The Murphy Builders Site

Posted: **Fri Feb 17, 2012 8:47 pm**

Wayne,

WD props on a 912S, with a compression ratio of 9.1, were not experiencing
any blade problems and it was believed because of the low 100 HP. My 912S
engine can only idle at about 1600 RPM or higher. But they are geared down
to 2.47 at the prop. Would the gearing make any difference with low RPM
theory? Perhaps now, in light of this suspected low RPM shocking of the
blades, more can be understood with the WD blade failures. I was always
suspicious of the low HP theory because an IO 240 is only 125 HP.
Bruce G

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Posted: **Fri Feb 17, 2012 8:47 pm**

A couple of commits to add to the Warp Drive crack/delamination problems, on
my IO 240 when you shut it down by pulling the mixture to idle cut off the
engine will quite often motor along very violently at 200 to 400 RPM for 2 to
5 seconds before stoping. I have observed this on other IO 240 powered planes
with wood, metal and Warp Drive propellers. I have my idle set at 650 RPM
with the metal Sensenich propeller. ( Very smooth idle )

If you are planning to install floats on your Rebel using the Warp Drive
propeller, 1100 RPM for water docking operations does not work. Even 500 RPM
is sometimes to fast.

Steve Sloan 536R

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Posted: **Fri Feb 17, 2012 8:47 pm**

HI Wayne

Very interesting comments on this subject. Thanks very much I will keep
this in mind in my ground handling techniques.

Best Regards

Brian #328R

-----Original Message-----
From: murphy-rebel@dcsol.com [mailto:murphy-rebel@dcsol.com]On Behalf Of
Wayne G. O'Shea
Sent: Sunday, March 11, 2001 12:04 AM
To: murphy-rebel@dcsol.com
Subject: Warp Drive Cracks

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To add to our previous discussions about Warp Drive crack/delamination
problems. Skystar Aircraft has issued a Service Alert for Warp Drive Props
installed on IO-240 powered kitfoxes.

It states, (like we have all been "theorizing"), that it is suspected that
this is happening because the low-inertia propellor and a high-compression
engine are resulting in abnormally high propellor blade "shock". The
interesting thing about their Service Alert though, is that it is suspected
that this happens at LOW IDLE RPM and not when making power!!!! They suggest
to never idle below 1100RPM. I rarely idle below 1200 to 1400 RPM, with the
mixture pulled VERY lean to keep the 100LL from fouling the plugs in my
O-320 and my propellor is holding up fine so far (KNOCK ON WOOD). I can do
this around here as I am flying off the grass/snow and don't have to really
taxi around anywhere with my private strip and a mid-field hanger. Anyone
with blade problems, do you idle down low 700/800 RPM while taxing those
long paved areas in Brampton/Hamilton etc. to save your brake pads or just
have the habit of pulling the throttle right back to the stop while sitting
idling???? Maybe we can do our selves a favour if we can nail down "pilot"
habit for idle speeds and the asociated W.D. problems.

Regards to all,
Wayne G. O'Shea
www.irishfield.on.ca

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Posted: **Fri Feb 17, 2012 8:47 pm**

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To: <murphy-rebel@dcsol.com>
From: Bob Patterson <bob.patterson@canrem.com>
Subject: Re: Warp Drive Cracks

Thanks for the update, Wayne ! I've heard that theory expressed
before, and it makes sense that there'd be less 'flywheel effect' at
lower rpm ...

<I> was always taught NEVER to idle below 1,000 rpm, as there
wasn't enough oil pressure to prevent damage to the crankshaft bearings
from the sharp thrusts from the pistons. Of course, those old 65 hp.
Continentals needed all the help they could get ! ;-)

....bobp

---------------------------------orig.----------------------------------
At 12:04 AM 3/11/01 -0500, you wrote:

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To add to our previous discussions about Warp Drive crack/delamination
problems. Skystar Aircraft has issued a Service Alert for Warp Drive Props
installed on IO-240 powered kitfoxes.

It states, (like we have all been "theorizing"), that it is suspected that
this is happening because the low-inertia propellor and a high-compression
engine are resulting in abnormally high propellor blade "shock". The
interesting thing about their Service Alert though, is that it is suspected
that this happens at LOW IDLE RPM and not when making power!!!! They suggest
to never idle below 1100RPM. I rarely idle below 1200 to 1400 RPM, with the
mixture pulled VERY lean to keep the 100LL from fouling the plugs in my
O-320 and my propellor is holding up fine so far (KNOCK ON WOOD). I can do
this around here as I am flying off the grass/snow and don't have to really
taxi around anywhere with my private strip and a mid-field hanger. Anyone
with blade problems, do you idle down low 700/800 RPM while taxing those
long paved areas in Brampton/Hamilton etc. to save your brake pads or just
have the habit of pulling the throttle right back to the stop while sitting
idling???? Maybe we can do our selves a favour if we can nail down "pilot"
habit for idle speeds and the asociated W.D. problems.

