LOP vs ROP RPM

[201er]

What kind of role does RPM and blade angle of the constant speed prop play between LOP and ROP operations?

 

For example it is possible to operate at the same RPM while ROP/LOP but the % power delivered to the prop will be different and thus the blade angle will be set differently by the governor. What effect does that have? Which achieves better prop efficiency? What about better speed?

 

Another observation is that you generally need a higher RPM (when already comparing WOT settings) to achieve equivalent power LOP that you would attain ROP at a lower RPM. How does this affect speed/efficiency? In other words what difference would operating 2500RPM ROP vs 2600RPM LOP? Both might achieve 75% power but which is faster? Which is more efficient (not for fuel but for prop)? Does running higher RPM (2600-2700) for the sake of attaining higher power LOP worthwhile? Or does the prop loose too much efficiency at those speeds?

[KSMooniac]

It is hard to quantify IMO and requires some flight testing for each engine/prop combo.  In my own case, I lost some top speed after upgrading to the MT prop because it appears to be optimized for lower RPM efficiency, and of course running lower power LOP compounds the problem if the goal is top cruise speed.  I am however, more efficient that I was with my old '77 McCauley prop.  My next temp is forced induction so that I can run 30" MP all the time and a 2400 cruise RPM.  

 

If you're really curious, you can do some flight tests at different altitudes and RPMs and see what you get.  You can then develop prop efficiency curves and figure out what is optimum for your goals.  testwest has some great data and modeling for a few different props (including mine now) on our J's, so you might ask him directly.  

[John Pleisse]

Both....LOP Ops and Prop efficiency are a function of altitude. Hmmmmmmm.

[carusoam]

In the O1,

At about 11,000'

LOP 165kts 12gph 2500rpm

ROP 175kts 15gph 2500rpm

I make the settings, blade angle is controlled by the governor...2500rpm is max for an unmodified O1.

More rpm will make more speed at the cost of FF and sound...

Does that help? Did I miss anything?

Roughly speaking, old memory...

Best regards,

-a-

[201er]

LOP 165kts 12gph 2500rpm

ROP 175kts 15gph 2500rpm

In both of these settings you are running the same MP and RPM. However, fuel flow is different and so is power. The governor keeps the prop spinning the same speed either way but it has to compensate for the decrease in power at the lower fuel flow by changing the blade angle. My question is if the change in blade angle by lowering power makes you fly faster or slower? And is it more or less efficient? And why?

 

Likewise what I want to know is if you increased RPM (like in a J) to achieve the same % power LOP that you originally had ROP at lower RPM, would you be flying faster or slower? Better prop efficiency or worse?

 

Who can answer my questions??

[DonMuncy]

At the risk of being chastised by everyone with actual knowledge, I think the question is not about LOP and ROP. Your prop does not know how the power is being generated. 150 HP is 150 HP whether it is ROP or LOP.

Thus it appears that the question is what is the result of a decrease in HP (whether it is by decreasing MP or going LOP).  If the prop speed remains unchanged, the plane will slow down. With less power it has to.

[201er]

Ok, sure. But what about flying 2 different RPM settings at equivalent power? 75% power at 2400RPM vs 75% power at 2600RPM? What about the delay that you get in LOP combustion?

 

From a recent flight it seemed that going 10GPH 75% LOP at 2600RPM was substantially faster than 12GPH 100ROP 75% power at 2400RPM. Yet it's still hard to get this to sink in that I could be going faster AND burning less fuel at the same time??? AND running cooler too! What's the catch?

[DonMuncy]

I went back and realized I had mis-read the question. My guess is that with the same power, your speed will be so close to the same, it won't be significant. But from a purely scientific standpoint, the prop efficiency will have to make some difference. But it will take an aerodynamic engineer, and probably one with a lot of study in prop design to give you the answer.

[201er]

Well I don't need hard numbers. I was just hoping that someone with rhetorical knowledge could clarify the concepts a little. Which is faster, why, etc?

