Aeromods must transfer force to be effective
 

That means they should be rigid and firmly affixed. I originally thought that getting the shape right was the most important thing. It turns out we want both, but the force transfer is paramount. An aeromod that isn't transferring force isn't doing much, and might even be counterproductive.

More thoughts and pressure stats here:
https://www.instagram.com/p/Cel1DD-L1K1/

I'm not certain. A tonneau cover can simply cover a void, isolating the air under it, and cut drag. I suppose there is force transfer going on there, but it doesn't seem proportional and mostly would be preventing the venturi effect by separating high and low pressure air. I haven't fully wrapped my head around the physics of it though.

rigid shape
 

The 'big-boys' call it aeroelasticity.
Forces acting on an under-engineered profile can actually deform it into a lifting panel, which subsequently rises to an angle of attack aggressive enough to get to burble point, then stall. The panel falls below the critical angle which initiated lift, and the 'system' begins the lift/stall cycle all over again, leading to the cyclic deformation known as 'flutter', which can bring an aircraft down.
Depending on the material, the degree of deformation, and the natural frequency, or harmonics of that frequency, the panel will just self-destruct.
There are YouTubes of model, Lockheed C-130 Hercules coming apart in a wind tunnel.
The easiest solution, yet the most difficult to fabricate, are 'compound' surfaces, where they curve in three axes. Like a hen's egg. The strongest and lightest structures have compound surfaces .
Most contemporary automobiles have some degree of compounding to their sheet metal. Strong and light!;)

A worst case would be a hoop and tarpaulin cover on a military truck.

I was driving through Northern California in the 1970s when as I met a Lincoln convertible the top tore loose at the header and turned into a roll-back as we passed.

That's the most readable post I've seen from you to date. No bullet points, and something resembling paragraph organization.

Last weekend I pulled a 16ft canoe on a trailer, mostly utilizing the canoe as extra storage, but also hoping to take it on the lake. I had all 4 interior sections filled with totes and tents, then covered the whole thing with 2 tarps. I tried to get them as taught as I could, but there was still some flutter at the end since it tapers so much creating that compound curve.

Seemed to be pretty aero though, as flat cruising didn't appear to suffer much in the way of fuel economy loss.

Next purchase I'm looking at a hitch mounted cargo platform for those instances where I just need more space, but not a boat. I'm torn between a sturdier looking lower profile, or the utility of higher walls.

https://i.pinimg.com/474x/cd/eb/65/c...57b3faec1b.jpg

https://www.uhaul.com/MovingSupplies...932&media=6255

In aeronautics there's really two kinds of drag

Induced drag and parasite drag

Everything related to induced drag would be load bearing this is the drag from the airfoil form

Parasitic drag could be load bearing. But it can also not be any more complicated than keeping air out of cowlings and spaces. Or not installing some thing onto the fuselage that protrudes into the slipstream. It is actually OK to use tape on commercial aircraft in some of these instances

A good example on our cars would be anything perpendicular to the airflow will need to be rigid enough to not flutter which would be worse than it not being there

Anything parallel to the airflow like a wheel skirt also doesn't need to flutter but at the same time sees much lower aerodynamic loads

Okay. Let me say it differently. The effectiveness of an aeromod is directly related to how well it can transfer force. I've made the bold claim that we want rigid aeromods, but the more accurate statement is: if we had two aeromods of the same basic shape, one rigid and one floppy, the rigid one will perform better. And there is a cap to the rigidity we need, i.e. the maximum amount of force that aeromod transfers onto the car frame under operating conditions.

Now, it's definitely the case that non-rigid surfaces can effectively harness aerodynamics. There are many examples: parachutes, kites, etc. And you are correct that a tonneau cover does, in fact, reduce drag. My statement is: if you put a rigid frame under the tonneau cover so that it more effectively transferred force onto the truck in addition to the fasteners, it would perform better. If you replaced the ribbed tonneau cover with an aerodynamically shaped bed cover that was made of rigid fiberglass or somesuch, it would perform even better. And if you calculated the total force on that cover due to aero and engineered it to withstand it, that would be the best it can do (rigidity-wise).

