EXTERNAL AERODYNAMICS

[phantomjock]

We’ve a lot of good discussions that have wound up in the “Internal Aerodynamics” thread. The topic I’m bringing is focused on EXTERNAL MODIFICATIONS, so thought I’d start as such.

Rather than ask to realign any of those items, I have done a quick catalog of EXTERNAL AERODYNAMICS.

Other Corvette External Aero Threads:


FRONT END LIFT:

Thread 'C3 front end lift (aero) question.'

Sep 9, 2009

I was going to post this in the tech section, but that area doesn't look like it gets much attention.
A fellow VM member (I'll let him identify himself if he wishes) mentioned that the lift at speed on his car is due to the bodywork area in front of the radiator. While I don't have any actual data myself, it seems reasonable that this area would contribute to the front end lift. Given that this area (actually, a volume) in front of the radiator is a diffuser, changing high velocity air into pressure (to increase efficiency of airflow through all the radiator fins), the slight pressure gain...

• 69427
• Replies: 95
• Forum: Classic Corvettes

FRONT SPOILER VS AIR DAM:

T

Thread 'Front Spoiler vs Front Air Dam'

Dec 11, 2021

The factory spoiler on my 69 is long gone. I am having bodywork done after many years of damage. So, I thought this might be a good time to replace the spoiler.

Just so you know, the car is 50% street driven and 50% track time, mostly HPDE and Time Trials.

I could get the regular spoiler as shown in the pic.
But I do run air ducts to keep the brakes cooler on fast tracks and therefore the Air Dam with holes for ducting is a plus.

I have two concerns:

1) What will they do to the handling of the car?

2) With the F-41 springs up front and the car lowered one inch, will these become...

• T-Guy69
• Replies: 2
• Forum: Classic Corvettes

REAR SPOILERS:

Thread 'Rear spoilers'

Oct 25, 2012

Has anyone done the research regarding effective rear spoilers for C3s? My working hypothesis is the spoiler would have to be above the roofline to truly be effective (don't care about whether or not it's a ricer look, but it must function)

While the gas tank is out, I want to brace the deck for a spoiler.... thus the timing of the question

• SuperBuickGuy
• Replies: 115
• Forum: Classic Corvettes

FRONT SPOILER:

Search results for query: front spoiler

www.vettemod.com

FENDER LOUVERS:

Search results for query: fender louvers

www.vettemod.com

WING:

Search results for query: WING

www.vettemod.com

DIFFUSER:

Search results for query: Diffuser

www.vettemod.com

SIDE SKIRTS:

Search results for query: side skirts

www.vettemod.com

First, I wanted to address the aerodynamic challenges of an early model C-3 Corvette.

Specifically the “flying Butress” design rear deck.

I had previously “labeled” this image, and I wanted to see if I could visit the same issues in a CF model. I have teaching myself CFD and it is a “natural” addition to my other aero background(s).

A simple model used for many CFD studies is the Ahmed. It is a relatively simple shape that students us to learn CFD and then run a model in the wind tunnel to verify and compare the results.

This model represents a C-3 Variant of an Ahmed model:



I loaded up the model int the software and ran some profiles. I used a range of speeds, and 40 meters per second is a pretty good one as it is pretty close to 90 mph. I ran the models at full scale to avoid any conversions and Reynolds Number issues.



This is the output of one of the runs:





You’ll note the similar areas of turbulence on the rear deck. Because this “slice” does not include the vehicle sides, the turbulence around the wheel mirror and outside of the rear deck is not shown.



The Baldwin/Motion modifications are often suggested as an aerodynamic choice to improve the airflow. This custom mod is an interpretation of that approach:





My block model CFD run to compare is here:



Easy to see the turbulence on the rear deck in this run has been eliminated.



Next, I wanted to see what the run would look like with a wing on the rear:

I won’t offer any conclusions regarding this run. My version of the software is limited to 1000 iterations, and it is difficult for the program to reach convergence with that limit. But it does give some general ideas. The software also restricts the number of streamlines and other bits of useful information – but it is free and a great learning tool.



One of our own forum members did a rear deck mod similar but not as drastic as a full Baldwin mod.

Here is denpos’:



“Back in the day” Ecklers offered parts to effect such a mod. One I think would be interesting and offer a CAD/CFD challenge is the rear Louvers seen on this catalog page:



Meanwhile, I have prepared a set of 18-20 variations on a C-3 early model. These include the Baldwin, a diffuser, splitter, air dam, etc. A smooth sample of one of the Motion versions is here:



For these I’ve removed all the trim, mirrors, bumpers, door handles, and blocked off the front radiator inlets. The software is much easier to run with fewer such complications. Perhaps less realistic – but close enough to compare these various mods.

I’m putting my pencils (and mouse) down on this project for a bit. I’m preparing the headers for ceramic coating. Then I need to get the aluminum cut for the belly pan, followed by some interior work. Wiring, fuel and brake lines, etc., etc. I just got the parts in to resolve a bump steer with the steering. These activities will be all in Elvira’s thread.



Cheers - Jim

 

[69427]

Jim, thanks for pulling these together for review. I'd forgotten about many of them.

The yellow '73 (IIRC) with the wool tufts is a test that I'm trying to do to my car this summer, and focusing on the side skirt area. Years ago I removed the stock under-door panels (I can't remember the correct name for them) and replaced them with lower/deeper (painted) Lexan pieces, hopefully accomplishing two objectives: covering up that unsightly transmission crossmember hanging under the car, and secondly, hopefully making it more difficult for air running along the sides of the car to swirl and enter under the car. But currently I have no data or evidence that perhaps all I've accomplished is the opposite, retaining undercar air that would have otherwise tried to exit outward. The wool tufts would give me a quick answer, but I'm a one man operation with the car, so nobody to monitor or video the area. I do have some very sensitive differential pressure gauges, but I'm not sure about getting good differential readings with the moving air.

