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…
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"!
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.
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.
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:
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?
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.
AnthonyQ wrote:
feathering stop pins which retract due to centrifugal force at around 800-1000 RPM. below that the prop stops against that pin in a high pitch position. Above that RpM the prop will feather if the oil pressure is lost.
If that is a quote of the POH, it contains unfortunate wording. What it means to say instead of “prop will feather” is that “the prop may be feathered”. As would be the case for any normal shut down, if you’re idling with fine pitch selected, and you shut down a piston engine, the prop will be in fine pitch when it stops.
The vitally important takeaway from that wording is that below that RPM, that prop will not feather! This is a vital thing to know if you’re flying that engine in a twin, as if the engine fails, and you allow it to slow below the 800-1000RPM, you will not be able to feather it. I was very clear about this when (re)writing the draft flight manual for an engine change in a well known twin – the manufacturer had not grasped this important point.
As an aside, and perhaps source of confusion, a PT-6 is feathered before it is shut down. If you don’t feather it, it will feather itself as the oil pressure bleeds off. I did this once as I was curious, shut down a PT-6 in fine pitch – the propeller spun for more than 6 minutes, and the engine had long before stopped. As the propeller came to a stop, it was feathering itself. The down side of this was that the gearbox was not supplied with pressure lubrication all that time, which is one reason it’s a poor idea to shut down this way. 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.
Airborne_Again wrote:
Picking up my nearest POH for a VP aircraft (TB20, Lyc. IO-540 engine), the power setting table includes 2200 rpm and 26.1 MP as one of the “recommended” settings for 75% power at MSL (and all the recommended settings in that situation have an MP greater than the rpm/100).
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.
Pilot_DAR wrote:
Interesting. Does that mean that when you walk up to the plane for the first flight, the prop is feathered? Can you reposition it to fine pitch for the start?
I believe there is a locking mechanism that keeps it at full-fine if you shut down with the blue knob all the way in….but I think if you lose the engine (and hence oil pressure) during cruise and do not go to full fine in time that it will feather….and stay that way… but I have not flown one so I may be wrong…
Extract from Mooney expert:
There’s feathering stop pins which retract due to centrifugal force at around 800-1000 RPM. below that the prop stops against that pin in a high pitch position. Above that RpM the prop will feather if the oil pressure is lost.
Pilot_DAR wrote:
There is some permission for some normally aspirated engines to be operated “over square”.
Please don’t talk about “oversquare”. I thought that OWT had been laid to rest. Of course an engine will be damaged if the MP is too high in relation to the rpm, but there is no magic relation between MP in inches Hg and rpm/100. Picking up my nearest POH for a VP aircraft (TB20, Lyc. IO-540 engine), the power setting table includes 2200 rpm and 26.1 MP as one of the “recommended” settings for 75% power at MSL (and all the recommended settings in that situation have an MP greater than the rpm/100).
