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Old 03-22-2014, 06:48 PM   #101 (permalink)
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Originally Posted by Hersbird View Post
I will Chaz, I'm working and posting from my phone today. I do get what you are saying but now have a new question. Why do I want any downforce? Does downforce come from drag? Forgive me I'm new to all of this.
Sorry if I seemed a little terse. I think I misspoke when I said downforce is added, I think that lifting force is reduced now that I look at the data again. The Coeficient of lift goes from .22 to .11 at 10 and to .36 when it reached 18, so rather than "Adding Downforce" it is "Reducing Upforce". So I'm guessing that's where part of the drag reduction is coming from.

Yeah, when you get a chance to look at the paper on a big ol computer sometime, I hope it makes sense to you, I really think this would optimize your fuel savings with your project.

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Old 03-22-2014, 06:53 PM   #102 (permalink)
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Originally Posted by aerohead View Post
Thanks Chaz,I printed out 119-pages of it.
The plan taper would change the equation a bit,as well as any diffuser.
Ahmed et al. got into this.I'll do a search for it.
Thanks again in advance.
You're Quite Welcome, and Thank YOU Phil for all the insights you've given all us over the years!!!

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Old 03-22-2014, 11:08 PM   #103 (permalink)
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Originally Posted by ChazInMT View Post
Here's what I'd do to the design. Everyone gets all geeked up over 20 rear angle, but I think that is too aggressive. Based on a paper I read by a Cal State Masters Thesis candidate exploring pickup truck aerodynamics, you get 97% of your aero gain by going only 10, but you maximize lift reduction. The usable space is way better with the design I illustrate here too. So in a sense, you get the best of everything, huge drag reduction, big lift reduction, and a usable design. Win Win Winny Win.

Reason you don't want to get too aggressive on the top taper, is that you start to create a large area of low pressure that wants to get filled in from the sides, this sets you up for a vortex generation situation which produces a lot of drag.

The paper I reference is here. Paper The table on pg. 85 sums it up.

Why this paper isn't the gold standard for truck aerodynamics is beyond me. The author Feysal Adem clearly put a huge amount of VERY knowledgeable effort into its creation, yet somehow everyone bases their opinions on what looks right to them instead of the real science Feysal did.

I can only take credit for putting 2 & 2 together here on this concept. All due credit rests with Feysal.


This looks good as to use-ability. The central assumption about a trailer is that it has work to do. And is not likely to be constantly underway, so "mpg improvements" can actually work against one.

Trailer suspension is critical. Not ever as sophisticated as the tow vehicle (as passengers are not allowed), and market economics place terrible suspensions aboard as a result. The exception is in fully independent suspensions such as DEXTER "Tor-Flex". And the wheel center (face) using that type should be no further inboard than the outer trailer wall. For leaf sprung, the leaf spring itself should be at trailer wall. These are not to be compromised. Note U-Haul trailers in this regard as they are very good.

I would not ever use 15" or smaller wheels/tires with ST ratings, but would automatically upsize to 16" and LT-spec tires. Disc brakes (and anti-lock) are also a no-brainer (cost impediment usually cited).

A page taken from 1940's travel trailer design (Wally Byam, et. al.) is that sidewall radius to roof and undercarriage should be 12-22 degrees. The longer the trailer, the more the sidewall pressure of winds increases along the length. Side winds are what roll trailers. This radius was used by travel trailer makers where the cost was the same or higher than the average American house circa 1960, and by Airstream to this day.

Thanks, ChazInMT


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Old 03-22-2014, 11:24 PM   #104 (permalink)
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Quote:
Originally Posted by ChazInMT View Post
The problem with this design is that a serious downdraft and consequent very low pressure area is created above the rear window and high presure air is moving along the side. So you have air moving in 2 directions and 2 differing pressures.

Can you say VORTEX? The drag created by spinning the the air dragging along 2 tornadoes is immense.

