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 17: Harry’s First Flight

N997CZ on the ramp at KHEF, ready for Harry's first flight
N997CZ on the ramp at KHEF, ready for Harry’s first flight

Three owners, three pilots — and as of this afternoon, all three have now flown the airplane. Flight 14 was Dan’s first flight; Flight 17 was Harry’s. Harry is no stranger to the family — he’s a partner in our RV-7 too, and a seasoned RV hand — but this was his first time at the controls of the RV-10. I flew the morning’s cruise-and-leaning card solo (Flight 16); in the afternoon I handed N997CZ over, stood out on the ramp with the camera, and watched the third member of the partnership take our airplane up.

The Numbers

Date2026-06-13 (afternoon)
Engine time~1.3 hr
Engine hours23.9 → 25.2
Max altitude~4,545 ft MSL (4,795 ft GPS)
Fuel used14.9 gal (totalizer)
Profilefamiliarization — no test cards
Conditionswarm afternoon (cruise OAT +63 °F)

No test-card link this time — like Dan’s flight, this was a familiarization sortie, flown rich and relaxed. The cards stayed in the binder.

From the Ramp

Harry took his time with it — a long, unhurried run-up and taxi before rolling, a good 24 minutes of ground time before brake release. That’s exactly how a first flight in a new-to-you airplane should go, and no surprise from someone who already knows his way around an RV.

Harry in the left seat of N997CZ before his first flight in the -10
Harry in the left seat of N997CZ before his first flight in the -10
Harry taxis out for his first flight in N997CZ

The Flight

Harry’s first takeoff in the RV-10

A proper get-comfortable hop: a cruise out to the southwest toward the Culpeper/Orange practice area and back, mostly down low (max ~4,500 ft MSL) and quick — about 167 knots true in cruise, full rich, no leaning. Nothing aggressive: no stalls, no steep stuff beyond gentle 30°-bank turns. Just stick time, getting the feel of the airplane’s weight, trim, and sight picture.

The ground track is a clean out-and-back — none of the test-pattern geometry of a card flight, just a flight with somewhere to go and back.

N997CZ's Flight 17 ground track — an out-and-back to the southwest toward Culpeper and Orange
N997CZ’s Flight 17 ground track — an out-and-back to the southwest toward Culpeper and Orange

A couple of systems notes from the ride. Cabin CO peaked at a benign 6 ppm during the low cruise — worth a mention because it confirms the detector was reading normally, which retroactively makes the flat zero on the morning’s Flight 16 the genuine oddity to keep watching. CHTs ran warm-but-fine on the rich, low-altitude profile (hottest cylinder 430 °F), and the attitude system stayed rock-steady the whole flight — fitting, since this airplane’s long-running attitude saga had finally closed out that same day, with all three sources verified healthy.

Harry taxis in after his first flight in N997CZ

Bottom Line

The whole partnership is now checked out in N997CZ. Harry has the airplane in his logbook, it behaved exactly as it should, and Phase 1 rolls on.

— Jim

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

N997CZ — Flight 15: Climb Performance (the Sawtooth)

N997CZ Flight 15 ground track — the day's flight over the Phase 1 test area
N997CZ Flight 15 ground track — the day’s flight over the Phase 1 test area

Flight 15 was the longest sortie of the program so far — three hours on the Hobbs — and it had one main job: measure how fast this airplane climbs, and at what speed it climbs best. That means a sawtooth: a stack of timed full-power climbs, each flown at a different airspeed, so the rate-of-climb-versus-speed curve falls out of the data.

It also carried over the one stall card Flight 13 didn’t get to — the accelerated/turning stall — so this post has a little of everything.

The Numbers

Date 2026-06-08 (morning)
Engine time ~2.9 hr (longest yet; ~3.0 hr Hobbs)
Engine hours 19.1 → 22.0
Max altitude ~7,700 ft MSL (7,961 ft GPS)
Fuel used 36.1 gal (totalizer) — matched the truck receipt (36.40 gal) to 0.3 gal
Cards flown 15-1 (accel/turning stall), 15-2 (climb set-up + cooling), 15-3 (sawtooth climbs)
Conditions cool-ish morning (cruise OAT +61 °F)

📄 Test cards: Flight 15 test cards (PDF)

The Sawtooth

The plan: two full-power climbs at each of 120, 110, 100, 90, 80, and 75 KIAS — twelve timed test climbs in all — each one taken through the same 4,000-to-7,000 ft band so the runs are directly comparable. Flown back to back, the altitude trace draws the maneuver’s namesake: a row of teeth, each a full-power climb up through the band followed by a descent to set up the next.

Flight 15 climb-test altitude profile — the literal sawtooth: each tooth a full-power climb through the 4,000–7,000 ft band, labeled by target KIAS
Flight 15 climb-test altitude profile — the literal sawtooth: each tooth a full-power climb through the 4,000–7,000 ft band, labeled by target KIAS

Timing each of those climbs gives the headline result — rate of climb against indicated airspeed:

Rate of climb vs indicated airspeed — raw sawtooth result (weight-biased; see text)
Rate of climb vs indicated airspeed — raw sawtooth result (weight-biased; see text)

Read the raw numbers and the airplane looks like it climbs best slow: roughly 1,000 fpm at 120 KIAS rising to ~1,470 fpm near 80 KIAS. But there’s a catch, and it’s an important one. The runs were flown fast-to-slow over two hours, so by the time we got to the slow climbs the airplane was lighter — it had burned an hour-plus of fuel. A lighter airplane climbs better, so some of that “80 knots is best” result is really just “the airplane weighed less by then.”

The honest takeaway: the raw best-rate speed sits around 80 KIAS, but after correcting for the weight change, Vy is more likely up around 85–90 KIAS. Pulling the clean, weight-normalized Vx/Vy out of this dataset is a data-reduction job still on the bench — a good subject for its own post.

Cooling Held

Climb testing is where you find out whether your cooling can take sustained full power. The card set hard limits — back off if any CHT hit 420 °F, level off and enrich at 450 °F. We never got close: every climb topped out between 366 and 411 °F, with Cylinder 5 the hottest (411 °F), exactly as the air-dam canary from earlier flights predicted. No aborts, plenty of margin.

The Carried Stall

Before the climbs, we cleaned up unfinished business from Flight 13: the accelerated / turning stall (card 15-1). Three banked breaks in each direction at about 25° of bank, breaking around 60–65 KIAS with the nose up and the G unloaded at the break. Textbook, and the last stall card in the deck is now checked off.

Cabin CO stayed benign all flight (peak 2 ppm), and the new overhauled AHRS #1 behaved — though the other attitude unit had a tell on this flight, which is a story told in full in the AHRS post.

