Ford Focus boat tail question

[fauxpaws]

Hello all you ecomodders!

I am new, but I have been lurking for weeks. I got hooked reading the Forkenswift and electo-metro threads (both awesome projects btw), but I am not ready to tackle anything near that scale. However, I recently read through the popular metro boat tail project, and I think something similar might benefit me.

Here are the details:
I drive a 2000 ford focus (4 door sedan) and I would like to add something similar to what MetroMPG added to his metro, but when I was drawing it up I saw the possibility of a far simpler design for my car.

Attached is a very rough picture (ms paint) of my proposal. In red is something that would be similar to MetroMPG's tail, and in blue is my proposed revision. I thought this would allow me to retain my view to the rear.

Here are the questions:
Would attaching some sort of fin array forward of the rear window help maintain attached flow in this case?
Do you think the combination of the turbulators and the smaller tail would improve the aero on this car, or would this be an exercise in futility?

Obviously, the only way to be absolutely certain is to try it and record the results. If there is a chance this could be functional I will build a prototype and perhaps do some tuft testing (but do not expect the level of detail shown in the MetroMPG thread...)

Looking forward to hearing from those with more experience. Don't hold back!

Fauxpaws

[fauxpaws]

Had trouble with the picture. Hopefully it is there this time...

[Vman455]

I've wondered about this myself, as a smaller tail would be much easier to build. But, does the air flowing down the sides of the car affect the efficiency of this as opposed to a full tail when it reaches the rear window area?

[Sven7]

Turbulators (which I'm guessing are essentially vortex generators) will work against your cause. You want smooth airflow- the point of the boat tail is to reduce turbulence as far back as possible. With the 20* angles and generous radii you should have attached, smooth flow the whole way back without issue. Tuft test to make sure.

Also be sure you make a belly pan to lead up to the boat tail to take more advantage of its benefits. You want the tail to continue smooth airflow, not start it. If you have turbulent air coming onto the tail, whether from VG's or a dirty under body, it will not work as well as it could.

[fauxpaws]

Sven7: Thanks for the reply. So if I understand correctly, you are suggesting that the flow over the rear window is already attached. I will have to tuff test that to confirm. I suspected it would not be attached, as the angle is >20*, and these vortex generators can be found on other vehicles (eg Mitsubishi Evo) in order to maintain attached flow. If I can confirm the flow is attached, then the project is that much simpler!

I was already planning on at least a partial belly pan and perhaps rear wheel fairings, in order to get that flow as smooth as possible.

[Sven7]

The 20* comment was aimed at your proposed boat tail.

The flow over your rear window is probably not attached. Flow over the roof is, though. In your diagram you show the boat tail beginning at the crux of the roof and rear window, where flow would normally separate. Instead of letting it to that, you guide it onto the boat tail and continue the smooth flow to the end.

The Mitsu has VG's to put more pressure on its wing, creating downforce (as I understand it). Engineers likely spent days in the wind tunnel tuning this and its purpose is not drag reduction. Creating vortexes takes energy and that energy is coming from the car's engine. Forget about VG's!

[fauxpaws]

Sven7: I see your confusion, you are looking at the red lines, which are similar to other tails I have seen. However, my proposal is to only build the blue lines, which begin aft of the trunk. This would obviously be less effective than beginning at the roofline, however it has the advantage of leaving rearward visibility unobstructed.

Now that you see what I am really proposing, what do you say? Is it worth my time, or is this not something that will be effective?

[Vman455]

Originally Posted by Sven7

The 20* comment was aimed at your proposed boat tail.

The flow over your rear window is probably not attached. Flow over the roof is, though. In your diagram you show the boat tail beginning at the crux of the roof and rear window, where flow would normally separate. Instead of letting it to that, you guide it onto the boat tail and continue the smooth flow to the end.

The Mitsu has VG's to put more pressure on its wing, creating downforce (as I understand it). Engineers likely spent days in the wind tunnel tuning this and its purpose is not drag reduction. Creating vortexes takes energy and that energy is coming from the car's engine. Forget about VG's!

Several forum members have tested VGs; the Mitsubishi engineers published a study on their VG design and was reported on by Autospeed in a 4-part article--the VGs resulted in a .006 reduction in Cd and slight reduction in lift, but it was never made clear if the baseline was with the wing or without. The purpose of the VGs in that case was clear, and their effect in the independent tests of Autospeed and some people here: flow does stay attached over the rear window better with the VGs, but in FE testing they don't seem to make a difference because of the slight drag penalty inherent in their use. What's intriguing about the OP's question, however, is the use of VGs as part of a comprehensive plan: keeping flow attached over the rear window and trunk to take full advantage of a smaller tail off the end of the car, the theory being that the smaller wake made possible by the smaller, easier-to-live-with tail will more than outweigh any drag created by the VGs while also improving aerodynamics more than the small tail would be able to with the air detaching over the rear window. I think it is certainly worth investigation and testing. But, I'm also no engineer--I just read a lot.

Originally Posted by fauxpaws

Sven7: I see your confusion, you are looking at the red lines, which are similar to other tails I have seen. However, my proposal is to only build the blue lines, which begin aft of the trunk. This would obviously be less effective than beginning at the roofline, however it has the advantage of leaving rearward visibility unobstructed.

Now that you see what I am really proposing, what do you say? Is it worth my time, or is this not something that will be effective?

More advantages than just rearward visibility; a smaller tail means less material and cost, less work to fabricate, and less intricate design (as you don't have to follow so many of the car's existing curves).

[fauxpaws]

I suppose what I really need to do is build it and see. Unfortunately, I lack the critical peice for testing: a Scangauge. I will proceed to build it anyway and submit pictures for your viewing pleasure. If I am feeling ambitious, I will add an array of tufts and show pictures of the flow without vgs, and with, so we can solve that debate for this vehicle at least. Perhaps we can learn something from that. Plus, it will be one more (possibly unique) way to attach these boat tail devices.

Fauxpaws

[basjoos]

Even if you don't have a scangauge, you can use coasting tests to check the effectiveness of your mods. If you have a mix of downhill coasts along your routes where you either; slowly lose speed, maintain speed, or, gain speed, then notice what change the aero mod has done to your car's performance on these downhills. As I added aeromods to my car, downhills where I used to slowly lose speed gradually changed to maintain speed, then to gain speed. Its all relative and you have no hard and fast numbers, but it is an easy way to test your progress. You have to make these coasting tests starting at the same speed at the top of the downhill and at close to the same ambient temperature and not drafting another vehicle to make valid comparisons, but this is something that is easy to do if you drive the same route at the same time every day.