An airborne jet train that flies on a soft porous track

This paper explores the quantitative feasibility of developing an airborne jet train that flies on a soft porous track within centimeters of the earth's surface at speeds approaching current commercial jet aircraft. The jet train employs a lift mechanism first proposed in Feng {\&} Weinbaum (2000) J. Fluid Mech. 422:282 and a nearly frictionless track suggested in Wu et al. (2004) Phys. Rev. Lett. 93(19):194501. Using an asymptotic analysis for large values of the permeability parameter H/$\surd K_{p}$, where H is the porous layer thickness and $K_{p}$ the Darcy permeability, we first show that it is possible to support a 70 metric ton jet train carrying 200 passengers on a confined porous material if its $K_{p}$ is approximately 5 x 10$^{-9}$ m$^{2}$. For this $K_{p }$one finds that the tilt of the planform is $<$ 0.1 degrees and the lift-off velocity is $<$ 5 m/s. Compression tests on a fiber-fill material with these properties show that the fibers contribute $<$ 0.2 percent of the total lift and hence the friction force of the fiber phase is negligible. Using jet engines of 10,000 lbf thrust, about 1/5 that of a 200 passenger jet aircraft, one is able to obtain a cruising velocity approaching 700 km/hr. This would allow for huge fuel savings, especially on short flights where much of the energy is used to climb to altitude and overcoming lift induced drag.