SR Chevy engine mount and Cowl

[Mike Kimball]

My engine mount is nearing completion. I just uploaded two new pictures to
the SR engine section. Finger straps and gussets still need to be added
here and there. Also, a horizontal tube between the two upper engine mount
points on the firewall will be added to stiffen the mount. If you pull the
upper bolt on one side as it is now the mount still has a lot of flex.
Instead of relying on the channel riveted to the firewall to account for
this we decided we wanted the mount to be stiff on it's own.

I discussed Ken's comments with my mount builder and I think most of Ken's
concerns are addressed by the cross pieces that are now visible in the most
recent pics that I uploaded. Downward forces on the rear mount points are
accounted for with the tube from the upper fuselage point to the point below
the rear engine mount point. Outward forces on the rear mount point are
accounted for with the cross pieces at the lower rear of the mount that
weren't in the original pics. The rear mount points are now the apex of a
four point pyramid. Although I know adding a tube from the top firewall
points to the rear engine mount points would add strength, I'm not sure it
is necessary with the current configuration. The tube in it's current
position definitely has less of a bending moment on it then a tube going
directly from the firewall points to the engine mount points would. I'm
going to take one last look at it this weekend and make a final decision
about whether or not to add another tube. Thanks again Ken for your input.

I am now looking ahead to the cowling. I wonder if Bob and others could
tell me more about the channels used to begin building the four door cowl?
I have asked Murphy to quote me on three channels just like the one on the
firewall between the upper mount points but five feet in length. I was
thinking that I could use those as the left, right, and upper channels to
start the four door cowl idea. I think they are 3-inch channels with 1-inch
flanges made from something like 0.040 or 0.062 aluminum. (I'm not in my
shop - just guessing from memory). Next, I still want to find an
off-the-shelf nose bowl that would work. Any ideas on that? Lastly, I'm
hoping that I can simply form the cowl skins by hand somehow since I don't
have a sheet metal roller. Any ideas on that? HELP! Thanks

Mike Kimball
SR044




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[Al Paxhia]

Hi Mike,
If you are planning floats, a fifth mount at the top of the firewall,
attached to the V-brace will provide rough water protection. There is lots
of movement in the front cage when operating in bad water. In prior posts I
noted that the v-brace has broken and most of the rivets holding the
instrument panel in have sheared in rough water. When building the airplane
I did add a second channel that runs between the top motor mounts on the
back side of the firewall. I have noticed, all the mounts that MAM sell now
have the fifth mount and it connects to the v-brace. If I do it over I will
update to the five point motor mount.
Al
----- Original Message -----
From: "Mike Kimball" <mkimball@gci.net>
To: "Rebel List" <rebel-builders@dcsol.com>
Sent: Friday, November 26, 2004 1:45 PM
Subject: SR Chevy engine mount and Cowl

My engine mount is nearing completion. I just uploaded two new pictures
to
the SR engine section. Finger straps and gussets still need to be added
here and there. Also, a horizontal tube between the two upper engine
mount
points on the firewall will be added to stiffen the mount. If you pull
the
upper bolt on one side as it is now the mount still has a lot of flex.
Instead of relying on the channel riveted to the firewall to account for
this we decided we wanted the mount to be stiff on it's own.

I discussed Ken's comments with my mount builder and I think most of Ken's
concerns are addressed by the cross pieces that are now visible in the
most
recent pics that I uploaded. Downward forces on the rear mount points are
accounted for with the tube from the upper fuselage point to the point
below
the rear engine mount point. Outward forces on the rear mount point are
accounted for with the cross pieces at the lower rear of the mount that
weren't in the original pics. The rear mount points are now the apex of a
four point pyramid. Although I know adding a tube from the top firewall
points to the rear engine mount points would add strength, I'm not sure it
is necessary with the current configuration. The tube in it's current
position definitely has less of a bending moment on it then a tube going
directly from the firewall points to the engine mount points would. I'm
going to take one last look at it this weekend and make a final decision
about whether or not to add another tube. Thanks again Ken for your
input.

I am now looking ahead to the cowling. I wonder if Bob and others could
tell me more about the channels used to begin building the four door cowl?
I have asked Murphy to quote me on three channels just like the one on the
firewall between the upper mount points but five feet in length. I was
thinking that I could use those as the left, right, and upper channels to
start the four door cowl idea. I think they are 3-inch channels with
1-inch
flanges made from something like 0.040 or 0.062 aluminum. (I'm not in my
shop - just guessing from memory). Next, I still want to find an
off-the-shelf nose bowl that would work. Any ideas on that? Lastly, I'm
hoping that I can simply form the cowl skins by hand somehow since I don't
have a sheet metal roller. Any ideas on that? HELP! Thanks

Mike Kimball
SR044




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[Mike Kimball]

Wow. Great responses right away on my cowl, mount, and seafins! I have
already asked MAM to send me the seafin kit based on Al's description of
performance improvement. Robin said they helped a lot on the radial Moose
as well.

