Cruise RPM

[Tommy]

Recent discussion brought up the issue of optimal 65% cruise power setting (fuel + oil burn, power delivered, and temperature control etc).

Mike Busch advocate low RPM - https://www.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2012-10_flying-efficiently.pdf

Others advocate high RPM. http://www.mooneypilots.com/mapalog/cruisepower.html

Who's right?

 

[Raptor05121]

They're both right, IMHO. Mike is talking about efficient cruising if I remember from the last time I read that article. The author of the Mooney article pretty much summarizes: "you're in a Mooney, get there quick".

I'm with Mike when local. 23/2300 nets me 10GPH at 2,000 feet for local flying. But, when I am actually going somewhere worth stretching the legs, I'll climb high and WOT/2500RPM and just deal with the fuel burn when I get to the pumps.

More often than not, I will turn the prop down just to cut down on noise. It seems to be exponential with speed. 2300rpm is much quieter than 2500, and only costs me ~5-7mph

 

[jetdriven]

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. 

Edited September 16, 2017 by jetdriven

[Bartman]

It depends on a lot of factors. Big tailwind I might reduce to 2,300 and increase to 2,500 for headwind. Flying above 10,000 I generally go higher RPM 2,500 or maybe even 2,600 if pushing the limits like 15,000 ft.  But for local flying 23x23 is good. 

But for everyday cruise at 8-10 K setting of 2,400 is my choice. 

[Hank]

It depends on altitude:

• Low cruise (flightseeing, short hops)--23/2300 up to 3000 msl
• Mid level (medium distances, level,offs)--22/2400 for 3000-6500/7000
• Traveling--WOT-/2500  Just one of the joys of carburetion, backing off the throttle until the MP needle moves

 

[midlifeflyer]

Whichever RPM gives me the smoothest ride.

[Bob - S50]

None of the above.  I've started cruising at 2600 RPM.  I believe (and hope to verify in the next few weeks) that the higher RPM helps keep my CHT lower and reduce blow by.

I think it depends.  If you have an engine that is timed at 25 BTDC, I think a higher RPM is better.  If you have an engine that is timed at 20 BTDC, I think lower RPM is better.  The reason being, lower RPM brings the peak pressure closer to TDC.  If the engine is already advanced, a lower RPM may bring the peak pressure earlier than the optimum 16-18 degrees ADTC.  On the other hand, if the engine is timed 20 BTDC, it may actually be getting the peak pressure after the optimum point.  Running lower RPM would bring it back closer to optimum plus reduce engine friction losses.

This is all speculation since I have no way of knowing at what point the peak pressure will occur for the two different timings.  I do, however, know that my A3B6D engine starts to complain below about 2400 RPM unless I'm at pretty low power settings.

Only flight testing will tell.

[ArtVandelay]

I agree with Bob, IIRC one of the Mooney evaluation articles mentions that the 2500 rpm was where it's most efficient. I have 20° timing, 2300-2600 is where the engine sounds the smoothest. I don't see why CHTs should be lower unless they are getting better airflow, if burning the same fuel amount, then the heat generated will be the same. Actually lower RPMs should have slightly less friction losses.

[Andy95W]

2 hours ago, midlifeflyer said:

Whichever RPM gives me the smoothest ride.

+1

[Mooneymite]

I voted "2300, or less", but that's not quite accurate.  My 'C' POH performance tables do not have settings for 2300.  They use 2350....I presume to keep operations well clear of the dreaded "red arc".

[carusoam]

Use Caution: 65% BHP is not about efficiency.  It is not an optimum.  It is about staying out of the red box. This allows for operating LOP. Which is all about using up 100% of the 100LL for power production and not cylinder/EGT cooling that is major part of running ROP.

The Who's right scale is...

• continuous from very low rpm to very high rpm.  
• Plane specific outlined in the POH.
• Some prop/engine combos have yellow zonal limitations....and a red line...
• slower rpms have benefits from smaller 'end effect' blade efficiency challenges.
• really slow rpm, at low MP has the (opportunity to have the) best % conversion of fuel to exhaust products.
• really fast rpm, at high MP has the most conversion of 100LL to exhaust products.

