I have a 1967 7KCAB with the old Champion inverted oil system that has the trapdoor in the sump and a clank tube oil pickup.
I don’t run more than 6 quarts in sump (which is actually the 5.5 qt spot on the dip stick. I suspect that’s why the oil pressure starts to fall around 45-60 seconds rather than the two minutes that is mentioned for that system. I suspect I’d get closer to 2 minutes with a full 8 quarts in the sump.
Given that the 7KCAB has Bendix fuel injection and a 1.5 gallon header tank with 3/4 gallon available for inverted flight, you really have to work at it to get the fuel to stop flowing. You can do it, but it requires rolling upright long enough to refill the oil sump then rolling inverted again before the header tank refills - and then rinsing and repeating until you finally exhaust the available inverted fuel in the header tank.
It’s a lot easier to exhaust the inverted fuel with the Christian inverted oil system as it has pickups in the sump and in the case and will pump oil continuously***.
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With the 7ECA you don’t have either inverted fuel or inverted oil systems, and you don’t have a header tank designed for inverted flight. I suspect during the vertical up line in hammer head you are unporting the oil pickup when all the oil drains to the rear of the sump, but you still have enough fuel in the carburetor bowl that you are still getting fuel to the engine. You also don’t have a negative G condition that would cause the float in the carburetor bowl to cut off the fuel.
Filling the sump with more oil would probably help, although at the cost of blowing a lot more oil put the breather.
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In general there are three different ways of providing fuel to the engine in inverted or negative g flight.
In the normal carburetor the fuel flow is float controlled and under negative G conditions the float shuts off the fuel. The early MK I and MK II Spitfires were at a disadvantage to the fuel injected Bf-109E which could push over into a dive, while the Spitfire had to first roll inverted to avoid the momentary power loss encountered with negative G. That allowed the Bf-109 to disengage from a fight more or less at will.
The British came up with an interim solution of placing a plate in the carburetor bowl with a small hole in it. That hole allowed the carburetor to operate normally when upright, but held most of the fuel in the bowl long enough to ensure fuel flow during brief negative G maneuvers - much like Champion’s trap door inverted oil system. (In fact, if horizontally opposed engines didn’t also rely on splash lubrication, a similar hole in the plate in the oil sump would have worked.)
However, in the mid 1930s Bendix-Stromberg invented a pressure carburetor. A pressure carburetor uses a Venturi like a regular carburetor bowl to sense the mass flow of air, but it is a float less design. The fuel is provided to the carburetor from a fuel pump under pressure, so it continues to provide fuel regardless of negative G. the first Bendix-Stromberg pressure carburetor appeared in 1936 on the early Allison V-1710 engine. Smaller models were also used in general aviation and you’ll find pressure carburetors on aircraft like the older Pitts Specials.
Around the same time, direct fuel injection was developed and has become the preferred method of supplying fuel to engines in inverted flight. That’s partly because fuel injection is fairly simply compared to the rather complex (lots of internal parts) pressure carburetor. Fule injected aircraft have a fuel control servo where the carburetor is normally located that controls the fuel distribution to the cylinders. The major difference is that the fuel is delivered to the cylinders rather than to common point in the airflow before the air reaches the induction tube for each cylinder.
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***Not all Christen inverted oil systems have the same number of pickups. Some Christian systems can experience oil starvation in knife edge flight (where the Champion system still does ok for a limited period of time.
Also, and to Bob’s observed loss of pressure, the Christian system relies on ball valves that react to the positive and negative G.
There is a single ball and weight in the oil separator to close off the breather line from the crank case in negative G flight. Crank case gases then vent through the oil return line from the breather to the sump.
There are also two balls valves in the oil valve. In normal upright flight one ball closes off the line from the breather tee, so that crank case gasses are routed to the breather rather than the oil valve. In inverted/negative G flight, the other ball closes off the normal oil pickup line so that oil feeds from the crank case vent line through the breather tee to the oil valve.
one potential problem is that those balls in the oil valve need to be moved on a regular basis. If not, they can become stuck in their normal upright positions and the Christian inverted system is suddenly just a regular oil system that won’t feed in inverted flight.
