increased fuel flow with decreasing altitude

erich weaver · #1 10-08-2012, 02:31 PM

I have read more than once in columns by Mike Busch and others how one generally must enrichen fuel flow when descending so as to compensate for the denser air that would otherwise result in a mixture that becomes too lean. However, with my constant speed IO-360B1B (AFP system) I have without exception observed that, without touching any controls, my fuel flow goes UP when descending. Why the discrepancy from what the wise men say? Is the AFP system self-compensating here?

thanks

erich

molson309 · #2 10-08-2012, 02:48 PM

Erich - I am assuming your manifold pressure is increasing as you descend - just like opening the throttle further would - which will cause your fuel flow to increase.

I see the same thing in my airplane.

Wayne Gillispie · #3 10-08-2012, 05:19 PM

I leave my mixture right where it was at cruise. I maintain my cruise speed during descent. I set prop to 2100 rpm. I pull throttle no more than 2" mp at a time to prevent shock cooling and to keep power below 65%. I don't touch the mixture until within a few miles of airport unless engine gets rough. If cht's gets near 250F in cool weather, I will enrichen/add some power as needed to remain at or above 250F. I try to start descents a little farther out in cool wx.

RV10inOz · #4 10-08-2012, 10:40 PM

Gents, you are not thinking outside the Lycoming / bendix square.

On the typical RV setup with a Bendix style fuel delivery system, or a carboy, as the mass airflow increases the fuel delivery almost follows perfectly. Sometimes the subtle differences mean a small tweek of the mixture once on descent.

I often leave FL130-150 and do not touch a thing until at the hangar door.

Now.....try doing this on a TCM/CMI engine in say a Bonanza! The fuel delivery is proportional to the fuel pump which is driven by the crankshaft, so while you descend, the engine goes leaner and leaner, and perhaps it may get to the point where the fire goes out, so the little red knob needs a bit more tweeking on the way down.

Does that help?

erich weaver · #5 10-09-2012, 12:00 AM

Quote:

Originally Posted by Wayne Gillispie

I leave my mixture right where it was at cruise. I maintain my cruise speed during descent. I set prop to 2100 rpm. I pull throttle no more than 2" mp at a time to prevent shock cooling and to keep power below 65%. I don't touch the mixture until within a few miles of airport unless engine gets rough. If cht's gets near 250F in cool weather, I will enrichen/add some power as needed to remain at or above 250F. I try to start descents a little farther out in cool wx.

Hmmm. Not sure how this is responding to my question.
Erich

RV10inOz · #6 10-09-2012, 02:55 AM

But mine did perfectly..... do I win the chocky frog???

Wayne Gillispie · #7 10-09-2012, 04:52 AM

Quote:

Originally Posted by Wayne Gillispie

I leave my mixture right where it was at cruise. I maintain my cruise speed during descent. I set prop to 2100 rpm. I pull throttle no more than 2" mp at a time to prevent shock cooling and to keep power below 65%. I don't touch the mixture until within a few miles of airport unless engine gets rough. If cht's gets near 250F in cool weather, I will enrichen/add some power as needed to remain at or above 250F. I try to start descents a little farther out in cool wx.

Sorry for surrounding it with other operational jargon. Now, can I have my choky frog?

RV10inOz · #8 10-09-2012, 05:17 AM

Wayne, plenty to go around

Russ McCutcheon · #9 10-09-2012, 08:59 AM

Quote:

Originally Posted by erich weaver

I have read more than once in columns by Mike Busch and others how one generally must enrichen fuel flow when descending so as to compensate for the denser air that would otherwise result in a mixture that becomes too lean. However, with my constant speed IO-360B1B (AFP system) I have without exception observed that, without touching any controls, my fuel flow goes UP when descending. Why the discrepancy from what the wise men say? Is the AFP system self-compensating here?

thanks

erich

I?ll take a stab at it, I have Bendix injection. The fuel servo is set up to get leaner as you retard the throttle so if you?re cruising at 10k and 22? let?s say LOP at 7gph. Now your descending, if you don?t retard the throttle as you descend I would expect the FF to increase as the manifold pressure increases, however if you retard the throttle to maintain no more than your original 22? the mixture will get leaner and FF will decrees. If you are retarding the throttle wile descending at some point during this decent I would expect the mixture to get lean enough so that the engine begins getting ruff at this point nudging the mixture in a bit will smooth it out but your still LOP.

Don?t know if this answers your question but I took a stab at least.

Wayne Gillispie · #10 10-09-2012, 09:48 AM

So far, desending from 13,500' to 1,500' pattern altitude my engine has not ran rough. I really thought it would. But then I compare where I have it during taxi and there is not much difference. Possibly because I am occasionally pulling the throttle out. Also by setting 2100 rpm, I am giving the fuel air mixture more time to burn. I am not an engine expert, but that is my best guess.