Windshield angle

[Indy Lonnie]

Are you trying to find an angle that will retain laminar flow/"flow attachment" above the cockpit, similar to what you would see with with a bodied car with a roof (and ideally a fastback)? Regardless of your angle, you are going to spill over and recirculate behind the windshield; you are just trying to bring that turbulent flow low enough so that it doesn't affect the wing as much.

Kyle.Engineers has a great youtube channel on aero and this video may help you understand the issue you are trying to solve. However, it doesn't address your main roll hoop.

Wow - good find! Basically I’m going to have to go high with the wing as my experiment showed. I’m going to rake the windshield back and see If I can lower the presently 20” height increase of the stands.
Thanks!

Pics below represent a Miata with the top on with windows up vs convertible down with windows down.

 

[Scott #321]

another option: this one was inspired by Ross and I copied for my car. Have been happy with the way it directs air away from my head and provides very noticable downforce at highway speeds. You will find that there are trade offs in reducing the air in you face and chasing the deamons of stagnant air or heat i the cab. I never had the DF windshield to compare to.

 

[SmsDetroit]

Here are some before and after with the 10deg lean back

 

[Desert Sasqwatch]

Keep going, you're getting closer.

 

[Indy Lonnie]

Here are some before and after with the 10deg lean back

Is this an increase of 10* more, or 10* from vertical? If it an increase, what is the angle from vertical now?
I never touched my windshield angle and it is sitting at 9* from vertical.

 

[Indy Lonnie]

Keep going, you're getting closer.

What is that angle from vertical?

 

[SmsDetroit]

Is this an increase of 10* more, or 10* from vertical? If it an increase, what is the angle from vertical now?
I never touched my windshield angle and it is sitting at 9* from vertical.

It is an increase from stock

 

[Desert Sasqwatch]

Same angle as the a-pillar - 33 degrees. Hmm, looks like the optimum angle for aerodynamic sweet spot for a Goblin, per @Indy Lonnie calculations.
And my line of sight is slightly above the top of the windshield - because sqwatchy.

 

[Indy Lonnie]

A good day of modification. I used a $3 harbor freight angle finder, vice and sheet metal vice grips to adjust the windshield frame. I had to grind on the lexan in a few places to git it to clear the hood. I also had to grind the corner of one side of the windshield frame to clear. I did have to redrill the outer windshield frame bolt holes about 1” towards the front of the hood. The old bolt hole was hidden by the frame. Windshield is now at 64* from vertical. Started at 81*.
Took it for a spin. The airflow is now a palms width from my head At 75 mph. The top of the windshield is a few inches above my sight line. I’m glad I stopped there. Any more and the top of the windshield would have been directly in my line of sight.

Next - I’m going to re-do my experiment and see if I have lowered the dirty air going over the car to get clean air to the rear wing.

 

[Rauq]

Did you rake the windshield back? I’m wondering how far back I can go without getting the airflow into my head.

I haven't done anything with the windshield except install it as it came to me. I believe DF raked the windshield back further on kits later than mine.

 

[Indy Lonnie]

Interesting read.

Convertible Aerodynamics

Human beings differ according to their passion, desire, and aim therefore the tools that we have used for centuries diverge as to satisfy our demands. This divergence can be seen in every field of...

sersol.weebly.com

 

[Indy Lonnie]

Quote from below: “Open top – In my aero tests at Watkins Glen, an open top reduced the wing’s downforce by 2.5x. At the time I didn’t know if this was turbulence or a change in wing angle, but at this point I’m leaning towards turbulence. On second thought, I probably won’t bother testing the airflow visualizer with an open top, because anyone using a wing and open top needs more help than I can give them.”

Visualizing Airflow for 2D and 3D Car Wings

As air moves over the top of a car, it follows the contour of the roof, changing in shape and direction. Most car wings are a 2D design, meaning they have the same shape across the entire wing, and…

occamsracers.com

 

[David]

Bringing this topic back up, did anyone ever come up with the best angle and height for the windshield? Was going to go the route of @Desert Sasqwatch and do the same angle of the cage. Then was thinking if a full screen was ever done, and if it helped with aerodynamic.

 

[Desert Sasqwatch]

Two factors will limit the angle. The a-pillar, if you have a track frame, and the clearance between your knuckles on the steering wheel and the windshield.

A taller windshield will push the air coming at it over the top of car moreso than the shorter windshield - where some of the air enters the cockpit area. It is difficult to determine which is more aerodynamic given the variation in items behind the windshield - roof panel (if track frame), rear view mirror, seat height, your head height - versus pushing more air directly with the windshield.

 

[Lonny]

From a performance perspective, if you are not running a front wing, the more you stand the windscreen up, the better the down force balance will be. It drives the air over the cockpit, chassis and rear wing, which all have aero drag.

Without a front wing and wind screen, as you increase speed the down force increases disproportionally at the rear of a Goblin.

I have done absolutely no testing to prove or disprove this theory.

 

[David]

I was thinking the same regarding the higher windshield given more down force. Would be nice to have an Aerospace Engineer do a wind tunnel test on the Goblin.

 

[Desert Sasqwatch]

Wind tunnel time is not cheap. Automotive type wind tunnels typically start at $600 per hour at the low end, $1200 per hour is middle of the road. They typically charge 4 hours minimum, plus set up and teardown time - some only work in 8 hour blocks that include the set up and teardown time. If you could book at $600 with 1 hour set up and teardown, it would cost at least $3600 for about 6 hours in the tunnel.

For testing, a baseline test would have to be established (city frame? track frame?) - with no windshield? Then what configurations will you be testing? Short/tall windshield? Different windshield angles? Side panels/no side panels? Splitter/no splitter? Wing/no wing? Different wing sizes? Different wing mounting heights? Just having a small number of variable parts to swap out would be a good sized expense.

Determining what to test and how far to go becomes the real driver to how many days you would be spending going between configurations to get accurate numbers. And is testing done at only one incident angle (0 degrees) and at one speed? A Goblin in a auto-x sees different speed profiles, than on a track or on the road. Do you test every 10 MPH? 20 MPH? Where is top end? 100? 120? 140? Just changing wind tunnel air speeds requires time for each change to normalize and establish accuracy.

Or would you just test your Goblin in its single, as built, configuration? It just costs money and finding a wind tunnel that you can schedule time with. Look forward to seeing the results.

 

[David]

Wind tunnel time is not cheap. Automotive type wind tunnels typically start at $600 per hour at the low end, $1200 per hour is middle of the road. They typically charge 4 hours minimum, plus set up and teardown time - some only work in 8 hour blocks that include the set up and teardown time. If you could book at $600 with 1 hour set up and teardown, it would cost at least $3600 for about 6 hours in the tunnel.

For testing, a baseline test would have to be established (city frame? track frame?) - with no windshield? Then what configurations will you be testing? Short/tall windshield? Different windshield angles? Side panels/no side panels? Splitter/no splitter? Wing/no wing? Different wing sizes? Different wing mounting heights? Just having a small number of variable parts to swap out would be a good sized expense.

Determining what to test and how far to go becomes the real driver to how many days you would be spending going between configurations to get accurate numbers. And is testing done at only one incident angle (0 degrees) and at one speed? A Goblin in a auto-x sees different speed profiles, than on a track or on the road. Do you test every 10 MPH? 20 MPH? Where is top end? 100? 120? 140? Just changing wind tunnel air speeds requires time for each change to normalize and establish accuracy.

Or would you just test your Goblin in its single, as built, configuration? It just costs money and finding a wind tunnel that you can schedule time with. Look forward to seeing the results.

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