Tag: vibration

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.

N997CZ — Chasing Vibration: Five Flights of ADAHRS Data, a Tumbling Horizon, and What the Community Helped Me Figure Out

If you’ve been following the N997CZ build journal, you know the first five flights have been a mix of exhilarating milestones and humbling detective work. Flight 1 was a dream. Flight 2 sent me home early with erratic oil temperature gauges. Flight 3 gave me two solid hours and flaps for the first time. Flight 4 found an alternator belt slipping. But threaded through all of them was something I kept noticing and not fully understanding: in every single flight, the PFD #1 artificial horizon has tumbled upside down at least once.

That’s not a nuisance — that’s a serious ADAHRS event. Once I started pulling the deviation data from all five flights and laying them side by side, the pattern became impossible to ignore. I posted the data to the Van’s Air Force build thread and the community response was immediate, deep, and directly useful. This post is about what the data shows, what the community has experienced, what Garmin tech support confirmed, and my current working theory: this is primarily a vibration problem — with CAN bus wiring issues thrown in as a secondary layer.

What Does “ADAHRS % Deviation” Actually Mean?

N997CZ actually carries three ADAHRS units, all integrated into the G3X system. The G3X Pilot’s Guide confirms that the G3X supports up to three ADAHRS sources, with the G5 functioning as a full participant in the cross-comparison and miscompare monitoring via CAN bus — not a standalone backup, but a third integrated ADAHRS. The G3X uses two GSU 25C units — ADAHRS #1 and ADAHRS #2 — plus the Garmin G5, all cross-checked against each other. Both GSU 25Cs are mounted on the sub-panel directly forward of PFD #1. The G5 sits in a separate location, which makes its deviation data a valuable third data point — if its profile looks different from the GSU 25s, it helps localize where the disturbance is worst. That data is being pulled and will be added in a follow-up.

Before diving into the charts, an honest caveat: Garmin has not publicly disclosed what % deviation actually measures internally. We asked. They didn’t specify. So everything below is interpreted with that uncertainty in mind. It could be a cross-check residual between the two GSU 25 units, or something internal to each unit — perhaps related to Kalman filter covariance, or how much raw IMU data agrees with the filter’s predicted state. Without Garmin’s definition, we can’t be certain.

Working assumption: % deviation is a proxy for disturbance severity — higher is worse, sustained high values indicate a real problem. Garmin tech support declined to specify what maximum acceptable values should be. The 999% hard pegs have not been confirmed as caused by CAN bus dropouts specifically. Garmin’s guidance: fix the CAN bus first, then re-evaluate what remains.

Hypothesis on the 999% spikes: These hard pegs likely occur during takeoff as RPM sweeps upward from idle. The rising forcing frequency sweeps through the natural resonant frequency of the ADAHRS mount — where vibration transmission into the sensor peaks. Once RPM stabilizes at cruise, the deviation drops to an elevated-but-lower baseline. This is consistent with classic mass-spring-damper resonance behavior and with community reports of problems being RPM- and power-setting-dependent.

Several things can drive this, in rough order of likelihood for a new RV-10: an unbalanced or un-clocked prop; GSU 25 mount resonance amplifying vibration at one sensor location; loose lower cowl hinge pins transmitting vibration into the firewall; and CAN bus dropouts causing the cross-check math to fail and peg at 999%.

Five Flights of Data

Blue = ADAHRS #1 | Teal/green = ADAHRS #2. A flat, quiet trace near zero is healthy. What you’ll see instead is large % deviation in both sensors from takeoff to touchdown across all five flights — sustained throughout every flight, not isolated spikes.

Flight 1 — April 11, 2026 (First Flight, 127 nm)

Flight 1 ADAHRS deviation chart
Flight 1: Large deviation in both units throughout. ADAHRS #2 (teal) consistently worse than #1 (blue), with a hard 999% peak at 13:00:38.

The very first flight and already there’s a clear problem. Both sensors show elevated deviation from shortly after takeoff through landing. ADAHRS #2 is consistently worse than #1, with a hard 999% peak around 13:00:38. The sustained asymmetry between the two units throughout the flight points to a physical difference in what each sensor location is experiencing. The horizon tumbled on this flight — PFD #1 only.

