SAVING FUEL

The shape of things to come

If the styling on the new Mercedes trucks is intended to reduce air resistance and generate fuel efficiency, then truck designers still have not got it right.

Compare both ends of these trucks with the Boeing 747 below, note the difference and ask yourself why the Boeing designers put the sharp end at the back.

Aerodynamic resistance can be explained in simple terms using a square box passing through the air. Ignoring skin friction which depends on the total area of the box sides, about 30% of the remaining air resistance can be attributed to frontal drag and the remaining 70% can be attributed to drag from the rear surface.

Since air resistance quadruples as velocity only doubles, air resistance is naturally more critical at the near supersonic speeds of the Boeing. However, it is still vitally important to the truck fleet operator who could save thousands of dollars a year in fuel costs with better equipment.

The difference in drag between front and back comes about because the air "hears" the object approaching and parts easily to let it pass through. But at the rear of the object the air becomes confused and curls and eddies behind the object (turbulence) and there is a sharp decrease in air pressure behind the object.

The object, rather than being held back by the air resistance at the front, is pulled back by the larger resistance at the rear.

A practicle way to prove this fact is to compare the visible difference in road dirt after a journey in bad weather. The dirt will be divided about 30/70 between front and back (assuming both ends have relatively flat surfaces).

The first major step must be to eliminate the large flat doors at the rear of the truck or trailer. That will be a challenge but it will not be impossible.

Wings or fairings attached to the rear of a tractor do little to lower air resistance and will substantially increase air resistance when traveling without a trailer.