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Fast idle at end of flight .. wisdom sought

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Fast idle at end of flight .. wisdom sought

Old 05-31-2007, 02:05 PM
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ww2birds
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Default Fast idle at end of flight .. wisdom sought

Hi guys,

On a few of my warbirds, I have had some issues with engines whose idle speed, at the same throttle setting, have greatly increased RPMs at the end of the flight vs. start/warmup and taxi.

Sometimes this can cause issues with the plane wanting to land much too fast, requiring a go-around and retrimming of the low-end. Not a lot of fun.

Since these are mostly well-cowled engines, I have assumed heating was perhaps an issue .. and I've seen it with both side-carb and rear-carb engines from multiple manufacturers.

Is this a familiar problem with some routine items to check? I've varied the low-end mixture, tried different throttle linkages (was worried about nyrods expanding when warm), and improving airflow to the carb, so far without much success.

Any thoughts from those who may have been there/done that?

Dave McQ
Old 05-31-2007, 02:20 PM
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Don M.
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Default RE: Fast idle at end of flight .. wisdom sought

I've been flying for over 20 years and I don't believe I've ever had a flight where a engine HASN'T picked UP idle RPM's at the end of a flight. Glows are worse than gas. I just always chocked it up to the engine leaning out because of lower fuel level, engine getting warmer, etc. etc. This is why I fly with a JR radio with a rachett throttle trim. It's just something I do when I'm landing is pull back 3 or 4 clicks on the trim and "come on down".

If there is someway to prevent this I've never figured it out []
Old 05-31-2007, 02:28 PM
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Default RE: Fast idle at end of flight .. wisdom sought

I had a very similar problem. My engine would be idling way high making it hard to land. After it touched down and was on the ground a bit the idle would slow down to where it should be. What ended up fixing my problem was going a tad richer on the top end.
Old 05-31-2007, 03:23 PM
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WRK
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Default RE: Fast idle at end of flight .. wisdom sought

Try setting your idle mixture a bit richer that will help.
Old 06-01-2007, 09:36 AM
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BillS
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Default RE: Fast idle at end of flight .. wisdom sought

I find engines will idle slightly faster at the end of the flight but only a few hundred rpm. However a few hundred rpm with larger size props is usually a higher landing speed than comfortable.

Several techniques help but don't fix the issue.
Lowest idle possible.
Slightly rich low needle will reduce the idle but often it will take 30 seconds for the idle to fall.

A reliable engine kill operated from the radio will absolutely assure a slow landing speed. I prefer a kill system that will immediately stop the engine from any throttle setting. Choke kill and linkage kill may be OK but not as positive as I like.

I find the extra few hundred rpm helpful to keep the engine alive while taxing and while flying but too fast for a comfortable landing. Usually I throttle up a few clicks anyway as soon as the airplane is on the ground.

Maybe one solution would be to use a few % of throttle cut or offset only from threshold to ground.

Bill
Old 06-01-2007, 05:15 PM
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Tired Old Man
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Default RE: Fast idle at end of flight .. wisdom sought

It comes from establishing the idle rpm setting with a cold engine. The idle will always gain rpm as the engine warms up. If you permit the engine to reach a normal operating temp prior to blasting off it's much easier to set a stable idle rpm.
Old 06-01-2007, 06:20 PM
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Default RE: Fast idle at end of flight .. wisdom sought

I read this same explanation from Silver a few months ago and stopped bumping up my idle just before taxiing out for take off. The problem is gone. Good advice.
Old 06-01-2007, 07:35 PM
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Default RE: Fast idle at end of flight .. wisdom sought

Thanks to SS that's the way I do it now too.

Karol
Old 06-02-2007, 08:27 AM
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ww2birds
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Default RE: Fast idle at end of flight .. wisdom sought

Thanks all for some good tips!

Dave
Old 06-02-2007, 10:02 AM
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Default RE: Fast idle at end of flight .. wisdom sought

Yep, set your throttle trim for engine idle speed AFTER landing...[8D]

I used to monkey with idle speeds on my glow engines (you know, bumping it up three clicks before takeoff then down three clicks to land) and carried that over to my first gassers. Needless to say I had a number of experiences where I wound up doing several passes down the runway bumping the idle speed down since it wouldn't land. But I think the answer is one that's already mentioned here, use a throttle/throttle mix for a flight idle speed setting which is just a bit higher and your comfort level with the thing will improve.

Most of the GS guys I fly with use a "flight idle" and that mix, once I set up the first model that way I did all the others as well. Depending on your radio, there can be many ways to set it up, but the mix I mentioned is usually the easiest to do. Most of mine seem to work well right around a 4 to 6 percent change on the radio but that will depend on your setup and servo arm/throttle arm geometry.

Try it, you'll LIKE IT!
Old 06-02-2007, 12:16 PM
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Default RE: Fast idle at end of flight .. wisdom sought


ORIGINAL: Silversurfer

It comes from establishing the idle rpm setting with a cold engine. The idle will always gain rpm as the engine warms up. If you permit the engine to reach a normal operating temp prior to blasting off it's much easier to set a stable idle rpm.
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