Prove that LOP climb is more efficient

[201er]

I'm not sure I can picture why an LOP climb would be more efficient than ROP. I do see how LOP can be more efficient in cruise. However, I see a few problems with the concept of LOP climb. I'd like someone to break things down and prove that LOP climb is more fuel efficient.

 

1) I realize that LOP climb wastes no fuel for cooling so fuel is only used for thrust.

2) LOP Climb takes longer to reach altitude because less power is lower climb rate

3) Laminar flow wing probably more efficient in cruise than in climb?

4) ROP climb gets you to altitude quicker which means you are at higher altitude and TAS sooner AND you are sooner to begin LOP cruise for efficiency

5) LOP climb starts with reasonable power but quickly dwindles off effectively to cruise power but being used to climb

 

Just some of my thoughts about LOP vs ROP in climb. What else can you tell me about it? Can someone lay the case and justify LOP as being more fuel efficient in the long run for climb in the span of an entire trip.

[jetdriven]

Perhaps if climbing into a headwind a LOP climb is more efficient, but if it is, the savings are on the order of less than a gallon.  I used the LOP climb from 300' on my 800 mile trip because that extra half a gallon mattered, I was climbing into a headwind of 20 knots, and I was only going to 2500 feet.

 

If you are climbing into tailwinds, perhaps the LOP climb is less efficient. It takes longer to realize the benefit of the increasing tailwind.  Besides, Mooneys dont climb very well anyways.

 

The only way to do it is to actually measure it.

[aaronk25]

I can tell you that anytime I'm LOP vs ROP i see 30-40% better mileage across the ground this is no differnt in climb.  I'm not sure if it saves .5 gallon more or less but just like lop cruise there is a certain power settings where applying more fuel only gives a bit more power but takes alot of gas.

 

Here are my climb numbers: everything forward 115kts and 850FPM 

LOP 110KTS and 650FPM or better.

 

So what is the percentage of MPG difference between the 2?  You got me know idea  . But its better.  When your ROP you have to leave the mixture firewalled and don't have a choice of bringing the mix back or you will cook it, at least until you get some altitude, but lop you can cut the fue by 40% and only give up maybe 15% power????  My best guess that ROP climbs waste 3.5GPH of fuel for cooling.  So if you climb for 10 mins you wasted 1 gallon?

 

I sure like seeing the fuel flow at 10.6gph instead of 17gph.  Also cleaner for engine.

 

Dont think the laminar flow wing has much to do with it as any speed of 110Kts or higher in the climb isn't plowing though the air.  Also remember fast you go more drag so that offsets the wings effeciancy.

 

I even do LOP decents.  Sure I give up 5kts in speed to do it, but I'll arrive with 30% or more gas in the tanks and cleaner plugs and less engine deposits.  

[201er]

BTW, I wonder if flying lighter by going LOP helps with anything? Say you're going somewhere that would normally require full fuel but LOP you can take 30% less fuel. If you actually do that and not top it, you probably save even more by having less weight to haul around?

[Alan Fox]

Remember this , Engines usually fail on climb out , and power changes , The little bit of fuel you save on a 2000 foot climb out is nothing compared to the altitude that might save your life if you need it.... Climb out at full power , 100%   If you can do this lop go for it , I dont think you can.... Climb out full rich , when you have enough altitude to make a safe forced landing , transition into a cruise climb and run whatever power setting you want.... Safety trumps economy... 

[AndyFromCB]

Perhaps if climbing into a headwind a LOP climb is more efficient, but if it is, the savings are on the order of less than a gallon.  I used the LOP climb from 300' on my 800 mile trip because that extra half a gallon mattered, I was climbing into a headwind of 20 knots, and I was only going to 2500 feet.

 

If you are climbing into tailwinds, perhaps the LOP climb is less efficient. It takes longer to realize the benefit of the increasing tailwind.  Besides, Mooneys dont climb very well anyways.

 

The only way to do it is to actually measure it.

