Fuel Flow at Takeoff In a M20J

[romair]

I see about 17.5 on takeoff. Reduce prop to 2500 soon after gear up. I start leaning to about 1250 after first 1000ft and keep that target egt in the climb every 1000ft or so. I transition to lop once I level off.

[N9201A]

I am also curious for the rationale of pulling the prop back in climb.  

 

When I checked out in an Arrow the instructor insisted on this "to never operate over square."  I asked him if he had read Deakin's Pelican's Perch articles on this and when I got a blank stare, I just complied.  When I did my multi training my CFI-ME owned the 310 I used, so I complied with his instructions because it was his airplane.  He at least read Deakin, but just disagreed.  Certain big-engined Contis (B and C brands) have take-off power restrictions, and other prop/engine combos are inordinately loud, so I understand those reasons.  

 

But none of those apply to a J.  I've left the prop firewalled in climb for 16+ years with no apparent adverse effects.  I also "cruise climb" by flattening the deck angle and accepting more speed but less climb angle for more visibility and good cylinder cooling with less drag, but my prop setting is irrelevant to that as one can climb, cruise or descend at 2100 or at 2700 -- not that one would.  

 

I am always interested in why other pilots do what they do, because I am always interested in learning something new.  Can anyone please articulate their rationale for pulling the prop to 2500/2400/2450 in climb?    

[Bob_Belville]

25/2500 was SOP 44 years ago when I started flying Mooneys. Pulled back to those numbers right after raising gear, flaps, & boost pump off. Always throttle first, then prop. We advanced throttle gradually to maintain 25" as we gained altitude.

 

You've probably heard the old story that explains these rules: a little girl who asked her mother why she cut the end off the ham before she put in in the oven. "Honey, because my mother always did it that way."

"But why?" "Let's call her."

"Because that's the way I always saw your grandmother do it."

So the little girl called her great granny who explained that the pan she baked ham in back then was too short.

[Bob_Belville]

Thinking about this a little more, I'm believe we pulled back to 25 square BEFORE we did about anything else... I think we were warned that full power was really hard on the engine. 

[jetdriven]

I was taught that too.  Now we know better. The cylinder pressure is no higher at takeoff power, the fuel flow and higher airspeed provides more cooling, the airplane levels off at cruise sooner which results in lower trip fuel burn.

[philiplane]

Pulling the prop back to 2500 at speeds equal to or greater than Vy results in a speed increase of a few knots with no loss of climb rate. Climbing at full RPM is best limited to Vx climbs where forward speed is less important that absolute Rate Of Climb. 

 

This applies to all airplanes,some more than others. Twins benefit more from the RPM reduction since they have two engines. 

[rocketman]

I see 17.1 GPH on takeoff in my 1982 201. Full rich to about 8,000 ft. I don't mind consuming fuel, like cool cylinders, and no chance of detonation. Maybe too rich, but ...well, it works for me! 1750 TTSN and all graphs, gauges, temps, compressions, and performance numbers are well in the green. 

[Bob_Belville]

Pulling the prop back to 2500 at speeds equal to or greater than Vy results in a speed increase of a few knots with no loss of climb rate. Climbing at full RPM is best limited to Vx climbs where forward speed is less important that absolute Rate Of Climb. 

 

This applies to all airplanes,some more than others. Twins benefit more from the RPM reduction since they have two engines. 

Interesting. The prop in low pitch is less efficient at a relatively low IAS, Vy. Is that charted somewhere?

 

My Scimitar prop is prohibited between 2350 and 2550 when MAP >24", so I routinely fly @ 2550. (Or 2350 now and then for sightseeing or max endurance.) With the relatively poor cowling on my old E I am always concerned with CHT and Oil temps in climb so I am usually climbing at 110 kias or more. Sounds like the 2550 might be better than balls to the wall in the en route climb mode. 

[jetdriven]

Pulling the prop back to 2500 at speeds equal to or greater than Vy results in a speed increase of a few knots with no loss of climb rate. Climbing at full RPM is best limited to Vx climbs where forward speed is less important that absolute Rate Of Climb. 

 

This applies to all airplanes,some more than others. Twins benefit more from the RPM reduction since they have two engines. 

 

 

That's completely wrong. In every airplane I have ever flown, max rated RPM is always going to deliver more horsepower to the crank and a better climb rate, or a higher cruise speed. It feels like it increases speed because it does the moment that you pull the prop back. The rotational energy of the prop turning at 2700 RPM has to go somewhere, so pitch increases to lower RPM. But it stabilizes at a lower airspeed.

