flyrotary@lancaironline.net Mailing List Archive

Some excellent points, Steve, as always.

Each system is different and as we know even "small" details can made a big difference if operating close to a critical point.

I agree the pressure in front of the pump inlet with two pumps running would be less than with only one pump drawing the fuel. I guess the question is how much less. I suspect the effect is not linear but related to the square of the flow volume/velocity.

Heating of the fuel seems to be the major factor in why a system seems to operate fine one day - until perhaps the hottest days/conditions and then problems occur.

In my case, I do have a small (1/2 pint) header tank on the firewall side which I return the Pressure regulator line to. In the early days before I shielded and added a blast tube to my set up, there were occasions on hot days when I noticed the fuel pressure start to wobble - turning on the boost pump stopped in in every case.

Now if I took off with only one EFI running - would I need the boost pump - don't know- not inclined to find out {:>). Actually, there have been a few occasions when I neglected to turn on my boost pump, but they were not on the hottest days - so not a true test.

My greatest concern is this regard - is pump failure on take off - if only one pump is on and fails, then it is going to take x amount of time to detect, recognize and take corrective action - and my reaction time is not improving with age {:>). Yes, I could set up some sort of automatic pressure sensor to turn on the second pump, but I dislike complicating my set up anymore that it is. So as you point out - you evaluate the scenario and make your choices.

The output of the two pumps would still be regulated to whatever pressure you have set in your regulator - but with two pumps pumping that would indicate that more fuel is flowing through the system and the excess is being dumped back into the return line (because no more fuel is being used by the engine).

So would this increased flow of cooler fuel through the system increase or decrease the probability of creating low enough pressure (with the cooler fuel) to cause vapor? Ah, the devil is in the details.

Basically, it would seem that anything you do to lessen the pressure drop and/or keeping the fuel cool (perhaps turning off one EFI, removing the coarse filter, increasing pressure in the line through a boost pump, fuel cooler, etc) reduces the risk of fuel turning to vapor. My assessment is that the positive effects of the boost pump can be relied upon with more confidence than most other "fixes" or preventive measures - but that assessment is not based on any testing, just my opinion.

The one I potential preventive measure/fix I have not looked into is the vapor by-pass/dump that I know a few folks are using. I search the archive but could not find a description of this method - anyone care to provide one?

Since I always take off with my boost pump on (produces approx 6 psi) whatever adverse effect running with two EFI pumps might have on the possibility of vapor lock appears to be alleviated by the addition line pressure on the EFI pump intake line.

My greatest concern is this regard is pump failure on take off - if only one pump is on, then it is going to take x amount of time to detect, recognize and take corrective action - and my reaction time is not improving with age {:>). Yes, I could set up some sort of automatic pressure sensor to turn on the second pump, but I dislike complicating my set up anymore that it is.

So like most problems we wrestle with - it’s a trade off.

FWIW

From: Steven W. Boese

Sent: Tuesday, August 09, 2011 10:12 PM

To: Rotary motors in aircraft

Subject: [FlyRotary] vapor lock

Ed,

Do you think your fuel system is somewhat unique with the use of the small header tank?

It seems to me that running two high pressure pumps simultaneously to deal with the possibility of failure of one pump increases the likelyhood of vapor lock in the supply to the pumps due to the doubled fuel flow requirement compared to running only one pump. This increased susceptibility to vapor lock would only exist during the critical phases of flight when both pumps would be running. The procedure used would depend on the operators judgement of which failure mode (pump failure or vapor lock) would be most likely. Like everything else, a compromise.

This may not have anything to do with Sam's system, operating procedure, or problem, just something to consider.

Steve Boese

On Tue, Aug 9, 2011 at 8:06 AM, Ed Anderson <eanderson@carolina.rr.com> wrote:

Further discussion with Sam leads me to believe that the highest probably cause was a "vapor lock" of some type in the fuel line. Sam stated the engine never stopped running (correction to my earlier report - where I stated it had quit), but just continued to sputter and failed to produce power.

The ignition checked out OK as did the rest of the system back in the hangar. The symptom of sputtering engine and no power would put a fuel system problem high on my list of suspects and given the high ambient temps (98F) and the run up before take off -- would leave me to believe those conditions could have lead to diminished fuel flow.

Sam is going to inspect his coarse filter to ensure it was not partially obstructed.

One item of note is that Sam does not have any sort of boost pump between tanks and his EFI pumps. In my early days of testing while doing power-on stalls (high power low cooling airflow) on a hot day, my engine started to sputter (unlike Sam I had plenty of altitude when it happened) and I noticed my fuel pressure fluctuating between 20 - 35 psi. I turned on my Facet boost pump (puts out approx 6 psi pressure) and it immediately squelch the problem. So I now take off/land with boost pump on and have never since had that problem.

We know that any obstruction in a fuel line produces at least a small pressure drop down stream - Given the suction of the EFI pumps and even minor restriction of the coarse filter - a lower pressure region would exist between filter and EFI pumps - elevated temperatures could be enough to cause some of the fuel in this area to flash into a gaseous state - which of course is not pumped very well. Its my opinion that a boost pump is a worthwhile addition in keep pressure in the low pressure part of the fuel system sufficiently high to prevent the fuel from percolating - particularly on HOT ambient days. However, make certain that the boost pump is of a type that does not obstruct fuel flow during a malfuction or when off.