Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph

[JackMcCornack]

Originally Posted by MetroMPG

Sorry to disappoint, but I'm using painter's tape too. Well, these tufts were stuck to gooey electrical tape, then onto a square of painter's tape. I save the tufts for the next project.

That's not a disappointment, that's an insight.

I'll bet you use scissors, too. I saw the uniformity and the black-with-light-border design, and, well, things said half in jest are said half in truth, and I wouldn't have been surprised to hear A) that some outfit in Detroit manufactures sticky-end tufts for this very purpose, and B) that you'd hunted them down.

A clever blend of technologies there; electricians' tape to hold onto the yarn, painters' tape to hold onto the car. I'll try that.

Artists talk about cheap turpentine, ecomodders talk about sticking yarn to their cars. Thanks for the help.

[Nerys]

this makes me REALLY want to build a removable tail for my Club Wagon for next summers trip to Colorado!

[aerohead]

Originally Posted by MetroMPG

Re CAD...



Sorry, should have said "CAD", in quotes. With a nod to Craig Vetter, I was talking about "Cardboard Aided Design".



Thanks for the input, Phil.

I'm a little nervous about those nervous looking side tufts. It's clear I could increase the top taper - those tufts are laying almost still right to the transom.

What I don't know - and maybe you or Bob can tell me - is this: should my goal be to taper as much as possible until I actually see evidence of separation through circulation/reverse flow, and then back off a bit?

In other words, will we achieve favorable drag results even as we increase the amount of "nervousness" in the tufts (evidence of tubulence and a thickening boundary layer) through further tapering?

It's clear I can taper the top more. Not sure about the sides though. Maybe you can decide when I post some video this week.

Given that Cd is supposedly constant in the range of speeds where road vehicles operate, why would a vehicle want to actively adjust its angle of attack? Is the nose up/tail down attitude a concession to low speed driving for clearance, making the top angle aerodynamically sub-optimal until the nose goes down?

Darin,I apologize,as I'm trying to catch up line by line.
With respect to the longitudinal centerline curvature,I enlarged my image of the Ford Probe-IV and put a protractor to the back,for the 45-mph full-down suspension orientation.
With the nose dropped and hiney sticking higher into the air,a straightedge laid across roof to trunk lip reads 17-degrees.
So it looks like your 15-degree final exit angle is "conservative." However,looking at the EV1 and it's boat tail,the straightedge also yields 15-degrees.Maybe conservative is good!
Kamm advocated no more than 10-degrees although his K-cars violate that rule,and the K-3 car which we have a photo of clearly shows the boundary layer separation and emergence of turbulent wake at the back of the roofline where the curvature begins to get "fast".
As far as the sides go that's a bit more tricky.Kamm started "narrowing" his roof at same time as it was curving down.
How the front of the car divides the airstream,and the forward architecture determines the local velocity and available kinetic energy of the flow at any given point.
Typically,the nose scoops more over the car and the energy there is greater than the sides and can sustain the most "bending."
I don't have Hucho's later book and I don't know if he goes deeper into side flow.
Jaray's/Klemperer's/Fachsenfeld/etc. work suggest that if you respect the teardrop form,as a half body of revolution,we can curve the sides just as we would the top.
But the teardrop doesn't have a "roof." And I'm more conservative,allowing only half what might work over the roof.No good science there,sorry!
I know that for any given length,that if I take the sin of 7-degrees,multiply that by the length of the run,and then "bend" a line off center that measurement,at that length,I get a curve the air will follow.
I'm going to do the full 22-degree Mair' maximum on the trailer,top and sides.And it's an unknown quantity.The VW worked at 20-degrees and NASA chose 20-degrees and you can see from their tufts that it's pretty clean.
I have extremely gentle curve transitions on the boat hull up to 22-degrees,then the rest of the body just shoots back at the constant 22-degrees.We'll see in a few weeks what happens.
I can get away with murder underneath because of the extra axle of the trailer.It's belly is a continuation of the T-100's 2.5-degree diffuser.
All the under-belly fairings and strakes are shaped to respect the SAE 10-degree departure angle.
By the way,your wheel fairings should have your rear tires below Cd 0.10 now for what you've done.They mimic the "chute" style underwing fuel tank and rear faired aero wheels which are at Cd0.10 (and do not have the lead fairing as your's).
I'll shut up and read more of your posts.Your boat tail is awesome!

[MetroMPG]

Originally Posted by aerohead

How the front of the car divides the airstream,and the forward architecture determines the local velocity and available kinetic energy of the flow at any given point.
Typically,the nose scoops more over the car and the energy there is greater than the sides and can sustain the most "bending."
I don't have Hucho's later book and I don't know if he goes deeper into side flow.
Jaray's/Klemperer's/Fachsenfeld/etc. work suggest that if you respect the teardrop form,as a half body of revolution,we can curve the sides just as we would the top.
But the teardrop doesn't have a "roof." And I'm more conservative,allowing only half what might work over the roof.No good science there,sorry!

