RE: Prop construction

Art,

Take a look at a jet airplane wing. You will notice that the topside is fairly unobstructed where the underside has all the bumps, pods, and engines (sometimes there are small vortex generators or turbulators on the top side of an airfoil but they act to keep the airflow attached to the surface).  In general, it is the airflow along the top side of an airfoil which is most important.  So, the spar should, in general, be on the underside.  Many think having a sparless prop is even better (a current trend for many in F1D). Jim Richmond often likes to put his spars on the very trailing edge of the prop, still on the underside, so the spar acts like a spoiler on the back of a car or a "Gurney flap" on a wing.  But, if there is a spar, having it on the underside of the prop or airfoil is generally better than putting it on the top side.

One can look at it in this way.  The air over the top of an airfoil goes faster than that on the bottom.  A topside obstruction will slow that faster air down.  An obstruction in the bottom side slows the already slower air down even more.  An obstruction on the top side usually lessens the lift (for example the topside spoilers that pop up on a glider to kill the lift).  The obstruction on the bottom side often increases the lift (as does a Gurney flap on a wing) but the increase in drag usually hurts more than the gain in lift helps.

Leo