aerothrust on semitrucks

This is for guys pushing 18 wheelers down those long, long roads; the guys that bring the food to your plate and all the other goodies of life.
For many years this architect has pondered the aerodynamic forces acting on those big boxes being shoved thru the air. As you may know, a box has some 14 times the aero-drag that an equal cross sectional fish shape has. This is why you see aeroshells going up on more and more cabs, to ease the airflow over the top of the box, approximating the ideal fish shape.

But that’s a passive answer to wave drag losses on a 9’wide, 14’ high, 35’ long box, is there a better solution? There could be. Think of the box as living in the drag half of a half wavelength, Compressive stress acting on the front, and Tensile stress on the back. Basically the front is snowplowing air to the sides, and being PUSHED back; and because the box blows a HOLE through the air, the low pressure tensile stress PULLS the box back, because that HOLE has to be backfilled.

Thus the dust that piles up on your back doors is a result of that tensile stress. Now consider a light cessna airplane taxiing down a runway, only its propeller is moving it. As a thrusting element the spinning blade has low pressure on the front surface and high pressure on the back surface.

You see then where this is going? How can we apply the THRUST concept to the semitruck box? Instead of C-front/T-back drag, we have T-front/C-back THRUST just like the airplane blade/propeller. A proposed solution: 3 rows of propellers in the plane of the sides/top on the front edges and another 3 rows on the back 3 edges. They are SIDEWAYS, not longitudinal axis like the airplane propeller. The purpose is to PULL air off the front plate and PUSH it in on the back doors, creating a T/C pressure imbalance, ie, aeroTHRUST instead of aeroDRAG.

Ok then, how much energy are we talking about? Basic calc : level, straight freeway lane at 70 mph. 70 mph = 102 fps, 9’x14’ cross sectional area = 126 sf x 102 = 12,935 cf. Air weighs .0807 #/cf = 1043#/sec that you have to pull sideways in the front(to the sides and top)to create a tensile stress region on the front panel.

But it’s W=mg, it’s not WEIGHT that matters here, it’s the air MASS that’s being pulled backward out in front. Thus 1043/32.17 = about 32 poundals-mass. Then WORK = Force acting through an average distance(W=Fxs). Since F=ma we have 32 poundals and a=51 f/s x 51’ to the centroid of the air-mass. That’s 83,232’-#-s, then divide by 550 '-#/s for 1 HP = 151.3 HP.

Now you see why you need a 350 HP diesel motor to plow/pull your way through the air? 150 HP in front + 150 HP in back = 300 HP. The question then becomes : could you improve things a bit by just backfilling the rear region with 3 rows of propellers and an electric generator running their motors?

They would be retrofit panels/rows on the back 3 edges like wings IN the plane(s)of the sides/top. Think of a stack of (5)2’x2’ summer fans jutting out from the back above the back bumper. Then another (4)across the top edge. They are on piano hinges to swing out of the way when you back up to a loading dock and its padded sides/top.

Another analogy here is the diesel locomotive : diesel engine runs generator runs electric traction motors powering the rail wheels. Can the same idea be translated into/onto the semitruck? Could it improve your mileage more than the aeroshell over the cab? Talk to an aerodynamic engineer…

Gas/fuel prices have dropped dramatically in just 9 months but you know it’s an illusion, when they shoot back up again, many of you will get squeezed out of business. Could this aerothrust concept give you the edge when that day inevitably comes? Go figure> W=P

I don’t want to be driving next to that semi when those props fail and come flying through my windscreen.

Your theory makes sense on paper. However the practicality of propellers on Semis might be a bit much for the department of transportation to swallow.

Are there any other ways to do what you are thinking?

Oh course the DOT will be all over it, on orders from the CBO, OMB, etc : do you know how much MONEY the fed and states make off of fuel taxes? They have NO INTEREST, as VESTED INTERESTS, in energy efficiency when it threatens its CASH COW. Thus the **** about “freedom from foreign oil” is a LIE from the get-go. It should be “freedom from gov’t fuel taxes” instead.

As to the propellers falling off and hitting your windshield, not anywhere as big a worry as getting RUN OVER by a 100,000 pound semitruck…The aeroshell on cab-tops is strongly secured to the roof, it won’t be any different with these rear propeller wing-stacks, they are too valuable to just tack on with tack welds.

You COULD do this with a turbine atop the cab, feeding high pressure air into pipes, then slotted outlets on the 6 edges(3 front, 3 back)but I suspect that would have a lot of line losses, whereas the propellers do the job right there where they are needed.

