Cooling shroud or air blast tube on fuel pump?

[Gary0747]

What is the consenus of having a blast tube of air or a cooling shroud hooked to the air tube for cooling the fuel pump?  I do not see anything in the parts manual.  I was also wondering how many have the third fixed cowl flap in the center to help cooling?  I would think that would help cool the fuel pump.  

Gary

67F

[Yetti]

 

[Shadrach]

My 67F had one as it came from the factory. If yours does not, than it's likely missing. If you operate in cold climates year round, it probably does not matter much. If your operations include hot and high DA days, you might encounter an engine burble at a most inconvenient time. 

[DXB]

Do the carb'd O-360 planes also have this fuel pump cooling shroud?  I'm perpetually obsessed with intermittent low FP readings in climb.

[Shadrach]

2 hours ago, DXB said:

Do the carb'd O-360 planes also have this fuel pump cooling shroud?  I'm perpetually obsessed with intermittent low FP readings in climb.

Do you notice any change in EGT when it happens? Does it happen regardless of OAT?

[DXB]

Just now, Shadrach said:

Do you notice any change in EGT when it happens? Does it happen regardless of OAT?

No change in EGT, no change in fuel flow, at any OAT, happens on about 1/4 of climbs,  totally random as far as I can tell during >1 year of observation.  But vapor lock in the line up to the transducer seems plausible.

[Shadrach]

1 minute ago, DXB said:

No change in EGT, no change in fuel flow, at any OAT, happens on about 1/4 of climbs,  totally random as far as I can tell during >1 year of observation.  But vapor lock in the line up to the transducer seems plausible.

Do you climb with the boost pump on?

[DXB]

Just now, Shadrach said:

Do you climb with the boost pump on?

I keep it on up to 1000 AGL, then turn it off, then back on for the rest of the climb in minority of instances where pressure reading drops slowly to <0.5psi.  Never had any issue in cruise, and FP actually reads too high when taxiing (with boost pump off).  

[Shadrach]

24 minutes ago, DXB said:

I keep it on up to 1000 AGL, then turn it off, then back on for the rest of the climb in minority of instances where pressure reading drops slowly to <0.5psi.  Never had any issue in cruise, and FP actually reads too high when taxiing (with boost pump off).  

I only ask because if you were running the boost in climb and it had an intermittent issue, it could manifest itself in this way. 

It could be the transducer or how it is mounted. <0.5 is no bueno per Lycoming. IIRC, 1lb is minimum. I think it may be an anomaly in how your system is reading fuel pressure more than an actual low pressure event. 

[takair]

This seems a little odd.  I have now heard of two similar events that ended up being minor leaks at the gascolator.  They would only manifest at higher power and lower ambient pressure (altitude). Basically, sucking air.  I would think the fuel flow would change a little too, but you do still have some pressure.  The electric pump applies positive pressure at the gascolator, thus plugging the leak.  In both cases, there was no evidence of leaking fuel on the ground.

[carusoam]

The C uses such low fuel pressure that the pressure changes with a change of attitude.

1) Picture a column of fuel. More specifically a vertical tube with fuel in it...

2) the bottom of the tube is exactly one square inch.  (This simplifies the math quite a bit)

3) fill the tube with a gallon of fuel.   A gallon of 100LL weighs approximately 6 pounds.

4) Magically, we have put 6 pounds of fuel over a square inch of surface area...  6psi

5) knowing that there are 231 cubic inches in a gallon, leads me to believe that my tube needs to be 231 inches tall, almost 20' to contain the six pounds.

6) Basically saying that a 20' column of fuel generates 6psi at the bottom. Or 1 psi for each 3.3 ft of the column.

7) how much pressure does the fuel pump generate when the plane is horizontal?  About 6 psi(?)

8) how much pressure does the fuel pump generate in a steep climb?  About 3psi (?)

9) Air leaking into the system can cause problems in FF sensors.  

10) Air leaking in can cause burning problems.

11) Air leaking in may not show up on a pressure gauge.

12) air or fuel leaking out will show up on a pressure gauge.

Going through the math this way, may include some errors.  But it does show a six foot fuel tube turned uphill is going to show the sensitivity of the low pressure system to attitude.

Anyone follow my logic on this one?  My memory of actual FP readings in my carbureted C is several years old...

I have never been a mechanic, I am only a PP.

Hope this helps spur the conversation.

Best regards,

-a-

[DXB]

16 minutes ago, Shadrach said:

I only ask because if you were running the boost in climb and it had an intermittent issue, it could manifest itself in this way. 