I just got an email from Robin at MAM and he says both the horizontal stab
mod and wing tip mod are for the SR. I don't know what's going on with the
Elite, but again - both mods are for the SR.

I just looked at the Rebel cowl photos. I have those on my desktop. I
could have posted them myself if I'd realized they were part of what was
lost. I sure wish I could see the inside view of the channels at left,
right, and top. But I think I understand what Wayne was describing about
using a piece of flat plate and some angle aluminum to build channels.
That's great because I have a lot of angle laying around. I just need to
get some plate.

Thanks again everyone!

Mike Kimball
SR044




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[Wayne G. O'Shea]

If you just had of asked last week Mike...I had the one side channel off, to
remove my battery from the firewall (to install it down the tailcone... to
make up for my hydraulic installation and to get ready for my amphibs in the
spring) and could have taken a shot.

Picture a flat sheet....then hinge halves sticking out each side and cut
short the same amount as the overlap onto the firewall lip and the cowl
lip...then backed up with a 3/4 x 3/4 angle with the perpendicular flange
flush with the flat sheets edge and the hinge eyes sticking out between
them. The angles are of course the same length as the flat sheet which
overlaps both lips so you can lock the flat and angle like a fork end over
the lips.

Wayne

----- Original Message -----
From: "Mike Kimball" <mkimball@gci.net>
To: "Rebel List" <rebel-builders@dcsol.com>
Sent: Friday, November 26, 2004 7:45 PM
Subject: Re: SR Chevy engine mount and Cowl

Wow. Great responses right away on my cowl, mount, and seafins! I have
already asked MAM to send me the seafin kit based on Al's description of
performance improvement. Robin said they helped a lot on the radial Moose
as well.

I just got an email from Robin at MAM and he says both the horizontal stab
mod and wing tip mod are for the SR. I don't know what's going on with

the

Elite, but again - both mods are for the SR.

I just looked at the Rebel cowl photos. I have those on my desktop. I
could have posted them myself if I'd realized they were part of what was
lost. I sure wish I could see the inside view of the channels at left,
right, and top. But I think I understand what Wayne was describing about
using a piece of flat plate and some angle aluminum to build channels.
That's great because I have a lot of angle laying around. I just need to
get some plate.

Thanks again everyone!

Mike Kimball
SR044




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[Ken]

Hi Mike

My thoughts are that you probably don't need a horizontal tube between
the two upper firewall points from a structural point of view. If you
look vertically downward on your
upper tubes it looks like they extend at close to a right angle off the
firewall. That means that unlike the Lycoming mounts, there will not be
large compressive forces trying to pull those two points together. You
can push and pull on a tube but you can't really transmit sideways
forces on a tube as it will simply flex or bend. Visualize the tube as a
piece of wire or string that
can be pulled or pushed on but easilly bent. If you want to add a
horizontal tube, I'd suggest that it would be more effective between the
lower firewall points as there it would reduce sideways compressive
forces transferred to the firewall as that is where they will appear on
this mount. I understand that the lower points are way stiffer than the
upper ones and I'm not saying that you need a lower tube, I'm just
saying that from the firewall's perspective it would be more useful on
the bottom if you feel you need it.

I like that you have addressed the yawing forces with the X bracing.
More comments embedded below:

Mike Kimball wrote:

My engine mount is nearing completion. I just uploaded two new

pictures to

the SR engine section. Finger straps and gussets still need to be added
here and there. Also, a horizontal tube between the two upper engine

mount

points on the firewall will be added to stiffen the mount. If you

pull the

upper bolt on one side as it is now the mount still has a lot of flex.
Instead of relying on the channel riveted to the firewall to account for
this we decided we wanted the mount to be stiff on it's own.

I discussed Ken's comments with my mount builder and I think most of Ken's
concerns are addressed by the cross pieces that are now visible in the

most

recent pics that I uploaded. Downward forces on the rear mount points are
accounted for with the tube from the upper fuselage point to the point

below

the rear engine mount point.

I'm not entirely comfortable with that last sentence Mike. If it were
mine I'd still move that tube from the upper firewall point back to the
rear engine mount (or add the extra tube). My concern is a sideways
buckling failure of the tube from the
lower firewall mount to the forward engine mount. That lower tube may
well be amply
strong but I think I'd want it analysed or tested for at least
8 g's of engine weight (8 g's would include some allowance for
gyroscopic loads) if
it were mine. If one of those lower tubes buckles you might well lose
the engine. Monitoring for cracks is fine for some things but not really
sufficient for a critical tube subject to sudden buckling failure
IMO. Sorry but FWIW I guess it still doesn't totally pass my eyeball
comfort test. As I said though I do tend to overbuild things.

