Tag: cruise performance

Flight 18: Cruise Numbers at 2400, 2500, and 2600 RPM

Flight 16 gave us our first real cruise card — a speed-power polar and a GAMI lean sweep at 2500 RPM. The obvious next question: how much does prop RPM matter? On a constant-speed prop you get to pick your cruise RPM, and the folklore says lower RPM is more efficient. Flight 18 set out to put a number on it.

The plan was simple: fly the same two F16 cruise cards again — the speed-power polar (cards 18-2 / 18-4) and the GAMI mixture sweep (18-3 / 18-5) — but at a low RPM (2400) and a high RPM (2600), back to back on the same flight, at the same 7,500 ft MSL I used on Flight 16 so the density altitude is comparable. That puts F16’s 2500 RPM run right in the middle as a reference, and lets me bracket the cruise RPM range with one morning’s data.

Solo, smooth morning air, 07:07 engine start to 08:56 shutdown, 1.8 hours on the Hobbs.

The headline: lower RPM, same speed, less fuel

Here are the three RPMs at the top of the cruise band — wide-open throttle, ~22.7″ of manifold pressure, 7,500 ft:

Prop RPMTASPowerFuel flowEconomy
2400167 kt68%14.8 gph11.3 nm/gal
2500 (F16 ref)168 kt70%18.7 gph9.0 nm/gal
2600169 kt73%18.0 gph9.4 nm/gal

Read the 2400 and 2600 rows — they were flown back to back on this flight, same air, same altitude. For all of 2 knots of true airspeed, pulling the prop back from 2600 to 2400 saved 3.2 gallons per hour — about a 20% improvement in economy (11.3 vs 9.4 nm/gal). On a three-hour leg that’s roughly ten gallons of avgas for no meaningful loss of speed.

And it wasn’t just the top end. At every power setting I tested, 2400 RPM held the same true airspeed as 2600 on less fuel:

Speed-power polars and economy at 2400 / 2500 / 2600 RPM
True airspeed (top) and economy (bottom) vs engine power, for 2400 / 2500 / 2600 RPM at 7,500 ft MSL.

The top panel is true airspeed vs engine power; the bottom is economy (nm/gal) vs power. The 2400 line sits at or above the 2600 line for speed while burning less — so its economy curve rides consistently higher. At a typical ~55% power cruise, 2400 returns about 12.6 nm/gal vs 2600’s 11.2 and 2500’s 10.1.

A caveat on the 2500 line. That data is from Flight 16 — a different day, a touch cooler, and flown full-rich for the whole polar. It brackets the picture but it isn’t a clean same-flight point, and at high power it actually runs richer than either F18 sweep, which is why it sits low on the economy plot. The trustworthy, apples-to-apples result is the 2400-vs-2600 comparison from this single flight, and that one is unambiguous: lower RPM wins.

Leaning: tight injectors, and RPM doesn’t change that

After each polar I ran a GAMI lean sweep — slowly leaning at a held manifold pressure and watching the order in which each cylinder’s EGT peaks. The spread between the first and last cylinder to peak is the GAMI spread, the classic measure of how evenly the fuel injectors are matched.

GAMI lean sweep at 2400 RPM
GAMI lean sweep, 2400 RPM — spread ≈ 0.55 gph.
GAMI lean sweep at 2600 RPM
GAMI lean sweep, 2600 RPM — spread ≈ 0.50 gph.

The result was tight and consistent:

  • 2600 RPM: 0.50 gph spread
  • 2400 RPM: 0.55 gph spread
  • 2500 RPM (F16 reference): ≈ 0.6 gph

All comfortably under the ~1.0 gph rule of thumb for “well-matched” injectors — and notably, the spread barely moved with RPM. The ordering was the same both sweeps too: Cylinder 1 runs leanest, Cylinder 4 richest. Same fingerprint we saw on Flight 16. Nothing to chase here.

