Whats the deal with counter-rotating props?

Mike Noel wrote:
if you slow it down
too much with the engines still developing thrust, it can flip over on its
back.


So you can't practice power on stalls? How about landing, on final
approach? Sounds pretty dicey. This is with both engines running?

This if from the August 2004 issue of AOPA Pilot:

'NASA's July 1971 final report reached three conclusions about the PA-30
Twin Comanche:

* At the stall, large rolling and yawing moments occurred as a result of
asymmetric wing stall. The left wing stalled, NASA said, at an angle of
attack about 2 degrees lower than the right wing.
* These rolling and yawing moments are larger than the corrective
moments produced by aileron and rudder controls.
* The airplane exhibits a flat spin under certain conditions involving
the use of asymmetric power.'

I interpret the first two items to apply to a Comanche with both engines
operative, with only the last referring to a single engine Vmc situation.


--
Mike Noel,
Tucson, Arizona

'Men never do evil so completely and cheerfully as when they do it from
religious conviction.'

-Blaise Pascal
"Mike Noel" wrote in message
. ..
I didn't notice anyone bring this up in the thread. The AOPA published
some
interesting stuff on the Twin Commanche without CR props since they were
giving one away last year. With the older style twin, if you slow it down
too much with the engines still developing thrust, it can flip over on its
back. One of the wings is effectively flying at a couple of degrees
higher
angle of attach due to the prop airflow. I think this is the main reason
the newer Twin Comanches went to CR props.

--
Mike Noel,
Tucson, Arizona

'Men never do evil so completely and cheerfully as when they do it from
religious conviction.'

-Blaise Pascal
"Dico" wrote in message
oups.com...
Hello,

We're looking into a twin and the Twin Comanche is on our list. I know
that the later models have the counter rotating props -- although i
don't know too much about what this means, other than its "better".

We fly a mooney now and thus I don't worry about the prop -- as long as
it keeps spinning.

Is there a web site that gives a good explanation as to why I want CR
props. There are hundreds of the earlier model Twin Comanches flying
without the CR props --- so what does someone with 300 hours single
engine time need to worry about? What actually goes wrong? And when
it does, what happens? I hear "critical engine" but it means very
little to me.

I like to fly and try to be very careful when I do fly... but I don't
follow too much aviation stuff other than how it affects me -- so I'm
not exactly a "buff", hence the above questions which may seem obvious
to many.

Why are we looking at a Twin Comanche? Because its a twin, safer for
IFR flight (perhaps this is only preceived), plus we live on the east
coast on an island so we're flying over water quite a bit. Also this
plane has decent speed and is an "economical" twin. We rarely fly with
4 people, so we don't need any more seats than 4.

Any help or links to help would be appreciated.

Thanks,

Dico

We're looking into a twin and the Twin Comanche is on our list. I know
that the later models have the counter rotating props -- although i
don't know too much about what this means, other than its "better".

You've gotten a lot of uninformed opinions here. I started countering
them, and after the first post decided it would be better to put it all
in one place. I have over 900 hours in Twin Comanches of various
flavors (including the CR), I've instructed in them, and I've worked on
them. Having said that, here's what you're dealing with.

There are exactly two things a counter-rotating prop does for you in
flight.

First, it relieves you of the necessity of applying right rudder on
takeoff or climb.

Second, it allows you to fly at a slightly lower airspeed in the event
the left engine fails without rolling over - which boils down to being
able to fly not just much too slow, but much, much too slow. The
reality of the situation is that the PA-30 should never be flying at
less than 90 mph unless
(a) the runway is made and you are decelerating in preparation for the
flare, in which case power is near-idle and an engine failure is
irrelevant,
(b) you are in ground effect accelerating after takeoff, in which case
an engine failure calls for an immediate idling of the throttles and a
touchdown straight ahead, or
(c) you are at a safe altitude doing training.

