Pilot_DAR wrote:
Yes, very true, even Lycoming will tell you that the airframe POH prevails over their own engine operating manual for operating their engine in an airplane. But, I doubt that Lycoming will recommend an oversquare power setting for their normally aspirated engines. I asked them recently, and the answer was they do not state any procedure for this.
I looked at the engine manual for the Lycoming (I)O-360 series. I can’t find any restrictions on MP/rpm combinations.
Pilot_DAR wrote:
The PT-6 exception is those engines installed in floatplanes, where there are stop pins, so the prop will stop on flat pitch, rather than feathered.
Why the difference for floatplanes?
Airborne_Again wrote:
I looked at the engine manual for the Lycoming (I)O-360 series. I can’t find any restrictions on MP/rpm combinations.
Look closer:
Airborne_Again wrote:
The PT-6 exception is those engines installed in floatplanes, where there are stop pins, so the prop will stop on flat pitch, rather than feathered.Why the difference for floatplanes?
When you start a PT-6 from the normally feathered blade angle, when you are ready to taxi, it is necessary to move the propeller out of feather into fine pitch. As the blade angle changes with this command, the blades will move from feather through very coarse pitch, toward fine, and eventually fine and then Beta, if selected. On the ground, you can hold this with the brakes (indeed, often hardly needed, as ground friction keeps you put. On the water, as the blades moved from feather through coarse, you’d be along for quite a ride, before you had command of blade angle to use Beta to control your speed. Thus, with the advent of the PT-6 powered deHavilland Turbo Beaver, blade latches were developed.
Thanks for posting the IO-360-A data. The manifold pressure limitation for continuous operation recommends running five inches maximum over square, e.g. ~28 inches at 2300 rpm, just like the O-320. The difference is that the limit applies when using 100LL fuel, not lower octane 80/87 as for the smaller engine. Bigger cylinders require higher octane fuel.
I suppose an engine with really large cylinders would be challenged to drive a fixed pitch prop that requires high MP with relatively low static rpm for takeoff and climb. Even worse if it were a high speed plane with a very coarse prop, something like the old Schneider Trophy racers would be the worst.
mh wrote:
Look closer:
I didn’t check the graphs for every single variant. E.g. the IO-360-L2A (page 3-40) doesn’t have such a limitation. In any case, the limitation is for continuous operation and still permits e.g. 1800 rpm and 25"!
Airborne_Again wrote:
In any case, the limitation is for continuous operation and still permits e.g. 1800 rpm and 25"!
Yes, they allow a considerable “oversquared” MP, but there is a limit on most O360 variants. Many Lake-Pilots recommend to run the aircraft on 1850 RPM and 24" consistently for noise if flying along congested coasts and reportedly the aircraft is extremely quiet then. I have tested it once and it is very smooth, but you don’t get anywhere…