The reason I mention this is I originally had some "loose" elements in my design, expecting the wind to shape them into the optimal curve. Which it did. But because those elements were loose, I wasn't getting the full benefit of the aeromod, because the wind was spending it's energy into deforming my surface instead of into smoothing and diverting it's own flow.

Most importantly, I've now realized that the optimal forces that I want on the vehicle were the opposite of what I was expecting. An aerofoil has a big push back right along the tip of the nose... and then is pulled outward by lower pressures at all other points. That means that surfaces (like my belly pan) which I though I needed to design to be pushed up I actually needed to make sure could be pulled down. That requires a significantly different bracing strategy that I was using before.

Anyhoo, I hope that's clearer. I'm not saying loose elements do nothing; Champrius 3.0 was a loose design and it got me past 60 mpg. I'm saying tighter more rigid elements do better. I'm hoping that it gets me past 70 mpg this time around.

3mm abs engine undertray will fail at about 180km/h speed unless its not reinforced.

Install few metal L-bars to make it stronger and you are good to go faster.

tonneau cover
 

As far as automakers are concerned, pickup trucks are just large sedans, without a trunklid.
Flow separates off the rear of the roof, and there's nothing to reattach to.
There's no pressure recovery.
High drag and high lift.
A tonneau cover is essentially a 'trunklid.'
If it's sufficiently close enough, vertically, and sufficiently 'far' longitudinally, it provides a surface for reattachment.
A vortex is captured on top of it, and the outer free stream will skim over the 'locked-vortex' as if it were a solid structure, plus 'touch' the back of the cover before separating.
If 'smoked', you'd see streamline filaments diverging as they decelerated down the 'contour' of the vortex, picking up pressure as they lost velocity.
This is the drag reduction.The higher pressure is communicated to the base, behind the tailgate, raising the base pressure, which lowers the pressure drag, the largest component of aerodynamic drag; and why we streamline.
And since slower, higher pressure air impacts the rear of the tonneau, it also kills most of the rear lift.
A 'half-tonneau' works better due to reattachment, plus the low pressure of the vortex core telegraphing under the cover, to the forward face of the tailgate, increasing the pressure differential between the front and rear face.
GM has the US Patent on it.
If you watch a vinyl cover, you'll notice a dip at the rear where air is attacking it.
At the front, behind the cab, it will look like Yoda's under there trying to get out, creating an upwards bulge in the fabric. Highs and lows.

This video gives a good feel for the relative pressures around a typical modern car, which directly translate into the forces one should account for.
https://www.youtube.com/watch?v=n1Hro6QpnRE
Enjoy!

There is a history from before your join date. I wouldn't follow an aerohead post with a reference to the other party involved, out of respect.

I've never understood whether the front wall under the half-tonneau is necessary or not. Is the pressure at the floor of the bed and at the tailgate similar or different?

The patent
https://www.freepatentsonline.com/4573730.pdf

Results on my truck

https://ecomodder.com/forum/showthre...oma-39148.html

more rigid... better
 

Clarence 'Kelly' Johnson, of Lockheed's Skunkworks is remembered for 'just enough and no more' in design.
For every pound saved on the U-2 Dragon Lady, they could get one more foot of altitude ( important when you're in the other guy's airspace, and he's got surface-to-air missiles to go with his radar ).
'Kelly' would engineer no further than necessary than what was necessary to satisfy an aircraft's performance criteria.
'Aluminum foil'- thick fuselage skin, which could be dented just by looking at it.
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My first aeroshell 'exploded' under aerodynamic loading, on it's maiden voyage.
My second aeroshell was destroyed one night by young goats which thought my pickup was a great thing to climb and jump off of.
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So yeah, some strength, rigidity, and toughness are more than welcome.
And since drag is a function of frontal area and shape, the rigidity guarantees that your shape will hold.
Take care of the shape, and the shape will take care of the pressures.;)

