Rounded corners vs. Straight edges on kammback

[drmiller100]

so, in another thread aero, myself and the "clan" talked at length about angles you can run on a car without danger of separation.

the consensus seemed to be 20 degrees on the top and sides was "safe", and 10 degrees was "safe" on the bottom.

Further, the consensus, as I understood it, if you have 20 degrees on top/sides, and have MINOR disturbances, the air MAY reattach, but be very careful of the disturbances.

If you run greater than 25 degrees no matter what happens in front of the angles, you are pretty much guaranteed the air won't reattach for small disturbances, and there are studies which show 25 degrees you can't even guarantee the air stays attached without the disturbances.

It seems to me the SMALLER the back of the car, in general, the less drag. So, a reasonable goal to me is to have as small a "back" of the car as reasonably possible.

To have as "small" of a back fo the car, you should run the steepest angles you can safely get away with.

Those are some basic fundamentals for building a boat tail In MY OPINION.

So. when you go to DO something on a car, you need some guidelines or places to start. It is all fine and good to make fun of me. It is all fine and good to make fun of my name, or my education, or whatever, but that kind of stuff does not help anyone DO things.

My goal was to point out things to really focus on which are IMPORTANT. I don't think the angles the tops and sides meet are important, and I don't think a rolled edge vs a sharp edge is important, but that is my opinion, and there are no studies which indicate I am significantly wrong in that opinion.

There are PLENTY of studies which show greater than 25 degrees is horrible. There are plenty of studies which show IN GENERAL the smaller the back of the car, the shorter the trailing vortices, the less drag there is.

One of the goals in any scientific or logical discussion between people wishing to learn is to NOT attack the people making the opinions. Calling someone a big meany is a waste of everyone's time, but for sure strokes the ego.

is the goal to stroke the ego, or is the goal to BUILD something, or is the goal to LEARN something????

Neon has challenges, as he is DOING. When you DO things, you run into lots and lots of variables, most of which are adjustable. Which variables are important? Critical? Pretty, but functionally unimportant?????

I've built a LOT of cars, and theory is all great, but building something which WORKS is much harder.

Best wishes to Neon - keep it up, good job, and please report back your findings. What you are BUILDING is very hard - the vast majority of readers on this site, and even the vast majority of posters to this thread will never undertake any project as hard as you are doing.

Thank you.

[freebeard]

aerohead -- The (possibly apocryphal) story I heard was the on his test flight, he couldn't figure out why he wasn't taking off so he looked behind and saw that the augmented exhaust was conforming to the fuselage and pointing like a blow-torch at his tail surfaces. He pulled back on the stick and hopped over the wall of the city of Paris and made a landing he walked away from. If it's not true, it's still a great story.

drmiller100 -- Well said. “If we knew what we were doing, it wouldn't be called research” ― Albert Einstein

[MTrenk]

lol, I'm just trying to urge you to seek out more education on the subject of aerodynamics.

Having an angle greater on top than on bottom will cause lift. Lift is not good on cars in general, and lift creates drag just as much as downforce does. I choose downforce, it likely won't get me killed as easily in crosswinds.

It is more efficient to keep all curves smooth, especially when no one here is going to do a CFD analysis of this design. Airfoils are curved. I would recommend we look at successful aero projects (i.e. NASA, Boeing, extreme eco cars, top speed record cars) and see how many hard edges to an arbitrary angle we see. Yes you can do tricks with hard edges, but you probably don't know how to do that unless you are a physicist, mechanical, aerospace, or computational engineering, or a really bored homeschooled kid.

How do you suggest someone be 'safe' about the potential disturbances of having a hard angle downward at 20 degrees behind the car?

Hopefully you read that question and realize that your 'opinions' on how aerodynamics work are not helpful, and could potentially be hurtful to those who also don't know anything about aerodynamics.

However you are completely right about the smaller back. Kammback = chopped airfoil.
The closer you are to an airfoil shape, the more efficient you are. Not an opinion, a well researched theory that has plenty of evidence (not a fact/proven, just well evidenced).

However at '25 degrees you are pretty much guaranteed the flow won't reattach,' your opinion is meaningless. I guess you haven't studied how car designers manipulate the different types of flow at the boundary layer: laminate and turbulent. This is where those hard line and vortex generators come into play, designed by people with engineering degrees and hundreds of hours of experience in CFD and wind tunnel research. Neither you nor I are capable of creating these effects reliably, but I'm planning on gaining this skill in the future, and actively pursuing knowledge on these skills.

I could go on forever and ever, but my point is not to say you are wrong. The point is that you are providing incorrect information that could potentially lead to a poor design that might in fact harm mpg for Steve's Neon. Pretty sure all Chaz cares about is proving you wrong. All I want to do is encourage you to increase your education, or stop spreading your black magic 'opinions' on how you've decided aerodynamics work without doing any testing of your own.

As far as the building thing goes, I'm the Project Manager for my school's Formula SAE team. We design and build our cars from scratch. If anyone shares your sentiment about building something, it's me. There's nothing like seeing a vehicle you helped design come to life, built by your own hands. In the competition there is a fuel economy section, so you have to balance speed and economy to get the highest points.

Right now my life is centered around learning about vehicle dynamics, and I joined these forums because I know that I can help people learn amazing knowledge that will help them achieve their dreams, like I'm doing right now with no money, no car, but thirst for knowledge. I don't have an ego. I can fit all of my belongings in one of those fold up cubbies you hang in your closet that act as shelves. lol

[Frank Lee]

The "good" angle on the top IS greater than the "good" angle on the bottom.

[MTrenk]

Yes, and this creates lift, which is not really ideal.
I like the 'good'. It's really convinced me...

[Frank Lee]

Don't take my word for it. Check your Hucho book, or EM, any good text, or your Great Uncle, that says the max angle for underbodies before separation.

[MTrenk]

Max angle based on what type of body shape? Airfoil? Large SUV? Small car? Solar panel car? This "max angle" can only be defined for each unique body that exists. Sure lots of road cars are similar, but I bet the Ferrari 458 has a different "max angle" than a Ford Focus.
Certain generalizations can be made in aerodynamics. This is not one of those instances.

[KamperBob]

Airfoils don't have to worry about ground effect. Road vehicles do. Hucho's compendium gives the topic much attention.

[drmiller100]

Originally Posted by MTrenk

Max angle based on what type of body shape? Airfoil? Large SUV? Small car? Solar panel car? This "max angle" can only be defined for each unique body that exists. Sure lots of road cars are similar, but I bet the Ferrari 458 has a different "max angle" than a Ford Focus.
Certain generalizations can be made in aerodynamics. This is not one of those instances.

If Neon wants to build something, or Basjoos wants to build something, or one of us who DO things rather than just ***** about it, we need a generalization to start with.

And, 20 degrees on top and sides is a good place to start, and 10 degrees on the bottom is a good place to start.

Go ahead and *****, or go ahead and offer a better angle to start with.

[drmiller100]

Originally Posted by 2000neon

What I am thinking for the hinge is two hinges spaced reasonably far apart at the top of the window. .

do you have a pick a part locally?

94-95 honda accords have a rear trunk hinges which "articulate" such that you can mount the hinges underneath the kammback and they will lift the kammback up and away. they come complete with gas struts to help you lift and hold the kammback up.

it might be you have glass right where the hinges would go - just trying to think of ideas......