Inverted with no flop tube

Hi Guys,
I'm probably going with the half raven oil system on my 7a but it got me thinking,if I installed the fully inverted oil system how long do you think I could safely sustain inverted flight with no flop tube in the fuel tank?Tanks are done and I want to keep it that way.
If you could get around 10 seconds or more I might consider the fully inverted system,if not it would be a waste of time and money and the half raven would be more than sufficient.
What do you guys think?

I have a full inverted oil system with no flop tube. I've flown 15 seconds without any noticible engine hesitation.

You did not say, but do you have fuel injection or carb. If a carb, I doubt you can get more than a second or two of negative g's.

Sorry,I meant to say fuel injected as well

The engine will run for 10 to 15 seconds but that is too long to run without oil pressure.

fuel level and attitude

How long you can run inverted without a flop tube also depends on how much fuel and what maneuver you're doing. I assume you're not just planning to fly inverted in level flight. I have no problem flying the Known Sportsman routines using the non flop tube tank. I'm only inverted with negative Gs for about 3 seconds. But doing half loops and setting up for inverted spins makes things go quiet without the flop tube. You need to come join us for some competition and FUN.
Bill McLean
RV-4
Slider
lower Alabama

Consider an Accumulator

Cmore said:

Hi Guys,
I'm probably going with the half raven oil system on my 7a but it got me thinking,if I installed the fully inverted oil system how long do you think I could safely sustain inverted flight with no flop tube in the fuel tank?Tanks are done and I want to keep it that way.
If you could get around 10 seconds or more I might consider the fully inverted system,if not it would be a waste of time and money and the half raven would be more than sufficient.
What do you guys think?

Click to expand...

Hi Cmore,
You might want to consider using the 1/2 Raven and an accumulator. Im flying with that setup and it works great as a way to maintain your oil pressure for 5-8 seconds of sustained zero-negative g. As Bill said, you really won't need more inverted capability than that for IAC Sportsman class aerobatics.

Check out this thread on accumulators - http://www.vansairforce.com/community/showthread.php?t=100090&highlight=accumulator

I can't speak to the fuel delivery question. I'm flying a carbureted engine so my motor quits anytime I turn upside down for more that a second or so! That's not stopping me though and you'll obviously be lots better off with an injected engine, flop tube or not.
Best regards,
Randy

I am using a half Raven pretty happy with it. Don't have inverted fuel. With my fixed pitch prop and IO-360 I can stay 3 seconds inverted with oil pressure coming down to 50 psi and 15 seconds for it to drop to 25psi ( based on my dynon logs) which is when I stopped as it's the min for Lycoming. There is no fuel issue that entire time so I guess I have more then 15 seconds worth of fuel. The only time I get my engine to quit is in spins and long sequences of consecutive rolls.

Got no experience with constant speed prop configurations. So your mileage may vary.

Before I put the 1/2 Raven in I lost lots of oil... . So that was well worth it. As I still haven't painted (160 hours now...) I am still debating if I should retrofit the flop tube or not as it will help with the roll issue.

I forgot to mention that I do have fuel injection. I'll be converting mine to a 1/2 raven system soon in the interest of saving weight. I don't fly inverted enough to justify the weight and complexity of a full inverted system.

Thanks guys,
What's the advantages of the accusump over the fully inverted system,it looks lighter but is it straightforward to install for the uninitiated like myself?

Oil Accumulator

Cmore said:

Thanks guys,
What's the advantages of the accusump over the fully inverted system,it looks lighter but is it straightforward to install for the uninitiated like myself?

Click to expand...

The best possible system would include both. Even with a full inverted oil system, during attitude transitions, especially at or near zero g, you may lose oil pressure as the system shifts between the positive and negative configuration. The accumulator provides a ready source of pressurized oil to fill in the gaps.

I did not find installing the accumulator to be difficult or technically complex. I installed the most complex of the available system configurations which includes an electric pressure controller, switch, and indicator light. You could go with the very simple installation which is just the accumulator without any valves or controls. You have to tap into the pressurized side of your oil system. That and fabricating a physical mounting location are the hardest parts of the job. But, even those are pretty straightforward. I did have to learn some new skills. It was my first time to make up my own hoses or bend aluminum pipe (didn't build my airplane). If you already know how to do those things, it should be a piece of cake. Installing my system took me about 10 hours of work which included a firewall penetration as I mounted the accumulator in the cabin cockpit.

I think one thing you have to consider is the complexity of the system vs. the benefit. E.g. F14av8r is completely correct that in 0g your inverted system won't help, but adding an accusump increases system complexity. So what's the odds of the hose to the accusump breaking which will lead to total oil loss vs. the decrease ( if any ) of engine live because you are at 0 g for a moment or two. Same is true for full inverted system. You can find reports where they have failed leading to the engine loosing all lubrication on the other hand what's the impact on engine live if your oil pressure drops for a moment.

As always in aviation it's a trade off. Doing my research I found many opinions what will happen to your engine if you fly inverted for a couple of seconds without inverted system but no data to back it up either way.

Oil Pressure

First, I'd like to have a cool name like Oliver Spatscheck!

But, I digress . . .