It could be the transducer or how it is mounted. <0.5 is no bueno per Lycoming. IIRC, 1lb is minimum. I think it may be an anomaly in how your system is reading fuel pressure more than an actual low pressure event. 

Notably the identical phenomenon happened with the old transducer for my original analog MAP/FP gauge and the new JPI900 that replaced it.  So I believe somehow the pressure in the line up to the transducer (which is significantly higher than the pump) is actually going to 0.  But Fuel flow decreases little or none during these events, based on pulling the data off the JPI.  My shop agrees with you that there's unlikely to be a real FP problem.

14 minutes ago, takair said:

This seems a little odd.  I have now heard of two similar events that ended up being minor leaks at the gascolator.  They would only manifest at higher power and lower ambient pressure (altitude). Basically, sucking air.  I would think the fuel flow would change a little too, but you do still have some pressure.  The electric pump applies positive pressure at the gascolator, thus plugging the leak.  In both cases, there was no evidence of leaking fuel on the ground.

This sounds consistent with my situation.  I've never seen any hint of a fuel leak.  I specifically asked them to check for leaks at annual and nothing was found.  Their conclusion was "meh...low winged carb'd planes do that."  That flashing red 0.0 annunciation on the JPI during climb continues to get my attention though.  

[N601RX]

How high is the gauge or pressure transducer above the carb? I would guess 24-36" in most applications?  I don't know the specifics of 100LL off the top of my head, but water head drops at a rate of about .43psi per foot.  100LL is lighter than water and would be slightly less.

[carusoam]

Fuel density is 6 eighths or thee quarters of water density...  6 pounds per gallon compared to eight pounds per gallon...

.75 X .43 = .32 psi/ft (head pressure of 100LL) using 601's memory...

1 psi per 3.3' (as calculated above) = 1/3.3 = .30 psi/ft

Good news: numbers off the top of one MSer matches the numbers calculated by another...?

Best regards,

-a-

[Yetti]

The gauge on an F is maybe a foot to 18 inches above the fuel servo.  But at about the same height as the fuel pump above the Fuel servo.   So it should be a pretty good indicator of the pressure at the outlet of the pump.

[Yetti]

On 3/2/2016 at 3:36 PM, Shadrach said:

My 67F had one as it came from the factory. If yours does not, than it's likely missing. If you operate in cold climates year round, it probably does not matter much. If your operations include hot and high DA days, you might encounter an engine burble at a most inconvenient time. 

Never had a burble. Don't have a shroud.  Did mount the scat tubing so it is blowing on the pump.   DA was 2800 feet on a 300 foot field.  Temp was 100 degrees.   Sat on a ramp before starting up.  Fuel temp was probably in the high 90s.  

[Tcraft938]

On ‎3‎/‎2‎/‎2016 at 10:04 PM, carusoam said:

The C uses such low fuel pressure that the pressure changes with a change of attitude.

1) Picture a column of fuel. More specifically a vertical tube with fuel in it...

2) the bottom of the tube is exactly one square inch.  (This simplifies the math quite a bit)

3) fill the tube with a gallon of fuel.   A gallon of 100LL weighs approximately 6 pounds.

4) Magically, we have put 6 pounds of fuel over a square inch of surface area...  6psi

5) knowing that there are 231 cubic inches in a gallon, leads me to believe that my tube needs to be 231 inches tall, almost 20' to contain the six pounds.

6) Basically saying that a 20' column of fuel generates 6psi at the bottom. Or 1 psi for each 3.3 ft of the column.

7) how much pressure does the fuel pump generate when the plane is horizontal?  About 6 psi(?)

8) how much pressure does the fuel pump generate in a steep climb?  About 3psi (?)

9) Air leaking into the system can cause problems in FF sensors.  

10) Air leaking in can cause burning problems.

11) Air leaking in may not show up on a pressure gauge.

12) air or fuel leaking out will show up on a pressure gauge.

Going through the math this way, may include some errors.  But it does show a six foot fuel tube turned uphill is going to show the sensitivity of the low pressure system to attitude.

Anyone follow my logic on this one?  My memory of actual FP readings in my carbureted C is several years old...

I have never been a mechanic, I am only a PP.

Hope this helps spur the conversation.