 

Hey Byron,

 

What do you mean Mooneys don't climb very well? Pound for pound, hp for hp, Mooneys have a higher service ceiling and climb rates than any other GA aircraft I'm aware off.

 

And I agree with n74795. I don't touch my throttle or reduce power until well over 3000 AGL feet has been reached, only then do I reduce to 34/2400 and accelerate to 120. I my Bravo it only takes about 2 minutes to hit 3000 AGL at 38/2575 and 105knots.

[jetdriven]

Our plane makes ~800 FPM at sea level but at altitude, say, 9K, the plane climbs 300 FPM at cruise climb, perhaps 450 at Vy. 

[aaronk25]

In IMC I climb Rop until I am well comfortable and well ahead of the plane.  In my machine i know exactly where to put the mix control and can do it by feel when making the initial mix pull to 10.6gph and only then do i glance quickly at the flow to ensure I put it in the right spot.  

 
If i have a heavy load on the plane I climb everything forward until 3000ft so I do modify procedure some in that aspect. 
 
I don't buy the argument that moving the mixture from full rich to 10.6gph at 1,000 feet is unsafe on the grounds that the engine might quit or if a engine is going to quite it will do so in some duration of time after take off and if you haven't climbed as high in that given period of time you have less altitude to work with, as reducing fuel flow to 10.6 actually reduces stress on the engine.   

 

Its also good to get away from the ground as quickly as possible though as thats were the bad stuff happens, so I get that to, but if it only reduce my climb performance by 200feet per min and I can still make 650FPM or so, thats ok for me.

--

[Alan Fox]

In IMC I climb Rop until I am well comfortable and well ahead of the plane.  In my machine i know exactly where to put the mix control and can do it by feel when making the initial mix pull to 10.6gph and only then do i glance quickly at the flow to ensure I put it in the right spot.  

 

 

If i have a heavy load on the plane I climb everything forward until 3000ft so I do modify procedure some in that aspect. 

 

I don't buy the argument that moving the mixture from full rich to 10.6gph at 1,000 feet is unsafe on the grounds that the engine might quit or if a engine is going to quite it will do so in some duration of time after take off and if you haven't climbed as high in that given period of time you have less altitude to work with, as reducing fuel flow to 10.6 actually reduces stress on the engine.   

 

Its also good to get away from the ground as quickly as possible though as thats were the bad stuff happens, so I get that to, but if it only reduce my climb performance by 200feet per min and I can still make 650FPM or so, thats ok for me.

--

Reducing fuel flow does not reduce stress in the climb ,it increases it with higher operating temps... the extra fuel cools the engine , there two ways to cool the engine at max power , Air (lowering the nose ) fuel (richer mixture) ........ Seroiusly , if worrying about a gallon or two of fuel , verses safety , you need to find a safer hobby.....Perhaps fishing , or collecting stamps....

[201er]

Reducing fuel flow does not reduce stress in the climb ,it increases it with higher operating temps... the extra fuel cools the engine , there two ways to cool the engine at max power , Air (lowering the nose ) fuel (richer mixture) ........ Seroiusly , if worrying about a gallon or two of fuel , verses safety , you need to find a safer hobby.....Perhaps fishing , or collecting stamps....

 

You're missing his point. We're talking about LEAN OF PEAK climb. What you say is true for typical Rich of Peak operations. Once you're below peak (or that point somewhere around 40F ROP, but that's subtle), going leaner and reducing fuel flow is cooler. If things are hot when you're lean of peak, then you have to reduce fuel flow and go even leaner. Increasing RPM should help make things cooler LOP as well.

[gregwatts]

I don't get it.....this obsession to save 32 drops of fuel while climbing.........IMO.....will only end up costing you more somewhere down the line. Climbing to 8000' at 800 fpm is 10 minutes of climb! Why would you need to climb LOP? If you want to save fuel, from 8000' and 10 miles out......shut the engine off and just deadstick it in.

