Third fuel pump on 912 iS engine installations

by james porter in 912iS Technical Questions 6 years ago

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Re: Third fuel pump on 912 iS engine installations

by geoffrey klestadt » 6 years ago

geoffrey klestadt

27 94 94

Now you've got me thinking.... i'm building a Zenith CH750 STOL - high wing, gravity feed to a header tank with the fuel system designed by Mark at Rotech Canada. I'm going to ask around if anyone has had these problems with this model, because a stumble let alone fuel vaporization on a STOL takeoff on a hot day (and we get a lot of them) is going to have unpleasant results.

Question for Ian; why did you use two facet pumps? Series or parallel? One for each wing tank? I guess I have room to fit them under the header tank and I have an extra switch hole on my panel:)

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Re: Third fuel pump on 912 iS engine installations

by Genival Pacheco » 6 years ago

Genival Pacheco

4 87 135

Hi guys,

To avoid cavitation you have to guaranty a sufficient NPSH (net positive suction Head).
The value will depend on the fluid characteristics and the working temperatures.
See the document attached

Cavitation-White-Paper_FINAL-2.pdf (You do not have access to download this file.)

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Re: Third fuel pump on 912 iS engine installations

by Peter Collins » 6 years ago

Peter Collins

1 9 0

My plane is a high wing, gravity fed to a collector tank then down though a very short, fat hose (with a course filter in line) to the pumps box which is at a lower level still - flat on the floor under the seats which also means they are in perhaps the coolest part of the plane. Rotax require the pumps to be low down, and having them gravity fed, ALL the way, should help them. I guess they don't deal well with low pressure on the suction side. And of course, an injected engine requires continuous fuel pressure so even the briefest interruption will cause a 'stumble'. Re-plumbing could be a pain, but going to Rotax-standard could be the answer. See the Rotax Builder Series for the collector tank experiments. The tank was higher than the pumps.

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Re: Third fuel pump on 912 iS engine installations

by Adrian Whitmarsh » 6 years ago

Adrian Whitmarsh

0 1 0

This may not help you but I had a fuel pressure issue on my 912UL recently. After changing in-line fuel filter, then all fuel hoses to ensure no blockage, then a new mechanical pump, then fitting an electric pump the cause was a tightly crimped vent pipe on the mechanical pump! What is your pump make? Mine is the Italian produced BCD. The vent pipe originally fitted was the clear plastic variety. It was cable tied to an oil hose. In 2 1/2 years had no problem but over time the plastic vent pipe must have been softened by the hot oil pipe the over a cold winter hardened into a very restricted shape such that there was insufficient clearance. That explained the intermittent nature of the problem. Once spotted I replaced it with a similar length of fuel hose as its thicker walled and has some heat resistance. Bingo, problem solved.
Hope this might help but if not, heads up for others to check this simple item on inspections.
Best regards
Adrian
UK

Thank you said by: RotaxOwner Admin

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Re: Third fuel pump on 912 iS engine installations

by james porter » 6 years ago

james porter

14 78 70

Just an update on further testing of my fuel system. I have a high wing airplane and both wing tanks currently have approximately 5 gallons of fuel each.
In static testing today I determined that the fuel system at the firewall has approx. 1.25 PSI (positive pressure).
When the fuel selector is "Both" (two 7MM fuel lines feeding a 5/16" fuel line to the firewall) when one fuel pump is activated the system pressure is .75 to 1.0 PSI (positive pressure). When both pumps are activated, the system pressure is "0" PSI (neither positive pressure nor negative pressure or suction).
When either "L" or "R" is selected there is a slight positive pressure when only one fuel pump is activated.
When both fuel pumps are activated there is a negative 2 to 3 PSI (negative).
It would seem that the two 7MM fuel lines are sufficient in this installation even when both pumps are used. Using "R" or "L" is sufficient when using only one fuel pump but would preclude the use of both fuel pumps due to the negative PSI (suction).
Positive fuel pressure is maintained when using only one fuel pump during all operations regardless of fuel selector setting.
Fuel vaporization may be best avoided by using only one fuel pump. (contrary to the suggested Rotax operating manual).

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Third fuel pump on 912 iS engine installations

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