[N1026F]

Understand the basics of leaning at altitude and enriching the mixture at lower altitudes. Then understand that on any given day, the density altitude on takeoff and cruise will require you to adjust your mixture in cruise to obtain the most efficient power/ efficiency profile. The best pragmatic solution is a quality engine monitor; the flip down cheat sheet is a very rough solution.

[jetdriven]

Amazing after 250 hours in the plane and 2 years, I still dont know exactly how much efficiency is from flying LOP or differing RPMs, and how much is from simply flying slower (reduced drag).  I am pretty sure the door has been kicked down on LOP ops, and operating at peak EGT is significantly (8-10%) more efficient than ROP.  you get a lot just leaning to peak EGT below 75% power.   Operating at 15-25 LOP is even more efficient, but the gains are very small from peak EGT. I did take a whole hour, more than once, to quantify this, and after ~100 LOP, the NMPG actually goes down along with speed.  It appears that ~15-25 LOP has the best NMPG accounting for the speed. The total change in NMPG was around 1, from 16.8 to 18.0, this at 50% power.  

 

So, lower RPM.  Sure, the NMPG goes up. But how much is total drag reduction from flying 3 knots slower (you will), and how much is from the mechanical efficiency of the engine and prop. If maximum range is your goal, less of both is going to be ideal.

 

For Mike, I would go up to 8,000, run it at 50 LOP 2500 RPM and measure the IAS, OAT, altimeter setting,  and FF. Then roll the RPM back to 2300 and measure all that again. You will have to offset the speed loss from lowering RPM, by richening the mixture closer to peak, to observe the same speed.  Measure the FF at both power settings that have the same IAS and TAS.  Let us know what you find, I an interested. I would guess you would get a .3 NMPG improvement by lowering RPM from 2500 to 2300.  What I have found is that lower RPM is slower and burns less fuel, it does get 0.5 maybe even 1.0 better NMPG, but you are also flying slower, and some of that gain is from that. You have to factor that out some way.

 

I have spent several hours fooling with this, and I keep coming up with something I learned from Jim R. a year ago. His airplane has only a single point EGT and a fuel flow meter.  Set RPM to 2500, set fuel flow to 8.5 GPH, or less FF and RPM, if I want to go slower. Look out the window.  Enjoy the 17.5 NMPG. If you can;t make your destination with adequate reserves, slow some more.

[1964-M20E]

Byron

 

I like the last part of your statement.  Set all controls to a good point and then sit back and enjoy the flight.

[John Pleisse]

Well I don't need hard numbers. I was just hoping that someone with rhetorical knowledge could clarify the concepts a little. Which is faster, why, etc?

 

This entire thread....a lotta numbers...not one altitude mentioned. Anyway, PM Norm, "Testwest", he'll get you a solid answer, quick.

[201er]

Amazing after 250 hours in the plane and 2 years, I still dont know exactly how much efficiency is from flying LOP or differing RPMs, and how much is from simply flying slower (reduced drag)... I have spent several hours fooling with this, and I keep coming up with something I learned from Jim R. a year ago. His airplane has only a single point EGT and a fuel flow meter.  Set RPM to 2500, set fuel flow to 8.5 GPH, or less FF and RPM, if I want to go slower. Look out the window.  Enjoy the 17.5 NMPG. If you can;t make your destination with adequate reserves, slow some more.

Feeling the same way!

 

As for altitude, it doesn't seem to matter much for LOP. That's another phenomena that baffles me is how you get pretty much the same TAS at different altitudes flying LOP. Lower, you have more air so you can run higher MP. Higher you, have faster TAS but less MP for LOP ops so it's a wash. Why? Dunno, I don't have a grasp on it yet!

[carusoam]

Is there a source for a graph of prop efficiency vs rpm, IAS, ALT

It seems for overall efficiency, I can target...

(1) lower air resistance by selecting a higher altitude. I am up there for gliding distance already, just shy of O2 requirement, for now

(2) increased prop efficiency by selecting a lower rpm. (this is not exact...maybe close?)

(3) increased fuel burn efficiency operating at or slightly below peak. WOT.

(4) selecting a destination that is down wind.

I have a new prop. I have not seen a booklet that should probably come with all those details...

