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Author Topic: THROUG THE FIREWALL MOROR MOUNTS - Revisited  (Read 2901 times)

Online Ken Culbertson

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THROUG THE FIREWALL MOROR MOUNTS - Revisited
« on: April 15, 2019, 02:49:37 PM »
Dannis Adamissin posted a design for"through the firewall" rear mounts for electrics back in 2011.  I found out too late that my new front mounts were just not strong enough for a Cobra 3520/14.  I toyed around with various alternatiives like metal brackets till I found his post.  What I am asking is do they work for a motor as large as the 3520/14?  I suspect they do.

Ken
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Offline Fred Underwood

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #1 on: April 15, 2019, 03:01:14 PM »
Rear mount of the Cobra will give significantly shorter bearing life than front mount.  There have been comments about that over a few years.  Lucien, the fellow at Innov8tive RC who was involved with the Cobra development, initially told me to rear mount.  He later changed that recommendation for CL to front mount due to our hard corners and load changes.....    He still liked rear mount for RC.
Fred
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Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #2 on: April 15, 2019, 04:22:34 PM »
Rear mount of the Cobra will give significantly shorter bearing life than front mount.  There have been comments about that over a few years.  Lucien, the fellow at Innov8tive RC who was involved with the Cobra development, initially told me to rear mount.  He later changed that recommendation for CL to front mount due to our hard corners and load changes.....    He still liked rear mount for RC.
I totally agree but I am faced with a situation that may not let me do a front.  Fuselage is too thin and there is no place to adequately anchor it.  I tried that and it took me a 1/2 hour to find the motor in the grass.  Luckily the battery stayed in and I was able to utilize my years of sailplane flying to gracefully get it down in one piece!  I am considering an aluminum bracket that looks like a Morris nose ring with wings that I can bolt to the doublers.  Dennis's mount sure is simpler.

Ken
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Offline Fred Underwood

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #3 on: April 15, 2019, 05:43:06 PM »
I used the x mount that came with the 3520 Cobra.  Mounting isn't the problem if you have the rear bulkhead.  There are a couple of thread about rear mounts and if I recall correctly, the answer was not to avoid rear mount, but select motors carefully based on bearing life.  Paul Walker and a few others went back to Plettenberg's at least partially because of bearing life and rear mounting.  Part of the reasoning is also that the bell of the motor may have harmonic oscillation worse at about our needed rpms, and that Cobras in particular may be at a bad harmonic frequency, so not just bearings.


Some info here
https://stunthanger.com/smf/gettin-all-amp'ed-up!/bearings-49917/msg516876/#msg516876
Fred
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Offline Dennis Adamisin

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #4 on: April 16, 2019, 12:39:03 PM »
Dannis Adamissin posted a design for"through the firewall" rear mounts for electrics back in 2011.  I found out too late that my new front mounts were just not strong enough for a Cobra 3520/14.  I toyed around with various alternatiives like metal brackets till I found his post.  What I am asking is do they work for a motor as large as the 3520/14?  I suspect they do.

Ken
First off, nice job digging up that old thread of mine!

The stresses on a through the firewall mount are the same as on a rear mount.  However with a big hole through the middle of the mounting plate, its important that the perimeter of the mount is properly anchored, especially near the mounting holes for the X-mount.  Another problem with a 35xx motor is whether the x-mount reaches past the periphery of the motor far enough so that you get enough material to mount to.

If the X-Plate is too small to cover over the hole in the motor mount (very likely with a 35xx other than the Arrowind) then you might try to build-in the mount.  Key to this is to have a hole in the nose big enough to install the motor, and have access to the motor mounting screw through the battery box.  Build-in your X-mount with access to the motor mounting screws, or use a Hunt "Hardnose mount, or even use 1/4" plywood, with brass tubing inserts and bolt the motor in from behind.

One way to GREATLY reduce the stress on a front mount is to add a rear bearing to keep the "south" end of the motor from moving around.  A relatively easy way to do that is to front mount the motor, but use the bolt-on prop adapter on the rear of the motor.  Get a bearing from Boca that fits the prop adapter and bake a bearing holder that you can bolt in behind the motor after it is mounted.  Last I heard Crist Rigotti was routinely using this kind of system.  Find one of his build threads in the electric forum for more info.