The downside is you get an engine where the prop, prop governor,
magnetos, alternator, vacuum pump, fuel pump, oil pump, and tach
adapter are nonstandard, and thus rarely stocked. On top of that, some
of those parts are used only on that one engine (the counter-rotating
IO-320) which is used on only that one airplane (the PA-30CR or PA-39)
which hasn't been made in decades and of which relatively few were
made. The impact on maintenance is spectacular.

The reason Piper modified the original PA-30 (in many ways - they added
control linkages, stall strips, and finally the counter-rotating props)
was to make it a more docile trainer. They didn't succeed. If you buy
the plane, make sure you find an experienced Twin Comanche instructor
to train you. No amount of time in Seminoles, Duchesses, Apaches,
Geronimos, or similar trainers prepares someone to fly a PA-30. I've
seen what happens when someone who just took an MEI ride in a Duchess
gets into a PA-30 - he can't hold on to the tail. You rarely see
PA-30's as trainers anymore, and for good reason. When MEI's with
50-100 hours multi time tried to teach in them, the carnage was
spectacular.

You do not need an STC to convert a straight PA-30 to CR. There is a
service bulletin from Piper for the conversion. All you need to do is
buy the parts and convert the engine by simple parts replacement. That
won't be cheap either.

With the low altitudes and cool weather of the East coast, the service
ceiling of the PA-30 is quite high enought (5800ft density altitude at
gross). Contrary to popular opinion, the PA-30 will fly just fine on
one engine, even at gross. BTDT.

Most of the PA-30's out there now have been modified to reduce Vmc.
The popular mod is the rudder fin (Knots-2-U and Robertson make them)
which improves lateral stability and reduces Vmc.

The real Vmc of a PA-30 without CR props is 80 mph. That's what was
designed in and verified by flight test at Swearingen. However, that
assumes you do everything right. Relatively small errors in technique
can raise that. This was fixed by raising the Vmc marking on the ASI
and in the AFM to 90 mph by AD (no changes were made to the airframe).

If you want more info, just ask

Michael

Michael,
You only left out one part: when a pilot does learn to fly the
PA30 and stay ahead of it, it's a blast to fly!! (especially the turbo'd
model)

-----Original Message-----
From: Michael ]
Posted At: Monday, January 30, 2006 11:54 AM
Posted To: rec.aviation.owning
Conversation: Whats the deal with counter-rotating props?
Subject: Whats the deal with counter-rotating props?

We're looking into a twin and the Twin Comanche is on our list. I
know
that the later models have the counter rotating props -- although i
don't know too much about what this means, other than its "better".

You've gotten a lot of uninformed opinions here. I started countering
them, and after the first post decided it would be better to put it
all
in one place. I have over 900 hours in Twin Comanches of various
flavors (including the CR), I've instructed in them, and I've worked
on
them. Having said that, here's what you're dealing with.

There are exactly two things a counter-rotating prop does for you in
flight.

First, it relieves you of the necessity of applying right rudder on
takeoff or climb.

Second, it allows you to fly at a slightly lower airspeed in the event
the left engine fails without rolling over - which boils down to being
able to fly not just much too slow, but much, much too slow. The
reality of the situation is that the PA-30 should never be flying at
less than 90 mph unless
(a) the runway is made and you are decelerating in preparation for the
flare, in which case power is near-idle and an engine failure is
irrelevant,
(b) you are in ground effect accelerating after takeoff, in which case
an engine failure calls for an immediate idling of the throttles and a
touchdown straight ahead, or
(c) you are at a safe altitude doing training.

The downside is you get an engine where the prop, prop governor,
magnetos, alternator, vacuum pump, fuel pump, oil pump, and tach
adapter are nonstandard, and thus rarely stocked. On top of that,
some
of those parts are used only on that one engine (the counter-rotating
IO-320) which is used on only that one airplane (the PA-30CR or PA-39)
which hasn't been made in decades and of which relatively few were
made. The impact on maintenance is spectacular.

