Cruise RPM

[bonal]

2450 for us but wasn't one of the choices 

[amillet]

On September 16, 2017 at 5:10 AM, Bob - S50 said:

None of the above.  I've started cruising at 2600 RPM.

Me too.  After installing a factory reman & new Top Prop it is smoother at 2600 than 2500 where I always used to cruise following Bob Kromer's advice of wide open throttle 2500 RPM.  

[TTaylor]

On 9/16/2017 at 5:33 AM, jetdriven said:

A 180-200hp Mooney is a relatively low horsepower airplane.  Bonanza and turbo 310 techniques such as 100f LOP and cruising at 2100 or 2200 RPM don't really work. For example, I found at 10,000 ft that a 2700 RPM cruise was the most efficient.  It had the fastest speed for the same fuel flow. Pull prop back and/or lean deeply, the airplane loses 13 knots TAS for a FF savings of 1 GPH. Especially when the RapM goes below around 2350. This may be a McCauley and 201 specific issue, but it exists. 

I have tried similar on the F, not enough data to confirm but I feel like 2300 is a few knots slower than 2500 with the same fuel flow at 10,000 feet.  I need a very stable day to try experimenting with the differences again.  For long flights I generally cruise at 2500 rpm and 9 gph in the 8,000 to 13,000 foot range.  

[peevee]

On 9/16/2017 at 5:33 AM, jetdriven said:

A 180-200hp Mooney is a relatively low horsepower airplane.  Bonanza and turbo 310 techniques such as 100f LOP and cruising at 2100 or 2200 RPM don't really work. For example, I found at 10,000 ft that a 2700 RPM cruise was the most efficient.  It had the fastest speed for the same fuel flow. Pull prop back and/or lean deeply, the airplane loses 13 knots TAS for a FF savings of 1 GPH. Especially when the RapM goes below around 2350. This may be a McCauley and 201 specific issue, but it exists. 

Cmi has advised to avoid cruise operations below 2300, which is why I run 23

[Bob - S50]

6 hours ago, Tommy said:

Plausible but then this means Mike had deliberately misled people... Hmmm.. interesting.

The only way we will know is to try it ourselves.  I think his point was that oversquare is OK.  I'm oversquare every time I takeoff, 2700/30.

[Bob - S50]

If we are looking for efficiency, we have to consider the prop too.  If the engine is more efficient but the prop is less efficient, which has more impact?

I saw an article referenced on Beechtalk about propeller efficiency.  Prop length and speed were the main determinants.  Based on that article I created a spreadsheet to get a ...rough... gouge about the most efficient prop speed.

At 150 KTAS

Mooney 74" prop --> 2470 RPM.  That may be why the MAPA article recommended 2500 RPM.

Cessna 177RG (same engine) 78" prop --> 2340 RPM

Bonanza 80" prop -->2275 RPM

The faster you go and the shorter the prop, the higher the RPM for maximum efficiency.  Interestingly, for the 2600 RPM I've started using, that is the optimum (roughly speaking) for a speed of 158 KTAS.

[Bob_Belville]

The Hartzell (Scimitar) Top Prop on my IO360A1A has a tach red zone from 2350-2550 for MAP > 24".

• Even though we cruise at altitudes high enough that WOT MAP is below 24", I normally climb at 2650-2700 and cruise at 2550, both at WOT (If above 8000' and clear of visible moisture I add RAM air).  
• Everyone on board has an effective David Clark headset so noise is not a consideration.
• Subjectively, istm the engine "likes" 2550 better than 2350.
• On a long flight I might pull the tach back to 2350 to check the effect on miles per gallon. Will the reduced FF justify the reduced GS.    

[Piloto]

At 2400 rpm my M20J consumes very little oil vs 2500 rpm. 2400 rpm is significantly quieter than 2500 rpm. But 2500 rpm gives me 3 kts extra at the expense of 1/2 gph.

José

 

[jetdriven]

With leaning it may not even be a half gallon penalty but yeah,  I see the same performance as you. It's 3 knots loss to go to 2400 rpm another 3 for 2300 rpm. But then it becomes nonlinear, around 5 knots and more for 2200

[Hank]

Coming home today, I climbed to 7500 and set 22/2400. Wasn't happy with speed or engine sound as I leaned it. Then as I richened it a bit, I found I was 25°LOP by accident. Didn't know my C would do that . . . Then I pushed my Halo tips another 1/8" into my ear and the funny noise improved. A few minutes later I noticed that I had inadvertently set 2300, so I pushed the center knob a bit and releaned--viola! Running like it was supposed to. 

So I can vouch that 22/2300 gives wonky performance, but runs smoothly at 25°LOP. But 22/2400 is so much better!

[takair]

On 9/16/2017 at 2:07 PM, Hank said:

Really? When did it change? My 1970 Owners Manual shows 1800, 1950, then 2300-2700 in increments of 100. My red arc is 2000-2250; I've heard that our IO-360 friends are redlined to 2300, though.

E model with Hartzell and IO-360-A1A has red arc of 2100-2350 RPM.  There are more variations to these red zones than I imagined...

[steingar]

Been running mine 23 squared for low altitude traveling, 23/21 for tooling around locally. I'm burning less than 9 gallons an hour. If I've sufficient altitude to really drive down manifold pressure I'll run the RPM up in some combination that gives about 70% power. But I'm VFR only, so I don't always get all the altitude I like.

i am by no means much of an expert about any of this.

[Hank]

Just now, steingar said:

Been running mine 23 squared for low altitude traveling, 23/21 for tooling around locally. I'm burning less than 9 gallons an hour. If I've sufficient altitude to really drive down manifold pressure I'll run the RPM up in some combination that gives about 70% power. But I'm VFR only, so I don't always get all the altitude I like.

i am by no means much of an expert about any of this.

For low level farting around, I use 2300 and adjust throttle as required. 2100 is in my red zone (2000-2250). Giving rides on Airport Day, i mix it up with 172s and Cherokees using 16-17"/2300 or less, aiming for 100-110 mphi.

[steingar]

Just now, Hank said:

For low level farting around, I use 2300 and adjust throttle as required. 2100 is in my red zone (2000-2250). Giving rides on Airport Day, i mix it up with 172s and Cherokees using 16-17"/2300 or less, aiming for 100-110 mphi.

By 23/21 I mean 2300 RPM and 21 inches manifold pressure.  I have the same red zone on my tachometer. I haven't tried to mix it up with Skyhawks, don't see the point of flying slowly. I did read someone here saying that you could go 19 squared or some such to duplicate Skyhawk speeds, but I don't think I've seen that in my POH, so I'm not in a big hurry to try it.

I didn't buy a Mooney to fly slowly.

[Hank]

Just now, steingar said:

I didn't buy a Mooney to fly slowly.

I didn't either, but when there's six 172s, a Cherokee 180 and me giving rides, following the same pattern around the area, going snow is kind of required. I'm not a fan of straight in approaches, but had to do them one Airport Day when the wind was blowing funny.

[peevee]

7 minutes ago, Hank said:

I didn't either, but when there's six 172s, a Cherokee 180 and me giving rides, following the same pattern around the area, going snow is kind of required. I'm not a fan of straight in approaches, but had to do them one Airport Day when the wind was blowing funny.

I'm really getting tired of tracon or tower asking me to slow down 50 knots. Go down or slow down guys, pick one.