Maximum angle for boat tail?

[aerohead]

Originally Posted by Bicycle Bob

That looks very oversimplified to me, too. The "ideal" shape is impossible, having no ground clearance. The Corvair Greenbrier wagon, which looked like the offspring of a VW van married to a Chinese Pagoda, still lucked out with a phenomenally low drag.

Bob,It's very oversimplified.It's maybe a "bridge" to demonstrate a form which is borrowed by every low drag "car".The "mirror" image takes into account ground clearance,so that's not an issue.If the Chevy Greenbrier is placed under the overlay,it's Cd drops from 0.38,to 0.16,with about 33% of the roof and body sides "Kammed" or" Mackeral-tailed","bob-tailed"",boattailed",whatever you want to call it.Every fuselage has it.Every canoe.Kayak.Submarine.Fast torpedo.Artillery projectile.Supersonic bullet.Whale.Dolphin.Bird.Varna.It's everywhere.----------- As far as "impossible",I'll be driving that come September.I have no pre-cognition of how it will play out,however,everything I've looked at since 1973 re-affirms my conviction,that for a ground vehicle,passive low drag can be achieved by no other means.

[winkosmosis]

The Sunraycer has a much more sleek front than that overlay. The front is not a 1/4 dome but instead oval sectioned

[MetroMPG]

Yes please, winkosmosis, let's start debating "pointy vs. blunt" front ends again.

As Phil mentioned, the front isn't the issue here. Assuming you have attached flow to the peak height of the roof (or max cross section area) - which almost all modern passenger vehicles do - the overlay provides a good basis for a tapered 3D form which retains attached flow from that point aft.

[RobertSmalls]

I believe that the answer to the question, "What is the correct boattail angle for Car X?" can only be satisfactorily answered by CFD simulation or wind tunnel studies. Remember that we're dealing with 3D shapes that need to be tapered in two dimensions.

I would love to compare the Cd of raindrops and cars of various geometries, but I can't get OpenFOAM to work.

[winkosmosis]

Originally Posted by MetroMPG

Yes please, winkosmosis, let's start debating "pointy vs. blunt" front ends again.

As Phil mentioned, the front isn't the issue here. Assuming you have attached flow to the peak height of the roof (or max cross section area) - which almost all modern passenger vehicles do - the overlay provides a good basis for a tapered 3D form which retains attached flow from that point aft.

My point is that the shape used in those overlays appears to be arbitrary.

[Blue Bomber Man]

Wink:

If you continue the nose of that car on the trend it was going to the ground, and the tail on its to the ground, I think you'll find that the shape is very similar to the one metro was showing in his overlays

[Bicycle Bob]

"I would love to compare the Cd of raindrops and cars of various geometries, but I can't get OpenFOAM to work."

Raindrops are oval, not "teardrop shaped" because without a wet trail, surface tension severely rounds their tails. Air pressure at speed can make large drops orient themselves as parachutes.

[aerohead]

Originally Posted by winkosmosis

My point is that the shape used in those overlays appears to be arbitrary.

Copy- Pro will be closed this weekend for the 4th of July celebration,so it will be the following weekend before I can post any new images.Without Hucho's book,it's probably counter-productive to discuss the "template." Without a handle on "ground-reflection" or "mirroring" it will be impossible for anyone in the aero forum to grasp the premise of the template.----------- I will venture that there is nothing arbitrary about it.Once everyone gets a look at their own car on top of a mirror,they'll immediately see how really "blunt" their car appears to the air.------------- Due to ground-reflection,and respecting that a 2.5:1 teardrop body of revolution has the lowest drag of any form investigated,it will become obvious to everyone,that to achieve the lowest drag in ground-effect,a vehicle would have to be 5-times as long,as it is tall ( including it's ground clearance).This IS Hucho speaking.You can ignore me,I'll just play messenger.---------------- Since the 2.5:1 teardrop has the lowest drag,and especially no separation whatsoever in it's all-important aft-body,when it is represented in ground-effect,it has no peer as an example of ideal aerodynamic low-drag form,bar none.---------- Anything "faster" will suffer separation and higher profile drag.Anything "slower" and it will suffer increased skin-friction and consequently higher drag.---------------------- The reason it is chosen is because it works.Any contemporary automobile (save some vans and SUVs) has undergone most of the mods CAR and DRIVER came up with in 1974.You would have expected that.And modern automobiles do not have separation at their fore-bodies.The challenge remains in the automobiles wake.And to do the full meal deal,as Darin says,a full 66% of a car's body must be "aftbody",in order to provide the gentle deceleration ramp necessary to allow the air to come to rest,losing all "motion",and without turbulence.That's all streamlining is.-------------- And it's worth repeating.The path used to displace the air,must be at least two-times longer in order for it to regain it's static pressure,re-assuming the position it had before your vehicle disturbed it.------------------------ The template provides a no-brainer architecture to follow in creating aft-body mods.It's a go-no-go if you will.If you're above or below the line,you mod at your own peril.No one is obligated to use it.And please,if there is one of you that can develop something easier to use that's risk-free,go for it! ------------------------- I'll get the images up in a week and a half.In the meantime,if you like,take a gander at the back of a every commercial jet which flies over.Look at the back of every nuclear submarine.Look at the back of every fast torpedo.Goodyear Blimp.FUJI Blimp.Met Life Blimp.Orca.Bottlenose Dolphin.------------------------ Not arbitrary.

[aerohead]

Originally Posted by winkosmosis

The Sunraycer has a much more sleek front than that overlay. The front is not a 1/4 dome but instead oval sectioned

The 1922 vehicle at the top of the graphic may look familiar------------------------------ Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - aerohead's Album: Book illustrations - Picture

[Bicycle Bob]

I'm still waiting for the reference on that "ideal" 2.5: 1 teardrop. My many other sources all say that trying to proceed from 5:1 to 3:1 will almost inevitably cause separation. And what if there is any modification to the forebody shape, for wheels or other practical considerations?