 

 

 

 

 

 

Just sayin'

[jetdriven]

Its incremental.  Used to be people flew radial engines full rich and burned 40 GPH.  Then flat motors arrived, and people flew those full rich at 20 GPH, man look at those savings.  Then, fuel injection.  Wow, 11 MPG now and 18 GPH.  Jeez, better than a new Studebaker Lark wagon.  Then the 70s fuel shocks.  WTF do you mean gas is 80c a gallon?  Better read the POH, learn to lean 100 ROP.  Wow, saving it now, we are burning 18 GPH in a Bonanza but traded her in for a Mooney and now I am burning 12.  Cut my fuel bill in half.

 

LOP changed everything.  But really it was applying 1940s radial airliner operations to flat engines.

 

I suppose the only sacred cows left are LOP climb and Carson's speed. If 100LL goes up 2$ a gallon more, half of us will be doing both.  If it hits 10$ a gallon, we will all be doing it.  If gas hits 12$ a gallon, I am out.

[Alan Fox]

You're missing his point. We're talking about LEAN OF PEAK climb. What you say is true for typical Rich of Peak operations. Once you're below peak (or that point somewhere around 40F ROP, but that's subtle), going leaner and reducing fuel flow is cooler. If things are hot when you're lean of peak, then you have to reduce fuel flow and go even leaner. Increasing RPM should help make things cooler LOP as well.

Lean of peak on climb out is not full power..... Its less than 100% power........  that means reduced climb....... That means reduced time to climb..... That means more risk.....

[aaronk25]

Reducing fuel flow does not reduce stress in the climb ,it increases it with higher operating temps... the extra fuel cools the engine , there two ways to cool the engine at max power , Air (lowering the nose ) fuel (richer mixture) ........ Seroiusly , if worrying about a gallon or two of fuel , verses safety , you need to find a safer hobby.....Perhaps fishing , or collecting stamps....

Maybe I will start a hobby, I always have like collecting stamps. Maybe I can stick them all over your airplane  .  Reducing fuel flow decreases internal cylinder pressure, which reduces stress.  The higher heat is in the fourm of exhaust gas.  The CHT actually come down about 10-20 degrees even if cowl flaps are reduce to trail.  

 

I didn't say I operate like this all the way, just respnding to the thread which is "prove that lop is more efficent" which I thought I did, didn't say it was safer.  I believe you have the same chance of that engine quitting after take of either way.  Lets not debate if its more likley to quit after a given amount of seconds or after somthing fatiuges because LOP and ROP would both have arguments.  

 

You can save some gas, didn't say I thought it was worth it just saying you can.  BTW I also fly 250 hours a year, so you can betch that it adds up!  

 

take a chill pill.......

[aaronk25]

Its incremental.  Used to be people flew radial engines full rich and burned 40 GPH.  Then flat motors arrived, and people flew those full rich at 20 GPH, man look at those savings.  Then, fuel injection.  Wow, 11 MPG now and 18 GPH.  Jeez, better than a new Studebaker Lark wagon.  Then the 70s fuel shocks.  WTF do you mean gas is 80c a gallon?  Better read the POH, learn to lean 100 ROP.  Wow, saving it now, we are burning 18 GPH in a Bonanza but traded her in for a Mooney and now I am burning 12.  Cut my fuel bill in half.

 

LOP changed everything.  But really it was applying 1940s radial airliner operations to flat engines.

 

I suppose the only sacred cows left are LOP climb and Carson's speed. If 100LL goes up 2$ a gallon more, half of us will be doing both.  If it hits 10$ a gallon, we will all be doing it.  If gas hits 12$ a gallon, I am out.

Makes alot of sence thanks for the educational tid bid.  Entertaining too, definatly the truth!

[Alan Fox]

Well at least if there are stamps all over my plane , I will know who did it....LMAO!!!!