floor and tailgate
 

Both are exposed to the core's tornadic low pressure.
Low pressure exposed to the floor might imply 'lift', however, the slower, higher pressure air, decelerating down the rear of the locked-vortex, slams into the top rear of the tonneau, appreciably behind the rear axle, creating a positive aerodynamic moment arm ( torque ) as Wolf Heinrich Hucho discusses in his 2nd-Edition.
Rear lift is significantly reduced, along with drag.
The Cd 0.315, GMC S-15, 'Syclone on the Salt,' used the patented 1/2-tonneau as part of the aero kit which helped it secure a land speed record at Bonneville. 210+ mph.
And considering that they started @ Cd 0.475, that's a pretty good hat trick!
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The US Patent mentions the 'partition' which was tested underneath the tonneau. If it had any significance, it didn't stick in my shriveling mind.:p

The strongest shell structure would be a geodesic spheroid.

https://ecomodder.com/forum/member-f...07-7-35-02.png

strongest
 

We'd have to consider the panel strength of each individual facet, each with a center of percussion, resonant frequency, plus harmonics.
The irregular perimeters of the facets would also open the door to 'superposition' and rogue waves communicating across the asymmetrical spans.
Reflections. Amplifications. Standing waves.
The superstructure would survive anything you threw at it, but if any panel got to 'singing' it could suffer repetitive bending fatigue, rupture, and ultimately failure.
Finite element analysis, CFD, or wind tunnel could reveal suspect areas.
( Any university undergraduate seniors, graduate students, want to take on a fascinating project ? ):rolleyes:

I've moved to geodesic-like design from the box-like thinking I used in earlier iterations. I had a great conversation with an actual aeronautical engineer, and he summed up how to build solid frameworks succinctly: "maximize interlocking triangles." ;)

Apparently triangles are the simplest building block that keep their shape under a variety of loads from different angles. So if we can interlock them, it adds a lot of structural rigidity. I'm finding triangles overlapping rectangles are particularly effective. The only issue I'm having is creating "smooth planes", because the framework bars have to go "over and under" each other, so to speak.

Not getting the over and under part.

If you look closely the design at #16 is based on an octahedron. There are four-way connections at the front, rear, top and sides. This is what allows the differential prolating.

Also, the triangles can be rendered as diamonds or hexagons. The triangulated shell over a diamond frame would [technically] be triangles over quadrilaterals.

edit: look what I found, A Wellington but instead of showing it's Lamella frame, it's under a canopy that is diamonds over squares.

http://www.airpowerworld.info/bomber...wellington.jpg

Embed is thwarted. :(

Sorry. I meant like basket weaving we get a tighter mesh by "sewing" struts over and under each other. It also keeps the face plane more flat, rather than having many levels at each junction we limit it to around two. The aluminum bars that I'm using are thick enough that it's difficult to even have a four-way interconnect, let alone attach a sheet on one side.

Hehe. In your picture I thought the airman was being impaled by the propeller! :p

Okay. If the 'struts' are flat bars I can see it.

To put it simply, triangles are the strongest shape. If you can take a shape and turn it into a collection of interconnected triangles, it will be significantly stronger.

geodesics
 

I spent some hours considering geodesic construction that I thought I'd share.
Only machines can define them
Specific surface feature identification and address location would be extremely complicated among humans
Only robots would be able to easily navigate the 'language' of the 'fractal' architecture
Only AUTOCAD could draw it
Only CAD-CAM could create tooling for it
It couldn't be 'stamped'
Glazings could not be installed, windshield, door glass, fixed quarter-windows, or backlight
I'm unsure how a body shop could repair it
I'm unsure how any cut-opening ( bonnet, door, boot ) could open and close without binding.
Even robotic painting would be problematic
Machine sanding would be impossible.
It couldn't be built from a mold.
Composite fabrics would not drape over it
Tooling would have to be in split sections, clamped together for layup, leaving 'flash' at all separation lines ( difficult to remove for priming and topcoats )
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Aerodynamically, we might be able to get way with murder in the forebody, but once past the roof apex, the angled prismatic nature of the 'panel', at non-cyclic intervals, could introduce such a transverse flow vector contamination to the boundary layer, as to introduce such an adverse pressure gradient, that it would be impossible to maintain attached flow on top or sides.
This is already an established problem with 'smooth' compound-curvature panel airship bodies in a crosswind according to NACA ( NASA ).
Just some thoughts.:rolleyes:

I appreciate them. Even the wrong ones. :)

This is the basic question I would like answered. Vekke was no help. Dimples work, why not raked edges (cough! Cybertruck cough!)

