Stainless Kammback for Previa. - Page 4 - Fuel Economy, Hypermiling, EcoModding News and Forum

alohaspirit · 08-07-2009, 07:52 PM

right on man

that thing looks great

cant wait for the numbers

orange4boy · 08-07-2009, 08:31 PM

I asked aerohead about this and this was his reply:

Quote:

(2) Sepp has reported that an open shell did not offer as much drag reduction as a closed shell on his Nissan truck.Bob Parson's open boattail for his Vanagon did not return as high a savings as the closed unit on my Transporter but his "lifestyle" dictated an open plan and he was happy to live with the difference.Open units may allow"circulation" which always has to be paid for at the pump.

So I closed it. I think it looks better too.

cfg83 · 08-07-2009, 11:18 PM

orange4boy -

Ok, wisdom from the master.

CarloSW2

orange4boy · 08-09-2009, 09:40 PM

I thought I would post these which I made last year. I can't remember the values I used but I have not been able to reproduce them since.

I notice now that the angle of the kamm on the second video is just a touch too steep as there is some separation there. Who knows how accurate these are. I have been scouring aerohead's images for the best angle for the afterbody. It seems to range from 10 to 22 degrees. I measured mine again and got 17 degrees on the actual kamm. The video is 22 degrees to the mid point and goes steeper after. I will have to do more tuft testing to be sure on the actual Kamm and I may change the angle depending on how that pans out. My tufting seemed good at 17 but I only got up to about 45 mph.

I updated the annotations to reflect my current understanding of the theory (corrections and clarifications welcome)

http://www.youtube.com/watch?v=7BJc9I1W2JE

http://www.youtube.com/watch?v=KJGkxHDrNr4

orange4boy · 08-13-2009, 01:27 AM

Results of first highway drive.

I have some relatives visiting from California who were more than happy to watch my tufts while on a highway run to another relative's home. Luckily, my brother in law is a serious tech nerd with a good understanding of aero.

The top section tufts were straight back, so that angle seems fine. The tufts on the joint between the top and side panels were perfectly parallel to the seam and straight and flat. This is good news because it means the top and sides are not spilling any air and are not creating turbulence.

There are some separation problems with the lower half of the side panels which I was half expecting. The tufts on the top half were good and straight but the bottom half were flapping like crazy on the coroplast section. This part of the kamm has greater angles than the top section. The side panel angles change from top to bottom due to some bad eyeballing during construction and a compromise because of the length of the aluminium stock. I bet they are making huge trailing vortices behind the Egg. This will
be easy to fix once I get home.

The separation on the sides seems to be such a penalty as to cancel out the benefit of the reduced drag on the top part. additionally, the open bottom of the kamm may be a big parachute. Seat of the pants coast down was about the same but possibly worse than with just the stainless extensions. The stainless extensions were a big improvement by the way so I think this is a tuning issue not a failure as such.

I think my trailing vortices are about this big.

Earth As Art - Karman Vortices

Tomorrow, I will try to pick up a taillight set so I can finish the lower half of the kamm.

I am half bummed but I think it's fixable and will make a huge difference when it's tuned.

Christ · 08-13-2009, 01:40 AM

Quote:

Originally Posted by orange4boy

Results of first highway drive.

I have some relatives visiting from California who were more than happy to watch my tufts while on a highway run to another relative's home. Luckily, my brother in law is a serious tech nerd with a good understanding of aero.

The top section tufts were straight back, so that angle seems fine. The tufts on the joint between the top and side panels were perfectly parallel to the seam and straight and flat. This is good news because it means the top and sides are not spilling any air and are not creating turbulence.

There are some separation problems with the lower half of the side panels which I was half expecting. The tufts on the top half were good and straight but the bottom half were flapping like crazy on the coroplast section. This part of the kamm has greater angles than the top section. The side panel angles change from top to bottom due to some bad eyeballing during construction and a compromise because of the length of the aluminium stock. I bet they are making huge trailing vortices behind the Egg. This will
be easy to fix once I get home.

The separation on the sides seems to be such a penalty as to cancel out the benefit of the reduced drag on the top part. additionally, the open bottom of the kamm may be a big parachute. Seat of the pants coast down was about the same but possibly worse than with just the stainless extensions. The stainless extensions were a big improvement by the way so I think this is a tuning issue not a failure as such.

I think my trailing vortices are about this big.

Earth As Art - Karman Vortices

Tomorrow, I will try to pick up a taillight set so I can finish the lower half of the kamm.

I am half bummed but I think it's fixable and will make a huge difference when it's tuned.

Wait... you think your trailing vortices are over 100 Miles long?!

LOL - I wonder if you noticed the scale indicator in the bottom right corner of that second pic?

orange4boy · 08-13-2009, 02:43 AM

vtec-e · 08-13-2009, 05:56 AM

Stick a couple of VG's in there and she'll be right!

ollie

orange4boy · 08-14-2009, 03:50 PM

Quote:

Stick a couple of VG's in there and she'll be right!

I love the idea of VG's too, but I can still change the angles easily at this point to fine tune it.

I had disappointing results with the mpg and seat of pants from the new back. I thought the angles were wrong. Top was 15-17 and sides were 15-20. I changed to 12 top and 12 sides but the van still seems to be sluggish on coastdown. It was much better with just the short extensions. It's possible the cooler weather is affecting the results. Just how much difference is there in air density between 85F to 70F? If it was working well one would think that the benefit would out-weigh the change in temps. One might be wrong, though.

Or could the half shell create some massive vortex, increasing drag, as postulated in previous post?

Of course it could be a dragging brake or some other increased friction like a toast bearing. I guess I should take the sucker off and try it that way. OR...Cardboard and ductape it for now.

I picked up my new taillights. I wanted red strip LED's but i couldn't find any DOT approved

.

orange4boy · 09-03-2009, 05:23 AM

Well, It turns out the problem was not the kamm back but sticking brakes. Not so much to get the wheels hot but enough to rob me of the gains from the kamm. This time it was the left rear drum.

New best of 36 mpg on the highway. MPG has been trending up.

I'm not sure when I'll have time to finish the bottom half. I did get the tail lights set up. Pics ready soon.