[201er]

Well I'm at least sold that LOP climb makes more sense than "25 squared climb." n74795, we had a poll (can't find it now cause it was on the old site) and more than half of mooney pilots said they reduce power for climb rather than climb full power. So given the fact that most pilots are doing that anyway, perhaps it's a better idea to reduce power in the climb by pulling the mixture back rather than doing this by pulling the throttle back! Air is free, gas costs money.

[Alan Fox]

All kidding aside , I dont climb 25 squared , that is not 100% ...I climb out at WOT full mixture , full prop.... Until I have a safe altitude.....Nothing wrong with LOP , but not on climb out..... Be safe , not sorry......  

[aaronk25]

201er. "Air is free gas cost money". I love it! That's the truth the more air the better!

[testwest]

Your climb speed is a very strong driver of efficiency. Just looking at the prop alone:

 

M20J, making 90% power at Vy (86 KIAS), 3000 feet, 2600 rpm, standard day, thrust horsepower is around 135hp. Increasing climb speed to 115 KIAS (Vz) while keeping everything else the same, increases thrust horsepower to 149, due to increased prop efficiency. So the same fuel flow gets you more thrust horsepower, if you just fly faster.

 

I don't do LOP climb because I want as much power to convert to altitude as I can while maintaining the forward IAS. What KSMooniac and I use is WOT/2700 RPM, Target EGT, 115 KIAS until the airplane won't climb 500 fpm, then 500 fpm until speed drops to Vy. This is the Vz profile.

 

Try a back-to-back CAFE score evaluation of an LOP climb versus a Vz, I don't think you could beat the CAFE score to top of climb of the Vz profile with any other method. Search CAFE on the forum for more background info!

[Dave Marten]

I don't do LOP climb because I want as much power to convert to altitude as I can while maintaining the forward IAS. What KSMooniac and I use is WOT/2700 RPM, Target EGT, 115 KIAS until the airplane won't climb 500 fpm, then 500 fpm until speed drops to Vy. This is the Vz profile.

Right on! For the J try the Vz profile!

[testwest]

For Byron, you should be able to get "around" 550 fpm at Vy, 9000' PA, gross weight and standard day. At 115 KIAS you would get only about 280 fpm, per Benchmark.

[Hank]

I can tell you that anytime I'm LOP vs ROP i see 30-40% better mileage across the ground this is no differnt in climb.  I'm not sure if it saves .5 gallon more or less but just like lop cruise there is a certain power settings where applying more fuel only gives a bit more power but takes alot of gas.

 

Here are my climb numbers: everything forward 115kts and 850FPM 

LOP 110KTS and 650FPM or better.

My C climbs faster than that on 20 less hp! WOT/2700/Full Rich, at Vy is 1000+ fpm at home.

 

So what is the percentage of MPG difference between the 2?  You got me know idea  . But its better.  When your ROP you have to leave the mixture firewalled and don't have a choice of bringing the mix back or you will cook it, at least until you get some altitude, but lop you can cut the fue by 40% and only give up maybe 15% power????  My best guess that ROP climbs waste 3.5GPH of fuel for cooling.  So if you climb for 10 mins you wasted 1 gallon?

You need to recheck your math . . .  10 minutes is 1/6 of an hour; 1/6 of 3.5 gph is not 1, it is 3.5/6 = .58, a half gallon. There's no need to lean in the climb unless you use the Target EGT method, as your temps will all rise until you get below peak. See above, I rarely climb for 10 minutes and don't flight plan where ½ gallon makes a difference. I respect the "golden hour" and plan to land with at least 9 gallons, so far haven't been below 11.

I even do LOP decents.  Sure I give up 5kts in speed to do it, but I'll arrive with 30% or more gas in the tanks and cleaner plugs and less engine deposits.  

Good for descents. I maintain cruise MP & EGT all the way down, easing throttle back and mixture forward. Makes for nice groundspeed. I learned the first time I flew an F that procedures don't always transfer well between carbureted and injected engines.

