yup, tailgate UP...

Read the engineers response about the "bubble". That is 100% factual. By lowering the tailgate or placing tonneau cover you are actually increasing forward drag as well as decreasing your downforce. The shape of a truck is basically a step down from the front of the vehicle. this is one of the most studied forms in fluid dynamics. this creates a low pressure zone behind the cab which in turn causes an upward and rear ward pull on the truck. the larger the step the more resitance (within limits). notice the teardrop shape has no step... hmmnnnn.
with the tailgate up you create a vortex that rotates clockwise if you were staring at the driver side door, this produces lift, but not much drag (ie less drag then bed down)...
so what you actually experience is a combination of this pocket cavitization and a step down effect. whichh effect dominates is dependant on the individual truck's design, speed, bed coverd/uncovered, tail up/down etc.
My persoanl opinion is I doubt it has ANY effect lower than 45mph, maybe minimal effect at 60ish and would start to be relevant in the 75-90 range, above that it can spell disaster (namely due to lift not to drag).
For cars like prostock trucks the lift is an overiding factor, and they have encapsulated beds. As far as drag... ITS A TRUCK!!!!! pretty much you are F'ed and you will only get minor improvements at best unless you have a full skirt from roof to tail. and that might not improve your airflow either!

I'll send you references if you want but, yeah pretty much the way it works... if you are wondering... My senior project is on Computational Fluid Dynamics.

Jay

Being a commercial pilot, I think my two cents has a reasonable value on this topic.

There are lots of different types of drag. Some of these include induced drag, form drag, parasite drag and profile drag. Ignoring all forms of drag caused by the shape of the body of the vehicle (some curved surfaces could produce lift, thus causing induced drag), a tailgate in the verticle position acts like a great bloody speed brake on the fuselage or wing of an aircraft. Whether the vehicle engineers design the cab to minimize tailgate drag in this configuration... I don't know? But... a flat board that is perpendicular to the relative airflow of the vehicle will create a good amount of parasite drag. For further information read the book "Aerodynamics for Naval Aviators". There are equations and text to describe all of this.

On some aircraft wing designs, vortex generators are installed on the top of the wings in order to delay boundary layer seperation of the airfoil. The small vortex generators actually create a small amount of induced and parasite drag. The funny thing about drag is that some types of drag (induced) decrease with speed, while parasite drag increases dramatically with speed. On a velocity vs. drag graph for a specific airfoil, the sweet spot is where the two intersect...

Getting off topic but... I don't think there is too much of a difference. The cab probably shields the tailgate's effect on drag. An interesting airflow probably does occur in the truck's bed that could cause the tailgate up scenario being the best or non the worst. If the tailgate was higher than the cab, definitely. It just depends on the truck's profile.

blah, blah, blah...... <img src="/forums/images/graemlins/sleeping.gif" alt="" />

Frank...


hmnn... more thoughts... (ground is the bottom of vehicle excluding tires)

lets think about this a little more thouroughly...
if the bed is REALLY short we can say it basically doesn't exist. assume equal air over top and bottom (invalid assumption) and what you get is a pull backward. if you have a LONG bed that is closed off on top you have a lower pressure zone above the bed which would cause air flow to be uneven and cause a pull both back and upward.

By removing the tonneau cover we now increase the "cab to bed" drop off, the bed to ground drop off remains the same (since the tailgate is still up); however the tailgate now acts like a scoop and the airflow sepparates. With the tonneau cover ON the airflow sheds from the cab and drops down towards the bed which increases the airs volume and decreases its speed (bernoilli), further more, the air comming off of the tail is forced up by the higher pressure air from under the truck which causes the air to collide with the rest of the airflow, this increases the drag and lift coeficients. with the cover OFF the airflow is split at the tail and has the opposite effect, the air inside the bed is flowing rear to front and heads upward at the cab. These airflows join without much tubulence or decline in pressure. Most drag and lift occur at the tailgate when the airflow separates.
Witht he toneau cover off, and the tailgate OPEN. The cab to bed drop is maximized and the bed to ground distance is minimized (essentially zero). The effect is basically the same as with the tonneau cover on except the relative lift is higher, and the relative drag is slightly less. Most lift occurs at the tail, and nearly all drag occurs at the cab/bed junction. (as a side note, youd be suprised how much additional lift the small air slot between the bed and tailgate can generate when the tail is down)

In conclusion, unless you are driving REALLY fast (ProTruck circle track) here arre the basics...
Most lift (low and high speed): tail down.
Least lift (low and high speed): tonneau
Most Drag (low speed): tail up
Most Drag (mid speed): tonneau
Most Drag (high speed): tail down

Mid speed being 55-80 mph or thereabouts.. i doubt it matters, maybe if you factor in lift as in decreasing the trucks weight??? But as it was posted earlier, better to remove the tailgate altogether.
for racing applications the lift issue becomes more important than the drag. i am sure we could kill the subject a bit more if we cared to.

Jay

I would be very curious to see details on different wind tunnel tests.

What I see here is a lot of examples, some contrary, some concurring, and a lot of sound reasoning. For example, the thread starter brings up what appears to be a valid water tunnel test concluding the tail gate down is better. I mean I have heard of the tailgate-up tests myself, but have also experienced positive effects of having it down. Others have too. My truck was not stock though. I think it is unreasonable to assume all who believe their trucks got better mileage with the tail gate down are deluded as it is unreasonable to assume those that see no difference are.

So what does all this mean? I think it means there's more going on here than meets the eye. Example, looking at the truck from the drivers side we have a clockwise vortex in the bed area. But, maybe air passing underneath creates an opposing vortex behind the tailgate. Do they interact? If so how? At what speed? Maybe at a given speed they annihilate one another? Maybe something entirely different is going on. I don't think there is a be all end all to this subject. I suspect differences between front/rear height, over all height, cap to bed geometry, average sustained speeds, body lifts, and who knows what else all come into play and interact.

I'm sure most of the data is not accesible but if anyone knows how to get it, I for one would love to read it.

Frank.

I think we already knew that, didn't we? <img src="/forums/images/graemlins/wink.gif" alt="" /> Just giving you a hard time. So disprove the ME students study in water. Since we don't really know his test procedure can you speculate why his study is flawed?

BTW, I think I believe the tailgate up is better, there seems to be more evidence.