After Shutdown

A few panel photos from the ramp after the flight — documentation of the day’s totals, not in-flight readings:

The G3X engine page after shutdown — CHTs already settling into the 320s–350s following the three-hour sortie
The G3X engine page after shutdown — CHTs already settling into the 320s–350s following the three-hour sortie
The Fuel Calculator at shutdown — 36.1 gallons used, matching the truck receipt to a third of a gallon
The Fuel Calculator at shutdown — 36.1 gallons used, matching the truck receipt to a third of a gallon
The PFD's ADAHRS source page at shutdown — three attitude sources, all in agreement at the end of a long day
The PFD’s ADAHRS source page at shutdown — three attitude sources, all in agreement at the end of a long day

Bottom Line

Three hours, twelve timed climbs, a carried stall finished, and cooling that never blinked. The sawtooth is in the can; the weight-corrected Vx/Vy reduction is the homework. Next up: cruise performance and leaning.

— Jim

N997CZ — The Attitude Problem: Convincing Garmin, Twice

If you’ve been following along since the early flights, you know this airplane has had a recurring character in its story: the tumbling artificial horizon. ATTITUDE and HEADING MISCOMPARE annunciations, deviation numbers pegged at their limits, an attitude display that would occasionally roll over and play dead during the takeoff roll or hard maneuvering. This is the post where that story gets its arc — how we isolated the problem with flight data, convinced Garmin to overhaul one attitude unit, proved the fix, and then used the same data to convince them about the second unit.

The Setup

N997CZ carries three independent attitude sources: two Garmin GSU 25C ADAHRS units (AHRS #1 and #2) behind the panel, and a G5 electronic standby. Three opinions about which way is up, continuously cross-compared — when they disagree, the system annunciates a miscompare and the pilot gets to wonder which box to believe.

From the first flights, the disagreements clustered in the high-vibration, high-acceleration regimes: the takeoff roll above all, plus stalls and slow flight. Two suspects emerged early: vibration (the prop was genuinely out of balance), and the attitude units themselves.

Fix One: Balance the Prop

The dynamic prop balance took vibration from 0.57 IPS down to 0.01 — a 50-fold reduction — and it helped every attitude source. Looking only at the big deviation excursions (reported deviation above 200%, takeoff roll through touchdown), all three sources improved markedly after the balance.

But it didn’t eliminate the problem. The large excursions kept coming, and they kept coming disproportionately from one box.

ADAHRS deviation by era — the worst unit flips from AHRS #1 to AHRS #2 at the swap
ADAHRS deviation by era — the worst unit flips from AHRS #1 to AHRS #2 at the swap

That’s the chart that tells the whole story, so let’s read it. Each group is an era: (A) original prop, original AHRS #1; (B) balanced prop, original AHRS #1; (C) balanced prop, new AHRS #1 with original #2; (D) both units overhauled — but we’re getting ahead of the story. The prop balance (A→B) drops everyone. But within every era, somebody is the outlier — and in eras A and B, it’s AHRS #1. Not just on average: AHRS #1 was the highest-deviation source on every single flight from 5 through 12. On Flight 12 it saved its best for last — in-flight re-aligns totaling almost two minutes, and a cross-source roll disagreement of roughly 136°.

Fix Two: Replace AHRS #1

Garmin agreed to overhaul AHRS #1, and the replacement unit went into the panel on June 6 — the same day as Flight 12’s fireworks. The before/after is about as clean as flight test data gets:

  • In-flight re-align events (“AHRS1 ALIGN”): 350 seconds total across Flights 5–12 → zero on Flights 13, 14, and 15.
  • Maximum cross-source roll disagreement: ~136° → under 10°.
  • Extreme deviation excursions (>500%): eliminated.
  • Attitude-horizon tumbles: Flight 13 was the first flight in the airplane’s life without one, and there hasn’t been one since.

Case closed on AHRS #1. Which is exactly what made the remaining data interesting.

The Plot Twist: The Baton Pass

Look at era C in the chart above. With the new AHRS #1 installed, the outlier didn’t disappear — it moved. AHRS #2, the original, never-serviced sister unit, became the highest-deviation source on every flight 13 through 15. The worst-of-three title passed from #1 to #2 on the exact flight the new unit went in.

In-flight ADAHRS re-align events per flight — after the swap, the re-aligns shift to AHRS #2
In-flight ADAHRS re-align events per flight — after the swap, the re-aligns shift to AHRS #2

Then Flight 15 produced the smoking gun. AHRS #2 — which had never logged an in-flight re-align in the airplane’s entire history — dropped its attitude solution and re-aligned for a cumulative 118 seconds, with the system annunciating “USING AHRS1” while the brand-new #1 carried the load. The roles from Flight 12 had exactly reversed.

Better still, from a diagnostic standpoint: the failure is reproducible on demand. It triggers in high angle-of-attack, full-power slow climbs — on Flight 15 it recurred in roughly eight separate events inside a single 30-minute window of climb-performance testing, all at a median of about 89 knots, nose up, full power. A fault you can demonstrate on command is a fault nobody has to take your word for.

Convincing Garmin, Round Two

All of the above went into a short data report to Garmin — the era comparison, the re-align timeline, the reproducibility recipe — with a simple argument: AHRS #2 now exhibits the same in-flight signature AHRS #1 exhibited before its overhaul fixed it; we respectfully request the same service. The analysis we sent Garmin — since updated with the post-replacement verification data — is here, for anyone fighting a similar battle: N997CZ GSU 25C ADAHRS Deviation Analysis (PDF).

Garmin agreed. The replacement AHRS #2 is in hand.

Fix Three: Replace AHRS #2 — and the Verification

Garmin’s overhauled AHRS #2 went into the panel after Flight 15. *Flights 16 and 17 (June 13) are the first this airplane has ever flown with both attitude units overhauled* — the verification flights I’d been waiting to write about.

The data did for #2 exactly what it did for #1. Measuring only the part of each flight that counts — from the start of the takeoff roll through touchdown, with the power-on/alignment window excluded — here is the before-and-after:

  • ATTITUDE / HEADING MISCOMPARE annunciation time: Flight 15 logged 346 seconds of attitude miscompare and 209 seconds of heading miscompare. Flights 16 and 17 each logged zero of both. These are the very annunciations that had survived the AHRS #1 overhaul — because #2 was still arguing — and they’re simply gone now.
  • AHRS #2 in-flight re-aligns: 118 seconds on Flight 15 → zero on Flights 16 and 17, with no “USING AHRS1” reversion. AHRS #1 stays at zero too. Both boxes now hold their solution from brake release to rollout.
  • Maximum cross-source roll disagreement: 0.8° on Flight 16, 3.0° on Flight 17 — against the ~136° this airplane once produced on Flight 12. No deviation tumbles on either unit.

That’s the whole arc closed. Look back at the re-align chart: the navy bars (AHRS #1) end at Flight 12, the lone orange bar (AHRS #2) stands on Flight 15, and after the second overhaul goes in at Flight 16 — nothing. Three healthy attitude opinions, for the first time in this airplane’s life.

