What are your cruise fuel flow settings for your Turbo?

n86121 · #1 02-04-23, 09:17 AM

Last Sunday was cold and rainy, so I started compiling various engine and airframe performance charts into something useful.

While there are billions and billions of tables, does anyone really fly using them?

I think in reality we

1) set manifold pressure, then
2) RPM, then
3) lean, ...and away we go!

Beyond that is just obsessive diddling with things

----

One 'constant' is INDICATED airspeed at SEA LEVEL for a given power setting.

From that INDICATED airspeed one can then use math to determine TRUE airspeed, adjust for pressure altitude, temperature etc. But we can only adjust power for a shown IAS.

The TCM engine manual has tables for rich to lean, for example.

The table below shows the limits from the TCM engine manual for my TSIO 360ABs

QUESTION #1: With GAMI do you lean BELOW the 'lean' values in the TCM manual? or...

QUESTION #2: With GAMI do you just run smoothly at the SAME TCM lean settings?

Below is the user table I derived from the various tables and manuals.

Next time I have time to spare I will flight check reality vs tables.

The efficiency values to right are MPG and speed, vs baseline of 75% at ROP.

For example, running the engine 75% power LOP would get you 20% better MPG than ROP with no loss in speed. Running 65% power LOP is 24% better MPG, with a loss in speed of 7%, etc.

Comments? Comparisons?

SET IAS @ altitude GAL/HR KNOTS/GAL LOP LOP
MP RPM PWR MPH KTS 5k 10k 15k ROP MID LOP ROP GPH LOP +EFF -SPD
32 28 100% 178 155 - - - 42 40 ---
28 25 75% 173 150 160 169 177 28 24 23 5.4 6.3 6.5 20% ---
24 70% 169 147 156 165 175 26 23 22 5.7 6.4 6.7 24% 2%
26 26 70% 169 147 156 165 175 26 23 22 5.7 6.4 6.7 24% 2%
24 65% 161 140 150 159 169 24 23 21 5.8 6.1 6.7 24% 7%
22 55% 151 131 143 152 162 21 19 18 6.2 6.9 7.3 35% 13%
24 22 50% 143 124 135 144 155 18 17 16 6.9 7.3 7.8 44% 17%

mshac · #2 02-06-23, 12:09 PM

I've never given it the level of thought shown in your post, but I flight plan 26 gph total in my P337H. That's at 75% where I usually get pretty close to the book cruise numbers. Close enough anyway.

I do not have GAMIs.

I fly pretty much the way you described: I set FF via the JPI to about 13 GPH each initially, then "diddle" with it as the flight progresses. The JPI has a "Lean Find" feature that I find very un-intuitive, so I hardly ever use it. I will lean back as I get to altitude if I can keep my CHTs under about 380 and turbine inlet temps under 1500.

I just had the baffling repaired on the rear engine and haven't yet made any flights of consequence, but before I had to give the rear a bit more fuel in order to maintain the same CHTs. I would often fly with FF of 12.5 up front and 13.5 for the rear. We will have to see if the baffling fixes change that.

Ryann · #3 02-08-23, 05:34 PM

Tangentially related, anyone else try seeing what the minimum possible fuel flow while staying aloft is for your plane? I needed to build a bunch of hours for insurance reasons while not going anywhere in particular very quickly, so I spent some time playing with this and got it down to about 12.5 GPH total (10500', 6.5 GPH front, 6 GPH rear at about 21" and 2250 RPMs). Couldn't get it much leaner without intermittent missing. 1975 P337G, no GAMI injectors on here. Photos attached showing EDM-960 display and airspeed.

Single engine with the front prop feathered at 10500' (while loitering over a rural airport with a 5000' runway) actually took more GPH to maintain altitude than with both engines running.

-Ryan

mshac · #4 02-08-23, 05:44 PM

That's good info on the min fuel flow. I see you were doing C172 type speeds but with an extra 50% fuel flow (vs the 8 gph of the 172).

I have the EDM 760 FF gauge installed. My understand is the 960 is a plug-and-play replacement for the 760 (won't fit in the same mounting spot though!) I think with trade-in, the upgrade only costs about $1500! I would have already done it, but it would require almost a complete redesign of my panel, so I've held off. Would LOVE to have one though!

I bet you could cut it down even more by caging the front engine. I hear that the Skymaster is the only airplane that EXTENDS its range when operating on only one engine!

Ryann · #5 02-08-23, 06:07 PM

Found the photos from my single engine minimum fuel flow test. Photos attached, looks like I ended up at 14.7 GPH on the rear engine to just barely maintain altitude close to Vx.

I'll try it again during the winter months sometime soon and see if I can get it down any further with cooler air available. And maybe see if I can maintain altitude at Vy for a little less drag.

mshac · #6 02-09-23, 01:38 PM

Thanks for the pics Ryann! I always say, if there's no pics then it didn't happen!

n86121 · #7 02-11-23, 03:37 PM

Once upon a time, I heard that a three letter agency has a 337 that provides top cover comms coordination, during certain circumstances, that powers waaay back, with low RPM, and goes for hours and hours and hour