Highflyer wrote:
"Montblack" wrote in message
("Rich S." wrote)

snip interesting shroud discussion because of increased drag

This might be urban legend, or might be stranger than fiction...

My next door neighbor back at Zuehl built a Zenith 601-HDS with a
Rotax 912 and electric prop. It's a heck of a setup.

Unfortunately, we didn't do this properly to collect data, but his
claim was that trimming the prop tip to a 45 degree angle (viewed
from the side) would increase thrust and efficiency both.

He claims he got a 200 rpm increase from that.
I can't really verify it, but -

When we were trying to trouble shoot his cooling problem we tufted
the cowl and took pictures. While doing that, I did notice that
the turbulence on the ground behind the prop was - gone(!).

I'd heard this same story from Tom Cassutt too, but, well, he was
having some old age problems then.

Anyway, _*IF*_ it is really working that way (and more testing is
definitely in order), I think what may be happening is as follows...

Air on the back side of the prop picks up a healthy "span wise" vector.

At the tip (forward facing angle - it's the back side, remember), the
angled part acts like a (suddenly) increased volume .

Increased volume - lowered velocity and higher pressure.

Might is be that this could be creating an invisible pressure fence
around the circumference of the prop?

Now, admittedly, it wouldn't be much good for keeping parachute
shrouds out of the prop...

But the weight, drag, and potential thrust / efficiency increase
sure look attractive enough.

Richard