erich weaver

erich weaver

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

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? :)
 
Wayne Gillispie said:
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.
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Hmmm. Not sure how this is responding to my question.
Erich
 
But mine did perfectly..... do I win the chocky frog??? :D

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Wayne Gillispie said:
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.
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Sorry for surrounding it with other operational jargon. Now, can I have my choky frog?
 
erich weaver said:
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
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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.:)
 
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.
 
ok, understand decreasing altitude results in increasing MP which results in increasing fuel flow with the AFP set up. Apparently this is not true for all fuel systems if I am understanding RV10InOz correctly.

erich
 
Carburetors sense air volume. Bendix RSA senses air mass (sorta). TCM continuous flow senses throttle plate position, with their turbo systems varying pump pressure by tying it to MAP. Since pressure carbs or backsuction carbs are about extinct, Bendix is the only one that compensates itself based on mass. That's why their mixture remains the most constant of the three during ascent/descent.
 
In my 172 with IO360 and Bendix injection and matched Gamijectors (similar to many RV setups here) I will cruise WOTLOP at about 2450, controlling my RPM (fixed pitch prop) with mixture. When I start descent I just dial it back leaner still to maintain that RPM range, and I notice that I do indeed have to periodically richen slightly as I descend to keep that RPM. As I go through about 4k' I go back to ROP and use throttle for power control again.
 
My RV-7A has an AFP setup and my Rocket has a Bendix. Both operate the same way while in flight - as MP climbs during a descent the FF goes up, and eventually I end up having to enrichen the mixture as well as retard the throttle.
 
erich weaver said:
ok, understand decreasing altitude results in increasing MP which results in increasing fuel flow with the AFP set up. Apparently this is not true for all fuel systems if I am understanding RV10InOz correctly.

erich
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My Bendix would do the same thing if you let the MP increase, With my RV-4 I can't let the PM increase much because of speed considerations so I am pulling the throttle back to avoid increasing MP and over speeding the airplane, this results in lower and lower FF for the same MP as I retard the throttle to maintain the same PM during the decent.
 
As best I can tell, and I am willing to be corrected, the AFP is esentially a Bendix/Precission system, in that they are MASS AIR FLOW devices.

The TCM and someone has mentioned already, are constant flow and with the ACFP (altitude compensating fuel pump) they adjust fuel flow based on the outside air pressure, and when well set up they work a treat.

airguy..... why not leave the mixture alone, only tweek it if it needs a small amount more, and still use your throttle all the way to the hangar. Your plugs and cylinders will appreciate it ;) .
 
RV10inOz said:
airguy..... why not leave the mixture alone, only tweek it if it needs a small amount more, and still use your throttle all the way to the hangar. Your plugs and cylinders will appreciate it ;) .
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Because the mixture is lean enough at cruise that the engine wouldn't run smoothly at low altitude or would stumble if I needed to go around and went to full power quickly. At least, I suspect as much - I have to admit I haven't tested that. I dislike setting myself up to run the engine "in the red box" and if things happened to go south at low altitude I'm going to need full power immediately without worrying about the mixture - better to have it already where it needs to be. For the short periods I spend in approach, the additional fuel burn at that low power setting is nearly none. My home field is literally in the middle of a bunch of cotton fields - my typical approach is steep and quick, I might spend 2 minutes total between 4000' and turning off the ruway.

You may have misunderstood me, my normal operation is wide open throttle on takeoff roll (fixed pitch prop), lean as needed for best power (balanced against cylinder temps) but still ROP during climb, then LOP (still wide open throttle) during cruise and control RPM with mixture. As I start my descent I stay WOTLOP and just go leaner to keep RPM down (reduced total power with increased airspeed in descent) until I hit about 4k' or thereabouts, when I pull the throttle back and push the mixture in.

I know an awful lot about LOP operations and I'll preach it long and loud, but I also know there are things I don't know. Care to tell me why you believe the plugs and cylinders would be happier otherwise? I always lean agressively on the ground, in just over 400 hours over 2 years I've only had 2 incidents which I believe to have been due to a fouled plug - and one of those I suspect my plane partner did not lean during taxi.
 
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Greg, not sure I misunderstood you, but I am sure you misunderstood my last post. Let me back up a second or two and we will get on the same page. :) By the way this is why I harp on about doing seminars, you can't do a 2.5 day course in a few posts on VAF, Mike Busch does some good webinars, and even in an hour or two he cant do it all justice.

OK so I see you are using the mixture control to descend, go leaner, less power and thats all good. You are living proof that the mixture knob/lever is the most effective power knob/lever ;).

Maybe I misunderstood you here when you said
As I go through about 4k' I go back to ROP and use throttle for power control again.
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And I took this to mean ROP= Full Rich, and maybe that is not really what you meant just that you richen up again to regain some power, be it LOP or ROP. Hey in that case, you are not doing low power plug gunking at all! :)

As for climb out, to keep the ICP and Theta PP in the optimimum spot, and to get the best climb for your $, the data shows that using a target EGT method to be the best. That is whatever egt you have as you pass through 1000' remember that number and lean back to it every couple of thousand feet. This will be kinder on your engine for sure.

Once above say 10,000 you may want to go in search of 75-80 ROP for the best power you can get.

I know an awful lot about LOP operations and I'll preach it long and loud, but I also know there are things I don't know. Care to tell me why you believe the plugs and cylinders would be happier otherwise?
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Nope that is the best for them...keep doing it!:)
 
Yup - we're on the same page now. On descent I don't go full rich - I will pull throttle until the RPM sags significantly then push mixture until I'm close (based on my experience with this airplane) to best power mixture, but definitely not full rich. Normally the only time I'm full rich is takeoff roll at low-altitude airports, usually start leaning to near best power during the climb at around 1000' AGL. I stay rich enough to keep CHT's happy until I level off for climb, after that it's WOTLOP operation until 2 mile final.
 
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