Free and Open Source software is wonderful. Since 2.80 or so, Blender has had a Geodesic add-on. Here is the contextual menu.

https://i.stack.imgur.com/itv2a.jpg
https://i.stack.imgur.com/itv2a.jpg

Review the Object Types and Object Parameters. All the Platonic shapes for primitives; I'm not sure the difference between Squish and Eccentricity. You can add a Superellipse in two dimensions or all three.

Autodesk's parametric NURBs modelling is in contention with Blender's Geometry Node Editor.

This is an answerable question. It depends on the Reynolds number regime we are in and the size of the boundary layer. At 65 mph, the boundary layer near the front of the vehicle is around 1/4" and near the back is 1"+ (depending on location and how good the aero is). The general rule of thumb is that for subsonic, subcritical flow any imperfection less than half the boundary layer isn't significant, and that flow will reattach right over it. That's why panel gaps of 1/8" are fine for passenger cars, but high speed racers actually care about them.

As long as the angle of curvature of the geodesic is under the constraints above, flow will have no problems remaining attached. At slightly greater angles it might reattach later on the surface, the same way flow reattaches to the side of a car after being disrupted by the front wheels / mirrors. At larger angles, there is too much shadowing and the flow doesn't to reattach. At that point, being a geodesic / sharp shape has nothing to do with things; the airflow is being deflected too much.

Anyhoo, there's already been CFD done on the Cybertruck. It's decent. Crap compared to fully aero vehicles, but great comparted to regular trucks. Because the general "silhouette" is vaguely aerofoil-like, the angularness doesn't count against it too much.

dimples
 

They don't work! Not on a full-scale automobile.
The only reason they're on a golf ball is because of Reynolds number. Even at 110-mph, coming off the clubhead of a driver or iron, the ball is too 'short' to achieve supercritical Rn.
Artificial surface roughness hastens the transition from a laminar boundary layer, to a turbulent boundary layer, the best thing for flow attachment and drag reduction.
The reason the golf ball has 'dimples' is that they don't stick 'out' and can't get knocked off. Sand glued to the ball would do just as well, but it wouldn't survive the rigors of of the game.
The stunt that Mythbusters pulled had to do with the crappy notchback design of the Ford. The only dimples that did any good to the Ford were the ones on the rear of the roof, preceding the top of the backlight, acting as vortex-generators, and helping with flow reattachment onto the trunklid / boot.
You cannot take the longitudinal flow on the aft-body of a vehicle and shove it sideways. You're creating the kind of pressure kink Hucho insists must be completely avoided. It's part of the fluid mechanics ground rules he hoped the readers would get from his 2nd Edition. It's 33.3% of what he hoped to get across.
I don't know anything about other editions.
------------------------------------------------------------------------------------
Please expand on what you mean by 'raked edges'. I'm not clear on what you're trying to convey.

Thanks Talos Woten.

I'll contemplate the 1/4-1" range vs dihedral anglesI notice for a prolated shape, the more severe angles are at the front end. The rear would be inverted for a box cavity.

