Silverado aerocap build thread

[Fubeca]

First tank with the aerocap - 15.4 MPG - dismall!!

Of course there was snow, 4wd, in-town driving, and -20F weather.

[sarcasm] I don't know why the aerocap didn't help[/sarcasm]

Next week we are having a heat wave - supposed to get up to 30*

[ChazInMT]

You guys hold a slavish devotion to this less than 15 degree thing, Just curious, do you ever consider the fact that when the airflow that is leaping over the roof of the truck, reaches the rear of the cab, it is already headed downwards at a 5 - 7 degree angle?

I think the thing to do is to put a 2 foot long piece of yarn on top of your tailgate in the middle go fast. If it drags behind your truck, then the air wants to be there naturally, drop the cap to the top of the tailgate. If the tuft whips wildly to and fro, or ends up remaining inside the bed of your rig....you should strongly consider raising the separation point up.

I have just posted two videos to Youtube of flow illustrator movies showing my truck with a 82” bed and a shortened version, I just guessed at the shortened version length. Flow Illustrator is not an exact program, so it isn’t critical that I be spot on. To that end, the real world air flow could be 8-12 inches above the tailgate, which is why a long tuft on the trailing edge of the top of the tailgate will tell us a lot.

So I ask, is this “12 degree roof line rule” based on a super streamlined shape? Or is it based on air which has just been violated by the front half of one of our Great American forms of transportation?

Take a look at the Vids. Tell me what you think.

Pax

http://www.youtube.com/watch?v=ZmxtDP8ujEg

http://www.youtube.com/watch?v=twcFjEmmLpk

[mjboks]

Originally Posted by Fubeca

First tank with the aerocap - 15.4 MPG - dismall!!

Of course there was snow, 4wd, in-town driving, and -20F weather.

[sarcasm] I don't know why the aerocap didn't help[/sarcasm]

Next week we are having a heat wave - supposed to get up to 30*

Do you have a grill block? That might help with the cold weather. I'm running into the same cold and snowy conditions that cut into the mpg but found the grill block seems to be helping a bit. No hard data to show that though.

[Fubeca]

The entire upper portion of the grill is covered with the same shrink plastic as the topper. It helps it warm up a little faster - but most of my trips on this tank are less than 15 miles.

The engine gets up to temp - barely, but the transmission doesn't come close.

[MetroMPG]

Originally Posted by Fubeca

First tank with the aerocap - 15.4 MPG - dismall!!

[sarcasm] I don't know why the aerocap didn't help[/sarcasm]

Thanks for the laugh!

(And for showing an understanding of "testing" that seems to escape so many.)

---

Originally Posted by ChazInMT

Just curious, do you ever consider the fact that when the airflow that is leaping over the roof of the truck, reaches the rear of the cab, it is already headed downwards at a 5 - 7 degree angle?

Air doesn't "leap" over the front of the cab roof and "fall" down again at the back.

Watching rain spray on the highway might give a person that impression (because displaced rain droplets are vastly more massive than air, so don't follow the same trajectory). In fact, air flow over a modern pickup's cab roof is parallel (tangental) to the roof surface. You can see this in smoke trace photos/videos.

So I ask, is this “12 degree roof line rule” based on a super streamlined shape?

I think it's based on established research results. Which apparently works reasonably well by properly applying the teardrop template that you see in this and other threads.

Nobody's suggesting it's a perfect approach. I'm no expert, but my guess is there's probably some wiggle room of a couple of degrees. But knowing how much work it takes to make a mockup, much less a "good" version of an aerocap, Kamm back, or a boat tail, I'd err on the side of a conservative slope that is likely to work, rather than end up with a shape that goes "a bit too far" and suffers detached flow, rendering my work useless.

Why re-invent the wheel? But if you really want to experimentally investigate what slope/arc works best, by all means go for it!