And just like Flight 16, it’s worth seeing each lean sweep in the same form as the polar table — fuel flow, power, speed, and economy as the mixture comes back. Watch the Power column: it barely moves (the throttle’s parked), so the leaning alone is buying the economy, and the speed holds right up until the very lean end.

2400 RPM lean sweep (manifold pressure held ~22.6″, 7,500 ft):

Fuel flowPowerTASEconomy
15.9 gph67%165 kt10.4 nm/gal
15.0 gph67%167 kt11.1 nm/gal
14.0 gph67%167 kt11.9 nm/gal
13.0 gph68%166 kt12.8 nm/gal
12.5 gph67%165 kt13.2 nm/gal
12.0 gph67%162 kt13.5 nm/gal
11.1 gph66%156 kt14.1 nm/gal

2600 RPM lean sweep (manifold pressure held ~20.5″, 7,500 ft):

Fuel flowPowerTASEconomy
16.3 gph66%164 kt10.1 nm/gal
15.0 gph66%164 kt10.9 nm/gal
14.0 gph64%162 kt11.6 nm/gal
13.0 gph65%162 kt12.5 nm/gal
12.5 gph65%161 kt12.9 nm/gal
12.0 gph65%157 kt13.1 nm/gal
11.5 gph66%156 kt13.6 nm/gal

The same trend the polars showed turns up here too: at a matched ~67% power, the 2400 sweep holds ~166 kt while leaning all the way down, bottoming out near 14 nm/gal — a touch better than 2600’s best of ~13.6. Lower RPM keeps winning, lean or rich.

Health check: cool, clean, and steady

  • Cylinder head temps: the hottest airborne reading was 412°F (CHT2, during the climb); the cruise sweeps stayed at or below 399°F. Everything well under the 450°F redline.
  • Carbon monoxide: cabin CO peaked at just 2 ppm (mean 0.03) — benign. Worth noting because Flight 16 read a flat zero all flight, which I’ve flagged as a detector anomaly; Flight 18’s normal, non-zero trace confirms the detector is alive and reading.
  • Attitude (AHRS): another flight since Garmin replaced the second AHRS unit, and it stayed rock-solid — cross-unit pitch differences ≤ 5.4°, roll ≤ 3.3°, and zero of the big deviation spikes that plagued the airplane before the swap.
Flight 18 ground track
Flight 18 ground track (KHEF test area).

All three sweeps, side by side

To put the three lean sweeps in one place — each cell stacks three values: economy (nm/gal), TAS, and mixture (degrees F rich or lean of peak EGT, ROP / LOP), at that fuel flow, all at 7,500 ft. A dash means that sweep didn’t reach that fuel flow:

Fuel flow2400 RPM2500 RPM2600 RPM
18.0 gph——9.3 nm/gal
166 TAS
178 ROP
17.5 gph——9.4 nm/gal
166 TAS
178 ROP
17.0 gph——9.9 nm/gal
168 TAS
176 ROP
16.5 gph—9.9 nm/gal
161 TAS
173 ROP		10.1 nm/gal
164 TAS
175 ROP
16.0 gph10.4 nm/gal
165 TAS
147 ROP		10.2 nm/gal
162 TAS
164 ROP		10.2 nm/gal
163 TAS
168 ROP
15.5 gph10.7 nm/gal
166 TAS
135 ROP		10.6 nm/gal
164 TAS
143 ROP		10.5 nm/gal
163 TAS
153 ROP
15.0 gph11.1 nm/gal
167 TAS
114 ROP		11.0 nm/gal
163 TAS
122 ROP		10.9 nm/gal
164 TAS
131 ROP
14.5 gph11.4 nm/gal
166 TAS
91 ROP		11.3 nm/gal
163 TAS
99 ROP		11.2 nm/gal
163 TAS
105 ROP
14.0 gph11.9 nm/gal
167 TAS
66 ROP		11.6 nm/gal
163 TAS
71 ROP		11.6 nm/gal
162 TAS
79 ROP
13.5 gph12.2 nm/gal
166 TAS
36 ROP		12.3 nm/gal
163 TAS
41 ROP		12.0 nm/gal
162 TAS
50 ROP
13.0 gph12.8 nm/gal
166 TAS
11 ROP		12.5 nm/gal
163 TAS
12 ROP		12.5 nm/gal
162 TAS
23 ROP
12.5 gph13.2 nm/gal
165 TAS
peak		13.1 nm/gal
161 TAS
peak		12.9 nm/gal
161 TAS
4 ROP
12.0 gph13.5 nm/gal
162 TAS
9 LOP		13.4 nm/gal
160 TAS
2 LOP		13.1 nm/gal
157 TAS
peak
11.5 gph13.7 nm/gal
157 TAS
26 LOP		13.7 nm/gal
157 TAS
18 LOP		13.6 nm/gal
156 TAS
2 LOP
11.0 gph14.1 nm/gal
156 TAS
43 LOP		14.0 nm/gal
154 TAS
28 LOP		—
10.5 gph13.9 nm/gal
147 TAS
49 LOP		——

