Think Flight: Using Simple Aerodynamics to Beat High Gas Prices

[Xist]

It always seems like the rear corners of cars are optimized for not hitting things, independent of aerodynamics.

So, rear skirts before a boat tail?

Undertray, too?

[aerohead]

Originally Posted by Xist

It always seems like the rear corners of cars are optimized for not hitting things, independent of aerodynamics.

So, rear skirts before a boat tail?

Undertray, too?

Yep!

[aerohead]

I ran the numbers for the Impreza dimensions.
The frontal area is approx. 23.815- sq-ft ( 2.212 meters-square ).
The L/ sq-rt of Af basically doesn't change.
'Kamm's' V-criteria does, increasing to 2.4643.
A comparison of seven cars with V's in the neighborhood, averaging 2.3372 yielded an average Cd 0.1475.
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If the car were given the TF wheel covers and tail as a 'new' car, over it's statistical, 12-year lifespan, for the EPA-weighted HWY portion of it's total mileage, the Impreza would return:
* approx. 600-million less Btu waste heat to the atmosphere ( entropy )
* 5,274 fewer gallons of fuel consumed.
* @ $ 3.00/gallon, saving $ 15,800.
* And lowering carbon-dioxide emissions by 52-tons.
The fuel savings would cover about half the new car price, in constant dollars.

[Cd]

He has part 2 up now. ��
https://youtu.be/4ykw_8lpjco

[Bicycle Bob]

Originally Posted by Cd

He has part 2 up now. ��
https://youtu.be/4ykw_8lpjco

I'm sure he does not need that step as a turbulator after two open wheelwells. It would be easy to put skirts on the rear as a blister running past the wheel openings. You could make a mold for one side, and then just pop a thin splash of fiberglass into the opposite curve for the other side before finishing it. Copies would probably fit many cars with a little trimming. Vacuum forming might be cheaper for production.

[Cd]

I was admiring just how flush that his wheels were with the body in stock form.
I cringe each time I see those huge 'elephant ear ' mirrors out destroying the drag though.
I notice he didn't add any plexi for windows in back, yet he used it to cover the lights after viewers suggested it.
If he added windows, he could have folded his mirrors ( or at least one ) back.
In this latest video, he added a low air dam, rather than creating a belly pan.
He is familiar with all sorts of materials for creating R/C planes, and I'm puzzled why he chose not to cover the underside in Coroplast ( with cutouts around hot bits )
The air dam added too much to the frontal area, and actually made things worse if i remember correctly.
I think maybe he chose the airdam due to it being the faster option.
He got really low on the departure angle of the tail too.
He's lucky to have not scraped it.
One thing I plan to do if I ever create a full tail, is to have this same angle, but have it flex upward and drop back down by using the floor mounted to a hinge and held up by wires.
He was asking for advice on how to get a more precise way of measuring the MPG over tank to tank fills at the pump.
I saw a few comments that mentioned the Scanguage, and I hope he reads them.
The guy is brilliant with R/C stuff and electronics, and I have been a fan of his videos for a while now without knowing who he was. ( Been into R/C since childhood, but just could never afford one )
One really clever thing he did, was to create a LIDAR controlled Ekronoplan ( one of my favorite machines ! )
He really does come up with some interesting stuff.
Looking at the shape of his latest tail, I'm reminded of others that have created them on this site.
He even used foamboard.
After looking at some at work today, I'm now interested in using it myself. If flexes to curves, unlike Coroplast.

I am reminded of Darin's ( MetroMPG ) work each time I see someone post a video like these.
It would be nice to see him mentioned, since he is one of the first to really put this stuff out there on Youtube in educational form along with A-B-A data.

[Xist]

Originally Posted by Massimo Tava

You could always buy some Flow-vis (short for Flow Visualisation) is a paint-like substance used for aerodynamic testing during practice sessions in Formula One. A high-contrast luminous colour, it is applied to an area of the car – e.g. one side of the front wing.