Outward forces on the rear mount point are
accounted for with the cross pieces at the lower rear of the mount that
weren't in the original pics. The rear mount points are now the apex of a
four point pyramid. Although I know adding a tube from the top firewall
points to the rear engine mount points would add strength, I'm not sure it
is necessary with the current configuration.

I'm not sure it is necessary either but consider that with it you would
have a redundant mount where any single tube or tube weld could fracture
and you
would likely have a normal flight to destination. I am
familiar with a six cylinder 0-300 on a bed mount where on 3 occasions a
cracked forward engine mount was discovered during inspections. The
current theory is that it cracked whenever an aileron buzz event
happened (not a Murphy aircraft). The aileron situation has now been
corrected but I am very thankful that the mount had some redundancy in
it since few airplanes glide without an engine...

The tube in it's current
position definitely has less of a bending moment on it then a tube going
directly from the firewall points to the engine mount points would.

Sorry but I don't understand what tube you are referring to here.

Another thought is if you look at a certified Cessna 0-300 bed mount
(pre 1967 I believe) you may notice that the front engine mount pads are
joined left to right. If done correctly that eliminates the front to
back twisting of the tubes as the weight of the engine pushes down on
the mounting pads. That twisting will also likely hasten buckling of the
lower tube on this design and is a (small) part of the reason I'm
expressing concern.

One interesting way of getting a feel for a structure like this is to
glue up a
model out of toothpicks or straws or something and then apply some
forces with your finger and see what flexes/bends/breaks ;)

I'm
going to take one last look at it this weekend and make a final decision
about whether or not to add another tube. Thanks again Ken for your

input.

I am now looking ahead to the cowling. I wonder if Bob and others could
tell me more about the channels used to begin building the four door cowl?
I have asked Murphy to quote me on three channels just like the one on the
firewall between the upper mount points but five feet in length. I was
thinking that I could use those as the left, right, and upper channels to
start the four door cowl idea. I think they are 3-inch channels with

1-inch

flanges made from something like 0.040 or 0.062 aluminum. (I'm not in my
shop - just guessing from memory). Next, I still want to find an
off-the-shelf nose bowl that would work. Any ideas on that? Lastly, I'm
hoping that I can simply form the cowl skins by hand somehow since I don't
have a sheet metal roller. Any ideas on that? HELP! Thanks

You may recall the occasional mention of these cowls being somewhat
flimsy when open. I liked the result that I got by using channels/panels
tapered wider at the firewall end instead of the 3" x 0.032 channels
supplied by MAM.

Ken
ps - a buckling failure is the failure you get when you compress two
ends of a straight rod and it snaps or bends in the middle. If you
simultaneously push sideways on that rod it fails much sooner of course.

Mike Kimball
SR044

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[Mike Kimball]

Thanks again Ken. I appreciate your comments. I've been avoiding it, but I
may even get my old structure analysis books out from when I studied that
stuff in college. (Let's see, 20+ years ago...) Some finite analysis has
already been done on various parts of the mount. It is being designed to
withstand 10 Gs. However, I, like you, believe in the credibility of the
"eyeball" test. It just plain looks like it would be a lot stronger with
the tube added between the upper firewall mounts and the rear engine mounts.
It would add a negligible amount of weight. If it doesn't get in the way of
anything why wouldn't I add the tube? I'm going to go stare at the mount
for a while and try to visualise force vectors, harmonic waveforms, the
nature of the universe, and the meaning of life. :-)

Mike

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com]On Behalf Of Ken
Sent: Friday, November 26, 2004 5:56 PM
To: rebel-builders@dcsol.com
Subject: Re: SR Chevy engine mount and Cowl


Hi Mike

My thoughts are that you probably don't need a horizontal tube between
the two upper firewall points from a structural point of view. If you
look vertically downward on your
upper tubes it looks like they extend at close to a right angle off the
firewall. That means that unlike the Lycoming mounts, there will not be
large compressive forces trying to pull those two points together. You
can push and pull on a tube but you can't really transmit sideways
forces on a tube as it will simply flex or bend. Visualize the tube as a
piece of wire or string that
can be pulled or pushed on but easilly bent. If you want to add a
horizontal tube, I'd suggest that it would be more effective between the
lower firewall points as there it would reduce sideways compressive
forces transferred to the firewall as that is where they will appear on
this mount. I understand that the lower points are way stiffer than the
upper ones and I'm not saying that you need a lower tube, I'm just
saying that from the firewall's perspective it would be more useful on
the bottom if you feel you need it.

