Advice on aero-cap shape

[hat_man]

Thanks Big Dave you've got me thining again. Hmmm.... use the hurt bed for the aerocap template to get a working model and make adjustments, then......hmmmm. Now my head hurts! Need to get out the sketch book again.

But on a serious note...does the 10* transistion go for the sides as well as the top?

[ChazInMT]

It was 4° (Press and hold Alt while you type 0176 then let alt go for a °) plus 6° for 10° total. The break should be be cheated forward a bit to more closely follow the template, I didn't figure exactly how far back it was in inches but it looks to be about 40% of the way.

Originally Posted by hat_man

But on a serious note...does the 10* transistion go for the sides as well as the top?

Yes. The sides should not taper in faster than the template


It all looks like you're on the right track except for the sides, you probably want to go from 60° to 30° front to back instead of ending up at 15° at the gate. I think that would optimize it just based on my "Spidey Senses".

[Sven7]

Here's a true side view with the template laid over it. This will be more useful in actually constructing the structure than looking at photos with perspective.



Sorry I couldn't get a plan view- it appears all the other blueprints on the net are the Euro-spec Ranger which is very different from ours.

FYI I have an SAE paper here from Texas Tech (#881874) that says an Aero Shell should net you 10-20% drag reduction. This will probably translate to 5-10% MPG improvement. With that and a nice smooth coroplast belly pan you could probably get 10-15% MPG over the stock body.

I agree with skyking though. If you could build a metal framework off of the frame it would use similar techniques as you'd use for the topper, but on a larger scale. Skin it with coroplast, sheet plastic or sheet metal. Having a smooth side will help you a lot and custom building the entire rear half could REALLY help you out!

[t vago]

I've been re-thinking this idea. It's counterproductive to have a significant downward angle on the cap, without having similar angles on the sides of the bed. The downward angle is going to cause the top air to also move down at an angle, and it's going to react with the air moving on either side of the truck, and it's going to cause power-robbing vortices to form on either side of the truck's rear end. I've seen this happen with my own aerocap.

I'm going to design version 3 of my aerocap pretty soon, based off the master's Naval Post-Grad School paper that LT Williams wrote...

http://www.dtic.mil/cgi-bin/GetTRDoc...c=GetTRDoc.pdf

I am going to do this because, using LT Williams' numbers, he would be able to get a 13% increase in fuel economy using only his aerocap. This is better than the 10% I am currently seeing with my version 2 aerocap.

[Sven7]

Will there be strange vortexes coming off the rear edges of the bed? I've thought about this and refrained from it because the bed will protrude from the aero shape. With a generous radius on a normal aero cap it shouldn't be too bad. Can't be worse than an open bed!

[hat_man]

T vago...That is the shape I had envisioned to start and then changed to increase the slope on the sides. I know nothing about aerodynamics and generally fly by the seat of my pants. The image you posted looks like the vertical angle at the top (nearest the cab) reamins fairly constant all the way down the sides. I "erased" some of your image and came up with this. With my bed being 10" wider at the cab than at the tailgate I'm not sure it exactly matches our "combined" picture. But if I made the rear match the tailgate width and the front match the cab width and kept the vertical side angle (still looks like 60* to me) the same for the lenght of the bed, and then the 4* slope for approx. 30" (in bed length) and then 6* more for the remaining 42", [whew, gotta catch my breath] how does that sound?

I'm not sure what the green area is, but it sure would make a nice cut out to be able to see the 3rd brake light.

Chaz and T vago...just to open up some discussion...I thought gradually sloping the sides inward (from 60* to 15*) would allow the air to flow better off the back. I remind you both that I know absolutely nothing about aerodynamics. It just looked "swoopier" to me. (You can both laugh now.) What about maybe like Chaz says and transition from 60* at the cab and graduating to 30* at the tailgate? The graduation being at the point that the top slope changes from 4* to 10*. I'll mark it with white spots on the pic. Is the problem with the air from the top (moving downwards) and the air from the sides (moving rearward) meeting up and causing problems? Does keeping the side angles at approx 60* for the full length of the bed negate this? I don't have a clue.

It sounds like I have the top slope pretty well worked out. Now the question is on the sides. I'd love to hear what you two think or anyone else that is interested. The more the merrier. Maybe someone else will be watching this thread and beat me to the punch and finish before me. LOL

And are there any thoughts on the other pic? If I keep the cover square and leave the 5" overhangs over the bedrails will this cause problems with trapped air or anything else? Or is my tapered idea better?

Thanks again to all.

p.s. Chaz...I tried the degree symbol thing and as soon as I hit the 1 it jumps to the top of the page. ??? Maybe the * will have to work for now.

p.p.s OK got it now. Grrrrr. Gotta use the number pad and not the numbered keys above the letters.

[hat_man]

Ok so I goofed...try this pic instead.

If both the "ribs" on the white dots are at 60* and the "rib" on the yellow dots is 30* then it remains constant at 60* between the white ones and gradually transitions to 30* along the reamining length of the bed.

Does that help?

[t vago]

From an aerodynamics standpoint? Transitioning from 60* sides to 30* sides should be at least somewhat beneficial, as long as the transition is gradual and there are no sharp edges. From a fabrication standpoint? Could be a challenge.

[aerohead]

Originally Posted by t vago

I've been re-thinking this idea. It's counterproductive to have a significant downward angle on the cap, without having similar angles on the sides of the bed. The downward angle is going to cause the top air to also move down at an angle, and it's going to react with the air moving on either side of the truck, and it's going to cause power-robbing vortices to form on either side of the truck's rear end. I've seen this happen with my own aerocap.

I'm going to design version 3 of my aerocap pretty soon, based off the master's Naval Post-Grad School paper that LT Williams wrote...

http://www.dtic.mil/cgi-bin/GetTRDoc...c=GetTRDoc.pdf

I am going to do this because, using LT Williams' numbers, he would be able to get a 13% increase in fuel economy using only his aerocap. This is better than the 10% I am currently seeing with my version 2 aerocap.

The Ford/Texas Tech 'aeroshell' with straight sides was good for 20% drag reduction and 10% mpg @ 55 mph.
My shell,with plan-taper gave 13% mpg,so reverse-engineering yields the 26% drag reduction.
So the numbers are pretty consistent.Members just need to decide how much money and human capital they're willing to invest in a project.

[aerohead]

Originally Posted by Sven7

Will there be strange vortexes coming off the rear edges of the bed? I've thought about this and refrained from it because the bed will protrude from the aero shape. With a generous radius on a normal aero cap it shouldn't be too bad. Can't be worse than an open bed!

I tufted mine and it showed nothing weird.The velocities must have been similar enough,top and sides,to prevent them.