The reason Piper modified the original PA-30 (in many ways - they
added
control linkages, stall strips, and finally the counter-rotating
props)
was to make it a more docile trainer. They didn't succeed. If you
buy
the plane, make sure you find an experienced Twin Comanche instructor
to train you. No amount of time in Seminoles, Duchesses, Apaches,
Geronimos, or similar trainers prepares someone to fly a PA-30. I've
seen what happens when someone who just took an MEI ride in a Duchess
gets into a PA-30 - he can't hold on to the tail. You rarely see
PA-30's as trainers anymore, and for good reason. When MEI's with
50-100 hours multi time tried to teach in them, the carnage was
spectacular.

You do not need an STC to convert a straight PA-30 to CR. There is a
service bulletin from Piper for the conversion. All you need to do is
buy the parts and convert the engine by simple parts replacement.
That
won't be cheap either.

With the low altitudes and cool weather of the East coast, the service
ceiling of the PA-30 is quite high enought (5800ft density altitude at
gross). Contrary to popular opinion, the PA-30 will fly just fine on
one engine, even at gross. BTDT.

Most of the PA-30's out there now have been modified to reduce Vmc.
The popular mod is the rudder fin (Knots-2-U and Robertson make them)
which improves lateral stability and reduces Vmc.

The real Vmc of a PA-30 without CR props is 80 mph. That's what was
designed in and verified by flight test at Swearingen. However, that
assumes you do everything right. Relatively small errors in technique
can raise that. This was fixed by raising the Vmc marking on the ASI
and in the AFM to 90 mph by AD (no changes were made to the airframe).

If you want more info, just ask

Michael

You only left out one part: when a pilot does learn to fly the
PA30 and stay ahead of it, it's a blast to fly!! (especially the turbo'd
model)

You're right - I left that out. I figured that part was obvious

Seriously - the plane had some rough edges, but nothing touches it for
comfort and efficiency in twins, and it is a blast to fly once you get
out in front of it. The turbo is OK, but the premier one is the Miller
Mod with IO-360's. Now THAT is a machine.

Michael

One thing that hasn't been mentioned in this thread is the tremendous
amount of pure BS that is published and retold as the truth concerning
both single and twin Comanches. Flight instructors check out new
Comanche pilots without having a clue as to what they are doing.
Mechanics work on Comanches and just make things worse.

Two tales concerning the landing gear illustrate the point. Perhaps you
recall the pictures of the Comanche landing gear up (in California, I
think). The newsies called it a Cherokee, but it really was a Comanche.
Anyway, a flight instructor was checking out a new Comanche pilot.
The gear failed to extend, so they went to the emergency gear extension
option. They removed the floor plate to access the release lever and
proceeded to pump the gear down for the next hour, but could never get
it to lock. Finally they landed gear up. The problem? The Comanche
gear has no hydraulics to pump down! It is all electrical and
mechanical. You simply push the lever all the way forward, and leave it
there (like the manual gear in the early Mooneys)! Of course, neither
pilot bothered to read the emergency extension procedure which was
written on the back of the access plate they removed as the first step.

The second tale concerns a Comanche pilot whose plane developed a
hesitation in retraction after gear up was selected. So he went to his
mechanic and described the symptoms. The mechanic thought a minute and
then opined that it was probably a problem with the hydraulic power pac.
There still aren't any hydraulics in the Comanche landing gear.

The International Comanche Society and the Comanche Flyer Foundation
have spent a bunch of money providing Comanche specific training for
flight instructors. Use them. While it is very important for a single
Comanche, it is vital for a twin. ICS can tell you who and where they
are. Similarly, find an experienced Comanche mechanic. It will save
you money in the long run.

Hank
Henry A. Spellman
Comanche N5903P

Dico wrote:
Hello,

We're looking into a twin and the Twin Comanche is on our list. I know
that the later models have the counter rotating props -- although i
don't know too much about what this means, other than its "better".

We fly a mooney now and thus I don't worry about the prop -- as long as
it keeps spinning.

