Think Flight: Using Simple Aerodynamics to Beat High Gas Prices

[Cd]

I came across the video two days ago.
I hate to admit it, but I was unaware that he is a big time YouTuber.
I was amazed at how many views he had on the video, and left a reply back to him with some 'advice' on how he could get better flow with different side angles. ( I should have just not said anything, since after seeing some of his other videos, I see the guy really knows his stuff. I feel like a fool now.)
It's great to see his video get so many views.
I just wish that he would have given some credit to Darin's 'Flea' design.
If you are into aero and like quality videos, have a look at his channel and subscribe !

[aerohead]

Originally Posted by redpoint5

I didn't watch, but I don't think aviation is a simpler frame for aerodynamics. The entire subject is extremely complex, perhaps with a few easy to remember "rules of thumb".

As with any subject, the basics are low hanging fruit, with mastery involving lifetime dedication and proclivity.

The thing about 'aviation' which sets it apart from road vehicle aerodynamics is that, the fineness ratios of fuselage, wings, and tail surfaces are typically so 'large' as to make flow separation impossible.
And in addition, virtually all fuselage have 'compound surfaces'.
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The Subaru remains a 'bluff' body, and pressure drag, which dominates total drag, is due the the intentional absence of the aft-body, which is responsible for the large wake's low base pressure.
The Subaru electric sibling ought to have significantly lower drag, due to a smaller radiator opening and a 'ideal' underbody.
Adding the same boat-tail to the BEV variant might boost battery range closer to 20%.
And optimizing the boat-tails' sides and bottom, push the improvement to over 20%.
AeroStealth and wife are leaving San Diego this morning, having spent the weekend at the 'Fully-Charged' BEV convention. We talked last night and he said no one presenting at the fair had any clue about aerodynamics and range.
This video could help stimulate a conversation.

[Blacktree]

I'm actually a little disappointed that he made the tail with such an aggressive diffuser angle. I figured he would know better. But I'm sure he'll get it figured out.

[aerohead]

Originally Posted by Blacktree

I'm actually a little disappointed that he made the tail with such an aggressive diffuser angle. I figured he would know better. But I'm sure he'll get it figured out.

Seems like the tufts were enough to convince him.
If he'd been able to see the elevation above ground for the 'source' flow traveling under the Subie, he would have never expected it to rise 'above' where it came from at the 'sink'.

[Xist]

I replied to as many comments as I could.
Does anyone use Discord? I want him to see my diagram!

https://discord.gg/b7BfHq2QJM



I haven't done one of those in years!

The roof looks pretty close to the template. I moved the bottom forward to line up with the bumper.

He said that he tried to match the body curves, but those probably detach at the front of the bumper.

I am not sure that the length is wrong, just too much taper in the front, and then not enough in the rear.

Hey, do you guys remember fanfold foam boards?
Do you think that he could use individual layers of that to build up the front and then either use a longboard to sand it down, or fill it in with 2-part foam?

I am sure that he would want to remove the foil first.

How hard would that be?

It was a pain to remove the perforated film from my fanfold foam.

I am sure that the foil is fine--until a rock or grocery cart hits it, but after he adds to it he would need to fiberglass it.

There were some questions about how much benefit different lengths of tail would provide.

Does anyone remember where those tables are?

Aren't there rules of thumbs for major mods, like 5% for an air dam, 10% for an undertray, and 20% for a boat tail?

Are those first two high? Maybe 3 and 5%, respectively?

I hope he isn't planning on leaving the license plate on the car. He talked about it and in one shot it was gone, but it sure seemed like he drove around with a boat tail and the license plate on the car.

I was trying to figure out what year his Imprezza is.

It seems like most comments were the same assumptions over and over--and some people really don't like making a small car longer.

How long do you think his goop will keep the wheel covers attached?

If he makes a proper boat tail, should he remove the hatch?

[freebeard]

Does anyone remember where those tables are?

This is where they should be, but I didn't check. Too busy linkifying the quote.

Reference threads on aerodynamics
Return to Main Page

Here are some threads with the most commonly used aerodynamic charts, graphs and pictures.

Index of Phil Knox Aerodynamics Seminars & Mod-data lists

Aerodynamic Streamlining Template

Pictures of cars with the ideal shape overlaid


Talk thread for this page: Wiki Hypermiling thread

--WeatherSpotter-9828 14:44, 6 September 2010 (EDT)

Aren't there rules of thumbs for major mods, like 5% for an air dam, 10% for an undertray, and 20% for a boat tail?

Rule of Thumbs are merely first approximations. The various elements are interactive; for example adding an air dam without a spoiler.

[Bicycle Bob]

Sometimes boundary-layer turbulators can save a marginal taper from separation. The Aero-Gare prototype flew OK, but they had to rough up the production ones to keep them flying straight, as they would develop separation on one side or the other.
I was surprised that the first bit of advice I got from a PhD who earned it was to avoid using a skin-on-frame nose cone like a glider, because gliders only get away with it by not having crosswinds like cars.

