Venting of Lycoming 0-290 D Engine

On Apr 27, 2:09 pm, "Phil" wrote:
Hello
I have been helping a young fellow EAA member with the restoration of a
EAA Biplane , he is using a rebuilt Lycoming 4cyl. , 125 H.P. 0-290 D engine
in this plane and has a 3/8 " O.D. aluminum tube running from the firewall
and exiting just ahead of the tailwheel , he wants' to keep the underside of
this fabric covered plane as clean as possible , the vent elbow that exits
the top forward area of the crankcase has an I.D. of approx. 5/8 " , he
plans to put a reducer to make the step from 5/8 " I.D. to approx 1/4 " I.D.
on the alum tube , this tube will go from the firewall to the tailwheel ,
the question is this , does the engine vent just relieve pressure and will
the long narrow tube cause any problem ?, am not an expert in the dynamics
of this of this area of the engine , any thoughts or help would be
appreciated .
Thanks
Phil Lohiser
EAA 12873

Unless that engine is really tight (pretty much zero ring
leakage) he'll end up with backpressure in the crankcase and will blow
the front crank seal out, losing oil at a good clip and maybe even
getting so much on the windscreen that he can't see where he's going.
If he flies long enough he'll run out of oil. And if by some miracle
it doesn't blow out, and then he flies in subfreezing weather, that
long tube is going to ice up immediately (water vapor condensing in
the tube, the vapor being an unavoidable byproduct of combustion) and
the sure thing will happen: blown seal and lost oil.
Cessna and Piper and Mooney and Beech and American Champion and
Taylorcraft and about a hundred others over the years have used
minimal lengths of 5/8" and 3/4" and 1" vent tubes for some very good
reasons, and those tubes often have a small hole well above the outlet
in case the outlet, being in the cold slipstream, ices up. A long tube
under the belly would ice up all along its entire length.
A slightly oily belly is much preferable to an engine failure.
It's pretty hard to improve on what the major manufacturers do with
their airplanes. They're concerned about oily bellies, too, but you
don't see long tubes under them. You can get the Airwolf oil/air
separator setup to supposedly extract the oil from the venting air,
and we have one on a 172, except that it doesn't work all that well.
Needs a vacuum pump on the system, too, to get the pressure to drive
the oil back from the separator to the case.

Dan