Where It Stands

The two original cores — the unit that started it all (S/N 5Q2001016) and its sister (5Q2002262) — go back to Garmin per their return instructions, and that closes the books. Two RMAs, two overhauled units, and a flight-data trail that called the shot both times before the wrench ever came out.

The villain is retired. Onward to the rest of the Phase 1 card deck.

— with thanks to Garmin support for engaging with the data all the way through.

N997CZ — Flight 14: Dan’s First Flight

Dan's new BMW wearing the N997CZ vanity license plate
Dan’s new BMW, already wearing the airplane’s callsign.

Let the record show the order of operations: Dan put the tail number on his car before he ever flew the airplane. That’s commitment. (In fairness — he offered the N997CZ plate to me first, before reserving it for his own ride. I passed. He didn’t hesitate.) So when Flight 14 rolled around, the man already had the callsign on his bumper; all that was missing was the airplane in his logbook.

Some context for newer readers: N997CZ has three owners — Harry, Dan, and me. I’ve flown every minute of the test program so far, because that’s how Phase 1 works best: one pilot, one airplane, building knowledge methodically. But thirteen flights and twenty-some hours in, with the stall series done and the systems behaving, it was time. Flight 14 was Dan’s first flight in the RV-10 — his airplane as much as mine.

The Numbers

Date 2026-06-07 (afternoon — second flight of the day)
Engine time ~0.9 hr
Engine hours 18.3 → 19.1
Max altitude ~6,140 ft MSL (6,490 ft GPS)
Fuel used 9.8 gal (totalizer)
Profile familiarization — no test cards
Conditions warm afternoon (cruise OAT +76 °F)

No test-card PDF link this time, and that’s deliberate: this was a familiarization flight, not a test sortie. The cards stayed in the binder.

Taxi Out

His first taxi out in the airplane:
Dan taxis out for his first flight in the RV-10.

First Takeoff

And the moment itself — Dan’s first takeoff in N997CZ:
Dan’s first takeoff in the RV-10.
Dan in the cockpit of N997CZ on the ramp
Dan in the left seat on the ramp.

What the Data Says About a Familiarization Flight

Even a no-cards flight leaves a data trail, and Flight 14’s log reads exactly like what it was: a thorough, unhurried checkout.

Ground track for N997CZ Flight 14
Ground track for Flight 14.
Flight 14 altitude and indicated airspeed vs time, with the air-work block shaded
Altitude and indicated airspeed — the shaded block is the slow-flight and steep-turn air work.
  • Air work: a focused fifteen-minute block of slow flight and steep turns. The slow flight walked down to 52 KIAS at ~5,400 feet — comfortably into the regime where this airplane has now been thoroughly characterized — and the steep turns ran to about 45° of bank, pulling a maximum of 1.66 G. Textbook checkout numbers.
  • Mixture: cruise fuel flow ran about 21 gph — essentially full rich. Thirteen flights of my leaning experiments did not transfer by osmosis; nobody leans aggressively on their first flight in a new airplane, nor should they.
  • CHTs: the warm afternoon, rich mixture, and lower altitudes made this the toastiest of the weekend’s three flights — cylinder 5 peaked at 428 °F, still under the 435 °F comfort line. All six stayed in the green.
  • Cabin CO: peak 3 ppm. Benign.
  • Attitude system: the freshly overhauled AHRS #1, two flights into its tenure, stayed perfectly quiet through all the maneuvering. Not a twitch — the second tumble-free flight in a row, in an airplane that had never logged one before.

Fuel Etiquette

After landing, Dan topped the airplane off on his own dime — about ten gallons. The partnership’s fuel accounting remains in perfect balance, and the totalizer reconciliation thread that runs through this blog barely noticed the handoff.

Bottom Line

Flight 14 added a second name to N997CZ’s pilot roster, and the airplane treated its other owner exactly the way you’d hope: honest slow flight, crisp steep turns, cool-headed avionics, no squawks added. The man had the tail number on his car; now he’s got the airplane in his logbook.

Me? Maybe someday I’ll register a 997RV plate — the partnership’s RV-7 — and close the loop. Then each of us can drive around wearing the other airplane’s callsign, and the fleet will be fully registered on pavement and off.

Engine time after Flight 14: 19.1 hours.

N997CZ — Flight 13: The Stall Block

Ground track for N997CZ Flight 13 over the Phase 1 test area
Ground track for Flight 13 over the Phase 1 test area.

Every flight test program has a day where you stop sneaking up on the stall and just go fly it. For N997CZ, that was Flight 13. The slow flight and first banked stalls on Flight 12 had shown the airplane was honest at high angle of attack; this flight was about systematically documenting where the wing quits — clean, with flaps, and with power on — and doing all of it with a lot of sky underneath.

It was also the flight that finally set the airplane free of round numbers: a new program-high altitude of 11,520 ft MSL (12,069 ft GPS).

The Numbers

Date 2026-06-07
Engine time ~2.2 hr
Distance 233.3 nm (GPS path)
Fuel used 24.1 gal (totalizer)
Engine hours 16.2 → 18.3
Max altitude ~11,520 ft MSL (12,069 ft GPS) — new program high
Cruise OAT +67 °F (warm day)
Loading solo, light weight
Cruise mixture leanest yet — ~11.5 gph

The Card Deck

Flight 12 only got partway through the stability cards before the slow-flight work used up the morning, so this flight’s deck rolled the leftovers in with the stall series — eight cards in fly order: area set-up, steady-heading sideslips, spiral stability checks, then the pre-stall set-up and recovery drill, and finally the stalls themselves — 1-g clean, full-flap, and power-on. (The last card, accelerated/turning stalls, didn’t get flown before fuel and the warm afternoon said go home — it carries forward.)

The sideslips and spiral stability points were flown and are sitting in the log waiting for their own data reduction — a future post once the numbers are out.

The full 16-page deck — a briefing script page for each card plus a boxed, cut-out kneeboard card for the cockpit — is here: Flight 13 test cards (PDF). Here’s one from the deck, the 1-g clean stall:

FT-13-5 kneeboard card — 1-g clean stall, document the break
The FT-13-5 kneeboard card — 1-g clean stall, document the break.

The Stall Block

Flight 13 altitude and indicated airspeed vs time, with the stall block shaded
Altitude and indicated airspeed vs time — the shaded block is 33 minutes of stall work, ending with the climb to the program-high 11,520 ft MSL (12,069 ft GPS).

The profile chart shows the structure: climb out of Manassas, stability work around 10,000 feet, then a sustained 33-minute stall block — that dense comb of deceleration spikes in the red trace, each one a deliberate walk down to the break and a recovery. Toward the end of the block the airplane steps up and up, topping out at 11,520 ft MSL (12,069 ft GPS) for the power-on series, then a long descent home.

