Aero difference between 67/8 and later hoods?

[texanidiot25]

In theory, the 69 and up hoods are less aerodynamic, forcing the flow of air to separate from the body. Basically, meaning it bounces off the hood's leading edge, and falls back near the cowl. In the pages I have down on this post, page/image A4-fig.16, bottom left corner illustrates this well.

Read your hearts out: Loads of stuff about front-end design on boxy things.

Aero development on the VW Rabbit, loads of info of how the design of the nose effects air flow.
A1: http://i42.photobucket.com/albums/e3...ROFILES/A1.jpg
A2: http://i42.photobucket.com/albums/e3...ROFILES/A2.jpg
A3: http://i42.photobucket.com/albums/e3...ROFILES/A3.jpg
A4: http://i42.photobucket.com/albums/e3...ROFILES/A4.jpg
A5: http://i42.photobucket.com/albums/e3...ROFILES/A5.jpg
A6: http://i42.photobucket.com/albums/e3...ROFILES/a6.jpg

Air flow on box-shaped vehicles
B1: http://i42.photobucket.com/albums/e3...ROFILES/b1.jpg
B2: http://i42.photobucket.com/albums/e3...ROFILES/b2.jpg

[69TowRig]

Originally Posted by LONGHAIR

You frontal area has changed.....it may not have been reduced in size, but it has sloped back quite a bit. (Change)

That is incorrect. In aerodynamic theory the frontal area (Af) is defined as the total cross-sectional surface area of an object that moves through the air. Sloping the hood will change the coefficient of drag (Cd).

Originally Posted by LONGHAIR

Cowl Induction hood? Taking advantage of the "low pressure area" in front of the windshield. (vacuum?)
Deflect objects? in the air....up higher?
That whole statement seems to contradict itself? It may not make it totally over the roof, but it has to be higher.
As pointed out earlier, that is the concept behind a "bug deflector" and why the "sun visor" manufacturers tell you not to use both together.

The mis-conception here is that you are bundling objects and the fluid (air) together. Solid objects (rocks, bugs, etc) cannot change velocity and direction anywhere near as quickly as the fluid, which can roll over the surface of the hood and "pile up" at the base of the windshield, (which produces a high-pressure area and is why cowl induction hoods work, forcing more air into the intake). The air still flows over the car, the objects get bounced (deflected) up wards. The truck is not shaped like anything like an airfoil and will not produce a vacuum effect (lift) over the surface of the hood.

In the lab you could probably measure the effect of the angle of the two hoods. In the real world I would expect very little difference.

[LONGHAIR]

That is incorrect. In aerodynamic theory the frontal area (Af) is defined as the total cross-sectional surface area of an object that moves through the air. Sloping the hood will change the coefficient of drag (Cd).

Way too picky here............Like I said, sloping the hood back does change the aerodynamics. I specifically said the size of the area may not have changed, but is "effect" certainly does. You have gone all "engineer" on this (simple) discussion.

Solid objects (rocks, bugs, etc) cannot change velocity and direction anywhere near as quickly as the fluid,

No one said it did.....but is enough in most cases. Yeah, the bigger (mass) bugs do still make it to the glass, up higher, but they make it. The smaller stuff goes right by though.

[jeffspower]

While you guys are getting technical, I'll throw in what I know to be true. I have both a '68, and a '72. Both on 33 x 12.50 tires, at nearly identical ride height. The '72 has FAR less wind noise & turbulence. You can feel & hear the difference as the air comes over & around the windshield. Windows up or down, same difference.

[BigSky71]

Originally Posted by texanidiot25

In theory, the 69 and up hoods are less aerodynamic, forcing the flow of air to separate from the body. Basically, meaning it bounces off the hood's leading edge, and falls back near the cowl. In the pages I have down on this post, page/image A4-fig.16, bottom left corner illustrates this well.

Thanks for posting that info, interesting reading for sure. I especially liked that part about the rain gutter. It didn't specifically focus on a rain gutter [drip rail] at the top of the windshield but on the side glass if you try to push a big honking ledge through the air you loose efficiency...

[BigSky71]

Originally Posted by jeffspower

While you guys are getting technical, I'll throw in what I know to be true. I have both a '68, and a '72. Both on 33 x 12.50 tires, at nearly identical ride height. The '72 has FAR less wind noise & turbulence. You can feel & hear the difference as the air comes over & around the windshield. Windows up or down, same difference.

That is a good comparison, nearly same setup on both trucks, and it agrees with what I think I can feel on the highway.

[Bowtie001]

Longhair, the engineers listening perked up solely because you used the term "frontal area" which is defined as the cross sectional area. What 69towrig said was absolutely correct, there are basically three factors, frontal area, drag coefficient and speed that determine the drag force. There was indeed a change, but it was not in frontal area. He originally said there was little change to drag coefficient and you disagree if I understand correctly. Interesting problem. Easy solution. Do a coast down test with hood one (multiple passes in both directions for consistency) and repeat with hood two. This is a surefire way to eliminate all other variables, and is a time tested way to measure your drag coefficient.

-Brian (yes, an engineer)

[texanidiot25]

If some one could be so kind as to provide a large pic of the 67-68 hood, from the side (basic side profile) I can create a video to show the effect of air moving over it...

Same program as I did for this using my 69.

YouTube - Truck bed test, open bed

I can create one to compare the 67/68 hood to the 69-72. It's not 100% accurate but provides a good visual tool to compare the two.