How suitable is an Element as a tow vehicle? - Page 4 - Fiberglass RV


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Old 01-05-2006, 06:09 PM   #43
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Hey Benny--------Best I can do on short notice.


Aerodynamic shape optimization
The motivation of this project is to solve large-scale shape optimization problems that arise in airplanes design when searching for the shape that provides the best aerodynamic performance. Efforts are here concentrated on developping efficient algorithms in order to calculate the shape gradients of the aerodynamics coefficients (drag, lift, moments) needed for gradient-based optimization. The results obtained on the adjoint of the Euler equations of gas dynamic are implemented in the 2005 release version of an industrial code for Computation Fluid Dynamics (CFD) (EDGE). The current research on adjoint equations concerns the Reynolds Averaged Navier-Stokes equations (RANS).
A topic that is naturally related to shape optimization is the deformation of the calculation grid. Changes in the shapes involve deformations of the computational grids (structured or unstructured). There are numerous methods to propagate nodal displacements from the shape to the interior of the calculation grid. The current effort is here on developing an efficient solver in the case where the displacements are solutions of an elliptic equation involving a discrete Laplace-like operator.

Commonly, this type of optimization puts requirements on the feasible geometries, that is on the parameterization of the shape. Some degree of smoothness as well as fullfiling constraints (volume, thickness, curvature or angles of wedges) are normal requirements. A parameterization strategy for 2D cases (1D shapes) was devised that meets these requirements and offers a general setting.

Because of the grid deformation it can be expected that the accuracy of the aerodynamic coefficients (drag, lift, moments) provided by CFD calculations can vary during the optimization. Here an attempt is made to introduce goal oriented control of the error in aerodynamic shape optimization using adjoint of the flow equations and adaptation of the position of the nodes in the computational domain.

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Old 01-05-2006, 06:28 PM   #44
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Ches...

Thanks eversomuch for clearing up the question for us!

Rog
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Old 01-05-2006, 06:34 PM   #45
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I very much doubt that a T@b is any lower drag than an egg of the same height and width. The rounded front-to-top and front-to-side corners of the egg are very important for minimising drag. The T@b gets only a rounded front-to-top corner - those 'square' front-to-side corners will generate lots of drag.

A full teadrop is only the best shape if the vehicle is very, very long - say 10 times as long as it is high. For shorter vehicles, a truncated teardrop is best, as identified by Dr Kamm (of the Kamm tail) in the 1930s.

On the teardrops forum, we discussed this subject and I did this drawing of a trailer that might be able to add no drag to the vehicle towing it (a Honda CRV in this case). It is even possible that it might be able to lower the total drag, but only by being very closely matched to one particular tow vehicle, being no larger than it and being towed impractically close behind it - some sort of fancy extending tongue would be required for going around sharp corners!

Andrew (in full nerd mode)
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Old 01-05-2006, 06:35 PM   #46
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Benny, I don't know how authoritative this is, but it's excerpted from: Car Design On-Line

Quote:
Aerodynamic Considerations

Aerodynamic factors, considered carefully, can improve many aspects of a vehicle. Some key aerodynamic considerations have been summarised here.


[(vi) refers to Road Vehicle Aerodynamic Design by R.H. Barnard]


Improvements at the front can be made by ensuring the ‘front end is made as a smooth, continuous curve originating from the line of the front bumper’. Similar results can be achieved through a suitably curved roof.

[b]At the rear of vehicles, the ideal format is a long and gradual slope. As this is not practical, it has been found that ‘raising and/or lengthening the boot generally reduces the drag”.

Results of research state that drag due to rear slope angle will be at its ‘peak at 30º and minimum at around 10º’.

Increasing the curvature of the roofline will also reduce the drag coefficient. Benefits are gained by bringing the roof line down at the front and rear. Simply ‘bulging the roofline up’ however, may cause such an increase in frontal area that any gains may be negated.

In plan view, rounding corners and ‘all forward facing elements’ will reduce drag. Increases in curvature of the entire vehicle in plan will usually decrease drag provided that frontal area is not increased. ‘Tapering the rear in plan view’, usually from the rear wheel arch backwards, ‘can produce a significant reduction in drag’. Under the vehicle, a smooth surface is desirable as it can reduce both vehicle drag and surface friction drag. ‘For a body in moderate proximity to the ground, the ideal shape would have some curvature on the underside.’

Smooth unbroken contours with favourable pressure gradients as far back as practical should be used.

Strongly unfavourable pressure gradients at the rear should be avoided; some taper and rear end rounding should be used.

The form should produce negligible lift.

The underbody should be as smooth and continuous as possible, and should sweep up slightly at the rear, There should be no sharp angles (except where it is necessary to avoid cross-wind instability). The front end should start at a low stagnation line, and curve up in a continuous line.

Glazing should be flush with the surface as much as possible.

All details such as door handles should be smoothly integrated within the contours.
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Old 01-05-2006, 06:45 PM   #47
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Benny opened a hornets nest
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Old 01-05-2006, 07:03 PM   #48
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Andrew: looks like minumum drag in a straight line, but - umm - how does this combo turn, with almost zero clearance between the front corners of the RV and back corners of the tug?
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Seems to me that as soon as you open up the necessary clearance between tug and tow to allow a decent turn radius, you lose a lot of the aerodynamic continuity. And first time you turn up a steep driveway, you may lose the spare and back window as well!

