6th Gen Accord Aerodynamics

[brick]

All right, you guys finally did it. After reading about all these aero mods I've decided to build a coroplast belly pan for the Accord. My highway mileage seems to have hit a plateau at 42-44mpg based on technique alone. I have no idea how much of a difference this is going to make, but anybody want to take a guess just for fun?

I should have the material (2 4x8 sheets worth) by some time next week, so in the mean time it's all about figuring out just how this should be done. Cutting and shaping will have to be done as I go, but what about fastening? Are screws the only way to do it right or is there a good alternative that doesn't involve putting holes in my car? I guess I don't mind screwing into plastic parts, but drilling into the steel seems like an invitation for rust.

Any other thoughts from the folks who have done it?

[Mighty Mira]

I'd like to know this too. I'd also love to know how to secure an aluminum undertray on a car without creating a huge sacrificial anode that will fall off the moment the area around the bolts corrodes away.

[MetroMPG]

Brick - I'm with you on the reluctant-to-drill-into-underbody concern, particularly since I live in the rust belt. I'm not sure where you live.

But there's really no other easy way to do it. I'm going to get around to doing the belly pan also, probably later this summer. I think we can take some precautions to minimize the chance that rust will start by oiling or greasing the holes once drilled, prior to fastening the coroplast. Also, you might want to consider stainless steel machine screws, so the heads don't rust (or the threads, inside the holes if they get wet).

The next concern is making sure the belly pan itself doesn't somehow trap water. I've already spotted 2 drain holes in the "frame" members that lead back from my front suspension lower mount points (ie inboard from the rocker panels). I'll have to accomodate those with corresponding holes in the plastic sheeting.

Mira: what about just using plastic washers? Seems like an easy fix to that potential problem.

[Mighty Mira]

Originally Posted by MetroMPG

But there's really no other easy way to do it. I'm going to get around to doing the belly pan also, probably later this summer. I think we can take some precautions to minimize the chance that rust will start by oiling or greasing the holes once drilled, prior to fastening the coroplast. Also, you might want to consider stainless steel machine screws, so the heads don't rust (or the threads, inside the holes if they get wet).

What is the material that you are drilling into? Is it steel? If so, using stainless steel will just act like a battery, eating away at the steel that you have drilled into.

I suspect that using galvanised screws would be the only way, as the zinc would preferentially get eaten away first.

Mira: what about just using plastic washers? Seems like an easy fix to that potential problem.

Hmmm. You would need some way of preventing the tray from moving around. The moment a screw touches the tray, you are going to get corrosion. Any ideas? Perhaps a tiny plastic washer to go inside the hole between tray and screw, and then a larger plastic washer to take the weight of the tray?

[basjoos]

I figure I've be needing to take the underbody panelling off from time to time when I take the car in for major scheduled service, at which time I'll inspect for rust and replace all of the 8-32 screws.

You may consider pressing rubber grommets into the drilled holes on your aluminum underbody panelling to keep it out of direct contact with the steel screws.

[brick]

I finally got around to doing one simple "mod" that may or may not be worth anything: removing the wind deflector from my sunroof. The roof is closed 99% of the time so I figure there's no point in having the extra 2" high x 30" wide piece of plastic on the roof. I think the car looks better without it, anyway.

[MetroMPG]

Brick, FYI:

March 18, 2004
The New A6: Body Structure

In all, the various improvements on the underfloor of the car reduce the drag factor by no less than 0.031 which, related to the car's overall drag coefficient, means an improvement by almost 11 per cent. Travelling at a speed of 180 km/h, this means a reduction in fuel consumption by approximately six percent compared with a comparable vehicle without suitable modifications at the bottom.

You can work the math backwards if you like, and estimate FE gains at hypermiler speeds.

[brick]

Good find! I developed a spreadsheet a few months back that approximates the power requirement to maintain a set speed on the highway, which allows me to plug in different numbers for CD, frontal area, rolling resistance, etc. If I go with the baseline numbers for my Accord (CD=0.33) it tells me that I need 21.03HP at 60mph and 104.0HPto maintain 112mph (~180kph). Reducing the CD by 0.031 to 0.299 results in a power requirement of 19.77HP @60mph and 95.48 @ 112mph, for a net reduction of 6.2% and 8.2% at 60mph and 112mph, respectively.

If you assume that their 11% is correct and just scale my numbers up, (remember that I estimated power, not FE), you get an 8.3% improvement at 60mph.

[brick]

Uh oh. I broke out the wrenches again! I still haven't had time to dedicate to the belly pan, but I made another change that might be worth something. My car came with the factory wing, which I'm pretty much convinced is worthless from an aerodynamic perspective. It's one of those things that's raised up about three inches from the trunk lid and shaped to mimic a down-force producing wing.






Anyway, now she looks like this:





I had to fill five holes where it was mounted, so I used some 1/4 by 1" carriage bolts with cut-down fender washers and the appropriate nuts to seal everything up from the rain. The beauty of it is that I can probably get the wing back on in 15 minutes flat if I decide that the car looks horrid like this (which I don't yet.) The plugged holes aren't exactly the stuff of show cars, but I'm going for function over form here. Now there's room for a functional lip spoiler should I choose to accept the challenge of designing one.

