OK Andrew, you got me thinkin!

[angib]

Didn't know much about it either. Was able to find this little site that was informative.

Sadly, the aerodynamics of road vehicles is still an area where the knowledge is largely proprietary - it's mainly held by carmakers and they ain't giving it away!

It's as least as complex as any area of engineering and an awful lot of good aerodynamics is in the detail, not the overall shape. It doesn't help that a lot of people's knowledge comes from advertising copy, including such lines as 'stunning aero design' (this translates as 'the aerodynamics are extremely poor but the stylists have done their best to disguise it').

If anyone is interested enough to buy a book on the subject, I'd recommend 'The Leading Edge: Aerodynamic Design of Ultra-Streamlined Land Vehicles by Goro Tamai. This book is just about the aerodynamics of solar-powered race cars but happens to contain probably the best 'primer' on aerodynamics ever written - and the solar car stuff is interesting.

The only book I've ever found with details of trailer drag is Aerodynamics of Road Vehicles: From Fluid Mechanics to Vehicle Engineering by Wolf-Heinrich Hucho but this book is getting towards 20 years old.

Andrew (in full nerd mode!)

[Arne]

Something about race cars, the try to aero the front but don't seem to be too concerned about the back... some of them have pretty squared off rears....

[mikeschn]

Aerodynamics don't sell cars. Emotions do. And to evoke an emotion that strong enough to sell a car, you've gotta have a great design.

Automakers don't do much with aerodynamics for that reason.

Mike...

[angib]

Automakers don't do much with aerodynamics for that reason.

Edit: US Automakers don't do much with aerodynamics for that reason.

If your gas cost $5-6 per gallon, like ours, aerodynamics would look more interesting!

Andrew

[mikeschn]

Andrew,

You're right of course... we've got a different culture here than in Europe. But right about now, if car companies were smart enough to offer the European cars over here, they would sell. Diesel in my town just jumped up to 2.43/gal.

Give me a diesel Opel instead of a gas guzzling Hummer any day...

Mike...

[xe1ufo]

Randy:

In my very humble opinion, that squared off back end WILL create a vacuum, which will increase drag and reduce your gas mileage. J.C. Whitney used to sell some drag-breaking curved "fins" that attatched vertically on the back end sides of an RV and horizontally along the back roof to break that vacuum. (Imagine a 36-inch diameter, 8-foot long pipe cut langth-wise into quarters.)

I am also concerned, Randy, for your safety in this design. What happens if you are sleeping in the trailer, while somebody else is cooking, and the galley (God forbid!) accidentally catches fire? If you want no doors up front, at least consider either getting one of the larger side-windows, or the larger roof vents that are specifically designed to also serve as escape hatches as well.

Just my 2-cents worth ...

Dr. Steve
Central Old Mexico

[rlphoto]

Randy:

In my very humble opinion, that squared off back end WILL create a vacuum, which will increase drag and reduce your gas mileage. J.C. Whitney used to sell some drag-breaking curved "fins" that attatched vertically on the back end sides of an RV and horizontally along the back roof to break that vacuum. (Imagine a 36-inch diameter, 8-foot long pipe cut langth-wise into quarters.)

I am also concerned, Randy, for your safety in this design. What happens if you are sleeping in the trailer, while somebody else is cooking, and the galley (God forbid!) accidentally catches fire? If you want no doors up front, at least consider either getting one of the larger side-windows, or the larger roof vents that are specifically designed to also serve as escape hatches as well.

Just my 2-cents worth ...

Dr. Steve
Central Old Mexico

Thanks for the areo tips Dr Steve! I will look into a rear fin. I got the windows in finally, at 4 windows at 18"x24" and they will certainly allow for escape if nesessary. Good thinking!

I have finnished the framing and will be posting progress soon!

Randy

[angib]

In my very humble opinion, that squared off back end WILL create a vacuum, which will increase drag and reduce your gas mileage.

I'm sorry to disappoint you, but if you think that the airflow over a teardrop follows the shape of its body, you are mistaken. The airflow will separate from the body just behind the point of maximum height and everything behind that will be turbulent wake and the (partial) vacuum you describe.

The only 'teardrop' that seems to have been designed for low drag is the Tiny Tears Boxter:


This tapers in height gently enough so that the airflow can stay attached right to the back corner. The benefit of this was discovered by a Dr Kamm in the 1930s, so this is not a new idea.

