The Murphy Builders Site

I have another question for my esteemed colleagues. I have been told that
the engine should be offset to the right about one degree and slightly
downwards as well. I was told by MAM that this was accomplished on the
prototype mount with a 3/8 shim on the upper left mount point. Leaving the
lower left mount point unshimmed I assume there were some other smaller
shims at the remaining two mount points. I also have a drawing from MAM
locating the prop center which is centered on the firewall. My question is,
should the prop center end up centered with respect to the airframe AFTER
adding the one degree offset down and to the right, or do I locate the prop
center exactly as shown in the drawing, then make the offset adjustment?
With about 46 inches from the firewall to the prop flange on my engine a one
degree offset moves the location of the prop about 3/4 of an inch. This
could have repurcussions on the cowling assuming I use MAM's cowling. If I
leave the prop centered with respect to the firewall/airframe that means the
back of the engine will be shifted to the left about 3/4 of an inch. I
don't know if the engine CofG being slightly to the left (something less
than the total 3/4 of an inch) will make much difference. I end up keeping
extra fuel in the right wing to keep things balanced already with me sitting
in the left seat.

Lastly, seems like someone on the list mentioned that slightly downwards is
a good thing. Judging by MAMs shimming they tend to agree. Anyone know
why? Horizontal stabilizer a bit too powerful? Too much downward force
back there? Firewall canted upwards slightly with respect to the overall
airframe design for level flight? What's the deal?

Mike




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Hi Mike.

It's all to do with P factor, torque and thrust line verses drag.
For those of us with the M14 we'd have to offset to the left due to the
opposite direction of prop rotation.
The down thrust is to balance drag as the thrust line is below it
reducing the pitch up tendency with an increase of thrust levers.
They engine c/g is irrelevant as you'd have to consider all the
accessories if one was concerned.
Yes you'd have to factor the offset before final fit of cowl.

Those are the short answers as not too many online with OSK goings on.
I'll let the esteemed ones confirm your requirements.
Regards,
Warren
Moose 029

Mike Kimball wrote:

I have been told that the engine should be offset to the right about one degree and slightly
downwards as well.

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I also understood that it was normal to offset the rear of the engine
and keep the prop in the center for a symetrical cowl which is what I
did on my Rebel. If you offset the front of the engine I think it tends
to cancel some of the effect of the offset thrust line. Somewhat like a
twin with one engine shutdown ;)
However I thought that about 2 years ago I heard Daryl say that he
recommended no offset of any kind for any of his aircraft which caused
me to wonder whether it really matters...
Ken

Warren & Marg Montgomery wrote:

Hi Mike.

It's all to do with P factor, torque and thrust line verses drag.
For those of us with the M14 we'd have to offset to the left due to the
opposite direction of prop rotation.
The down thrust is to balance drag as the thrust line is below it
reducing the pitch up tendency with an increase of thrust levers.
They engine c/g is irrelevant as you'd have to consider all the
accessories if one was concerned.
Yes you'd have to factor the offset before final fit of cowl.

Those are the short answers as not too many online with OSK goings on.
I'll let the esteemed ones confirm your requirements.
Regards,
Warren
Moose 029

Mike Kimball wrote:

I have been told that the engine should be offset to the right about one degree and slightly
downwards as well.

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Hi Ken: For what it is worth I have been flying my O320 equipped Rebel for
two years now with no Engine offset (Unless it was built into the Murphy
Dynafocal mount) with no ill side effects. Climb is straight forward with
just a little right rudder required (normal) and none required at cruise
because I have a stiffer spring on the right side.If one where to add a
rudder trim tab (electric) you could get real lazy with the feet if you
wanted to.( I would not recommend the lazy part though as you do need to pay
attention on the ground with a tail dragger) Bob J Rebel "652"




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Hi Bob
Interesting.
I have an older conical Lycoming mount from MAM here that does have both
right offset and a bit of down tilt so I copied that for my Subaru. I
have a second hand drawing of the prop position taken from a Dynafocal
that suggests they are offset too but I don't know for sure or whether
they are all the same.
The archives suggest that one can get away without offsetting the
vertical stab as well !
Ken

Robert and Olga wrote:

Hi Ken: For what it is worth I have been flying my O320 equipped Rebel for
two years now with no Engine offset (Unless it was built into the Murphy
Dynafocal mount) with no ill side effects. Climb is straight forward with
just a little right rudder required (normal) and none required at cruise
because I have a stiffer spring on the right side.If one where to add a
rudder trim tab (electric) you could get real lazy with the feet if you
wanted to.( I would not recommend the lazy part though as you do need to pay
attention on the ground with a tail dragger) Bob J Rebel "652"

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Ken That is quite possible that the offset was already built into the mount.
I never checked it. The cowl I made was the 4 door aluminium version which
started by mounting the nose bowel on the front of the engine and working
from there. Next time I am out, I will check with a tape measure from common
points on each side of the aircraft to the spinner point and see what kind
of difference I can come up with. I did not add any offset on my fin
either - It is dead center of the fuselage and that is obviously not a
problem. When I was building some one on the web wrote in that if you built
with 3/4 offset you would be flying in circles all day or standing on the
rudder to compensate. Best regards. Bob J. Rebel 652




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When I built my engine mount I added some right offset but no down tilt
which was Daryl's recommendation at the time. I didn't move the rear of the
engine over so as a result my prop is about 3/4" off the centreline of the
fuselage. You can see the offset on my plane but you have to look carefully.
Drew

At 04:11 PM 7/31/2004 -0400, you wrote:

Hi Bob
Interesting.
I have an older conical Lycoming mount from MAM here that does have both
right offset and a bit of down tilt so I copied that for my Subaru. I
have a second hand drawing of the prop position taken from a Dynafocal
that suggests they are offset too but I don't know for sure or whether
they are all the same.
The archives suggest that one can get away without offsetting the
vertical stab as well !
Ken

Robert and Olga wrote:

Hi Ken: For what it is worth I have been flying my O320 equipped Rebel for
two years now with no Engine offset (Unless it was built into the Murphy
Dynafocal mount) with no ill side effects. Climb is straight forward with
just a little right rudder required (normal) and none required at cruise
because I have a stiffer spring on the right side.If one where to add a
rudder trim tab (electric) you could get real lazy with the feet if you
wanted to.( I would not recommend the lazy part though as you do need to pay
attention on the ground with a tail dragger) Bob J Rebel "652"

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Drew





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