[MetroMPG]

Damn the back of the car looks like an NSX with the spoiler! Particularly in black. Never noticed that look before.

But, um, alright for better aerodynamics!

[brick]

Yeah, I'm not going to lie. I do like the way the car looks with the spoiler, which is precisely why Honda produced it. But what I'm really trying to figure out is what impact it really has on the aerodynamics of the vehicle.

The spoiler is shaped a little like an upside-down airfoil, with the top being relatively flat and the bottom relatively rounded (blunter side to the rear.) Assuming that this shape is viable, it would tend to accelerate the air between the trunk surface and the spoiler, and directing it upward right as it exits the trunk lid surface. **BUT** That's the behavior that it would exibit in free stream air. More than likely the airfoil shape is ineffective due to flow separation and turbulence so close to the surface and relatively close to the rear window. That window is well raked, but I don't think it looks like a sufficient angle as to prevent detachment. There's also the fact that the auxiliary third brake light is a very dirty feature and the word "Accord" is written in large letters that are recessed 4-5 millimeters into the entire bottom surface of the wing. So, yeah. How about that air foil?

I also found a flow-vis image of a similar wing as used on an Infiniti G20 in that SAE publication I have. That particular wing produces down force in roughly the manner described above, but also produces a much larger wake behind the vehicle, indicating increased drag as a side effect.

[MetroMPG]

Here's what to do. I'm sure you're familiar with tuft testing.

Stick some rows of yarn on the trunk lid and figure out some way to be able to record their behaviour or have someone else drive the car and you watch it. If the yarns at the back of the lid are streaming freely back, you've done a good thing.

I think it's likely, as you say, that flow separates on the rear window. But I'd be surprised if it isn't reattatched by the end of the trunk lid. Tuft testing will tell you that for certain, and then you can feel good without doubt about taking the spoiler off.




Tuft testing a Lexus LS 400 - note separation at the forward end of the lid (tuft is lifted in turbulence) and reattached at trailing edge (tufts streaming back).

[Mighty Mira]

Originally Posted by MetroMPG

Here's what to do. I'm sure you're familiar with tuft testing.

Could you go over the theory behind tuft testing and what exactly to look for?

I figure that my main modus operandi once I start aero modding will be to do a combination of electronic timing assisted coast down testing (via a microcontroller with VSS signal input), and instantaneous FE metering.

Since we are modifying the Cd or Cd*A I figure that the best place to measure improvement is that particular spec. But if the tuft testing can provide a quick and dirty method of testing, so much the better.

Particularly, I'm imagining manipulating a coroplast boattail until tuft testing indicates that the flow is detached.

[brick]

I am familiar with tuft testing, but haven't tried it yet due to the fact that I really don't have many options for an assistant around here. (If only I were still in school...) I could probably get a good idea of what's going on at the rear glass by taking some over-the-shoulder stills with the digicam. Better yet, I just had a brainstorm about that digital camera. Supposedly it can double as a webcam, and could stream a live feed to my laptop. I'm thinking mount the camera somewhere on the back shelf of the car, stream the video to the lappy 386, check de email (strongbad fans?), capture, host, party like a bunch of physicists at a start trek convention. It might not be as good as a view directly from the rear or side, but could be about good enough, yeah? That would make it really easy to do runs with and without the wing to see what's going on with the air flow. The other reason I've wanted to do this is to see what kind of turbulent low pressure region exists on the rear windscreen, leading to possible VG experimentation.

[MetroMPG]

Originally Posted by Mighty Mira

Could you go over the theory behind tuft testing and what exactly to look for?

I started a new thread on the topic, since it's relevant beyond just 6th gen. Accords.

> Tuft testing

[MetroMPG]

Originally Posted by brick

stream a live feed to my laptop. I'm thinking mount the camera somewhere on the back shelf of the car, stream the video to the lappy 386

That's a great idea! And you can post clips too.

[brick]

Tuft testing pics are in! Not the greatest but not too terrible considering that I did this solo. All I did was rig the camera high in the rear window, connect it to the laptop via USB, and set it to capture at the highest frame rate available of one every 5s. (It's not exactly a new camera.)

First pic showing the setup: two tufts along the edge of the trunk lid, one about in the middle, and two that are very close to the glass.



Second pic showing the tufts at 60mph:


And another at 60mph:



Hooray for attached flow! You can see that the tuft in the middle of the lid pointed pretty much straight back, and it looks like the two at the edge are straight back and wrapped around as expected. The two by the glass are clearly in turbulent flow, indicating a low pressure region.

[MetroMPG]

That's awesome!! You get my vote for best post of the month.

And congratulations on the attached flow at the lip.

Lip extension, anyone?

Seriously, good work.

[SVOboy]

Dudeness, that's good stuff. I'll second darin's vote!

