RPM vs LOP engine smoothness.

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Caldemeyn
Master Sergeant
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Joined: 01 Feb 2011, 11:21
Location: Poland

RPM vs LOP engine smoothness.

Post by Caldemeyn »

Hi all

I watched this interesting video: https://youtu.be/A1fmNFWNHD4?t=2490 from savvy aviation about oversquare operation and there was a part where Mike told a story about a cirrus pilot who always had misfires when operating LOP, turns out he was operating at too high engine rpm, where the fuel/air mix didn't have enough time to be extracted of energy and finished burning in the exhaust. After the pilot rectracted the combined throttle/prop lever to 2500 rpm/wot, the engine run great again.

I wanted to see if i would get some engine hesitations if i operated LOP at high rpm so i got into the comanche (i know, a different plane :) ) put it at 5500 feet, 1800 rpm, WOT throttle and max LOP i could get, then i increased the rpm steadily to maximum, the engine hesitated a bit until it stabilized at that rpm and there was no problem otherwise. There is a part in this video where the phenomenon is explained in more detail, a bit before the cirrus story: https://youtu.be/A1fmNFWNHD4?t=1920

Is this just not taken into account in the current airplanes or maybe the engine is different enough for this to not be a problem ?

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DHenriques_
A2A Chief Pilot
Posts: 5711
Joined: 27 Mar 2009, 08:31
Location: East Coast United States

Re: RPM vs LOP engine smoothness.

Post by DHenriques_ »

Caldemeyn wrote: 24 Jul 2021, 10:10 Hi all

I watched this interesting video: https://youtu.be/A1fmNFWNHD4?t=2490 from savvy aviation about oversquare operation and there was a part where Mike told a story about a cirrus pilot who always had misfires when operating LOP, turns out he was operating at too high engine rpm, where the fuel/air mix didn't have enough time to be extracted of energy and finished burning in the exhaust. After the pilot rectracted the combined throttle/prop lever to 2500 rpm/wot, the engine run great again.

I wanted to see if i would get some engine hesitations if i operated LOP at high rpm so i got into the comanche (i know, a different plane :) ) put it at 5500 feet, 1800 rpm, WOT throttle and max LOP i could get, then i increased the rpm steadily to maximum, the engine hesitated a bit until it stabilized at that rpm and there was no problem otherwise. There is a part in this video where the phenomenon is explained in more detail, a bit before the cirrus story: https://youtu.be/A1fmNFWNHD4?t=1920

Is this just not taken into account in the current airplanes or maybe the engine is different enough for this to not be a problem ?
It's absolutely amazing to me that the technicians and mechanics involved with this Cirrus "issue" including the people at Continental didn't ask the pilot immediately about his cruise power settings.
This is a perfect example of why aircraft maintenance is so costly.
Dudley Henriques

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AKar
A2A Master Mechanic
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Joined: 26 May 2013, 05:03

Re: RPM vs LOP engine smoothness.

Post by AKar »

I didn't check out the video yet, but one thing I notice is this combined power lever causing confusion. In at least some Cirrus aircraft, this is not an electrical control, but is connected to the engine fuel control and the governor via two mechanical cables. It is, according to flight manual, rigged to provide ~2500 rpm at "cruise power settings", and 2700 rpm at full power. I have never flown nor worked on Cirrus in any position, but instinctively I don't like that feature.

-Esa

Caldemeyn
Master Sergeant
Posts: 1101
Joined: 01 Feb 2011, 11:21
Location: Poland

Re: RPM vs LOP engine smoothness.

Post by Caldemeyn »

Yeah, from what i know, the last bit of range on the lever controls the rpm between 2700-2500 while keeping the throttle at wot. After that both the rpm and throttle are reduced. Im mostly interested in the lack of engine hesitations in a2a planes at aggressive lop and high rpm which, according to the video, should be happening ?

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AKar
A2A Master Mechanic
Posts: 5208
Joined: 26 May 2013, 05:03

Re: RPM vs LOP engine smoothness.

Post by AKar »

It is true that, in aircraft engines, a high rpm at very lean mixtures (aggravated by very cold conditions) results in still ongoing combustion when the exhaust valve opens. This can result in exhaust popping, distinctively audible outside of the aircraft.

In faster-revving, more modern (yet fairly classic) engines, the increasing rpm tends to result in more turbulent flow in the combustion chamber, resulting in better air/fuel mixing. This, to some extend, offsets the decrease in combustion speed of somewhat lean mixtures. I've got no knowledge really on whether these effects have any role in relatively slow-turning aircraft pistons, with rather conventional combustion chamber and piston geometries.

-Esa

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