The Golden Shower Case for LOP

[201er]

Here's a scenario for J drivers but we can find similar cases for other models. Good case why flying slower is cheaper for the same time to destination. 

900nm, something like NJ to Orlando. You can choose economy LOP cruise 140ktas, 8gph, 60% power or max performance cruise 160ktas, 12gph, 75% power. Using $5/gallon as typical.

Economy:

6.4 hours, 51g, 1.5hour reserve, $257 fuel

 

Performance:

5.6 hours, 68g, -0.3 hour fuel, $338 fuel

 

The slower cruise can skip a fuel stop and go nonstop. Not enough fuel to make it the fast way so a fuel stop is mandatory. Fuel stop takes about half an hour to get the fuel, pay, and use the bathroom plus some time for the extra climb. End result is arriving to the destination 900nm away at the same time as slowing to skip the stop.

That's an $80 bathroom stop since you arrive to the destination no sooner. Talk about a golden shower!

[201er]

Now I understand there's different situations, traveling with kids, loaded cabin with less than full fuel capability, etc. But you guys flying solo most of the time... cmon.

[N6758N]

But Mike, what about the added expenses of replacing your exhaust system more often due to the LOP operations...

 

 

[201er]

42 minutes ago, N6758N said:

But Mike, what about the added expenses of replacing your exhaust system more often due to the LOP operations...

I had to replace the exhaust when I bought it after last owner flew it ROP.

[Marauder]

You guys having watching "Spirit of St. Louis" too many times. You need to start watching more "Top Gun". Full afterburners baby! Burn, baby, burn.

[INA201]

2 hours ago, bluehighwayflyer said:

Amen. I put 1000 nm and 8 hours on my J two days ago at between 5.5 and 7.5 GPH LOP in order to get the range I needed. I find the J's efficiency to be remarkable. 

I'm in the process of installing fuel flow(jpi 900)and you've peaked my curiosity. What kind of numbers are you seeing with these flows, altitudes, mp, etc? I never entertained the idea that 5.5 was in the toolkit.  1000nm nonstop?  I'm in a 78J btw. 

[flyboy0681]

Interesting thread Mike. What are the differences in time and burn at 60% ROP?

[Bob_Belville]

I came back from MA yesterday. 5.5 hours, cruised @ 8.1 gph LOP, 8000' 2350 rpm & 21.7" (full throttle w/o RAM Air) so that I could non-stop against headwinds. 46.1 gallons. Probably about 140 ktas though flying the mountain ridges up/down drafts make it hard to know accurately. (I landed with only about 8 gallons remaining which I would not normally do. Weather was perfect and I was keeping a very close eye on fuel.) We're beginning the install of the bladder expansion tomorrow which will give us more options. 

[jaylw314]

If you're flying IFR, the time for the fuel stop is more like 45-60 min for filing the new flight plan and picking up your clearance.  Even if you do those in the air after takeoff, you need an extra 15 minutes on the ground to prepare or just to keep your head from exploding.

In the M20J, flying lower and slower also gives you slightly better fuel economy, but both have their limits.  Flying down the west coast you have to fly over mountains and weather which limits the "lower."  Your spouse's bladder endurance and willingness/ability to use a portable toilet may limit the "slower."

On the other hand, when I told my wife the portable toilet would save us $75 and and 45 minutes, she promptly went for it, bless her soul

 

[201er]

48 minutes ago, flyboy0681 said:

Interesting thread Mike. What are the differences in time and burn at 60% ROP?

I never really do 60% ROP, don't see any point. Power is low enough and things are cool enough you could just as well be at peak. That would probably run you about 8.5GPH compared to 8.0 slightly LOP. So mostly the same numbers. But if you were going to stupidly throw fuel at it to make it 100ROP for best power at 60%, then about 10.5GPH. Wouldn't make any sense to go far rich of peak because CHTs and ICPs will be low enough right down to peak. For more performance, rather than enrich just add throttle or RPM.

[201er]

42 minutes ago, jaylw314 said:

If you're flying IFR, the time for the fuel stop is more like 45-60 min for filing the new flight plan and picking up your clearance.  Even if you do those in the air after takeoff, you need an extra 15 minutes on the ground to prepare or just to keep your head from exploding.

