Hood-Windshield Transition

[aerohead]

Originally Posted by Sularus

But He has also stated if you don't clean up the front, the back will not do much good. It is a total package, and I have seen multiple people suggest starting at the front and working your way back to realize the most benefit.

J

It's tough attempting 'one size fits all" comments.Most modern cars,vans,SUVs,and especially 'cross-dresser' (cross-over) SUVs are plenty 'clean' enough,save for the likes of Wrangler,H2,H1-type vehicles.
For the exception of grille-blocking ( standard now on many late model cars ), bellypanswheel skirts,wheel fairings,diffusers,wheel covers,the front ends of the majority of vehicles are plenty 'clean.'
If I'd done nothing other than the boat-tail to the 1984 CRX I would have ended up just about as well off.
This is really where the big money is.

[aerohead]

Originally Posted by Joenavy85

how's this look??

Joenavy,what you've imaged is a good plan.I recommend the Aerodynamic Streamlining Template as a guide for your curvatures,top,sides,and bottom.
If you get this part right,you could always add more length in the future.

[aerohead]

Originally Posted by Sularus

Then why are we talking about boattails. Working from the front to the back is both cheaper before you get to the boat tail, but easier as well. This is about hood-windshield transition.

This is what was done to aerocivic. As far as the angle goes I am not sure, but I would think an angle meeting up with the top edge of the windshield where the roof begins would help and it somewhat bisects the difference of the hood-windshield transition.
Aerocivic - aerodynamic mods for maximum fuel economy - aerocivic.com boat tail storage space

J

Walter E.Lay investigated all this and published in 1933.Hucho refers to it specifically in his 1986 book.
Lay found that for a pure pumpkinseed or one with a conventional windshield cut in ( with minimum radius respected) the drag remained un- changed.
From wind tunnel still photos of similar shapes,I'd be willing to bet a dollar to a donut that the Liberty's front end is already clean enough to ignore with respect to a boat-tail.Just my opinion.

[aerohead]

Originally Posted by Sularus

Then why are we talking about boattails. Working from the front to the back is both cheaper before you get to the boat tail, but easier as well. This is about hood-windshield transition.

This is what was done to aerocivic. As far as the angle goes I am not sure, but I would think an angle meeting up with the top edge of the windshield where the roof begins would help and it somewhat bisects the difference of the hood-windshield transition.
Aerocivic - aerodynamic mods for maximum fuel economy - aerocivic.com boat tail storage space

J

I recommend you get a peek at Hucho's book.
The doctorate-level mechanical engineers who spend an entire career in and around the best full-scale wind tunnels in the world are the ones telling us to essentially ignore the front ends of modern vehicles and focus on the back.

[aerohead]

Originally Posted by Blu3Z3rg

There is a thread here but i can't link it due to only having 3 posts yet. I will quote it instead - it's in my bookmarks.

To quote....
1. Narrowing the body at the front and rear,
2. Side window angle (22° and 30°),
3. Length of front bumper,
4. Angle of grille (0°, 12.5°, 25°),
5. Angle of hood/bonnet (5°, 7°, 9°),
6. Angle of windshield (25°, 30°, 35°),
7. Angle of rear window (30°, 35°, 40°, 45°, 50°),
8. Inward angling of rear pillar,
9. Angle of trunk/boot (4°, 8°, 12°),
10. Geometry of rear spoiler.


The effect of each of these elements on the drag coefficient was tested, then the propotional change in Cx (Cd) was calculated. Here is an ordering of those effects:

1. 46.5% - Angle of hood/bonnet (element #5)
2. 17.0% - Angle of grille (element #4)
3. 11.7% - Narrowing the body at the front and rear (element #1)
4. 11.5% - Angle of rear window (element #7)
5. 3.9% - Inward angling of rear pillar (element #8)
6. 1.9% - Relationship between rear window angle and spoiler
7. 1.7% - Relationship between windshield and rear window angles


As the above list shows, the hood/bonnet angle has the largest effect on the drag coefficient Cx of a sedan shaped vehicle. Increasing it from 5° to 9° reduced Cd from 0.52 to 0.47.

Bear in mind that all this was written in the mid-1980s.The Liberty already embodies virtually all these technologies!

[d0sitmatr]

Originally Posted by euromodder

You won't get the full potential for improvement of a boattail until you sort out everything in front.
But you do gain a serious improvement even if the front of the vehicle is a brick.


Streamlining a brick : see what the air resistance does when you add fairings to a brick .

the brick observation doesnt quite work so well with most vehicles, since they arent actually "brick shaped" your forgetting that most vehicles have a windshield and roof that is almost 24" above the rest of the body.

