Aerodynamics

Introduction to the aerodynamics of fully faired recumbent bikes

The air resistance can be described by the formula W=Rho/2*cd*A*V² .
If you like to go faster with a certain power input, this can occur by reduction of the cd value and the frontal area A ( the air density Rho is fixed by air pressure and temperature and just can get influenced e.g. for record attempts by the choice of a suitable location etc. ). Because of the space requirement of the driver the frontal area can be reduced just down to certain limits ( in fully faired bikes 0,25-0,4 m² as a rule ). But the drag coefficient cd holds extreme potential especially in comparison to unfaired bikes. The cd in "normal" upright bikes can reach heights causing giddiness up to 0,75 . Against that the lowest cd I know in fully faired bikes lies under 0,07 . This means a reduction of more than 90% just by a better form design.
The air resistance W can be divided into a pressure part and a skin friction part.
The pressure resistance includes form mistakes and the resulting formation of eddies.
If there´s no complete pressure compensation possible by flow separations any more, a resistance force arises called pressure resistance. But you can reduce it to a minimum by perfect shaping. In the windtunnel shape mistakes can be localized well by smoke.
The skin friction resistance is caused by shear stresses between body and flow. In automobiles the friction part normally lays distinctly under 20%, by which progress can be made mainly by reduction of the pressure resistance. In comparison with fully faired bikes it is possible here, because of having less handicaps ( comfort, number of passengers, etc. ), to shape the form near the absolute optimum, which means that the "streamlines" meet again at the tail almost without loss of pressure / eddies. By that the friction part increases very considerably and therefore you have to pay attention to it much more. Roughly estimated the friction part at Magic Scooter 2 lies between 75 and 90 % !
The friction resistance also gets much less the smaller the surface area of the fairing is. So you don´t just have to reduce the frontal area but also to design the total vehicle as compactly as possible without risking a flow separation again at the tail.
You can divide friction resistance into a laminar and a turbulent part. On the front part of the fairing the flow is first of all laminar, but turns then to turbulent flow at a point depending on shape and surface which means a distinct increase of resistance in the following area. Therefore you have to try to hold the flow in the boundary layer as far as possible laminar. This seems reachable by the use of laminar profiles with the biggest width at the first 50% of total length. Because of that the flow gets accelerated longer and stays longer laminar on this accelerating distance. But with laminar profiles there´s the danger of flow separation by the shortened distance of tail tapering. The consequence would be a serious increase in resistance.
A further possibility to hold the stream laminar consists in optimizing the surface especially on the front half because there´s no possibility to return when the flow has been made turbular somewhere by a bad surface or so.
But is the flow separated by a mistake in shape at a certain area, so there´s the possibility by increase of roughness to catch the flow again by this definite supply of energy and by that to reduce the resistance ( the pressure resistance ! ).
But with a trick you can even "tame" the turbulent part of the boundary layer and at least partly turn it into laminar, namely by vacuuming the boundary layer and thereby also the resulting "mini eddies" through small holes in the surface. The required power to vacuum would be much less than the realized reduction of required drive power. The main problem here is the extensive expenditure necessary for research and conversion into practice.