Power Settings

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Kurt h
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Power Settings

Post by Kurt h »

Had this in the public forum buried in a thread but thought it might generate interest here.

Why does the 75% power setting stop at 5000 ft in the POH? Originally I thought this meant WOT was ok above 5000 (because 75% power could not be exceeded), but did some more research with the Lycoming power chart for the IO360M1A. With some work I was able to put together 75% settings up through 8000 ft. Between 5000 to 8000 I am typically not full throttle. Above 8000 it is typically full throttle to reach 75%, or less of course at higher altitudes.

What settings do you use above 5000 since our POH guidance for 75% power stops there?

I'll attach my power chart when I get back to my computer.
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Re: Power Settings

Post by Kurt h »

Here is the Lycoming power chart and the power settings for 75% I put together from it.
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Re: Power Settings

Post by Tommy »

There is no reason not to fly full throttle at every altitude unless of course you want to burn less fuel at the lower altitudes or go slower.
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Re: Power Settings

Post by Kurt h »

Tommy, I thought the POH limits us to 75% power for continuous cruise operation (full power only for climb). If not then certainly the discussion is not necessary. I think a Lycoming rep told me to limit to 75% as well, but I know they tend to be conservative.
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Re: Power Settings

Post by Rich »

Kurt h wrote:Here is the Lycoming power chart and the power settings for 75% I put together from it.
Sorry, Kurt, I have to question your power chart accuracy. I think you're misinterpreting the Lycoming charts and the realities of a less-dense atmosphere at hight altitudes. Basically you are showing unattainable manifold pressures at the higher altitudes.

Absent turbo- or super-charging 75% power is not going to be attainable above somewhere around 7500-8000 ft, except on extremely cold/high-pressure days. This is due to the standard pressure lapse in the troposphere of about 1 inch per thousand feet. So you go from roughly 30 inches of absolute pressure available at SL to 22.5 at 75,00 ft. This ~75% of standard of standard. Any higher altitude it just keep dropping off. This is consistent with what I find in the Lycoming chart using the instructions on the chart.

And consistent with what s flight I took yesterday: 9,000 ft, pressure happened to be standard, temp maybe 3 deg C above normal. WOT at 2300 RPM was about 21.5 inches MAP. This jives with

The real reason the Diamond chart doesn't go beyond 5,000 ft is due to the old limitation (which still persists in the POH) of max continuous RPM of 2400. Lycoming and both prop manufacturers are fine with continuous 2700 RPM. If you run the Lycoming chart at 7,500 ft for 2700 RPM, you'll see the max available MAP is right at 22.5 inches and coming in from the other side at 22.5/2700 and drawing the intersection you'll come in right at 135 HP.
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Re: Power Settings

Post by Chris B »

Piston engine management is a sufficiently complex topic that several really smart guys created an excellent training course (link) on the topic. IMO the $400 cost is money well spent considering the crucial importance and value of our engines. Lean-Of-Peak/Rich-Of-Peak is actually not the focus of the course, which is primarily geared to understanding enough about the engine to identify incipient issues and diagnose symptoms. Highly recommended!!

Condensing this wisdom into a short post is not possible.

But if your engine runs smoothly Lean-Of-Peak EGT, then the short operational summary is that any LOP cruise power setting that keeps the cylinder heads relatively cool is OK. Opinions vary, but <380F is generally considered "relatively cool" CHT (aluminum alloy is half-strength at ~400F).

Note that Lycoming's charts are "maximum power mixture" (typically ~100F ROP). This is great for marketing (maximum HP!), but generally the worst possible place to run an engine for longevity. :cry:

Tommy is correct that wide-open throttle is usually fine. In fact, the engine is most efficient WOT (minimum pumping loss). But the aircraft is often much more efficient at less than full-throttle. Drag on the DA40 escalates rapidly after ~115 kts IAS. Assuming reasonable winds, this is why I try to fly ~10-12k'. Wide-open throttle, ~140 kts TAS, ~8 gph. :thumbsup:

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Re: Power Settings

Post by Rich »

Chris B wrote:
Note that Lycoming's charts are "maximum power mixture" (typically ~100F ROP). This is great for marketing (maximum HP!), but generally the worst possible place to run an engine for longevity. :cry:

Chris
Not true for this engine. In the manual for this engine Lycoming specifically states to not lean on the rich side less than 150 degF for best power.
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Re: Power Settings

Post by Chris B »

Rich wrote:Not true for this engine. In the manual for this engine Lycoming specifically states to not lean on the rich side less than 150 degF for best power.
Hi Rich -

OK, but in terms of the (bad) effect of peak pressure, 150F ROP operation is very close to "~100F." ;)

While the exact temperature varies, the important part is "maximum power mixture." Except at high altitude (where a NA engine can't make high power at any mixture), "maximum power mixture" is generally a bad place to run an engine.

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Re: Power Settings

Post by Tommy »

Kurt h wrote:Tommy, I thought the POH limits us to 75% power for continuous cruise operation (full power only for climb). If not then certainly the discussion is not necessary. I think a Lycoming rep told me to limit to 75% as well, but I know they tend to be conservative.
You can go above 75% as long as you are pushing enough fuel through the engine to keep the temps. where they should be. Yes, your efficiency does start dropping off as your speed increases. However, that's pretty much the way it is with any engine including diesels. If you want to go fast, your going to burn more fuel turbocharged or not. A turbocharged plane will actually be slower at the lower altitudes (under 8,000' +-) than a normally aspirated plane. There's always a trade off.
I'll go 200 kts. all day up to 8,000' msl. I'll also burn 30 gal. hr. +- all day doing it. The cost of flying a normally aspirated twin.
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Re: Power Settings

Post by Kurt h »

Rich wrote:
Kurt h wrote:Here is the Lycoming power chart and the power settings for 75% I put together from it.
I think you're misinterpreting the Lycoming charts and the realities of a less-dense atmosphere at hight altitudes. Basically you are showing unattainable manifold pressures at the higher altitudes.

Absent turbo- or super-charging 75% power is not going to be attainable above somewhere around 7500-8000 ft, except on extremely cold/high-pressure days. This is due to the standard pressure lapse in the troposphere of about 1 inch per thousand feet. So you go from roughly 30 inches of absolute pressure available at SL to 22.5 at 75,00 ft. This ~75% of standard of standard. Any higher altitude it just keep dropping off. This is consistent with what I find in the Lycoming chart using the instructions on the chart.
Maybe I am not using the power chart right, but let's go with the 75% power settings from the POH at 2400 rpm. Manifold pressure at 3000' = 24.8", 4000' = 24.5", and 5000' = 24.1" where the chart stops. The relationship is fairly linear, so my extrapolation is 6000' = 23.7" (or close) for 75% power. At full throttle, 2400 rpm, and 6000', my manifold pressure is significantly higher than 23.7". Thus my assumption that I am making > 75% power.

I agree that 8000' and higher you cannot typically reach 75% power due to thin air.
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