Wide Open Throttle??

[Medflyer]

After flying about 100 hours in my '68 M20F I am starting to get the feel of the plane and where it likes to fly as far as MP and RPM.  Looking at the performance charts in the POH and experiencing real life makes me question the WOT technique.  I don't have the POH beside me so the numbers may be a bit off...but the premise is there.  By dropping the MP down to 22" yes, I'll lose about 6 knots of IAS, however, that brings the fuel burn down from 14 to 10 GPH.  I can understand Ricky Bobby wanting to go fast (if you aint first, you're last) mentality...believe me, that's why I went with the Mooney in the first place...but shaving 4 GPH at the expense of 6 knots or so seems to be a great trade-off.  So a three hour trip is now a 3 hour and 10 minute trip saving around $60 in fuel (at $5/gallon).   Just seeing what others think about this.  

Have a great day and safe flying!!

 

[Ragsf15e]

2 minutes ago, Medflyer said:

After flying about 100 hours in my '68 M20F I am starting to get the feel of the plane and where it likes to fly as far as MP and RPM.  Looking at the performance charts in the POH and experiencing real life makes me question the WOT technique.  I don't have the POH beside me so the numbers may be a bit off...but the premise is there.  By dropping the MP down to 22" yes, I'll lose about 6 knots of IAS, however, that brings the fuel burn down from 14 to 10 GPH.  I can understand Ricky Bobby wanting to go fast (if you aint first, you're last) mentality...believe me, that's why I went with the Mooney in the first place...but shaving 4 GPH at the expense of 6 knots or so seems to be a great trade-off.  So a three hour trip is now a 3 hour and 10 minute trip saving around $60 in fuel (at $5/gallon).   Just seeing what others think about this.  

Have a great day and safe flying!!

 

You really can’t talk power settings without the rpm, altitude and mixture. It sounds like you might be flying really low?  Maybe pulling it way back is reasonable down low, but in general, if you want efficiency and don’t care about speed, roll the prop back 2400 or lower and pull the mixture back to lean of peak or farther (if you’re not going anywhere).  The airflow at wot is the most efficient.  If you’re not already lean of peak and prop back, pulling the throttle wouldn’t be my first choice.

[Slick Nick]

What Rags said. Throttle should be the last adjustment you make, after RPM and mixture. Especially when it’s efficiency you’re after, keeping the throttle plate wide open allows the engine to “breathe” with the least restriction. 

[ArtVandelay]

WOT above 6500’, I generally fly 8500’ or above on x-country trips.

[Medflyer]

18 minutes ago, Ragsf15e said:

You really can’t talk power settings without the rpm, altitude and mixture. It sounds like you might be flying really low?  Maybe pulling it way back is reasonable down low, but in general, if you want efficiency and don’t care about speed, roll the prop back 2400 or lower and pull the mixture back to lean of peak or farther (if you’re not going anywhere).  The airflow at wot is the most efficient.  If you’re not already lean of peak and prop back, pulling the throttle wouldn’t be my first choice.

Ragsf15e,  

  Thank you for the reply.  I generally fly in the 7,500-9,500 range with the prop at 2,500.  The lean of peak is an interesting topic as well.  The POH does give a max cruise and max efficiency mixture setting, however, they are both on the rich of peak side, one at 100 degrees and the other 25 degree rich of peak.  Is it common practice to operate lean of peak when the POH only uses rich of peak?  When you mention the airflow at WOT is at the most efficient, what do you mean?   Airflow through the engine?  

 

[KSMooniac]

The POH is *way* out of date and runs counter to prudent engine management in this era with great monitoring instruments.  25-50 ROP at >65% power is the WORST place to run your engine if you care about it.  POH's and performance data were optimistic at best, marketing BS at worst back then, and likely a little bit today.  It was all about bragging rights so numbers/settings were published to get max speed, not most efficient or longest-lasting engine.  Many of us are LOP converts after learning all about it, and yes, running WOT is the most efficient throttle setting in terms of airflow since the butterfly plate obstructs airflow the least when fully open, so you get more air into the engine.  I run WOT in my J (close enough to your F) from takeoff until setting up on an approach or slowing for pattern entry.  There is no good reason to put it anywhere else unless just farting around and/or building time inefficiently.   

[Ragsf15e]

34 minutes ago, Medflyer said:

Ragsf15e,  

  Thank you for the reply.  I generally fly in the 7,500-9,500 range with the prop at 2,500.  The lean of peak is an interesting topic as well.  The POH does give a max cruise and max efficiency mixture setting, however, they are both on the rich of peak side, one at 100 degrees and the other 25 degree rich of peak.  Is it common practice to operate lean of peak when the POH only uses rich of peak?  When you mention the airflow at WOT is at the most efficient, what do you mean?   Airflow through the engine?  

