'97 Jeep Cherokee XJ 4x4 overland rig

[mannydantyla]

Originally Posted by freebeard

I don't get the first one.

Yeah I didn't do a very good job of putting it into words, I'll have to make an illustration. I think this should do the trick:

Originally Posted by freebeard

Those deflectors to keep the back window clean add frontal area, and therefore drag.

Oh ok, I guess it's to avoid this then:

[freebeard]

^^A good example.

As for cutting up a lift gate; I suspect you'd do better to start with strips of flat sheet. What you're proposing is similar to the boxed cavity I mentioned in Permalink #48. Also the Koenig-Fachsenfeld (corrected spelling) tearing edges mentioned in permalink #28.

[mannydantyla]

I just ordered this $20 OBCII scan tool that you connect to with your iOS or Android divice via wifi. http://amzn.to/2GwmCTt

I'll need to download a $5 app though to use it but if I like it maybe I'll find a used iPad or something to keep in the car permanently.

Originally Posted by freebeard

As for cutting up a lift gate; I suspect you'd do better to start with strips of flat sheet. What you're proposing is similar to the boxed cavity I mentioned in Permalink #48. Also the Koenig-Fachsenfeld (corrected spelling) tearing edges mentioned in permalink #28.

Exactly!

Originally Posted by freebeard

http://ecomodder.com/forum/showthread.php/random-wind-tunnel-smoke-pictures-thread-26678-17.html#post544437

This is a similar shape. The vertical edges are 'hotter'. two triangular fins from the outer edges down toward the taillight would be a good start on a boxed cavity. Here — according to the text is an explanation, if you can see Photobucket images: http://ecomodder.com/forum/showthrea...tml#post542668

Two triangular fins from the outer edges down toward the taillight... ??

I can't see the photobucket image in the second link, either

I don't know why I didn't do this sooner but I looked up the drag coefficient for the Jeep Cherokee. Cd is a whopping 0.52!

Also, folks are saying that for a box cavity to be effective, air flow needs to be pretty smooth to begin with and a diffuser really helps. http://ecomodder.com/forum/showthrea...uck-26367.html

Once the OBCII wifi scan tool arives in the mail, I'll make a cardboard mockup and do some testing...

Can this tell us anything??

[freebeard]

PRK is Phil Knox AKA aerohead. Another example based on the
Vanagon that popular in the Aerodynamics subforum is this:



What it tells us is that the humped back of the bubble top has a lower Cd (0.40) than the baseline 0.44, even though the CdA would be higher. Upstream effects.

[mannydantyla]

What about this? Should I also test a boxed cavity where the extension panels are "blown" as the Georgia Tech folks call it? https://www.slideshare.net/Hondafana...-backed-trucks



They claim 5-6% improvement in fuel economy. They also tested a Suburban but it's unclear how that worked out for them, at least I can't make heads or tails of it.



It's not the best slideshow ever but it's interesting. Raises more questions then it answers for me though.

Another image by Aerohead/Phil Knox/PRK:

http://i1271.photobucket.com/albums/...titled1_16.jpg


Something about the panels being 42% the length of the body to get maximum benifit. That's far too much for me. I saw somewhere that someone said 4" is enough to get a significant improvement. Length of my jeep is 167 inches, 4 inches would be 2.4%, and 70 inches would be 42%.

That seems absurd to me. Almost half the length of the vehicle! I don't know about that but I also don't know much (at least I know enough to know that!)

This isn't boding well but I still want to test it. Either with a scan tool or by doing the down-hill speed thing.

[freebeard]

Thanks for the link. I've seen Georgia Tech's work as PDFs. The competition for pneumatically blown slots is plasma actuators. I like blown slots because VWs have 1500 cubic ft/minute of cooling air exiting a duct at the bottom rear.



The picture to go with aerohead's drawing:



...done by a neighbor of his. From here; where there are links to box cavity threads.

A full boat tail based on Thee Template is a heavy burden, but following Mair, and looking at the width rather than overall length:



You're doing pretty well at half the 5-6ft width/height.

[mannydantyla]

Here's the design I came up with:



Here's how that might look:



So is it a roof spoiler? partial kammback? boxed cavity? partial boxed cavity?

Sorry but if it's going to be permanent then aesthetics are at least somewhat pertinent for me.

Even at that small of a size, a guarantee you that I'll get sticky notes stuck to my windshield at least once a week saying "Sweet spoiler, broh!!" I live in a college down (Lawrence, KS) and park near the football stadium to go to work every day, and yesterday someone stuck a sticky note to the windshield saying "broh! nice roof rack bro!" and I'm still scratching my head wondering if they were laughing at my expense or genuinely complementing my welding skills or somewhere in between.

If it was the size of this kammback then people would be shouting out their windows at me daily:



http://ecomodder.com/forum/showthrea...-6-a-6069.html

^^^ That's a good thread, btw. Although he never says how long it is, but it looks like 18" to 24" to me. So maybe twice the length of the one I designed, but it goes all the way to the bumper.

However, the bottom is not closed off. Which flies in the face of this quote from aerohead:

Originally Posted by aerohead

Your cardboard mockup is more like a box-cavity,and for it to work,it needs to be as low as the bottom of your bumpers bottom,and it needs a 'floor'.It cannot be open on the bottom unless there is a back on it,and the back is airtight.

(I really hate photobucket, btw)

That quote is from this thread - http://ecomodder.com/forum/showthrea...k-29583-4.html - in which a fellow SUV ecomodder tries a cardboard kammback/boxed cavity, and sees only a 1% increase in MPG over stock and after several days worth of testing.

So, given all these things, can I realistically expect maybe 1/3 the results that MetroMPG got with that minivan? Which would be a little over 1% so I would go from 15 mpg to 15.15 mpg which is insignificant IMO.