Each cell stacks economy (nm/gal), TAS, and mixture (degrees F rich or lean of the engine-average peak EGT). Only the 2600 sweep was run up into the rich 17–18 gph range; only 2400 was leaned past 11 gph (the dashed corners). Since this engine runs a conventional left mag against the SDS electronic ignition on the right, treat the absolute ROP/LOP numbers as approximate; the trend across the sweep is what matters.

Read the table by the second value in each cell — true airspeed — and 2400’s advantage is plain: at any fuel flow from about 16 down to 12 gph, the 2400 sweep holds roughly 3–5 knots more TAS than 2600 for the same fuel burn (and a couple of knots over 2500), at equal-or-better economy. The sweet spot is a lean cruise around 12.0–12.5 gph — right at peak EGT or a hair lean of it (see the mixture value) — where 2400 returns ~13.2–13.5 nm/gal and still makes 162–165 kt. You’ve leaned into the efficient range and barely given up any speed, while at that same fuel flow 2600 is down near 157–161 kt. The three only converge at the very lean end (~11.5 gph and below), where speed tails off for all of them. Lean or rich, the lower RPM keeps the airplane moving faster on the same gas — and ~2400 RPM at ~12–12.5 gph looks like the efficient-cruise sweet spot.

Bottom line

If you fly an RV-10 (or really any constant-speed-prop airplane) and you want range, the cheapest “modification” available is your right hand on the prop control: pull the RPM back. On N997CZ, 2400 RPM cruises at essentially the same speed as 2600 while burning about 3 gph less. The injectors are well matched at any RPM, the engine stays cool, and nothing about lower-RPM cruise costs you anything but a few knots.

Next I’d like to repeat this in smoother, more stable air and tie it to a fuel-flow endurance number — but the trend is already clear enough to change how I set cruise.

Test cards flown: Flight 18 (cards 18-1 through 18-5). Cruise analysis and charts generated from the G3X data log. See also Flight 16: Cruise Numbers and Three Healthy Horizons for the 2500 RPM baseline.

N997CZ — Flight 16: Cruise Numbers, and Three Healthy Horizons

N997CZ's Flight 16 ground track — long, straight test legs for stabilized cruise points
N997CZ’s Flight 16 ground track — long, straight test legs for stabilized cruise points

After the climb sortie, Flight 16 turned to the next block in the Phase 1 deck: cruise performance and leaning. Pick one altitude, hold it dead steady, and step through power and mixture settings, logging speed and fuel flow at each stabilized point. The ground track tells you what that looks like from above — long, patient straight legs instead of the racetracks and sawtooths of the maneuvering flights.

It was also quietly historic for a different reason: this was the first flight with both attitude units overhauled (more on that below).

