Aero Standie Build Log

[Bluebunny]

<snip>
Here's the final design. The geometry of the roof is limited in that it must be 4' high at the rear, and less than 80" at its maximum. The front bulge can be varied, but I found this current shape to be pretty aerodynamic.
Sadly, this gets a drag force of 0.317 vs 0.175 for the car without a trailer, so it looks like it's going to need 80% more fuel... However, my car model is actually an impreza which I just resized and raised off the ground a bit, so maybe that throws the calculations off. I hope.

EDIT: I realised the crosstrek was slightly off-center in the last analysis. I re-centered it and ran again, and drag dropped to 0.25 vs 0.175 for the car alone. This is in line with my calculations, using estimated frontal areas from the model. I don't know why flow analysis scales my models down to ~ 0.005 m = 6 ft, but whatever.

Rgambord,
something still seems fishy-fishy. If I am correct, you are using a 1/10 scale model, with a full-scale 62mph wind? I cannot cite offhand how scale effect works, but a 1 meter long combo rig traveling at 62mph changes the Reynolds number significantly from the real world of a 32 feet long rig traveling at 62mph? I have not looked at scaling in quite some time, but I'm pretty sure that your rig is not traveling at a reasonable speed? Research "scale effect in aerodynamic modeling"?

Believe it or not, model airplane builders as a community have some awesome knowledge and abilities and published works in modelling aerodynamics! Kind of obvious if you think about it...they might need to make a model replica plane fly maybe at R=8000, where the real plane might fly at R=200,000! Sum doe's folks gots sum big brains!

The best solution is for you to FULL scale model the critter! You can DO it! Good luck! When you get it all figured out, I'll hire you to do my rig!

[rgambord]

<snip>
Here's the final design. The geometry of the roof is limited in that it must be 4' high at the rear, and less than 80" at its maximum. The front bulge can be varied, but I found this current shape to be pretty aerodynamic.
Sadly, this gets a drag force of 0.317 vs 0.175 for the car without a trailer, so it looks like it's going to need 80% more fuel... However, my car model is actually an impreza which I just resized and raised off the ground a bit, so maybe that throws the calculations off. I hope.

EDIT: I realised the crosstrek was slightly off-center in the last analysis. I re-centered it and ran again, and drag dropped to 0.25 vs 0.175 for the car alone. This is in line with my calculations, using estimated frontal areas from the model. I don't know why flow analysis scales my models down to ~ 0.005 m = 6 ft, but whatever.

Rgambord,
something still seems fishy-fishy. If I am correct, you are using a 1/10 scale model, with a full-scale 62mph wind? I cannot cite offhand how scale effect works, but a 1 meter long combo rig traveling at 62mph changes the Reynolds number significantly from the real world of a 32 feet long rig traveling at 62mph? I have not looked at scaling in quite some time, but I'm pretty sure that your rig is not traveling at a reasonable speed? Research "scale effect in aerodynamic modeling"?

Believe it or not, model airplane builders as a community have some awesome knowledge and abilities and published works in modelling aerodynamics! Kind of obvious if you think about it...they might need to make a model replica plane fly maybe at R=8000, where the real plane might fly at R=200,000! Sum doe's folks gots sum big brains!

The best solution is for you to FULL scale model the critter! You can DO it! Good luck! When you get it all figured out, I'll hire you to do my rig!

I got my degree in biology, so this is all stuff I'm learning as a hobby. I don't really understand reynold's numbers, but I'm going to see if I can fix the units. It's full scale in sketchup, but when I export it and import it into the flow analysis, it scales it down really small. I'll play around tonight and see if I can get it to work.

[KCStudly]

I've seen that scaling issue when the parent file was built using default metric units as the base (i.e. 1 unit equals 1 mm) whereas the intended scale was SAE units (i.e. 1 unit equals 1 inch). Check to see if the scale factor is off by a ratio of 25.4 to 1.

[rowerwet]

don't forget the other sides of the drag problem! the airflow that goes along the side of the car and then around those square corners on the front and rear. Those plumb sides and square corners all along the side create massive drage and induce vorteces in your wake.
If you are really trying to make a super smooth path for air to flow around the tear, something shaped more like an Airstream trailer is going to do a better job.
whatever computer simulations say, wind tunnel testing is still the best way. Only with super computers cranking through tons real world test data, have car manufacturers been able to model without going back to the wind tunnel many times. And that level of computer ability is a recent thing.

[rgambord]

Well, I don't have a wind tunnel and there's nothing I can do about the front corners. I can't put a complex curve in the materials I am using. I hope the computer simulation is wrong. 774 N vs 364 N @ 60 mph. If it's correct, I'll see my mpg cut in half on the freeway.