The POH also shows this. Do you have anything different?

Explain this scenario. I pretty much always cruise at WOTLOP and 2400-2500 RPM. Often, when given a different altitude in cruise, I simply turn off the altitude hold and set the prop to 2700 RPM. The airplane climbs at the trimmed airspeed. One button and one lever.

 

below is the POH, 21" is the maximum available MP at 10,000'.  The TAS is lower for lower RPM than 2700.

[Bob_Belville]

Byron, the charts indicates significant differences in fuel flow implying the mixture know might be changes as well as the prop to get the charted speed differences.?

[jetdriven]

The fuel flow varies but so does the percentage of power. Manifold pressure and leaning technique in this graph is the same, so the only difference is RPM. Lower RPM makes less power and that results in less airspeed.

[sy308]

In my J, according to the JPI, it is below 16 gph (15.4 the last time I looked at 25 squared) and I am at 700' airport elevation.  I generally don't look at full throttle because I am too busy flying the airplane.  I have GAMI injectors.  I flight plan for 10 gph but it is generally between 8.5 and 9.5 when lean of peak.

 

We checked this morning and it was 17.5-18 gph full throttle just after take off.

[N201MKTurbo]

I checked on my last flight it is 17.7 GPH.

[KSMooniac]

Your fuel flow might be lower than standard due to density altitude in AZ... the RSA-5 adjusts automatically for density...and hot and/or high elevations will "call" for less fuel flow.

[N9201A]

That's completely wrong. In every airplane I have ever flown, max rated RPM is always going to deliver more horsepower to the crank and a better climb rate, or a higher cruise speed. It feels like it increases speed because it does the moment that you pull the prop back. The rotational energy of the prop turning at 2700 RPM has to go somewhere, so pitch increases to lower RPM. But it stabilizes at a lower airspeed.

The POH also shows this. Do you have anything different?

Explain this scenario. I pretty much always cruise at WOTLOP and 2400-2500 RPM. Often, when given a different altitude in cruise, I simply turn off the altitude hold and set the prop to 2700 RPM. The airplane climbs at the trimmed airspeed. One button and one lever.

 

below is the POH, 21" is the maximum available MP at 10,000'.  The TAS is lower for lower RPM than 2700.

Screen Shot 2014-07-08 at 7.41.27 AM.jpg

 

+1!!

 

Another way of testing in cruise:  With aircraft trimmed in level flight, and MP and prop at cruise values, simply reduce RPM 2-300 rpm.  You will descend.  How is that possible if lower rpm = MORE power (which it must if speed increases, then thrust must be increasing).

 

I often initiate a descent by reducing RPM.  Then just reduce MP on the way down IAW your favorite technique...but that's another posting!

[N9201A]

25/2500 was SOP 44 years ago when I started flying Mooneys. Pulled back to those numbers right after raising gear, flaps, & boost pump off. Always throttle first, then prop. We advanced throttle gradually to maintain 25" as we gained altitude.

 

You've probably heard the old story that explains these rules: a little girl who asked her mother why she cut the end off the ham before she put in in the oven. "Honey, because my mother always did it that way."

"But why?" "Let's call her."

"Because that's the way I always saw your grandmother do it."

So the little girl called her great granny who explained that the pan she baked ham in back then was too short.

 

"25 years ago" -- Did you like me have that discussion with someone, between drags on a cigarette, driving to the airport without seat belts...?  It IS progress, right?

 

This is what's great about this forum and pilots interacting generally:  The critical thinking steepens the learning curve and makes those of us willing to invest even a little time much more informed.  There is so much to know even without the Old Wives' Tales (can I say that now or is it Old Spouse's Tales?)

 

My CFI-I checkride I was all fire and brimstone, like the Gordo Cooper portrayal in The Right Stuff.  That examiner had never seen the likes of me before, he was really going to learn something.  About five minutes in he began a conversation about airspeed...easy, right...that slowly (thankfully, still fast enough!) underscored that "there was a whole 'nother can of whupass" to be opened!  

 

Even knowing what I needed to know in order to instruct, I didn't understand it as well it could be understood.  There was a whole other level of understanding beyond what mere competency required.  It was an enlightening experience that for me reinforced how important it is to keep asking questions and getting better.

 

I appreciate everyone in these discussions because enable us to "open that can" and really drill down on what we know...or THINK we know.  And I want to keep learning.