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03-23-2014, 02:12 PM
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Join Date: Jun 2006
Posts: 316
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Fuel Flow Limits
All I?m saying is that the fuel flow limits established by Lycoming that are to be used when calibrating RSA servos are +/- 2%. That puts the fuel air ratio at .086 to .089 for most of the 4 cylinder and 260 HP 6 cylinder Lycoming?s. So David, what you are saying is that Precision is setting the fuel servos to the lean limit on the new parts flow sheet and this is failing operation on the engine. I may point out to you that Lycoming has Service fuel flow limits that are deemed within their parameters for detonation and cooling. These fuel flow limits are +/- 6% and that puts the fuel air ratio to .084 to .095. Even with a .084 fuel air ratio, it is about 180 degrees F rich of peak, which is still rich enough to keep the engine happy. Running 250 rich of peak is putting the fuel air ratio around .091 to .092, which is out of the rich limit for new parts calibration. I guess I am curious why supposedly your shop is finding so many servos from Precision set out of the lean limit. We are using the same limits your guys shop uses and we do not get rejected servos from the field. It just seems strange that being lean 2% from the mean on the new parts fuel flow limits would cause a problem.
Basically on most RSA-5?s the fuel curve is pretty linear from 8 GPH on up. That means if you set the mixture at altitude and the fuel flow is above 8 GPH then increase the power (RPM or MAP) the fuel flow will increase but the fuel air ratio will remain the same (not touching the mixture control). So the amount of EGT ROP would remain the same. With fuel flows less than 8 GPH, WOT; the fuel mixture tends to go richer. This again is a function of the fuel curve that Lycoming prescribes for these units.
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03-23-2014, 03:46 PM
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Join Date: Sep 2007
Location: pittsburgh pa
Posts: 533
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My recent experience as a data point-
I recently changed out cylinders on my 180 HP IO-360 with a Bendix mag and an LSE electronic ignition. I used ECI with ECI 9:1 CR pistons and porting by Lycon. I immediately saw high CHT's on what had previously been a very cool running engine. I reset timing back from 25 BTDC to about 23 BTDC. Some small improvement was seen in CHT's. After rotating the mag drive gear 180 degrees I was able to get 20 BTDC. CHT's improved somewhat (slower to go high and a tighter spread) but high CHT's (425 F or so) persisted at full power during takeoff and climb out.
I was seeing 17.5 GPH during this period. After talking with Don, I did the peak to full rich check and was getting about 150 degree deltas. My T/O numbers were 17.5 GPH and EGT's about 125-145 degrees ROP.
After much thought, more reading and more thinking, I sent the RSA fuel control down to Don with a request to richen the F/A ratio. Kyle called me immediately upon receipt and said that the control calibrated exactly where Lycoming said it should and if I richened it, I would be going off the norm. We discussed it further and went with about a 10% richer calibration.
After a few quick flights, it appears that I am now getting 18.8 GPH at full rich T/O and seeing EGT's 200 degrees ROP. CHT's began their slow post takoff climb and I figured "here we go again", but they leveled nicely at 380 degrees during full power climb, about 50 degrees lower than before.
For my particular engine (9:1 and ported) the extra fuel seems just the ticket. I sense better WOT power although that is subjective until I can run some reliable 3 leg speed tests.
This fuel flow would now seem more commensurate with the 190+ HP I was expecting and seemed not to get.
Thanks Don and Kyle, excellent service including taking the time for a nice discussion about fuel controls in general and my personal situation in particular.
I think that this all supports the general points made in this discussion.
I still contend that many who are suffering from high CHT's need to look very critically at timing, fuel flows and cooling system. Having one out of three right may not be enough to achieve acceptable (less than 400) CHT's.
Great discussion and so much to learn here!!! Thanks all.
__________________
Gary Reed
RV-6 IO-360
WW 200 RV now an Al Hartzell for improved CG
Last edited by gereed75 : 03-23-2014 at 04:39 PM.
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03-23-2014, 04:49 PM
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Join Date: Sep 2007
Location: pittsburgh pa
Posts: 533
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Two other quick points -
I have the often maligned Hall effect triggering option on my Lightspeed Plasma II. One advantage to it is the timing is infinitely and easily adjustable with this unit as opposed to the crank fire sensor which, to my knowledge, is difficult to adjust.
In addition to setting the WOT fuel flow, Don's guys also correctly set the idle fuel flow. As he has posted, there is now no need to lean during idle/ground ops. Thanks again Don!!
