To bypass or not to bypass, that's the question!

Hi RW,

"Be aware that just because you have a high wing you can still have low pressure at your fuel pump.  This is due to the location of the pump in relation to the fuel tanks.  In a climb attitude you may have little to no gain in pressure by just gravity feed. "

I am astonished by this statement.

Assuming we are talking about a carburettor engine - they require fuel flow, pressure has little to do with it.We use pressure as a defacto indicator, that fuel is being delivered (flowing) to the carby bowl (s).

For a high wing (fuel tank) the owner would be best establishing what part fuel (reserve quantity), no pump, delivery rate (L/min) is, before making the decision to complicate the fuel delivery system with an additional (boost) pump. For best assessment the aircraft should be in a climb (tail down) attitude. Should the flow be adequate, to sustain engine operation, there is no advantage in installing a boost pump & every reason not to.😈

"- relying only on gravity feed is a little outside my comfort zone, so that's why I prefer to have the elec pump."

Humans are creatures of habit. It's likly you have been using an aircraft fitted with a boost pump, that you switch on, automatically (habit), when anticipating a high engine power requirement. Not having a switch makes you uneasy (comfort zone). This is understandable but not logical.

"- my fuel installation doesn't have a return line and it's not straightforward installing one. My hope with the elec pump is to have a way to push trapped air bubbles through to the carb and thus avoid vapor lock (the pump is installed in the cockpit, where temperatures are lower than under the cowling)."

What you are describing is how Rotax 9's were installed before the recommendation to fit a fuel return line came out. Many Rotax flew (are still flying?) with no fuel return line ie not necessary for the engine to run.The return line is specifically to address the possibility of fuel vapour forming in the delivery line. Fuel vaporization is  most likly in warm-hot ambient temperatures  after the engine has been run (is hot) shut down for a short period and restarted.

If you fly in a cool climate, you may never experince vapour lock. 

The presence of a fuel shut off valve, in the carby float chamber, will reduce the chances of effectively removing vapour bubbles in the fuel delivery system.  In line fuel vapour, close to the carby, will likly be vented. As soon as liquid fuel fills the float bowl, flow will be stopped, by the valve. Any vapour still in the fuel line will remain until the valve opens. This may lead to intermittent liquid fuel flow and engine running.

I am not advocating against installing a boost pump.

I am advocating for a logical/reasoned approcah to the decision to install one.

😈

Hi Sean

I think you are not understanding the Bing CV design that Rotax uses.  It is designed for use with a fuel pump, not for gravity feed.  Flow can only be achieved if we can fill the float bowls at all operational power settings.  Measuring the flow must be down after it travels via the entire fuel system and this includes the internal parts of the carburetor.  

The use of the mechanical fuel pump in the 912 UL and ULS requires, due to the pressure from the pump, a smaller inlet valve seat.  On gravity feed systems in motorcycles these seats are as much as double what the seat is on a pressure feed fuel system has.  In Mikuni it was common to see a 3 to 3.5mm seat on larger displacement engines.  The smaller displacement did not need the flow and would drop down to 2 to 2.5 on engines less than 250 cc for example.  

The Rotax Bing 64 has a 1.5mm seat, this is to accommodate the fuel pressure from the mechanical pump and any inline aux pumps that may be in the system.  To attempt to get full power with gravity feed would in all likelihood fail.  

Cheers

See the Bing drawing and specs for the CV 64

RW,

IF the carburettor requires pressurised fuel to run, why does it have the float bowl open to atmosphere?

Any incoming fuel pressure will be reduced to ambient, in the float bowl.

IF I am correct, a gravity feed system will ensure continued engine operation - may not be up to rated power.. 

What you are describing is a fuel supply flow problem, due to a dimensional restriction.  Pressure is  being used to overcome a restriction of fuel delivered to the float bowl.

If the float bowl is open to atmosphere - the delivery pressure will have little to do with carburetor operation, other than to keep the float bowl full.

Sems to me that quite a few high wing/tanks Rotax powered aircraft  are not fitted with a boost/auxiliary fuel pump, as the builder has measured  fuel flow at the carburettors & found it adequate for sustained engine operation, in the event of a mechanical pump failure..

😈

Hello again Sean...I give up.  As someone who has worked with many many OEM I have never seen one of them actually do a flow test on their systems.  We can argue they should be the fact is they dont.  The mechanical pump is needed since Rotax has no idea if they are a tractor or pusher or high wing or low wing.  The only backup one can call out is an aux pump given the literally hundreds of applications.  

I want my float bowl full all the time.  Given that I always want the security of having enough fuel in the event of a mechanical pump failure. 

Cheers