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Building Tips and technical articles. => Building techniques => Topic started by: Mike Griffin on December 23, 2017, 09:07:45 AM
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I am curious about the methods that people build the crutch for a full body fuselage. A typical crutch might consist of the two maple motor mounts, a F1 and F2 plywood former. In what sequence do you build the crutch and how do you keep it square while building it? At what point do you drill the holes for the engine bolts and how do you keep them accurate? If you have pictures, please post them. Has anyone ever come up with a jig in which to build the crutch?
Thank you
Mike
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Well the way I do it is cut a piece of balsa the thickness of the hard wood mounts so I can glue it cross grain between the hard wood. This get the hard wood square and then I glue the formers in place. The other way is to cross grain balsa sheet over the hard wood. D>K
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Thank you for posting this Doc. Looks like a very accurate way to do it.
Mike
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On some I do as Doc "doos", more often I use a motor mount drill guide from Jim Lee to set the space of the mounts, then apply a cross piece or cross pieces inside the mounts using epoxy. Then the F1, F2, etc are installed. The motor must fit and the tank must fit. These are allowed for before I go any further. Guess why? LL~ H^^
I know Ty. I learned the hard way on that too.
Mikeik
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The engine itself is the jig.
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The engine itself is the jig.
Yes it would be Paul.
Mike
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Mike,
The engine crutch assembly is rather fun to build, and is almost impossible
to construct wrong.
Accurately cut out the 1/4"-3/8" balsa filler block that will go between the
hard maple motor mounts. Using slow setting epoxy, securely clamp
these pieces down to a piece of glass, scrap granite, or 3/4" particle board with
a melamine surface. It's preferable to clamp this down with another piece of
flat material on top. After this cures, you can add F1, f2, and F3 insuring that
they are square to the crutch.
In the photo, you can see that I like to drill the holes for mounting the engine
at this time. By having the assembly square to the work surface (in this case
the 3/4" Melamine covered particle board) you can calculate and offset what you
might want on the engine. Clamp your engine in place, adjust for desired
offset (if any) and use the commercial tool pictured for 'spotting' the hole locations
which are later completed on a drill press.
Piece of cake !
Cheers.
Warren Wagner
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I do what Doc does for the crutch, and then (more because I can than because it's really absolutely necessary) I use an X-Y table on my drillpress, or my cheap Chinese milling machine to drill the holes "to drawing". If you're a beginning machinist note that it's a good idea to mark out the holes first, sanity-check them against the engine, and THEN drill -- making a perfect set of holes in the wrong place is embarrassing, as is making a set of holes that's exactly 1mm too narrow, or 1/10th inch too short fore and aft, or some such.
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Folks:
I'm still trying to get my mind around this "crutchy" thing. What is it, why build one, and is it part of the finished aircraft? At least this inquiring mind wants to know. Thanks!
Dave Mo...
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It's the engine bearers and the fuse formers. Here is a pic of mine in my box car chief with the glue curing.
You've built them, maybe just hadn't seen the term.
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I believe that some people still glue the motor mounts and plywood (or balsa) doublers to each fuselage side and start from there. It is harder to get the engine bearers flat...this is kinda "old way", but it can work ok if you're careful. Engines like flat mounting bearers. D>K Steve
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I do a variation of what Warren does, you cant go wrong. Well, you would have to try real hard to mess it up using that method. At this stage I also add phenolic motor pads and sand in a .75° downthrust taper into the pads.
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Well the way I do it is cut a piece of balsa the thickness of the hard wood mounts so I can glue it cross grain between the hard wood. This get the hard wood square and then I glue the former's in place. The other way is to cross grain balsa sheet over the hard wood. D>K
Doc & Mike,
While that works great on .35 or smaller engines, I would use 1/2 balsa cross-grain between the mounts on 40 to 51 size engines. On larger engines like the old thumping ST 60, I would add to pieces of 1/2” square basswood behind the F 1 former and in front of the F 2 with 1/2” cross-grain between them. Use slow cure epoxy and clamp down and allow to dry. Drill and mount the engine and your ready for the plywood doublers and balsa fuse sides.
Later,
Mikey