Best regards,

-a-

I love this and the way you explain it.  When I was considering a C, I talked with an owner where his was in the shop completing annual.  All seemed well until the A&P told him to turn on the fuel pump.  Fuel spilled behind the nose wheel and it had never done that before.  It was from the gascolator.  So they put new O rings in, on with the fuel pump and it leaked again.  Both were perplexed.  I looked in the cockpit and noticed the fuel selector in the off position.  Just using reverse logic, but no real knowledge of the system, I asked if that could be part of the problem.  I got a dirty look and a lot of reasons why that was a stupid ask.  However, my thinking was, you commented it's never done it before and I seriously doubt you fly long with the fuel shut off and I doubt it is leaking that bad while you're flying because while you may not see it, you'd be putting a lot more fuel in the tanks after an hour.  To appease me, the A&P said, turn fuel to left tank and turn fuel pump on.  Shazam, no leak.  I'm thinking that having the fuel pump on without the fuel selector on caused a condition opposite of what the O rings were designed to take.  Needless to say, my other big lesson was, "this A&P is not going to do my pre-purchase inspection when I find a Mooney".  That may have been a valuable item learned.

[cliffy]

Am I missing something here?

We have a "captured" system with a pump providing the pressure to the captive system 

I can see a pressure drop IF you have a "tube" vented to the outside and a drop in vertical altitude by angling the tube BUT we have a captive environment with pressure being applied by the pump. Think of your tube now with a piston and a 6# spring pushing down on it keeping the pressure the same whether it is vertical or tilted (makes no difference). The spring and piston takes the place of the fuel pump. We are not a gravity feed system vented to the outside air. 

Secondly, you're talking about tilting the vented tube enough to change the effective height to 10' instead of 20'. What is the angle you have to pitch to to get that response? Any geometry students out there? We have a known angle ( 90 degrees and a known C squared distance of 20 feet:-) How does that pitch angle compare to a normal climb angle? Not even close. 

So my postulation is that angle of climb has no bearing on fuel pressure in a captive system. The fuel pressure variance is caused by other factors in the fuel system itself. 

What is "vapor lock" in a captive system? Bubbles forming on the inlet side of the pump due to the lowering of pressure enough for production of bubbles in the fuel stream. The pump has a check valve on the inlet and outlet side of the diaphragm. The diaphragm is moved up and down by the fuel pump rod being moved by a lobe on the cam of the engine with every engine rotation. The volume of the area above the diaphragm increases and decreases with every rod movement. As the volume increases a check valve is opened on the inlet side of the pump and fuel is "sucked' into the pump cavity from the fuel line. When the pump cavity is decreasing in volume by the rod pushing in on the diaphragm the inlet check valve closes and fuel is pushed out the outlet side of the pump through another check valve producing a slight pressure in the pump cavity as it does so. When the pump "inhales," whatever it takes to unseat the inlet check valve lowers the pressure in the pump cavity. If the fuel is hot enough to be near "vapor lock" state then the lowering of the pressure "inhaling" will cause bubbles to form coming in to the pump. Now we try to push the fuel out to the engine and we are trying to compress "air" and some fuel. The fuel pressure drops because of the air in the system. Just like opening a warm Coke bottle on a hot day or one out of the fridge on a cold day. Much more chance of a boil over on the hot day than the cold day. Lowering the pressure takes "air" out of suspension in the liquid.  

Heat magnifies the issue. Bubbles lower fuel pressure due to their compressability and the finite volume of the engine pump. Solid liquids are not compressible (for this discussion). 

Why do we have an electrical fuel pump feeding the engine driven pump? For engine driven pump failure (although if the diaphragm fails we will see fuel coming out the engine pump drain line at the bottom of the cowl ) (and if the pump diaphragm is damaged enough for it to be leaking, any fuel we push through that pump, under pressure from the electric pump, may go out that same hole in the diaphragm. Enough may get to the engine to sustain operation but if you need to turn on the electric pump to keep the engine running, other than just in the climb,  you might just think about getting your rear on the ground as quick as possible as you might have  a big fuel leak going on outside the cowl and being blown along the fuselage bottom) and, to provide a positive pressure of fuel to the engine driven pump to reduce the possibility if cavitation and vapor lock. 

With the electric pump on there "should" be no reduction in fuel pressure in climb with high temps. Mine works just that way. On a hot day with just the engine pump on I'll get a drop in fuel pressure in a long climb. I turn on the electric pump and it comes right back to 6 psi and stays there. Once up higher in cruise (and cooler air) the electric goes off and the fuel pressure stays due to less "cavitation" in the engine driven pump in cooler temps. 

Shields are up!