"Montblack" wrote in message
...
("Rich S." wrote)
snip

It would seem that the most elegant solution lies in a prop shroud. No
moving parts, an increase in safety and possible efficiency improvement.

I've heard said improvements in efficency, with prop shrouds, haven't
transfered well from the chalkboard to the airframe.


They have and they haven't. The shroud gains efficiency by minimizing the
tip losses of the propellor. These energy losses are the rotating
propellors version of the wingtip vortices spun off the wing tips as a
concomitant of generating lift. With the prop they are the blade tip
vortices spun off the blade tips as a concomitant of generating thrust.

A tip plate can effectively increase the apparent aspect ratio of the wing
( blade ) and thereby increase its efficiency because of the resulting
decrease in the induced drag.

For this to work properly with a prop shroud the tip clearance must be very
small. Ideally approximately zero! :-)

Of course the shroud itself contributes friction drag that lowers
efficiency. This increases with airspeed. As long as the friction drag of
the shroud is smaller than the reduction in induced drag it provides to the
prop tips the shroud increases the efficiency of the shrouded prop above
that of the equivalent unshrouded prop. The folks at the Aerodynamic Magic
Works down at Mississippi State found that the tradeoff occurs at around 140
mph. Below that speed you gain efficiency with a shrouded prop. At about
that speed it really doesn't make any difference either way. Above that
speed the prop shroud gives a decrease in overall efficiency.

The shrouded prop begins to become more efficient again as the prop RPM
increases, especially at higher airspeeds where standard unshrouded
propellors begin to get into trouble with excessive airspeeds into the prop
disk. With a high RPM engine at high airspeed and high altitude the shrouded
propellor get quite favorable again. These days we call shrouded propellors
in this regime "fanjets." They have allowed modern "jet" aircraft to get
the specific fuel consumption of their engines down from around 1 pound of
fuel per pound thrust per hour, to something slightly over half that amount
of fuel. Making possible airplanes like the 747 and its "ilk."

Highflyer
Highflight Aviation Services
Pinckneyville Airport ( PJY )

10th Annual Pinckneyville rec.aviation flyin coming up May 19, 20, and 21.
Email Mary at if you are planning on attending and which
days so that she can purchase sufficient groceries. We don't want to run
out of steaks! :-)