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  #1  
Old 08-15-2018, 01:46 AM
dpemmons dpemmons is offline
 
Join Date: Apr 2017
Location: San Francisco, CA (KDVO)
Posts: 17
Default Intermittent high fuel flow

Last fall I bought a 2010, D180-equipped RV-12 with about 150 hours TT. I've been flying it regularly since May with 100LL and had a couple experiences of very high fuel flow:

The first was in late May during straight and level cruise on a long cross country from Norcal to Socal. 5500ft/7660DA, 23.4"/5170rpm at 97kt IAS/110kts TAS, I noticed fuel flow indicated about 8GPH and stayed around that level for something like an hour. The level in the tank upon landing indicated much higher than planned fuel use as well so I think the reading was roughly accurate. On the second leg of the trip it started out normal (6-ish) and then climbed again to 7-8GPH for maybe a half hour. Then with no changes to the throttle or anything else it went down to 5.9GPH and stayed there.

On my return trip a couple days later I saw around 5.9-6 the entire way under similar conditions; speed, altitude, etc.

Earlier this month I experienced a related event. At 6000ft/7900DA, WOT 24"/4950rpm at 87kt IAS/98kt TAS climbing at about 400fpm I watched as fuel flow increased from 6GPH to 8.5 over the course of about a minute. Leveling out at about 7000ft/9000DA I pulled the throttle back about 2" and stayed at about 85kt IAS, 21.2"/4520rpm for several minutes but I never got below 6.8GPH.

This doesn't seem to happen when I'm down low around sea level. That same day, at 1400ft/2900DA, 26"/4820rpm at 101kt IAS / 105kt TAS I saw 4.9GPH indicated.

I believe the k-factor is relatively accurate based on what the fuel totalizer says compared to what I actually end up with in the tank, but to be honest I haven't kept accurate enough records to know precisely. I've weighed the carb floats and they're all within spec. The engine is from 2010 but has the 893114 fuel pump installed.

I believe the prop is pitched too coarse as I've never seen the tach above about 5200rpm, compared to the Sling2 I flew previously that would happily do >5500rpm WOT and could get up to 5800 when descending with throttle in. Perhaps prop pitch explains why my fuel economy is generally poor? I don't think it explains why the fuel flow would intermittently be so high, however.

Any thoughts?
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  #2  
Old 08-15-2018, 06:53 AM
BigJohn BigJohn is offline
 
Join Date: Nov 2008
Location: Gloversville, NY
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I believe you have two issues.

1. It sounds like your prop is indeed set with too much pitch, based on the RPM and airspeed numbers you are seeing. You did not say what RPM you are seeing in a static runup at WOT. That would be a place to start. Based on the age of the airplane I think you will be facing the repetitive prop inspections or hub replacement required by the recent Sensenich SB, and that will require re-pitching the prop anyway, and that will be an opportunity to adjust pitch to Van's recommendations using the procedure in the current KAI.


2. Some RV-12's of your airplane's vintage experienced intermittent high fuel flow indications. These occurred primarily at altitudes of above 5000'. My airplane (Ser. # 120176) was one of these. Mitch Lock, then the Van's East Coast rep and builder of the blue demonstrator airplane also experienced the issue. The high flow readings were indications only, and as I recall were noted with both the D180 and early Skyview systems. It was discovered that the flow indications would return to normal as soon as the electric boost pump was turned off by pulling the fuse. Considerable effort was put into figuring this behavior out by myself, Marty Santic (and others), Van's staff, and Dynon. The eventual fix was to install a boost pump switch so the electric pump could be turned off during cruise at higher altitudes. Personally I was very comfortable with that procedure as a guy with a lot of time in Piper's and other low wing airplanes. I just added a couple of entries to my checklist. This was admittedly a band-aid solution, but as far as I know a true cause and solution was never arrived at.
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  #3  
Old 08-15-2018, 07:06 AM
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Piper J3 Piper J3 is offline
 
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Location: Hinckley, Ohio
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I can’t speak to fuel flow other than to say wiring connection or intermittent ground problem can cause your symptoms.

You have too much prop pitch if you can’t easily attain engine red line in cruise flight. Some benchmark engine speeds:

• 5150 rpm in initial climb at ~ 900 fpm.
• 115 knots at 5400 rpm in cruise
• 5400 rpm cruise should have throttle pulled out about 1”

See http://www.vansairforce.com/communit...9&postcount=81 for how to set prop pitch.

Also, you may not be aware, there is a service bulletin on your prop hub http://www.vansairforce.com/communit...d.php?t=162407
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  #4  
Old 08-15-2018, 09:09 AM
Mich48041 Mich48041 is offline
 
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Location: Riley TWP MI
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It is not good for the engine to be lugging. The prop should be re-pitched.

I would not shut off the electric pump in flight.

If there really is a high fuel flow and not an indication problem, some possible causes are vapor lock or a fuel leak.
Gasoline vapor in the fuel line can result in fuel flowing easier which can make the flow sensor turbine spin faster.
Installing the new fuel tank vent kit will slightly pressurize the tank to help prevent vapor lock.
Engine driven fuel pumps can leak which will cause increased fuel flow.
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  #5  
Old 08-15-2018, 01:18 PM
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DaleB DaleB is offline
 
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Location: Omaha, NE (KMLE)
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We've been chasing a similar issue. Once in a while we get a "FUEL FLOW HIGH" alarm, with flow showing as high as 10+ GPH. In our case it seems to almost always be above 7000 DA, consistent with what others have experienced. The fact that the builder had installed a switch (well, a pull-able breaker) for the electric fuel pump makes me think this has always been going on. There's no way the fuel flow is actually that high, but the alarm is annoying when solo and much worse if you have a passenger. One day it happened when I was on a longer trip; descending from 7500 to 5500 stopped the issue. And yes, pulling the fuel pump fuse "fixed" it, but I'm on a quest to discover and correct the root cause.