This is a good point. It's an even clearer win IFR. Not to mention the added work load of doing an additional approach/IFR plan in one day.

[201er]

Just now, bluehighwayflyer said:

6.5 GPH LOP will be around Carson's speed and is a good power setting if you want to maximize efficiency while still covering some ground.  I don't see any reason to set power lower than that unless you are also factoring in an endurance target like I was doing. 

Actually Carson's speed isn't a power setting. Not even a speed really, it's an angle of attack. But simply put it's around 120KIAS. So the power required to reach Carson's speed will vary some with altitude but the true airspeed flown at the Carson's IAS will increase with altitude until you can't generate enough power.

[Guest]

The flip side of flying slower is more airframe time, engine time and more frequent inspections.

Clarence

[Browncbr1]

I knew AOA would come up!  

[201er]

10 minutes ago, M20Doc said:

The flip side of flying slower is more airframe time, engine time and more frequent inspections.

Clarence

And more pilot logged time 

[jaylw314]

23 minutes ago, bluehighwayflyer said:

Sorry but I really can't answer your questions very well.  My flight two days ago was irregular and subject to some outside influences that would make no sense to emulate for most folks under most circumstances.  In other words, I was building time.

Generally speaking I recommend just setting fuel flow at 8 to 8.7 GPH LOP  (60 to 65 percent) and looking out the window.  6.5 GPH LOP will be around Carson's speed and is a good power setting if you want to maximize efficiency while still covering some ground.  I don't see any reason to set power lower than that unless you are also factoring in an endurance target like I was doing. 

Wow, it's nice not to have mountains around!  My last flight down the west coast I had to cruise at 12,000 MSL, and I could only get up to about 7.6 gph running WOT and 2400 RPM at peak (about 135 KIAS).  LOP would have been painfully slow.

Isn't Carson's speed something like 1.31 x best glide speed?  That's about 125 KIAS for most M20's?  I suppose if you want to maximize your airspeed x fuel economy, you'd fly something like 13,000 MSL at WOT and 2400 RPM.  That would probably be about 125 KIAS.

Edited November 28, 2017 by jaylw314

[nels]

When I did my return from Denver Front Range airport to I69 near Cincinnati a few weeks ago I could have easily done the trip back non stop but opted to refuel near Mexico, Missouri just to pick up cheap fuel. The desent and time killed on the ground was well over two hours. Total flight time was a little over six hours but if I would have gone straight through I'm sure it would have been more like 5.5 hrs. The refuel airport wasn't too far off my course but still had to deviate somewhat to get there and then had to descend and climb back out.

[mike_elliott]

2 hours ago, Bob_Belville said:

(I landed with only about 8 gallons remaining which I would not normally do

you have a small bladder Bob...

 

[bradp]

Do they do time based inspections (like 100 hr) in Canada?

[mccdeuce]

I have never been able to get my J setup to run smooth LOP.  With both magnetos and with one electric. 

Bigger issue I have is I seem to get a fuel smell in cockpit when LOP. Which makes no sense to me on why LOP vs ROP would create this smell. 

I do really like the idea of extended flight with only a few knot loss. 

[Ned Gravel]

16 minutes ago, bradp said:

Do they do time based inspections (like 100 hr) in Canada?

Yup.  100 hrs or annually, whichever comes first.  Just like y'all.

[Hank]

15 minutes ago, Ned Gravel said:

Yup.  100 hrs or annually, whichever comes first.  Just like y'all.

That's only on rental aircraft here. My plane only requires an annual inspection.

[Hank]

30 minutes ago, Ned Gravel said:

Yup.  100 hrs or annually, whichever comes first.  Just like y'all.

Wow, that Canadian "ya'll" slipped right past me! Ned's been traveling, eh?  

[Hank]

@201er, you should google "golden shower". Try it on me, you'll be lucky to run LOP for the next week or so . . . .

On the other hand, reduced fuel burn to make the same trip in the same time is a good thing. But difficult to do in a C.

[Bob_Belville]

1 hour ago, mike_elliott said:

you have a small bladder Bob...

 

Be nice Mike... you'll be old someday... (I limited myself to 1 cup of coffee at breakfast but that wasn't good enough. I'm going to revert to my old practice - a one gallon Prestone antifreeze jug on board for such trips.