Deene Odgen:
On injected systems, they are, obviously, required, one on the engine and a backup for starting and standby. For the Bendix and Airflow injections systems, these pumps operate at about 20 psi.
On carb engines, I think Alan is right. The rule is that you should have no pumps or 2 pumps. I wouldn't operate without pumps since the BD-4, unlike the C-150/172, has very little fuel head at climb attitudes. With a single pump system the problem comes should that engine pump fail. I used a RV4/6 type Facet pump inline right ahead of the gascolator when I had the O-360 carburated engine.
Roger Mellema
Newsletter #19
In the last newsletter I reported on an accident where a
BD-4 experienced an engine failure shortly after take-off. They had a lot of
trouble finding the cause but it turned out to be a failure of the mechanical
fuel pump. This is really scary as most of us have flown for years with just a
mechanical fuel pump.
Steve Mahoney got very worded as he regularly carries his family over some very
ugly terrain and he has always assumed that the system would gravity flow no
matter what happened to the fuel pump. He bypassed the fuel pump and had no
problem until just about lift-off. The engine would not go over about 1600 rpm.
He then thought about all the airplanes that have always flown without a fuel
pump (C-1 50, C-1 72, Piper Colt, Piper Tri- pacer, etc.) and wondered why a BD
should be different. He then hooked up the fuel pump again and found that the
engine had a hang-up at about the same rpm, but by pumping the throttle, he
could get it to full power. Steve found that the float in the carburetor had a
small shiny place on it where it was rubbing the side of the float bowl. With
the float bent so that it didn't rub, he found that the airplane flew fine with
or without the fuel pump.
I am not sure quite what lesson we should learn from this. I almost makes you want to try flying without the pump so that you will be sure that your installation will work with gravity only flow. The Marvel-Scheibler carburetor requires a fuel pressure of about 0.2 lb/sq. in. to work properly. A cubic inch of gasoline weighs 0.026 lbs. Theoretically, a 7.7 inch head of gasoline should give sufficient pressure to allow proper operation.
Roger Mellema, George Wittet
Newsletter #19
George Wittet called me some time ago and suggested using 4 Facet electric fuel pumps to assure good fuel flow for the BD-4. He put two pumps in each wing root, one hooked up to each fuel line. He has 4 switches in the panel so any or all of the pumps can be selected. He says that this system works fine and the airplane will cruise with no pumps'on'. The pumps each have check valves in them to prevent backflow into the tank when only one pump is turned 'on' on a side.
This pump arrangement is especially good for low wing airplanes as it is always better to 'push' gas rather than 'suck' it (sucking enhances the chance of vapor lock). This system may add a little weight but it sure allows you to get gas from the very back of the tanks for take-off and climb! These pumps are a little noisy but only when there is still air in the lines. You couldn't hear them over the engine and wind noise anyway. I have two of these mounted under the instrument panel right now for my V-6. Jim Huber informed me that these pumps are available with "flare" fitting rather than the hose fittings on the ones I have. I have found that hoses tend to flatten too much when they bend and that they flatten more over time. I may be ordering the flared fitting model soon for my V-6 conversion so that I can use aluminum lines. Another solution suggested by Tim McGinnis is to use polyurethane tubing. It is extremely resistant to chafing and can be bent into a tight radius if it is heated some. This tubing is recommended by Rutan and is available from Spruce and Specialty.