Denny Adamisin
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Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #5 on: April 16, 2019, 12:56:52 PM »
First off, nice job digging up that old thread of mine!

The stresses on a through the firewall mount are the same as on a rear mount.  However with a big hole through the middle of the mounting plate, its important that the perimeter of the mount is properly anchored, especially near the mounting holes for the X-mount.  Another problem with a 35xx motor is whether the x-mount reaches past the periphery of the motor far enough so that you get enough material to mount to.

If the X-Plate is too small to cover over the hole in the motor mount (very likely with a 35xx other than the Arrowind) then you might try to build-in the mount.  Key to this is to have a hole in the nose big enough to install the motor, and have access to the motor mounting screw through the battery box.  Build-in your X-mount with access to the motor mounting screws, or use a Hunt "Hardnose mount, or even use 1/4" plywood, with brass tubing inserts and bolt the motor in from behind.

One way to GREATLY reduce the stress on a front mount is to add a rear bearing to keep the "south" end of the motor from moving around.  A relatively easy way to do that is to front mount the motor, but use the bolt-on prop adapter on the rear of the motor.  Get a bearing from Boca that fits the prop adapter and bake a bearing holder that you can bolt in behind the motor after it is mounted.  Last I heard Crist Rigotti was routinely using this kind of system.  Find one of his build threads in the electric forum for more info.
You really nailed it.  The "X" is really too big and the wonderful folks at Cobra sent me a three hole prop adapter and a four hole motor.  I have 4 hole adapters from some smaller motors with 5mm shafts that should fit for that bearing.  Thanks for the tips.

Ken
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Offline peabody

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #6 on: April 16, 2019, 01:52:08 PM »
I believe that Bob used firewall mounts so that he could achieve a narrower (lighter) fuselage and that he sawed the conventional mounts off of the engine (lighter).
Then he added the fitting to fill the fuselage and wings with Helium.


Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #7 on: April 16, 2019, 02:39:14 PM »
I believe that Bob used firewall mounts so that he could achieve a narrower (lighter) fuselage and that he sawed the conventional mounts off of the engine (lighter).
Then he added the fitting to fill the fuselage and wings with Helium.
How did he seal the leadouts?

Ken
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Offline Kim Doherty

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Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #9 on: April 17, 2019, 07:07:03 AM »
Ken, do you mean like this?

https://stunthanger.com/smf/gettin-all-amp'ed-up!/light-weight-electric-motor-mount/msg327451/#msg327451

Kim
Close.  That will work on a smaller motor but the 35xx series needs more support.  I have come up with a light weight solution that may/or not work.  It is an aluminum frame that has sides that epoxy and bolt to the fuselage doublers behind a Morris nose ring.


Ken
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Offline Dennis Adamisin

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #10 on: April 17, 2019, 08:28:08 AM »
Tom Morris builds his electrics around 3D printed motor mounts.  He has some available for 35xx Cobra's.  Have you checked with Tom?
« Last Edit: April 17, 2019, 10:22:51 AM by Dennis Adamisin »
Denny Adamisin
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Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #11 on: April 17, 2019, 10:30:40 AM »
Tom Morris builds his electrics around 3D printed motor mounts.  He has some available for 35xx Cobra's.  Have you checked with Tom?
They don't appear to be on his site but I will check with him.  I doubt that anything available will fit in this narrow nose.  I only have 1/16" clearance on the sides with the 3520/14 as is.  Rookie mistake in design.  I built the fuselage for a 2820/12 before I was convinced that the 28 was too small even on a 5s so I ordered the 3520 not realizing that it was 43mm in dia, not 35.  The cavity is 45.5mm wide.  Plenty of room top and bottom for cooling.  You know after 60 years of doing this stuff it sure sucks to be a rookie again!  Next ship I will  be designing the fuselage around the motor mounts not the reverse.