 

The poh power settings are a starting place but we’ve learned a lot about operating these engines efficiently and safely over the past 50 years.  If you have an engine monitor that shows all 4 CHTs and EGTs, lean of peak is going to be your best bet.  My ‘68F poh mentions peak egt as “economy” and that works fine up high where you have pretty low power from the thin air anyway.

So if you have an engine monitor, at those altitudes (which are where i fly) try lean of peak.  You need to ensure you’re just barely lean of the LAST cylinder to peak.  Then check your CHTs.  You’ll findd they are cooler than rich of peak and you are a couple (3ish kts) slower on ~8.8gph compared to your previous 11ish gph.  You can look up the APS course or all the threads on lean of peak ops.  For engine health, keep chts at least below 400 long term.  Cruise less than 380 is what i use.  Lean of peak that is usually easy.

Yes, airflow efficiency is max at wide open throttle and if you’re lean of peak, that really helps efficiency too.

Edited October 14 by Ragsf15e

[Medflyer]

Seriously,

  Thank you all for the replies, they are greatly appreciated!  I figured there was more to the story than a 50 year old operator's manual!  Rags, do you run your prop around 2,400....or are you a 2,500 guy?

[MikeOH]

@Medflyer

What with your observation about trading a little speed for lots of fuel savings, you are already an ideal LOP flyer!

If I'm going somewhere it's WOT from takeoff to when I have to slow for pattern entry.  Usually I run 2550 rpm.  I control power with the red lever  When LOP power is pretty close to 15 x Fuel Flow (really, 14.9).  So, for example, if I set fuel flow to 9 gph I'm making about 135 H.P. which is about 67% power.

[Ragsf15e]

6 minutes ago, Medflyer said:

Seriously,

  Thank you all for the replies, they are greatly appreciated!  I figured there was more to the story than a 50 year old operator's manual!  Rags, do you run your prop around 2,400....or are you a 2,500 guy?

I use 2700 for takeoff and all the climb until I level off, then I normally use 2500.  You can try 2400-2600 to see what works best for your prop/smoothness.  Realize that backing off the rpm will slow you down some as it’s effectively reducing power.

 I’m usually going somewhere far-ish when I fly, so I use just lean of peak or peak to get the most speed and efficiency out of my fuel.  That being said, if you’re up above about 11000’ or higher, getting the most power possible from your engine is probably what you’ll want, so something like 100rop.

[toto]

I’ve always used Bob Kromer’s advice for running my J, and his recommendation is 2500rpm. I always just set it and forget it 

[AndreiC]

I think I may have an explanation for the big difference between 14gph and 10gph with only a modest difference in speed. The POH requires (at least in my 1970 E) full rich above 75% power. So if at WOT at your altitude you’re at 76% power, the manual says to not lean at all, hence a huge fuel flow of 14gph. You back the throttle just a bit, and bam: you’re at 74% power, and the rule now allows you to lean to 100 degrees ROP (per manual) which gets you down to 10gph with only a tiny change in power and speed. 

But this whole dichotomy is there because of the rigid 50 year old rules “above 75% power you must be full rich, below you run at 100 deg ROP”.  These days it seems we understand that you can also lower power with mixture, so that even if your MP at WOT + RPM appear to say you are above 75% power, by reducing fuel flow (through mixture) you can get the engine to put out less power, and thus stay safe. 

For example: at 7500 on a standard day, WOT (23") and 2500 rpm is 76.2% power with a fuel flow (full rich) of 14.0 gph. Running 22" of MP you are just below 75% at 72% power, and can lean to 10.5 gph to be 100 deg ROP. But if you leave throttle wide open and RPM at 2500, but nevertheless lean to flow 9 gph, you’ll be making 9*15 = 135 hp or 67.5%, which is a very safe setting to be at.

The only catch is that not all engines run well (smooth) at such a LOP setting, but most Lycoming IO360s do, and the efficiency is better when there is no throttle plate to mess up with the flow of air.

[Shadrach]

10 hours ago, Medflyer said:

Seriously,

  Thank you all for the replies, they are greatly appreciated!  I figured there was more to the story than a 50 year old operator's manual!  Rags, do you run your prop around 2,400....or are you a 2,500 guy?

