Cowl Flaps?

[Toobuilder]

...While some of the exact failure modes are not known there have been a number of fatal accidents due to fuel plumbing...

Not going to rehash the debate concerning what's better, But I will say that if you are not comfortable with a particular method, then dont use it. Personally, I replace every hose I can with hard line. Hose is only used on my airplanes where there is relative movement. I have many years and hours across several airplanes doing it with no failures.

Use the method that meets requirements, fabricate and install the components correctly, and inspect regularly. That's aircraft design 101.

 

[MacCool]

Just as a brief followup, these are the undercowl temps I recorded yesterday using Pete Howell's datalogger. It uses LM34's, so multiply Y-axis temps by 100. They were secured to the fuel pump, red cube, and gascolator. About 55 minutes from startup to shutdown, and after shutdown until it was clear that temps were all decreasing. Not as hot under there as I anticipated.

 

[NTex]

Mac, before I spent time and money on cowl flaps, I'd revise the fuel system.

The boost pump should be behind the firewall, where it is cool. Elbows should be minimized in the plumbing between the tank selector and the boost pump. Bring the fuel through the firewall with a SS bulkhead fitting, then run a short, insulated teflon hose directly to the engine driven pump. The red cube should be downstream of the engine driven pump. It's a moderate flow restriction. And do you see a fuel flow change when you switch the boost pump to ON?

Dan curious why you recommend a stainless steel bulkhead fitting vs a carbon steel. Just to match the firewall material or something else?

 

[DanH]

Dan curious why you recommend a stainless steel bulkhead fitting vs a carbon steel. Just to match the firewall material or something else?

Water and/or alcohol content in fuel can cause corrosion with a carbon steel fitting, which sheds rust flakes into the system. I once removed one with what looked like steel wool inside, after finding some in the Bendix inlet screen.

 

[MacCool]

Mac, before I spent time and money on cowl flaps, I'd revise the fuel system.

The boost pump should be behind the firewall, where it is cool. Elbows should be minimized in the plumbing between the tank selector and the boost pump. Bring the fuel through the firewall with a SS bulkhead fitting, then run a short, insulated teflon hose directly to the engine driven pump. The red cube should be downstream of the engine driven pump. It's a moderate flow restriction. And do you see a fuel flow change when you switch the boost pump to ON?

Yeh, chatting at breakfast the other day with some of the local RV brain trust, there’s genuine puzzlement about the FF fuel line routing decisions by the OB’s engine guy when the airplane was built. Gascolator, component location, 90° fittings all seem to be non-standard, or at least deviate from what many here would call "best practice". I’ll probably have to find someone to revise them for me and I've tentatively lined that up for when he gets free. In the meantime, the airplane is functional and temps illustrated by Pete Howell’s datalogger are not outrageously out of line. My A&P and I did dissect the Tempest EDP that we just changed out based on suspicions of the asymptomatic fuel pressure variations at hot-day taxi that I saw. It appeared to be fine, diaphragms intact. I don’t have much concern about functionality or safety. I appreciate the help in this thread.

 

[DanH]

Gascolator, component location, 90° fittings all seem to be non-standard, or at least deviate from what many here would call "best practice". I’ll probably have to find someone to revise them for me and I've tentatively lined that up for when he gets free. In the meantime, the airplane is functional and temps illustrated by Pete Howell’s datalogger are not outrageously out of line.

Mac, I seem to have missed the graph. True, the recorded temperatures are not outrageous, but in flight they are rising above the best practice value of 140F for fuel. If I recall correctly, that number came from Don Rivera, and represents the rough range where some of the more volatile fractions in 100LL begin making vapor bubbles. Throw in some trip points, like the sharp internal corners in 90's, and things get worse. I'll bet you can improve the system. And kudos to both you and Pete...nice work.

 

[MacCool]

Mac, I seem to have missed the graph. True, the recorded temperatures are not outrageous, but in flight they are rising above the best practice value of 140F for fuel. If I recall correctly, that number came from Don Rivera, and represents the rough range where some of the more volatile fractions in 100LL begin making vapor bubbles. Throw in some trip points, like the sharp internal corners in 90's, and things get worse. I'll bet you can improve the system. And kudos to both you and Pete...nice work.

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I agree that my fuel system needs to be addressed. Your original suggestion of getting some LM 34s to get actual data and Pete Howell’s completion of his datalogger, as well as the fact that he’s only 30 RV minutes away represented a fortunate congruence. I learned a lot and I see an opportunity to make my airplane a better airplane …thanks.