Efficient Cruising

[Cwalsh7997]

Hey guys I wanted to know your opinion on the most efficient cruise setting. I've hear people say around 19 squared which will give me around 5gph but is it ok to run the engine below 2100-2350rpm (which it doesn't recommend in this range)?? Please lemme know?

[Cwalsh7997]

Keep in mind this is for me being a young pilot (17) that's trying to build some hours and I'm paying for the fuel.

[carusoam]

Do you have any rpm limitations?

They are printed on your tach and covered in your POH under limitations...

Often they are specific to engine, prop and airframe.

Other than that flying with low power can lead to some pretty low cylinder temps.

Did you get the engine monitor?

Best regards,

-a-

[Hank]

There's more than one definition of efficiency. For me, it's not the lowest possible fuel burn to stay in the air, it's the least fuel burned for the flight.

Look at your performance charts, anything there is safe. Divide distance by shown airspeed then multiply by shown fuel flow for several desired power settings. You will never achieve this, because it is all no wind, but it will show projected efficiency.

If you want maximum time aloft for your fuel dollars, fly at Carson's Speed, Vz. There are several threads about this here, and more available online at your favorite search engine.

Happy flying!

[1964-M20E]

Like Hank said what are you doing, going some where or boring holes in the sky?

 

Other than the RPM restrictions you can run the engine anywhere in it power envelope.  The lower you go on power the slower you will go down to the stall speed then you stop flying but that is know by any pilot.  I have on occasion, when just boring holes in the sky, backed down to the lowest MP I could get and keep the plane in the air and yes I was down to the 5 to 6 GPH range.  Essentially MCA with minimum power.

[carusoam]

Adding all the above together....

Study some glider technique. There are two sources of lift you may be able to harness...?

Low power, lean close to peak, Vz, add ridge or solar lift = long hours aloft.

Bring the G bottle...

-a-

[atn_pilot]

Hey guys I wanted to know your opinion on the most efficient cruise setting. I've hear people say around 19 squared which will give me around 5gph but is it ok to run the engine below 2100-2350rpm (which it doesn't recommend in this range)?? Please lemme know?

 

I don't know about the F model, but my C has 1950 RPM as one of the recommended power settings in the manual. The older Cs had a POH that went down to 1850 RPM. I pull it back to 1950 pretty regularly unless I'm in a hurry.

[PMcClure]

The charts in my F had a "Maximum Duration Setting" for minimal fuel burn. I don't remember the settings but I recall that the duration was almost 10 hours. Maybe your POH has something similar?

[Bob - S50]

For just building time and not trying to go anywhere, try best engine out glide speed.

 

As already mentioned, Carson's speed works if you are actually going somewhere, about 1.3 times best glide.

 

I like a quote from a John Deakin article: "I didn't buy a fast airplane to fly slowly!"

 

Bob

[cujet]

When light, I can back the RPM down to below 2000, manifold pressure to below 20 and lean to 5.5GPH. At this point, I'm going 110-125MPH. In no wind conditions, it makes for some very efficient flying. 

 

However, flying from F45 to LAL (for sun n fun) took me over 2 hours in a headwind, including the LAL approach which clearly adds some time. I may not have saved any fuel at all getting there when compared to operating at 25/25, 10GPH and 1 hour of flight time. All I accomplished is an extra hour on my engine, prop and airframe. Those costs can be quantified.

 

For example: (note, my example is simply one possibility, your results will vary)

 

Prop, $6/hr total cost (includes purchase price, overhauls, mx and lifespan) 

Engine, $25/hr total cost (includes the $15 per hour overhaul costs and mx over it's lifespan)

Airframe, $20/hr total cost  (includes depreciation, repairs, and so on) 

 

That's roughly $50/hr operational costs, not including fuel. Or put another way, that extra hour of operational expense could have purchased 12.5 gallons of 100LL at 2IS at $3.99/gal. 

 

So, additional time on your components may not be saving you as much as you think in the long run. In my case, it cost me $50 to save 6 gallons of fuel. 

 

By the way, I am local to you. 

[Hank]

When giving rides on Airport Day, mixing in with Skyhawks, I generally use 2300 and back off MP to hold around 110 mph indicated, call it 16-17" or so. That kept me from running over the Skyhawk in front of me. Lean a lot at low power settings, too!

[Jerry 5TJ]

If you want to build time, leave the gear down.

[1964-M20E]

If you want to build time, leave the gear down.

that just take all the fun out of it. 

[jlunseth]

+1 on best glide, given that you want to build hours and not distance.  Best glide will give you the lowest L/D ratio.  As far as power is concerned I would just pick an RPM, set the MP where you need it for best glide, and then use the old trick of pulling the fuel back until the engine runs rough then putting it in until the engine smooths out again.  At a power setting that low it does not matter much where the fuel is set as long as the engine is happy and the flow is as low as you can get it.  You could experiment with RPM's to see if you get a lower fuel flow at lower RPM settings but I am guessing that at some point that will be a game of diminishing returns. 