Sometimes when just burning holes in the sky -- maybe 3-4000 MSL, 100 knots or so -- I'll watch the fuel flow reading go from 5-ish GPH to over 7, and vary up and down quite a bit. No indication of carb leaks, fuel going overboard, or anything else. Floats are fine, new gaskets. Don't know quite what to think.

My working theory is a combination of vibration and the mounting of the transducer. Maybe it's mogas related. Ours is a D180 equipped RV-12 completed in 2011, 400 hours, unleaded mogas almost exclusively. We're going to get the prop balanced and see if that helps. I'm toying with the idea of using vibration isolators to mount the transducer, but I don't want to create any fatigue problems with the fuel lines.

As for the prop pitch, you can check the static RPM and see. Ours was too low when I bought it, about 4700 as I recall. It should be 5000, or very close to it. De-pitching it to the correct static RPM helped fuel economy as well as speed and climb.
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  #6  
Old 08-16-2018, 07:24 AM
BigJohn BigJohn is offline
 
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Quote:
Originally Posted by DaleB View Post
……………. One day it happened when I was on a longer trip; descending from 7500 to 5500 stopped the issue. And yes, pulling the fuel pump fuse "fixed" it, but I'm on a quest to discover and correct the root cause...………………….

.........…. Maybe it's mogas related. Ours is a D180 equipped RV-12 completed in 2011, 400 hours, unleaded mogas almost exclusively...……..
Dale,

I wish you well on your quest to find the root cause. I hope you are more successful than I was 6 years ago. I hated the idea of not getting to the bottom of the issue, but finally decided to treat the symptoms with the addition of a simple switch, rather than beat my brains out looking for the true cause/fix. I justified this decision by virtue of the fact that I was able to prove to myself that this was strictly an indication of high flow, not an actual high flow, and thus not a safety issue.

As to fuel, all incidents that I experienced were with 100LL.

Happy hunting!

John
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  #7  
Old 08-16-2018, 11:25 AM
RFSchaller RFSchaller is offline
 
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This seems odd to me because my experience with high fuel flow is always accompanied by low fuel pressure and quickly remedied by turning on my electric pump (I rewired my electric pump to the unused nav light switch). It happens in hot weather (I live in PHX) and appears to be due to fuel flashing to vapor in the line upstream of the red cube (my assumption based on the indications and reaction to starting the electric pump). What you are seeing is something different.
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  #8  
Old 08-27-2018, 02:48 AM
dpemmons dpemmons is offline
 
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Location: San Francisco, CA (KDVO)
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I repitched the prop yesterday. My goodness is it a pain to get both blades to the same angle!

The adjustment cylinder initially indicated "5", and the pitch stop ring's tabs were sufficiently wide that I couldn't change the pitch more than a couple tenths of a degree. I shaved a bit off the tabs on the stop ring and figuring I was way over pitched took out 1.6 degrees .. which I think got me close to "3", but it's hard to see with the spinner plate in the way. Static WOT went from 4700 to 5100 so I was worried I over did it, but decided to test fly anyway.

I flew this afternoon from KDVO up through the Sierras to Lake Tahoe and then Carson City, NV for fuel, and then back. Would have been a beautiful flight if not for all the smoke, so it was probably fine I didn't have a passenger.

The plane performed nicely. Idles smother, climbs better and feels quieter with less vibration. I leveled out and stabilized at 5500' (7500' DA) WOT for a few minutes and saw 24.1"/5580rpm at 106kts IAS/119kts TAS. That seems better. That was without wheel pants.

And she did great up in the mountains - still plenty of climb left in her at 10,000'/11,500' DA, and taking off from Carson with full fuel at 7000' DA was a non-event.

Related to fuel flow:

I saw pretty high indicated flow most of the way to Tahoe - 6.5-7.5 GPH, but saw 5.3ish indicated most of the way back. I was flying with about an inch of throttle out the whole time. Ignoring time on the ground, actual use for the whole trip was somewhere around 5.3gph. I think I'm seeing the same indication issue as DaleB and BigJohn...
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  #9  
Old 08-27-2018, 10:57 AM
DHeal DHeal is offline
 
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Your prop pitch adjustment will be much easier and far more accurate if you use an electronic level to set your pitch angles. There is much written here in this RV-12 forum regarding the appropriate equipment and this process.

Regarding your high fuel flow: Use actual fuel consumed over time -- i.e., actual gallons bought and put into the tank divided by operating time (Hobbs and/or Flight Time) to get a more realistic value for your average GPH. You can then adjust the rate of the Fuel Flow Sensor on the Dynon to generally match this average rate. You will likely find that the GPH shown on the Fuel Flow Sensor is generally accurate over the total course of the flight -- its instantaneous accuracy is not all that good due to the fuel system's basic design.
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  #10  
Old 08-27-2018, 11:57 AM
RFSchaller RFSchaller is offline
 
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How did you set your K-factor on the red cube? Maybe it just needs to be recalibrated if the high fuel flow indication is steady. Originally it sounded like yours was spiking.
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