Ken
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Offline Dennis Adamisin

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #12 on: April 17, 2019, 12:46:26 PM »
They don't appear to be on his site but I will check with him.  I doubt that anything available will fit in this narrow nose.  I only have 1/16" clearance on the sides with the 3520/14 as is.  Rookie mistake in design.  I built the fuselage for a 2820/12 before I was convinced that the 28 was too small even on a 5s so I ordered the 3520 not realizing that it was 43mm in dia, not 35.  The cavity is 45.5mm wide.  Plenty of room top and bottom for cooling.  You know after 60 years of doing this stuff it sure sucks to be a rookie again!  Next ship I will  be designing the fuselage around the motor mounts not the reverse.

Ken

One "cheeky" solution would be to lop off a section of the fuse in the vicinity of the motor and graft on a external fuse side doubler - in the shape of a cheek cowl -  on either side.  That will gain you the thickness of your fuse side/doubler on each side and still carry the load back into the main fuse.    Should enable you to build a bulletproof motor mounting system and still be quite stylish!
Denny Adamisin
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Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #13 on: April 17, 2019, 01:18:01 PM »
One "cheeky" solution would be to lop off a section of the fuse in the vicinity of the motor and graft on a external fuse side doubler - in the shape of a cheek cowl -  on either side.  That will gain you the thickness of your fuse side/doubler on each side and still carry the load back into the main fuse.    Should enable you to build a bulletproof motor mounting system and still be quite stylish!
If this doesn't work I will have no choice.  Problem is that it is a "Classic".  I could fudge on the width but adding the cheek cowl might be a stretch.  Are photo-shopped pictures of the originals legal for Classic documentation? LL~
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Offline Dennis Adamisin

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #14 on: April 17, 2019, 02:20:55 PM »
If this doesn't work I will have no choice.  Problem is that it is a "Classic".  I could fudge on the width but adding the cheek cowl might be a stretch.  Are photo-shopped pictures of the originals legal for Classic documentation? LL~

Cool Design!
The cheek only needs to be 1/8" thick to do its job.  Surround it with some punky balsa and you should be able to blend it in pretty close to invisibly.  Of anyone notices you can either
A. Fess up (mistakes happen, and you are not gaining any performance for the structural enhancement)
B. Claim that it was part of the original intended design.  (MOST folks use this option to explain away any number of Classic or OTS oddities...)

Denny Adamisin
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As I've grown older, I've learned that pleasing everyone is impossible, but pissing everyone off is a piece of cake!

Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #15 on: April 17, 2019, 02:40:17 PM »
Cool Design!
The cheek only needs to be 1/8" thick to do its job.  Surround it with some punky balsa and you should be able to blend it in pretty close to invisibly.  Of anyone notices you can either
A. Fess up (mistakes happen, and you are not gaining any performance for the structural enhancement)
B. Claim that it was part of the original intended design.  (MOST folks use this option to explain away any number of Classic or OTS oddities...)
That is the ship I flew as a Senior at the 1965 Nats - I had hair!  The original flew on a Fox 35.  It was controlled free flight but man could that plane corner.  Funny you should mention "B" because the third one was designed with a "Shoestring" like nose so I could mount the engine sideways.  Never got built, life and Uncle Sam kept me pretty busy for the next 10 years.

Ken
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Offline Dennis Adamisin

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #16 on: April 17, 2019, 03:42:09 PM »
That is the ship I flew as a Senior at the 1965 Nats - I had hair!
Ken


I flew at the 65 NATs too.  I still have my hair, but it no longer has ANY color... 

Seriously I think the outside brace approach is completely viable, even on a Classic bird and could help you out of the structural challenges.
Denny Adamisin
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Offline Brent Williams

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #17 on: April 17, 2019, 05:12:08 PM »
I see many classic ships with the aft end of the fuselage slightly widened to allow for an adjustable elevator horn, carbon pushrod and ball link arrangement.  I doubt you would get much flack for a slight increase in nose width.
I believe that pots and kettles can get along...
Laser-cut, "Ted Fancher Precision-Pro" Hard Point Handle Kits are available again.  PM for info.
https://stunthanger.com/smf/brent-williams'-fancher-handles-and-cl-parts/ted-fancher's-precision-pro-handle-kit-by-brent-williams-information/

Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #18 on: April 17, 2019, 05:19:41 PM »
I see many classic ships with the aft end of the fuselage slightly widened to allow for an adjustable elevator horn, carbon pushrod and ball link arrangement.  I doubt you would get much flack for a slight increase in nose width.
I believe that pots and kettles can get along...
Since the only documentation I have is me holding the plane at the 65 Nats, they would have to know my exact height to get the nose width and I will never tell  VD~