What do you think of this one. I made it for another thread in 2015 -- a bellmouth difusser.

https://ecomodder.com/forum/member-f...14-1-42-00.png

One big vortex generator to order the wake.

edit:
I didn't say you can shove it. :)

sideways
 

It's the angled triangular facets which would impel the flow sideways off their optimum path.
The sharp angles around the corners are also 'spikes'.
They're also deflecting the flow 'away' from where the local streamline would 'go' if on a streamlined contour.
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If you can imagine an air molecule above the body, as a roller lifter, the atmospheric pressure as the valve-spring tension, and the body as a camshaft lobe passing under the roller lifter; there is a limit to the 'contour' of the camshaft lobe, after which the spring tension is insufficient to hold the roller follower in contact against the cam. The roller 'floats'. It 'separates.'
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The lowest pressure on the car is just ahead of the windshield header.
All the air would like to go there.
That's okay in the 'front' of the car.
It's the worst thing for the back of the car.
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Beyond the roof apex, even on a perfectly streamlined body, the air is moving towards a higher pressure than where it's at, at the rear of the body.
The only thing that keeps the air moving rearwards is, shearing forces from the local streamline, just above the boundary layer 'strafing' kinetic energy into the boundary layer, 'pinning it down', like a machine gunner, against the body's surface.
If the body surface falls away too radically, the shearing force from above just richochets off the top of the boundary layer, without imparting any energy.
When this happens, the boundary shrugs its shoulders and says ---- it! It always wanted to go where the windshield was, and that's exactly what it attempts to do, rolling up into eddies as it tries to climb backwards up the back of the car, against the flow, then completely lifting off the surface as the local streamline attacks it, blooming into full turbulence.
The energy balance necessary to keep the flow attached is a very delicate affair.
'(C)hange in a pressure distribution is highly significant for the origin of drag.'
Hucho, 2nd-Ed, page-117.
' The pressure drag is explained by the deviations of the pressure distribution in actual flow.' Hucho, page-124.
' (I)t is very important to design a rear body surface which brings the divided streamlines smoothly together .' Hucho, page 61.
'(P)ressure drag is the largest component in the aerodynamic drag. Its minimization is the true objective of motor vehicle aerodynamics.' Hucho, p.119

An interesting idea. If you are heating the air in the center, this is effectively a turbine that produces thrust! :turtle:

By the way, generally speaking, one wants to avoid sharp / cleaving edges at the front of a vehicle. The reason is because air rarely is coming on exactly straight along the main axis. Even modest crosswinds (10 mph) at highway speeds (65 mph) create angles that sharp edges shadow... and thus produce lots of counterproductive turbulence. If we look at real turbines:
https://storage.googleapis.com/mcp_2...-1920x1000.jpg
we can see that the leading rim is always a rounded shape. The reason why rounded fronts/noses are universal is because they still function well even when airflow is off angle to their primary axis.

The other reason why we want round noses instead of sharp edges is they also produce the lowest drag, when integrated by off angle. A way to produce low drag is to have the smallest area being "pushed back" against airflow. In a curved nose, that's basically a knife edge for an aerofoil and a tip for missile. If air comes off angle, the high pressure area is still small. But in a sharp / angular / flat surface, when air comes off angle, it often exposes some surface * some trig function that produces appreciable counterforce.

That's actually the biggest mistake I made when I first started my mods. I only thought in the two dimensions of the car silhouette, instead of the full 3D, let alone considering off axis flow. As Spock would say: "He is intelligent, but not experienced. His pattern indicates two-dimensional thinking." :)

As for the VGs, they produce turbulent flow and should be used with caution. The only use case where they are beneficial is when they reduce or prevent even worse turbulent flow. Their best use is to try to reattach separated flow earlier to a surface than it would otherwise. If we introduce a VG into a laminar situation, it actually hurts performance. So, if the backside of your toroidal turbine already has a smooth tail with minimal wake, it's uncertain injecting turbulent air into it would actually help.

No need to sell me on round noses, I own a Superbeetle.

A little off topic, but this is something I want to do with my project car. The idea is to make a rear diffuser, and route the engine exhaust into the front opening. For that car, fuel economy isn't really a concern. But less lift and/or drag would make it faster on the racetrack. :cool:

It's not *entirely* off topic. The basic gist of improving fuel economy is more advantageously rearranging the forces acting on the car, in particular the aero forces.

If you are going for a racing car, a common tactic is to angle the radiator to lean forward and then use a shaped hood bonnet. That gives cleaner flow at the nose, improved radiator performance, and downforce on the front wheels. It's also convenient, since everything is right there are the front, which means you can do professional ducting on the airflow. I almost did that to Champrius before I discovered I got better fuel economy increasing the hood insulation.

