel-efficient!

I've realised I've given examples where the small vehicle has a low Cd and the big one has a high Cd, but these are not related:
- the old (very small) Mini had a Cd value of about 0.50, while
- the Mercedes (and Dodge) Sprinter panel van has a Cd of 0.36.

Does any of this help or explain? I worry that I may just be adding confusion, so I'd better stop!

Andrew

[Chip]

nope,, that put things in more perspective, for me,,, cda is how to compare objects of different size, shape etc,,,

thanks for the lessen in fluid dynamics,,,

and somebody said ya cant teach an old dog new tricks,,,,bark bark,,,

chipper