Comment

Comment by Bob Pustell on April 25, 2020 at 9:47pm

Alain -- if you really want to be able to run both pumps together I would suggest using a parallel arrangement for the pumps rather than having them in series. That way you are not doubling the pressure. There are several discussions here about how to set up a parallel pump situation. Basically you need a one way flow type check valve at each pump so that if a pump dies the flow from the good pump will not go backwards through the dead pump but will instead go onwards to the carb. With a parellel pump arrangement you would double your flow capacity with both pumps on but would not double your pressure. 

Your idea of putting a pressure regulator on the line would work but now you are introducing a critical single point of failure. If that one regulator fails you will not have proper fuel flow to the engine. Parallel pumps with check valves would be a much less risk inducing arrangement, in my not so humble opinion.

Comment by Alain Dufresne on April 12, 2020 at 10:24am

Thanks guys, I still think using 2 pumps for Takeoff and landing. Losing power at those critical moments is too.... stressful. I will see if I can add a pressure regulator after the pumps. That way I can respect the carb limit and ensure redundancy on the fuel feed. Thanks again.

Comment by christopher fryatt on April 12, 2020 at 8:30am

I gotta agree with bob, im using using the WW design since my AC was originally corvair powered. When going to the continental I opted to eliminate the oil pressure switch. The carb used is the same and HP ratings similar. I have seen that Vashon Ranger POH, O200 powered but gravity feed.... has the fuel pressure redlined at 4.8 psi

Comment by Bob Pustell on April 11, 2020 at 6:08pm

I would respect the pressure limits given in the books for the carb and not use the two pumps simultaneously. I agree that that having both on for take off and landing seems better intuitively but I would worry about putting that much pressure on the carb's float needle. William Wynn is a smart guy and his installations use series pumps wired to a single switch as described by Clint Gosch on Tuesday's response. If the engine so much as farts, flip that switch to the other pump but only run one pump at a time. William Wynne added a brilliant (in my opinion) "extra" by wiring the primary pump through an oil pressure sensitive switch that makes contact with oil pressure. In the event of a crash the engine would presumably stop running and the oil pressure loss would therefore turn off the fuel pump so that you will not be pumping fuel in a post-crash situation. The standby pump is not wired through the oil pressure switch so in the event the switch should fail the standby pump will still be active. You start the plane with the standby pump turned on so you will have fuel pressure for the start. Once the engine is running you switch to the primary pump. Both pumps get checked for every flight using this procedure.

Also, since the pump is required for flight, the power for the pumps should be hot wired to the battery, not run through the master switch. That way if you need to shut off the master switch in flight to control an electrical problem your fuel pumps will still work from battery power to allow the engine to keep running until you get to a landing location. Just be sure to shut off the pump before leaving the plane our you will run your battery flat. That clunk, clunk, clunk sound the pump makes should remind you to turn the pump off, but be sure before leaving the plane.

My two cents.

Comment by Clint L Gosch on April 7, 2020 at 6:35pm

I have a service manual for the MA3 series that describes a pressure test. The only two pressures they prescribe are 0.4psi and 6psi.

I used inline fuel pumps and wired them to a two pole, three position switch. Up position powers the primary pump, down powers the aux pump, middle switch position turns both off.

Comment by Alain Dufresne on April 7, 2020 at 2:13pm

Thanks Christopher. I was planning to duplicate the dual pump operation for take-off and landing as I do on my PA-28. The mechanical pump is always on but I put the electrical pump on for these critical phases. I am still wondering if twice the pressure is an issue for the carb itself.....

Comment by christopher fryatt on April 7, 2020 at 9:24am

I see about 4.5 4.6 psi, dual inline pumps but only use one at a time, unused for back up