Ken
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Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #19 on: April 18, 2019, 10:46:04 AM »
Cool Design!
The cheek only needs to be 1/8" thick to do its job.  Surround it with some punky balsa and you should be able to blend it in pretty close to invisibly.  Of anyone notices you can either
A. Fess up (mistakes happen, and you are not gaining any performance for the structural enhancement)
B. Claim that it was part of the original intended design.  (MOST folks use this option to explain away any number of Classic or OTS oddities...)
Dennis - I felt I owed you a follow up.  I sketched out what it would take to add the outside doubler which is probably the best solution but the damage was only to the first 1/2" (It really did fly away clean).  I just couldn't muster the energy to completely refinish the nose so I am giving it one more chance to ruin my day before doing it your way.  I have attached a couple of shots.

Just out of curiosity, what age level were you at the '65 Nats?  I watched every flight from JR through the Walker flyoff so I probably saw you fly!

Ken
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Offline Dennis Adamisin

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #20 on: April 18, 2019, 06:52:10 PM »

Just out of curiosity, what age level were you at the '65 Nats?  I watched every flight from JR through the Walker flyoff so I probably saw you fly!

Ken

I was 11 yo, flying a red/grey Buster in Jr.  Crashed my first flight when it inexplicably ran out of gas.  Soldiered on with a scotch-tape repair!
Denny Adamisin
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Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #21 on: April 18, 2019, 08:35:45 PM »
I was 11 yo, flying a red/grey Buster in Jr.  Crashed my first flight when it inexplicably ran out of gas.  Soldiered on with a scotch-tape repair!
I was in my last year of Senior. Flew the plane in this thread with a solid white wing and Stearman red everywhere else.  Put up a couple of really decent patterns then got screwed by the Navy.  Judges forgot to put my appearance points on the sheet and I didn't find out till after it was all over and we picked up our scorecards!  That was in the 40 point years too!   Ever since then I have found a way to sneak a peak before the final results were posted!  How did they do all that adding and checking before computers?  In fact that may have been before even calculators and for sure before cell phone cameras documenting everything!

I had a solid dark blue Buster back then and I think I may remember yours but not from the Nats.  Did you by any chance fly the Southwesterns back that far?  Memories tend to merge at my age.

Ken
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Offline Paul Walker

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #22 on: April 20, 2019, 04:26:25 PM »
Here is what I have been doing for years. This is looking forward. The motor is on a X mount and it attaches to that bulkhead. Very strong.

Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #23 on: April 21, 2019, 09:06:53 AM »
Here is what I have been doing for years. This is looking forward. The motor is on a X mount and it attaches to that bulkhead. Very strong.
This is exactly what I am going to do from here on.  Thanks!

Ken
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Offline Paul Walker

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #24 on: April 21, 2019, 10:29:36 AM »
This is exactly what I am going to do from here on.  Thanks!

Ken

Note that the motor loads from the rear, and the blind nuts are on the front side of the bulkhead.

The other thing I do is to shorten the shaft so the collet just touches the front of the can. This way there is less mass cantilevered out front. Helps the bearing life be as long as possible.

Offline Steve Helmick

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #25 on: April 23, 2019, 06:57:07 PM »
If this doesn't work I will have no choice.  Problem is that it is a "Classic".  I could fudge on the width but adding the cheek cowl might be a stretch.  Are photo-shopped pictures of the originals legal for Classic documentation? LL~


Folks apparently need to read the Classic Rules. There is nothing in there about DQ'ing an entry for modifications. There might be a slight adjustment in appearance points, since absolutely nobody uses the <optional> authenticity points.   D>K Steve
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Online Ken Culbertson

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Re: THROUG THE FIREWALL MOROR MOUNTS - Revisited
« Reply #26 on: April 23, 2019, 07:46:59 PM »

Folks apparently need to read the Classic Rules. There is nothing in there about DQ'ing an entry for modifications. There might be a slight adjustment in appearance points, since absolutely nobody uses the <optional> authenticity points.   D>K Steve
Probably right on all counts.  I'll just fly it.

Ken 
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