[ryoder]

I have found that 115 to120mph is best for low cost flight. Any slower and cooling suffers, oil pressure is low, and the engine runs rough. I run at 2300 and various MP settings. It's almost like flying a fixed pitch prop in cruise and pattern entry.

[Mooneymite]

This question dove-tails into a question I asked about max-endurance speed/power settings for holding.

 

http://mooneyspace.com/topic/6024-holding-power-settingsbest-econ-speed/?hl=%2Bmooneymite+%2Bholding+%2Bspeed#entry77146

 

Seems like we're always interested in flying fast and getting to our destinations efficiently, but there really are no numbers that I'm aware of for just staying aloft going nowhere.

 

When my son was building time in our 'C', he used 1900 RPM a lot, but I really don't know what kind of performance he experienced.  I do know fuel-wise, it was cheaper than a C-172 for similar speed.

[Hank]

Gus, a C-model at normal cruise speed is cheaper than a 172 for the same trip, too. It should use about 10% less fuel while going ~40% faster. Can't swear for the OP's F.

[mike_elliott]

Conner, A friend of mine Jonathan Paul, knows how to milk every mile out of a Gallon of avgas. His IO360 and prop combo is the same as yours, but he has a less draggy airframe.

A good read here

http://www.jonathanpaul.org/pdf/NonStop.pdf

[jetdriven]

When light, I can back the RPM down to below 2000, manifold pressure to below 20 and lean to 5.5GPH. At this point, I'm going 110-125MPH. In no wind conditions, it makes for some very efficient flying. 

 

However, flying from F45 to LAL (for sun n fun) took me over 2 hours in a headwind, including the LAL approach which clearly adds some time. I may not have saved any fuel at all getting there when compared to operating at 25/25, 10GPH and 1 hour of flight time. All I accomplished is an extra hour on my engine, prop and airframe. Those costs can be quantified.

 

For example: (note, my example is simply one possibility, your results will vary)

 

Prop, $6/hr total cost (includes purchase price, overhauls, mx and lifespan) 

Engine, $25/hr total cost (includes the $15 per hour overhaul costs and mx over it's lifespan)

Airframe, $20/hr total cost  (includes depreciation, repairs, and so on) 

 

That's roughly $50/hr operational costs, not including fuel. Or put another way, that extra hour of operational expense could have purchased 12.5 gallons of 100LL at 2IS at $3.99/gal. 

 

So, additional time on your components may not be saving you as much as you think in the long run. In my case, it cost me $50 to save 6 gallons of fuel. 

 

By the way, I am local to you. 

We bill~32$/hr for dry rate plus gas.  65% power at any altitude is the lowest trip cost with 5$/gal fuel. Cheaper fuel biases the airspeed up, but flying below 65% power, or 8.7 GPH is more expensive per trip than 65%.  Unless you can skip a fuel stop.  That costs around 25-30$, and flying slower, although incurring a fuel cost penalty, will save the fuel stop cost and reward you with a lower total trip cost and a low shorter elapsed time ad well.

[N9405V]

Use the charts in your owners manual.

[Hondo]

Jack Norris, aeronautical engineer, technical director for the Voyager, wrote a great book, The Logic of Flight the Thinking Man’s Way to Fly. http://propellersexplained.com/

Jack goes to great lengths to make it easy for the layman to understand.

 

David Rogers, aeronautical engineer and ATP provides a simple rule of thumb for propeller efficiency at altitude. http://www.nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf 

 

LOP operation has been discussed a lot here, but brake specific fuel consumption is at a minimum about 35 degrees F LOP.  Flying there could save you a fuel stop and reduce your total trip time.

[DaV8or]

Like Hank said what are you doing, going some where or boring holes in the sky?

 

Other than the RPM restrictions you can run the engine anywhere in it power envelope.  The lower you go on power the slower you will go down to the stall speed then you stop flying but that is know by any pilot.  I have on occasion, when just boring holes in the sky, backed down to the lowest MP I could get and keep the plane in the air and yes I was down to the 5 to 6 GPH range.  Essentially MCA with minimum power.

 

I've done that to to see if it's possible to fly the Mooney as though it were a Cessna 152 in terms of cruise speed and fuel burn. In practice, yes, it is possible, but actually not that easy and not worth it at all. The Mooney handles like crap at this cruise speed.

[carusoam]

http://www.nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf

If you are having a problem with Hondo's link above... See if this one works better

-a-

[aaronk25]

NOPE.....15-20LOP is best break specific fuel consumption....based on 20-25 degrees before top dead center timing.....

[Hondo]

aaronk25,

 

Could be.  

http://11hc.44rf.com/manuals/engine-prop/lycoming/aerosportspower-library/360_series_ops_manual/sec_3a-operating-instructions.pdf

See page 3-7

 

With an IO-360-A3B6D  25TDC timing, I cruise at 30 LOP.