[aerohead]

Chaz,it's not really about angles at all but rather an angle which occurs at a particular point along a roofline or the flanks of a vehicle.
For a short-bed truck,the angle measured at the back of a cap which will support attached flow might only be 12-degrees.
A long-bed truck would be able to support a steeper angle due to it's increased length.
For both short and long-bed truck,no angle may work,unless a gentle transition of curvature is present upstream of the "angle".
Two aerodynamic templates are available for you to use at the aero forum.
The first can be found at "Permanent Kammback for Metro" and the second at "Aerodynamic Streamlining Template".
From either,you will see that the roof curvature begins almost in perceptibly at first,building greater degree of curvature the further down field.
Without the curve,your flow will likely separate immediately,with little benefit to you.
The science of pickup truck airflow is well established now since 1988 thanks to the folks at Texas Tech University and the SAE Paper they published.No guesswork is necessary today.You can copy what they've done and go about your business with a working cap or you can ignore the science and modify at your own peril.

[Fubeca]

Phil,

If you get a second to put some mental work in my direction - could you answer a quick question?

I've read your post on ambient temperature and its effect on drag and fuel efficiency.

I noticed when I did my initial ABA style testing that my cap only increased my mileage by about 4%. Does the cold dense air increase the skin friction significantly? Would this cause the mileage increase with the topper to be less in the cold than it might be in hot air?

Or, is the opposite true, does an aerodynamic improvement with a lot of surface area have an increased effect on MPG improvement in the cold because aerodynamic drag is more pronounced in the cold air?

I'm curious as to whether my mpg difference will be consistent as the weather warms up or if when the weather is warmer I might see something more like 10% increase with the topper.

I do understand the non-aero effects of the cold, and during my test tried to mitigate them by testing with and without the topper as fast as I could make the change, and ensuring the truck was fully warmed up before the test. I also do not think the test was definitive and look forward to further testing.

[mjboks]

Fubeca,

If you know the specs on you truck, there's a great ecomodder tool to play around with that allows you to adjust air density based on temperature. It might work if you work backwards from your results to get your improved Cd and then play around with air temp.
Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

Boks

[aerohead]

Originally Posted by Fubeca

Phil,

If you get a second to put some mental work in my direction - could you answer a quick question?

I've read your post on ambient temperature and its effect on drag and fuel efficiency.

I noticed when I did my initial ABA style testing that my cap only increased my mileage by about 4%. Does the cold dense air increase the skin friction significantly? Would this cause the mileage increase with the topper to be less in the cold than it might be in hot air?

Or, is the opposite true, does an aerodynamic improvement with a lot of surface area have an increased effect on MPG improvement in the cold because aerodynamic drag is more pronounced in the cold air?

I'm curious as to whether my mpg difference will be consistent as the weather warms up or if when the weather is warmer I might see something more like 10% increase with the topper.

I do understand the non-aero effects of the cold, and during my test tried to mitigate them by testing with and without the topper as fast as I could make the change, and ensuring the truck was fully warmed up before the test. I also do not think the test was definitive and look forward to further testing.

Fubeca,the additional skin friction of the cover is completely over shadowed by the reduction in profile drag.
The colder denser air this winter is going to cut significantly into your mpg.
In addition,your tire's rolling resistance is up a tick.All your lubricants require greater "pumping" power,and even when fully "warmed" may still be way below SAE "cold" temps.
I waited a whole winter for my bellypan to show up on the radar.
Winter modification can be really frustrating.
A topper like bondo has done is going to provide about a 10% mpg improvement on the highway in "normal" weather.
I'd tough it out 'til Spring and see if your mpg doesn't creep up with the warmer temps.
My T-100 will lose almost 9% HWY mpg in the winter.If you're seeing 4% in "nasty" weather,you may be ecstatic come May.

[Fubeca]

Thanks!

I'll just wait anxiously for warmer weather

I don't know all of the parameters for the online tool. I'll have to see what I can come up with.