smooth is green

UPS used to run commercials bragging that they kept their planes immaculately clean because a clean plane has less drag and saves energy.  They didn't mention how much energy it takes to produce the water and soap, or to treat the water after it's been used.  Most of us didn't really care all that much, either.  As airplane prices start to rise with the new geopolitical unrest, people are paying more attention and realizing that sometimes, little things can mean a lot.

One of the most difficult basic physics concepts to accept is that no energy is required to keep an object moving at constant speed if there are no forces acting on the object.  Force is required to create acceleration, not velocity.  You must supply a force to throw a ball in the air, but in the absence of any external forces the ball would go on forever in the same direction once it got started.  

The primary reason why this is so difficult for students to accept (and why even very intelligent people like Aristotle got it wrong) is because a person is highly unlikely to encounter a situation in which an object is not subject to friction.  Unless you live in space, every object you encounter is going to be affected by gravity, friction and air resistance.  This remains one of my primary pet peeves about how we teach physics.  Why do we insist on starting out by telling students things that contradict everything they have experienced in life thus far?

Air resistance is the friction created when air molecules bounce off a surface.  While the up/down forces are important for keeping the plane in the air, we are also interested in the force the air molecules exert in the direction opposite to the direction in which the plane is moving.  The sum of the tiny forces exerted by the very large number of molecules that oppose motion is called drag.  Drag is one of those external forces that require the plane to constantly supply energy.  The higher the drag, the more fuel required to keep the plane moving at a constant speed. 

Drag depends on a lot of things, including the density of the air (lower density means fewer molecules encountered during the same time), the cross-sectional area of the plane (a bigger area hits more molecules) and speed (a faster plane hits more molecules in the same time). 

The concept of area is interesting when you're talking about friction because the critical parameter isn't so much the macroscopic area, but some type of surface area that takes into account the surface roughness on a micro- or even nano-scopic sense.

I've discussed this in reference to surfaces, like wood and skin, but it applies to everything:  It may look and feel smooth to the eye, but If you look at any surface closely enough, you will find roughness on some scale.  As you might imagine, a perfectly smooth surface will produce less drag than a rough surface.

Aerodynamics is very important to racecars when engine power is limited.  When going to tracks like Daytona and Talladega, NASCAR teams apply dummy decals to the bare metal of the car's body.  They paint the car, and then remove the decals.  Keeper decals are applied in the relief cavities made by the paint, and then the car is clear coated and sanded smooth.  Even the roughness associated with the raised edge of a decal is large enough to be of concern to a race team.  A small difference in drag can have a big impact in results when races are won and lost by thousandths of a second.

Commercial aviation isn't concerned as much about speed as it is with cost.  According to portfoilio.com, fuel prices usually account for 30 percent of operating costs; however, when oil prices keep rising (as they are doing now with the increased unrest in northern Africa), they can account for 40 percent of the cost of a flight.

I'm suspicious of 'magic numbers' like the $100/barrel mark, but it makes sense that there exists a tipping point for air travel that probably isn't too removed from the $100/barrel mark.  Ticket prices rise with oil prices.  At some point, ticket prices are so high that businesses stop approving travel and leisure travelers stay closer to home.  The airlines cut prices to attract travelers, and end up flying planes that are actually losing them money.   Unless, of course, they find ancillary fees they can tack on for checking bags, drinking soda or reading magazines. 

In December 2010, U.S. airlines used 929 million gallons of fuel at a cost of 2.141 billion dollars.  A 1% decrease in fuel comsumption represents a savings of 21 million dollars in just one month.  When fuel prices rise rapidly, so does industrial interest in technologies that can make even small improvements in efficiency.  Dirt sitting on the surface of the plane makes the surface rougher and increases drag; however, washing is not without cost.  You have to pay for the person power, the water, the tools and the detergent. Wouldn't it be nice if there were a way to Scotchgard against surface roughness?

That's a little harder said than done.  Planes undergo extremes in heat and cold, and if you think the wind on a day with a -20 F wind chill feels like needles on your face, imagine the force a surface must sustain moving at 500 mph at 35,000 feet. Anything applied to the surface of the plane has to be flexible enough to sustain the thermal cycling of the surface as it gets warm and cold, and has to be able to bond to the surface without coming off.

