Boat tail on trailer vs. deflector in front: The beginnings of a clean slate project

[skyking]

I have to fab up both bumpers, the grille, tailgate, bed liner wood, louver system for the grille......
It will be a while
I could not help myself: I walked around it yesterday with the critical look you get when you splash some penofin oil in your eye. A few eye drops later I was giving it the bloodshot stare of a serial sleepwalker.
I think I can use some 4MM Okoume marine ply and bend it to match the top back, then straighten it out to three panels in about 1 foot aft. Hard mount a skirt like that across the top and down the sides to square them.
Hinge my swinging panels from that.
Top piece will not be a curve but three tapering panels, sides will be straight.
That is why I was asking angle because that is what I will get, not a radius.

[euromodder]

Originally Posted by skyking

What is the maximum angle I can use and still get a benefit from this partial boat tail?

If you can't get a continuous curvature like the template shape, use a 12° angle.

[skyking]

Thank you

I presume a smaller angle on the side panels then? At 12 degrees with a hypotenuse of 8', I get a drop of 4.3' and length normal to earth of 6.75'.
12 degrees on the sides and it would come to a point in the back.

[fidalgoman]

There is a lot of information in the world of aircraft aerodynamics. Somewhere from the back of my mind the number 14 degrees maximum angle from the angle of attack before reaching stall. That's maximum under ideal conditions. Zero in this case would be paralell to the road. That's why people use vortex generators to patch problems.

[skyking]

I spent a lot of time hanging out under the truck yesterday while I was mounting the Cumaru (Brazilian Teak) running boards. It would be very, very easy for me to do a full belly pan! That running board is the right height, about the same as the frame and cross members.
I could get at least back to the rear differential with one.
If I pan the truck, does that mean I can omit a front air dam, if I get it all brought together smoothly?

[Christ]

Yes and no.

The pan will help, but an air damcan stop some of the air from goering into that tight area between the truck and the road, and whether that's going to benefit you more or less seems to be on a case by case basis.

[skyking]

I read about that, and what happens is you get to a point where the increased frontal area of the dam starts to hurt you. That point seems to move around
I would think speed has some bearing on that.

[euromodder]

Originally Posted by skyking

I read about that, and what happens is you get to a point where the increased frontal area of the dam starts to hurt you. That point seems to move around

Both will help - you still want to reduce the air going underneath the vehicle.
But there's indeed a point where lowering the airdam and increased frontal area starts to hurt your fuel economy.
Where exactly the turn-over point is, is hard to say.


With a square rear end, it's not beneficial to add a shallow diffusor aft of the wheels.
But if you do a full belly pan + boattail (i.e. sorting out the squarish rear end), you can gently (about 4° max) angle up the rear end belly pan, from about the rear wheels aft if the structure permits that.
(Well, that's how it would be on smaller vehicles.)

[euromodder]

Originally Posted by skyking

I presume a smaller angle on the side panels then? At 12 degrees with a hypotenuse of 8', I get a drop of 4.3' and length normal to earth of 6.75'.
12 degrees on the sides and it would come to a point in the back.

If I take 4 ft as half the width (and height) and 12° angles , I end up with a boattail that'd have to be nearly 19 ft long before the ends meet in a point.

Tan 12° = [width/2] / [boattail length]

[boattail length] = [width/2] / Tan 12 = 4' / 0.21 = 18.8 ' to meet up in the middle.

One of us got their math wrong

[skyking]

That would be me. I was using Cosine adjacent and I borked it up.