RPM

[Tommy]

Been reading about the best cruise setting for M20J. One author says 2500 rpm no matter what but Mooney Flyer seems to advocate as low RPM as possible!

Which is better for the engine in the long run??

[NotarPilot]

Recently I've been using 2200 RPM and 27" MP down low, say 2000 MSL or less to about 25" MP to WOT and 2200 RPM at 5000 MSL and above.  I fly LOP and can usually get my fuel flow to about 7.8-7.9 GPH.  

 

I've based most of my engine management habits around what I've heard Mike Busch preach.

[Tommy]

Wow that's quite over the square!!

What's the rationale?

[Jeff_S]

The manufacturer says that for that engine and that airframe, 2500 is the ideal cruising RPM. But others (like Mike Busch) argue for a lower RPM as that gets more efficient use of the horsepower from the engine (less friction, as I recall). But I think it really just depends in several factors that are entirely up to you. For example, you can cruise at 2400 RPM and WOT all you want (don't worry about the old wives tale of "over square") but you will be going a little bit slower than if you cruise at 2500 RPM and WOT.  For me it was about 3-4 knots.  For some people, they just find the RPM setting where they think the engine is "smoothest" and leave it there.

 

I think more important than the RPM is how you manage the fuel mixture. Whether you choose LOP (me) or ROP, just make sure you keep the CHTs in the comfort zone of 330-350 degrees. That's what will have the most positive benefit for your engine.  You can choose to slow it down and enjoy the ride, or rev it up to get there faster...just maintain those CHTs and your engine will be happy.

[Jeff_S]

Wow that's quite over the square!!

What's the rationale?

As I wrote (while those other posts were happening!) the "over square" problem is a myth with our horizontally opposed engines. There was some truth to it with the big radial engines of the  WWII era, but for our engines you can discount that entirely.  As noted, view the webinars of Mike Busch (http://www.savvymx.com) and you'll learn a lot.

[NotarPilot]

What Jeff said.

[John Pleisse]

Mike Busch's ideas are well developed theories and practices from years of experience, not the gospel.

[Lood]

I find that 2450 rpm is the smoothest setting in my F. I must confess that I haven't tried lower rpm settings, but 2500 or 2550 certainly return quite a bit more noise and there seems to be quite a bit more vibration.

[Piloto]

At higher RPM the engine consumes more oil, so this is another factor to consider. On my I I0-360 I get 7 to 8 hours per quart at 2500 rpm but at 2400 I get 10+ hours per quart.

 

Jose   

[Jibsheet1]

Another question to go along with the OP. Is there a difference between a three bladed and two bladed prop when considering the best cruise rpm? I am also flying a J.

[Bartman]

In my J 2500-2700 RPM produce the best vibration free engine operations. If I go below 2400 noise decreases, but vibration increases slightly. I always climb at 2700, but tend to cruise at 2450 as it is the best trade off between noise and vibration.

As Jose stated RPM also affects oil consumption significantly. My first few trips several years ago were at 2700 in cruise. I no longer cruise there due to increased oil consumption.

[Jeff_S]

As to the difference between 2- and 3-bladed props, since I've only ever had the 3-blade I can't say for sure. Most people will say that due to the extra weight and drag, the 3-blade is costing me up to 5 KTAS. They may be right, but I don't feel like changing it out right now.  As to RPM, probably just the same guidance as already noted applies...if you have a preference for what feels smoother, use that, but realize that at any power combo the 3-blade is going to be a bit slower than a two blade (in cruise...climb is a different matter!).

[Hank]

People say the same bad things about my 3-blade prop too, but I still manage to achieve book speed. Plus it climbs like nobody's business, especially in the winter.That's ROP, 2500 above 7000 msl. At lower altitudes I reduce RPM. I generally run 23/23 at 3000 and 22/2400 from 4500-6500/7000. Your injected 200 hours engines will do more.

[Ftlausa]

I have never flown the 2 blade prop, I am making book speed with the 3 blade prop as well, plus the climb is good. 

[M20F]

At higher RPM the engine consumes more oil, so this is another factor to consider. On my I I0-360 I get 7 to 8 hours per quart at 2500 rpm but at 2400 I get 10+ hours per quart.

Jose

I would add my oil temp gets high at the higher RPM's and it causes the oil pressure to dance in the yellow. Part of that I think is related to having SWTA cowling with the oil cooler relocation and some poor baffling but for me running a more squared manifold /RPM setting seems to yield same speed/GPH with no side issues. I find every plane is a bit different so what works for one doesn't always work for another.

[orionflt]

I have been playing with WOT and 2000 rpms and all altitudes, I have had very little drop off of speed (no more then 5 kts) compared to running at 25/25 or even WOT/2400 at higher altitudes like I had been doing, but my fuel burn has dropped by 1-1 1/2 gallon per hour and everything is smooth.

 

brian

[pinerunner]

I think calling the over-square rule an old wives tale may be going a bit too far. While I do violate it myself I have a good engine monitor, watch my CHT's, and generally have a care about how I'm doing it. 

 

If you set up a worst case scenario and go full throttle, ROP 100 degrees for maximum possible power, then pull the prop back say to 1800 RPM, and do it in a climb to make the cooling worse I bet you will induce detonation and preignition, even though Busch and Deakins have said its hard to get detonation. 

 

I don't think you should let the old rule be a straight-jacket to you if you want to try things out that you've researched carefully and have the instrumentation to monitor well. But I also don't think those old timers were stupid and even today I think the rule of thumb still has value.

 

I do buy into Deakins advice in Pelicans Perch to drop RPM's in a low power setting like descent to help keep the CHT's up and think that might have particular value to folks flying in the winter. 

 

Note that if you're LOP the manifold pressure doesn't give power anyway so the rule should probably be reworked and based on RPM's and fuel flow.

[jetdriven]

Why lower rpm in descent at all? You give up a lot of speed in the climb for a long period of time. If you take the additional speed in descent it lowers block time.

Also, my j runs 360-380 in cruse and consistently 330 in descent without touching anything. After landing it's 290.