Motor Mounts

[Leester]

On the Miss BJ I'm building the plans show 7  1/4" lightening holes in each motor mount. Is this the way to go??  Doesn't this weaken the mounts??  Advice appreciated.  Thanks    The fusalage is profile, and the engine will be a L&J Fox 35.

[Bob Reeves]

Holes are well behind the main stress points of the mount.. Normal operating procedure to lighten hard wood mounts.

[Bill Little]

Hi Lee,

As Bob said, it is a fairly common procedure for many builders.  Some others do not do it.  Seems to work either way! 

I often use my circular saw to relieve the back side of the maple mounts when they're going in a full fuselage plane with a modern engine.

Bill <><

[Bill Sawyer]

When I made the Argus kits I slotted the motor mounts for inserting the 1/4" balsa cross grain. This produced a mortise and tenon joint that is the strongest woodworking joint - it is used where leg rungs go into the legs as an example. I have done this for my planes for years and recently I used this system in the Playboy kits as well. When I did the Playboy I epoxied the mounts together and trimmed them to length.

[Leester]

Looks good Bill.

Just finished drilling, my mounts are 5 5/8" long each was sloted 1/16" for engine clearence and weight was 20gms combined. After drilling the weight was 15gms. My scale only does 5gm increments, but individually weight was 5gms ea. So for sure you save 5gms up to 9gms. FWIW

[Ralph Wenzel (d)]

On the Miss BJ I'm building the plans show 7  1/4" lightening holes in each motor mount. Is this the way to go??  Doesn't this weaken the mounts??  Advice appreciated.  Thanks    The fusalage is profile, and the engine will be a L&J Fox 35.

In a Stunt News a couple of years ago, Noel Drindak (I think) advocated drilling a 1/4" hole lengthwise (from the rear, naturally). The article suggested that any hole drilled through the M/M would create multiple stress risers.  FWIW

[phil c]

the real question is, do you really need to take 5 or so grams out of the nose on a profile plane?  The holes may very well accumulate more than 5 gr. of epoxy when you glue the mounts in.  I'd really question this.  If you reallly decide you need to get rid of 5 gr. up front, take a belt sander and take a few extra strokes along the the plywood doublers to smooth, round, and fair them in.  that will take off the extra weight quickly and easily and add some pretty curves besides.