How do I shim a 4 bolt backplate?
09-14-2003, 10:21 AM
#1
Banned
Thread Starter
Join Date: Dec 2001
Location: Monroe,
NC
Posts: 1,332
Likes: 0
Received 0 Likes
on
0 Posts
How do I shim a 4 bolt backplate?
This thread occurred during the RCU software switchover and I'm attempting to bring it over from the old site with as faithfully as I can reproduce it.
If you need to shim a 4 bolt backplate mount, how is the best way?
I've seen a pack of firewall shims at the LHS, but they aren't open on Sunday.
I've never had to do this, but can I simply use washers on two of the four bolts? Or do I need to put washers all around, and add extra washers on the side that needs shimming to keep from distorting the backplate mount?
Originally posted by PylonWorld on 09-06-2003 11:35 PM
I've seen a pack of firewall shims at the LHS, but they aren't open on Sunday.
I've never had to do this, but can I simply use washers on two of the four bolts? Or do I need to put washers all around, and add extra washers on the side that needs shimming to keep from distorting the backplate mount?
09-14-2003, 10:22 AM
#2
Banned
Thread Starter
Join Date: Dec 2001
Location: Monroe,
NC
Posts: 1,332
Likes: 0
Received 0 Likes
on
0 Posts
How do I shim a 4 bolt backplate?
Originally posted by JohnBuckner on 09-07-2003 1:35 AM
Using washers is a surefire way to cause distortion and the washers will dig in to the wood causing lose bolts frequently. I,ve done it several times where I screwed up and was bit off on the thrust line by starting with a 2 1/4 square eighth inch ply and just taper sanding it with 60 grit sanding block to fit behind the sq. mount. Easy and can get the angle just right and won.t distort the mount.
John
__________________
"Keep your controllines tight"
Using washers is a surefire way to cause distortion and the washers will dig in to the wood causing lose bolts frequently. I,ve done it several times where I screwed up and was bit off on the thrust line by starting with a 2 1/4 square eighth inch ply and just taper sanding it with 60 grit sanding block to fit behind the sq. mount. Easy and can get the angle just right and won.t distort the mount.
John
__________________
"Keep your controllines tight"
09-14-2003, 10:25 AM
#3
Banned
Thread Starter
Join Date: Dec 2001
Location: Monroe,
NC
Posts: 1,332
Likes: 0
Received 0 Likes
on
0 Posts
How do I shim a 4 bolt backplate?
Originally posted by PylonWorld on 09-07-2003 9:29 AM
I put washers on the bottom bolts and loosely tightened the bolts. With washers about 1/32" thick, it reduced the downthrust to about 1/2 degree.
I'm on my way to make a plywood shim.
09-14-2003, 10:27 AM
#4
Banned
Thread Starter
Join Date: Dec 2001
Location: Monroe,
NC
Posts: 1,332
Likes: 0
Received 0 Likes
on
0 Posts
How do I shim a 4 bolt backplate?
Originally posted by JohnBuckner on 09-07-2003 9:47 AM
Sorry, I neglected to say that taper sanding is real easy if the 21/4 wide ply is left six or more inchs long to have something to hang on to while sanding, just make a mark where it will be cut off.
John
__________________
"Keep your controllines tight"
Sorry, I neglected to say that taper sanding is real easy if the 21/4 wide ply is left six or more inchs long to have something to hang on to while sanding, just make a mark where it will be cut off.
John
__________________
"Keep your controllines tight"
09-14-2003, 10:29 AM
#5
Banned
Thread Starter
Join Date: Dec 2001
Location: Monroe,
NC
Posts: 1,332
Likes: 0
Received 0 Likes
on
0 Posts
How do I shim a 4 bolt backplate?
Originally posted by PylonWorld on 09-07-2003 10:29 AM
I used 3/32" aircraft grade ply. Since there are 3 1/32" plys I could use the plys as guides, and I wouldn't be moving the engine forward very much.
Without the shim, it measured almost 2 degrees negative to the wing ... now with 1/32" removed from the top of the shim it measures less than 1 degree negative. I started to go for 0, but I decided that I would just make a second shim so I can change it at the field if the first one isn't enough.
09-14-2003, 10:31 AM
#6
Banned
Thread Starter
Join Date: Dec 2001
Location: Monroe,
NC
Posts: 1,332
Likes: 0
Received 0 Likes
on
0 Posts
How do I shim a 4 bolt backplate?
Originally posted by MDP on 09-07-2003 12:18 PM
I have found the best way to correct the firewall incidence, is to shim & mud the mount. I use small pieces of 1/32" hard ply about 1/4" square. When you have the right amount of shims, in the correct spots, tack them in place on the firewall with CA. Wax the back of the mount, mix up some epoxy with micro balloons (mud), coat the mount with the mud, and screw the mount back on to the fuselage again. Clean off excess epoxy before it sets up, and wait to cure. If you are fast, you can use 5 minute epoxy. Now pop off the mount, the plywood is buried in the mud, and you have a perfect sold firewall, with no sanding........