I agree with Oliver that a momentary loss of oil pressure probably isn't a big deal. That lubricating fluid is probably still hanging around for a few seconds even if the gauge / sensor says the pressure has dropped. The question, of course, is how much is too much. I prefer not to dabble in that arena. I don't know enough about the lubrication characteristics of a Lycoming to have an opinion. I do know that having oil pressure is a good thing though so I elect to maintain mine through all my aerobatic figures!

Great discussion!
Randy

Randy,
If I just went with the simple setup as in an accusump with no electric bells or whistles is it just a matter of pluming it to the oil cooler and pressurizing it to 10psi, fixing it to the airframe and thats about it.
What happens then,does it fill up when you start the engine,drain when you stop the engine and push oil into the engine anytime you have 0 or negative in between times.I see some guys have it mounted on the engine side of the firewall,would this be ok?

Basic Accumulator

Yes, the basic setup would be a single hose to a T fitting. The accumulator will fill when you start the engine and discharge anytime it sees low pressure. It essentially acts as a surge suppressor (a basic accumulator function). You can definitely mount it on the engine side of the firewall. The manufacturer cautions against exposing the accumulator to excessive heat as the seals on the piston can be damaged but there are several documented firewall forward installations on VAF that seem to be working well.

Here's what you give up with the basic install:
- You can't pre-oil the engine before start
- Some of your initial start up oil pressure will be used to pressurize the accumulator. This is also true of a low oil pressure situation such as you'll encounter during sustained negative / zero g flight.
- You won't have any indication of the operation of the accumulator other than your oil pressure gauge
- You can't isolate the accumulator in the event of a leak in the accumulator itself

Frankly, I don't think any of those are big deals and you'll gain most of the benefit of the accumulator with just the basic install. I wanted more control and, since I have a carbureted engine (it quits anytime I get less than .5 g), I didn't wan't the accumulator recharging until the engine pump restored pressure to greater than 55 PSI. I also like the idea of pre-oiling even though I fly my airplane two or three times a week on average. it's just really cool to power on the master, flip the accumulator switch, and see the oil pressure come up to 40 PSI before i start!

The electric pressure valve does add complexity and weight though so it is certainly a tradeoff.

If you can't find room for the 2 quart unit forward of the firewall, consider the 1 quart unit. You won't have as much sustainment of pressure but it will still act as a pretty good shock absorber!

Thanks Randy,you may have just talked me into it and I sure like the idea of a pre oiler as well but for now I'm trying to stick to the KISS mentality to get airborne,is it a fairly straight forward job that you could do later?
Where did you put your T,was it on the oil cooler or accessory case?

Accumulator Location and Upgrade

Installing the valve at a later time would be possible and not overly difficult IF you plan for it in your initial installation. The valve assembly is fairly large (about four inches tall and 2.5 inches diameter) so you'd need to make sure you leave room. And, you might want to run the wires now. I highly recommend installing an indicator light so you can see when the valve opens. Finally, make sure you allow for access to the air servicing port and either keep a sight line to the gauge or install a remote gauge.

My T is on the oil cooler inlet. I would have preferred the outlet but it was too tight a bend for the hose. At some point, I may change it over to a firewall mounted manifold arrangement which would be neater. It's working well with the T right now though.

Here's a shot of the valve and associated hardware:

Thanks again guys you've been a great help as usual,stand by for the next question as I'm sure it won't be long.

Accusump

Just trying to wrap my head around this.
You installed the Accusump with the optional valve correct?

Accusump

Yes, I installed the Accusump with the Pro series Electric Pressure Valve, a switch, a remote inflation port, and an indicator valve that lights when the valve is open.

Cmore was (is?) considering installing just the accumulator (accusump) without the valve which would be simpler but provide somewhat less capability.

f14av8r said:

I agree with Oliver that a momentary loss of oil pressure probably isn't a big deal. That lubricating fluid is probably still hanging around for a few seconds even if the gauge / sensor says the pressure has dropped. The question, of course, is how much is too much. I prefer not to dabble in that arena. I don't know enough about the lubrication characteristics of a Lycoming to have an opinion. I do know that having oil pressure is a good thing though so I elect to maintain mine through all my aerobatic figures!

Click to expand...

Hey Randy, even though nobody in the aerobatic community really uses accumulators (except for certain CS props) or is concerned about the idea of lubrication issues associated with slight oil pressure fluctuations (even with inverted systems), I can't argue against peace of mind being a wonderful thing! We all get good life out of our Lycs, subjected to some pretty hard flying. I consider it a non-issue. Run it hard, run it regularly seems to work best for these engines. The serious acro community is pretty minimalistic when it comes to the idea of any added cost, weight, and complexity to their airplanes, especially when it may be a solution to a problem that doesn't really exist. Not a dig, but the RV community is definitely much more gadget-oriented. Good luck with the acro plans! Eric

Hi Eric,
Good feedback and I agree with you almost completely. If I had a full inverted oil system, fuel injected engine and a counter-weighted CS speed prop, I would not feel the need for an accumulator. One of the things I like most about my, admittedly compromise solution, is that it keeps oil pressure available for the prop. A prop overspeed with my non-counterweighted prop is the thing I'm most interested in preventing. I also agree that the sling lubricated components in the engine would probably be fine for quite a few seconds of no oil pressure flight but I'm not so sure about the pressure lubricated components.

No doubt the accumulator adds complexity, weight, and another potential point of failure. I do love the fact that my oil pressure is nicely sustained during my inverted maneuvers though!

Thanks again for the feedback,
Randy