[aerohead]

I froze the screen of the video, and Al captured it on a memory stick, then printed out 11" X 17" copy, at the scale of my x-ray overlays.
Using the dimensions of an Outback, the Think Flight came in at approx. 252" overall length, a 60" elongation.
Length / Height = 3.7672
Length / height = 4.2637
Length / width = 3.3753
Length / three-ratio average = 3.80206
Af estimated @ 85.5% of gross frontal area = 29.656 sq-ft.
Length / square-root of Af = 3.8562
forebody = 49.5% of L
Aft-body = 50.5% of L
The elongation effectively creates a 'double-ended' body
Verungungsverhatnisse ( V ), ( aft-body length / h ) = 2.1654
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I found six cars with L / square-root of Af similar to Think Flight. Cds ranged from 0.20, to 0.137, with a 6-car average of Cd 0.1568.
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I found six cars with V similar to Think Flight. Cds ranged from 0.19, to 0.137, with a 6-car average of Cd 0.1558.
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The comparison cars are:
1983 GM Aero 2002 concept
1984 GM Chevy CITATION-IV concept
1985 Ford PROBE-V concept
1993 GM EV1 LSR
2006 LOREMO
2016 M-B IAA concept
2017 Eindhoven University Stella Vie
2019 GAC ENO. 146
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With all- wheel- skirts, wheel covers, a full belly pan, diffuser, and 'corrected' boat-tail, there's a high probability that Think Flight would indicate near Cd 0.156 or less.
About a 50.9% drag reduction.
And 'back of the napkin' mpg improvement, at 65-mph, of 25.5%, or around 44.5-mpg.
If the same mods were applied to the 2023 Subaru Solterra BEV, we'd be looking at around 155-mpg-e.
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PS, before I forget, I measured a 19-degree downslope angle on top.
TF reported that he used 20-degrees for the sides.
I measured 20.5-degrees for the diffuser.
The upper profile is very similar to Mair's 1969 torpedo, Volkswagen's 1981 'Flow-Body', and the 2007 AST-I contours.
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PPS, over the weekend I'll go over the Subaru Impreza Wagon specs now that I have them.

[aerohead]

If this links, the point of interest is the 'elevation' of the lower smoke filament passing under the XL1, and the 'elevation' of the actual diffuser panel at it's trailing edge.
You'll notice that VW doesn't expect the lowest streamline at the rear to 'recover' to an 'elevation' which is 'higher' than where it originated from.
The 'sink' is the same as the 'source.'
https://www.vwvortex.com/threads/vol...48569&slide=35

[aerohead]

Originally Posted by Xist

I replied to as many comments as I could.
Does anyone use Discord? I want him to see my diagram!

https://discord.gg/b7BfHq2QJM



I haven't done one of those in years!

The roof looks pretty close to the template. I moved the bottom forward to line up with the bumper.

He said that he tried to match the body curves, but those probably detach at the front of the bumper.

I am not sure that the length is wrong, just too much taper in the front, and then not enough in the rear.

Hey, do you guys remember fanfold foam boards?
Do you think that he could use individual layers of that to build up the front and then either use a longboard to sand it down, or fill it in with 2-part foam?

I am sure that he would want to remove the foil first.

How hard would that be?

It was a pain to remove the perforated film from my fanfold foam.

I am sure that the foil is fine--until a rock or grocery cart hits it, but after he adds to it he would need to fiberglass it.

There were some questions about how much benefit different lengths of tail would provide.

Does anyone remember where those tables are?

Aren't there rules of thumbs for major mods, like 5% for an air dam, 10% for an undertray, and 20% for a boat tail?

Are those first two high? Maybe 3 and 5%, respectively?

I hope he isn't planning on leaving the license plate on the car. He talked about it and in one shot it was gone, but it sure seemed like he drove around with a boat tail and the license plate on the car.

I was trying to figure out what year his Imprezza is.

It seems like most comments were the same assumptions over and over--and some people really don't like making a small car longer.

How long do you think his goop will keep the wheel covers attached?

If he makes a proper boat tail, should he remove the hatch?

1) it's about wake area.
2) if you reduce wake area by half, you've reduced drag by half ( Variable trailer design drastically cuts aerodynamic drag,' SAE International Paper 2013-01-2414,' by Mario Hirz and Severin Stadler, Graz University of Technology.
3) before you boat-tail though, one wants to accomplish all the optimizations upstream of the tail, to provide fully-attached onset flow, or you risk compromising the tail's performance.
4) the tail itself must be 'streamlined.' At no point, top, sides, or bottom may the pressure increase exceed that seen in a 'streamlined' surface.
5) the 'top' is the easiest part, as the carmaker has already 'begun' to reduce the cross-section there, and the flow is typically attached until the rear edge.
6) the sides, while cambered, are typically not as 'boat-tailed' as the roof. And the flow on the sides of the vehicle is typically 'slower' than on top, is at a higher pressure already, is less 'displaced', has lower kinetic energy, and cannot survive as much cross-section reduction as on top without separation.
secondly, as you're creating a 'fastback' form, the fastback requires the highest degree of tumblehome, of the basic three body types.
7) The underbody has the most feeble flow, and asking a diffuser to position a separation edge at a 'higher' elevation than that from which the underbody flow originated at the front is a recipe for disaster.