Just another experiment on my list of stuff to do.

 

[rtj]

 

[rtj]

 

[phantomjock]

Damnit!
rtj - you just gave me another assignment!

I recall there is a relationship to the height of a gurney flap and the chord length.
Hmmm - would you wanna bet the same (or close) could hold true for body length and (NASCAR-style) spoiler height?
Added to my many items I find interesting.

Cheers mate!

 

[phantomjock]

63497: Just another experiment on my list of stuff to do.

Boy-oh - boy don't I know it!

Thanks partner for the good comments.

 

[phantomjock]

Reference the post in meme links...

 

[69427]

An item I've been interested in trying out, and had mentioned to Jim in passing quite a while back, is some sort of small cowl structure (preferably in clear Lexan) to cause some airflow stagnation at the base of the windshield. My goal would be to increase the air pressure at the windshield and base for a touch more central downforce on the car, and more drag during braking (the extra pressure at the cowl might possibly improve the airflow into the L88 hood and engine output to modestly offset the drag component at WOT in the straights). Whether any of this will be noticeable or measurable I don't know. I'm just out of better ideas to try to improve the car's performance while also keeping a mostly stock/sleeper looking vehicle.

 

[rtj]

Keeping the stock look makes it tough.

Maybe try raking the body a bit (lower the front and/or raise the rear).

The floor you put in under the engine compartment might like more rake.

How about a traditional looking chin spoiler that is really a modern splitter.

 

[rtj]

He put some thought into this. Not sure about the thick center, but the diffusers on the ends are known to work.

 

[rtj]

Here is his channel. He has more vids on front splitter and rear diffuser.

Nic's Garage

Updates on restoring / resto-modding the 1969 Corvette.

m.youtube.com

 

[phantomjock]

An item I've been interested in trying out ... is some sort of small cowl structure (preferably in clear Lexan) to cause some airflow stagnation at the base of the windshield.

Two very good goals here:
1. Increase the air pressure at the windshield; " the extra pressure at the cowl might possibly improve the airflow into the L88 hood and engine output.."
2. More central downforce on the car, improved braking {more drag during braking)

Goal 1. More Air Pressure at Windshield (BASE):
"Whether any of this will be noticeable or measurable I don't know. "
I think so. A manometer could get the info for the cowl, and pressures in the engine bay. I do recall "back-in-the day" hoods being opened (slightly) on a run to boost air into carbs. Again, measurement would tell.

I like your thinking on some Lexan - clear. Just focusing on the base of the windshield - it would be interesting to see if a Gurney Flap could make a difference. Here again - A-B-A testing. For testing you could use double stick tape and a strip of aluminum angle. For simplicity I'd jut focus on the center of the hood. Focus on the first task - looking for air to get into the engine bay. I'll dig out the ration of height vs location. The difference between 0.75" and 1.0" might be noticeable. As well as location for and aft on the hood centerline. Further forward shorter Gurney Flap and taller the further aft you go. Good time to do some Tuft Testing. This would show flow - but not pressures. With either the manometer or tufts - Be Careful driving with a camera!

Goal 2. Central Downforce:
"I'm just out of better ideas to try to improve the car's performance while also keeping a mostly stock/sleeper looking vehicle."
The most simplistic, and unseen change to increase downforce is under the vehicle. A splitter with dffuser sections will create 2 - 4 times the downforce of a "plywood splitter." The splitter could be fabricate to have the silhouette of a Pace Car Splitter, and the added benefits of "sorta" canard ends, as suggested by rtj. A full belly pan sloping down from front to rear, lowest point where you want most DF (close to CG). Slow rise to the rear with maybe a "break" to 23 degree (max) at the rear tires to exit? Not optimum, but might get most of what you're looking for. The addition of some rake is always (almost) good. Again, shout-out to rtj.

Somewhere I have a sketch of the inverted airfoil on a C-3 belly pan. For now, consider this as an approximation:

You can visualize the slope from the front under engine ( the dotted lines) to the lowest point and rise again to the rear. A lot of effort, adds some weight. Interesting possibilities to consider. I too am nearing this possibility as I am dropping the bellypan and floor 2 inches. I am looking at how much slope I can get from the front , under the engine, steering, etc and then aft to the rear suspension. etc. If I can't, at least I'll try and make it flat, with rise in rear.

The "more drag during braking" could occur if/when the nose dives during braking. Again from just an aero perspective, the "shaped" belly pan would be seeing a slightly larger angle of attack and could introduce some more lift - or stall. In either case thee would be more drag - making a minor contribution to slowing the car.

A lot there. Hope you can find some useful nuggets.

Cheers - Jim

 

[phantomjock]

I'll add a few points. First found my chart:



Then I had a bit of thinking and sketching:


So I think worthy of a quick tuft test, short angle double stick tape and see. May not get the extra pressure at the base of the wind screen - depending on engine bay pressure difference. The turbulence may be re-energized enough to go where you want. If/when I have a moment, I'll try a cfd. But given the restrictions on the free version, may not show much pressure increase.

Here is a sample that you can "fiddle with." It is one I ran a few months back. It shows a nice "trip" point [yellow to green] on the hood for a forward most position for a gurney. Then look at the area just aft of the rear deck spoiler. notice how much lower the pressures are there. Might happen at the windscreen base that way too? Testing required.


As a side note - you can see the general flow beneath is really "asking" for some sculpting. This was basically a flat bottomed diagram/model.

Cheers - Jim