charlie, if spiral prop wash is the reason for p-thrust ever wonder why a tail
dragger with the tail down experiences so much more sidesways turning than the
same aircraft at the same speed with the tail horizontal?

The "p-factor," of the upcoming and downgoing propeller blades having
different angles of attack, is the least signifcant reason for asymmetric
propeller thrust, although the most often touted. I came across a little
article in Flying years ago which did the math, and the effect is very
small. There are something like five reasons that a tail-dragger will yaw
one way, the p-factor being only one.

In my previous boat, Stella B, (www.ctlow.ca/StellaB/StellaB.html), I had
about the same asymmetric thrust whether the outdrive was trimmed in or out.
So much for prop angle.

The main effect is from the spiral prop wash. In reverse, where asymmetric
thrust is virtually always more pronounced, the top half of the prop wash
vortex strikes the hull, and pushes it sideways. The effect will vary
depending on the underwater hull shape, the angle of the prop shaft, design
of prop ...

So, a right hand propeller in reverse turns counter-clockwise. The top half
of the spiralling prop wash is moving to port, and pushes on the hull,
yawing the stern to port. The bottom half othe spiral is mostly in clear
water, pushing on nothing.

I hate to say "end of discussion," because that sounds arrogant, and I still
have things to learn, but I am very sure that for most practical purposes,
that is it.

Charles

====

Charles T. Low
- remove "UN"
www.boatdocking.com/BDPhoto.html - Photo Contest
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

"trainfan1" wrote in message
...

Using your theory, Steve, the stern would walk the same way whether the
prop was a left hand or right hand wheel... and this is not the case.

I have always been under the impression, on straight inboards, that the
lower half of the propellor, the arc furthest from the boat/hull
surface, is the one that does the most work/thrust (eg. surface piercing
drives, etc.). The prop has better "bite" in the less turbulent water
away from the hull. This has been my experience too...

Correct Craft & Century inboards: RH prop, walks to port in reverse,
favors left turns in forward... (the opposite of your explanation if
taken to the extreme of a 90 degree propshaft angle).

Mastercraft inboard: LH prop, walks to starboard in reverse, favors
right turns in forward. (again, the opposite of your explanation if
taken to the extreme of a 90 degree propshaft angle).

The rotational force of the shaft/prop is transferred primarily to the
attitude of the boat on a single screw craft.

Rob

On 05 Jun 2004 18:24:40 GMT, (JAXAshby) wrote:

steve, bone up on "P-torque" or "P-thrust" or more accurately "asymetrical
thrust". It is a common problem for aircraft with "conventional" landing gear
(meaning two wheels up front and one on the tail), and has been fully
understood since the early days of WWII.

That's only a factor when you're moving quick enough for the angle of
attack to be different for different parts of the prop if it's tilted
relative to the flow of the fluid. In the case we're talking about here
it's not a factor at all because we're talking about throwing it in
reverse and when the boat is not moving there is prop walk. If the prop
is not moving relative to the fluid, the angle of attack is the same all
around no matter what direction it's facing.

Steve

charlie, you don't understand what "action/reaction" means. the prop wash has
not have to "push" on anything but the prop, just like a rocket ship in space.

The top half
of the spiralling prop wash is moving to port, and pushes on the hull,
yawing the stern to port. The bottom half othe spiral is mostly in clear
water, pushing on nothing.

gene, it is nice to see that you came to your weak senses and agree with me and
the rest of the world.

gene, you are one horse**** pilot, if you are any pilot at all, for not
being
able to tell the difference between conventional gear and trikes.


Oh, I can tell the difference.... and the similarities.

Let me inform you a tad. when the aircraft is rolling with the tail down,
the
angle of attack one side of the prop circle is greater than on the other
side,
because of the forward movement of the craft. The side of the prop with
great
a of a developes more thrust and pulls the aircraft forward faster than the
other side of the prop. than means the aircraft pulls to one side. This is
often (erroneously) referred to a P-Torque, because it feels like the engine
is
torqueing the aircraft to the side. In fact, it is P-Thrust, or asymetrical
thrust that is causing the aircraft to veer.


You are amazingly close to correct. However, perhaps you should
consider that the angle of attack changes with respect to relative
wind.... and that changes when the taildragger (and the tricycle gear
to a lesser extent) transitions to flight attitude. You knew that,
right? Or did you think that they just popped up into the air?

be carefull, gene. Remember what the FAA taught you, "Safety is no
accident".


So... this is evidence that you are an authority on the FAA, too! How
many FAA certifications or designations do you hold?

You need to do some boning up on your pilot skills, gene, or maybe hang up
your
googles.


You can see all of this in your monitor? Having visions?...... are
you wearing those too-tight speedos again?
--
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

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

including P-51's?

charlie, if spiral prop wash is the reason for p-thrust ever wonder why a
tail
dragger with the tail down experiences so much more sidesways turning than
the
same aircraft at the same speed with the tail horizontal?


Uh..... duh.....

.......could it be because tail draggers have larger vertical surface
areas on the empennage to maintain directional control at slow ground
and air speeds?
--
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, you dum-dum. exactly as I said, you do not understand "action/reaction".
go call some junior high school science teacher and ask him/her to explain it
to you, then come back here. you won't have need for further discussion once
you understand just what "action/reaction" means.

charlie, you don't understand what "action/reaction" means. the prop wash
has
not have to "push" on anything but the prop, just like a rocket ship in
space.

The top half
of the spiralling prop wash is moving to port, and pushes on the hull,
yawing the stern to port. The bottom half othe spiral is mostly in clear
water, pushing on nothing.



ROFLMAO...... prop powered rocket ship in space! I wonder what is
pushing on *that* prop.......???

Scotty........ please beam Jax up..... has di-lithium crystals have
apparently cracked up.
--
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

oh, damn. here we had hope for you.

gene, it is nice to see that you came to your weak senses and agree with me
and
the rest of the world.




Yet another ill founded and incorrect conclusion. Whatever you
aren't, you *are* consistent.
--
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

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