But if a trailer is being towed behind a truck, or indeed any vehicle higher and wider than the trailer, most efforts to reduce aerodynamic drag are a waste of time since the trailer is sitting in the wake of the tow vehicle. Then the main objective should be make sure the trailer is sitting inside the wake - for example, that the bottom of the trailer does not stick down below the bottom of the tow vehicle.

Andrew

[rlphoto]

My Trailer is 4 in higher in the back than the front. And a foot higher than the tow vehicle in the front. How can I help theis situation?

[denverd0n]

The airflow will separate from the body just behind the point of maximum height and everything behind that will be turbulent wake and the (partial) vacuum you describe.

I might be wrong, but what I have learned about aerodynamics as a result of being a pilot tells me that this does not happen with an airplane's wing. With an airplane the air flow stays attached to the wing until very close to the trailing edge, unless the wing is at an extremely high angle of attack. So, why would the airflow over a teardrop be any different?

Or have I missed something?

[sseibe]

Airflow will stay attached on laminar flow wings, with a specialty design for that. Regular airfoils have the air leave the surface much sooner. Keeping the wings clean will assist in keeping the airflow close to the surface. Thus, bugs hurt your efficiency twice, gas mileage and cleaning time.

[angib]

I might be wrong, but what I have learned about aerodynamics as a result of being a pilot tells me that this does not happen with an airplane's wing.

You're absolutely right. But airplanes' wings have a very gradual taper from their maximum height and so the airflow can stay attached. In contrast the taper of a teardrop shape is severe and so the airflow cannot stay attached - exactly like the situation on a stalled airplane wing. You never see airplane wings that are half as high/thick as they are long (fore-and-aft), which is the proportions of a teardrop.

The teardrop shape is just styling, like the shape of a VW Bug/Beetle that was a contemporary design - and that actually has a very high drag coefficient (Cd>0.50).

Andrew

[angib]

My Trailer is 4 in higher in the back than the front. And a foot higher than the tow vehicle in the front.:

That 4" is hardly earth-shattering. The rest of your design looks sensible and with the 4" rise in the roof, you can be sure the airflow won't detach and so it is likely to be more aerodynamically stable than anybody else's trailer!

The one thing you could look at if, as I seem to remember, you are towing with a regular sedan, is to get the height of the trailer down behind the car as much as possible. If you're using a HF trailer, you'd want to turn the axle over as many people have done - that's 2-3" off the height straight away. Similarly, avoid putting big wheels/tires on it - not only have they got more drag themselves, but they make the trailer sit higher.

But, mostly, don't worry about it!

Andrew

[xe1ufo]

Andrew:

The reason I got interested in TearDrops in the first place was that I found them for the first time in some photos of VW bug gatherings! We have 2 Bugs: my daughter´s is a 1986 and mine a 1994. (Yes, VW bugs were still made here in Mexico until 2003.) Now Andrew, you got my interest even higher, speaking of the high effeciency of the bug´s airflow.

Oh, the specs on the VW Bugs say that the Bug can tow 880 pounds, with a tongue weight of 55-82 pounds. Can a usable TearDrop be made that light?

Sorry for all the questions. I am just learning.

Dr. Steve,
Central Old Mexico

[angib]

Now Andrew, you got my interest even higher, speaking of the high effeciency of the bug´s airflow.

Sadly, I'll be disapointing you again! A high drag coefficient indicates high drag and so the high ineffeciency of the bug's airflow. Its drag coefficient of about 0.50 is worse than all the SUVs on the road today (well, except the Hummer H1/H2)! Its drag is much less because it's so much smaller, not because it's aerodynamic.

The new Beetle is a good illustration of how important detail design is in aerodynamics. Its 0.38 drag coeff is 25% better than the original Bug, despite having the same general shape.

Can a usable TearDrop be made that light?

Steve Wolverton built a teardrop without a galley hatch, the 'Cowper', that weighed 485lb with a 45lb tongue weight, which I think is a record for this forum:



Steve has sold the Cowper and hasn't been seen here since April, but if you search on 'steve wolverton' and/or 'cowper', you should find out about it. Sadly Steve's website no longer has any of the photos on it, so that little pic above may be all there is.

Andrew