Good explanation, thanks.

I also basically guessed at side angle. I arbitrarily chose 10 degrees, and then eyeballed it by holding up a cardboard triangle with a 10 degree angle while laying out the strings.

One interesting result of my imprecise construction techniques that resulted in the boat tail being skewed somewhat to the passenger side: I've effectively tuft tested two different side angles. I haven't measured them yet, but will this week and will report back.

FYI, I measured the bottom angle: it's 4 degrees from the undertray to about a quarter of the way along the length of the boat tail, where it transitions to 7 degrees. And we saw from the tufts that it had attached flow to the end.

[Christ]

Originally Posted by MetroMPG

Good explanation, thanks.

I also basically guessed at side angle. I arbitrarily chose 10 degrees, and then eyeballed it by holding up a cardboard triangle with a 10 degree angle while laying out the strings.

One interesting result of my imprecise construction techniques that resulted in the boat tail being skewed somewhat to the passenger side: I've effectively tuft tested two different side angles. I haven't measured them yet, but will this week and will report back.

FYI, I measured the bottom angle: it's 4 degrees from the undertray to about a quarter of the way along the length of the boat tail, where it transitions to 7 degrees. And we saw from the tufts that it had attached flow to the end.

Most probably because of the belly tray you have installed, do you agree?

IOW, for the average Metro owner, the angle that maintains attached flow will probably be much less, perhaps half of that, due to extremely turbulent flow to the fore end of the transition to smooth surface.

[aerohead]

Originally Posted by MetroMPG

Good explanation, thanks.

I also basically guessed at side angle. I arbitrarily chose 10 degrees, and then eyeballed it by holding up a cardboard triangle with a 10 degree angle while laying out the strings.

One interesting result of my imprecise construction techniques that resulted in the boat tail being skewed somewhat to the passenger side: I've effectively tuft tested two different side angles. I haven't measured them yet, but will this week and will report back.

FYI, I measured the bottom angle: it's 4 degrees from the undertray to about a quarter of the way along the length of the boat tail, where it transitions to 7 degrees. And we saw from the tufts that it had attached flow to the end.

I seems like the underside has got to be compromised so it doesn't get scrubbed off at the first driveway.
At the GM exhibit in Epcot Center,their Citation concept car had an articulated active rear valence/diffuser panel which lowered for highway travel.I've envisioned something like this with a simple cable-pull and latch.
Looking at your numbers,if you were starting around Cd 0.30 before the tail,then the 15% mpg improvement would have you in Cd 0.21 territory
Basjoos-style movable front skirts would knock 9%( based on original Cd ) off.Full rear skirts maybe a few ticks more.
You're so close to sub-0.2 I can smell it! Thanks for doing all that,the meticulous records,video,ad infinitum............. "And the trophy goes to......"

[RobertSmalls]

Originally Posted by MetroMPG

FYI, I measured the bottom angle: it's 4 degrees from the undertray to about a quarter of the way along the length of the boat tail, where it transitions to 7 degrees. And we saw from the tufts that it had attached flow to the end.

So a steeper angle may be possible. And it would certainly make a boat tail easier to live with. If your underbelly were as smooth as, say, 3-wheeler's, might a 10° angle be possible? It would result in a much shorter tail.

[MetroMPG]

Originally Posted by Christ

"And we saw from the tufts that it had attached flow to the end."

Most probably because of the belly tray you have installed, do you agree?

IOW, for the average Metro owner, the angle that maintains attached flow will probably be much less, perhaps half of that, due to extremely turbulent flow to the fore end of the transition to smooth surface

Even without the belly pan, I had attached flow - very turbulent, but no recirculation - in the middle of the tail right to the end. But it definitely got better with the undertray. Ultimately, yes it makes sense that a smooth undertray would permit a steeper angle before flow separation than without one.

Originally Posted by aerohead

if you were starting around Cd 0.30 before the tail,then the 15% mpg improvement would have you in Cd 0.21 territory

.21? Woohoo!

Thanks for doing all that,the ... video

Oh, right, the video. I still need to put something together to post on YouTube. Weekend project.

Originally Posted by RobertSmalls

If your underbelly were as smooth as, say, 3-wheeler's, might a 10° angle be possible?

Only one way to find out. (Involves sacrificing some of the raw materials from your knitting kit though.)

[MadisonMPG]

MetroMPG, can you post the top right picture (the original) that is in post 1?

[gone-ot]

Originally Posted by brucey

Mine showed up absolutely nothing. But it also was a lot steeper. I fail.

...nah, we're just one of the outliers in a Six-Sigma world.