Aerocap Performance (Why are we seeing wide variation in reported MPG savings?)
 

In the past two months a couple of people here have built aerocaps and have posted some short term test results. There has been an A-B-A test posted by one of them and this data has been posted on the home page of ecomodder as being conclusive, a 4% increase in fuel efficiency.

In all fairness, I have made claims on the other end of the number line, a 20% increase in fuel efficiency. I should have reported this percentage as being able to achieve as high as a 20% increase in fuel efficiency, which I truthfully have on a few occasions. I am sure if I were to perform an A-B-A test this number would not hold and my percentage would be lower.

The wind tunnel testing I have had done on the Aerolid is all I can offer. In the A2 wind tunnel test the Aerolid achieved a delta Cd of -0.062. This test was with the lid on my 2006 F150 which Ford says has a baseline coefficient of drag of .42. I plan to do an A-B-A test but not until the weather warms up.

So as I try to get the Aerolid to market, the incorrect 4% fuel efficiency improvement which greets one who signs onto ecomodder and sees Fubeca's A-B-A result, which in all fairness to Darin, I feel is extremely flawed due to atmospheric conditions at the time of the test. The following information was compiled by Darin in 2007 in a report on how cold weather effects fuel efficiency and he claims aerodynamics are effected by the density of cold air.

http://ecomodder.com/forum/showthrea...leage-220.html

Within this thread you will find a link to an earlier thread at MetroMPG.com in which data he compiled is presented showing the declination in fuel efficiency as air temperatures lower. He also goes further in reason number 9, Aerodynamics, how aerodynamic drag increases by 2% with every 10 degree drop in temperature.

I owe Darin and and the good folks here at ecomodder a great amount of thanks for getting the Aerolid alot of exposure. Inversely, a reported 4% gain in fuel efficiency is all an aerocap will get you is not the kind of exposure you want if you are looking to get an aerocap to market.

I cannot offer an A-B-A result yet. All I can offer are the two wind tunnel tets which were done on the Aerolid. I can also offer my road test results which showed a solid 15% increase in fuel efficiency during the two years I ran the Aerolid on my 2006 F150. Check out round.boater's fuel log since he got his Aerolid in May of 2009.

So Darin let's work together to get closer to the truth. Fubeca and Chaz have worked hard on their lids and I hope they do not get dismayed by poor fuel efficiency results due to cold weather. If they can hold out until spring, they will be pleasantly surprised.

Bondo

The wide variation in results puzzle me. My results were more in line with Bondo's and my lid is crude, crude, crude.

Methinks we have a wide variation in test methods. Testing in mid-winter invites wide variability.

Bondo, if we are to go by your wind tunnel test, if its a .06 decrease in drag from .42, does that mean a 14% improvement in aerodynamics?

I'm looking forward to buying an Aerocap for my 2009 NISSAN FRONTIER, eh hem, even single-digit improvements would be a lot more than I can accomplish with wheel discs and grill covers.

Hi Brett -

First: let me say I have huge respect for what you're doing and I hope you become a zillionaire because of your cap. I praise it to everyone I talk to about pickup truck aero.

The Cd info you provided is fantastic:

Regarding Fubeca's number, don't forget we have to compare apples to apples, and we're not in this case. This is important:

1) He tested at a much lower speed than you did! At 55 mph, this is reflected in the smaller % difference he measured vs. the % difference you will see at 75 mph. If he had tested at a slower speed it would have been even lower (less than 4%), and if you had tested at 85 mph, your % results would have been even greater.

2) He's using a different truck,

3) His cap design is different from yours

4) Neither your or Fubeca's tests is "lab quality", so let's not get hung up on the decimal points, if you know what I mean.

All of these things make it impossible to directly compare his results to yours. (Also I don't think the home page suggests that all caps are created equal or that everyone should expect only 4% from an aeroshell.)

Unless you're testing the same truck at the same speed(s), in the same conditions, it's apples & oranges.

But we can work with your numbers:

If we plug the values for your truck into the aero/rolling drag formula for calculating power requirements across a range of speeds, here's what we find:

Using your vehicle (Brett's 2006 F-150)...