The headline numbers, all flown solo at light weight:

Series (engine-verified) Break Notes
Power-off, early series 52–58 KIAS idle/low power, ~10,000 ft — likely the clean (flaps-up) cards, per fly order
Power-off, later series ~48 KIAS three events, 48.0–48.7, idle/low power — likely the full-flap cards, per fly order
Power-on (departure) 55–59 KIAS 16–18″ MAP / ~2,500 RPM, +14 to +18° pitch — verified from engine data

A word on how those rows are labeled, because honesty matters more than tidiness. The flap-position sensor is one of the program’s open squawks — this flight’s log records raw counts from −521 to +9, which is to say, nothing usable — so no row above can claim a data-verified flap setting. What the data does verify is power: the engine channels cleanly separate five power-on breaks (16–18 inches of manifold pressure, 2,500 RPM, nose up +14° or more) from seven power-off breaks at or near idle. The flap attribution rests on the card fly order — the clean stalls were scheduled before the full-flap ones — plus a physics cross-check: flaps lower the stall speed, and sure enough the later power-off series broke a good seven knots slower than the earlier one. Likely, not proven, until the new flap sensor goes in.

The numbers also pass the placard test once you account for weight. The placarded stall speeds at gross are about 61 knots clean and 52 with full flaps; stall speed scales with the square root of weight, and this was one pilot and partial fuel, far below gross. Scale the placard numbers down and you land almost exactly where the airplane did: high-50s breaks for the (likely) clean series, 48 for the (likely) full-flap series. The G-trace adds one more correction — the breaks happened partially unloaded, 0.74–0.88 G, which reads lower still. And through all of it the airplane behaved: buffet warning, a straight-ahead break, conventional recovery, every time.

FlySto’s pitch trace makes the hour of stall work visible at a glance — every tooth in that comb is a nose-up walk toward the break and the nose-drop after it, and the tall +14 to +18° peaks late in the block are the power-on series, right on the card’s target attitudes:

FlySto pitch chart for Flight 13 — repeated pitch-up/break cycles through the stall block
FlySto’s pitch chart for Flight 13 — every tooth in the comb is one stall cycle; the +14–18° peaks are the power-on series.

One more layer from Flight 11’s calibration work: at these speeds the airspeed indicator reads about a knot and a half low, so calibrated break speed is closer to 49–50. Every knot of that bookkeeping matters when these numbers eventually set the approach speeds.

Can You See the Buffet in the Data?

The pre-stall buffet is unmistakable in the seat — the airframe starts talking to you well before the break. A fair question is whether the data logger hears it too. The answer: yes, faintly. The G3X logs at 1 Hz, and real airframe buffet shakes at several cycles per second or more, so the log catches only an aliased, heavily muted echo of what the pilot feels. But it’s there: in eleven of the twelve stall events, the roughness of the normal-acceleration trace climbs to two to five times its smooth-air baseline in the final seconds before the break.

One Flight 13 stall entry: IAS decay, G-trace roughening in the buffet, the break, and the recovery
One stall entry from the block — airspeed bleeding down, the G trace fraying in the buffet window, the break, and the recovery.

The chart above is one entry from the block: airspeed bleeding down, the G trace starting to fray in the shaded buffet window, the drop at the break, and the firm recovery pull after. The amplitudes look small — a few hundredths of a G — but remember the sampling: the cockpit experience is a much louder version of what survives into a once-per-second log.

Engine: Leanest Cruise Yet, and a Plot Twist in the CHTs

The mixture story keeps marching: median cruise fuel flow was ~11.5 gph, the leanest of the program — a long way from the 18–23 gph full-rich break-in flights.

FlySto CHT chart for Flight 13: all six cylinders vs time with color bands
FlySto’s CHT chart for Flight 13 — one climb-out peak brushing the caution band, then a busy green-band sawtooth through the stall work.

The FlySto chart shows the whole flight: one hot moment on the climb-out where the pack peaks just over 420 °F, brushing the caution band, then a busy, healthy green-band sawtooth as the stall work cycles power up and down for an hour. The plot twist is which cylinder topped the chart: cylinder 2 at 422 °F — not cylinder 5, the climb-cooling canary from the last three flights (it logged 419). One warm-day flight isn’t a trend, but the ranking shuffle is noted and goes in the watch file.

The CO Detector Read Zero — Which Is Suspicious

Cabin CO logged 0 ppm for the entire flight. Taken at face value that would be the best result of the program — but every slow-flight-heavy flight from 6 through 12 showed at least a few ppm, and this was the most aggressive high-AoA profile yet. A sensor that suddenly reports perfect silence on exactly the profile that always made it talk is more likely asleep than victorious. Verifying the detector is on the squawk list; until then, this flight’s CO data gets an asterisk, not a trophy.

Also On This Flight

  • A freshly overhauled attitude unit went into the panel the day before this flight — and through an hour of stalls and high-AoA work, the attitude display never so much as twitched. By my count that makes this the first flight in the airplane’s life with no attitude-horizon tumble — the cross-source roll disagreement that hit ~136° the day before stayed under 10° through all of it, and it has stayed that way since. (The story has a sequel brewing: the #2 attitude source has started miscomparing against the newly healthy #1.) The full saga, and the data behind it, is getting its own post.
  • Manual flying practice: about an hour of the flight was hand-flown — stall work is hand-flying by definition, and the autopilot got the cruise legs.

Squawks

  • Accelerated/turning stall card not flown — carried forward to a future sortie.
  • CO detector read zero all flight — verify it’s actually alive before trusting the result.
  • Flap position indication still broken — and it stings more on a flight like this one, because it leaves the stall-speed-versus-configuration record resting on the card sequence and pilot recollection instead of data. Replacement sensor is on the parts list.

Bottom Line

Flight 13 closed out the bread-and-butter stall series: power-off breaks walking down from the high 50s to 48 knots indicated as the flaps (most likely) came out, power-on breaks in the high 50s — all of it right where the placard predicts once you do the weight math — honest manners throughout, and a new program-high 11,520 ft MSL (12,069 ft GPS). The engine ran its leanest cruise yet, cylinder 2 stole the hot-cylinder crown for a day, and the CO detector’s perfect silence earned suspicion instead of celebration. One card carries forward, and the stability data is in the can awaiting analysis. The envelope is opening up.

Engine time after Flight 13: 18.3 hours.

N997CZ — Flight 12: Slow Flight at 9,500 Feet

Ground track for N997CZ Flight 12 over the Phase 1 test area
Ground track for Flight 12 over the Phase 1 test area.

Flight 11 was a marathon — two and a half hours of precision circles for the airspeed calibration. Flight 12, six days later, was the opposite kind of flight: shorter, higher, and slower. The plan was to start working through the stability test cards, and the headline act was a slow-flight series flown higher than this airplane had ever been.

