"Tony" wrote in message
oups.com...
If one uses the Cessna 377 Skymaster as an example, it's pretty clear
a pusher prop is more efficient than one pulling. The data seems mixed
though on q tip props -- any leads as to where there may be quantative
data? What I found suggested the extra length of the prop might just
as well go to increasing its radius as in decreasing end vortix
effects. Finally, does anyone know of any work on small airplanes
using a ducted prop (shrouding it rather than using the q tip bent
end? I'm not worried about ease of flying (the problem with pusher
props are pretty obvious) but rather with converting the horsepower
delivered to the prop into usable thrust, that is, getting the
greatest efficiency (miles per gallon is a crude way of expressing it)
for a small single.


On the subject of shrouded props, yes studies have been done, but I did not
link to them, so do not know where to find them.

There are very few flying examples of shrouded light plane props. A shroud
is tough to justify, because of a couple reasons. Prop to shroud clearance
has to be VERY small, generally under 1/16", for any great improvement of
efficiency. Therein lies the rub.

Maintaining the close clearance is tough, because the shroud would have to
be very stiff, and strong, to not flex and hit the prop. That means weight.
That means it will have to increase efficiency a good deal, to justify
carrying the extra weight around.

Also, the shroud would have to be mounted to the engine mount, (more
complexity, more weight) or else the engine would have to be hard mounted.
That is because the soft mounts of the engine would allow the engine (and
prop disc) to move around (and most of them move around a LOT), and if the
shroud were mounted to the fuselage, you can imagine that there would not be
close prop clearance for very long. g

Also, there is also the subject of the shroud and that dreaded drag. Of
course, we don't want to add anything to our airplanes that stick out in the
wind that does not need to be there. A shroud and mounts is something, and
a pretty big something at that. So more drag means it will have to produce
even more efficiency.

Humm, it seems like we have now said that it has to increase efficiency a
couple times, and we have now used up all of the possible efficiency gains
just to balance the disadvantages.

What does it mean in the long run? We have a more complex, heavier
airplane, for no noticeable improvement in performance.

That's why you don't see many shrouded airplanes running around out there.
--
Jim in NC