There is a slightly more efficient shape for creating drag, but not by much.
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Originally Posted by aerohead View Post
The FKFS (Koenig-Fachsenfeld/Kamm) tested something similar to the r-pod,except that it had inboard wheels.It was reported at Cd 0.45.
The r-pod,with exposed wheels and fenders would be higher.
The 'classic' teardrops would suffer the same attached longitudinal vortices as r-pod just as ChazInMT has illustrated above.
The rooflines are all too fast,the pressure builds over the aft roofline in advance of the side flow,we get separation up there,and the pressure differential causes higher pressure air from below and on the sides to race up there attempting to reach equilibrium.As the flow fields collide at different velocity and pressure,they coil up just like a mesocyclone,spinning into horizontal tornadoes.
The sharp edges rob about 16% of Cd potential.The gap sucks energy.If they don't have belly pans,there's some more loss.
A 'gutted' VW microbus 'trailer' would pull as well as a shrunken Airstream,at Cd 0.43 (free-air),lower in train behind a tow vehicle.It would have more usable interior volume than a 'teardrop.'
Wow, so you guys are saying that the cool looking teardrop trailer design is crap? Unless it has a taper in the plan view also? I'd like to get educated, can you point me to some literature on this? My fluid dynamics education is very rusty.
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Old 03-22-2014, 11:32 PM   #105 (permalink)
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Originally Posted by Hersbird View Post
. . Then I thought a 5th wheel design instead might be better but then I'd have to get a pickup that I would otherwise not use where the minivan is a daily driver.

. . . My goal here is to be able to tow it at about 65 getting 15mpg or better. A normal box trailer seems to drop mileage to 10 or worse.

You may wish to read on Airstream Trailer Owners Community what others are finding with Airstream trailers and latest turbodiesel tow vehicles. MPG above 20 is now fairly easy.

Plenty of satisfied owners with gasser minivans. The real savings is always in purchase price, not just vehicle spec. The latter is central, but the former is where one really makes out. Don't sweat a hard MPG number so much, but find the tow vehicle that best suits solo use and can also tow the trailer. Reducing solo miles and driving the remaining miles for best FE can help subsidize the fuel cost of towing. Find this balance and satisfaction will outweigh particular MPG numbers.

The trailer is the all-important choice as the right one (aero, all-aluminum) will last decades and the tow vehicle will change several times. My folks kept their TT 27-years and used two TV's in that time. I started from scratch and pull a 35' TT at an average 15-mpg with a 3/4T truck (sig) versus the 8-mpg my folks saw with the same brand and a 28' pulled by a Cadillac.

A/S was the bargain brand of this TT type. But, like Chevrolet, built them by the thousands. An enormous price range to cover new and used. No reason, IMHO, to reinvent the wheel if it is a TT you want. But, beleive me, you'll break ground with a strong FE conciousness.

65-mph is a little high. 58-62 mph is the range for highway speed for FE (best travel time and best brake time/distance versus reaction speed; greatest peripheral vision). Aero resistance climbs too high and too fast above 60-mph.

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Old 03-23-2014, 12:37 PM   #106 (permalink)
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In Montana we are sort of know for our no speed limits. Yes we do have limits again but it is common for big rigs to be 65-70 and everybody else 80 and that is not just interstates. There has been a long history of those speeds being proven safe in Montana and part of it is the added courtesy drivers showed each other when the no speed limit law was in place. Reinstalling the current limits has actually greatly increased fatalities. Montana: No Speed Limit Safety Paradox
I just wouldn't feel right being such a hindrance to traffic at much less then 65. Plus distances are great here and adding an hour or more to a trip is just not worth the extra gas when work is long and vacations are short. Maybe if I was retired.

I also question some mileage claims diesel owners seem to put out( and I used to be one with a 2005 duramax), plus where I live regular unless is 3.22 while diesel is $4.00 so 20 isn't that great. Plus they are probably talking those new Mercedes and Jeeps that cost $45-85k on top of an expensive trailer in the first place. My van is a KIA sedona worth maybe $7k and gets 22 normally at those 75-80 mph speeds. It does have good power for a minivan and we have towed our old 22' pontoon with it. I do plan on doing some comparision mpg tests once I'm done borrowing some other light campers using the same road and tow vehicle.

Last edited by Hersbird; 03-23-2014 at 12:47 PM..
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Old 03-23-2014, 11:01 PM   #107 (permalink)
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Hersbird -- You know what you really don't want? A vortex street. Buffeting that can amplify into sway.

Slowmover -- I always listen when you speak, but I have trouble with a radius expressed in degrees.
Quote:
A page taken from 1940's travel trailer design (Wally Byam, et. al.) is that sidewall radius to roof and undercarriage should be 12-22 degrees.
Is that like 1/16th to 1/8th of a circular arc? Or following Mair, the angle where it might as well be a straight line from there on?

bikenfool -- Yup, teardrops are crap. Here's my solution. The rear entry is a valid criticism. But it would be a cot with storage on either side and a kitchen that butterflies open at the hitch end. A sink would fold out of the top of the fender (you don't see).