If you want to combine the radiator with the rear diffuser, I'd put the outlet into the wake ejected above the diffuser plane. A thin linear rectangle 15% of the area of your inlet on top of the diffuser would be perfect. That ejects the air directly into the wake, harnesses the maximum low pressure for suction, and provides tiny but real amounts of thrust and downforce on the real wheels.

The biggest issue I would see is your airflow ducting. Optimally you'd want a squished horizontal oval shape of consistent cross sectional area to go directly from the radiator to rear end of the vehicle. But that would most likely inconveniently intersect the middle of the cabin, like in freebeard's pic.

https://ecomodder.com/forum/member-f...ed-stinger.png

A vertical Coanda nozzle fed by engine cooling air and/or exhaust. On the left.

It's easy when the engine is at the right end of the car. :)

Maybe I should revisit this. in 2014 I couldn't merge the boat tail and car body.

Hey freebeard! Is that your Superbeetle?

I didn't understand what you were trying to do the first time you showed it. If you are trying to make an integrated boat tail / radiator exhaust, then the optimal design is to start with that right pic. Then figure out what your radiator inlet and outlet areas are going to be. Find where the cross section of tail is equal to the outlet area and then truncate it there. Run a duct with that cross sectional area from the radiator to that new opening. Put on a ribbed rim with a flame and call it the Beetlemobile. :D

By the way, you want the tail to be symmetric from above but asymmetric from the side. The Beetle has most air go over the whole upper body, leaving the underside with less airflow. If the radiator intake is from underneath the vehicle, that further lowers where the exit plane should be. With certainty the optimal height is somewhere between a) at most the halfway point of the car body excluding wheels (i.e. the undertray and top of car) and b) at least the halfway point of the nose (same undertray plane and where hood meets windshield).

Do you have access to pressure measuring equipment? If so, then we could precisely determine where the exit plane should be. If you don't mind doing a mockup first, you could also tuft test. Start at the midpoint between a) and b) and then look at the tufts. Then tweak the exit plane up or down until you get the smoothest most attached flow.

You could also get some real gains by rerouting the radiator flow. I don't know how the Superbeetle is set up, but if those lines under the rear window are the exhaust, it's likely that air has to take a sharp turn to get up into the radiator inlet, then another sharp turn to get to the outlet. The optimal airflow is to gently deflect from underneath (or the sides), then take the straightest path to the outlet, with as gradual an angle change as possible. So if the radiator is at the bottom of the boot (say), then have the duct run along the bottom of the inside of the tail. The less we force air to zig zag around, the less drag we get.

As for fastening, my newest love are Rivnuts. If the body metal is still solid, you could easily Rivnut the tail onto the body with a good seam. If you have a tow hitch (or can put one on), then just engineer things so that most of the tail weight lies on the hitch. That will further reduce the stress on the tail / body fasteners. Otherwise, put the main weight on the back bumper / something else reasonably solid.

Hope this helps!

No. The picture dates to 2014, two years before I got the SUPERBeetle. I did a number of design, but in the software I was using, I could't merge my additions.

The Coanda design follows the Tropfenwagen, The one in the upper right follows Breer's work at Chrysler in the 1930s. Neither considers engine cooling.
Here's the thing. The whole engine is radiator. The Beetle engine compartment is a plenum. Cooling air is drawn by a squirrel-cage fan. All cooling air exits two ducts about 3x6".

Here're some other designs from the same time
https://ecomodder.com/forum/member-f...four-tails.jpg

And the boat tail mockup from 2014:
https://ecomodder.com/forum/member-f...9-100-0629.jpg

Wait a minute the Superbeetle is in the picture :confused: I sold the Baja Bug last week ($500).

Cool stuff. My first car was a '74 Beetle. Unbeknownst to me, it had a spun rod bearing. So I had to rebuild the engine before I could drive it. I was a high school student, with no prior experience building engines. So that was quite a project!