Incidentally, although aerodyamics can help with gas mileage in a head wind, I'm more concerned about the effect of sidewinds, which can be dangerous as well as exhausting for the driver. There, the rounded roofline of the fiberglass egg may be a definite advantage over slab-sided RVs.
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Old 01-05-2006, 09:03 PM   #49
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Ken:

In response to your comment on pricing of a SCAMP, I have the price list from SCAMP. I am really close to ordering a new SCAMP to replace our aging tent trailer.

The price list shows a 13 foot 'standard' at $6,995 and the 13 foot 'package' at $7,995 (US dollars). Since our Canadian dollar is quite strong, this means a new 13 foot SCAMP (package version) is about $9,300 Canadian. The 'package' includes a 3 way fridge, battery pack, gravel shield, screen door, window over range, rear opening window, additional cabinets and some other items (all over and above the standard items). (quite a bit less than the T@B here in Canada). I considered the T@B but so far am definitley leaning towards a new SCAMP.

SCAMP also provides a great DVD showing the trailer and how it is made. It is worth ordering (free) if you are serious about them.

I would love to see one first...anyone reading this with a newer SCAMP 13 footer live near Calgary?

Art.
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Old 01-05-2006, 09:34 PM   #50
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Art, call up Scamp and they will hopefully give you names of owners nearby who will show you their trailers. Us owners are Scamp's volunteer sales force (although they may throw a few bucks our way if we show to a later buyer), and are always happy to get more people Scampin' down the road.

Patrick
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Old 01-05-2006, 09:47 PM   #51
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Well, well...and I was positive that for minimum wind resistance the ideal shape would be a wedge with a square back........oh, well, back to the drawing board.....anyone around that has experience with wind tunnel testing?.......you would think that the new stickies with the sloped fronts would also have sloped rears to improve fuel economy in the tow vehicles and increase sales of trailers with that configuration..... also you would think that the manufacturers of ammunition would make a double ended bullet that would have a flat cap on the back end inside the casing and would disengage from the bullet as it leaves the casing leaving a double ended pointed bullet the would have a higher velocity, less arc, and more distance.....would be a heck of an improvement........but alas, no one`s done that yet.....Benny
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Old 01-05-2006, 10:18 PM   #52
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I am not Capt. Aerodynamics, but I am sure that all variations of designs have advantages and disadvantages.

The Element, as much as I love mine, probably will not win any awards from the aerospace folks, it is basically a brick with an engine in it. It actually is more stable in light winds WITH the trailer strapped to it. I was getting pushed around the hiway here on my way to work today. The only other car I have had that hamdles as bad in the wind was a VW Bus.

So, the question is, does the trailer shape make that much of a difference when being towed by a box?

As someone said I said (And I twernt lieing...) I just got 20 mpg towing in the most horrible conditions one could tow in safely. Or in a lot of folks safety realms, NOT safely to even try.

20 mpg towing anything WITH anything is pretty darn good. Is eeking out a few more drops per gallon really that critical?
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Old 01-05-2006, 10:30 PM   #53
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Well Gina, Mercedes is experimenting with a 4 person vehicle that bases it`s shape on a boxfish which isn`t far off the Element shape....it will do 100 Kilometers on 4.3 liters of fuel ........about 60 miles per gallon ain`t too shabby don`t you think? ....anyway so much for the aerodynamics......anyway, glad to know you made it to Oregon and back safe and sound considering the elements, (pardon the pun), that you had to deal with.....Benny
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Old 01-05-2006, 11:09 PM   #54
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...you would think that the manufacturers of ammunition would make a double ended bullet...that would have a higher velocity, less arc, and more distance.....would be a heck of an improvement........but alas, no one`s done that yet...
Actually, Benny, they have. Boat-tail bullets have been around for ages, and do deliver a flatter trajectory.
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Old 01-05-2006, 11:33 PM   #55
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Aerodynamocs of the Element (Or blowfish car) aside..

Ken, you have a rig that gets outstanding milage for a rig it's size without towing. There are enough of us on this board, and the EOC that tow, to now say that it has a proven safe and economical track record towing 13 footers and other trailers of similar weight.

All of us report milage from 19-20 MPG. Even if you improve your milage by a couple miles per gallon by getting something that in theory has better aerodynamics, thats only a pitance of a savings. I doubt you will actually improve anything with a T@B tho.

Say you even get 5 mpg over our 20. Every 100 miles, you save around 2 bucks. Woo Hoo!

I certainly would find better value in NOT saving that pitance to have the extra space of an egg. Price vs. actually "cost", be it $$ or space, or a combination of both.
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Old 01-06-2006, 02:58 AM   #56
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If you want efficient fluid dynamics (air is a compressible fluid) examples, look at the design of birds, whales, porpoises, aircraft and nuclear submarines --- they are effectively an element running backwards, with a blunter nose in front tapering to a wedge in back, they have now added the underwater blunt nose to ships -- I would expect an element to get excellent mileage in reverse, esp in higher gears

Regarding bullets, aerodynamics isn;t the only factor -- that last push of expanding gas against the flat rear imparts energy. That's why the cupped Minie Ball was superior to the round ball bullet.

Automakers, likewise, are trying to pack as much volume into an attractive package as possible, and aerodynamics is just a factor. Also, aerodynamics may be perceived in the eyes of the consumer, not real -- If race cars are actually designed for stability over aerodynamics, but fans think they are aerodynamic, then they will design cars to look like race cars.
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