SR-2500 Prop Location
Posted: Sun Feb 19, 2012 4:31 pm
I finally got the SR-2500 prop location info from MAM uploaded to the
SR/Moose - Misc section of the archives. MAM passed on another tidbit that
was crucial in building the engine mount. They told me that in addition to
the thrust location shown on the drawing, the thrust vector should be
slightly to the right and down. OK. How much? They told me they used a
3/8 spacer (of some sort) in the upper, left (pilot's side) engine mount
location on the firewall. A little math gave me a 0.55 degree offset to the
right and a 0.91 degree offset down. The mount was built with these offsets
in place. The mount was built with the entire engine rotated appropriately,
keeping the C of G in the middle. In addition, there are lots of dimensions
identifying the location of all four firewall engine mount locations among
other things. After my firewall was riveted in place I double checked those
measurements. Mine was very close but there were a couple of measurements
that were slightly off. I'm talking 1/32, or 1/16 off in a couple of
places. Tiny, but taken into account when making a firewall mockup out of
steel plate to begin the mount construction. Lastly, the engine C of G
location for the Chevy is different than what's identified on the drawing.
I found the C of G (fore and aft) by the simple means of balancing the
engine on a piece of tubing on a table. Not precise, but precise enough.
The good news is that the C of G of my Chevy is about 3 inches closer to the
firewall than the Lycoming even with a generous amount of space between the
water pump and the firewall, helping to offset the greater weight.
I also did a huge amount of research on Lord mounts. I finally settled on
one that is used in an aerobatic airplane used by a Canadian team with as
much horsepower as my plane and regularly pulling unbelievable Gs. I don't
have the part number handy, nor do I remember the plane it is used in, but I
can dig out that information if anybody is interested. One nice thing about
the Lord mounts I chose is that both halves are the same and they are
oriented vertically, one above the other. The weight of the engine rests on
the lower half with the upper half not doing much of anything unless I fly
poorly and exert lateral forces on them. If I manage to fly the plane
enough to start wearing out the bottom ones I can probably just switch them
around. I believe there are Lord mounts where one half is different than
the other and you must get them in right and can't swap them later.
Everything came together very well. Because of troubles I've run into I
have had the unfortunate experience removing and later remounting the engine
twice now. Performing this feat on the tarmac using an automotive engine
hoist takes less than three hours for removal and the same to remount,
including cowl removal/replacement and all hookups. It's a good thing the
plane isn't an inch taller or my engine hoist wouldn't reach high enough.
:-)
Mike
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SR/Moose - Misc section of the archives. MAM passed on another tidbit that
was crucial in building the engine mount. They told me that in addition to
the thrust location shown on the drawing, the thrust vector should be
slightly to the right and down. OK. How much? They told me they used a
3/8 spacer (of some sort) in the upper, left (pilot's side) engine mount
location on the firewall. A little math gave me a 0.55 degree offset to the
right and a 0.91 degree offset down. The mount was built with these offsets
in place. The mount was built with the entire engine rotated appropriately,
keeping the C of G in the middle. In addition, there are lots of dimensions
identifying the location of all four firewall engine mount locations among
other things. After my firewall was riveted in place I double checked those
measurements. Mine was very close but there were a couple of measurements
that were slightly off. I'm talking 1/32, or 1/16 off in a couple of
places. Tiny, but taken into account when making a firewall mockup out of
steel plate to begin the mount construction. Lastly, the engine C of G
location for the Chevy is different than what's identified on the drawing.
I found the C of G (fore and aft) by the simple means of balancing the
engine on a piece of tubing on a table. Not precise, but precise enough.
The good news is that the C of G of my Chevy is about 3 inches closer to the
firewall than the Lycoming even with a generous amount of space between the
water pump and the firewall, helping to offset the greater weight.
I also did a huge amount of research on Lord mounts. I finally settled on
one that is used in an aerobatic airplane used by a Canadian team with as
much horsepower as my plane and regularly pulling unbelievable Gs. I don't
have the part number handy, nor do I remember the plane it is used in, but I
can dig out that information if anybody is interested. One nice thing about
the Lord mounts I chose is that both halves are the same and they are
oriented vertically, one above the other. The weight of the engine rests on
the lower half with the upper half not doing much of anything unless I fly
poorly and exert lateral forces on them. If I manage to fly the plane
enough to start wearing out the bottom ones I can probably just switch them
around. I believe there are Lord mounts where one half is different than
the other and you must get them in right and can't swap them later.
Everything came together very well. Because of troubles I've run into I
have had the unfortunate experience removing and later remounting the engine
twice now. Performing this feat on the tarmac using an automotive engine
hoist takes less than three hours for removal and the same to remount,
including cowl removal/replacement and all hookups. It's a good thing the
plane isn't an inch taller or my engine hoist wouldn't reach high enough.
:-)
Mike
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