Hi All members:
The 4130 chromemoly seamless pipe for the engine
mount.
What is the appropriate size for building a engine
frame, as the included drawing of Paul L. (
copyright P.L )
The outside diameter and wall thickness ?
7/8 Inch OD x 0,069 “ Wall thickness ?
or 0.120 W
1 inch x OD ?
Cant find any specifications for such a design.
Sent from my iPhone
Le Roux Breytenbach --
Homepage: http://www.flyrotary.com/
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For perspective, the RVs
with up to an IO360 use 3/4" x 0.049" tubing. But as
Bobby & Andrew pointed out, crippling is the
most critical issue, and all of the RV's tubes that
are loaded in compression are much shorter than the
bottom-most tube in that pic. I'd be most concerned
about that tube; the pair that are arrowed wouldn't
concern me that much.
Those aren't the only
issues; you need to look at the load paths within
welded joints, also. The strongest welds are the
ones that are stressed along their length. A tension
load perpendicular to a weld line is kinda 'worst
case'. You obviously want to avoid loading tubes in
bending, but there are situations where it *might*
be acceptable, or even better. Ex: there are some
'bed mounts' (what that drawing effectively shows)
for some Continental 6cyl engines that use a
much larger horizontal member (left arrow in the
pic), and attach the compression member (bottom most
tube in the pic) near the mid point of the
horizontal member. This loads the horizontal in
bending (requiring upsizing), but shortens the
bottom compression member a lot, reducing crippling
loads on it 'Everything is a compromise.' ;-) Be
sure that your airframe's 'stock' motor mount
doesn't include a horizontal member between the two
upper firewall attach points. RVs don't need the
tube, but some composite or wood designs might not
have the compression strength across the top of the
fuselage that's needed to resist those two points
trying to come together under positive G loading.
Something I *wish* I could
have done with my mount is to incorporate a feature
found on some radial engine mounts. They often have
the 4 firewall attach points being a U shaped
bracket bolted through the bottom of the U, to the
firewall, and the motor mount termination a vertical
tube at each corner. By pulling the bolts on the top
& bottom of one side, the entire mount &
engine can be swung to one side to service stuff on
the
firewall or the back side of the engine. Trickier to
do with a mount like a taildragger RV, which has the
gear leg sockets incorporated in the mount.
I was fortunate enough to
have two different engineers, both with structural
experience, critiquing my design attempts, and one
of them is also an excellent welder.... :-) If you
can find an engineer or structures guy to look over
your shoulder, it would be a good thing.
Charlie