Regards to all,
Wayne G. O'Shea
www.irishfield.on.ca

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Posted: **Fri Feb 17, 2012 8:47 pm**

To add to our previous discussions about Warp Drive crack/delamination
problems. Skystar Aircraft has issued a Service Alert for Warp Drive Props
installed on IO-240 powered kitfoxes.

It states, (like we have all been "theorizing"), that it is suspected that
this is happening because the low-inertia propellor and a high-compression
engine are resulting in abnormally high propellor blade "shock". The
interesting thing about their Service Alert though, is that it is suspected
that this happens at LOW IDLE RPM and not when making power!!!! They suggest
to never idle below 1100RPM. I rarely idle below 1200 to 1400 RPM, with the
mixture pulled VERY lean to keep the 100LL from fouling the plugs in my
O-320 and my propellor is holding up fine so far (KNOCK ON WOOD). I can do
this around here as I am flying off the grass/snow and don't have to really
taxi around anywhere with my private strip and a mid-field hanger. Anyone
with blade problems, do you idle down low 700/800 RPM while taxing those
long paved areas in Brampton/Hamilton etc. to save your brake pads or just
have the habit of pulling the throttle right back to the stop while sitting
idling???? Maybe we can do our selves a favour if we can nail down "pilot"
habit for idle speeds and the asociated W.D. problems.

Regards to all,
Wayne G. O'Shea
www.irishfield.on.ca

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Posted: **Fri Feb 17, 2012 8:47 pm**

Yes the gearing matters a great deal. You may notice that one of the
holy grails of psru's (speed reduction units) is to get some kind of
viscoelastic torque damper in the drive to the prop. This is not to
dampen the piston power pulses but rather to tune the much more
destructive torsional vibration range to below the normal operating
range of the powerplant. Thus it is even more important to keep most
engines with speed reduction units idling above a minimum of 1000 or
1200 rpm (or higher). Ever watch the rattle and shake of a zenith 601
with a rotax 912 while it starts up or shuts down? The airframe is
momentarily a torsional tuning fork. Unfortunately it is not usually
obvious if the crankshaft is resonating instead of the airframe (until
something breaks).

If you are in the range of torsional vibration, the destructive forces
(torques) are easily an order of magnitude higher than the piston power
pulses. This is possibly why the higher compression engines are more
problem at low idle and sub-idle rpm than they are at full power.??
Direct drive airplane engines are certainly not immune to this problem
but I know even less about that. I will venture that the problem range
rises closer to idle speed with a lighter prop and that a rubber damper
would indeed reduce the problem rpm by making the rotating system less
stiff. And with that, I have exhausted most of my knowledge and
conjecture. Wayne's question about idle rpm may be very relevant.

Ken

Legeorgen@cs.com wrote:

Wayne,

WD props on a 912S, with a compression ratio of 9.1, were not experiencing
any blade problems and it was believed because of the low 100 HP. My 912S
engine can only idle at about 1600 RPM or higher. But they are geared down
to 2.47 at the prop. Would the gearing make any difference with low RPM
theory? Perhaps now, in light of this suspected low RPM shocking of the
blades, more can be understood with the WD blade failures. I was always
suspicious of the low HP theory because an IO 240 is only 125 HP.
Bruce G

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Posted: **Fri Feb 17, 2012 8:47 pm**

Received: from smtp.interlog.com ([154.11.89.176])
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To: <murphy-rebel@dcsol.com>
From: Bob Patterson <bob.patterson@canrem.com>
Subject: Re: Warp Drive Cracks

Hi Bruce !

The gear reduction is the secret ! Most re-drives have gone
to great lengths to reduce the power pulse problem, as it was
destroying their drives ! They usually have blocks, wedges, or
bushings of urethane to absorb the pulses, and, of course, the
gearing itself reduces the effect, as well.

There haven't been any reported W-D problems on 912s or
Subarus with re-drives. Another good argument for using 'modern'
engines !! ;-)
.....bobp

---------------------------------orig.---------------------------------
At 04:49 PM 3/11/01 EST, you wrote:

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Wayne,

WD props on a 912S, with a compression ratio of 9.1, were not experiencing
any blade problems and it was believed because of the low 100 HP. My 912S
engine can only idle at about 1600 RPM or higher. But they are geared down
to 2.47 at the prop. Would the gearing make any difference with low RPM
theory? Perhaps now, in light of this suspected low RPM shocking of the
blades, more can be understood with the WD blade failures. I was always
suspicious of the low HP theory because an IO 240 is only 125 HP.
Bruce G
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Posted: **Fri Feb 17, 2012 8:48 pm**

Wayne

I think you hit the nail on the head there.

