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Old 10-14-2019, 12:39 PM   #21 (permalink)
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Don't forget that the taper in the top view will be different, as it's applied at the centerline of the vehicle. Applied like that, it's only ~4' "tall", so you'll get a much more aggressive taper on the sides.

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Old 10-14-2019, 01:42 PM   #22 (permalink)
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In the next week I am planning on building a boattail...
Still on schedule?
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Old 10-14-2019, 01:58 PM   #23 (permalink)
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Quote:
Originally Posted by ennored View Post
Don't forget that the taper in the top view will be different, as it's applied at the centerline of the vehicle. Applied like that, it's only ~4' "tall", so you'll get a much more aggressive taper on the sides.
Hi Ennoread,

Are you saying that I might be underestimating the taper based in my calcs from the side photos? That I might be thinking it is 15 degrees or so when it is actually more?

If so, could you elaborate because I don't quite get it. I did wonder about that because I am calculating from the photo as it is a 2 dimensional object and obviously it is not. Is that what you are referring to by the view from the top? I don't get the ~4 feet reference?

Thanks, Bill

PS
I cut a piece of plywood for the base last night at the 15 degree mark and it feels like the taper is less than the NASA truck or the RV Aero vehicle I mentioned, so I am inclined to think you are on to something here.
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Old 10-14-2019, 02:06 PM   #24 (permalink)
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Originally Posted by freebeard View Post
Still on schedule?
I started building base yesterday. Moving slowly but that is okay. I am hitting road soon so at some point I have to just commit and build it but I am still questioning my numbers a little.

As I said in my other post the taper I cut at 15 degrees last night "looks" less than the NASA photos or the RV Aero thread I referenced. I do realize at some point I just have to commit and build and hope for the best.

My daughter is home until Tuesday so having time with her is more of a priority right now but I plan on hitting the road soon - so if it is going to happen, it is going to happen soon!

Bill
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Old 10-14-2019, 02:16 PM   #25 (permalink)
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It's possible to overthink things.
Quote:
Premature Optimization
Premature optimization is the root of all evil -- DonaldKnuth

In DonaldKnuth 's paper " StructuredProgrammingWithGoToStatements ", he wrote: "Programmers waste enormous amounts of time thinking about, or worrying about, the speed of noncritical parts of their programs, and these attempts at efficiency actually have a strong negative impact when debugging and maintenance are considered.
Undershoot and you have a bigger wake; overshoot and you have separation creeping up your posterior.

edit: You might do well to focus on construction details. Are your panel's materials flexible or rigid. Curved is better than flat, but has to be fabricated, for instance.
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Old 10-14-2019, 03:22 PM   #26 (permalink)
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This help?



You'd need a top view to actually fit the template to, but it's pretty much a box.

Point is, when applied to the side view, the template barely curves in at all in 36". In the top view, it would curve in much more. Still not as much as the Dryden van, or aero RV, but more.
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Old 10-14-2019, 04:35 PM   #27 (permalink)
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It's the same curve.

Side taper is double the top taper, but it's a half-body. The bottom (being a different case) upsets the symmetry with the top.

If you truncate The Template you find there're diminishing returns at the pointy end. Which leads back to Mair.

The pertinent question is how tight can the initial curve be (given the truncated length). It's not simple — case in point the greenhouse of the GM EV-1. It's 'too fast'.

Once again, 'premature optimization...'.
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Old 10-15-2019, 12:04 AM   #28 (permalink)
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I went back and re-ran my calcs based on looking at the NASA van and the RV Aero Mod boat tails from the rear view and reconfirmed that the taper is in the 14 to 18 degree range.

Having said that, I am going to forge ahead at this point with construction. My daughter heads back to college tomorrow so that will free up some time. Here's hoping the weather also cooperates!
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Old 10-31-2019, 02:46 AM   #29 (permalink)
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In the next week I am planning on building a boattail on my 2004 GMC Savana 3500 Shortbus Camper. I am looking at doing something in the 2 to 4 foot range for length. I would like to keep it on the shorter side but could be swayed based on fuel savings. At the moment I am leaning toward a compromise of 3 feet or so.

I am currently getting 11.25 mpg. The engine is the Vortec 6L Short Block V8. Overall length of rig is 20'.

I guess my main question has to do with whether there is an ideal taper for roof, sides and base? My memory from reading through a lot here in the past is that maybe 10 or 12% would be ideal but I wanted to double check that number.

All other factors being equal I would lean toward a smaller taper for storage space reasons but that is definitely secondary to potential fuel savings.

Any help here would be greatly appreciated!

Thanks,
Bill

PS
I did read the "Aero RV (custom boat tail for '95 Ford E-350 Class C motorhome)" thread and noted that several folks thought his taper was too aggressive but I couldn't find what would have been better % taper in that thread.


1). How many miles per annum will this RV be used?

2). At a 12% improvement, what is the annual reduction in gallons used? (45?)

3). Even at $3/gal, and the RVíer average of 5,000-miles per annum, itís maybe a little over $100 in savings.

A bus like that really isnít a highway vehicle. Metro loop, point to point, is about it.

Look at the differences in school buses and highway passenger service buses. What sticks out is ground clearance. A school bus doesnít benefit from lower cladding . Adds weight and makes inspections more difficult.

A Trailer-Tail has a LOT of force being applied to it while underway. Find a truckstop and walk up to have a look at the structure. Itís the right idea, but to have it stay attached may preclude use of rear emergency door.

Straight trucks (26í Penske rental as example) have some of the very worst aero imaginable. And itís that exposed underside that does it. The Fed-Ex Custom Critical guys blow everyone off the road (find a pic).

The key to lowering motorhome fuel burn is planned use. I can show that a 5-mpg Moho can use less fuel on a trip than something that gets four times that much: itís about capacities for propane, water & food.

The important question to answer is: How many nights aboard before a re-supply run is necessary.

Thatís the fruitful avenue.

The genuinely low transportation fuel cost route is where the family car is used to tow an aero travel trailer. Again, capacities matter. But the real savings is reducing the headache of a second drivetrain to maintain & repair. Not enough annual miles accrue to stave off dry rot with Mohos.

The cost of an RV is the total expenditure over (X) years divided by nights aboard (Y) in that period.

Step back for the big picture.

(If you keep it, see Roger Marbles site on RV tires. Mandatory reading).

.

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