Bill Freeman wrote:

Make sure that the scoop is thin with knife edges since
you want to avoid any external diffusion with this internal
diffuser.  External diffusion would be caused by wide
rounded lips on the scoop and would increase the inlet
pressure, but at the expense of mass flow.  With a
narrow inlet like that you need full freestream velocity
into the inlet to get the massflow you need, and then
slow it down and increase the pressure in your nicely
shaped internal diffuser.

Bill

Jeff Spitzer wrote:

I only partially agree with this post and those that follow, Bill.  The
parts I agree with are 1)if the internal diffuser is right then a small
radius (not knife-edge) inlet could have less drag. 2) The scoop MUST be
out of the boundary layer for the internal diffuser to work properly.  The
part I disagree with is that a large radius inlet will "cause" external
diffusion.  Like most systems the flow into the inlet will be based on an
equilibrium condition.  The back pressure of the downstream system (and
resulting volume flow vs. inlet area) determines how much external
diffusion takes place, not the radius of the lip.  If teh system flows
freely and equilibrium for the chosen inlet area allows the streamlines to
flow in straight, they will.  

In fact if you add energy, like Perry, then
the streamlines can be convergent into a large radius inlet.  The only
price of the large radius is the frontal area as you point out and some
acceleration of the free stream flow around the outside that would
otherwise not result.  

I never like to see knife-edge because it is so
intolerant to flow changes.  Remember, the flow through the radiatorMUST
change due to OAT, altitude, airspeed, power setting, etc.  A different
amount of diffusion is required for each case.  I suggest some reasonable
lip radii and a splitter to separate the boundary layer.

Jeff Spitzer

The Aircraft Rotary Engine Newsletter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org