How does a piston engine cope with g force?

[ben5575]

I understand how a dry sump in a car engine works having had them in Caterhams etc, but how is a piston engine protected from oil surge when it's pulling +6/-4g?

Is it in a sealed circuit or something? Is there handy idiot proof youtube video that explains it?

[robinessex]

Easy. The oil tank on a dry sump engine is virtually full, so G forces have no effect on oil movement, hence the oil pump supply isn’t compromised. That’s the only issue you have to worry about, the oil supply pickup being left high and dry due to G loads. Keep it in the bottom 1/3 of the oil tank, and it’ll be fine. Interesting Caterham if it sees +6/-4G though !!

Edited by robinessex on Tuesday 19th June 09:28

[CrutyRammers]

Given this is boats, planes, trains I guess he's thinking about aircraft

[robinessex]

In that case, the oil tank would be completely filled, and the inlet pipe located centrally I guess. The oil in the engine would be a bit aerated, that's why the dry sump pump sucks return oil at about 3 times the flow into the engine feed.

[Krikkit]

You would also think about multiple pickups from the oil tank, as well as a bigger capacity.

As a benchmark, a Merlin engine was expected to burn 6-20 pints of oil per hour of running, so they carried a significant volume with them.

Edited by Krikkit on Tuesday 19th June 11:34

[eharding]

Aerobatic Lycomings and Continentals typically use a Christen style inverted oil system.

http://musclebiplane.org/htmlfile/invert.php

You can still end up with a fair amount of oil going over the side via the breather when giving it a good whanging though. The oil breather in my old Pitts emptied into the 4:1 join of the exhaust manifold, and I remember the first time I tried a Mulleroid tumble the cockpit suddenly filled with smoke at the top, much to my concern. For a split second I thought the thing was on fire, and I was going to have to step over the side.

After I sold out of the Pitts, they relocated the oil breather so that rather than draining into the exhaust it helpfully dripped onto the right brake shoe, which I always thought was a retrograde step, rather than doing what should have been done from the start and running a breather line down to the tail.

The radial in the Yak has a different system, using a reservoir allowing a limit of 2 minutes (allegedly) of inverted flight (as Paul Bonhomme found out many years ago in a Yak 18T at Waltham, it isn't prudent to explore how accurate that 2 minute figure really is....)

[ben5575]

eharding said:

... I remember the first time I tried a Mulleroid tumble the cockpit suddenly filled with smoke at the top...

Yes I can imagine that might have proven a little scary!

Thanks for that link - looks like it's all held in place by a couple of ball bearings. Seems a simple solution hidden within a fiendishly complex bit of piping.

[eharding]

ben5575 said:

Thanks for that link - looks like it's all held in place by a couple of ball bearings.

Indeed - the secret of adequate aerobatic oil pressure is to keep your balls clean.

You'll often find the ball valve mounted at a slight angle to the vertical, so that when flying a perfect up-line, generally at high power, the balls are firmly registered to one orientation rather than floating - and causing a loss of oil pressure - which would be the case if it were completely level.

The same applies on a down-line, but extended vertical down-lines aren't flown with the throttle wide open (and if they are, they don't tend to last long, one way or the other).