Biggest gains are at the back???

[Ken Fry]

Originally Posted by COcyclist

I just feel there may be some gains to be had at the front too. For many of us, a full boattail presents visibility, added length and parking problems. Should we as Ecomodders, do more work on the front before concluding that it is boattail or nothing?

It's worth considering. The fronts of many cars are "pretty good" already, because a front that is good for aero purposes can also be good for styling (e.g., faired-in headlights). So there are (for many cars) opportunities for wake reduction that can be considered low-hanging fruit (rear fender skirts being an obvious one). Closing in the underside is another low-hanging fruit.

Some front gains are expensive to achieve -- raking the windshield being a good example. Others require removal rather than addition of material, which can be hard work (the removal may be easy, but the re-enclosing in nice smooth bodywork can be hard).

At both ends what looks right is not always, in fact, lower drag.

But your basic contention is correct, I think, we should look at the whole car: there are opportunities everywhere.

[aerohead]

Originally Posted by COcyclist

I have been pondering this for a while since observing that many posts about work done on the front of a vehicle get shot down in flames. (Picture Snoopy with smoke pouring out of his doghouse) Also, what I personally consider to be a couple of our leading authorities on this site (Aerocivic and Aerohead) have reshaped the fronts of their vehicles, as well as streamlining the back. Then I am reading a post from MetroMPG about the 2012 Chevy Malibu ECO aerodynamic tweaks and came across this:

10 counts: Underbody panels - two in the mid-body area under the floor pan on either side of the center tunnel, and two in the rear area covering the fuel tank and rear area on either side of the exhaust

10 counts: Rounded front corners - from the bottom of the fascia up through the headlamps - help air flow smoothly along the Malibu's body sides

10 counts: Tire deflectors positioned forward of the front tires act as "mini-air dams" to minimize wind disruptions

7 counts: The closed upper grille on select models pushes wind to the sides of the Malibu

7 counts: Outside rearview mirrors are specifically designed to deflect wind without "upsetting" the airflow

7 counts: Shutters in the lower grill opening on select models open and close automatically to maximize aerodynamic efficiency. This increases cooling airflow to the engine under certain conditions, such as under high-engine loads at low speeds, and reduces aerodynamic drag when extra cooling is not needed

5 counts: The front air dam redirects airflow to minimize aerodynamic disruptions

5 counts: The notch angle of the vehicle - the angle from the top of the rear glass to the trailing edge of the decklid - was optimized to reduce wind drag

2 counts: An integrated decklid spoiler incorporates a crisp, trailing edge that helps separate air from the rear of the Malibu.

The "rounded front corners" is 10 counts and the "decklid spoiler" is only 2 counts. Hmmm. I don't dispute the science that a full teardrop shape is essential for lowest drag in optimized vehicles. I just feel there may be some gains to be had at the front too. For many of us, a full boattail presents visibility, added length and parking problems. Should we as Ecomodders, do more work on the front before concluding that it is boattail or nothing?

If you have a body-on-frame vehicle,like a Chevy Colorado/Hummer H3,you can take some liberties with the upper structure,as it is basically superfluous to 'strength.'You could yank off the top,discard,and fabricate a Ford Probe-IV body to bolt back on.And with active suspension,PRESTO!, you've got Cd 0.15.And no boat-tail!
With unit construction as in all passenger cars, you're dealing with a structure which has undergone finite element analysis for strength,crashworthiness,mass production,etc.,and they're a completely different animal if you take a plasma cutter,Sawsall,or what have you to attempt major alterations.
The thing about boat-tailing as Hucho tells us:
*The front of your car is already okay as far as attached flow and any additional streamlining will pay diminishing returns.
*If you go after the rear,you maintain all the multi-billion dollar engineering which went into the car,you're taking nothing away,only adding drag reduction which wasn't available from the car lot.
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When Chevy boat-tails the Malibu they get Cd 0.12.When they fair in all the tires and wheels the Cd goes below 0.10.
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There is only so much which can be done with the front.And yes,I have cleaned up the front of my vehicles,but no more than what Korff recommended in 1963,or R.G.S.White in 1968,with his recipe for Cd 0.24.
I'm going for sub-Cd 0.10 and that will require the best onset flow I can get to the rear if I'm to crack 40-mpg with the truck.I've seen 47.9 mpg pulling the trailer under 'favorable' conditions,so there's still fruit on the tree.But these gains,other than the enclosed front wheels,will be coming from aft-body mods.
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If you really want to idealize the front you're going to need the $3,000 windshield and all the mods to make that fit.That's a hard one for most folks to swallow.

