Power setting question

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Alfredson007
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Power setting question

Post by Alfredson007 »

So, i presume any possible combination when MP and RPM needles are on the green are acceptable? If i cruise along with 2000rpm and i briefly use high manifold pressure (28" for example), is that BAD habbit? I don't mean to cruise with such power, but to use it for a 10-20 second power requirement.... (because i am lazy using prop control for situations like these..:)

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Great Ozzie
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Re: Power setting question

Post by Great Ozzie »

Alfredson007 wrote:So, i presume any possible combination when MP and RPM needles are on the green are acceptable?
•Go to the A2A C182 Manual Performance Charts (p. 52-57) for your chosen cruise altitude. Pick a power setting and set accordingly.

Alfredson007 wrote:If i cruise along with 2000rpm and i briefly use high manifold pressure (28" for example), is that BAD habbit?
YES!

:P
Rob Osborne
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Scott - A2A
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Re: Power setting question

Post by Scott - A2A »

Alfred,

You are flying a high performance plane, so the mindset is completely different to a lower powered fixed pitch. Be judicious with power changes.

In general, you don't want to add power (high power) to an engine that is "tied down" so to speak (low RPM). When the prop is pulled back, it's like you are driving a car in 5th gear at 20 mph. You don't just mash down on the accelerator otherwise the engine will start to groan. This is bad for the engine, and these engines have a minimum RPM for a given manifold pressure.

Follow the cruise charts as Rob suggested, but also, as long as you are in the green RPM and Manifold pressure arc, you are OK. That's what the green ranges are for.

Lastly, just so you can get used to the sound, set the prop to 2000 RPM and apply full power at sea level and LISTEN to the engine. It is over working. You can hear it. Get used to that sound, as sound is another warning sign you use in an airplane and we faithfully captured this.

Scott.
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Alfredson007
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Re: Power setting question

Post by Alfredson007 »

Thank you for your answers.

I am familiar with how to use constant speed prop etc, i know i should generally add rpm before adding more throttle etc, and that high mp / low rpm combinations should be avoided, although i've seen many real world manuals, beech bonanza for example that allows VERY wide range of accepted power settings, only the very top extremes are not RECOMMENDED, ie, having very high MP and very low RPM.

The thing is, i am pretty lazy with charts, also, since i have no axis for RPM, i am lazy with the prop too. That's why i generally prefer fixed pitch planes. I also try to fly realistically, that's why i usually add rpm before adding power etc... although, i think that if i have like 2200rpm and i add 28" for 10 seconds wouldn't do any harm in real life...?

Anyway, since i have no axis for cowl flaps and prop, etc. this plane is solely an IFR trainer or if VFR only for cross-country flying. Not gonna do repeated patterns or any maneuvers with it... and that's fine, i got the extremely good Piper for that.

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Great Ozzie
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Re: Power setting question

Post by Great Ozzie »

Alfredson007 wrote:The thing is, i am pretty lazy with charts, also, since i have no axis for RPM, i am lazy with the prop too. That's why i generally prefer fixed pitch planes. I also try to fly realistically, that's why i usually add rpm before adding power etc... although, i think that if i have like 2200rpm and i add 28" for 10 seconds wouldn't do any harm in real life...?
Hey Alfredson,

Apologies for the short answer... I wanted to reply with "something" to get you started before shooting out of the house this morning.

I have no idea about your example of 28/22 what it would actually do. How I approach the matter is: would this be something I would do? No, because I look at it as going into that region of being a test pilot. Something may break, maybe not. Maybe not this time, but maybe it sets up a problem that will happen down the road.

It's one of those things where, treat it with care and respect to get the best opportunity for a good return on that investment.

From your quote above... well... this a perfect opportunity to check those charts and to use good technique! RW speaking... no way would I use a power setting unless it came from the charts. And... you could see by taking a few minutes to look around at different altitudes, that 23"/2400rpm (top of green arcs) will provide (roughly) less than 80% MCP (maximum continuous power).

