Introduction: Hello everyone.

Okay Rich, I'll bite. You must have thought up a way to do that,
but it escapes me right now. Explosive bolts like they use in
the space program?

Don W.

Rich S. wrote:
"Don W" wrote in message
. com...


You wouldn't need a brake to get a fully feathering prop to stop. Even
with the propeller stopped it seems that getting the chute to deploy
without snagging risers on the prop would be problematic


Don............

I see you did not pick up on my suggestion, so I'll take it one step
further.

If the *prop* is the problem, jettison the _________. (Fill in the blank)

Rich "Engines are expensive, too." S.

"Don W" wrote in message
om...
Okay Rich, I'll bite. You must have thought up a way to do that,
but it escapes me right now. Explosive bolts like they use in
the space program?

Oh heck, Don. I was just trying to focus your problem solving skills on
where I thought the real problem was. It's not with the engine, of course -
besides, they're too hard to convince that leaving the airframe is the
proper thing to do.

I remember when I was building Esmeralda, I gave a bit of thought on how to
prevent the engine from departing the building in case I should ever toss a
prop blade. CG shift making the airframe unflyable, y'know.

I like the lathe tool idea, but there's others that might work. Since it's a
pusher, the prop would normally like to stay on something like a splined
shaft. You could have a snap-ring retainer that would resist it coming off
during an idling descent. To jettison, a rubbing block would push the snap
ring out of its groove and wind drag would yank the prop back and off the
shaft.

If it is a controllable pitch unit with separate blades, there must be a way
to retain the center section and just lose the blades. I think some of the
composite props have that happen unintentionally!

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

Rich S.

("Rich S." wrote)
To jettison, a rubbing block would push the snap ring out of its groove
and wind drag would yank the prop back and off the shaft.

The Wile E Cyotte in me is seeing a spinning prop (flipped horitontal)
coming back to get me ...and/or the chute lines. Didn't factor that one in,
did ya? :-)

http://img.thefreedictionary.com/thu...e_E_Coyote.jpg


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.

http://www.midwaysailor2.com/blaine/optica.html
OA7 Optica - pusher prop shrouded aircraft.
(ANE) Anoka County-Blaine, MN
D.A.D. Days, 2003


Montblack

"Montblack" wrote in message
...
("Rich S." wrote)
To jettison, a rubbing block would push the snap ring out of its groove
and wind drag would yank the prop back and off the shaft.

The Wile E Cyotte in me is seeing a spinning prop (flipped horitontal)
coming back to get me ...and/or the chute lines. Didn't factor that one
in, did ya? :-)

Zee small parachute for zee prop is trailing out of zee spinner.

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.

I said possible! It depends on who's stating the numbers. . .

http://www.moller.com/

Rich "Wanna see the elepahant??" S

Montblack wrote:
("Rich S." wrote)
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.

http://www.midwaysailor2.com/blaine/optica.html
OA7 Optica - pusher prop shrouded aircraft.
(ANE) Anoka County-Blaine, MN
D.A.D. Days, 2003


Montblack

That is one funky looking airthingamabob!

Don W.

"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! :-)

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

"Richard Lamb" wrote in message
ink.net...

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

So . . . we've gone from feathers to Force Fields?

Fah!

Rich S.

Rich S. wrote:

Oh heck, Don. I was just trying to focus your problem solving skills on
where I thought the real problem was. It's not with the engine, of course -
besides, they're too hard to convince that leaving the airframe is the
proper thing to do.

Yeah, and the other thing is that you usually want them to stay firmly
attached.

I remember when I was building Esmeralda, I gave a bit of thought on how to
prevent the engine from departing the building in case I should ever toss a
prop blade. CG shift making the airframe unflyable, y'know.

Yep, I know of one fatal accident that started with the engine trying to
fly on its own.

I like the lathe tool idea, but there's others that might work. Since it's a
pusher, the prop would normally like to stay on something like a splined
shaft. You could have a snap-ring retainer that would resist it coming off
during an idling descent. To jettison, a rubbing block would push the snap
ring out of its groove and wind drag would yank the prop back and off the
shaft.

If it is a controllable pitch unit with separate blades, there must be a way
to retain the center section and just lose the blades. I think some of the
composite props have that happen unintentionally!

Okay, you just made me think of a way that would work for a controllable
pitch prop. Just make the retainer hub keyed to fit the prop, and when
you rotate the prop just right, it slings its blades.

Check list item: Make sure that the propellor control detent lock is
installed properly. WARNING: NEVER OPERATE THE PROPELLOR CONTROL IN
THE DETENT POSITION EXCEPT IN AN EMERGENCY REQUIRING THE BALLISTIC
CHUTE!!


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

Rich S.

I finally figured out what you mean by a "prop shroud". Duh... Yeah
that could work.

Don W.

"Don W" wrote in message
. com...

I finally figured out what you mean by a "prop shroud". Duh... Yeah
that could work.


Pushy Galore used to have one, but I couldn't find a picture taken before it
was removed.

Rich S.