The Numbers

Date 2026-06-13 (morning)
Engine time ~1.7 hr
Engine hours 22.2 → 23.9
Test altitude 7,500 ft MSL, 2500 RPM (density alt ~8,900 ft)
Max altitude ~7,550 ft MSL (7,905 ft GPS)
Fuel used 22.7 gal (totalizer) — matched the truck receipt (21.70 gal) to ~1 gal
Cards flown 16-1 (cruise set-up), 16-2 (speed-power polar), 16-3 (mixture sweep)
Conditions cool morning (cruise OAT +55 °F)

📄 Test cards: Flight 16 test cards (PDF)

The Speed-Power Polar

The heart of the flight: hold 7,500 ft and 2500 RPM, then step the throttle down through a range of manifold pressure, letting the airplane stabilize at each setting. The stabilized points (power is the G3X’s own computed engine percentage):

MAP Power Fuel flow TAS Economy
22.5″ 70% 18.7 gph 168 kt 9.0 nm/gal
21.5″ 67% 17.6 gph 164 kt 9.3 nm/gal
20.5″ 64% 16.8 gph 162 kt 9.6 nm/gal
19.3″ 60% 15.9 gph 156 kt 9.8 nm/gal
17.9″ 55% 14.5 gph 149 kt 10.3 nm/gal
16.9″ 52% 13.7 gph 141 kt 10.3 nm/gal
14.8″ 46% 12.3 gph 133 kt 10.8 nm/gal

It’s the classic trade, made concrete: near wide-open (~22.5″, 70% power, 18.7 gph) the airplane trues a brisk 168 knots but returns only 9.0 nm/gal; pull the throttle back toward 15″ (about 46% power) and you give up roughly 35 knots of true airspeed to gain about 20% in fuel economy — up past 10.8 nm/gal. At a fixed mixture, every extra knot of speed costs efficiency: the balance every cross-country pilot strikes.

Two caveats before anyone quotes these figures. This is not the airplane’s top speed. Every point was flown at a fixed 2,500 RPM — not the 2,700 the engine turns at full power — and at 7,500 ft with the mixture full rich, so even the 168-knot top row is leaving speed on the table compared to full throttle, full RPM, and a more efficient altitude. This is a controlled comparison, not a speed record. And it is not best economy. Full rich is the thirstiest way to make any given power; the real fuel savings come from leaning, which is exactly what the next card went after. Read the table for the shape of the power-speed-fuel trade at one condition, all else held equal — not for the best the airplane can do at either end.

Here’s the whole sequence as flown — throttle stepped down in stages at a fixed altitude, with the airspeed settling out at each new power setting:

Flight 16 speed-power polar — MAP and fuel flow (dashed, right axis) stepped down while IAS and TAS (solid, left axis) bleed off
Flight 16 speed-power polar — MAP and fuel flow (dashed, right axis) stepped down while IAS and TAS (solid, left axis) bleed off

The Lean Sweep (GAMI Spread)

Then the part I’d been looking forward to: card 16-3, the mixture sweep. Hold the airplane at 7,500 ft and 2500 RPM, leave the throttle alone (manifold pressure parked at ~21.3″, wandering maybe ¾ of an inch), and pull the mixture back slowly — about 16 down to 11 gph — while the G3X logs all six cylinders’ exhaust gas temperatures. Each cylinder’s EGT climbs to a peak and then falls; the fuel flow at which it peaks tells you how rich or lean that cylinder runs relative to the others. The spread between the first and last cylinder to peak is the GAMI spread — the headline number for how well your fuel injectors are matched.

Flight 16 GAMI lean sweep — six EGTs (solid, left axis) rise, peak (★), and fall; CHTs (dashed, right axis) overlaid. GAMI spread ~0.6 gph
Flight 16 GAMI lean sweep — six EGTs (solid, left axis) rise, peak (★), and fall; CHTs (dashed, right axis) overlaid. GAMI spread ~0.6 gph

The result is a good one: a GAMI spread of about 0.6 gph. Cylinders 1, 2, 5, and 6 peak first (around 12.5 gph — they run slightly leaner), and Cylinders 3 and 4 peak last (around 11.9 gph — slightly richer). Anything under ~1 gph is generally considered well-matched and capable of smooth lean-of-peak operation, so this engine’s injectors are in good shape right out of the box.