 

With the IO550, is that 65% bhp of...

• 240 hp?
• 280 hp?
• 310hp?
• 350hp?

O1s were limited to 2500 rpm. A great set-it and forget-it set point... using MP to set 65% of 280hp...

The 310hp Os can climb at 2700.  With full sound and fury!

2550 rpm is a given cruise number for the TopProp that is a compromise between full speed ahead, but just short of full sound, with a little less FFury...

I choose alts above 10k for better glide distances.  Less crowded airspace.  Power is limited to below 65% by air density. Using 10°F LOP to convert all the 100LL without leaving much of anything else on the table....

 

Going deep LOP?  

• The focus of this power setting is maximum aero-efficiency, max fuel efficiency, and maximum distance...
• Consider using Carson's speed, minimum mp, and slow the prop down too...
• great for maximum distance, and lowest fuel used.
• terrible if you want to go fast.
• Too much power/speed left on the table...

 

An interesting way of looking at this challenge...  

• If you fly with the MAPA key number power settings for your bird...  
• the Rpm/100 + MP = a %hp constant.
• which means you can substitute 100rpm for 1" of MP to get the same hp, generally speaking.
• Are you willing to give up an 1" of MP or 100rpm?
• Are you willing to take on the extra FF and noise by adding 100rpm, when MP is already limited by altitude?

 

cruiseam style:

• Altitude selection: high altitude for best engine out glide options.  Altitude also limits my NA MP / %bhp...
• Oxygen selection: short of the O₂ levels for best convenience.
• Throttle Setting:  Full throttle for highest power available to produce highest speed.
• RPM Setting: 2550 rpm, recommended by the prop manufacturer/STC...(?)
• Mixture Setting: a few °F LOP for minimum wasted raw fuel sent out the exhaust.
• The extra 50rpm is like getting a 1/2” MP over the O1.   Without opening the air intake to Bugs and dirt...
• FF is increased to support the extra 50rpm.  Resulting in a higher speed.

 

Overall, there isn't a...

• set-it and forget-it rpm
• always use this rpm
• that works all the time
• for everybody

 

in flight planning... a pilot would want to consider a specific rpm for each aspect of the flight... since the topic is cruise rpm... also consider.... cruise climb and cruise descent rpm settings...

 

This is a couple of different ways a PP may look at the challenge... I'm not a CFI...

Did anyone read this far?

Best regards,

-a-

[peevee]

I run 2300, I don't notice a speed difference between 23 and 25, it's smoother and quieter, and our tach is accurate at 24 or 2450 so it saves a few pennies on mx costs per mile.

[Mooneymite]

Mooney owners generally like efficiency, so lower RPM ought to be favored.  

Perhaps it's an OWT,  but I've always heard that lower RPM is more efficient because it keeps the tips further below Mach.  However, we can all agree that 0 RPM is problematic.

The P-3, which can fly for hours on a teaspoonfull (well,  military teaspoonfuls) of fuel, flew at a constant 1000 RPM regardless of power setting.  Of course, they were 14' props.  

[Hank]

2 hours ago, Mooneymite said:

I voted "2300, or less", but that's not quite accurate.  My 'C' POH performance tables do not have settings for 2300.  They use 2350....I presume to keep operations well clear of the dreaded "red arc".

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.

[Mooneymite]

Just now, 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.

Yep....same red arc.  Just the power setting tables are apparently different.  Mine show 1950, 2350, 2400, 2500, 2600 and 2700.  No 1800 RPM numbers in my book.

[Hank]

2 minutes ago, Mooneymite said:

Yep....same red arc.  Just the power setting tables are apparently different.  Mine show 1950, 2350, 2400, 2500, 2600 and 2700.  No 1800 RPM numbers in my book.

Going by memory, it's not on my phone. But 1800 sounds familiar. It's posted in Downloads here.

[carusoam]

77C POH indicates... 1950rpm as the low end on the engine performance charts... 