I think you could achieve some gain just by angling you hood up near the windshield. by raising your hood 1-2 inches at the hinges Im betting you would see a much better airflow transition. it doesnt take much, aerodynamics are very close to aqua-dynamics. air and water flow very similar over surfaces.

[fjasper]

Originally Posted by fjasper

A little rounded hump right behind the windshield might help. It looks like the roofline slopes, so a small hump could smooth the transition, reduce pressure drop, improve pressure gradient on the roof, and leave you with more energy to make the curve onto the Kammback, without increasing the profile. I've seen something like it on the front of a bus, IIRC.
...

If it's like most rigs, especially 4WD, the undercarriage is probably where you'll find the biggest easy gains, though. Air dam is quick and easy, belly pan allows for better diffuser performance and better ground clearance. Steep approach angle on the jeep should make the air dam less likely to scrape than on other rigs.

I didn't get a good look at the picture before, it looks like it already has a nice smooth windshield-roof transition. So nevermind.

If the undercarriage is very rough-exposed solid rear axle, big stuff hanging down into the airflow, I'd do belly pan first. Higher ground clearance vehicle like this, the belly pan will almost certainly work better than an air dam, plus it makes your tail treatments more effective. (Hucho)

If the belly is like on most new cars, reasonably flush and even, probably tail treatment will give the biggest gain.

[fjasper]

Originally Posted by aerohead

I recommend you get a peek at Hucho's book.

Aerodynamics of Ground Vehicles

[aerohead]

Originally Posted by Blu3Z3rg

There is a thread here but i can't link it due to only having 3 posts yet. I will quote it instead - it's in my bookmarks.

To quote....
1. Narrowing the body at the front and rear,
2. Side window angle (22° and 30°),
3. Length of front bumper,
4. Angle of grille (0°, 12.5°, 25°),
5. Angle of hood/bonnet (5°, 7°, 9°),
6. Angle of windshield (25°, 30°, 35°),
7. Angle of rear window (30°, 35°, 40°, 45°, 50°),
8. Inward angling of rear pillar,
9. Angle of trunk/boot (4°, 8°, 12°),
10. Geometry of rear spoiler.


The effect of each of these elements on the drag coefficient was tested, then the propotional change in Cx (Cd) was calculated. Here is an ordering of those effects:

1. 46.5% - Angle of hood/bonnet (element #5)
2. 17.0% - Angle of grille (element #4)
3. 11.7% - Narrowing the body at the front and rear (element #1)
4. 11.5% - Angle of rear window (element #7)
5. 3.9% - Inward angling of rear pillar (element #8)
6. 1.9% - Relationship between rear window angle and spoiler
7. 1.7% - Relationship between windshield and rear window angles


As the above list shows, the hood/bonnet angle has the largest effect on the drag coefficient Cx of a sedan shaped vehicle. Increasing it from 5° to 9° reduced Cd from 0.52 to 0.47.

I looked at my SAE Paper# 860211,by Ford of Germany,at Cologne.They investigated these areas also.
Working with a baseline 'standard' 10-degree slope hood and 57-degree windshield,increasing the angle by either raising the cowl,or raking back the windshield netted only about 3% drag reduction.
I suspect that the report of 46.5% has the decimal point in the wrong place do to a slip-up on the part of a proof reader.I believe that 4.65% would better represent their actual findings.
I believe that Carr's research,cited in Hucho's book will concur.

[aerohead]

I walked past a Liberty on the way over here.We've had some rain and the owner hadn't washed the Jeep yet.
From the dried rain water tracings left on the Liberty it would indicate mighty fine, energetic ,and fully-attached flow over the windshield header and around the A-pillars.
* On close inspection of the car I would remark that an add-on nose would allow you to seal the entire grille leaving only a shark-mouth slit at the bottom.
* A deeper,wrap-around airdam would prevent air colliding with the suspension members.
* minimizing the front wheel opening gaps would help.
* dropping the rocker panels down another 5-inches ( 125mm ) would be good.
* blending the rocker panels into full rear wheel skirts would guarantee better lower onset flow to the back.
* flush wheel covers.
* bellypan later with diffuser.
* slip-on tail of any length.( the Liberty has only a 2.425 L/H ratio and without additional length it's going to be a rough go getting the drag down appreciably.Hucho suggests L/H = 5.0 as a goal for low drag which equals L/D=2.5 in ground-reflection.The 'Template' is modeled on this protocol.
You're at Cd 0.421 right now,about same as Grand Cherokee.Wrangler is Cd 0.55,down from Cd 0.8 in 1971.