Inverted Fuel & Oil?

[Eric Page]

I'm curious to know what's involved in the design of inverted fuel and oil systems. Is there some sort of flip-flop fuel pickup in the tanks, or a small, dedicated header tank to feed the engine during inverted flight for a limited amount of time? What about oiling? I have no guesses on that.

When I finally get around to building a plane, these are features I'd be interested in including, but I'm pretty fuzzy on the details! Thanks.

[Hiperbiper]

I'm curious to know what's involved in the design of inverted fuel and oil systems. Is there some sort of flip-flop fuel pickup in the tanks, or a small, dedicated header tank to feed the engine during inverted flight for a limited amount of time? What about oiling? I have no guesses on that.

When I finally get around to building a plane, these are features I'd be interested in including, but I'm pretty fuzzy on the details! Thanks.

For the explanation on the inverted oiling system for aircraft engines look no farther than the Big Dog on the Block:
http://musclebiplane.org/htmlfile/invert.php


You've got the fuel thing figured out pretty well...a flop tube follows the fuel in the tank during manuvers. The only difference is in the size and type of tank.
Most high wing flip-floppers have the fuel tanks in the wings and a header tank up under the panel. Gravity feeds the header tank (2-3 gallons) during straight-and-level-boring-a$$ flight which feeds the motor. During inverted flight the motor uses the header tank fuel which is NOT being replaced by the main tanks so your inverted flight time is limited by the amount of fuel available in the header...
In other aircraft there is a large acro tank that can be used in all phases of flight and is switched on (assuming there are other tanks) manually by the pilot. In the Pitts S2A the only fuel tank has a flop tube thereby keeping the pilot from mucking anything up...
My Hiperbipe on the other hand has a 26 gallon fuel tank with a soild pickup tube good for positive "G" only but a flick of the valve gives me 13 gallons of inverted pleasure...
Pitts pilots are not allowed to fly my airplane...the whole "two fuel tank" thing just messes 'em up....

I'm certian there are other fuel setups out there but these are the most basic and workable.

Hope this helps...

Chris

[Bill Greenwood]

You don't need all those flop tubes and special pickups in your fuel tank.
There is a firm in L A, that is importing av fuel from Australia.
It's a little expensive, but really works well for inverted flying. You may need to keep one tank of Amerkin fuel for starting and takeoff however.
Just to go www.downunderpetro.com for info.

By the way, stock T-34s had the flop tube in the dry sump oil tank. Thankfully mine had not been "updated" and still had it. There was an inspection a d as some of the tubes had come loose in the past.

I never had the desire to hang upside too long, just in passing through, not having any bats or opossums in my family background.

[WLIU]

I will offer the info that many people miss that modern inverted systems are in the suction part of the plumbing. Both the oil and fuel lines feed the suction ports of their respective pumps. That info drives the choice of hose types and the types of fittings used. For example, your AN right angle fitting is the equivalent of 3' of straight hose in terms of resistance to flow. And in a tight engine compartment, hose bends can be an issue. Aviat sells a spring that is installed in the upper hose that makes a relatively tight bend and then plugs into the accessory case of a Lyco. The hose has a bend tighter than the recommended minimum bend radius and the spring keeps it from collapsing due to the bend and the suction of the oil pump. I will note that you can not use a spring made of too large wire or you create flow turbulence that the oil pump does not like.

The inverted oil systems also use gravity actuated ball sealed valving that needs to be oriented correctly and the valve seats must be clean to seal properly in a suction system. The Christen system has 3 balls controlling flow. I will note that you want the stainless steel balls. The original polished mild steel balls corroded and stopped fully sealing after a number of hours. Especially the upper ones where they were exposed to the acids that are in the crankcase venting part of the plumbing. Gotta regularly fly inverted to keep those balls clean....

For the flop tube, the choice of hose is driven by that hose being totally immersed, internally and externally, in fuel for most of its life. Most hose installations do not involve lots of movement in service. A flop tube is expected to move a lot. Aviat issued an AD because their hose manufacturer appears to have made some sort of change that did not affect most customers, but in the inside-the-fuel-tank application, the hose stiffened over time and stopped flopping all of the way from the bottom to the top of the fuel tank. So the engine would stop inverted....

I will note that complex fuel systems and inverted flight are not a good combination. Most pilots don't fly enough inverted to be able to talk on the radio and switch fuel tanks while making an inverted steep turn. So simple is best.

Realistically, if you are just going to do loops and rolls, you do not need any inverted systems. I friend competes in IAC Sportsman in a clipped wing Cub with no inverted systems. A lot of his figures are not the prettiest, but he gets through the program without skipping any figures and with scores higher than zeroes.

