stunthanger.com
Building Tips and technical articles. => Building techniques => Topic started by: mark eisenhut on December 25, 2006, 08:51:54 PM
-
this might sound stupid but i cant seem to keep the motors tight on my profile planes. the washer and nut just keep going deeper into the mount. do i need to put like aluminum straps on the back side,bigger washers?
thank you
mark eisenhut
-
Are you using 1/8 ply for doublers ? Use blind nuts
-
this might sound stupid but i cant seem to keep the motors tight on my profile planes. the washer and nut just keep going deeper into the mount. do i need to put like aluminum straps on the back side,bigger washers?
thank you
mark eisenhut
One thing is to drill the holes big enough to use 1/8th" ID brass tubes to help keep from crushing. 4/40 bolts will go inside, no problem.
An aluminum plate on the back side will work, too.
As Lee said, Blind nuts seem to "pull in" less, but I get a *little* to "happy" sometimes and pull them up too tight, too! **)
Bill <><
-
I use some small carbon tube through the mount. Keeps the maple from crushing.
-
I've started using nylocks with blind nuts. Might seem like overkill, but I get the bolts good and tight with the blind nuts, and then use the nylocks as a lock nut. Seldom, do I have to re-tighten; they look pretty good, too. FWIW Glen
-
I use some small carbon tube through the mount. Keeps the maple from crushing.
Hey Randy,
I wish I were that "Hi Tech"! I can't get CF tube around here, just the brass. ;D
Hope you had a great Christmas Day. y1
-
Are you using 1/8 ply for doublers ? Use blind nuts
Good point and also of some importance is what type of plwood. Lite ply doesn't have the crush resistance of good 5 ply plywood. I like blind nuts mount right on the maple engine bearers. Maple has a much greater resistance to deformation than does the best plywood (usually birch). Very hard to crush hard maple with blind nuts!
-
Use theaded brass inserts. The 4-40 size work well on motors up to 40-46. You do need to take a lot of care in marking and drilling the holes so they line up with the motor. The screws can't angle over a bit to take up any alignment slack.
I install the inserts in the motor mount before assembling the fuse. Drill the holes centered on the mount(usualy 3/8 in. wide) and spaced for and aft for the motor. Soak the holes with epoxy and stir it around with a piece of music wire to make sure every thing is soaked. The holes shouldn't go all the way through the mount. Mop out any major excess with a piece of paper towel. Put the insert on a short Allen head screw with a plain nut between the insert and the screw head. Use the Allen screw to screw it into the mount. The hold the nut, back out the screw a turn and then crack the nut loose and remove the screw.
When both mounts are done trim them to fit the engine and bolt them in place. Then fit the mounts into the fuse front, shimming and trimming as needed. Tack glue in place and then complete the front end.
You can install the inserts into an already completed plane too. If the doublers are soft ply I'd trim it away under the lugs first so the full length ofthe insert is in hardwood.
-
Drill 2 mm diameter holes through the mounts then tap them with a 4-40 tap for the mounting bolts and you'll have no problems.
I know I'll get flamed for suggesting this, I always do, but hundreds of flights with no problems says it works and works well.
-
first I'm using what ever plywood that is in the kit. i liked the idea of brass tubing with blind nuts. so last night i started rebuilding the clapped out motor mounts. now they have a brass sleeve glued in with epoxy and i epoxied the mounts. then i put blind nuts that have some epoxy on them also. the motor never fit this good and it's tight. hope it stays that way. thank you for all of your suggestion.
mark eisenhut
-
I agree with Glen's suggestion. Put the blind nuts in place as normal and use engine bolts 1/4" longer than needed. Add nylon insert lock nuts that bottom on the face of the blind nuts. It may be belt and suspenders, but this way you don't need to crush the wood mounts by overtorqing the mount bolts and you never have a loose engine. A nut driver is a handy installation tool for the lock nuts.
Steve
-
I have always just used an internal star lock washer under the screw head with blind nuts. Never had an engine come loose on a built up. On some profiles I have had to re-tighten the screws once or twice (this is part of your preflight check isn't it).. Eventually the plywood will quit crushing and they will stay tight ;D
-
One trick for this problem that nobody has mentioned is to loosen up the mounting screws after every flying session. If not after each session, then anytime the plane will not be used for several weeks. Even rock hard maple can take a set if the engine mount screws are left torqued down for extended periods.