The Numbers

Date 2026-06-06
Engine time ~1.5 hr
Distance 156.5 nm (GPS path)
Fuel used 18.5 gal (totalizer)
Engine hours 14.8 → 16.2
Max altitude ~9,090 ft MSL (9,535 ft GPS) — highest of the program (so far)
Cruise OAT +58 °F (mild morning, ~13 °F warmer than F11)
Autopilot ~94% of airborne time (more on that below)
Config 7 qt oil; burn well under one tank, no tank switch

Climb High, Fly Slow

The idea behind doing slow flight way up high is simple: altitude is recovery margin. If the airplane is going to do something rude at high angle of attack, I’d rather it do it with 9,000 feet underneath me than 3,000.

Flight 12 altitude and indicated airspeed vs time, with the slow-flight series shaded
Altitude and indicated airspeed vs time — the shaded block is the slow-flight series at 9,100–9,500 ft.

The profile chart shows the shape of the flight: climb out of Manassas, work in the mid-8,000s, then push up to the program-high 9,090 ft MSL (9,535 ft GPS) and settle into about thirteen minutes of slow-flight work between 9,100 and 9,500 feet. The red trace tells the story — a string of deliberate decelerations, each one walked down slowly, held, and recovered, over and over. The slowest stabilized point was about 56 KIAS at 9,145 feet.

That block wasn’t all straight-ahead slow flight, either. Working down the stability card deck — the FT-12/FT-13 card set is here (PDF) — the points included longitudinal static-stability checks at 55% power (trim the airplane, displace 10 and then 20 knots either side of trim, and watch how it comes back), flaps-up power-off stalls banked both ways at 10°, 20°, and 30° of bank, and a set of power-on stalls at 55% power. The power-on stalls turned out to be the interesting ones — more on that next.

Remember from Flight 11 that the airspeed indicator reads a knot and a half or so low down at these speeds — so true calibrated speed at that 56-knot point was around 58, and true airspeed at that altitude meaningfully higher still. The margins were real, which is the whole point of doing this work upstairs.

The Autopilot Kept Tapping Out

One genuinely interesting system behavior surfaced during the slow-flight series: the G3X autopilot repeatedly dropped offline with a “Fail / Inertial miscompare” annunciation, then re-engaged — about four minutes of dropouts across seventy airborne minutes, and the trigger was specific: the power-on stalls. It wasn’t just the autopilot being cautious, either — the attitude horizon itself tumbled several times during those stalls. In the data, the disagreement between attitude sources reached roughly 136° of roll — the worst cross-source split of the entire program. Outside the high-AoA block the autopilot was rock solid; it flew about 94% of the airborne time.

“Inertial miscompare” doesn’t mean the system knows which attitude source is lying. Per the G3X installation manual, data from the two ADAHRS units is continuously cross-compared and a disagreement is simply annunciated — the system flags the mismatch and lets the pilot sort it out. (Interestingly, the G5 standby only joins the comparison if the system has degraded to a single ADAHRS, so there’s no three-way vote while both primary units are alive.) Figuring out which unit is actually bad happens one level down: each ADAHRS runs its own internal integrity monitoring, and a unit that loses confidence in its own solution drops it and re-aligns — at which point the system switches over to the survivor. Long-time readers will recognize this as the ADAHRS-deviation theme that’s been running since the first five flights — it gets provoked by exactly this kind of high-AoA, low-airspeed maneuvering. The decision coming out of this flight’s debrief: change out AHRS #1 and see what the data says. The replacement unit went into the panel that same day. That story deserves its own post.

Carbon Monoxide: Best Slow-Flight Result Yet

This one is encouraging. Every slow-flight-heavy flight since the CO detector went in has shown cabin CO in the 5–10 ppm range, consistent with the firewall heat-door leak theory. Flight 12 was the most slow-flight-intensive profile yet — and CO stayed benign: peak 5 ppm, mean 0.5.

Cabin CO across all twelve flights
Cabin CO across all twelve flights — Flight 12 is the cleanest slow-flight result since the detector went in.

Two honest caveats. First, the log shows a scary-looking 250 ppm spike — that’s the CO detector’s power-up warmup transient, recorded in the first ~50 seconds on the ground with the engine off, decaying steadily as the sensor warms. It’s a self-test artifact, not cabin air. (The trend script now auto-discards it.) Second, since I still haven’t flown the deliberate tape-the-heat-doors test, a quiet flight doesn’t tell me the leak is fixed — just that this profile, on this cool morning, didn’t pull much exhaust in. The isolation test stays on the list.

The Cylinder 5 Canary: Holding, Not Climbing

The air-dam saga continues, with a hopeful data point. Recap: before Flight 10 I removed the cooling air dam in front of cylinder 1, which fixed #1’s chronic heat but started costing cylinder 5 in the climb — its temperature-adjusted peak climb CHT marched 401 → 410 → 427 °F across Flights 9–11.

FlySto CHT chart for Flight 12: all six cylinders vs time with normal/caution/limit color bands
FlySto’s CHT chart for Flight 12 — all six cylinders (raw temps) with the comfort bands painted behind them.

Here’s the whole flight at a glance, as FlySto renders it from the G3X log — all six cylinders, raw temperatures, with the comfort bands painted behind them. The climb-out spike just after takeoff is the hot moment of the flight, topping out around 407 °F, and everything spends the rest of the sortie comfortably in the green. You can even see the slow-flight series as that choppy stretch in the middle, and the brief dip to ~260 °F right before it — the power-off deceleration entries.

Flight 12: 408 °F. Still the hottest cylinder in the climb, but it fell back to roughly the Flight-10 level instead of continuing to rise — helped, no doubt, by climbing into cooler air up high. Meanwhile cylinder 1 keeps enjoying its freedom: coolest jug on the engine, with cruise temperatures (OAT-adjusted) down around 327 °F. Cruise CHTs across the board were the lowest of the program — the reward for flying high in cool air. Verdict: the canary is alive but still in the mine. Watching.

Fuel: The Totalizer Earns Its Keep Again

A small satisfying footnote. The totalizer said the flight burned 18.5 gallons; the fuel truck that topped the airplane off twenty minutes after shutdown delivered 18.30 gallons. That’s agreement to 0.2 gal, and it keeps the program-long truck-versus-totalizer reconciliation within a fraction of a gallon. The fuel-flow system has earned trust the fuel-quantity gauges haven’t (the right float is still stuck — see squawks).

Cruise mixture ran richer than Flight 11 — about 15.5 gph against 14.5 — mostly because I spent the flight flying maneuvers instead of babysitting the red lever.

Squawks

  • Flap position sensor broke again — the May 31 workaround (remounting the worn pot to ride the healthy end of its range) didn’t hold, and this time the failure ran the other way: it wouldn’t let the flaps come up. On Flight 11 the same sensor had refused to put them down, forcing a no-flap landing. Two flights, two opposite refusals — enough. The fix chosen: reconfigure the VP-X so the flap switch works as momentary — hold the switch and the flaps drive up or down, no listening to the position sensor at all. (Since completed, with Dan’s laptop hooked to the VP-X.) The replacement for the 20-year-old Ray Allen pot stays on the parts list.
  • VP-X pitch and roll trim speeds need to come down — trim runs too fast.
  • AHRS attitude tumble / inertial miscompare under the power-on stalls — see above; next step chosen: swap AHRS #1.
  • Right fuel-quantity float still stuck (known item; totalizer is the authority).
  • Heat-door CO isolation test still not flown — benign CO this flight is encouraging but inconclusive.

