Maximum angle for boat tail?

[theycallmeebryan]

I was curious as to how the idea of (4 length : 1 Height) ratio would apply to my Ninja 250. So i drew a diagram in photoshop to figure it out.



I took into consideration where my head would be for the highest point.
The lighter fairing meets the 4 length : 1 Height ratio completely.
It would be 15.4 feet long!!
The dark fairing is perhaps a more realistic and streetable approach.
It would be a realistic 8.55 feet long.

Not too far off from what the prometheus EV ninja looks like.

[Bicycle Bob]

I'll bet it looks a lot better in plan view. Air takes the easiest way around, knowing nothing of theory.

[pvan93]

Try googling NACA and report 291 for a start. The lowest drag came from a Fineness ratio of 2. A NACA search for airforms and struts also brings out some interesting information that might be applicable to cross winds.

A referance somewhere (may have been here) said the nose should have the shape of a 2 to 1 ellipse, while the tail should have a shape of a 3 to 1 ellipse. Anyone know? Just bought a trailer for my boattail. Would like to verify the 3 to 1 ellipse referance before I build anything.

[theycallmeebryan]

Originally Posted by pvan93

Try googling NACA and report 291 for a start. The lowest drag came from a Fineness ratio of 2. A NACA search for airforms and struts also brings out some interesting information that might be applicable to cross winds.

A referance somewhere (may have been here) said the nose should have the shape of a 2 to 1 ellipse, while the tail should have a shape of a 3 to 1 ellipse. Anyone know? Just bought a trailer for my boattail. Would like to verify the 3 to 1 ellipse referance before I build anything.

What a great read! Heres the link if anyone wants to check this out:
http://naca.central.cranfield.ac.uk/...report-291.pdf

Thanks for the heads up, im scanning through it now.

[Bicycle Bob]

Well, that was tedious, digging out a 1927 study, but it says at NACA UK Mirror report description page
that the lowest drag was on the 4.62 fineness ratio. This was at a very low Reynold's number, which may account for all the Zeppelins being much finer.

I see that figure 13, presumably the source of this discussion, does not seem to agree with figures 7 and 9

[KJSatz]

Originally Posted by theycallmeebryan

I was curious as to how the idea of (4 length : 1 Height) ratio would apply to my Ninja 250. So i drew a diagram in photoshop to figure it out.



I took into consideration where my head would be for the highest point.
The lighter fairing meets the 4 length : 1 Height ratio completely.
It would be 15.4 feet long!!
The dark fairing is perhaps a more realistic and streetable approach.
It would be a realistic 8.55 feet long.

Not too far off from what the prometheus EV ninja looks like.

Hey I was wondering if you (or anyone!) would do me a favor. I don't have photoshop, and I can't download the Gimp or anything because I only have Internet at work right now. Anyway, I'd really like to see my Civic inside of a half-teardrop such as the one you have there. It's a 2008 Sedan; a good side view can be seen with this image:



Thanks a ton!

[rkcarguy]

I read somewhere that the magic # was 12 degrees at a time to have no separation....but I agree there is so much more to it than that.

[Hasbro]

So, understanding that there are a lot of other factors that come into play, if the top slope in pic is 19 deg. and bottom of red area is 4 deg, is this in the ballpark?

[pvan93]

I'm sorry I opened that bag of worms. Everyone seems the grab onto that 4.62 number. 291 is a good report to start with. Like I said, try searches for airforms, and another for struts. For breadtrucks (me), motorcycles, or anything that catches crosswinds like a sail, a long boattail might not be the best option.

[evolutionmovement]

What about a vertical fin for stability?