Overall though, this aerothrust idea may be a wash, energy-wise. The energy you save in C/T drag with T/C thrust is gobbled up by the transfer case-run generator and individual motor inefficiencies/blade drag. Still though, semitruck energy useage is a large fraction of the transportation picture, any improvement there is more than welcome. After all, you see aeroshells going up on more and more truck cabs. Why do truckers buy/install them if they weren’t doing any aerodynamic good?

Truckers buy and install them on their own? For the most part air dams are standard equipment on 95% (if not more) of trucks today. Secondly, your idea would require massive propellers in order to provide any “aerothrust” as you call it. Your theory doesn’t even work on paper. Comparing a large rectangle with spinning propellers to the streamlined design of any aircraft is about the same as comparing and orange to a dung beetle.

If you really are an architect, stick to what you know best. Like drawing building plans. Don’t try and re-design every thing on the planet just because you’re an “architect” and you think can do everything better than a trained professional. There is no conspiracy in the trucking industry to keep the aerodynamics down so that they keep using foreign oil. Diesel fuel and gasoline is a minute percentage of petroleum based products. If it really was the intent of truck manufacturers to create a less efficient truck, why have so many smaller trucking lines and a few large ones cut back or gone out of business completely due to increasingly higher fuel costs?

Just because you through some “Basic Calc” down on the board to distract some “artsy designers” doesn’t mean they are going to fall for your insane ideas.

I’m not an authority on this, but it seems to me there are some other efficiencies to be considered besides just aerodynamic drag.

While turbine thrust will reduce drag and back-suction, isn’t the energy in/power out ratio considerably worse for props and jet engines than for drive systems that engage the ground?

I suspect there is a reason why ground and water craft don’t use aerothrust unless they have to: there are fan boats, for example, but they’re only used in places where the water is too shallow or weed-choked to use regular propellors. As for cars – I’d think after 120 years of development, and many attempts at jet and prop powered vehicles, such vehicles would be far more common if they were actually more efficient.

Does anyone have any solid numbers on this?

I would encourage you to look at how much energy it will take these fans to refill the empty hole behind the semi. I suspect that it would balance out. Plus, you need to take into account reliability. Trailers especially get bounced around quite a big (we lost a container of glass shades when it was dropped at the port, don’t worry the container was fine!). You need to take this into account.

As for conspiracies, the newer aerodynamic semis actually improved fuel economy by 1mpg, which is huge considering the consumption of those behemoths.

Interesting. Complete nonsense, but interesting. By the way, it’s positive and negative pressure, not stress. Stress occurs in a material. You should probably take fluid mechanics before you start “solving” engineering problems like this.

Complete nonsense, but interesting.

Why complete nonsense? His theory from a qualitative standpoint stands up…but quantitatively I have no clue.

In other words, this may be a bunch of big word smoke and mirrors BS, but I have no clue where to start to debunk the theory…do you?

Sorry, I guess I should be more specific. He is essentially saying that he’s pulling air off the front of the truck and pushing it back into the rear. In the end, the same displacement of air occurs, only this time he’s using props or turbines to accomplish what the truck was already doing, only with way more mechanical losses. The only way to truly decrease drag is either accelerate the air flow around the truck or decrease the area of high pressure regions.

The only way to truly decrease drag is either accelerate the air flow around the truck or decrease the area of high pressure regions.

And to do that you would need to change the shape of the trailer to be more streamlined than squared off, correct? Like a cone shape in the back of the truck similar to the tail of an aircraft without the rudder and elevators.

Changing the design of a semi-trailer would be quite an undertaking. Just think of all the industries affected, trucking, shipping, rail systems…

Yes it would. I think the most efficient shape would be like an ice pick on the front and back, but that’s probably not the safest way to go. There were some guys doing fluid flow modeling on semis here at K-State, and I think there are still some small improvements to be made.

Good thread.

Not up on all my fluid dynamics/calc, to get into the finer points of your math, but to me, the aerothrust concept doesnt pass the “smell test”.

That is, as already mentioned, the extra energy required, weight, fuel, and space these things would take up compared to the savings seem off. And dangerous perhaps too.

A little over engineered too i think.

Wouldnt a more elegant solution be something like having a big tunnel hole through the cabin and and the container load to keep a portion of the airstream in one piece work? I would think that something like that may help the aerodynamic drag as the windspeed and pressure of the wind going around would then need to increase to catch up with the wind going through at a faster rate like over the top of a wing. Also no fans, motors or much modification needed. Im not sure the math, but makes sense in my head somehow. Imagine a “tube” travelling through the air (hole first) compared to a cylinder (round surface first) and you get the idea.

Or how about keeping the volume of everything the same, but changing the proportions? If the problem is the big front surface of the truck cab, and big back surface of the container, how about making everything 1/2 as tall, but 2x as long? Imagine a 1/2 height container that would stck on loading to be a full container size for shipping (not tall stuff mind you), and a cab thats more low like an opriginal hummel or one of those airport firetrucks.