Is there a web site that gives a good explanation as to why I want CR
props. There are hundreds of the earlier model Twin Comanches flying
without the CR props --- so what does someone with 300 hours single
engine time need to worry about? What actually goes wrong? And when
it does, what happens? I hear "critical engine" but it means very
little to me.

I like to fly and try to be very careful when I do fly... but I don't
follow too much aviation stuff other than how it affects me -- so I'm
not exactly a "buff", hence the above questions which may seem obvious
to many.

Why are we looking at a Twin Comanche? Because its a twin, safer for
IFR flight (perhaps this is only preceived), plus we live on the east
coast on an island so we're flying over water quite a bit. Also this
plane has decent speed and is an "economical" twin. We rarely fly with
4 people, so we don't need any more seats than 4.

Any help or links to help would be appreciated.

Thanks,

Dico

"Dico" wrote in message
oups.com...
Is there a web site that gives a good explanation as to why I want CR
props. There are hundreds of the earlier model Twin Comanches flying
without the CR props --- so what does someone with 300 hours single
engine time need to worry about? What actually goes wrong? And when
it does, what happens? I hear "critical engine" but it means very
little to me.


I would like to throw an interesting (if maybe false? I have no personal
experience or a cite.) factoid into this discussion.

I have read that the P38 Lightning was equipped with counter rotating props
that rotated in the opposite (down going blades on outboard side of engines)
from normal direction resulting in an aircraft with two critical engines and
much increased yaw force when either engine failed. This was done despite
control issues because it resulted in 15 knots extra top speed as compared
with CR props using typical configuration (down going blade on inboard side
of engines)

Someone more knowledgeable may be able to explain why.

Private wrote:
: I would like to throw an interesting (if maybe false? I have no personal
: experience or a cite.) factoid into this discussion.

: I have read that the P38 Lightning was equipped with counter rotating props
: that rotated in the opposite (down going blades on outboard side of engines)
: from normal direction resulting in an aircraft with two critical engines and
: much increased yaw force when either engine failed. This was done despite
: control issues because it resulted in 15 knots extra top speed as compared
: with CR props using typical configuration (down going blade on inboard side
: of engines)

: Someone more knowledgeable may be able to explain why.

Interesting. Perhaps less downward propwash on outboard ends of wings? Or
maybe propwash vorticies "lifting" the fuselage rather than "swatting it down?"

Just WAGs on my part.

-Cory


--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

Private wrote:
I have read that the P38 Lightning was equipped with counter rotating props
that rotated in the opposite (down going blades on outboard side of engines)
from normal direction resulting in an aircraft with two critical engines and
much increased yaw force when either engine failed.

This is definitely true, in the sense that the props did counter-rotate
opposite to what is now normal. However, the whole critical engine
thing is way overrated. Vmc is a control speed, not a performance
speed. It is quite a bit lower than Vyse, which is as slow as you ever
want to fly with anything close to full power (other than in a training
situation) whether you have an engine out or not. In other words, it's
really not a big deal, which is why it's rare to see counter-rotating
props in anything other than a trainer.

This was done despite
control issues because it resulted in 15 knots extra top speed as compared
with CR props using typical configuration (down going blade on inboard side
of engines)

That doesn't sound right. My understanding is that the reason was to
make the plane more maneuverable (using differential thrust for yaw).
Of course you could get the same effect by using a bigger rudder, but
that would mean more weight and drag, so maybe it boils down to the
same thing - a cleaner, lighter, and thus faster airframe with the same
maneuverability.

It is also my understanding that the extra maneuverability was rarely
used due to the complexity of engine management involved. Late in the
game, a prototype was built with single-lever engine controls. Richard
Bong, one of the most famous of the WWII aces, died test flying it when
an engine blew up.

Michael

"Michael" wrote in message
oups.com...
snip
Late in the game, a prototype was built with single-lever engine controls. Richard
Bong, one of the most famous of the WWII aces, died test flying it when
an engine blew up.

Michael

I thought he died in one of the first jets... perhaps a P-80?

Joe Schneider
8437R



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