[COcyclist]

Originally Posted by aerohead

The thing about boat-tailing as Hucho tells us:
*The front of your car is already okay as far as attached flow and any additional streamlining will pay diminishing returns.

In other words, there are returns to be had but they will be minimal (as in, hard to see at the pump) on most modern passenger cars?

Originally Posted by aerohead

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When Chevy boat-tails the Malibu they get Cd 0.12. When they fair in all the tires and wheels the Cd goes below 0.10.
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Just for clarification, the Cd 0.12 is hypothetical?, based on the research if Chevy had boattailed the 2013 Malibu Eco or did they test a boattail on that car?

So the Malibu would go from Cd 0.29 to Cd 0.12 if they only added a full boattail that follows the template??? That means overall drag would be roughly 1/3 of what the production rear end allows. If this is the case, I can see why it is worthwhile to pursue drag reduction at the back of the vehicle.

[aerohead]

Originally Posted by COcyclist

In other words, there are returns to be had but they will be minimal (as in, hard to see at the pump) on most modern passenger cars?




Just for clarification, the Cd 0.12 is hypothetical?, based on the research if Chevy had boattailed the 2013 Malibu Eco or did they test a boattail on that car?

Also, the Malibu would go from Cd 0.29 to Cd 0.12 if they only added a full boattail that follows the template?? That means overall drag would be roughly 1/3 of what the production rear end allows. If this is the case, I can see why it is worthwhile to pursue drag reduction at the back of the vehicle.

Basically speaking,the Cd 0.12 has been observed repeatedly since 1921 when Jaray had Klemperer test his 'pumpkin seed' in the Zeppelin Werke wind tunnel.
The 1987 GM/AeroVironment Sunraycer is a contemporary example.It's reflexed a bit,but of course it's a solar car,maximizing the PV array area and designed for 40-mph crosswind gusts from Australian land trains.

[Sven7]

1987 GM Sunraycer by Tyler Linner, on Flickr


Oldsmobile Aerotech by Tyler Linner, on Flickr


1988 Pontiac Banshee Concept by Tyler Linner, on Flickr

Notice that these all have smooth, rounded noses.

I taped the gaps on the Probe for awhile and found no noticeable improvement. However if the nose were reworked I'm sure it could do better. It's just that the time/money would be better spent making a full, smooth belly pan.

[Ken Fry]

Originally Posted by COcyclist

Just for clarification, the Cd 0.12 is hypothetical?, based on the research if Chevy had boattailed the 2013 Malibu Eco or did they test a boattail on that car?

It's beyond hypothetical. And well beyond "theoretical".

A pet peve of mine is the incorrect use of "theoretical" (and I am not suggesting that you have used "hypothetical" incorrectly here). In the very early Aptera days, they announced a Cd of .06 and a mileage of 330 on a diesel straight (non-plug-in) hybrid. They later said this was "theoretical." The problem is it is not "theoretical" in a meaningful sense, because they are not using the right theories: they are using one convenient one when 10 apply. It is as if one were to say, "a standard Honda Accord can theoretically get 100 mpg (whisper: provided the engine efficiency is 100%)". (The problem is that other relevant theories dictate the engine efficiency will not get remotely close to 100%.) The sun and moon are theoretically the same size because they subtend similar visual angles. (But, oops, they are different distances away.)


From 3000 miles away (from Aptera land) I was able to say "no, that shape with wheels will equate to .15 or so, but only if they do a good job of attending to all of the details. No, 330 real mpg is not plausible." So I later declared my self a human CFD machine, because I could do better from 3000 miles away than they could do right there in California. In the X prize competition, Aptera got almost 200 MPGe on electricity, making the vehicle about twice as efficient as a Leaf. (This is about as expected, given the Leaf's high weight and mediocre aero.) The Leaf, on gas, would get 30ish mpg (as the Juke does). The Aptera, on gas, would get 65 mpg. (That is significantly different than 330 mpg.) (For perspective: the VLC gets better than 100 mpg on gas, and the VLCE gets about 350 MPGe on electricity.)

