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  #1  
Old 08-18-2020, 01:26 AM
rv8ch's Avatar
rv8ch rv8ch is offline
 
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Default Airflow Straighteners

My FM-200 is running a bit rich, and Don speculates that it's due to the very turbulent air coming in the FM-200, which is causing the "sensor" to feel that there's more air coming in than reality.

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This seems to also be a common problem with MAF sensors on cars, and the solution to that seems to be something called an "airflow straightener". It seems one of the popular ways to do this is with an aluminum honeycomb structure just before the sensor.

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Seems companies like Bosch include this feature in some of their MAF sensors.



Does anyone have any experience using these on a Lycoming engine? Another option is of course to make the air coming into the throttle less turbulent, but that would require a lot of work on my intake, filter, cowl, etc. that I'm not really interested in taking on now. Another option is to get a leaner main jet in the FM-200 which I've ordered, but seems like another workaround. Thanks for any tips.
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  #2  
Old 08-18-2020, 09:44 AM
Freemasm Freemasm is offline
 
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Default Not exactly, but

I have applied a lot of flow conditioning devices for flow measurement; usually industrial gas turbine fuel flow related. Being that you are interested in a "permanent" installation downstream of any filtration, I'd be careful. Any failure scenario would probably dramatically block air flow; possibly worse. You can calculate and model it all day long but without the new device being instrumented for (mainly flow induced) vibrations, you would be rolling the dice a bit (English phrase about risk, sorry) regarding potential metal fatigue.

What kind of MAF sensor is it, hot wire anemometer? Other?
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  #3  
Old 08-18-2020, 09:51 AM
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skylor skylor is offline
 
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Default

Quote:
Originally Posted by rv8ch View Post
My FM-200 is running a bit rich, and Don speculates that it's due to the very turbulent air coming in the FM-200, which is causing the "sensor" to feel that there's more air coming in than reality.
How rich is "a bit" rich? What is your full power fuel flow (and field elevation)?

Skylor
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  #4  
Old 08-18-2020, 10:10 AM
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Default 19 GPH and 1421 ft

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Originally Posted by skylor View Post
How rich is "a bit" rich? What is your full power fuel flow (and field elevation)?
At full rich I see 19 GPH at 1421 ft. I was thinking it should be closer to 16 or 17 GPH.
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  #5  
Old 08-18-2020, 11:28 AM
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Quote:
Originally Posted by skylor View Post
How rich is "a bit" rich? What is your full power fuel flow (and field elevation)?
"A bit rich" is likely only a symptom of the problem, sort of an average end result. Turbulence means instability, nearly instant, moment to moment variations in the dynamic and venturi pressure at the sample points. That pressure drives the movement of the air diaphragm, thus the metering ball.

I don't know for sure, but the AFP design may be a bit more sensitive to turbulent inlet flow given its single pitot opening and small, high gain venturi. The Bendix style would tend to average out pressure variations across the diameter of the inlet, as it has four pitot tubes and a large diameter venturi. Good design is artful compromise. AFP's small high gain venturi provides better signal at low flow without choking high flow.
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  #6  
Old 08-18-2020, 03:46 PM
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Default turbulence

Quote:
Originally Posted by DanH View Post
"A bit rich" is likely only a symptom of the problem, sort of an average end result. Turbulence means instability, nearly instant, moment to moment variations in the dynamic and venturi pressure at the sample points. That pressure drives the movement of the air diaphragm, thus the metering ball.

I don't know for sure, but the AFP design may be a bit more sensitive to turbulent inlet flow given its single pitot opening and small, high gain venturi. The Bendix style would tend to average out pressure variations across the diameter of the inlet, as it has four pitot tubes and a large diameter venturi. Good design is artful compromise. AFP's small high gain venturi provides better signal at low flow without choking high flow.
It's starting to make more sense to me now. I think this was what Don was telling me but I wasn't getting it.

I guess the best solution would be a new air intake, but that's going to take weeks of fiberglass work.

Another option is the "straightener", obviously installed in a way that will stay in place - not 100% sure that this can be done without more fiberglass work, but much less than a new intake.

And the short term workaround is a leaner main jet, which Don is sending me. He didn't sound very optimistic that this would "solve" my problem of too rich at full rich mixture setting. I can of course just move the red lever, which is what I'm doing now, but it feels weird to not be at full rich on takeoff and landing at such a low altitude (1400 ft).
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  #7  
Old 08-18-2020, 04:37 PM
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Mickey, is your FF calibrated?

Are the numbers of burned fuel correlate with the amount you need to top off the fuel tank?
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  #8  
Old 08-18-2020, 04:57 PM
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Quote:
Originally Posted by rv8ch View Post
I guess the best solution would be a new air intake, but that's going to take weeks of fiberglass work.
Not necessarily. Got some photos, lower end of your snorkel?
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  #9  
Old 10-01-2020, 03:43 PM
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Default Frankensnorkel

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Originally Posted by DanH View Post
Not necessarily. Got some photos, lower end of your snorkel?
I'm kind of embarrassed to show this, but this is my snorkel.

http://www.rv8.ch/frankensnorkel/







It's hard to tell, but it's not as smooth as the Van's snorkel. I feel I have a lot of room to make it better. At least there is no place in there that has a smaller cross section as the FM-200 intake. But it is far from smooth with laminar flow.
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  #10  
Old 10-01-2020, 07:32 PM
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Default

The actual shape might be more of a concern than the interior surface finish. While developing the new snorkel for the RV 14 we discovered that the shape of the turn into the airflow performance servo is very sensitive. The explanation that Dan gave a few posts back about the differences in how the RSA versus airflow performance servos meter inlet air is I think a likely explanation. Because of this, before going to far, you may want to send your home made one to Don and ask him to flow test it for you
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