Get yourself a camera with crappy video, I got mine for 20 3 years ago,

[brick]

My camera does have crappy video, but for some reason I can't get the remote capture software to grab it via the USB link. If you click through all of the frames really fast it looks like a crappy video but I guess that's not the same .

Glad you guys are getting a kick out of this, because I sure am. The next time I do this will probably be an attempt to assess the low pressure region on the rear glass. (And obviously another trial with a lip spoiler once I make it.) Anybody want to see what vortex generators really do?

[MetroMPG]

Brick, I may be wrong here, but I believe as long as you've got attached flow at the rear of the deck lid, you can effectively ignore the turbulence at the bottom of the rear glass. It's a bubble, entrained in laminar flow.

Where you would want to deal with that low pressure is on a vehicle that didn't achieve flow reattachment by the end of the deck lid.

I'd skip that part and head straight to lip extension prototyping

[brick]

Prototyping in progress...slowly. I'm starting with coroplast since that's what I have handy at the moment.

[Mighty Mira]

Metro, I am no expert on this... but he might want to try some sort of gentle roof spoiler if the flow is detaching somewhere at the roof/window junction.

Again, I am no ultimate authority on this, but shouldn't drag be in large part the vector sum of pressures on the car? I.e. a bubble, even if enclosed in laminar flow, is still a low pressure region and hence you still have a region of high pressure at the front of the car that is directed to oppose the car's motion, and two regions of low pressure at the back of the car. One being the enclosed bubble and the other being the vacuum behind the trunk? Add them both together and they still point backwards.

Force = Pressure * Area (N/m^2 * m^2 or pounds per square inch * square inches).

This stuff is confusing, because I'm not sure which rule to ultimately go by, conservation of momentum or vector sum of pressure. Seems that (at least according to the NASA site) both theories are correct. I'm not sure what you were taught in school or reading science textbooks, but I got quite a bit of exposure to the incorrect "equal paths" theory explained in the link. I feel a little vindicated that it never made sense to me at the time and now I have something better to go with.

At least from the pressure point of view it is possible to verify via tuft testing that there is a vacuum at the back of the vehicle, and we know there will be positive pressure at the front of the vehicle.

Just thinking out loud as to how to explain what is the difference between a bubble and a boattail via the "Newton" method of conservation of momentum, with a boattail you only have skin friction slowing down the air, whereas in the other case the air might follow a similar path but with different acceleration due to accelerating towards the pressure... hence a net change in momentum towards where the car is going.

[brick]

I'm going to have to ditch coroplast as a material to make a spoiler out of. Metal is looking like the best way to get a working prototype, so I'm going to stop by Home Depot to look at what they have for aluminum flashing and a better cutting wheel for the rotary tool. After looking at the lid again I think it will be alright to start with a straight spoiler as long as it can curve just enough to match the crown of the trunk lid, which is slight.

[brick]

New plan: I spent my lunch break at Home Depot and found some 6" wide aluminum flashing and a hand seaming tool. The vision that I have in my head is a lip spoiler that sits on top of the trunk lid and uses the 20mm wide, 20 degree angle suggested in the aerobytes article. I'm going to have to roll the edge, maybe even double it over entirely for strength. I think I also want to use two of the existing mounting holes to restrain the ends rather than counting on mounting tape to force it to follow the curve of the trunk lid. It probably won't be as pretty as the spoiler on the new Civic coupe that I followed into the parking lot, but it should get the job done if I can wrestle it into the right form. I'm just worried that it will be short on strength, in which case I might need to go back for some heavier gauge stuff.

[SVOboy]

I can't wait to see some pictures! Did you just buy a big roll of flashing?

[brick]

Yeah, that was the least expensive way to go (under $5 for 10'). It seems really thin, which is why I'm worried about rigidity. But what I have drawn up should have some inherent strength since it isn't just one straight bend, but three bends that roughly follow the contour of the lid. Kinda hard to describe without pictures, but you'll see once I get it into roughly the shape I want. The loads on the working surfaces shouldn't be too high since it's such a shallow angle, anyway. I just need to make sure it doesn't flap in the breeze!

[brick]

Meh. So it turns out that metal working is a learned art, and I have quite a bit of learning to do! Here's my first attempt. At least you can see what I'm shooting for:





I think that the metal is a bit too thin, and I'm probably not quite using this tool right. I really need to get rid of the waviness where I've bent the metal.

[brick]

I should buy stock in Home Depot or something.

I went back and upped the ante on both materials and tools. First, the hand seamer turns out not to be the right tool to get the nice, clean, straight bends that I'm after. So the first thing I picked up was the 12" bending tool that I passed over yesterday (which was stupid because it's the less expensive of the two tools.

The other thing I picked up was a 60" length of heavy-gauge aluminum ductwork. (I don't know what it's called but it forms 1/2 of a rectangular duct.) It seems like really durable stuff, which should make it a little easier to work with and much more robust once the product is in use.

[SVOboy]

It's looking good! Better than I could have done.

Consider www.theimagehosting.com for my friendly image posting,

How have you been planning to attach this thing?