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Drafting using no fuel

There once was a mention in this article of a claim that a motorcyclist drafted behind a truck for hours and most of the time just "coasted". This coasting must have meant that the aerodynamic drag on the motorcycle was negative, and that the wake of the truck provides enough forward thrust to overcome the rolling resistance of the motorcycle. Is this feasible? In [1] it shows the drag coefficients of two cylinders of equal diameter, the first cylinder shielding the second from the wind. It turns out that for close spacing (the second cylinder being approximately less than one cylinder diameter behind the first (leading) cylinder, the drag coefficient is negative (the aerodynamic drag is reduced by about 130%). That is, the wake of the first cylinder creates a forward aerodynamic thrust on the second cylinder, pushing it in a direction opposite to the wind. This experiment is like the second cylinder was "drafting" behind the leading cylinder. Somewhat similar results are shown for two circular disks, one behind the other (but the induced thrust isn't as pronounced).

The above result tends to support the feasibility (in some cases) of drafting without applying any motor-driven thrust to the drafting motor vehicle. Another question is that of stability. If the induced thrust (due to the negative drag coefficient) increases the closer the drafting vehicle gets to the "truck" (leading vehicle) then the vehicle behind the truck might accidentally crash into it. But it turns out that for the cylinders, there is a stable zone about 1.7 diameters behind the leading cylinder. If motor vehicles behaved similar to cylinders then for drafting behind a truck: move closed to the truck and the drag goes positive, tending to push the drafter back away from the truck. And conversely, should the drafter move back further away from the truck the drag coefficient goes negative, providing forward thrust. But perhaps not strong enough negative to overcome rolling resistance. When the trailing cylinder is close to the leading cylinder (within 1/2 of the cylinder diameter) the negative drag (per the curve in ref. 1) stays nearly constant. However, there are not enough data points in this region to accurately determine the shape of this curve so the negative drag may not actually be constant in this region and a stable situation might exist.

Of course the above results don't prove that no-fuel drafting is feasible, but indicate that it might be. Additional wind tunnel testing would be of interest to resolve (if only partially) this question. Has anyone done this? David S. Lawyer 07:52, 14 December 2015. David S. Lawyer 18:58, 14 December 2015 (UTC)

References

  1. ^ Hoerner, Sighard F. "Fluid Dynamic Drag" (2nd ed.) Published by the Author 1965. Chapter VIII - Interference Drag, p 8-1, Fig. 2

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Effects of curvy roads

Curves are sometimes included in new street construction where flat land permits straight. Surely, curves increase tire wear and increase fuel consumption. I have been unable to find data regarding this. 199.168.104.122 (talk) 13:17, 18 December 2019 (UTC)[reply]

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