Weight: 4,840 lbs
Base Cd: 0.42
Cd with aeroshell: 0.358
Frontal area: 33 sq. feet (* this is an estimate: I couldn't find the value online, and EcoModder's CdA list has a value of 31.5 sq. feet for the previous generation 2004 F150 Lightning, lowered and probably slightly smaller. For comparison, a 2002-2008 Ram 1500 is listed at 35.1 square feet.)
Crr: .0125 (estimate)

Based on the Cd difference (and adding 150 lbs - est. - for the aero shell), let's calculate power requirements (in Watts) and look at the % differences to travel at 55 mph and 75 mph, at 25C and -10C (taking air density changes into account):

• Stock truck Cd 0.42 (open bed)
  
  @ 30 C / 86 F on summer fuel (click the links to open the tool with all values entered)
  55 mph requires 17,758 Watts (~21.7 MPG US)
  75 mph requires 37,271 Watts (~14.1 MPG US)
  
  @ -10 C / 14 F on winter fuel
  55 mph requires 19,452 Watts (~19.5 MPG US)
  75 mph requires 41,566 Watts (~12.4 MPG US)

• With aeroshell Cd 0.358 (plus 150 lbs - est.)

@ 30 C / 86 F on summer fuel
55 mph requires 16,319 Watts, 8.1% better than stock in same conditions (~23.6 MPG US)
75 mph requires 33,381 Watts, 10.4% better than stock in same conditions (~15.8 MPG US)

@ -10 C / 14 F on winter fuel
55 mph requires 17,763 Watts, 8.6% better than stock in same conditions (~21.3 MPG US)
75 mph requires 37,042 Watts, 10.9% better than stock in same conditions (~14.0 MPG US)

Big Dave, I think you're right on both points.

numbers
 

Brett,I come up with same modified Cd as Darin ( 0.358 ).
From Kelley & Holcombe at GM in the 1960s,they'd say that the 14.8% drag reduction would net you a 7.4 % mpg increase at 55-mph,and 8.8 % at 70 mph.
These relationships assume a constant BSFC which in the real world does not exist.
The other context,is that when this research was conducted,pickup Cds were in the neighborhood if 0.55.
In 1988 when Texas Tech published their paper,it was right after GM's introduction of their body in white,all-new pickup,which cut the Chevy and GMC drag from Cd 0.535,to 0.45.
In Tech's paper,they mention that the aeroshells benefit can be matched by a re-design of a pickup.
It's probable that modern trucks may not respond to aero mods as was seen with the F-150 tested by Tech.
The only claim they can make, is that an F-150 of that era,would see a 20 % drag reduction with the aeroshell.And this would be under SAE test protocals,with results normalized to standard temp and pressure,something very difficult to carry out.
Your shell mirrors the architecture of the aeroshell so closely,and respects parameters understood very well,for going on a century now,that as far as 'performance' your shell would have to fall at the high end of the curve.It's just that good.
To learn the most from our mods,we would have to do as comprehensive as testing as SAE.Short of that,we're going to see some 'scatter'.

I learned something new today.
 

Thank you Darin for taking to time to post very imformative information that I had not calculated. I suffer from being extremely right brained and my math skills are lacking. I wish to thank you for telling people about the Aerolid also. One truth is that financiual gain is a motivator to me but it is not the only motivating factor I have to see the Aerolid go to market. I would love to create jobs for others and a job for myself. After being out of work for 15 months with no unemployment, you realize the value of a job.

Again, I have learned something today and it all hinged on divulging the delta Cd number. I wish now I would have done this much sooner. The points you make about the differences in the aerocap construction, speed and truck type between my truck and Fubeca's definitely would produce a variance in the test results.

I agree also with Big Dave on how we do have a wide variation of test methods which do produce wide variation.

What would be so cool is for us all to get together at the A2 wind Tunnel for some testing. They are a great bunch down there and they were just charging about $350.00 an hour for testing which isn't bad. That is how I could afford it. We could fugure out what kind of test we would want to do, not just, pickups, and get to meet each other and have alot of fun too!