Temperature is my top priority for power settings. I manage temps almost exclusively with mixture. I find the IO360 A1A in my 67F feels smoothest if not the quietest at 2500rpm, so that is where I run it in cruise.  I run all the air through the engine that it will take unless I am trying to land. I use mixture to keep temps in a range that is appropriate for the OAT.  For instance, this morning, I flew 3 short legs of 26nm, 50nm and 67nm. Shuttling around the DC metro area. I did each leg at 2500msl except for a brief stint at 3000 to get over a ridge.  OAT at cruising alt was just over 40°. For each leg I was leaned to about 10.6 GPH in cruise. That translates to about 80% power around 25° lean of peak., CHTs all <330°, oil at 180°. Good speed, good economy, good temps and simple operations. This method is easy, only one knob to manage. 

[Pinecone]

You can look up the AVWEB Pelican's Perch articles (by John Deakin) on Lean of Peak Operations.

John was an early proponent of LOP, along with George Braly and Walt Atkinson.  They did a lot of testing and research on the topic back in the 1990s on the old Compuserve AVSIG forum.  And they stirred up a lot of controversy.  They also started APS and taught LOP for many years.  The in person seminars stopped a while ago, but I see that one is being held in Australia, so maybe they will come back.

The funny thing, to me, is that several of the people who are massively pro LOP, started as nay sayers.

I agree that temperatures are important to monitor.  I keep my CHTs under 380 (Continental).  If the temps creep up, I go a bit LEANER (more air, more cool) or crack the cowl flaps open.  I am also turbo, so need to keep my TIT down.

 

[Slick Nick]

For what it’s worth, my home airport is at 4000’ ASL so I’m already down a couple inches of manifold. I usually leave the throttle WFO from takeoff until top of descent. I run 2700 for takeoff, 2600 for climb, and anywhere from 2500-2300 for cruise depending on how high up I am. Then, I lean to go either rich or lean of peak depending on if I’m flying fast or to save gas. Don’t overthink it, you can always change your mind. 

Edited October 15 by Slick Nick

[A64Pilot]

Many things discussed here.

First that WOT is most efficient, the reason is pumping losses, it takes energy to create a vacuum that you have at reduced throttle, that energy is wasted, but how much energy is it? WOT is one reason Diesels are more efficient they are always WOT even are at idle, in fact there is no throttle.

They are Kings of LOP.

It exists, it’s real, but I have tried to measure it with a well instrumented airplane and the difference wasn’t measurable. It’s there but almost certainly the increased fuel mileage from slowing down greatly overwhelms it.

Second that you need an engine monitor or at least four cyl head temps and four egt temps to run LOP. You don’t if you’re after fuel efficiency, Lycoming says power settings below 75% you can’t hurt the engine at ANY mixture. Personally to be extra safe I use 65%. But here is the thing try getting above 65% above 7500 or so while near peak and reduced RPM as in 2300 for me as my engine runs smooth there, no other reason for me to pick it other than she is smooth and quiet, others may be higher or lower. I say if your after fuel efficiency because the biggest difference is power reduction, that saves more fuel than anything else and your not after fuel milage if your high power, yes I understand the theory of as fast as I can be and burning the least amount of fuel. What I am saying is if you value low fuel burn over speed, if you do then you will never cruise above 65% power, unless of course you need the speed, fuel burn be damned.

But to add to that Lycoming has stated in several pubs that for max engine life to cruise at 65% power or lower. I don’t want to buy an overhaul so 65% power or less cruise sounds good to me. Now there is nothing wrong with the idea of “I bought a Mooney to go fast not to save fuel” Nothing at all. But at least the J model can easily fly on less fuel per distance travel than even a Piper Cub, a J and I’m sure others, just I don’t have experience with others can be an extremely fuel frugal aircraft probably as good or better than even a lot of LSA’s. I don’t have any LSA experience either somI can’t give you a list.

Three, lower RPM is more efficient, it is for at least three reasons, the prop is more efficient, there is less frictional loss in the engine, but mostly from reduced engine power, roll back the RPM power drops with it. Again I tried to measure it and really couldn’t but not because it wasn’t there but because unlike a turbine we have no torque meter and without a way of knowing torque output you can’t be certain of power output, you have to go by charts and they aren’t accurate enough to split hairs, and we are splitting hairs here.

The Toyota Prius is or was the King of striving for efficiency, it did several things to eke out tiny increases in efficiency because they all add up. One thing it did was as it had a nearly infinitely variable transmission it strove to operate at full throttle and would pull the RPM down until power made was the power requested by the driver, the Prius spends quite a lot of time at WOT at higher speeds, that gave it both an increase of near zero vacuum in the intake and of course decreased friction from the slower turning engine. So those things are real, but if yiu were Hyper milking a Prius the by a large margin the single biggest effect you could have was to slow down. 