There's an inherent penalty for coatings:  American Airlines applies minimal paint to their planes because paint adds mass.  In space flight, where the cost per pound is even more significant, not painting the external fuel tanks on the space shuttle saves 600 pounds.  An unmanned launch costs more than $10,000 per pound, so even saving a few pounds on a spaceflight represented a major cost advantage.  It's not as significant on airplanes, but the coating's benefit has to offset the monetary and weight cost of applying the coating.  They can't be too expensive to apply, they have to last long enough that they aren't constantly having to take planes out of the fleet to re-apply the coating, and for commercial airlines, they have to be adaptable to the branding and required identification.

Wax was one of the first vehicle coatings that made the surface smoother and shinier.  Wax is applied to the car and forced into the pores and scratches by physically pushing it in, then removing any extra wax.  It's a lot of work, but the thin coating that results from a well-applied waxing repels water, increases shine, and smooths the surface.  Teflon or silicone coatings also have been used - those are normally small particles of the filler that are suspended in a liquid.  The liquid flows onto the surface, the particles are supposed to fill in the indentations, and then the liquid solidifies.  The smoothness of the surface, however, is going to depend on the size of the particles and how effectively you can get them into the nooks and crannies.  As you can see at left, the smaller the particle, the smoother the final surface.  Nanoparticles can get into smaller indentations and there is a lot of effort in developing drag-reducing nanomaterials for everything from yachts to the blades of wind turbines.

EasyJet, a UK aviation company, is testing out a new nanoscale coating for their planes that promises to effectively reduce drag by about 40%.  That reduction is expected to translate to about a 2% savings in fuel consumption.  Although that doesn't seem like much, EasyJet estimates it would save them about 22 million dollars from their annual fuel bill of about 1.2 billion.

The coating is called TripleO protective system, which has the acronym "ooops".  This was not unfortunate, it was intentional, as the princiapl from the company that developed this system owns a chain of auto repair stores.  The material being tested on the British jets uses an unspecified nanotechnology that the company says crosslinks with the paint to form a durable coating.  At less than a tenth the diameter of a human hair thick, the additional mass of the coating is negligible. 

One of the problems with coatings is getting them down into the nooks and crannies so that they form a really strong bond with the surface they are coating.  Delamination or spalling is the term used when a film separates from the surface it is covering.  It's what happens when your nail polish flakes off.  TripleO overcomes the problem of how to get the material down into the cracks by washing the plane first with oxalic acid.  The materials on the company's website note that this creates a positive charge on the surface, so I'm guessing that the acid dissolves a very thin layer of paint, leaving a bunch of atoms desperately looking for something with extra electrons that can offset the positive charge.  The company's polymer-based coating is negatively charged, so the surface actually pulls the emulsion into the crevices.  The coating bonds with the paint to form a smoother surface.  As a bonus, the surface also repels dirt, which reduces how often you have to wash the plane.

EasyJet is running an experiment. They've coated eight planes and will compare the fuel mileage of those planes with the other 192 planes in their fleet.  If the fuel savings are significant relative to the cost of the application and maintenence of the coating, they'll coat the remaining planes. If EasyJet saves save 2% of their fuel costs, that would correspond to 40 million dollars a month if the mainstream U.S. Aviation fleet followed suit.  Possibly more important than how much money the company saves on fuel is that the planes are burning less fuel and thus generating fewer emissions.  It seems like a win-win situation.

TripleO has worked with the auto industry, aviation (including British military planes) and even yacht racing, where drag is a major issue in speed.  If I ran TripleO, I would ship a couple quarts of the product to Charlotte in time enough to get it on a stock car at Talladega in April.  If their claim of decreasing drag by almost 40% is true, I can't see how it isn't a perfect solution: it's lightweight, clear, compatible with paint and it's not illegal.  Yet.

 

Comments

[cgoodrich]

Interesting article. I would have liked to have seen a discussion of laminar flow and how that affects drag.

Also, something that may seem counter-intuitive is that pool cues actually slide better over the fingers when the shaft is rougher, instead of smoother. Probably something about the co-efficient of sliding friction in there.

[BigKingKen]

I second the earlier comment on drag. This is a great article, but the treatment of drag is a bit simplistic. Laminar vs turbulent flows and where they are tripped make a big difference. Also a decent amount of drag can be attributed to the difference in pressure behind and in front of an object. And the cross-sectional area of an airplane is indeed a good indicator of how much drag it will have, it is more of a happy correlation than a direct result. The actual calculation of drag doesn't use it.

[Don SinFalta]

If by $2.141 billion you mean $2141 million, 1% of that is $21 million, not $200 million.

[Ianargent]

The weight penalty of paint, and the consequent removal of same for fuel savings, is nothing new. Late WWII aircraft such as the P-51 mustang and the B29 Superfortess dispensed with all-over paint jobs for just that reason.