And two items came off the list before this flight, in a Friday repair session: the upside-down left magneto switch from the Flight 11 squawks, and the remaining flap close-out work.

Bottom Line

Flight 12 took the airplane higher than it had ever been and slower than it usually flies, in the same thirteen minutes. The slow-flight handling was honest, the CO behaved, the cylinder 5 canary stopped climbing, and the totalizer matched the fuel truck to a couple tenths. The one system that complained — the autopilot’s inertial miscompare under high AoA — is the same attitude-source thread this airplane has been tugging since flight one, and it was about to get a lot more attention.

Engine time after Flight 12: 16.2 hours.

N997CZ — Flight 11: A Multi-Speed Airspeed Calibration

Ground track for N997CZ Flight 11 over the Phase 1 test area, with the DAR-approved pentagon boundary (KFRR–KHEF–KXSA–KFVX–KSHD) and landing airports marked, on OpenStreetMap tiles
Ground track for Flight 11 over the Phase 1 test area, with the DAR-approved pentagon boundary and landing airports marked.

Most of the flights so far have been about finding problems — a CHT that ran hot, an ADAHRS that wouldn’t behave, carbon monoxide where there shouldn’t be any. Flight 11 was different. This one had a single, deliberate job: figure out how honest the airspeed indicator actually is, across the whole speed range, in one sortie.

It turned into the longest flight on the airplane so far, and it produced the cleanest dataset I’ve collected yet. It also made me go back and admit I’d gotten an earlier piece of analysis wrong. More on that below.

The Numbers

Date2026-05-31
Engine time~2.5 hr (longest flight to date)
Distance285.6 nm (GPS path)
Fuel used30.4 gal (totalizer)
Engine hours12.4 → 14.9 hr
Cruise altitude~6,500 ft
Cruise OAT+45.5 °F (cool morning)
Autopilot~64% of the flight
Configflaps up throughout — including, it turned out, the landing; 7 qt oil

Thirty-plus gallons through the totalizer is more than one tank holds (30 gal/side), so this was a mid-flight tank-switch flight — only the second one in the program. Everything stayed comfortably inside the test-area boundary you can see in the opening image; the cluster of little boxes and circles southwest of Culpeper, up around Louisa, is where all the calibration work happened.

The Purpose: Why Fly Circles at Eleven Different Speeds

The airspeed indicator shows IAS — indicated airspeed, the raw pitot-static reading. What you actually want for performance numbers is CAS (calibrated airspeed, with the airframe’s position error removed) and ultimately TAS (true airspeed). The gap between IAS and CAS — the position error — depends on where the static port sits, how the air flows around the fuselage, and it often changes with speed. A system that’s nearly perfect at cruise can be a few knots off in slow flight, which is exactly where you care most.

My two earlier calibration points (Flights 7 and 9) were both stuck around 145–148 KIAS, because that’s just where the airplane happened to be when I had clean data. Flight 11 was the flight to fix that — to deliberately hold a series of different indicated airspeeds, all the way from cruise down to near stall, and measure the error at each one.

How I flew it. The trick to measuring true airspeed without a calibrated airspeed reference is to let GPS do the work. If you fly a full 360° at a constant indicated airspeed, your GPS ground-velocity vectors trace out a circle: the radius of that circle is your true airspeed, and the center is the wind. No compass, no assumptions — just geometry.

So I flew a stack of constant-IAS loops at ~6,500 feet:

  • First half — autopilot, heading bug walked around the cardinals, stepping down: 154, 139, 119, 98, 94 KIAS.
  • Second half — autopilot again, stepping back up: 130, 138, 144, 150, 155 KIAS.
  • Then, hand-flown — the autopilot won’t hold below about 95 knots, so I clicked it off and flew the slow ones by hand: 89, 82, 75, 71, and 66 KIAS.
Flight 11 chopped into constant-IAS segments, ground track colored by airspeed, with the transitions between segments in gray
Flight 11 chopped into constant-IAS segments — ground track colored by airspeed, transitions in gray.

After the flight, the data-reduction problem was just chopping that long log into the individual stabilized-airspeed loops — fourteen of them, from 66 to 155 KIAS — and trimming each one down to the clean, full-circle, constant-speed portion. Then each loop gets its own GPS circle fit.

The Airspeed Results

Here’s the headline, and it’s not what I expected: the airspeed indicator reads low. The airplane is genuinely flying a couple knots faster than the dial shows — a little at slow speed, and about three knots at cruise.

Airspeed-system position error vs IAS for Flight 11, with the recomputed Flights 7 and 9 points, from the GPS circle wind solve
Position error vs IAS for Flight 11, with the recomputed Flights 7 and 9 points.

Position error (IAS minus the GPS-derived calibrated airspeed) at each tested speed:

IAS (kt)Position error (kt)Reads…
66−1.8low
71−1.7low
75−1.5low
82−1.8low
89−1.5low
94−2.8low
119−3.5low
130−3.2low
138−3.2low
144−2.7low
150−3.4low
154/155−3.0 / −2.9low

So roughly −1.5 to −1.8 kt down in the slow-flight band, growing to about −3 kt in cruise. The circle fits were tight — RMS residuals of 0.4 to 1.6 knots — and the wind solution came out consistent across every single loop (8–14 kt out of the northwest), which is a nice internal sanity check: if the loops were sloppy, the winds wouldn’t agree.

Practically, this is good-news-bad-news. It means your true cruise and climb speeds are a hair better than the panel suggests. It also means that on final approach, when the indicator says 80, you’re really doing about 81–82 — worth knowing, not alarming.

The part where I was wrong

When I wrote up Flights 7 and 9, I concluded the pitot-static system was “essentially clean — basically zero position error in cruise.” That was wrong, and the error was mine.

Converting GPS true airspeed back to calibrated airspeed requires the air density ratio, and the formula I’d used computed it incorrectly — it was effectively using the pressure ratio at standard temperature instead of the actual density ratio. I caught it while setting up Flight 11’s analysis, checked the corrected formula against the standard-atmosphere tables (it matches exactly now), and reran everything.

With the fix, those two old cruise points come out to about −2.9 and −3.9 kt — right in line with Flight 11’s cruise cluster. And it resolved a puzzle that had been bugging me: previously the indicator looked perfect but the G3X-computed true airspeed read 3–4 knots low, which made no physical sense — they should move together. With the corrected density ratio they do move together, because there’s just one error: the static system reads a little low, and that propagates into both the indicated and the computed-true airspeed. One bug, two symptoms.

