Kevin's event/inverted oil/accumulators

flyeyes · #1 10-20-2008, 04:01 PM

Kevin's event has made me wonder again about something.

I have an IO-360 C1D6 with basically a stock setup. I don't have a flop tube or inverted oil system.

I have no interest in sustained inverted flight.

What I am interested in though is brief episodes of inverted flight, like doing a "real" slow roll as opposed to a sloppy aileron roll, or firmly establishing a line on a cuban eight.

While doing trail acro, I have occasionally had to "push" a little negative to keep lead in sight if I picked a bad line at the start of a maneuver. I haven't seen any drops in oil pressure (and the EFIS hasn't either, although the one-second intervals on the datalogging could easily miss a brief drop)

I didn't consider the full inverted oil system to be worth the weight and expense, but I'm wondering if an accumulator might meet my needs.

What say the engine gurus?

frankh · #2 10-20-2008, 04:33 PM

a few engines that use accumulators..I have both an accumulator and an invert system...works great.

without the accumulator I saw dips in the pressure as it went from positive to negative...Didn't like that so installed an accumulator from Moroso...1.5 Qt..I also pre-lube the engine on start up with it.

Although it is debateable how well that part works.

Had a friend doing acro with no invert and he had a prop overspeed event..accumulator took care of it.

Frank

Walt · #3 10-20-2008, 05:02 PM

My stock engine had an "auto" cut off mechanism to prevent any possibility of an overspeed in neg G conditions.... it was called a carburator. It also came with free belly lubrication system of aeroshell 50W

asav8tor · #4 10-20-2008, 05:08 PM

Quote:

Originally Posted by flyeyes

Kevin's event has made me wonder again about something.

I have an IO-360 C1D6 with basically a stock setup. I don't have a flop tube or inverted oil system.

I have no interest in sustained inverted flight.

What I am interested in though is brief episodes of inverted flight, like doing a "real" slow roll as opposed to a sloppy aileron roll, or firmly establishing a line on a cuban eight.

While doing trail acro, I have occasionally had to "push" a little negative to keep lead in sight if I picked a bad line at the start of a maneuver. I haven't seen any drops in oil pressure (and the EFIS hasn't either, although the one-second intervals on the datalogging could easily miss a brief drop)

I didn't consider the full inverted oil system to be worth the weight and expense, but I'm wondering if an accumulator might meet my needs.

What say the engine gurus?

Not a guru but I did have the inverted oil system installed and took it off.

If you are going to do sustained neg G of course you need the system. But for the conditions you describe a full up inverted system wouldn't do anything for you. For the inverted system to work the following events have to take place:

1. Neg G flight
2. All the oil spills to the top of the motor
3. The oil runs out the breather to a check valve
4. The check valve ball moves to the top (now the bottom)
5. The oil is now routed from the breather to the sump screen

The flying you describe would not give enough time for all of that to happen it takes at least a couple seconds of sustained neg G for the events to take place. If you don't push less than zero G none of it will happen. Hanging around at Zero G is actually bad. Where is the oil? Where is the check valve? I considered the Zero G +/- a quarter G to be a transitory state only. This is a problem with or without inverted oil.

The goal is to keep the oil pump from gulping air.

flyeyes · #5 10-20-2008, 05:38 PM

Quote:

Originally Posted by asav8tor

The flying you describe would not give enough time for all of that to happen it takes at least a couple seconds of sustained neg G for the events to take place. If you don't push less than zero G none of it will happen. Hanging around at Zero G is actually bad. Where is the oil? Where is the check valve? I considered the Zero G +/- a quarter G to be a transitory state only. This is a problem with or without inverted oil.

The goal is to keep the oil pump from gulping air.

Yeah this basically my reasoning as well.

What I don't really know is if one of these accumulators will supply oil for a reasonable time (like 3 seconds) with a safety margin and no sequelae?

Obviously going negative will unport the oil pickup, and the accumulator will begin dumping into the engine as long at it has enough oil, but what happens when you get positive "g" again and are now refilling the accumulator? Can you get air in it and "vapor lock?" Will the oil pressure fluctuate or drop until the accumulator refills? Or can you safely go negative for a few seconds and not worry about it?

Geico266 · #6 10-20-2008, 06:13 PM

Flying a Rotax 912 there is a 3-5 second max for inverted / negative G before problems arise. Is there a minimum amount of time a Lycoming is "rated" for this condition?

mannanj · #7 10-20-2008, 07:23 PM

Quote:

Originally Posted by Geico266

Flying a Rotax 912 there is a 3-5 second max for inverted / negative G before problems arise. Is there a minimum amount of time a Lycoming is "rated" for this condition?

Geico:

IIRC Lyc's can go up to 30 seconds w/no oil pressure. What you're mean nasty ole constant speed prop might do in the meantime----is apparently up for conjecture!

glenn654 · #8 10-20-2008, 09:15 PM

I'm not familiar with an "accumulator".....will someone enlighten me please?

Glenn Wilkinson

flyeyes · #9 10-20-2008, 09:43 PM

Quote:

Originally Posted by glenn654

I'm not familiar with an "accumulator".....will someone enlighten me please?

Glenn Wilkinson

An accumulator is a small tank that is plumbed to the high pressure side of the engine oil system. When the engine is running, it stores oil and pressure (think of a balloon inside a metal sphere-as you fill the balloon with oil you compress the air between the outside of the balloon and the inside of the sphere).

When everything is working normally, the accumulator just sits there, with its oil at the same pressure as the engine. If engine oil pressure drops for any reason, the oil will run out of the accumulator and maintain pressure for a bit. as the engine oil pump recovers, it will take over, and simultaneously refill the accumulator.

Some accumulators have a valve which can be closed on shutdown, storing oil under pressure. This valve can be opened before the next engine start, releasing pressurized oil into the engine and theoretically reducing startup wear.

They are often used on race cars to prevent engine damage from lateral gs which may unport oil pickups.

Some twin engined aircraft, especially trainers, have accumulators to store pressurized oil to bring the props out of the feathered position after an inflight shutdown.

pierre smith · #10 10-21-2008, 05:30 AM

Quote:

Originally Posted by glenn654

I'm not familiar with an "accumulator".....will someone enlighten me please?

Glenn Wilkinson

Hi Glenn,
I've seen them at racetracks, made of a small fire extinguisher and mounted upside down on the side, within the driver's reach, with an on/off valve. As he said, you can turn it on, or open the valve, as the engine is cranked. You'll have oil pressure on the bearings before the engine starts.

Regards,