are "counter-rotating" and "contra-rotating" interchangeable terms?

I'm doing some research, trying to find information on airplanes with
counter-rotating props, but I'm getting a lot of junk hits from
articles that use "counter-rotating" when they should mean
"contra-rotating".

I was always taught that "contra-rotating" was when you had two sets of
blades spinning from the same hub, as in this: http://tinyurl.com/vxu9a
and "counter-rotating" meant two different engines on a twin, each
spinning in opposite directions, ala the Seneca and Twin Comanche.

But according to google, more than half of the instances of
"counter-rotating" are used to describe "contra-rotating". Are all
those people wrong, or is the term interchangeable?

(by the way, I'm trying to compile a list of counter-rotating twins, if
anyone knows of any that are not listed he
http://en.wikipedia.org/wiki/Counter...ing_propellers please post)

Yea,

The Wright Flyer.


Karl
"Curator" N185KG



"buttman" wrote in message
ups.com...
I'm doing some research, trying to find information on airplanes with
counter-rotating props, but I'm getting a lot of junk hits from
articles that use "counter-rotating" when they should mean
"contra-rotating".

I was always taught that "contra-rotating" was when you had two sets of
blades spinning from the same hub, as in this: http://tinyurl.com/vxu9a
and "counter-rotating" meant two different engines on a twin, each
spinning in opposite directions, ala the Seneca and Twin Comanche.

But according to google, more than half of the instances of
"counter-rotating" are used to describe "contra-rotating". Are all
those people wrong, or is the term interchangeable?

(by the way, I'm trying to compile a list of counter-rotating twins, if
anyone knows of any that are not listed he
http://en.wikipedia.org/wiki/Counter...ing_propellers please post)

Counter-rotating: prop tips rotate inboard

Contra-rotating: prop tips rotate outboard

Counter-rotating: you have to run to keep up with the cash register.
Contra-rotating: you have to run to keep up with the sales people.

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
for Email, make the obvious change in the address.

Counter rotating props are on two engine and shafts, they
rotate both clock and counter clockwise. On the P38 the
blades rotated away from the fuselage. Modern practice is
to have the blades rotate toward the fuselage and thus have
the lowest Vmc.

Contra-rotating is having two propellers on the same shaft
rotating in opposite directions to cancel torque effect and
gyroscopic forces.




"john smith" wrote in message
...
| Counter-rotating: prop tips rotate inboard
|
| Contra-rotating: prop tips rotate outboard

On Fri, 10 Nov 2006 23:21:53 -0600, Jim Macklin wrote:
On the P38 the blades rotated away from the fuselage.
Modern practice is to have the blades rotate toward
the fuselage and thus have the lowest Vmc.

Kind of depends upon your point of reference, doesn't it? No matter what,
the props are going to rotate towards the fuselage... It just depends upon
whether your point of reference is the top or the bottom of the fuselage...

P-factor and rotation is always, by convention, based on the
top blade moving downward.



"Grumman-581" wrote
in message
news | On Fri, 10 Nov 2006 23:21:53 -0600, Jim Macklin wrote:
| On the P38 the blades rotated away from the fuselage.
| Modern practice is to have the blades rotate toward
| the fuselage and thus have the lowest Vmc.
|
| Kind of depends upon your point of reference, doesn't it?
No matter what,
| the props are going to rotate towards the fuselage... It
just depends upon
| whether your point of reference is the top or the bottom
of the fuselage...
|

"Jim Macklin" wrote in message
...
P-factor and rotation is always, by convention, based on the
top blade moving downward.


I can't think of a situation where the top blade would move any direction
but down. ;-)

The most common rotation pattern in modern twins is for the top blade(s) to
rotate towards the fuselage.

KB

Just for the heck of it..... in comparison...

On a twin engine BOAT , the prop tips rotate tips outward at the top
of the arcs for best handling and fuel econmy..

The boat handles like a pig if this is set up in backwards, and the
fuel economy plummits..

Dave



On Sat, 11 Nov 2006 07:31:52 -0500, "Kyle Boatright"
wrote:


"Jim Macklin" wrote in message
...
P-factor and rotation is always, by convention, based on the
top blade moving downward.


I can't think of a situation where the top blade would move any direction
but down. ;-)

The most common rotation pattern in modern twins is for the top blade(s) to
rotate towards the fuselage.

KB

On Nov 11, 8:51 am, Dave wrote:
Just for the heck of it..... in comparison...

On a twin engine BOAT , the prop tips rotate tips outward at the top
of the arcs for best handling and fuel econmy..

The boat handles like a pig if this is set up in backwards, and the
fuel economy plummits..

Dave

For what it's worth, I found this on one of the boat news groups
regarding screw rotation.

this discussion (the discussion being direction of rotation of twin
screwed boats) there are several other factors to consider. If you look
at the keel of the German Z- Boats, they had a concavity to the stern
section of the boat running from where the keel began to rise deck ward
back. The theory here was that this would tunnel the water into each
prop. What it did was creat a situation where the boat would suck its
stern down with a strong reverse command.
In combat warships we are always trying to improve our turning ability
with out stepping out side the rules. There has been experimentation
with both applications. We have seen the same thing that Peter
mentioned, but not with a large amount of speed increase.
We have found that with the props rotating inward, two things are noted
that are considered an improvement in relationship to the props
rotating outward. 1) That there is a slight decrease in the cavitation
noted with strong reverse commands and that the ship will get up and
move a little quicker. This is thought to be caused by a decrease in
water turbulence caused by the water and air ( there is air to the
underside of the boat) running off of the keel as the prop tends to
pull water in from the outboard aspect of the boat. 2) Perhaps more
important is the slight improvement in the turn radious noted with the
inboard turning props. This is suspected to be caused by the water from
the props pushing against the outboard surface of the exposed turning
rudder.
In the great scheme of things what we have noted in combat ships is
that as you increase the size of the prop you increase speed and
cavitation ( especially if that prop starts to get larger than the
water flow from the keel can feed). As you increase the rudder size you
decrease the turning radious, but you will increase the sudden rocking
of the boat as you turn th rudder over and you will decrease the speed
of the boat.