Best Taper for Boattail on Shortbus Camper

[stcyrwm]

Hersbird, Here is take 2. Is this right? Pretty minimal taper for the first few feet:

[freebeard]

Originally Posted by OP

I am looking at doing something in the 2 to 4 foot range for length.

Let go of the Template. It has a very narrow range of application, and is not fit for your purpose.

In 2-4ft, pace Mair, l/d (~3/6) give a 50% reduction in Cd. Of this one component.

The top and sides are straightforward, with a curve, slant or box step; the bottom needs special consideration due to the wake from the rear wheelwells.

If that's your vehicle here's a drawing I did for someone else. Hersbird proved out the pillars on the front corners.

...the theory (from others on the forum):

[stcyrwm]

Originally Posted by freebeard

Let go of the Template. It has a very narrow range of application, and is not fit for your purpose.

So if I let go of the template then I need some help from someone on how to size the boattail, which I feel more confused about at this point than when I started (not necessarily a bad thing given that I might have been on the wrong track in the first place???).

The original idea was to do something similar to the RV in the thread titled "Aero RV (custom boat tail for '95 Ford E-350 Class C motorhome)". Several people commented that the angle of the curves he used were too much which brings me back to how does one calculate how to build something where the angles are not "too aggressive"?

Originally Posted by freebeard

In 2-4ft, pace Mair, l/d (~3/6) give a 50% reduction in Cd. Of this one component.

I don't honestly know what this means other than maybe I am way over my head trying to do this, because of my own confusion reading this? I do get that a 50% reduction in Cd is good but everything before that is basically like another language, that unfortunately I probably don't have the time to learn right now.

Originally Posted by freebeard

The top and sides are straightforward, with a curve, slant or box step; the bottom needs special consideration due to the wake from the rear wheelwells.

Hmmm, "straightforward" but again if I let go of the template I have no idea where to begin other than eyeballing it based in the other thread I mentioned or going back to the original percentages I mentioned???

[stcyrwm]

I am going to try asking this a different way.

From what I have learned about boattail tapers it seems they could be as minimal as 5 degrees (based on bus diagrams that are from Hucho I think) and according to Hucho/Buchheim & Mair shouldn't ever go steeper than 22-23 degrees (that is via Aerohead).

Orbywan in his "Aero RV" thread that I referenced earlier in this thread seemed to get pretty good results from a taper on his boattail even though it was over the recommended 23 degrees max.

Assuming I am on the right track here I am trying to figure out which would be the best taper if I did something similar to orbywan's boattail.

Here are the 2 extremes that I am looking at (this is view from the rear of bus/camper):



Option A is with a 5 degree taper and is more inline with the aerodynamic template.

Option B is with the max taper recommended by Hucho/Buchheim and Mair.

Obviously I could also do anything in between.

- So do I stay with the 5 degree taper that is closer to the aerodynamic template and go with option A?
- Do I go with the max recommended degree taper and go with option B, because at least in that case I have orbywan's experince to fall back on?
- Or do something else???

Thanks for any help,
Bill

[freebeard]

I don't honestly know what this means other than maybe I am way over my head trying to do this, because of my own confusion reading this?

Given a 2 to 4 foot extension (Averaged as 3ft), following (pace being italicized because Latin) the work of Mair (shown above at Permalink #2) a three foot extension on an approximately six foot wide vehicle would approximate the first data point shown. Interpolating the vertical axis gives a drop from ~0.17 to ~0.06Cd. This result must be combined (somehow) with the Cd of the other aspect of the aeroform.

Paging aerohead!

I still haven't found a good example of a stepped box. See the wakeboards here:

and this:

https://ecomodder.com/forum/showthre...tml#post603215

The general idea is that at the truncation you choose, there is a line on the Template that can be achieved with a flat/slanted piece, a curve that starts smoothly from the forebody, or a straight piece inset appropriately. The latter can be useful to exposing taillights. In each case the trailing edge is in the same place.

You bus ain't no teardrop. Instead of the Thee Holy Template, consider the NASA van.

https://ecomodder.com/forum/showthre...tml#post323990
There'll be more about it if you dig. But this profile gives the gist, and follows Mair.

[stcyrwm]

Hi Freebeard,

Thank you for your help and your patience!

The NASA van photo is definitely helpful. I could easily use that as a template for what I am doing. Interestingly enough the taper on the NASA van seems pretty darn close to what orbywan used for his RV Aero mod and he had really remarkable results according to his reports.

For what it is worth I did some rough calcs on the taper of the NASA van and came up with 14 degrees. Then I went back to the oerbywan thread and while there isn't as good a photo to measure from, his taper came out by my calcs at 16 degrees - roughly equivalent to the NASA van.

Okay, feeling much more confident to give this a stab...

Thanks again, Bill

[freebeard]

Okey-doke.

That covered, look at Taking aim at air drag again. Ignore the 20% lost to the trailer gap. You still have the proportions of front vs rear vs undercarriage. Things like the high momentum mudflap and vortex strakes apply to you situation more than something borrowed from race cars.

For the fairing for over the windshield, look to Luigi Colani's trucks.

https://ecomodder.com/forum/showthre...html#post70669

[oil pan 4]

Model it after what the tractor trailers are doing with their boat tail.

[freebeard]

Model it after a vegetable steamer.

Go from four-sided to eight-sided in the first two feet, then have a collapsible box cavity that extends from 2 to 3 feet when it's opens like flower petals.

[Hersbird]

Yeah, it looks like the template wouldn't work well for you.
Here it is on my VW showing how VW followed it, and how to continue the lines If I were to make a boat tail or aero trailer for it.

On of the guys on here is the Airflow Truck company builder. This is their latest truck and the trailer tail looks like it doesn't have much of a taper either coming off of a box.