Best regards,

-a-

[Skates97]

My '65 C Manual says avoid 2,100-2,300 rpm in cruise. Performance charts have data for 1,800, 2,300, 2,400, 2,500, and 2,600.

The '77 C Manual says avoid 2,000-2,250 rpm in cruise. Performance charts have data for 1,950, 2,350, 2,400, 2,500, 2,600, and 2,700.

I'm not sure why the change in the red arc range or the data that they have in the performance charts. I would be interested in hearing people's thoughts on that.

For cruising around it really depends on how much of a hurry I'm in. Usually it's either 2,400 or 2,500. I may try out 2,300 to see what the sound difference is and if I like it better.

[carusoam]

Thanks, Richard!

I was with Hank, thinking 1800rpm came from somewhere...

But my 65C manual is gone with the schwin. Aka sold with the plane...

Best regards,

-a-

[Bob - S50]

6 hours ago, teejayevans said:

I agree with Bob, IIRC one of the Mooney evaluation articles mentions that the 2500 rpm was where it's most efficient. I have 20° timing, 2300-2600 is where the engine sounds the smoothest. I don't see why CHTs should be lower unless they are getting better airflow, if burning the same fuel amount, then the heat generated will be the same. Actually lower RPMs should have slightly less friction losses.

For a given power output, lower RPM gives higher CHT because it pushes the peak pressure closer to TDC.  That creates higher peak temperatures which produces higher CHT's.  If you keep the MP the same but at a lower RPM, the power produced will be less and the CHT will go down.   If the engine is already operating closer to TDC than optimum, anything closer will not only provide less power to the crank, it will increase loading on the crank and bearings.  I suspect the difference in temperature due to friction is negligible compared to the difference in temperature due to high peak pressure.

[201er]

Depends on altitude.

[Tommy]

7 hours ago, Bob - S50 said:

For a given power output, lower RPM gives higher CHT because it pushes the peak pressure closer to TDC.  That creates higher peak temperatures which produces higher CHT's.  If you keep the MP the same but at a lower RPM, the power produced will be less and the CHT will go down.   If the engine is already operating closer to TDC than optimum, anything closer will not only provide less power to the crank, it will increase loading on the crank and bearings.  I suspect the difference in temperature due to friction is negligible compared to the difference in temperature due to high peak pressure.

But that's not what Mike's data showed. Same power setting, the one with higher RPM and lower MP has higher CHT and EGTs. Read his article. 

[Tommy]

I wonder as a group, we can do some experiement ourselves. After all, we all have the required measuring instruments - cht / egt / FF / smart phone with a noise-measuring app. Shouldn't be too hard. Fly half of the leg on high RPM / low MP setting and the other half the other way round.

@jetdriven Byron's observation is very different from Mike's. Could well be different engine but you would've thought Mike Busch would say something like "but the opposite applies with a Lycoming IO-360" given how much data he has! 

[Bob - S50]

10 hours ago, Tommy said:

But that's not what Mike's data showed. Same power setting, the one with higher RPM and lower MP has higher CHT and EGTs. Read his article. 

I did read the article when it was posted in another thread.  And I suggested that it wasn't comparing apples to apples.  The low RPM setting plus MP adds up to 48.  The high RPM plus MP only adds up to 46.  I suggested that in the low RPM case he must have been operating much further LOP than he was in the high RPM case in order to get the same fuel flow.  The leaner the mixture gets, the slower it burns (until it won't burn at all).  That pushes the peak pressure point further from TDC which reduces peak pressure and CHT.  I would like to have seen his readings at 2100/25 and the same fuel flow.

[Tommy]

5 minutes ago, Bob - S50 said:

I did read the article when it was posted in another thread.  And I suggested that it wasn't comparing apples to apples.  The low RPM setting plus MP adds up to 48.  The high RPM plus MP only adds up to 46.  I suggested that in the low RPM case he must have been operating much further LOP than he was in the high RPM case in order to get the same fuel flow.  The leaner the mixture gets, the slower it burns (until it won't burn at all).  That pushes the peak pressure point further from TDC which reduces peak pressure and CHT.  I would like to have seen his readings at 2100/25 and the same fuel flow.

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