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
Date
2026-06-13 (afternoon)
Engine time
~1.3 hr
Engine hours
23.9 → 25.2
Max altitude
~4,545 ft MSL (4,795 ft GPS)
Fuel used
14.9 gal (totalizer)
Profile
familiarization — no test cards
Conditions
warm 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 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
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.
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
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
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
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 glanceG3X 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.
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
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
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)
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 sortieThe Fuel Calculator at shutdown — 36.1 gallons used, matching the truck receipt to a third of a gallonThe 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.
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
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
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.
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 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 Flight 14.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.
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:
The FT-13-5 kneeboard card — 1-g clean stall, document the break.
The Stall Block
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’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 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’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.
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.
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 — 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’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.
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
Date
2026-05-31
Engine time
~2.5 hr (longest flight to date)
Distance
285.6 nm (GPS path)
Fuel used
30.4 gal (totalizer)
Engine hours
12.4 → 14.9 hr
Cruise altitude
~6,500 ft
Cruise OAT
+45.5 °F (cool morning)
Autopilot
~64% of the flight
Config
flaps 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, 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.
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
−1.8
low
71
−1.7
low
75
−1.5
low
82
−1.8
low
89
−1.5
low
94
−2.8
low
119
−3.5
low
130
−3.2
low
138
−3.2
low
144
−2.7
low
150
−3.4
low
154/155
−3.0 / −2.9
low
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.
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:
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:
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:
#
RPM
IPS
Heavy spot
Fix applied for next run
Running weight state after fix
1 (bare prop)
2105
0.57
345°
AN4-7 + nut + 1 washer at 180°
180°: AN4-7 N+W
2
2144
0.34
334°
AN4-7 + nut + 1 washer at 150°
180°: AN4-7 N+W · 150°: AN4-7 N+W
3
2135
0.23
322°
Upgrade 150° to AN4-10 + nut + 2 washers
180°: AN4-7 N+W · 150°: AN4-10 N+2W
4
2096
0.22
329°
150°: AN4-10 → AN4-11; washers 2 → 4
180°: AN4-7 N+W · 150°: AN4-11 N+4W
5
2104
0.15
335°
One more washer at 150° (bolt stays AN4-11)
180°: AN4-7 N+W · 150°: AN4-11 N+5W
6
2082
0.13
339°
150°: AN4-11 → AN4-12; washers 5 → 6
180°: AN4-7 N+W · 150°: AN4-12 N+6W
7
2080
0.09
340°
180° → AN4-10 N+W; 150° adds washer → AN4-12 N+7W
180°: AN4-10 N+W · 150°: AN4-12 N+7W
8
2100
0.10
346°
Add washer at 180° → AN4-10 N+2W
180°: AN4-10 N+2W · 150°: AN4-12 N+7W
9
2084
0.09
323°
150°: AN4-12 → AN4-13; washers 7 → 8
180°: AN4-10 N+2W · 150°: AN4-13 N+8W
10
2082
0.08
316°
One more washer at 150° → AN4-13 N+9W
180°: AN4-10 N+2W · 150°: AN4-13 N+9W
11
2109
0.06
307°
Add a thin (half) washer at 150° → AN4-13 N+9.5W
180°: AN4-10 N+2W · 150°: AN4-13 N+9.5W
12
2077
0.01
350°
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:
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:
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.
N997CZ Flights 1–10, color-coded, overlaid on the Phase 1 test-area pentagon. Green dots are airports where I’m approved to land; red dots are boundary corners where I am not.
Every experimental amateur-built airplane has to do its first few dozen hours of flying inside a defined geographic test area. That’s a Phase 1 requirement, written into the aircraft’s operating limitations and signed off by the FAA’s Designated Airworthiness Representative (DAR) when the airplane is issued its airworthiness certificate. You can fly anywhere you want inside the area; you can’t fly outside it until Phase 1 is complete.
I went into the conversation with my DAR with an ambitious proposal: twelve airports scattered across central Virginia, several of them anchoring the boundary of the test area and all of them on the approved landing list — places I could put the airplane down on a runway if anything went wrong. The DAR took a look and came back with a constraint from the FAA: the approved landing list had to be reduced to no more than six airports. Twelve was too many; six was the cap.
So we kept the boundary pentagon and pared the landing list down. Two of those concepts are now separate:
The boundary — the pentagon that encloses the test area — is defined by five waypoints:
KFRR — Front Royal-Warren County (NW corner)
KHEF — Manassas Regional (NE corner, my home base)
KXSA — Tappahannock-Essex County (SE corner)
KFVX — Farmville Regional (S corner)
KSHD — Shenandoah Valley Regional / Staunton (W corner)
The landing list — the six airports where I’m actually approved to put the airplane down — is six airports, three of which happen to also be on the boundary:
KHEF Manassas (boundary + landing)
KFVX Farmville (boundary + landing)
KSHD Shenandoah Valley (boundary + landing)
KHWY Warrenton-Fauquier (interior landing)
KCJR Culpeper Regional (interior landing)
KCHO Charlottesville-Albemarle (interior landing)
All six show up on the map as green dots. The remaining two boundary corners — KFRR (Front Royal) and KXSA (Tappahannock) — are red dots: they help define the shape of the test area, but I’m not approved to land there.
The constraint behind all of this came from a rule of thumb the DAR worked through with me: at the airplane’s cruise speed (~160 kt), I should be able to reach a runway within roughly a 30-minute leg. Thirty minutes at 160 knots is 80 nautical miles, and the six landing airports we kept are positioned so that no point inside the pentagon is more than that 80-nm window from one of them. That’s a reasonable margin while the engine is still breaking in.
The map at the top of this post overlays all ten Phase 1 flights so far on that pentagon. You can see how much of it I’ve actually been using: the first six flights (F1–F6) stayed close to KHEF — local pattern work and the early shakedown profiles. Starting with F7 I pushed out toward the Casanova / Culpeper practice area in the western part of the pentagon, and F8 / F9 / F10 all worked roughly in the same neighborhood. Most of the pentagon’s southern reach is still unflown.
There’s a lot of test card left to fly inside this shape, and plenty of room to do it.
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