__________________
Gary Reed
RV-6 IO-360
WW 200 RV now an Al Hartzell for improved CG
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03-23-2014, 07:08 PM
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Join Date: Jun 2006
Posts: 316
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To clarify post 81. The fuel flow limits I have been referencing are for ?stock? compression and magneto ignition. I haven?t seen fuel flow limits for modified engines. In that case we do as the customer specifies understanding the ramifications of just setting the unit rich as they request. Most of the time the stock settings work fine as they are, typically the settings are set a bit rich. I do not know if Lycoming has done detonation and cooling tests on all the combinations of compression ratios and ignition systems available in the homebuilt world. Obviously modifying the stock configuration gets us into the ?experimental? part of what is required for the fuel curve. Still the basic physics prevail. You start cranking more HP per cylinder than the cylinder was designed to handle, and maybe the cooling fin design is not quite adequate, or as Dan stated the cooling system has to be optimized for these engines to operate correctly with the existing cylinder design. We saw this type of phenomenon on the M-14 engine when it was hopped up. In fact as far as I know, there are no published fuel flow curves for the Silver Hawk parts lists. I haven?t seen them, but maybe they exist somewhere.
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03-26-2014, 08:08 PM
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Join Date: May 2008
Location: Brisbane Qld. Aust.
Posts: 2,271
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Don,
In reply to your question above, the last of the FCU's that have been sent to Riverina Airmotive, the flow was 119PPH at the test reference airflow.
This is clearly out of spec as a new unit should be at least 124. It has been sent out to the customer now at 131 PPH I believe.
This one was an Avstar, the last one a Precision Airmotive. Numbers in the 10-13% range are what we are seeing as the required increase.
For anyone else watching this thread, think about this. If you had to choose, would you take a max flow rate FCU or a min flow rate or even a half way flow rate FCU?
Remembering that you can always make the max rate FCU flow less at will, but pushing as hard on the red knob as you can you can't make a min spec flow more. 
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______________________________
David Brown
DYNON Authorised Dealer and Installer
The two best investments you can make, by any financial test, an EMS and APS!
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03-27-2014, 06:19 PM
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Join Date: Jun 2006
Posts: 316
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I?m assuming you are re-setting these units for experimental use. In that realm you can do pretty much what you want. We even calibrate RSA-5?s to run ethanol for some RV group in South Dakota. The flow specs you are referring to I believe is test specification 11263-01. This flow spec. is used on RSA-5 servos that are installed on IO-360B series, IO-360A series (angle valve engines) and IO-540C (260 HP) series engines. Per the spec the fuel flow at 1400 PPH airflow is 120.0 to 125.2 PPH fuel flow. As a note the Service limits for this setting are 117 to 132.8 PPH. As a PART 145 Repair Station we have to set the units we overhaul to the new parts limits (120-125.2). Since our existence as Repair Station in 1997 and previously while employed at Bendix, we have never had a fuel controlled returned for lean operation in any of the applications this servo is used in the certified world. We use pretty much the same limits (fuel air ratio) for setting our experimental units (FM-series). This too has worked quite well with only the ocasional re-cal like what was described in post 82. We generally have more cases of rich operation than lean, which I suspect, has to do with inlet configurations on these experimental installations. The limits for this fuel control were established in 1961 when the fuel control was certified and they are still serving us (in the US) well.
Limits are limits, and 119 are out of the new parts limits. Likewise 131 is way out of the rich limit for new parts and is near the rich limit for the Service limits. Maybe I need to check with Wilshire Engineering in Sidney, to see if he is experiencing the same lean operation you are. Again if these instances are set for experimental installations all bets are off.
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03-28-2014, 04:32 AM
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Join Date: May 2008
Location: Brisbane Qld. Aust.
Posts: 2,271
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Don,
The majority of new units will be installed and if out of limits on the low side, never detected. People do not know how to pick it.
I guarantee unless one of these units ends up in an aircraft with an EMS, and someone who knows what they are looking at, they will be none the wiser.
If you ask around you will get very few if any responses. This does not mean they are all set correct.
__________________
______________________________
David Brown
DYNON Authorised Dealer and Installer
The two best investments you can make, by any financial test, an EMS and APS!
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03-28-2014, 05:37 AM
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Join Date: Jun 2006
Posts: 316
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David,
Maybe you and you associates need to purchase all your new and overhauled fuel controls from Airflow Performance.
I see an opportunity in your area.
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