ChazInMT -- I'll have to digest that paper. So far I've run it through Summarize to see what it's about:
Quote:
In Chapter 4, the problem formulation developed in Chapter 3 were used to study flow over a pickup trucks with add-on devises: Tonneau cover, Rear Roof Garnish, tail plates, Airdam, Traditional canopy, Aerocap with rear inclination angel of 50,100,120,150 and 18.770.

...By comparing the pressure distribution plot on the symmetry plane of the pickup truck with Tonneau cover, shown in Figure 4.1.2(a), with that of the baseline model in Figure 4.1.2(b) it indicates that the pressure distribution plot over the rear end of the Tonneau cover is larger than the pressure distribution plot over the under body of the vehicle.

...Similarly, the total pressure behind the cab of the truck with Tonneau cover is about - 6.39* Pascal as shown in Figure 4.1.4 and it is higher than that of the baseline truck, which is -8.71* Pascal as shown in Figure 3.17, signifying a reduced aerodynamic drag and lift in the case of the pickup model mounted with Tonneau cover.

...By comparing the static pressure in Figure 4.3.2 with that in Figure 3.16, it is seen that the static pressure acting on the tail gate of the base line truck is about -3.55* Pascal which have a suction effect at the rear of the vehicle.

...Figures 4.6.3, 4.6.4, 4.6.5, 4.6.6, and Figure 4.6.7 shows the total pressure contour in the symmetry plane and over the surface of the pickup model with Aerocap when the rear inclination angle is specified as α=50, 100, 120, 150 and 18.770 respectively.

...Figure 4.6.8 to Figure 4.6.12 shows the static pressure contour in the symmetry plane and over the surface of the model truck with Aerocap when the rear inclination angles α=50, 100, 120, 150 and 18.770 are specified, respectively.

...The thesis studied the flows over a pickup truck with add-on devices: (1) Tonneau cover, (2) Rear Roof Garnish, (3) Tail plates, (4) Airdam with 3in and 6 in clearance from ground, (5) Traditional canopy, Aerocap at 5 different rear inclination angles, and (6) a 3D curved Aerocap.
The bolded part is at Summarize's minimum setting. Looks about right.
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Old 03-24-2014, 03:55 PM   #108 (permalink)
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Originally Posted by bikenfool View Post
Wow, so you guys are saying that the cool looking teardrop trailer design is crap? Unless it has a taper in the plan view also? I'd like to get educated, can you point me to some literature on this? My fluid dynamics education is very rusty.
I'll try for a link to the Full-boat-tail trailer for T-100 thread,page 25,mid-page,there's some graphical drag tables which might help.Nope.I don't know how to link threads which exist on the same forum sorry.
Here's from the hard way:


























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Old 03-24-2014, 06:40 PM   #109 (permalink)
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Do you have more info on this setup? Looks similar to my single wheel boat tail project...
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Old 03-25-2014, 06:06 PM   #110 (permalink)
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Do you have more info on this setup? Looks similar to my single wheel boat tail project...
This is a rear shot of the motocross motorcycle-based 1-wheel trailer I constructed for the CRX back in the 90s.There's a side view in the Phil Knox Aerodynamic Photos Albums.
I made only one test trip with it,across Texas,New Mexico,into Colorado,and back.At speeds up to 80-mph trying to keep up following my friend (who doesn't know what speed limits are)to Copper Mountain,Colorado;and 11,000+ foot mountain passes,I averaged 48-mpg for the trip.
In the other photo you can see black soot on the side of the trailer from the tailpipe.It turned out that 2-vacuum lines on the carburetor had hardened and broken completely through,leaving a vacuum leak causing a rich mixture.
I've never taken time to complete the project.With only a crude upper gap-filler,and nothing for the sides or underneath I figure there's still a little fruit to pluck.
The trailer maintains the original bike frame and fork.1-inch square tubing was welded into a triangulated space frame with opening lid to the saddle-bag pods inside.Two adults could bivouac in there.She was skinned in pre-finished Masonite,pop-riveted to the tubing,with a full plywood smooth belly.
The front axle was cradled in a custom hitch and secured with 4-C-bolts,kinda like Burt Munro did on the back of his Holden with the Indian Scout coming and going to the beach speedway.
The dorsal fin was really there to reassure me that she was back there,as you can't see the trailer at all from the driver's position.I put a red clearance light atop the fin which I could see at night,and a vertical LED brake light strip up the back of the fin as a 3rd-brake light.
Since the fork maintained its original rake,the trailer leaned during turning.She pulled like she wasn't there.
The weather hasn't been kind to her although she could be restored and improved upon.We'll see.

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