On Jan 2, 7:35*pm, Mxsmanic wrote:
writes:
Most "smaller" aircraft engines are certified to produce full
rated horsepower at full rated RPM for the full TBO of the engine.
Look up the TCDS sometime for any engine you want. See the FAA
website. If the POH specifies a five-minute limit at full throttle ort
full RPM or both, then that limit should be stuck with. If the POH
doesn't say it, then you'll likely find, in the cruise settings
charts, RPMs as high as redline and the fuel flows and airspeeds
expected for that setting. Fixed-pitch props are often designed so
that full throttle in level cruise will give redline RPM or something
very close to it. When we break in a new Lycoming, we operate it as
they say, which is with the last half-hour of the 3.5 hour flight at
redline RPM, which takes full throttle at around 5,000 feet. Sea level
will be similar, since the higher power generated there is absorbed by
the higher prop drag and thrust created.

Interesting.

I found the page I had looked at before, for the Bonanza. *It's a chart that
shows manifold pressure (from 20 to 25.5) vs. RPM (from 1700 to 2700). *On the
left side there's a shaded area that says "Not recommended for cruise power
settings." *There's a bell-shaped area in the middle that says "continuous
operation at peak EGT permitted." *There's a squared-off section on the right
(with 25 in. and 2500 RPM as its upper right corner) that says "Continuous
operation at EGTs hotter than 20° below peak EGT (rich side or lean side) is
not approved in this area."

First, what's the difference between "not recommended" and "not approved"? *I
have the feeling that this wording is not chosen at random. *What bad things
might happen in each of these areas of the chart?

Second, why would peak EGT be okay for certain pressures and RPMs, but not for
the highest combinations of RPM and pressure? *If it were just an issue of
exhaust heat alone, I'd expect no distinction to be made--peak EGT would
always be okay. *The fact that this isn't stated implies that peak EGT in
combination with certain pressures and RPMs implies other changes in the
engine state that are potentially bad or harmless--what might those be? Maybe
cylinder heat temperatures or something? Do aircraft normally have CHT gauges
in addition to (or in place of) EGT gauges?

Third, why doesn't the manual give a specific time limit? *How long does
temporary have to be before it becomes continuous? *What would be an example
of each?

Here is a clue. Peak egt is different for different power settings,
and too high an exhaust gas temperature can do bad things to valves
and pistons. Highest temps occur near peak power. We control the
temperature by adjusting the mixture so there is less than
stoichometric combustion to keep those temperatures controlled.