Can I just fix it in the avionics?

Short answer: no. I went looking for a place in the G3X to enter an IAS-to-CAS correction and checked the installation manual to be sure. The G3X Touch has a “Zero-Airspeed Calibration” (a zero-point/sensor-noise step), but no facility to load a position-error curve. So this ~2–3 kt isn’t something I can dial out in the box — it’s a known characteristic of the airframe, to be handled with awareness and maybe a small placard. CAS ≈ IAS + ~2–3 kt in cruise. Done.

Also On This Flight

A calibration sortie is a long time at altitude, so a few other threads picked up data along the way:

  • Cylinder cooling. Before Flight 10 I removed the cooling air dam in front of cylinder 1, which cooled #1 nicely but looked like it might be starving cylinder 5 (the rearmost on that bank) of climb cooling. Flight 11 confirmed it: temperature-adjusted, the peak climb CHT on #5 has now climbed 401 → 410 → 427 °F over the last three flights, and #5 is now the hottest cylinder in the climb. Cruise is unaffected — it’s purely a climb-cooling cost. That’s a watch item.
  • Carbon monoxide. Still there. Peak ~9 ppm, showing up during the slow-flight portions, consistent with the firewall heat-door leak I’ve been chasing. The tape-the-heat-doors test still hasn’t been flown.
  • Mixture. This was the leanest cruise yet (~14.5 gph), continuing to lean out as I dial in the engine.
  • Autopilot. I bumped up both the roll and pitch servo max-torque settings in flight to firm up how it tracks — useful while flying precise boxes.

A No-Flap Finale

The flight ended with an unplanned test point. Coming home, when I selected flaps for landing, nothing came down: the flap position sensor had gone haywire in a way that made the VP-X — which drives the flaps based on that sensor’s feedback — refuse to run them. I only pieced that together late in the landing sequence, around the base-to-final turn, which is a poor place to start troubleshooting. There were two good outs available: the VP-X allows manual flap control from an on-screen override on the MFD, and a go-around was on the table the whole time. With the runway made and the airplane stable, I took the third option — keep flying the clean airplane I’d been flying all morning and make a no-flap landing. It was a non-event: carry a few extra knots, use a little more runway, and move the troubleshooting to the ground, where it belongs.

The diagnosis: the Ray Allen POS-12 flap position sensor — a 20-plus-year-old hand-me-down from our RV-7 — has a potentiometer worn out at one end of its range. I remounted it to ride on the healthy end of its travel, which brought it back to life for now, but a sensor that old and probably full of dust has earned retirement: a replacement went straight onto the parts list. (Spoiler from the squawk file: this story isn’t over.)

Squawks

  • Flap position sensor failed in flight — see “A No-Flap Finale” above. It didn’t touch the calibration data (the whole profile was flown flaps-up), but it did decide how the flight ended. Replacement sensor on the parts list.
  • Left magneto switch is installed upside down.
  • Fixed before the flight: a loose bolt on the left flap (a mechanical item, separate from the indication problem above).

Bottom Line

Flight 11 was the airplane’s longest flight, its cleanest dataset, and the one that finally mapped airspeed error across the whole envelope: the indicator reads about a knot and a half low slow, about three knots low at cruise, and there’s nothing to fix in the avionics — just something to know. It also cost me a little humility, since getting there meant finding a mistake in my own earlier math — and a no-flap landing, courtesy of a worn-out sensor. That’s flight test: the airplane keeps you honest, and so does the arithmetic.

Engine time after Flight 11: 14.9 hours.

N997CZ — Dynamic Prop Balancing: 0.57 IPS Down to 0.01

After the first five flights it was clear that N997CZ was sitting on more vibration than I wanted. The most visible symptom was the ADAHRS percent-deviation values logged by the G3X — they were chronically high during cruise on every one of the first five flights, and Garmin tech support had pointed out that high deviation values are often a sign that the attitude solution is fighting more vibration than it wants to. So when the airplane went into the hangar for the 25-day maintenance gap between Flight 5 and Flight 6, dynamic propeller balancing was one of three jobs on the list (alongside the CAN bus rewire and the left fuel-gauge float fix).

This is the story of that balance.

What dynamic propeller balancing actually is

Static balance — the kind you can do with the prop off the airplane on a balance stand — gets the prop to the point where it doesn’t have a preferred resting position. Dynamic balance is the next step: it deals with the residual imbalance that only shows up when the engine and prop are running at their actual operating RPM, with the actual installed combination of crank, flywheel, starter ring, spinner, and prop all spinning together as one mass.

The DynaVibe Classic balancer (RPX Technologies) does it with two sensors:

  • An accelerometer bolted to the top of the engine case, “as far forward as possible for maximum sensitivity… mounted vertically, perpendicular to piston travel.” The standard technique is to pull one of the case bolts along the top of the engine and reinstall it through the bracket that holds the accelerometer.
  • An optical pickup mounted on the same bracket, “approximately six inches behind the back of the propeller,” with its beam aimed at a small piece of reflective tape stuck to the back of the spinner backplate or starter ring. The optical pickup gives the balancer a once-per-revolution timing reference so it can tell the balancer not just the magnitude of the vibration but the angular location of the heavy spot.

(Quotes are from the DynaVibe Classic User Manual v1.09.)

With those two sensors hooked up, you start the engine and run it up to a steady cruise RPM — around 2,100 RPM in our case for the Lycoming IO-540. The balancer reads out two numbers: the vibration magnitude in inches per second (IPS), and the clock-angle of the heavy spot measured against the reflective-tape index. Then you stick trim weights opposite the heavy spot, run it again, see whether the magnitude dropped and where the new heavy spot ended up, and iterate.

The DynaVibe puts the result on a clear scale:

DynaVibe Dynamic Propeller Balancer form, showing the IPS magnitude scale: Extreme (≥1.25), Rough (0.25–1.00), Fair (0.15–0.25), Good (0.07–0.15), Excellent (0.04–0.07), Perfect (0.00–0.04). Polar chart for marking heavy-spot location.

The bands: Perfect under 0.04 IPS, Excellent through 0.07, Good through 0.15, Fair through 0.25, Rough all the way up to 1.00, Extreme above that. Anything in the Good band or better is acceptable for general aviation; Excellent or Perfect is the target if you have the time and patience to chase it.

Trim-weight construction

The trim weights on this airplane go on the starter ring gear, which has twelve bolt holes spaced every 30°. We assembled the weights out of standard hardware: AN4 bolts in various lengths, AN4 washers, and AN4 lock nuts. AN4 is a ¼”-diameter aircraft bolt; the dash number is the length in eighths of an inch — so an AN4-7 is ¼” × ⁷⁄₈” long, an AN4-10 is ¼” × 1¼”, an AN4-13 is ¼” × 1⅝”, and so on.