Again, thank you for running the numbers for me on my truck with and with out the Aerolid and with a variance in temperature. I have learned alot in my years in design but one thing I found out is you can never know it all, you learn form others. As this post is titled, I learned something new today.

Thanks again,

Bondo (Brett)

Thanks Phil!
 

I appreciate you running the numbers on my truck also. You and Darin both have a better developed left hemisphere of the brain than I. I must then contribute the higher percentage of fuel efficiency I experience in my truck, which tends to be much greater than 8.8%, to my driving habits.

The human factor is a very important element to the operation of any machine designed to be operated by a human. In that operation, certian controls can be excercised to achieve a desired result. This result, though influenced by the human factor, is still a real world result, though it may not be by the numbers.

Thanks,

Bondo (Brett)

One more thing Aerohead.
 

It does make me feel good when you said the architecture of the Aerolid follows correct aerodynamic parameters. In tooling the Aerolid, it was pure sculpture and nothing else. I had no information to go on. I am very fortunate to have arrived at an aerodynamically functional design purely by chance.

When you posted you're now legendary aerodynamic streamlining template, I immediatley copied it and overlayed it upon a picture of my truck. I was very pleased, and relieved, to see the curve of the Aerolid matched the template so closely.

I appreciate all you have done here on ecommoder and without people like you and Darin who understand the numbers and calculations of Engineering, there would be no progress. Sculpture is art, form can be functional. Aerodynamics is a wonderful blend of art and Engineering.

Bondo

Art plus engineering, I agree!

But I don't think it's a fluke that some designers are able to sculpt flow-friendly shapes without the benefit of CFD or training (or template overlays). Aero isn't necessarily intuitive, but some people just seem to get it. The Varna Diablo comes to mind as well.

Airplane people have a saying: "If it looks right, it probably is right."

Viz: The LanceAir.

But this is not always true at the lower Reynolds numbers that cars and trucks develop. Viz: The M-B "Boxfish."

I have to say I'm a little sad that my topper is on the home page with bad pictures and bad testing :o (but that is just my pride speaking). It does make me happy that a person can make it in a day +/- with cheap materials and basic tools.

I'm also pleased that a person can get real MPG improvement. I'd just like to be able to clarify how much and at what speeds.

I really need to do some more rigorous testing at a higher speed (typical freeway speeds). I would also like to do better tuft testing and perhaps make a few tweaks to clean up some airflow.

My butt-o-meter says ~15% is probably about right for my cap. I'll do A-B-A if anyone in Denver would help me lift the thing on and off. One of the big benefits for me is that I can take it up to 75 without watching the mpg's completely crater. (And FWIW, I'm still the slowest vehicle on the road where I do that)

Bondo, when are you going to design an underpanel that will bolt into the stock running board / step bar holes and fill the gap between the frame and the body?

Still waiting for someone to build a 50 mpg car that can tow 5000lbs and has room for me to sleep in the back...

I agreed when Big Dave said testing in winter conditions is challenging... but I also didn't see anything in the description you wrote up of your test that jumped out as being "bad".

(Or I wouldn't have posted it on the home page!)

I really wish I had time to duplicate that test and do some more testing at a higher speed. With the temperature and the crosswinds, I just don't know how valid the test was.

Of course it could just be the sour grapes of my dashed hopes :)

That's a very tall order, and part of me wonders if it's really necessary. I'm pretty sure it would not be possible with aerodynamic improvements alone.

It might be possible with a small donor truck, a large budget, ride height reduction, an aerodynamic rebody, and an open mind about acceleration and the appearance of the finished product. I'm picturing a chopped and teardropped Ranger with a low, feet-forwards seating position and an imported 2.0L Ford diesel.

However, the more reasonable thing to do would be own a 20mpg truck for towing your bulldozer, and a Prius for commuting, sleeping in, and towing 1000lbs.

How about a Prius, which already gets 50mpg, slightly basjoos'd, with an adjustable rear suspension, tougher frame / hitch, and just enough power for the load? It doesn't seem that far off. In the mean time I'll do what I can with my truck. I finally found a coroplast supply here in Denver so I can start working on the underside in addition to the top. With the Aerolid already on, I'm not sure there is much to be gained north of the wheel wells...

