On 05 Jun 2004 22:44:53 GMT, (Shen44) wrote:

the basic issue (propwalk) is caused by prop rotation and all
other factors may assist or decrease it, but the basic "phenom" occurs when you
rotate the prop.

============================================

Of course, but what is the cause of the asymetric thrust? THAT is the
question. Everyone agrees that a prop with a horizontal shaft still
exhibits prop walk, implying that the bottom the prop is more
efficient at providing thrust than the top. Lots of theories have
been provided but none that seem totally convincing since prop walk
still exists to one degree or another on deep props, that have plenty
of hull clearance.

On Sun, 06 Jun 2004 00:28:20 -0400, Wayne.B
wrote:

On 05 Jun 2004 22:44:53 GMT, (Shen44) wrote:

the basic issue (propwalk) is caused by prop rotation and all
other factors may assist or decrease it, but the basic "phenom" occurs when you
rotate the prop.

============================================

Of course, but what is the cause of the asymetric thrust? THAT is the
question. Everyone agrees that a prop with a horizontal shaft still
exhibits prop walk, implying that the bottom the prop is more
efficient at providing thrust than the top. Lots of theories have
been provided but none that seem totally convincing since prop walk
still exists to one degree or another on deep props, that have plenty
of hull clearance.

That's probably because it's a combination of several effects that all
contribute. Anyone who throws any theory out there that contains as
part of it's explanation some way that the thrust on one part of the
prop is different than on another part of the prop is probably correct.
Also, any explanation that contains part of it's explanation some way
tha the thrust from the prop gets translated into a rotational force on
the boat is probably correct.

For instance, no matter how deep the prop, the water on the lower side
is still going to be under ambient higher pressure than the water on the
top. So by moving the prop far away from the hull you may have
minimized the effect of hull turbulence on the top part of the prop.
But you haven't done anything about the pressure difference.

Steve

no matter how deep the prop, the water on the lower side
is still going to be under ambient higher pressure than the water on the
top.

the pressure difference due to water depth is inconsequential. The pressure
change is less than 0.5 pound per foot of depth. the compressibility of water
is near zero. even on a 15" prop, the center of effort difference between top
and bottom blades is less than about a foot. These ar truly miniscule forces
as compared to the force needed to move a multi-thousand pound boat in a
noticeable fashion.

wait a minute. wasn't it you that claimed the alleged p-thrust was actually
spiral prop wash on the rudder?

Yup, that was you. So how come you are now telling us that making the rudder
*bigger* would stop this alleged p-thrust?

dumb.

including P-51's?


Especially the P-51.......

.....it is a lesson in what happens with a design employing too small
control surface(s) for slow speeds. The P-51 was a purpose built
aircraft made to fly high and fast.... ground handling and slow speed
flight was sacrificed to this end. It was impossible to manually hold
this aircraft on the runway if you forgot to trim the rudder. Ground
handling, take-off, and landing in this aircraft was so poor that some
British divisions suffered losses of as much as 60% just trying to
learn how to fly the thing.

Uh..... ever flown one? Wanna go there?

--
23' Grady White, out of Southport, NC.

http://myworkshop.idleplay.net/cavern/ Homepage
http://www.southharbourvillageinn.com/directions.asp Where Southport,NC is
located.
http://southharbourvillageinn.linksysnet.com Real Time
Pictures at My Marina
http://www.thebayguide.com/rec.boats Rec.boats at
Lee Yeaton's Bayguide

Ok, teach...... tell me about how a prop-powered space ship
works.....

I said nothing whatsoever about a prop-powered space ship. you did.

I did say that "action/reaction" -- as in either a rocket ship OR of a boat
prop -- means that neither "pushes" against anything at all. Than means that
thrust comes from the action/reaction of the prop and water NOT action of the
prop and reaction of the hull.

dumb, you are.

gene, I am sure you don't realize it, but your post below states you believe
aircraft spiral left as they fly.

way to go, gene.

wait a minute. wasn't it you that claimed the alleged p-thrust was actually
spiral prop wash on the rudder?

Yup, that was you. So how come you are now telling us that making the
rudder
*bigger* would stop this alleged p-thrust?

dumb.


Jax your post is idiotic.



(2) The rudder is the only tool available to *counteract* the left
turning tendency (including "p-thrust") of the aircraft. The bigger
the rudder the more command available at slow speeds. Unless, of
course, JaxWorld pilots take off dragging the right brake....what do
they do when the main gear comes off the ground?
--
23' Grady White, out of Southport, NC.

http://myworkshop.idleplay.net/cavern/ Homepage
http://www.southharbourvillageinn.com/directions.asp Where Southport,NC is
located.
http://southharbourvillageinn.linksysnet.com Real Time
Pictures at My Marina
http://www.thebayguide.com/rec.boats Rec.boats at
Lee Yeaton's Bayguide

gene, your comprehension of English is sadly lacking. you agreed with this
statement AND disagreed. You agreed by stating the spiral prop wash caused the
aircraft to turn (thus smaller rudder is better) and disagreed by stating that
a larger rudder will stop the spiral prop wash.

English does confuse you, doesn't it gene.

(1) Please cite the post where I said that a smaller rudder would stop
"p-thrust."

On 06 Jun 2004 17:11:03 GMT, (JAXAshby) wrote:

no matter how deep the prop, the water on the lower side
is still going to be under ambient higher pressure than the water on the
top.

the pressure difference due to water depth is inconsequential. The pressure
change is less than 0.5 pound per foot of depth. the compressibility of water
is near zero. even on a 15" prop, the center of effort difference between top
and bottom blades is less than about a foot. These ar truly miniscule forces
as compared to the force needed to move a multi-thousand pound boat in a
noticeable fashion.

While it's true that the pressure difference is tiny between the top and
bottom of the prop, it is there and does contribute a tiny amount so it
should be factored in along with everything else. One special case that
could occur as a rare condition is when there is cavitation at the top
of the prop and not at the bottom due to the pressure differential.

Steve

Another wrong assumption, Jax.....


it was not an assumption, gene. it was a statement of fact.

gene, learn English and then continue to post here.

From: "Gene Kearns"
Date: 6/6/2004 5:12 PM Eastern Standard Time
Message-id:

On 06 Jun 2004 20:54:30 GMT, (JAXAshby) wrote:

gene, your comprehension of English is sadly lacking. you agreed with this
statement AND disagreed. You agreed by stating the spiral prop wash caused
the
aircraft to turn (thus smaller rudder is better)

I never said rudder. I said, "....larger vertical surface areas on
the empennage...." The rudder is a mixed blessing, but is the only
control surface capable of offsetting the left turning tendency.....
sorry you are confused.

and disagreed by stating that
a larger rudder will stop the spiral prop wash.

English does confuse you, doesn't it gene.

(1) Please cite the post where I said that a smaller rudder would stop
"p-thrust."




Apparently, your confusion arises from the fact that you don't
understand the subtle differences between the terms "stop" and
"counteract." Here's a hint..... you aren't going to "stop" it as
long as you use one propeller per shaft.




--
23' Grady White, out of Southport, NC.

http://myworkshop.idleplay.net/cavern/ Homepage
http://www.southharbourvillageinn.com/directions.asp Where Southport,NC is
located.
http://southharbourvillageinn.linksysnet.com Real Time
Pictures at My Marina
http://www.thebayguide.com/rec.boats Rec.boats at
Lee Yeaton's Bayguide