LondonMike wrote:
…John Deakin makes very clearly…
I’m afraid I have never heard of this gentleman, but many many years at the university studying engineering taught me to stay away from any limits one of my colleagues has set on machinery designed by him 
My car (a Fiat, whose design engineers do not have the best reputation to put it mildly) for example will be 18 years old soon and has 360,000km on the clock. The engine has never been opened or received any maintenance other than precautionary replacements of the cam belt. I strongly doubt that it would have lasted that well had I developed a habit to cruise at full throttle… But then, a 1990ies car engine is somewhat dissimilar from a 1940ies aero engine.
Anyway, I hardly ever operate aspirated aero engines above 9500ft these days.
LondonMike wrote:
One of the points John Deakin makes very clearly is that this sympathetic reduction in power (with the exception of engines with a time limit on maximum take-off power) is actually potentially harmful to an aviation engine with fixed ignition timing.
Although it is true that reducing the RPM moves the peak pressure point back closer to TDC, this also depends on your mixture (the richer ROP, the slower it burns, and the leaner LOP the slower it burns). If the MP/RPM combination is documented in the POH, I don’t really see why you shouldn’t pull the RPM back to that value.
In the case of the DA40 I mentioned, the AFM actually explicitly states that you should reduce the RPM to 2400 RPM when clear of obstacles. This was however essentially due to the fact that the 3-bladed propeller (MT?) originally fitted did not meet the noise certification limits at fine pitch. The actual airplane was however fitted with a 2-bladed Hartzell on which there was no such limitation.
In the case of the SR22 you have a single lever that controls RPM and MP so whatever that gives you should be OK. And down low the airplane climbs plenty fast at 2500 RPM and WOT (120 KIAS and 800 fpm)
RobertL18C wrote:
LondonMike I always operated the Lycoming -540s at wide open throttle altitude which gave me around 65% in typical airways FL. They would make TBO plus 20% and in the USA no doubt would have gone well beyond that.
Yes, but how many -540s are fitted with a fixed pitch propeller? 
I assume you did not do that with the blue lever fully forward.
Aviathor wrote:
In the case of the DA40 I mentioned, the AFM actually explicitly states that you should reduce the RPM to 2400 RPM when clear of obstacles. This was however essentially due to the fact that the 3-bladed propeller (MT?) originally fitted did not meet the noise certification limits at fine pitch.
I have an 3-bladed MT prop which is limited to 2500 rpm continuous in the AFM versus the 2700 rpm engine redline. After considerable written communication between the owner of MT and the previous owner of my plane, it was established that the reason for the limitation was German noise certification, not an engineering reason, with the additional comment “but surely you won’t want to fly around at 2700 rpm continuously, and it is nice to be quiet anyway”.
Correct, RPM in the cruise was 2400, but on a fixed pitch you have top of the green as a limitation – although I recognise the RPM indicator may not always be checked for accuracy.
Not sure when top of the green RPM in the C-90 (2350 RPM) equates to WOT, but probably around 10,000’?
I don’t like to operate any machinery at full throttle for extended periods of time, be it aircraft, car or even lawnmower, so I always throttle back a little even if the manual would allow otherwise
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You probably meant, not operate at 100% maximum continuous power for extended periods of time. For example, all diesels operate at full throttle (they don’t have one), the ‘throttling’ of a dirty airfilter is normally viewed as bad for the engine rather than good, and a normally aspirated aircraft at ‘full throttle’ 12,000 feet will be producing something like the sea level long range cruise power and be below red line RPM. Keeping away from RPM limits, temperature limits and maximum power outputs probably all lead to longer engine life, throttling may or may not be part of achieving those objectives.
A few separate points:
there seems to be one that the Lyco engines like the 540 are best cruised at 65% power, as the best compromise between engine life, fuel burn, etc. Peak EGT or LOP of course. This power level is reached with a wide open throttle and peak EGT at somewhere vaguely around FL100.Actually 65% is achievable all the way up to FL120 at ISA with WOT and the blue lever fully forward provided you lean for best power. But I would not fly at MAX RPM even if that was allowed for continuous operations. I think that on most engines 2500 is a reasonable maximum but I do not have any hard facts to back that claim up with. It’s more a gut feeling I have acquired over the years.
At FL100 you can reduce the RPM by a couple of hundred and still achieve 65%.
provided you lean for best power
Sure, but it’s some 20% extra fuel burn for hardly any extra speed.
I have operated DA40-180’s for hundreds of hours at around FL100 on long (typically IFR) flights.
The RPM limitation of the original AFM has been removed in an addendum. Limiting this engine to 2400 RPM literally cripples the aircraft’s performance in all flight regimes. I also believe it can be damaging at low altitudes with an unfavorable combination of RPM, MP and FF.
In my experience, the sweet spot (at altitude) for the IO-360 M1A is between 2450 and 2550 RPM and wide open throttle.
I almost never fly low (only if a headwind forces me to or sight seeing at 22/22).
Since this is a normally aspirated engine, it loses power with reduced atmospheric pressure, (sorry for stating the obvious!)
As soon as WOT MP is below 25 inches I operate at WOT as much as possible.
When WOT MP is between 22 and 24 inches, it is possible to chose RPM freely between 2250 and 2600.
Thanks to the G1000’s engine management page, it is possible to experiment.
This is actually a good way to avoid getting bored and sleepy on a long x-country.
Sometimes reducing RPM to 2300 makes a big difference in FF with only one knot of speed loss.
When MP drops below 22 inches at altitude, it becomes evident that higher RPM are required.
I think this engine sounds very happy at up to 2550 RPM and still very OK at 2600.
Normally I do not exceed 2550 RPM in cruise, but when really high 2600 gives a welcome boost.
65% power (at peak EGT) burns between 9 and 9,5 GPH with CHTs around 360 F (note: PowerFlow exhaust makes 20 extra HP and costs 1 GPH more)
The DA40 XL made 142 knots and the XLS easily hit 145 at this setting. This is not consistent with book values which have not been updated.
At 75% power (ROP) the engine wants 10 to 10.4 GPH. The XLS will do a constant 150 KTAS, even more when lightly loaded.
in 900 hours, I have never had any engine issues that would point to additional wear and – remarkably – never ever have fouled plug issues.
The climb performance of the DA40 is also brilliant if one bothers to extract maximum power from the engine and its very good cooling system, but this is off topic….
Disclaimer: the above is my contribution based upon experience with the Lycoming IO-360 M1A equipped with a tuned exhaust and fine wire plugs.
Other engines WILL have a different behavior.