The dashed lines on the chart are the cylinder head temperatures (right axis), and they tell their own reassuring story. As expected, each CHT peaks just slightly rich of its EGT peak, and the hottest any head got during the entire sweep was about 407 °F (Cylinder 2) — comfortably below limits the whole time, even at the richest, highest-EGT settings. Leaning this engine doesn’t cook it. (One honest note on method: the curves are read straight off the logged data; with a conventional left mag and the SDS electronic ignition on the right, the absolute EGT picture carries that timing asymmetry, but the relative peak ordering — which is what the spread measures — is robust.)

And here’s the bonus the sweep makes vivid: leaning buys efficiency far more cheaply than throttling back does. Hold the same ~21″ of manifold pressure the polar started at, and instead of closing the throttle, just lean the mixture — the airplane still trues about 163 knots on 13.3 gph, a tidy 12.3 nm/gal (and leaner still, up to ~14 nm/gal). Back on the speed-power polar it took ~18.7 gph of throttle to make that same ~163 knots, at just 9.0 nm/gal. Same speed, roughly a third less fuel. That’s the whole point of leaning, made concrete.

Here’s the sweep in the same form as the polar table above — but watch the Power column, because that’s the whole story. On the polar, economy only improved as power (and speed) came down. Here power holds at ~65% the entire time; leaning alone buys the efficiency, and the speed barely moves until the very lean end:

Fuel flow Power TAS Economy
16.0 gph 64% 162 kt 10.2 nm/gal
15.0 gph 66% 163 kt 11.0 nm/gal
14.0 gph 66% 163 kt 11.6 nm/gal
13.3 gph 66% 163 kt 12.3 nm/gal
12.5 gph 65% 161 kt 13.0 nm/gal
11.5 gph 66% 157 kt 13.7 nm/gal
11.0 gph 65% 155 kt 14.0 nm/gal

(All held at ~21″ MAP and 7,500 ft — the same condition as the GAMI sweep above.)

Cooling and CO

Nothing dramatic, which is the goal in cruise. CHTs stayed comfortable — Cylinder 5 the hottest at 421 °F, everyone else lower, all well under limits. Cabin CO read essentially zero the whole flight. That’s roughly what you’d expect from a stabilized cruise with no sustained slow flight — this airplane’s CO ingress shows up in high-angle-of-attack work, which wasn’t on today’s card — but a flat zero earns an eyebrow rather than a victory lap: Flight 13 also read zero, yet the same detector logged a normal small reading on Flight 17 later the same day. Whether F16’s zero is genuinely clean air or a detector that simply wasn’t reading is still on the verify list.

Three Healthy Horizons

Here’s the quiet milestone. The overhauled second attitude unit (AHRS #2) was installed before this flight, so Flight 16 is the first time N997CZ has flown with both ADAHRS units overhauled. The verification was clean: through the whole flight, from takeoff roll to landing, the two units disagreed by less than a degree in roll, there were no re-aligns, and — for the first time in a long time — zero attitude or heading miscompare annunciations. After a long-running saga, the airplane finally has three attitude opinions that all agree. The full story is in the AHRS post.

After Shutdown

Two panel photos from the ramp after the flight — documentation of the final engine state, not in-flight readings:

G3X engine page after shutdown — the sortie's fuel used and economy at a glance
G3X engine page after shutdown — the sortie’s fuel used and economy at a glance
G3X engine temperature page after shutdown — all six CHTs and EGTs, cooled down
G3X engine temperature page after shutdown — all six CHTs and EGTs, cooled down

Bottom Line

A clean set of cruise numbers (12+ nm/gal on the table), a tight ~0.6 gph GAMI spread that says the injectors are well-matched, cooling with margin to spare, and a verified-healthy attitude system. A productive morning’s worth of test cards. Next: the rest of the systems and performance cards.

— Jim