Angled tonneau cover questions

[ChazInMT]

Originally Posted by Angel And The Wolf

Everything I've read says 15% is optimal for preventing slip stream separation You want the air to stay as close as possible to the skin to prevent turbulence

Sorry, gotta interject, First off, this is a flat cover, so the break from cab to flat cap top will really mess up any chance of keeping it anywhere near "Attached" at 15°. Secondly, the 15° thing is the Steepest you can be and not separate.....any thing Less....will maintain attached flow as well. Thirdly, The drag coefficient (Cd) between 5° and 15° (on an optimally shaped cap, will be within 5% of optimal.

So, As I've stated before, a 5-8° slope will yield the best results here, anything more will start to increase drag and cut into the usable space under cover.

[freebeard]

A flat surface at any angle is suboptimal. Especially if it doesn't start at the maximum camber at the top.



Start flush and curve it to 22°. Respect the length/diameter for truncation. The section can be circular, square or squircular.

[Angel And The Wolf]

Originally Posted by freebeard

A flat surface at any angle is suboptimal. Especially if it doesn't start at the maximum camber at the top.



Start flush and curve it to 22°. Respect the length/diameter for truncation. The section can be circular, square or squircular.

Why 22 degrees? Why not the recommended 15 Degrees? Air speed does not decrease towards the rear.

[freebeard]

Why is there air? It starts and ends [relatively] motionless after the passage of the car. Velocity and pressure trade off with each other.

Where the 1930s Zeppelin designers used an airfoil section with a continuous curve from one end to the other, Mair was working on torpedoes, terminating a tubular body. 22° just falls out of the science.

I used 45° not 2x22° for simplicity's sake when I did these experiments:

[Angel And The Wolf]

I have drawn a study of the Prius with 15 degree tail section. The only area left vertical is the license plate. This could be made of a pipe frame and lightweight material stretched over it.

https://www.youtube.com/watch?v=qaqFdn-c8-c

[freebeard]

Your design takes the 15° ball and runs with it.

Some points though

There is a positive pressure gradient the front and a negative one at the rear. So the front can be bluff. That shovel nose isn't important until ~250mph.

Your extended approach and departure angles work against you aerodynamically. The problem with the symmetrical side view is you have free air above the car and a plenum with one moving wall (whichever way you look at it) below. So you have interference drag at any reasonable ride height.

Theory has the vehicle as a half-body, with a 'ghost' vehicle inverted under it. Underbody flow is minimized and managed with tunnels and difussers.

I would describe your rear end as a high-tailed stinger. Here's an example from Breers in the 1930s:

I compared the high boat tail with a Tropfenwagen style here:

The one on the left uses a Coanda nozzle in the truncation because the Beetle has a supply of pumped engine cooling air ducted to exactly the right spot.

Flat taillights would loose maybe 0.005Cd. and maybe 12-15" in length.

[Angel And The Wolf]

I can forget the front nose piece. Most drag savings is gained at the rear. I can't leave the lower angle of the tailpiece at 0 Degrees. It would drag on driveways I could raise the tail floor up, and make it 0 Degrees, but I don't know what that would accomplish. I guess I'll just have to build the boat tail, and test it on a two way run, after setting a reference trip without it.

[freebeard]

That's the dilemma with a full boat tail. One solution is BamZipPow's Dark Aero single-wheeled trailer.

Another is a fenced diffuser to mangle the underbody flow, making little vortexes out of big ones.

[Angel And The Wolf]

Originally Posted by freebeard

That's the dilemma with a full boat tail. One solution is BamZipPow's Dark Aero single-wheeled trailer.

Another is a fenced diffuser to mangle the underbody flow, making little vortexes out of big ones.

I do have another design, a one wheeled tapered trailer:

https://www.youtube.com/watch?v=FW_7pQiwy1c

[freebeard]

There ya go.

A ball-and-socket connection and everything.