Don.......Why are you changing the firewall incidence ? Is this on your Viper ?
PS......Make it 0' incidence. You are not building a trainer/sport plane are you ?
I have found the best way to correct the firewall incidence, is to shim & mud the mount. I use small pieces of 1/32" hard ply about 1/4" square. When you have the right amount of shims, in the correct spots, tack them in place on the firewall with CA. Wax the back of the mount, mix up some epoxy with micro balloons (mud), coat the mount with the mud, and screw the mount back on to the fuselage again. Clean off excess epoxy before it sets up, and wait to cure. If you are fast, you can use 5 minute epoxy. Now pop off the mount, the plywood is buried in the mud, and you have a perfect sold firewall, with no sanding........
Don.......Why are you changing the firewall incidence ? Is this on your Viper ?
PS......Make it 0' incidence. You are not building a trainer/sport plane are you ?
09-14-2003, 10:33 AM
#7
Banned
Thread Starter
Join Date: Dec 2001
Location: Monroe,
NC
Posts: 1,332
Likes: 0
Received 0 Likes
on
0 Posts
How do I shim a 4 bolt backplate?
Originally posted by PylonWorld on 09-07-2003 12:29 PM
I had to have about 3/32" of up trim. When I checked the wing to engine incidence there was almost 2 degrees of downthrust. Checking the stab to the engine showed almost the same thing.
So instead of shimming the wing and the stab, I'm going to try shimming the engine up first. I shimmed it up with plywood per John's recommendation about 1 degree.
I have a Futaba 8UAS and it took 17 clicks of trim to get to level flight. And each click is 4 trim steps. Fortunately I took off on high rates or I would not have been able to climb. I love digital trim, except for when you need to do it in a hurry.
Thanks for the technique for a permanent fix.
Check your Viper and let us know how it reads. Someone else has already reported a similar incidence issue.
09-14-2003, 10:35 AM
#9
Banned
Thread Starter
Join Date: Dec 2001
Location: Monroe,
NC
Posts: 1,332
Likes: 0
Received 0 Likes
on
0 Posts
How do I shim a 4 bolt backplate?
Originally posted by PylonWorld on 09-07-2003 1:19 PM
There is nothing funky about my wing saddle. The stab saddle was a little high on the right front side, and I made a minor adjustment so that the stab would line up. I mean really minor, just a couple of scuffs with a 220 block (or a fine rasp) on the high spot.
The wing does not seem to have excessive lift. In fact it feels very similar to an MH-18B and is close in speed to a NACA 66, if not as fast.
I sent tech support a message last night, and suggested that they check some production samples to the design specs.
I can't imagine why they would do it on purpose, unless the wing generates a lot of lift with a 428 engine on front. Then it would seem that the downthrust would be even more of a problem.
09-14-2003, 10:55 AM
#10
Banned
Thread Starter
Join Date: Dec 2001
Location: Monroe,
NC
Posts: 1,332
Likes: 0
Received 0 Likes
on
0 Posts
RE: How do I shim a 4 bolt backplate?
I spoke to the designer on Monday and Tuesday and everything is supposed to be 0-0-0.
My 1 degree shim was not enough.
I bought some .60 sized [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXE953&P=7]Ernst Thrust Plates[/link], but before I made a shim from them I thought about how hot the backplate mounts get. So I did a test with a covering iron at an estimated temperature, and they melted, and that took them out of contention.
So I made a new shim from 1/8" ply that tapered to 1/16" at the top this brought the incidence of the engine to within 1/4-1/2 degree negative.
We test flew again, and the plane still needed a lot of up trim, but it ballooned on reducing the throttle.
I went home and made a 1 degree shim from the Ernst Thrust Plate, since my 2 degree shim would protect it from the heat enough.
Finally the plane stopped ballooning, and actually dove a little on power reduction. I need to make a 2.5 degree shim and try it.
Even with the shims, the plane still needed a lot of up trim, so now I have to address the stab incidence.
The first photo below is the Ernst thrust plates, and the second photo is my 2 degree shim.
My 1 degree shim was not enough.
I bought some .60 sized [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXE953&P=7]Ernst Thrust Plates[/link], but before I made a shim from them I thought about how hot the backplate mounts get. So I did a test with a covering iron at an estimated temperature, and they melted, and that took them out of contention.
So I made a new shim from 1/8" ply that tapered to 1/16" at the top this brought the incidence of the engine to within 1/4-1/2 degree negative.
We test flew again, and the plane still needed a lot of up trim, but it ballooned on reducing the throttle.
I went home and made a 1 degree shim from the Ernst Thrust Plate, since my 2 degree shim would protect it from the heat enough.
Finally the plane stopped ballooning, and actually dove a little on power reduction. I need to make a 2.5 degree shim and try it.
Even with the shims, the plane still needed a lot of up trim, so now I have to address the stab incidence.
The first photo below is the Ernst thrust plates, and the second photo is my 2 degree shim.
