I had a Bendix PS5 pressure carb on a 150 HP Lycoming in a home built Pitts S1. It worked great for full inverted flight. Fortunately, the PS5 never required any repairs during my ownership of the Pitts. This was around 1974 - 1976. I'm thinking the PS5 was the best inverted fuel control device that preceded fuel injection systems of today. Perhaps someone more familiar with the history of the evolution of inverted fuel system technology could comment.
There are a few errors in your description. First, at 6:35 you state that when the throttle is opened the impact pressure in chamber A will increase and that's simply not true unless there's some external reason for the pressure feeding the carburetor to increase such as a ram air inlet with increasing airspeed of the aircraft that the carb is running in. Second, the pressure drop across the metering orifice is not a constant, it's a function of the incoming pressure and flow velocity (gpm). The flow velocity is further affected by the valves downstream of the orifice. Also, your description would easier to follow if you didn't mix "suction" and pressure WRT the forces on the fuel pressure regulator poppet valve. The "fuel metering force" is simply the spring force plus the impact pressure (chamber A) minus the lower venturi pressure (chamber B). That force (plus the spring force) is what opposes the fuel pressure in chamber D so the complete force equation would be S (spring) + A (impact) - B (venturi) = D (fuel). BTW the force generated by the spring plus the diaphragm between A and B presses directly on the "rivet" in the diaphragm between B and D and the fuel poppet stem is directly attached to the diaphragm between B and D. If the rod actually penetrated the right diaphragm fuel could leak from D to B. There's another fairly subtle effect that's not mentioned anywhere in the documentation as well. If you look closely at the cutaway diagram you can see that the diameters of the two diaphragms are different and this affects the ratio of the air metering pressure's effect against the fuel pressure.
Really great video! I'm studying for Powerplant Written Exam. There's a question:" An aircraft engine equipped with a pressure-type carburetor is started with the...?" Answer: "primer while the mixture control is positioned at IDLE-CUTOFF position." Do you know *why* this is the starting procedure for a pressure carburetor? Why not have the mixture control set to lean or rich? I'm reading that once the engine starts you immediately move the mixture out of the CUTOFF position. But why is it necessary/desirable to start the engines in the IDLE-CUTOFF position? And it seems like with it in the IDLE CUTOFF position little to no fuel would be available for starting. I don't get it! Thanks for the great video.
you will flood the engine with mixture in full rich position. using primer there is enough fuel for it to get quite a few revolutions giving you time to advance the mixture.
Great video!
I had a Bendix PS5 pressure carb on a 150 HP Lycoming in a home built Pitts S1. It worked great for full inverted flight. Fortunately, the PS5 never required any repairs during my ownership of the Pitts. This was around 1974 - 1976. I'm thinking the PS5 was the best inverted fuel control device that preceded fuel injection systems of today. Perhaps someone more familiar with the history of the evolution of inverted fuel system technology could comment.
There are a few errors in your description. First, at 6:35 you state that when the throttle is opened the impact pressure in chamber A will increase and that's simply not true unless there's some external reason for the pressure feeding the carburetor to increase such as a ram air inlet with increasing airspeed of the aircraft that the carb is running in. Second, the pressure drop across the metering orifice is not a constant, it's a function of the incoming pressure and flow velocity (gpm). The flow velocity is further affected by the valves downstream of the orifice.
Also, your description would easier to follow if you didn't mix "suction" and pressure WRT the forces on the fuel pressure regulator poppet valve. The "fuel metering force" is simply the spring force plus the impact pressure (chamber A) minus the lower venturi pressure (chamber B). That force (plus the spring force) is what opposes the fuel pressure in chamber D so the complete force equation would be S (spring) + A (impact) - B (venturi) = D (fuel). BTW the force generated by the spring plus the diaphragm between A and B presses directly on the "rivet" in the diaphragm between B and D and the fuel poppet stem is directly attached to the diaphragm between B and D. If the rod actually penetrated the right diaphragm fuel could leak from D to B.
There's another fairly subtle effect that's not mentioned anywhere in the documentation as well. If you look closely at the cutaway diagram you can see that the diameters of the two diaphragms are different and this affects the ratio of the air metering pressure's effect against the fuel pressure.
Just what I needed ❤
Nice video! Thank you!
Thank you!
Really great video! I'm studying for Powerplant Written Exam. There's a question:" An aircraft engine equipped with a pressure-type carburetor is started with the...?" Answer: "primer while the mixture control is positioned at IDLE-CUTOFF position." Do you know *why* this is the starting procedure for a pressure carburetor? Why not have the mixture control set to lean or rich? I'm reading that once the engine starts you immediately move the mixture out of the CUTOFF position. But why is it necessary/desirable to start the engines in the IDLE-CUTOFF position? And it seems like with it in the IDLE CUTOFF position little to no fuel would be available for starting. I don't get it! Thanks for the great video.
you will flood the engine with mixture in full rich position. using primer there is enough fuel for it to get quite a few revolutions giving you time to advance the mixture.