Something that AeroHead alluded to in his post, is that wind tunnel testing numbers do not relate exactly to real world numbers.

Let me give you an example...

I have been performing coast down testing for the last two months. One thing that gave way to the research was that:

At 55mph, 65% of the drag is due to air resistance
35% of the drag is due to Crr

The faster you go, the more the ratio shifts to air drag.

If one sees a 20% air drag benefit in the wind tunnel, then the only way that this will show a 20% benefit on a test track is to be going about 200 mph were the air drag portion swaps out the Crr portion.

As you slow down from this speed then Crr starts to become are larger factor again, and reduces the affect of the aero gains from the mods.

Jim

Crosswinds? Which direction? That could at least change the symmetry of the flow field.

I agree with Darin's apples-oranges point. Not only different shaped caps, they are sitting on different trucks. I would expect the latter to have the bigger effect. My 2c.

Cheers
KB

I've read people saying that rear wheel skirts did HURT their FE. I've read people saying that adding side skirts and/or a belly pan didn't change a thing. But I've never seen anyone not reporting FE gains with an aerocap. I think that fact alone speaks for itself. There is a concensus behind such a product, but of course YMMV depending on how you drive.

The crosswinds were not quite perpendicular to the road I was driving. I did run both directions and average the runs.

Thanks for keeping your fuel log up to date.
 

You have been much more diligent than I on keeping a fuel log. Thanks!

Funny you say that about the side skirts. I have a scale clay model done of a design for some and plan on constructing a set this summer.

Glad you are enjoying the Aerolid,

Bondo

Something which bears mentioning I think, or at least emphasized. The variations among "Pick-up" trucks are amazing. Since I've taken an interest in the aerodynamics of pickups (and really anything that moves through the air now, truth be told) I am amazed at the height differences, wheel/tire size differences, 4x4 vs 2wd, and cab/bed length differences. There is certainly no "One size fits all" remedy here for an aerocap advantage. I believe my extended cab 6.5 ft pickup really only has 15-17% room for improvement if I were to design the ultimate aerocap. I'm certain I have gotten better mileage with my cap. On my cross country 2500 mile trip I averaged about 16.2 mpg. Granted I was much more careful about my driving habits, I averaged 8 mph less on the whole. But I usually would have expected to see 14.5 mpg for an average. So that's a solid 10%, and with warmer temps, I'm hoping my average will climb to 17 mpg or better.

Also, as far as any fuel savings goes, at least an aerocap is a definite saving feature that has the potential to pay itself back over the life of it, there are very, very few accessories that people put on their vehicles which make this claim.

If you can recall which direction the cross wind was coming from when that video was shot and give ballpark numbers for vehicle speed and wind speed then we could interpret the effective flow field. I'm suggesting the cross wind effected the upflow shown along side the aero cap. If you happened to shot both sides they could be compared. :)

Cheers
KB

Sorry for the confusion - there was little wind on the day of the video. I was speaking of the original ABA testing of my topper.

Gotcha. The updraft does not seem like a cross-wind anomaly then but a real effect in your application.

Thanks!
KB

I think the updraft is caused by either an effect of the large low pressure area we create by having the downslope AeroCaps, or, it is just how it is with GM style pickup bodies. I have the same updraft on mine as evidenced by the water scum trails.

Or maybe the updraft is not caused by the cap at all. Come to think of it, rain water on my side windows (pushed around the A pillar) tends to crawl uphill. Given the amount of side taper maybe the cab itself is the cause. Any test data on side windows laying around?

Cheers
KB

Interesting you should mention side skirts. I made up a side skirt setup using an aluminum fabrication and rubber conveyor belt material. It worked OK as a complement to my air dam. I took it off for the winter. Eventually the snow will melt or so they tell me.

Word of advice. Don’t get too greedy on how close to the road you get. My side skirts were three inches above grade and they dragged some., eventually ripping the rubber. Maybe 4.5 or 5 inches would be more practical, but that far up may be losing the desired effect. I’m thinking about re-using the structure to support strakes.