Still though I will try and test it. Today the scan tool is supposed to arrive in the mail, but conveniently I broke my iphone last night! Which it needs to connect to. I could use my fiance's maybe, idk.

(don't mind me, I'm just going to continue dumping my brain out into this post)

You see these roof spoilers on every new hatchback and SUV being made today and they seem to be getting longer and some are even going over the sides too.





So I guess that's what I'm going for.

BTW, that last one is the 2017 Lexus RX. I found this:

How the new 2016 Lexus RX cheats the wind - Lexus


And this: Novel Spoiler Design Reduces Fuel Consumption for Minivans, SUVs - Green Car Congress

A team of researchers has used the principles of fluid dynamics and numerical simulation to design a new rear spoiler for bluff-backed vehicles (such as minivans and SUVs) that can reduce drag and lift [...] by 5% and more than 100%, respectively, when the new spoiler is attached to it.

Are there any vehicle in production using a spoiler like this?

Is 5% a lot?

If I take this data...



...and ignore the "1.9d" part and follow Mair's assumption that it's a function of the width not the length, then I get this:



And that shows that I can get the same 5% reduction in bluff body drag with only 5 inches of "partial boxed cavity" as I call it.

Yeah I know I'm speculating big time. Mainly that a partial boat tail has the same aero benifit as the "partial boxed cavity" that I'm talking about.

[freebeard]

It looks like your diving pretty deep. Your proposal (in white) looks like a pretty standard Kammback such as you see on Metros, etc., with proper curvature.

I'm really skeptical about that 'novel spoiler' in yellow. It's on a ~10° slope to start, there will be no laminar flow there to fill that narrow throat at A. It's a diverging duct. It might work better if it's cut back to B and convex from B to C.

But that's what wind tunnels are for.

As far as aerohead's quote, I'd point to Morelli's Fluid Tail. His post is Photobucketed, but here are links to the Morelli paper:A New Aerodynamic Approach to Advanced Automobile Basic Shapes

SAE wants $28, others will let you download it. It shows the FIAT Punto.

edit:
Of course as soon as i hit submit, I proceed to find this thread from 2008: http://ecomodder.com/forum/showthrea...info-2072.html. Here's Morelli's Punto

[mannydantyla]

Originally Posted by freebeard

Of course as soon as i hit submit, I proceed to find this thread from 2008: http://ecomodder.com/forum/showthrea...info-2072.html. Here's Morelli's Punto

Bingo!

The text to go with the illustration:

"Compared to the standard blunt end (a), adding what they call attikas (fascias or parapets; flat, parallel panels) (b) delays separation and moves the tail vortices back (reduces Cd by up to 10%); these can be set inward (c), further reducing the size of the „dead zone“ (reduces Cd by 0.06, which is about half as good as the ideal tapered tail); the open-ended Kammback made of flat panels (d); and „Fluid Tail" (e)."

So we finally have a name for the half partial kammback and half box cavity dsign: open ended kammback.

Now I just need to actually do this and test it.

My OBCII wifi dongle arrived in the mail yesterday, for $20 it broadcasts the obdii outputs over wifi to any device with an app that can read them. There's dozens of apps for android, iOS, etc. that can do this, some of them free and some of them $5 or less.

My iphone broke two days ago, however. So last night I tried to use a $50 Kindle tablet. I tried 3 or 4 apps and either they wanted to use bluetooth only instead of wifi, wouldn't work with that specific obdii dongle because it wasn't the same brand as the app develop, or it just simply couldn't connect to it for unknown reason. These were all the free apps, I didn't want to buy an app as the Kindle Amazon account is connected to my fiance's bank account and not mine.

I could try using my 2010 MacBook Air but it's battery is on it's last leg and I'm doing a last second backup to an external hard drive as I type this. Thankfully I got the power cord to cooperate, it's damaged and won't work 90% of the time.

I'll have to fix the iPhone I guess before I can read instant MPGs.

One more update: I bought some nice mono shocks to replace my crappy gas shocks. This might not seem like an ecomodding upgrade but this will allow me to run at a higher tire pressure much more comfortably. The ride quality on the Jeep is extremely harsh: it has two solid axles instead of independent suspension, and the rear axle uses leaf springs (so it can haul heavier loads) which makes it even worse. At least the front uses coil springs but they're fairly stiff at 150 lbs/in. Oh, and the offroad tires have a very stiff sidewall, to prevent damage from rocks. All of this makes for a very very harsh ride at 40 psi.

[freebeard]

Ride quality is important. I had a shop put adjustable struts in the front of my Superbeetle and they ordered the boy-racer part. It was so stiff if I put my elbow on the driver door arm rest, in 5-10 miles I'd have pain in my collarbone. Fortunately () I totaled it and got the stock spring rate back during the repair.

Slashdot points to a Macrumors story — 10% off on refurb iPhone 7s. https://hardware.slashdot.org/story/...-7-plus-models

I'm flying by the seat-of-pants protocol. The Superbeetle has a mechanical speedometer and electric gas gauge and two warning lights. My new-to-me pre-OBDII Dasher adds an electric clock, temp gauge and a third warning light for the glow plug. Don't even have a cell phone.

A note about the open ended kammback: You will notice that the upper half has a screen with a half-circular cutout on the bottom edge. This is a perforated base plate, halfway between open and boxed.

I saw an amazing example of what screens can do at the Darko wind tunnel. They have a wall of 2" welded wire mesh in the return part of the loop. The air goes ten feet past it, around a corner, into the throat of the tunnel and then thirty feet down the tunnel, into the test section. They got an improvement in laminar flow on the tunnel wall.

I think there is a place for perforated plates parallel to the airflow, like the louvers on a old hot rod.