Dry sump engines with remote oil tanks are a discussion for another time.

Hope the info above is useful.

Wes
N78PS

[Eric Page]

Thanks much for the info, guys. That answers my questions about "how" very well.

Realistically, if you are just going to do loops and rolls, you do not need any inverted systems.

See, that's why this forum is so valuable. Wes just saved me a lot of money and effort!

Gentle positive G stuff is pretty much all I'm interested in. I discovered some years ago that I have a low G tolerance and don't much like the experience, so there's no hard stuff or competition in my future. I'd just like to be able to have a little more fun than straight and level once in awhile.

And before anyone reminds me, I will definitely get some training before I attempt anything remotely aerobatic, especially given my unfriendly relationship with G.

Thanks gents!

[WLIU]

Glad to be able to answer the question behind the question. My friend with the 75 Cub has his engine go quiet several times during his Sportsman program. The propeller keeps on spinning so as soon as positive G comes back all of the engine noise comes back. I will note that the oil pump keeps pumping so long as the propeller is spinning so his oil pressure suffers less than his fuel pressure. The local competitors regard the guy as something of a hero as his age is approximately the same as his horsepower and he keeps doing it for the challenge. Says that next year he might upgrade to 85hp.

I will note that momentary interruptions in oil pressure are not grossly harmful to the engine. Expect a lower TBO but the result is increased wear not immediate damage.

Best of luck,

Wes
N78PS

[Eric Page]

Good info. Thanks again, Wes.

[B_Berggren]

As I've been reading the acro threads, there are several (many actually) references to planes... in competition.... that may not be aerobatic planes. (172's, Cubs, Citabria, etc...) Some of these planes, like the ones referenced in this post, don't have inverted systems and I have to assume (possibly incorrectly) that they are not "authorized" (whatever that means) for acrobatic flight. In other words, like my plane, the POH may have a utility catagory certification with +4.4 / -1.76 G's with a notation of "no inverted flight".

Realistically, if you are just going to do loops and rolls, you do not need any inverted systems. I friend competes in IAC Sportsman in a clipped wing Cub with no inverted systems. A lot of his figures are not the prettiest, but he gets through the program without skipping any figures and with scores higher than zeroes.

What are the restrictions on "legally" flying a non-aerobatic plane in competition? Flying for fun outside competition? Referencing the quote above, "if all you want to do is loops and rolls you don't need an inverted system" If a plane does not have an inverted system, can it still be legal to fly inverted for any length of time albeit a very short length of time as in a roll or loop?

[Jim Rosenow]

B_B

Your question should probably be in a separate thread, but I'll creep to your topic for a minute. I can't imagine you'll find any contest that will allow a non-aerobatic-certified aircraft in...think liability. "Flying for fun" outside of the stated operating limitations of the airplane makes you a test pilot, as the factory that produced it hasn't done it or didn't like it when they did it...probably not somewhere to go.

Your airplane examples can be used to highlight different types of certification...in order....the 172 is a non-aerobatic (tested to lower g-loading, etc) aircraft. See 'test pilot' above. Before someone gets on me...there are exceptions...many 172's are legal for spins.

The Citabria is an aerobatic category aircraft per certification, and has a list of permitted maneuvers and entry speeds on a placard in each airplane. Many do not have an inverted fuel/oil system. They fly the 'inside' (positive-g) aerobatic maneuvers OK without them.

The Cub (assuming it's an original Cub) was certified under older regulations which listed a 'never exceed speed' for the aircraft. You were allowed by regulation to do anything the aircraft could/would do as long as you did not exceed that speed. With all due respect to the 200 gorgeous Cubs at OSH, I would be extremely loath <- (when was the last time you saw THAT in a post :-) to do that unless I knew the airplane very well inside and out.

As an aside, the Aeronca Champ (circa 1946), from which the Citabria evolved, was in the last category with the Cub, but the re-certification to then-current standards when the Citabria came out changed the limitations on the airframe type.

Hope this helps!

Jim

[WLIU]

Hello,

All aerobatic competitions in the US are organized and run by International Aerobatic Club chapters. IAC rules essentially specify that to be accepted for entry at a contest, the airplane must have a Acrobatic or Experimental airworthiness certificate. And the Contest Director and the Contest Jury have the final say.

If you aircraft operating limitations do not state that your airplane can fly the figures in the program, your airplane will not be accepted for a contest.

Best of luck,

Wes
N78PS
IAC National Judge