I like that you have addressed the yawing forces with the X bracing.
More comments embedded below:

Mike Kimball wrote:

My engine mount is nearing completion. I just uploaded two new

pictures to

the SR engine section. Finger straps and gussets still need to be added
here and there. Also, a horizontal tube between the two upper engine

mount

points on the firewall will be added to stiffen the mount. If you

pull the

upper bolt on one side as it is now the mount still has a lot of flex.
Instead of relying on the channel riveted to the firewall to account for
this we decided we wanted the mount to be stiff on it's own.

I discussed Ken's comments with my mount builder and I think most of Ken's
concerns are addressed by the cross pieces that are now visible in the

most

recent pics that I uploaded. Downward forces on the rear mount points are
accounted for with the tube from the upper fuselage point to the point

below

the rear engine mount point.

I'm not entirely comfortable with that last sentence Mike. If it were
mine I'd still move that tube from the upper firewall point back to the
rear engine mount (or add the extra tube). My concern is a sideways
buckling failure of the tube from the
lower firewall mount to the forward engine mount. That lower tube may
well be amply
strong but I think I'd want it analysed or tested for at least
8 g's of engine weight (8 g's would include some allowance for
gyroscopic loads) if
it were mine. If one of those lower tubes buckles you might well lose
the engine. Monitoring for cracks is fine for some things but not really
sufficient for a critical tube subject to sudden buckling failure
IMO. Sorry but FWIW I guess it still doesn't totally pass my eyeball
comfort test. As I said though I do tend to overbuild things.

Outward forces on the rear mount point are
accounted for with the cross pieces at the lower rear of the mount that
weren't in the original pics. The rear mount points are now the apex of a
four point pyramid. Although I know adding a tube from the top firewall
points to the rear engine mount points would add strength, I'm not sure it
is necessary with the current configuration.

I'm not sure it is necessary either but consider that with it you would
have a redundant mount where any single tube or tube weld could fracture
and you
would likely have a normal flight to destination. I am
familiar with a six cylinder 0-300 on a bed mount where on 3 occasions a
cracked forward engine mount was discovered during inspections. The
current theory is that it cracked whenever an aileron buzz event
happened (not a Murphy aircraft). The aileron situation has now been
corrected but I am very thankful that the mount had some redundancy in
it since few airplanes glide without an engine...

The tube in it's current
position definitely has less of a bending moment on it then a tube going
directly from the firewall points to the engine mount points would.

Sorry but I don't understand what tube you are referring to here.

Another thought is if you look at a certified Cessna 0-300 bed mount
(pre 1967 I believe) you may notice that the front engine mount pads are
joined left to right. If done correctly that eliminates the front to
back twisting of the tubes as the weight of the engine pushes down on
the mounting pads. That twisting will also likely hasten buckling of the
lower tube on this design and is a (small) part of the reason I'm
expressing concern.

One interesting way of getting a feel for a structure like this is to
glue up a
model out of toothpicks or straws or something and then apply some
forces with your finger and see what flexes/bends/breaks ;)

I'm
going to take one last look at it this weekend and make a final decision
about whether or not to add another tube. Thanks again Ken for your

input.

I am now looking ahead to the cowling. I wonder if Bob and others could
tell me more about the channels used to begin building the four door cowl?
I have asked Murphy to quote me on three channels just like the one on the
firewall between the upper mount points but five feet in length. I was
thinking that I could use those as the left, right, and upper channels to
start the four door cowl idea. I think they are 3-inch channels with

1-inch

flanges made from something like 0.040 or 0.062 aluminum. (I'm not in my
shop - just guessing from memory). Next, I still want to find an
off-the-shelf nose bowl that would work. Any ideas on that? Lastly, I'm
hoping that I can simply form the cowl skins by hand somehow since I don't
have a sheet metal roller. Any ideas on that? HELP! Thanks

You may recall the occasional mention of these cowls being somewhat
flimsy when open. I liked the result that I got by using channels/panels
tapered wider at the firewall end instead of the 3" x 0.032 channels
supplied by MAM.

Ken
ps - a buckling failure is the failure you get when you compress two
ends of a straight rod and it snaps or bends in the middle. If you
simultaneously push sideways on that rod it fails much sooner of course.

Mike Kimball
SR044

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[Ken]

Be Careful Mike
Anyone of those things can cause headaches, loss of memory, and cause
you to skip forward in time several days.
I shudder to think of doing them all at once... ;)
Ken

Mike Kimball wrote:

snip

I'm going to go stare at the mount
for a while and try to visualise force vectors, harmonic waveforms, the
nature of the universe, and the meaning of life. :-)

Mike

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