Yes, I do fall to those low RPM idle Gods.
On start-up I feel that I am trying to save wear on the engine if I can idle
at a low RPM until the oils heats up a bit.
I too, do lean the mixture out during idle to prevent foul-up. But idling at
over 1000 RPM on pavement requires brake pressure.
Sure its nice to sit there with little pressure on the brakes, but obviously
my train of thought will change.

Rick Ford

----- Original Message -----
From: Wayne G. O'Shea <oifa@irishfield.on.ca>
To: <murphy-rebel@dcsol.com>
Sent: Sunday, March 11, 2001 12:04 AM
Subject: Warp Drive Cracks

*-------------------------------------------------------------------------*

The list archives are located at:
http://www.dcsol.com:81/public/default.htm
username: rebel password: builder

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To add to our previous discussions about Warp Drive crack/delamination
problems. Skystar Aircraft has issued a Service Alert for Warp Drive Props
installed on IO-240 powered kitfoxes.

It states, (like we have all been "theorizing"), that it is suspected that
this is happening because the low-inertia propellor and a high-compression
engine are resulting in abnormally high propellor blade "shock". The
interesting thing about their Service Alert though, is that it is

suspected

that this happens at LOW IDLE RPM and not when making power!!!! They

suggest

to never idle below 1100RPM. I rarely idle below 1200 to 1400 RPM, with

the

mixture pulled VERY lean to keep the 100LL from fouling the plugs in my
O-320 and my propellor is holding up fine so far (KNOCK ON WOOD). I can do
this around here as I am flying off the grass/snow and don't have to

really

taxi around anywhere with my private strip and a mid-field hanger. Anyone
with blade problems, do you idle down low 700/800 RPM while taxing those
long paved areas in Brampton/Hamilton etc. to save your brake pads or just
have the habit of pulling the throttle right back to the stop while

sitting

idling???? Maybe we can do our selves a favour if we can nail down "pilot"
habit for idle speeds and the asociated W.D. problems.

Regards to all,
Wayne G. O'Shea
www.irishfield.on.ca

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Posted: **Fri Feb 17, 2012 8:48 pm**

Rick, on the Warp Drive installation that eventually led to cracks and delamination,
what torque did you have on the bolts, both the 3/8 hub prop bolts and also the 1/4
inch blade clamp bolts? I'm not suggesting this had anything to do with the problem,
but just curious what you were using.

Rick and Cathy Ford wrote:

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Wayne

I think you hit the nail on the head there.

Yes, I do fall to those low RPM idle Gods.
On start-up I feel that I am trying to save wear on the engine if I can idle
at a low RPM until the oils heats up a bit.
I too, do lean the mixture out during idle to prevent foul-up. But idling at
over 1000 RPM on pavement requires brake pressure.
Sure its nice to sit there with little pressure on the brakes, but obviously
my train of thought will change.

Rick Ford

----- Original Message -----
From: Wayne G. O'Shea <oifa@irishfield.on.ca>
To: <murphy-rebel@dcsol.com>
Sent: Sunday, March 11, 2001 12:04 AM
Subject: Warp Drive Cracks

*-------------------------------------------------------------------------*
The list archives are located at:
http://www.dcsol.com:81/public/default.htm
username: rebel password: builder

*-------------------------------------------------------------------------*
To add to our previous discussions about Warp Drive crack/delamination
problems. Skystar Aircraft has issued a Service Alert for Warp Drive Props
installed on IO-240 powered kitfoxes.

It states, (like we have all been "theorizing"), that it is suspected that
this is happening because the low-inertia propellor and a high-compression
engine are resulting in abnormally high propellor blade "shock". The
interesting thing about their Service Alert though, is that it is
suspected
that this happens at LOW IDLE RPM and not when making power!!!! They
suggest
to never idle below 1100RPM. I rarely idle below 1200 to 1400 RPM, with
the
mixture pulled VERY lean to keep the 100LL from fouling the plugs in my
O-320 and my propellor is holding up fine so far (KNOCK ON WOOD). I can do
this around here as I am flying off the grass/snow and don't have to
really
taxi around anywhere with my private strip and a mid-field hanger. Anyone
with blade problems, do you idle down low 700/800 RPM while taxing those
long paved areas in Brampton/Hamilton etc. to save your brake pads or just
have the habit of pulling the throttle right back to the stop while
sitting
idling???? Maybe we can do our selves a favour if we can nail down "pilot"
habit for idle speeds and the asociated W.D. problems.

Regards to all,
Wayne G. O'Shea
www.irishfield.on.ca

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