Lowest safe rpm for cruise ?

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Caldemeyn
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Lowest safe rpm for cruise ?

Post by Caldemeyn »

Hi all

As the title says, should i really not go any lower than 2000 rpm ? What could be the consequences (prop governor kaputt ? valves sticking ?) I read in some lycoming manuals that it could be operated at 1800 ? Would i be lugging it this way ? Vibrating it to pieces ?

Its tempting to run as low as possible, better prop efficiency, less frictional losses in the engine, less wear and tear, better fuel economy etc ?
I always try to fly at wot, lowest rpm, max leaning to roughness and high up at close to carson speed.

Also would it be best to operate at altitude where the engine can go at its max continous manifold pressure at wot and lop ? (Thats not that high for this 182 i think, i haven't flown this plane in some time though)

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Scott - A2A
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Re: Lowest safe rpm for cruise ?

Post by Scott - A2A »

You will hear many arguments on both sides. While I have my own position, it still is just my position and considering the complexity of a combustion engine, it's quite a grey area to determine things like this. But some general things I do know which has led me to be a believer in low RPM / High manifold pressure, especially for Lycoming engines.

1. In general, combustion engines run more efficient at lower RPM and higher manifold pressures. For the Lycoming, 60% power at 1800 RPM is more efficient than 60% power at 1900RPM, etc. all the way to the redline. By more efficient I mean, you will produce more horsepower with less fuel.
2. If the Lycoming engine manual says you can run continuously at 1800 RPM, that means their engineers endorse this (you need to consider prop limitations too)
3. My "gut" ultimately tells me what I think an engine likes. Some Lycoming / prop combinations don't like 1800RPM, but better at 1950RPM, or 2100 RPM. If it feels good at 1800 RPM and Lycoming says it's OK, that's good IMO.
4. Lower RPM, lower wear and tear, longer life
5. Lower RPM, lower noise

Scott
A2A Simulations Inc.

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

Re: Lowest safe rpm for cruise ?

Post by Caldemeyn »

I did a short flight to test a bit, standard atmosphere, 6500 feet, wot, hard lop, all that allowing me to be at max continous manifold pressure and 1800 rpm, the engine seemed to be alright, didn't vibrate, run a bit cooler than when at 2000 rpm, around 7 gph, overall it looked happy :)

Also i decided to go all out for a short time :) and lowered the rpm as much as possible, it resulted in around 1200 rpm, 3.7 gph, temps going down hard and speed settling at about 100-90 kias :D now thats what i call economy :mrgreen: the engine didn't seem to complain, stumble or vibrate itself to pieces though so...profit ? 8) my valves would get morning sickness from this :lol:

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Scott - A2A
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Re: Lowest safe rpm for cruise ?

Post by Scott - A2A »

BTW an AccuSim combustion engine actually simulates these efficiencies. I admittedly never tested cruising at 1200 RPM in a Lycoming, but it is probably projecting into that area pretty accurately. Both in terms of engine efficiency and airframe at those higher angles of attack.

Scott.
A2A Simulations Inc.

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

Re: Lowest safe rpm for cruise ?

Post by Caldemeyn »

I reckon that irl nobody would like to putter around at 1200 rpm :mrgreen: (also pilots are averse to experiments :) ) maybe if the goal was to stay in the air the longest possible time ? Im wondering if such low rpm would damage the counterweights on the prop in some time ? On the other hand, even at wot, my leaning is so aggresive that the engine is producing very little power so it should be safe ? It would be a whole different ball game to try that at ROP i think. Hard LOP operations allow for some truly interesting feats while keeping the engine safe :)

Also would such low chts (they were in 300s when at 1200rpm ) cause any problems with lead scavenging agents in the oil, damaging the valves? (100LL is a b :evil: )

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