There are many reasons sailplanes look the way they do. There are many reasons solar racers look they way they do. It is not coincidence that they look very "clean" at the front, as well as at the rear. It is no coincidence that solar racers have impossibly skinny tires.

.12 [thanks, Neil] is solar race territory. A Malibu cannot look anything like the "eco" does and come remotely close to that, boat tail or not.

Sharp leading edges (parallel to the ground plane) work well, BTW. Rounded leading edges are only required if the angle of attack varies substantially, but in a car it doesn't change significantly relative to the ground plane. Once cars can fly, that will change.

So, if you pull out the stops, you get something that looks like this:

(Photo of Nuna5 from gizmag)

No Hucho template upper curve, no blunt leading edge: just impeccably clean everywhere, not just at the tail.

The Probe 4 is "all wrong" -- sharp at the front, blunt at the rear -- but had very low Cd -- about the same as the Aptera, which "looks" right -- like a teardrop.

The Malibu numbers must be taken with a grain of salt, but represent possibilities for a particular shape going to another closely related one. (it is a particular cleanup of a particular car.) The numbers clearly show that lots can be done at the front of many cars. One can assume that they are exaggerating the aero work, because the fuel efficiency is awful, especially around town where a hybrid should excel. The Malibu is actually a "heavier" hybrid than the Insight (with a 15 kW motor generator vs a 10 kW one) but does far worse than the only slightly smaller new Insight, and tons worse that the Prius (which is mid sized, like the Malibu). The Prius gets essentially double the fuel efficiency of the Malibu on the urban cycle.

It is as if they name it "eco" to disguise the fact that something went wrong in the engineering. "31 combined mpg vs 50 in the Prius. Embarrassing. Let's call it 'eco' to fix it." Certainly, the aero is much worse than the Prius's, because unless they are breaking new ground in just how inefficient engines can be, the expectation would be that the Malibu engine is about 10% less efficient than the Prius (so with comparable aero, you'd expect the highway ratios to be 10% apart: 50 for the Prius, 45 for the Malibu).

I've already said this, but your contention that it all counts is correct. The Mooney 201 and 231 aircraft were famously clean, because of attention to a great many details.

[COcyclist]

Originally Posted by Ken Fry

So, if you pull out the stops, you get something that looks like this:

(Photo of Nuna5 from gizmag)

No Hucho template upper curve, no blunt leading edge: just impeccably clean everywhere, not just at the tail.

Thanks. I appreciate all the replies to this thread, but I am still learning, which is why I ask. Isn't the Nuna5 solar racer partly shaped for maximum solar panel space? I imagine something like this...
Human Powered Vehicles (HPV)
... if you pull out all the stops.

Originally Posted by Ken Fry

I've already said this, but your contention that it all counts is correct. The Mooney 201 and 231 aircraft were famously clean, because of attention to a great many details.

Can you post a photo?

[NeilBlanchard]

Ken,

Did you mean the solar racer has a Cd of 0.12 or 0.012? I suspect it is the former?

[Ken Fry]

Originally Posted by NeilBlanchard

Ken,

Did you mean the solar racer has a Cd of 0.12 or 0.012? I suspect it is the former?

Thanks, it is indeed the former! I've been thinking about tires too much recently, I guess.

[Ken Fry]

Originally Posted by COcyclist

Can you post a photo?

Here's one, but I think the landing was a little too agressive:


Here's one flown more conservatively.


Roy LoPresti was the guy responsible for the cleanup leading to the 201 (201 mph on 200 hp). His son (I assume) David seems to be doing similar stuff.
Mooney M20 Cowling Modification - LoPresti Speed Merchants

Yes, the solar racer designs are definitely driven by the need for lots of panels. I was thinking mainly in terms of leading edge. The leading edge of the Nuna could be fattened and rounded, but drag would go up, as long as there is little angle of attack change across the edge.

The HPVs are naturally more torpedo shaped, and the nose is then subject to the effects of cross winds. These shapes generate lift to the side in a crosswind. A rounded nose would permit this lift to develop over a wider range of crosswinds, whereas a vertically sharper edge would stall earlier. In this picture, there are both approaches.



There would be some crosswind (with an apparent wind angle of perhaps 12 degrees) where one shape would be producing lift and the other would have stalled. The lift, in either case, tends to blow the rider off course and increases both aero drag and tire drag.
The stall would be expected to produce more drag. Experience has shown, I guess, that the Varna shape works better overall.