[rowerwet]

if you were building in foam it would be easy to layer it and then sculpt the radiuses you wanted, or with plywood walls you could still make 1' radiused corners with some sculpted foam bridging between panels.
Another thing to think about is the radius that plywood will bend before it snaps. some of the curves you have drawn would be tough to create even in 1/8" bendy plywood.

[rgambord]

if you were building in foam it would be easy to layer it and then sculpt the radiuses you wanted, or with plywood walls you could still make 1' radiused corners with some sculpted foam bridging between panels.
Another thing to think about is the radius that plywood will bend before it snaps. some of the curves you have drawn would be tough to create even in 1/8" bendy plywood.

I don't want to mess with foam. It's expensive and I can't skin it with the temperatures hovering around 0 F. I'm using FRP on the roof. It's very flexible.

[Bluebunny]

you're getting warmer...but your trailer model is now 30 feet wide. Keep plugging! Is your trailer actually wider than your tow? Does not seem correct...If it is, then yes, I can see why that wide brick (from top view) will cut your mileage in half at speed. Get your model to scale. Look at it it viewing from profile view AND from above to be sure the scaling is correct in all dimensions. Don't push go until all the dimensions are very reasonable. Accurate modeling is key; pushing "go" is the easy part. Never more true than computer modeling; poo-poo in is worse than guessing. We're cheering for you!

Maybe the trouble is the tow vehicle "insert" is dragging certain parameters with it that muck-up the translator? Consider - tow the trailer with a simple brick to see if ditching the tow "insert" will solve your scaling issues?

[rgambord]

you're getting warmer...but your trailer model is now 30 feet wide. Keep plugging! Is your trailer actually wider than your tow? Does not seem correct...If it is, then yes, I can see why that wide brick (from top view) will cut your mileage in half at speed. Get your model to scale. Look at it it viewing from profile view AND from above to be sure the scaling is correct in all dimensions. Don't push go until all the dimensions are very reasonable. Accurate modeling is key; pushing "go" is the easy part. Never more true than computer modeling; poo-poo in is worse than guessing. We're cheering for you!

Maybe the trouble is the tow vehicle "insert" is dragging certain parameters with it that muck-up the translator? Consider - tow the trailer with a simple brick to see if ditching the tow "insert" will solve your scaling issues?

That's the wind tunnel size. The model is properly scaled.The trailer is the same width, but 18" taller. Frontal area with the trailer is ~1.6 x normal, and my model is showing ~1.8 x as much drag. I can see that.

[Bluebunny]

congratulations! The end view is helpful to explain the drag. The nice profile curve will sure help with some of that area. good work!

[Bluebunny]

Additional comment; you are getting significant lift from the rig profile. Maybe try running in your program the extreme of a flat roof and again at a shallower angle after the front curve. See how it affects the drag (should hurt it a little)? Won't add beauty, but may help you decide the most practical shape (if your inside is getting pinched). You could alternatively give the roof a little up-kick in the last foot or two if you get concerned. The effect of lift here may lighten the grip of the trailer wheels, and may give the trailer some torque; down on the tongue and up in the rear. At some point, just shut me up and grab some wood and a saw. Thanks for sharing!

[Wolfpack]

I like your new profile, and I look forward to watching your build.

I have doubts if the drag alone from your trailer would cut your MPG in half. Consider the ultralight campers common in many parts of the world, that are towed by cars like yours. They really can half your MPG, but they are taller, longer, wider and 4x heavier than what you're planning.

[Pmullen503]

Have you found any real world data on the mileage reduction that can be expected towing a tear?

[Bluebunny]

Have you found any real world data on the mileage reduction that can be expected towing a tear?

Hey Pmullen, that sounds like an interesting thread; effect of tow vehicle versus effect of trailer size versus cruising speed, etc. Efficient smaller tows I suppose would get hit the worst, with 50% or more reductions, with big tow monsters hardly noticing what's back there...

Rgambord,
Have you decided to proceed with your build? With your latest proposed design? Were looking forward to hearing about it!

[rgambord]

Have you found any real world data on the mileage reduction that can be expected towing a tear?

Hey Pmullen, that sounds like an interesting thread; effect of tow vehicle versus effect of trailer size versus cruising speed, etc. Efficient smaller tows I suppose would get hit the worst, with 50% or more reductions, with big tow monsters hardly noticing what's back there...

Rgambord,
Have you decided to proceed with your build? With your latest proposed design? Were looking forward to hearing about it!

The pictures above are the currently proposed design. I am waiting for an axle to arrive before I start building. I didn't pay upfront, and there was no contract, so I consider it a possibility that the axle will not arrive, or the place I ordered it through will change the price on me. I need to finish the floor before I start working on the body -- I have plenty of time, so taking it piece by piece.