I can get 20 NMPG at 120 kts, but I can’t stand to fly that slow, but I can tolerate 135 pretty well, that gives me 8 GPH and that gives me 16.9 NMPG. Many couldn’t stand 135 kts I’m sure. This is down low because the majority of my flights are short, if I went high then I could do better, but you have to calculate the fuel burned in the climb, and the increased wear at climb power (good luck trying to figure out what that is  )

Edited October 15 by A64Pilot

[Shadrach]

Once you know an airframe/engine combo well, you can just about lean it in your sleep.  I am a big proponent of engine monitors.  However, I can easily lean an IO 360 to LOP without an engine monitor by using airspeed and engine roughness. Could I get the very best out of it, no…but plenty adequate.   It would be much, much, harder and more dangerous to lean to best power without a monitor.

[A64Pilot]

Now on Conti’s and or Lycomings other than the IO-360 you may need an engine monitor, I had everything on my IO-540W1A5D, Gami injectors, tight Gami spread etc and the darn thing just wouldn’t run smooth LOP, while my blueprinted but otherwise stock IO-360 will continue to be smooth so deep LOP that it’s lost too much power, way too deep LOP for efficiency. Very few engines do LOP as well as our IO-360’s.

So we have the poster child for LOP engine, in a very efficient airframe, that’s why I think we can get such good fuel milage if we slow down

On edit The IO-520 in my C-210 did very well LOP, and that airplane could get very efficient if you slowed down, but it too had a Laminar flow wing, maybe that was part of it

On edit, I won’t run best power at higher power, ever, if I’m high power I will be well rich of best power, very little power is lost by going rich, you just waste fuel, that why we takeoff so Rich. Not much power lost but a lot of engine cooling is gained by throwing fuel away.

Only time I would run best power is at high altitude maybe trying to climb in standing wave turbulence or something, but I don’t see myself ever doing that kind of flying again

Edited October 15 by A64Pilot

[Utah20Gflyer]

If I had to choose between reducing throttle or reducing prop rpm I would reduce prop rpm because at least theoretically you would get better performance per gallon of fuel burnt.  It’s two different ways of accomplishing the same thing.  The only reason I wouldn’t is if there was a problem with running smoothly or vibration issues at lower rpm.     I normally cruise between 2300 and 2400.  I very rarely will use 2500. 

[philiplane]

The four cylinder Lycoming camshafts are optimized for 2450 rpm. That would be your optimum cruise RPM setting. 

[A64Pilot]

1 hour ago, philiplane said:

The four cylinder Lycoming camshafts are optimized for 2450 rpm. That would be your optimum cruise RPM setting. 

By optimum and optimized you mean what exactly?

[philiplane]

2 hours ago, A64Pilot said:

By optimum and optimized you mean what exactly?

camshafts are ground to deliver specific power at a desired rpm. No different than street, RV, or race cams for auto engines. The Lycoming cams are designed to get peak torque at 2450.

[Pinecone]

I have also heard that our props (at least the one on my 252) is optimized for 2500 RPM.

I normally cruise at 2300 RPM.  I need to do some testing to compare 2300 and 2500 at the same % power.

I am a big proponent of LOP.  But if you are NA, and flying high, you may need to run at best power mixture to deal with the loss of power due to the thinner air.

[Shadrach]

@Medflyer to build on my earlier comments. The type of flying I was doing yesterday morning illustrates why WOT LOP power settings down low are an excellent option, if one understands how to do it properly. 

For the most part I was cruising at 2500’, wide open throttle, ram air open and 2500rpm. At 2500’ I enjoyed ground speeds in cruise of 178-186kts when eastbound but fighting the winds on the return trip still yielded 112-122kts. All while burning less than 11gph with CHTs <330°. 
I’m increasingly convinced that this is the best way to run an IO360. YMMV.

[varlajo]

1 hour ago, Shadrach said:

For the most part I was cruising at 2500’, wide open throttle, ram air open and 2500rpm. At 2500’ I enjoyed ground speeds in cruise of 178-186kts when eastbound but fighting the winds on the return trip still yielded 112-122kts. All while burning less than 11gph with CHTs <330°. I’m increasingly convinced that this is the best way to run an IO360.

 

 

 

Wait, what? 
This violates (almost) everything I've held sacred! At 2500 ft, my SOP would have me fly at 24"/2400 rpm (KN=48), ram air closed, mixture leaned to 6.9 gph, and observing an IAS of 118 kts. Going to try your way tomorrow, but you owe me an engine if it goes boom