Thread: Interesting thing

Originally Posted by Dougal

The Cd is a correction factor, it corrects the actual projected area to an equivalent projected area. Your description is correct, but your interpretation isn't.
It is measured and and applied to correct the formula.
1. A 93 defender 110, 95 discovery and 94 defender 90 all have a Cd of 0.55.

2. There have been many studies done on US pickups aerodynamics, they do better with the tailgate up than down. They do even better of course with a streamlined canopy fitted.

1. Drag coefficients published by manufacturers (and parrotted by everyone else) are in many cases very suspect (I don't know whether this is the case with Landrover but I would want to see proof it wasn't rather than the reverse). There are two reasons for this - in most cases they are measured on models in wind tunnels, and do not properly account for the roughness of the underbody. This is particularly important for vehicles like the Defender with a very high ground clearance. The second reason (probably less important for unstreamlined shapes like Landrovers) is that for most body shapes the Cd is very sensitive to small changes in body attitude, which means that the quoted figure is only achieved in practice when loaded exactly as the test model. It would be surprising if all body styles of Defender 90, 110 and the Discovery had the same Cd, regardless of the fact that the published figure is the same, although I would not be surprised at their all being similar, as they are all more or less boxes, and even more important the very similar underbody must represent a fair bit of the drag - but agreement to two decimal places does sound unlikely.

2. I am surprised to hear this - certainly in the US pickups are commonly operated with flowthrough tailgates or no tailgate, although this may well reflect marketing rather than anything else. In my own experience, tailgate or no tailgate on my big trailer is good for more than 10kph in maximum speed when empty, although I will admit that the tailgate on the trailer is a lot further back than on a ute.

John

Originally Posted by pawl

Ummm.....I would have to agree with JDNSW here. Cd is the drag Coefficent . Well, thats what I was taught in Helicopter Aerodynamics years ago.
It is a dimensionless number used to describe the drag or resistance of a shape through air or water.

A coefficient of any type is a correction factor, whether it's for fluid drag, friction or otherwise. It corrects the ideal situation to fit with a measured result.
They have to be dimensionless (unitless) otherwise they unbalance the equations they're correcting.

As for landrovers all having pretty similar Cd's, well they're all at the upper end of the scale, so getting it worse is difficult.
Even a hummer is listed as being better (0.5).
With a dyno plot of actual wheel power and a measured top speed you could work it out yourself.

Here's the most comprehensive list I've found.
The Mayfield Company Homepage - Coefficient of Drag Tables and Curves
Landrover list the D3 Cd as 0.41 in their tech specs, but they also say it has 24 valves per cylinder.

Everyone is right. Cd = drag coefficient, and is basically a correction factor.

For values of Re < 1 (Stokes flow), Cd = 24/Re. (Re = reynolds number) - so Cd values can go up to around 1x10-5.

For Re > 1000 (where vehicles usually operate), Cd ~ 0.44 for spheres, and my texts say the ranges are from about 0.04 for (subsonic) planes, and up to almost 1 for vehicles such as "delivery vans".

Dougal is right on the ute tailgate issue. It was even on mythbusters! IRC, both having the tailgate up and having a tailgate replacement of wide plastic mesh were better than having it down or removing it.