Expert: Paul Soderman - 5/7/2008

Question
I have a 13000lb 5th wheel RV that has a rounded front but a square rear end. my question is can I use something like vortex generators to help make my RV more aero dynamic to help save diesel? in a nut shell my question is how do I make a shoe box aero dynamic? we pull it with a 06 F350 ....anything we can do to help make it have less drag?

Answer
Hi Wayne
A shoe box is always a problem aerodynamically, but there may be a few things you could do.  First, sharp corners are bad.  S. Hoerner showed years ago that a volkswagen bus with sharp corners had much more drag than one with rounded corners as they now have.  See http://www.sciencedaily.com/releases/2005/01/050106113455.htm
for one such concept applied to a truck.

An even better approach may be to try boattail plates on the aft end as studied by my colleagues Jim Ross and others at NASA Ames on big rigs in the wind tunnel.  See https://www.llnl.gov/str/May03/McCallen.html
Jim may be able to get you a full report that describes how the boattails work (James.C.Ross@nasa.gov).  Jim could also give you more ideas - he is an expert on drag.  My understanding is that the boattail concepts have not caught on with the industry because of conflicts with backing into a dock.  But in your case, that should not be a problem.

For even more ideas, google truck aerodynamic drag.  You are on the right track.  Aerodynamic drag goes up as velocity squared and power required to overcome drag goes up as velocity cubed.  Any improvement will save you money.

Good luck.
Paul