The catch: each bolt has limited thread engagement for the lock nut, so you can only stack about one or two washers on a given bolt before you have to step up to the next longer bolt size. Bumping the bolt up a length adds the equivalent of a washer or so of weight all on its own, which gives the balancing procedure a slightly quantized feel — you’re not adding weight continuously, you’re adding it in discrete steps of either a washer or a bolt-length-bump.

The session — twelve runs, eleven adjustments

Initial run, before any trim weights were added: 0.57 IPS at 345° at 2105 RPM. Solidly in the Rough band.

What follows is the entire in-hangar log:

Handwritten iteration log from the prop balance session: starting at 0.57 IPS @ 345° and converging through twelve trim-weight adjustments to a final reading of 0.01 IPS at 350°.

Decoded as a table — each row is one measurement, the “Fix applied” column is what we added or changed between this run and the next, and the rightmost column is the running weight state the airplane was carrying when the next measurement was taken:

#RPMIPSHeavy spotFix applied for next runRunning weight state after fix
1 (bare prop)21050.57345°AN4-7 + nut + 1 washer at 180°180°: AN4-7 N+W
221440.34334°AN4-7 + nut + 1 washer at 150°180°: AN4-7 N+W · 150°: AN4-7 N+W
321350.23322°Upgrade 150° to AN4-10 + nut + 2 washers180°: AN4-7 N+W · 150°: AN4-10 N+2W
420960.22329°150°: AN4-10 → AN4-11; washers 2 → 4180°: AN4-7 N+W · 150°: AN4-11 N+4W
521040.15335°One more washer at 150° (bolt stays AN4-11)180°: AN4-7 N+W · 150°: AN4-11 N+5W
620820.13339°150°: AN4-11 → AN4-12; washers 5 → 6180°: AN4-7 N+W · 150°: AN4-12 N+6W
720800.09340°180° → AN4-10 N+W; 150° adds washer → AN4-12 N+7W180°: AN4-10 N+W · 150°: AN4-12 N+7W
821000.10346°Add washer at 180° → AN4-10 N+2W180°: AN4-10 N+2W · 150°: AN4-12 N+7W
920840.09323°150°: AN4-12 → AN4-13; washers 7 → 8180°: AN4-10 N+2W · 150°: AN4-13 N+8W
1020820.08316°One more washer at 150° → AN4-13 N+9W180°: AN4-10 N+2W · 150°: AN4-13 N+9W
1121090.06307°Add a thin (half) washer at 150° → AN4-13 N+9.5W180°: AN4-10 N+2W · 150°: AN4-13 N+9.5W
1220770.01350°Done — achieved 0.01 twice with DynaVibe averaging (the instrument floor)Final installed: 180°: AN4-10 N+2W · 150°: AN4-13 N+9.5W

A couple of things worth noting from the table:

  • The heavy-spot angle moved around the clock as the magnitude came down. It started at 345°, walked counterclockwise to 307° by step 11, then jumped to 350° on the final run. Once you’re in the Good / Excellent range, the angular position is increasingly noise-dominated — small changes in RPM, OAT, or run-to-run engine settling can shift the indicated heavy-spot by tens of degrees while the magnitude barely moves.
  • The step backward at #8 (0.09 → 0.10) is the classic instrument-floor signature. When you’re under ~0.1 IPS, the noise floor of the balancer is comparable to the imbalance you’re trying to chase, and small runs that look like regressions are usually just measurement scatter. We worked through it and kept converging.
  • The final two readings used the DynaVibe’s averaging mode — it takes multiple-revolution samples and reports a stable mean. We saw 0.01 twice in a row in that mode, which is essentially the floor of the instrument. Calling it done was an easy decision.

The convergence as a polar “bullseye” plot — each dot is one measurement, walking from the Rough band on the outer edge in toward Perfect at the center:

Polar convergence chart of the 12 balance measurements. Concentric bands show the DynaVibe IPS categories (Rough / Fair / Good / Excellent / Perfect). Dots are color-coded from red (first measurement, 0.57 IPS) through green (final, 0.01 IPS), connected by a path showing the order of measurements. Each dot is annotated with its measurement number, IPS value, heavy-spot angle, and the trim-weight configuration that was in place when that measurement was taken.

End to end the magnitude trended:

0.57 → 0.34 → 0.23 → 0.22 → 0.15 → 0.13 → 0.09 → 0.10 → 0.09 → 0.08 → 0.06 → 0.01 IPS.

Twelve runs, eleven trim-weight changes, the better part of an afternoon, and the airplane went from Rough to Perfect. A ~57× reduction in cruise vibration magnitude.

What it bought me in flight

The whole point of doing this was to clean up the ADAHRS environment. The G3X logs the ADAHRS percent-deviation on every flight for all three units — the two GSU 25C ADAHRSes on the sub-panel, and the G5 standby on the main panel — so the question was whether the flight-after-flight median deviation actually came down once the prop was smooth.

It did:

Bar chart of median and 95th-percentile ADAHRS deviation across all 10 N997CZ flights, for ADAHRS #1, ADAHRS #2, and the G5 standby. A vertical dashed line at F5.5 marks the dynamic prop balance. Pre-balance medians cluster at 80–115%; post-balance flights F7–F9 drop into the 18–58% range.

Each panel is one ADAHRS source: the two GSU 25Cs and the G5 standby. The dashed vertical line is where the prop balance happened — between Flight 5 and Flight 6. Median deviation values pre-balance hovered in the 80–115% band across all three units. The first post-balance flight (F6) was a short low-altitude shakedown and showed only modest improvement, but F7 through F9 — the first three flights at altitude after the balance — settled into the 18–58% band. Median deviation roughly halved on the GSU 25Cs and dropped by nearly 75% on the G5.

That last detail — that the G5 showed the biggest relative improvement — fits the geometry: the G5 lives on the main instrument panel, further aft from the firewall and the engine, with more structural compliance in the path between it and the source of the vibration. With the engine vibration that was reaching it now ~57× smaller, the residual deviation on that unit is dominated by other sources, and those sources are small.

What it didn’t fix

The thing dynamic prop balancing does NOT fix is the takeoff tumble — the PFD #1 attitude going upside down on the takeoff roll, every flight, 1–11 seconds after takeoff power application. That phenomenon was the original reason I started looking at ADAHRS data, and it has persisted unchanged through both the CAN bus rewire and the prop balance. Once two of the three leading environmental suspects (CAN noise, engine vibration) have been ruled out, what’s left is the GSU 25C hardware itself, and that’s now an open Garmin case against Service Bulletin SB 2144. (More on that in a future post.)

The takeaway

If you’ve got an experimental airplane with a fresh engine/prop install and you haven’t done a dynamic balance yet, it is — based on this single data point, take it with appropriate salt — worth the afternoon. The before/after on cabin feel is obvious, the before/after on ADAHRS deviation is measurable, and getting into the Excellent or Perfect band is achievable with patience and trim washers.

Twelve runs. Rough to Perfect.