Oil breather cowl exits and materials

zkvii · #1 02-08-2007, 02:40 PM

Hi,

We have the first section of the oil breather sorted (VANs supplied rubber hose) and have a number of options with the remain length AL, rubber, combination of the two.

What are the pros/cons of putting the breather exit in trail, vertically down, should it be straight cut, angled, should it finish behind the cowl lip?

Are we aiming for a positive pressure in the crankcase or a slight vacuum? Any performance gains one way or the other?

Trying to understand the function as well as the installation,

Carl

robertahegy · #2 02-08-2007, 04:31 PM

I positioned the opening of the metal tube over the exhaust pipe per Van's instructions. This will help keep the tube warm in winter to prevent freeze-ups and keep you from popping the front seal on the engine. A slit should also be put in the tube above the outlet incase you do freeze up. Moisture and fumes are drawn out of the crankcase via this tube, so you don't want to angle it to the airstream and pressurize your crankcase. Neutral or slight vacuum is best. Van's also supplies the metal tube, incase you did not get it.

Roberta

Kyle Boatright · #3 02-08-2007, 04:53 PM

Trust me, you absolutely do want the end of the breather beveled so it faces the slipstream and adds slight pressurization to the crankcase.

My tale was told in the RVator about 5 years ago, but the short story is that I lost about 1.5 quarts of oil in 15 minutes due to the combination of a breather with the end beveled the wrong way and a dipstick that didn't get tightened during the preflight. The impact of this was that the vacuum created at the end of the breather sucked a fair amount of air through the dipstick tube and that moving air carried a bunch of oil with it.

I landed at my home field and had no idea there was a problem until after I parked the airplane and saw a bunch of oil drips under the airplane. As it turned out, the belly of the airplane was covered in oil.

Remember, this was on a 15 minute flight. If I'd flown for another 30 minutes, things might have gotten interesting.

Before I flew again, my AI and I went through every imaginable check and the only thing that made any sense was the backwards bevel on the breather exit and the unsecured dipstick.

erich weaver · #4 02-08-2007, 05:21 PM

Wow. Good info to know Kyle. Seems like that could happen to just about anybody.

erich weaver

robertahegy · #5 02-08-2007, 05:46 PM

The tube that Van's supplies curves slightly forward but is angle cut towards the rear. This would suggest a negative pressure on the tube as air passes by it. I have never experienced oil loss due to having my crank vent tube aligned this way, but I haven't flown with the oil cap loose, either. I get 20-22 hrs per qt. Attached is a shot of the tube over the exhaust pipe. It's possible it could pressurize or be neutral.

Roberta