 

I'm supposing that the 115 KIAS climb speed (Vz) is Carson's speed, which I always thought was used as an endurance cruising speed rather than a climb speed. I climb as close to Vy as Oil Temp will permit on the green stripe, and can easily be at 9000' msl, power set and trimmed, within 15 minutes of turning the key. Home field = 567' msl. You injected folks with more power should easily beat me.

 

If a simple Mech. Engineer could work his way through your MS thesis on efficient flying, could you post a link or send me an abstract or something? I apparently didn't absorb as much from the previous Carson discussions here and elsewhere as I thought I did.

[jetdriven]

For Byron, you should be able to get "around" 550 fpm at Vy, 9000' PA, gross weight and standard day. At 115 KIAS you would get only about 280 fpm, per Benchmark.

Thats about right, Norm. I am always trying to maintain 115 KIAS in the climb and around 8K it peters out badly.  I cant stand seeing 100 knots on the groundspeed.   Have you modeled a LOP climb to 8,000 vs target ROP? I wonder is there is any savings at all.  I would guess perhaps .2 less fuel, and an additonal .1 on the tach, so you save 1$ on fuel and spend and extra $3.20 on variable costs.

[testwest]

Great post Hank!

 

A comparison of climb rates is quite a bit more complex than most people think. There are a host of factors that affect rate, and although the instantaneous climb rate at any condition is somewhat interesting, the real parameters of interest are time, fuel and distance to climb. That's why the chart is listed that way in the earlier POHs.

 

And predicting climb rate at any other condition than Vy (required by the FAA for certification) requires a rather lengthy set of performance calculations, which cost a lot of money. Mooney did publish a "cruise climb" schedule in some of the latest POHs...there is one for the Encore and some of the Ks. I'll take a look and post a picture in a bit.

 

Hank, what is your published Vy speed? Carson's Speed is about 1.3 times that, however, Carson's has never caught on as a cruise speed because the resulting power is just too low, unless you are at a very high altitude. And winds aloft are an overriding factor in cruise velocity-made-good, so efficient flight (max CAFE score) has as much to do with flight planning (choice of altitude) as it does with engine operation.

 

In a nutshell, my thesis shows that Carson's Speed is a pretty ideal climb speed, subject to performance limitations (500 fpm)...which is the Vz climb profile. Jack Norris touched on this in his book "The Logic of Flight", but he did not discuss target EGT or factor in any consideration of performance limits or winds. John Eckelbar wrote about cruise climbs in his book "Flying High Performance Singles and Twins", he discusses a cruise climb of about 1.15(ish) times Vy, but does not rigorously show the efficiency (speed and mpg in proportion).

 

201er wrote: "Can someone lay the case and justify LOP as being more fuel efficient in the long run for climb in the span of an entire trip."

 

Actually, you can do that yourself!

 

An LOP climb may be more fuel efficient, but it may (will) take more time. You can't make full rated power lean of peak with these engines. So the time to climb to any target altitude will be longer. And LOP ops above around *roughly* 70% power run a higher risk of detonation unless you are significantly lean of peak on the richest cylinder.

 

And you will quickly get to a performance limit (500 fpm or less) where it is pretty hard to keep climbing consistently.

 

For the span of an entire trip, choice of cruise altitude to maximize your CAFE parameter for the trip is a much bigger factor than climb technique...and the tradeoff of a reduced-power LOP climb means a "little" more time down low where you don't get the advantages of higher true airspeed for a given IAS.

 

For Byron, I have not yet modeled an LOP climb in Benchmark.....but it would be possible to do a back-to-back flight test comparison of Vz to whatever LOP climb technique one would want to evaluate. You need a handheld GPS with track recording capability and an accurate fuel totalizer. The test cards and the procedure are in the thesis, if I can attach it. You would need to change the procedure for any LOP climb (I compare the published POH Vy technique to Vz), and compare the resulting CAFE scores at top-of-climb for whichever technique resulted in the longest distance to climb.