Maximum angle for boat tail?

[aerohead]

Originally Posted by 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.

Think curves instead of angles.

[aerohead]

Originally Posted by 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?

From Mair's boattail research,you could conceivably go as steep as 22-degrees.But if you do not slowly transition into that angle,you'll lose the flow to separation.Check out his graphic from Hucho's book .We posted it recently,it's in the Phil Knox's aerodynamics photo archive.

[aerohead]

Originally Posted by evolutionmovement

What about a vertical fin for stability?

Dr.Morrelli put his under the car,behind the rear wheels for his "banana" car.

[aerohead]

Originally Posted by bombloader

Bad idea if your worried about crosswinds. A vertical fin would tend to turn the vehicle into the wind, like a weathervane. This is why airplanes have them, it keeps the nose pointed into the relative wind. You want a car or motorcycle to stay pointed down the road, not swerve into the wind. Besides, you would add drag.

If the car is going to yaw,then this is the preferred way to do it.Cars are designed such that the center of pressure is always behind the CG,so in a wind gust,should the car yaw,it DOES turn into the wind,cancelling the yawing-moment.Otherwise,as the nose turns with the wind,it catches more air,increasing the yawing-moment,and can reach such strength,that the steering is overwhelmed and you crash.

[aerohead]

Originally Posted by bombloader

I think I was little confused by Bicycle Bob's post. He was referring to Cp of the car in the yaw direction, I took it to mean in pitch which is the only way I heard it talked about in pilot training. Got it now So to make things clear, I was talking about what aircraft are designed to with the CP and CG in pitch for stability. Bicycle Bob, you were talking CP in yaw correct? Anyway, I still think I'm correct about the vertical fin though. Two reasons:
1) Directional stability(yaw) in a car comes from traction. Even race cars producing downforce are just trying to generate more traction. Therefore, CG in relation to wheel placement is probably more important than CG related to CP.
2) Observationally, I've noticed that the poorest handling cars in high winds are typically aerodynamically dirty ones like my Jeep. Cleaner vehicles seem to shrug of winds better.
Take my my nonexpert opinion for what its worth, it would seem strange if a well designed boattail actually decreased stability. If I had the problem, I'd probably question whether I actually achieved the drag reduction I aimed for.

My thought is that CG and wheel placement will have a tremendous effect on the cars polar-moment-of-inertia,and how it handles under transient loading.------------ With respect to CG and CP,auto fatalities have been strongly linked to aerodynamic instability.Front-drive cars have an advantage with low-drag forms,as their mass is concentrated towards the front of the car,and relatively low to the ground,both helping to keep the CP behind the CG,and roll-moments to a minimum.---------------- As to boattails,I've never added more than 4-feet.That was an inflated structure on the T-100.It was good for 5-mpg with a half-tonneau.

[evolutionmovement]

So the fin would be a good idea for stability after all? The MG EX181 had one that didn't add frontal area, and my design for an aero-mod Miata has one off the back of the teardrop canopy, no higher than the highest point (I want it to echo the D-Type Jaguars and so I can mount a taillight up the vertical trailing edge for extra visibility).

[bombloader]

Maybe if its not overdone. And it still seems like a bit of band aid fix to me. In other words, I wouldn't use one until I ran into actual stability problems. Keep in mind that while swerving into the wind is more desirable than swerving away, if the car already has no problem with stability in crosswinds, there is no need to make it move one way or the other. The idea is aerodynamic neutrality. Keep in mind that you add a boattail to the back of the car, your already moving the CP back. Now, if you made your boattail really heavy then it might move the CG back significantly too, but if you making mods that add that much weight to the car your kind of defeating you purpose in hypermiling I think at the beginning of this part of thread, you were thinking off adding the vertical fin because you thought that the boattail might decrease stability, and looking at what I've learned on this thread, I think that's pretty unlikely. It'll probably have a neutral or positive effect on your stability.

[aerohead]

Al Glidewell's Copy-Pro,where I use his computer, was to transition to Verizon FIOS and Pakistan couldn't communicate with India,so they mistakenly killed his internet service last Friday,and service was not restored until Tuesday.All that said,I was unable to post...................... I did locate all the related materials,and if Al isn't swamped we'll get all the images posted this coming Saturday.----------------------- Anyone with Hucho's book will already have all the data.Just look for "mirror-image" in the index and read about Paul Jaray's research and development of this concept.-------------------- And a correction.I credited airship bodies for Hoerners "shape",and while these bodies are contained within the "mix",along with ellipsoids,the specific reference I employed for the template is "streamlined bodies of revolution" listed in Hoerner's table,also in Hucho.The source for the table is given for Hoerner's book of 1965,but I found the original source in his 1951 book which is from Berlin,1944.----------------------- Also,for anyone who does not have an elementary text on Fluid Dynamics,I strongly recommend you hit the used book stores in your area and see if you can score one of these.These books will have all the tables for drag coefficients for all shapes investigated for 2-and 3-dimensional flow.See you all Saturday.

[evolutionmovement]

Part of the reason I want the fin is the visual link to vintage endurance cars and aircraft (hence also the sliding canopy), part to run a vertical taillight up it as the asymmetry should get more attention (if that will be a problem), and part expense as otherwise, I think it would look dumb as a teardrop with the back half metal and doing the whole thing clear plastic is more money and will need to be replaced due to UV at some point (I can make the simpler form in the center that slides myself). I don't think stability will be much of an issue at all with a Miata with about two-thirds the stock frontal area and a CD easily somewhere in the .20s, but this discussion raised some questions I wanted to look into. I don't mind a little compromise for aesthetics, but not if it's a drastically negative effect on the functionally.

[aerohead]

Al Glidewell is scanning my materials.He has other work to do also.I hope we can get them posted.------------------------------------------------------ I'm going to put this stuff on a new thread"Aerodynamic Streamlining Template" in the Sticky:Index of Phil Knox's Aerodynamics Seminars & Mod-data lists,where some of my other stuff is.I apologize for any inconvenience,it's just that it 's less apt to get lost there and everyone without Hucho's book will need this data.See you all over there.