Improving aerodynamics (of '87 Nissan Bluebird)

[landspeed]

Hi!

Just wondering if anyone had any ideas for improving the airflow over the rear of my car, as I think the shape is about the worst it could be (see attached picture!).

When driving in the rain today (after stopping so the rain had hit the rear window for a while), the water went into straight lines going down the window - does this mean there was attached flow?

[Peakster]

Just start experimenting! Does rain get on the rear window if you don't stop and let the droplets hit the window for a while? By the way, your car is really nice and low to the ground, so you got that aero-issue down pat!

exec_2.jpg

[MetroMPG]

What's the angle of your rear glass?

A 10 degree angle is considered optimal for the slope at the rear of a vehicle, with Cd worsening as you increase up to about 30 degrees at its worst. Of course, the sides should also be tapered in if possible as well (also following the 10 degree rule).

Peakster's drawing is the right idea, but I think the slope could be a bit more.

If you didn't want to make a rear structure to get that 10 degree shape, you could also accomplish something similar with a "wing" (not really a wing). See http://max-mpg.com/html/tech/main.htm

Other things to consider:

- partial grill block
- smooth wheel covers, and/or
- front/rear wheel skirts
- smooth under trays
- passenger mirror delete

There's more, but there are some to think about.

[Peakster]

Originally Posted by MetroMPG

A 10 degree angle is considered optimal for the slope at the rear of a vehicle, with Cd worsening as you increase up to about 30 degrees at its worst.

MetroMPG, what happens if the angle is more than 30 degrees?

[MetroMPG]

Cd starts to improve again.

30 degrees is approximately the transition (as far as airflow is concerned) between the definition of "notchback" (if it's got a trunk - perhaps "wagon" if it doesn't?) and "fastback".

Though beyond 30 degrees, the improvement in aero isn't equal when increasing vs decreasing. IE, a fastback of 15 degrees will likely be better than a notchback of 45 degrees, all else being equal. (The notchback would have a larger wake.)

[MetroMPG]

If you were to ask me what makes 30 degrees so bad for Cd, I'd have to say I don't know for sure. Should look that up tonight.

[Peakster]

Originally Posted by MetroMPG

Though beyond 30 degrees, the improvement in aero isn't equal when increasing vs decreasing. IE, a fastback of 15 degrees will likely be better than a notchback of 45 degrees, all else being equal. (The notchback would have a larger wake.)

That would make a lot of sense. I remember my Pontiac Fiero (90 degree angle from roof to rear window) having a giant wake when white smoke came billowing out of my engine compartment when a coolant hose ruptured on the Trans Canada Highway.

[MetroMPG]

I've got "Road Vehicle Aerodynamic Design: An Introduction (Paperback)" by R.H. Barnard

Wanna swap books, clencher?

[landspeed]

I was wondering about a Kamm-back wing - but it would look a bit strange.

When I am driving, no rain hits the rear window - is that good thing?

[The Toecutter]

Your rear angle is like the worst possible in regard to drag!

Build a Komm-wing for that *****, ASAP!

You could also do a partial grill block, rear wheel skirts, full underbelly, removal of passenger mirror(or better yet, cameras replacing mirrors), removal of exterior plastic trim pieces, lowered ride height, smooth wheel covers or hubcaps, front and rear wheel spoilers, shaved door handles, small plastic or lexan windscreen in front of the windshield and raked properly may help reduce front turbulenace, shaved door handles, side skirts, rear boattail or partial boattail, among other modifications.

They each have different levels of cost and difficulty to implement. If you want the car to retain a professional-looking finish, some of these aeromods could get very expensive(such as fiberglassing an entire rear boattail). However, if you're willing to settle for coroplast or cardboard tacked on, it can be very cheap.

[landspeed]

I'm thinking of full underbelly as one of the easier ways to improve drag,
without sacrificing appearance. I may also try at least partial rear wheel
skirts. Wing mirror cameras are a possibility, but also expensive.

Do you think a set of vortex generators across the roof would help at
all, without doing the full kamm wing etc?

Or... How about something like this? (see picture) - surely that would negate the
effect of the rear window angle (although the drag due to the size of the rear wake
would still be as large).

[The Toecutter]

Do you think a set of vortex generators across the roof would help at
all

From all of the data I've seen, not at all.

How about something like this? (see picture) - surely that would negate the
effect of the rear window angle (although the drag due to the size of the rear wake
would still be as large).

Looks like it would. Try making a mock-up and experimenting.

[MetroMPG]

Originally Posted by The Toecutter

From all of the data I've seen, not at all.

Can you point to some data?

[MetroMPG]

I think you'd be better off with something like this:





Try to get as much taper as possible (top & sides) approaching 10-12 degrees before you cut it off. Reduces wake size.

[landspeed]

Originally Posted by MetroMPG

I think you'd be better off with something like this:





Try to get as much taper as possible (top & sides) approaching 10-12 degrees before you cut it off. Reduces wake size.

I like the look of that - might try it soon!. Just need to find some coroplast, or whatever they call it in the UK, then try some SuperMID tests!. I will post back when I manage to get a mockup ready!.

On another note, fitted my EBH today (it is a 3kw circulating one - 'Kenlowe Hotstart') as shown here: http://www.kenlowe.com/pre-heaters/cars/diy.html
I'm really pleased with it, and tried it out for the first time this morning (my car was covered in ice). Will try it again tomorrow before going to work!.

Toecutter : I am also thinking of a grille block - my plan is to get a grille from the scrapyard, completely block it (because air can flow through some other gaps anyway), and use it in the winter, until it gets to hot to be useful. I also have my eyes on an aluminium rad from a Primera (G20 Altima), at the same scrapyard - smaller, lighter, better heat dissipation, and it means I can block a certain amount of the lower grille too!. Also, as it is smaller, it contains less coolant, making it lighter still in use.

[MetroMPG]

Good stuff. FYI, the results are likely to be small - I'd predict a 1-3% FE improvement @ 55 mph based on doing something similar to my car: http://www.metrompg.com/posts/boat-tail-prototype.htm

Not trying to discourage you. Go for it.

Thanks for the EBH link.

[MetroMPG]

Wow - that's a massively powerful EBH! Only 20 minutes plug-in recommended. Some good info on that site.

[landspeed]

I bought it on ebay, and the guy tried to rip me off - when I plugged it in, the water boiled within 10 seconds (it has a cup-sized amount of water inside). I removed it, took it apart, and it turns out the motor was seized solid. Using some oil, a mole wrench, and plenty of time, I managed to unseize it, and it now works perfectly - but the person selling it must have thought it was broken!. I like it so much, that I'm going to get one for my mums car as well!.

I think the aero mods are worth it, as the cumulative fuel saving could be quite good!. Also, given how bad the aero is on my car (the lower engine bay is just open to the road, it has that rear angle for the hatchback), I might get bigger savings!.

I want aluminium for the under-body stuff (at least for the engine bay) so need to find a source for that. However, I have found out that, in the UK, 'Coroplast' is called 'Correx', and is pretty cheap, so a partial kamm-back prototype might not be that far off!.

I'm now off to make that Circuit that Yoshi told me to do (for SuperMID!)

[MetroMPG]

It actually has a pump in it. Nice.

I mistakenly assumed it was convective (didn't read the site closely).

[MetroMPG]

(NOTE: moved the airtabs discussion to the VG thread:

http://www.gassavers.org/showthread.php?t=995)