It would be nice if the power settings (abbreviated) were on the visor. This I have seen and it is handy (I never used the visor anyway - that's what cool aviator glasses are for. :P ). If you print up your checklists, wouldn't hurt to do the same for the performance chart settings. Or maybe just copy those for some basic settings at each altitude (like 55%, 65% and 75% for std temp).

-Rob
Rob Osborne
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AKar
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Re: Power setting question

Post by AKar »

My points follow. :)

One thing to understand is that on its own, limiting the engine's rpm by using prop lever alone is not a bad thing. Remember, these naturally aspirated engines produce less power at lower rpm, all else left untouched, and this is actually used to give different engines their different ratings at factory.

Remember the 172R? There we did exactly that - the coarser prop on R forced its engine to lower rpm where it produced only up to 160 hp, but on the other hand it was quieter and arguably more sensible plane in cruise (there is a reason these coarser props are called 'cruise props' and finer are 'climb' or 'power props'). If anything, the IO-360-L2A 'forced down' to 160 hp probably lasts longer.

When having a plane with constant speed propeller, a commonly used technique to reduce noise is to pull your prop back just a bit when safely airborne. Maybe some 70-100 rpm will do already. I don't think it's harmful to the engine, maybe even the opposite. Remember, the engine is already limited to its rated power by propeller resistance! We loose a couple of horsepowers when we pull the prop back a bit, but we also loose quite a bit of prop noise.

As Scott implied with his car analogy, what you want to avoid is using excessively 'low gear', that is, insensibly low rpm relative to the engine's effort. The IO-540-AB1A5 is relatively slow turning engine already, so you don't want to be too rough when handling its prop. When in climb, or otherwise flying low where very high powers are available, I'd vote for using 2300-2400 rpm all the time until you deliberately set the mp to some lower value.

You ask what would 28'' and 2000 rpm do? Note that the 28 inches imply that you're almost at sea level there, I'm not sure what situations would make such a settings practicable. But anyway, without looking up or anything, I'd think that would put the engine way out of its comfort zone. Note that the engine has quite large ignition advance, at 23°. At high power settings, what happens if you reduce the rpm too much, the peak pressure in the cylinder occurs relatively closer to the piston's top position. This decreases engine efficiency, increases maximum pressures and translates into high CHTs. Not sure you might even get some detonation. Having the engine at full rich may save something as this slows down the combustion.

Picking up some charts and finding your settings from there is a way to do it. But I'm a lazy chart reader too, and when I fly longer trips I want to be economical (just to have something to do, if nothing else!). Therefore I climb to some higher altitude, generally somewhere at or above 8500 ft [**] where I simply don't have more than 75 % of power available, no matter what I did! This allows me to leave the throttle wide open; I use the altitude as my throttle. I bring my RPM to around 2100 usually, and find a nice lean of peak mixture setting, as the engine runs beautifully smooth. Very lean mixture slows down the combustion too, helping to push back the peak pressures in the cylinders to where they're supposed to occur, but even if it didn't, in this thin air the engine pressures shouldn't get high enough to cause any instant harm almost no matter what I did, excluding some really stupid things!

You might notice that putting together good engine settings is quite like chasing multiple moving targets! That's why the charts are good, they've done the hard part for you, but I think half of the fun is trying to learn to think about what you're doing with your engine, and why, in real time. :)

Oh btw, just to show you some of those moving targets! Notice, that I said the engine power gets lower when you decrease the rpm. But if you cruise at some partial power setting and try it out, check out the manifold pressure. It should actually rise when you decrease the rpm (and the power)! Why is that, can you figure it out? :wink:


-Esa

Edit:
[**] Note that driving at full throttle at 8500 ft may be, in many circumstances, pushing the engine and you can make some really stupid cruise settings if not careful. It gives better performance than higher altitudes, but that performance is of course extracted from the engine which is working harder.

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