Originally Posted by Gungan Works

Flow vis paint is just Mineral oil and fluorescent dye.

Okay, but what do you do with it?

Originally Posted by Leo Jackouski

You could use a pressure sensor to measure your air pressure in the tail section. Then pressure drag is equal to this multiplied by the area of your truncated cross section.

Originally Posted by drippingwax

When I was in Afghanistan many semis had cloth over their grills.
What about a dust filter with vertical pleats?

Originally Posted by Leo Jackouski

A good starting point is 13° on the top, maybe the same 13° on the sides, and 7° on the bottom. Take it down to at least 30% of the maximum cross section. But the more the better. Then having a really sharp, crisp, trailing edge is super important to get clean flow separation.

This guy again?!

Originally Posted by Excalibur John Deere

You’re a roof rack on top of the roof is like fins they are actually a bonus the way you have them now. Your boattail needs to also have that same affect upper and lower Slope. And don’t forget to give Wire mesh a try

I don't have any idea what he thinks he is saying.

Someone mentioned variance in accelerating and I remembered people here adding hand throttles, then wondered about adding a servo or something so that it accelerates up to speed the same way every time, then you can use cruise control.

You would just need an immediate way to deactivate it.

Originally Posted by olsonspeed

Reel to reel recording tape is now the preferred material for tuff tests.

Originally Posted by Alan John

I've seen people use choroplast (plastic cardboard) to smooth out the underside of their cars. Lots of info on Ecomodder

Get our name out of your mouth! The last thing we need over here is activity!

Originally Posted by Samuel Hopely

Filling in the rear of the tail surely hurt you a bit. The plate cover is a large surface for the low pressure wake to act upon. Remember that it's pressure region's acting on a moving body itself that contribute to drag! Having the tail hollow the first time around meant that realistically only the edges of the foam were subject to wake forces, with the gradient increasing to probably around atmospheric when you reach the body. With such a better optimized curve on the tail, you had way higher velocities at the bluff cutoff, so every square cm had an even more severe pressure drop to contend with vs the former. If you cant get a point (which you probably can't achieve efficiently anyways) find a way to leave her hollow!

In reply:

Originally Posted by Lumber Jack

Well, the old design created a large cavity, and there is no way to tell what the internal cavity pressure was without some further analysis. Sometimes it can result in higher pressure when you create desirable recirculation inside the cavity, but in this case I suspect recirculation was minimal. So it was probably transmitting the wake recovery pressure, or a lower pressure, up against the hatchback and bumper inside the cavity. Which means not much of a reduction in base drag. The new closed boattail shape is probably better.

Excalibur John Deere pointed out that you get better fuel economy at slower speeds.

Originally Posted by Mijc Osis

Its a pity that an overhang like that on the back of a vehicle is illegal. You are only allowed to overhang by so much of the wheelbase.

This says that is for cargo, but I am sure that is open to interpretation.

Originally Posted by turbogto

First

I finished reading the comments?!

Many comments recommended installing a separate gas tank and weighing it before and after.

How did they do that on the Centurion?

[Xist]

Originally Posted by chubscub

You also have surface area drag and ground interference drag. Without a wind tunnel and many iterations it will be difficult to get even close to an optimum.

.

[freebeard]

	
	

			
				

					Originally Posted by Xist
					
				
				
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Youse yor words.

[aerohead]

I'm appreciative of the project.
Some impressions:
1) without the belly pan we might be looking at a Cd 0.035 penalty.
2) compromising the diffuser, due to the missing belly might add another Cd 0.02 penalty.
3) lack of rear skirts could mean another Cd 0.0.006 penalty.
4) and since a 'fastback' body is the most aerodynamically sensitive to tumblehome, there's a little extra fruit there for the taking.
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All of which adds to build complication and expense. And outside the scope of the project.
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The 'skirted' configuration reflects an aggravated frontal area situation, so whatever the Cd was, it would be a little lower, if adjusted for the higher Af.
I give it all a big thumbs-up!
Thanks again for sharing it.