? Im the first one to say im no expert, just tossing in my thoughts with the rest of the left field ideas here.

hmm… maybe i should be off to the USPTO site now in case Big Oil is spying on our forums…


I saw an experimental inflatable fairing a while ago, which inflated behind the trailer at speed to enable airflow to rejoin in a more stable manner. Same principal as they use for that fairing which is bolted over the rear of the space shuttle while in transit aboard the 747.

I think the mechanical pump to keep it inflated used a fair amount of energy too, although I imagine you could divert a small amount of airflow inside it to keep it at high pressure?

Another possibility, more KISS, is vortex generators:

Ah, inventors. :unamused: Propose the most complicated possible solution to a problem, selectively ignore basic physics to such a degree that your “solution” actually makes the problem worse, then cry conspiracy when no one jumps to license your patent. No offense. I’m sure you’re a nice guy, but engineering generally requires you to actually do the math. The real math, not that stuff you’ve got up there. It’s a bit like someone trying to design a building with no knowledge of basic structures or the requirements of building codes.

Having more than a few thousands of miles in eighteen wheelers under my seat I found this interesting. If you can get passed General Alexander Haig, check out the following video…

Then see:

And not a single fan, turbine, mirror, pully, whistle, or bell required among them.

Now let’s discuss redesigning the…

Yes, most of you are right, aerothrust to PULL air off the front and PUSH it onto the back with rows of propellers would take MORE energy than it’s worth, considering generator/motor inefficiences + blade drag losses. But neither are you Edisons.
N Harris has it right, a companion form to the aeroshell, (the “fish head”) : a “fish tail” : a torpedo form on the rear surface. It could be inflatable OR it could be self inflating. A perforated rubber sheet, like stretchable nylon stockings, that is a pyramidal gum drop shape when fully extended by the rearward tensile stress.
Thus at 70 mph it pokes out some 10 ft to 20 ft to a point allowing the air to collapse in the laminar mode rather than the violent turbulence on flat truck back doors. Then when the truck stops it pulls back on the back doors like a sagging nylon stocking. Or it could be an accordian-fold.
The 9’x10’ frame is piano-hinged on one side and swings 180 deg onto the box side when loading/unloading. Of course you’ll need some kind of latch to securely hold the frame during travel, either magnets or physical latches.
Cost? Oh maybe $1K to $2K installed, something like the cost of the aeroshell. And the mpg improvement should be about equal to the aeroshell aerodynamic improvement.
Another aspect is alternating Karman Vortices(how fish “walk”) : solid tail vs inflatable vs the loose perforted rubber sheet(permeable FABRIC in the generic sense) : how will each perform with those side motions if and when they occur?
Generally speaking though, how many FISH are there with SQUARE/FLAT tails? Or BOX shapes? And yet all objects moving through a fluid are sort of “fish”, even semitrucks. They’ve already done the “head”, now we need to add a “tail”. HEY, any FISH can tell you THAT…

When I saw the topic to this post, I initially thought of the Mercedes Bionic concept car form a couple of years ago. They used the boxfish as their inspiration:

That blowfish looks like a tasty little morsel to a FAST barracuda. How does it stack up aerodynamically with other designs? That high rear end looks like LOTS of SUCTION.

No, nature has evolved fish to be as fast moving and energy efficient in a fluid as possible, or get EATEN. Unless the blow fish has some other defense, it’s going to be some faster fish’s FOOD.

Aerodynamically, semitrucks are “fish” moving in an ocean of air. The fastest and most energy efficient ones SURVIVE, the others…fall into the lawyer’s hands, a kind of SHARK…

Despite its boxy, cube-shaped body, this tropical fish is in fact outstandingly streamlined and therefore represents an aerodynamic ideal. With an accurately constructed model of the boxfish the engineers in Stuttgart were able to achieve a wind drag coefficient of just 0.06 in the wind tunnel.

DaimlerChrysler utilised the findings from this research during the development of the Mercedes-Benz bionic car, a fully functioning and roadworthy compact car with a length of 4.24 metres and space for four occupants plus luggage. With a Cd value of just 0.19, this concept vehicle is among the most aerodynamically efficient in this size category.

Sorry, but I just can’t see how the square, flat back of a semitruck is in any way, shape, or form aerodynamically efficient. Why do fish have tapered-to-a-point tails and bullet noses? The reason dust piles up on the back doors is the tensile stress from the HOLE that the box blows in the air. That tensile stress = a partial vacuum = drag = energy loss. Semitrucks need fish TAILS as well as fish HEADS(aeroshell over cab).