Best regards,

-a-

[Hank]

Too bad #4 isn't always controllable . . . But I did enjoy an actual tailwind on Saturday. It's not often a C-model holds 160+ knots groundspeed. 

[Piloto]

Another factor to consider when optimizing for best nmpg is winds aloft. Here is an example

 

TAS=135kts, Head wind=100kts, GS=35kt, FF=7.0gph, nmpg=5,0, fuel: 60gals, Range=300nm

TAS=150kts. Head wind=100kts  GS=50kt, FF=8.5gph, nmpg=5.9, fuel: 60gals, Range=354nm

 

The above may be an extreme case but not unrealistic above FL200. In some instances when facing a strong headwind the lower speed may not give you the best range and for some planes (C172) you may actually be going backwards. Of course you may try going to lower altitude but when crossing the Sierra Nevada westbound that option may not be available. For performance using TAS is the way to go but for actual range calculation use GS. You do not want to find out when is too late that you may have to ditch.

 

José 

[kmyfm20s]

Ok, sure. But what about flying 2 different RPM settings at equivalent power? 75% power at 2400RPM vs 75% power at 2600RPM? What about the delay that you get in LOP combustion?

 

From a recent flight it seemed that going 10GPH 75% LOP at 2600RPM was substantially faster than 12GPH 100ROP 75% power at 2400RPM. Yet it's still hard to get this to sink in that I could be going faster AND burning less fuel at the same time??? AND running cooler too! What's the catch?

This is me not knowing crap but some props are optimized for a specific RPM and you may be at your optimum RPM at 2600. I actuality I believe it would be blade angle but for how most of us conceptualize it, would be RPM. Since the governed maintains RPM by constantly adjusting blade angle. I think a better comparison would be going ROP at 2600 rpm with less MP to make 75% vs LOP at 2600 rpm and 75%, then do both at 2500 rpm. This would give you the answers your looking for.

[testwest]

So, this is one of those questions that Benchmark is tailor-made to answer. Let's put an M20J at 8000 feet, best power, WOT and RPM pulled back to 2400. You are making 135 hp at 11.1 gph or so. Here is a graph of a McCauley C212 under those conditions:

 

 

Note the thrust horsepower is about 118 hp.

 

Now consider running LOP to get the same power, it takes about 2550 RPM, 9.2 gph and you are still going about 155 knots or so.

 

 

Note the thrust horsepower is exactly the same.

 

Remember, you are much better off picking at altitude where your CAFE parameter is maximized for the wind and route. If you like WOTLOP and 2400 for cruise, just put those numbers into the fltplan.com advanced burn profile and then run the wind matrix button off the flight log page. It's really handy, and it's free.

[carusoam]

Norman,

Do you have any hints for me to access and run this program for a new engine/prop for my Ovation?

Best regards,

-a-

[testwest]

Sure!

 

1. You are going to need a Mac. No way around it. I bought a MacBook Air just for this program, and it has become my favorite laptop by far. I run Parallels and Win7 anytime I need that environment for other programs, like JPI EZTrends.

 

2. Download the program free, here: http://www.seqair.com/benchmark/index.html

There is a MASSIVE amount of information on this website on how to run Benchmark. Be sure to read through all of it before you contact the software author.

 

3. You can perform flight tests for your airplane's performance, or input the perf data from the POH like I did.

 

4. Have fun!

[carusoam]

Thanks Norman,

Now I'll have to ask her if I can borrow her work computer. I'm probably better off buying the new Mac....

Best regards,

-a-

[Cris]

Thanks Norman,

Now I'll have to ask her if I can borrow her work computer. I'm probably better off buying the new Mac....

Best regards,

-a-

Or maybe replace that Ipad one with an Ipad 3? Would that work?

[testwest]

Benchmark runs under the Mac OS, currently OS10.* something or other, named after a large cat. It does not run under iOS like on the iPads. That may be a future development of Benchmark, but such development isn't currently driven by any financial motivation.

[carusoam]

Tense conversation...

Honey, can I borrow your work computer to run a Mooney simulation?

Tip toe tip toe.......

I'd rather go buy an iPad 3 at this point....

-a-