Made a Kamm extension for Mirage.Should it work?

[3dplane]

We are used to seeing the template so much that one glance can tell that it might came out a bit too aggressive in both planes.







I've been driving with it for a few days but it has been windy until today.

So I finally had a chance on my way home today to do a little tuft testing.

Low light conditins did not let me get a good picture so I will have to draw it on top of one of the images to explain.

In short: Sides looked ok with the yarn straight back but on top only the center one pointed straight back. The rest on top pointed inward toward an imaginary point about three feet behind the car at the center.

Kind of like this:



It just makes no sense to me if I got the twin vortex going on making the yarn pointing inward on the top then why is it not pointing up at the sides of my contraption?

Anyhow I appreciate your input on this!

[Frank Lee]

If the tufts weren't fluttering I think you are in good shape.

[2000mc]

I bet if you could see tufts on the factory kammback they would point in similar directions

[cowmeat]

The straps you're attaching the top of the Kammback to the car with will create some drag when wind gets under them, like a bike rack strap does. They are going to negate a big part of the aero benefits, and at 60 mph they'll vibrate like a guitar string. It would work a little better with a different attachment up front.

The Kammback itself looks awesome, nice work if it will hold up! Maybe a little aggressive on the side curvature, but it looks a ton better than the one I put on the Festiva that was actually made for the car! Give it a couple of tanks to see if it has a positive affect on mpg. Mine actually gained me about 2 mpg from what I could tell.

[3dplane]

Originally Posted by 2000mc

I bet if you could see tufts on the factory kammback they would point in similar directions

Good point! I did not think of that.

Originally Posted by cowmeat

The straps you're attaching the top of the Kammback to the car with will create some drag when wind gets under them, like a bike rack strap does. They are going to negate a big part of the aero benefits, and at 60 mph they'll vibrate like a guitar string. It would work a little better with a different attachment up front.

The Kammback itself looks awesome, nice work if it will hold up! Maybe a little aggressive on the side curvature, but it looks a ton better than the one I put on the Festiva that was actually made for the car! Give it a couple of tanks to see if it has a positive affect on mpg. Mine actually gained me about 2 mpg from what I could tell.

New car and I did not want any scratches or holes drilled on it so that is how I could mount it. Good thing the strap is longitudinal to the airflow so minimal amount of interaction is going on with the wind.
Thanks on the complement! It seems to stay on pretty good sofar.

[3dplane]

Originally Posted by Frank Lee

If the tufts weren't fluttering I think you are in good shape.

They were straight with minimal amount of wiggle!
Thanks for the input Frank!

[aerohead]

If the roof inclination is too 'fast' the downwash will be held down the center by a macro version of the effect created by VGs,except the vortices are large and can contribute significantly to drag.
The tufts are pointing into a lower pressure/faster flow.
If the contour is like the 'Template',all the tufts will blow straight back,vortice free.

[2000mc]

Originally Posted by aerohead

If the roof inclination is too 'fast' the downwash will be held down the center by a macro version of the effect created by VGs,except the vortices are large and can contribute significantly to drag.

With that in mind, I noticed the back edge of the factory kammback curls up. Seen most easily in the first picture. If that lip is effective at redirecting air toward where the template would fall, could the added kammback reduce that effect, and be worse than the factory setup?

[aerohead]

Originally Posted by 2000mc

With that in mind, I noticed the back edge of the factory kammback curls up. Seen most easily in the first picture. If that lip is effective at redirecting air toward where the template would fall, could the added kammback reduce that effect, and be worse than the factory setup?

It's a good question and I'm not sure that I can give you a decent answer.
Here's what comes to mind:
*If the original contour was fast,the up-kicked edge would help with reattachment.Since the roof isn't a pure squareback,the slanted hatch can create lift,and the 'wing' could help with that (induced lift/hydroplaning at speed).
*The up-kick could also just be a styling gesture.(unless Mitsubishi published about it we may never know for sure)
*If the up-kick actually functions and the addition is too fast,then there's a chance that the flow will overshoot the beginning of the Kamm addition and reattach downstream a bit.Texas tech ran into this with their cab-wing research,as a hinge was used for the wing attachment,and as they lowered the wings,it created a bulge which actually created lift!
I don't think that the Kamm extension can hurt lift,as it removes the vertical force vector over the roof rear,moving all the low pressure into the wake,acting normal to the flow direction,like a ships stern.It also probably helps decelerate the top flow before separation,helping to raise the base pressure just a smidgeon.
I'd be inclined to keep it,'n maybe play around with angles a bit,'n see what the tufts do.
I really like how the plastic is formed.I've broken so much of it,it's nice to see success with fabrication.

[3dplane]

Well,the good must be balancing out the bad because tonight I did a quick A,A-B,B test.

There is a high point on the road on my last leg to the house,and as much as I did not want to remove this thing,it kept bugging me wether it's doing any good or not.

So here is what I did. The high point(or top of hill) is marked with the double yellow lines starting,this is where I pop the car out of gear.

Hold steady speed 60.5 MPH (for some reason easier to get than 60),get to mark,pop out of gear and coast to another landmark 0.6 miles away,then to another at an additional 0.7 miles away.

Kamm extension on first run: 60.5 MPH - 36 MPH - 15 MPH
second run: 60.5 MPH - 36 MPH - 15 MPH

Kamm extension removed:

First run: 60.5 MPH - 36 MPH - 15 MPH
second run: 60.5 MPH - 36 MPH - 15 MPH

Freaking identical so I did not see a reason for 3 runs each.

So it really doeas not do any measurable difference,therefore while I have it off,I will do some tweaking to it to raise the rear when ever I get a chance and I guess, do it again.

This might be a while though as work takes up my play time.