Flight 2 — April 17, 2026 (Abbreviated, 68 nm)

Flight 2 ADAHRS deviation chart
Flight 2: Same sustained pattern. ADAHRS #2 consistently higher than #1, with another 999% peak at 17:07:09.

Identical story to Flight 1. Large deviation in both sensors from takeoff to landing, #2 running consistently worse than #1. The repeatability across two flights makes it clear: this is not random noise. The horizon tumbled again.

Flight 3 — April 18, 2026 (Two Hours West, Flaps First Time)

Flight 3 ADAHRS deviation chart
Flight 3: Both units elevated throughout. Notable concurrent peak at 11:13:06 — #1 at 229%, #2 at 233% simultaneously.

The two-hour flight shows the same sustained high deviation. What’s notable is a concurrent peak where both sensors reach similar values (229% and 233%) at the same moment — suggesting the disturbance is either broad enough to affect both locations equally, or the CAN bus dropouts confirmed by Garmin are contributing.

Flight 4 — April 18, 2026 (Alternator Belt Discovery)

Flight 4 ADAHRS deviation chart
Flight 4: Sustained deviation throughout, #2 again significantly worse than #1. Peak: #2 at 999%, #1 at 26% at the same timestamp.

Large deviation in both sensors across the whole flight, #2 running consistently worse again. Flight 4 was when we discovered the alternator belt was slipping, though the ADAHRS deviation pattern is consistent with every other flight.

Flight 5 — April 19, 2026

Flight 5 ADAHRS deviation chart
Flight 5: Both units show large, sustained deviation throughout — consistent with every prior flight.

Both sensors show large sustained deviation from takeoff to landing, same as every prior flight. The consistent, high deviation across all five flights is itself the story: this is not a problem that comes and goes — it is present in every flight, start to finish.

Coming next — G5 data: The Garmin G5 is mounted in the main instrument panel, further aft than the two GSU 25Cs on the sub-panel. The sub-panel is tied directly to the firewall via a fore/aft rib, making it a near-direct receiver of engine vibration. The main panel is at the end of a longer structural path with more joints and compliance. If the G5’s deviation runs lower than the GSU 25s — which early recollection suggests — that would directly implicate the sub-panel’s structural coupling to the firewall. That data is being extracted and will be added in a follow-up.

What the Van’s Air Force Community Has Experienced

I posted the data to the N997CZ build thread on VAF and the response was both immediate and sobering: this is a well-known problem, and multiple experienced builders have been down this exact road.

The GSU 25 Mount Location Is the Central Issue

Both GSU 25s are mounted on the sub-panel in the upper-left corner — close to the display, accessible, tidy. But multiple builders reported this location is problematic on the RV-10, and several noted a frustrating catch: Garmin’s ground-based vibration test passes in essentially every position. The only meaningful test is in flight at full power.

The community consensus is that stiffening the sub-panel — adding 0.063 angle stock, doublers, or bracing — doesn’t reliably solve the problem. Making the bracket stiffer just makes it a better conductor of vibration rather than isolating it. Multiple builders who resolved the problem did so by relocating the GSU units entirely: some to the back of the GDU display (despite Garmin’s manual discouraging this), others to a remote aft mount.

The community’s blunt summary: ground vibration tests are a waste of time for diagnosing this. You can’t simulate full power and flight conditions on the ground, and the GSUs will pass the test in any position — then fail in flight. The only data that matters comes from the air.

The Cowling Hinge Pin Fix (An Unexpected One)

One builder shared a fix I wouldn’t have thought to look for: the standard Van’s lower cowl piano hinge vertical pins. Their ADAHRS problem persisted through a sub-panel stiffener and a GSU hardware swap under the Garmin service bulletin. The breakthrough came when someone noticed the lower cowl vertical pins allowed a tiny amount of flex — detectable by hand-pressing the sides of the cowl against the firewall. Replacing those pins with slightly oversized ones resolved the AHRS drift over 30+ hours of subsequent flying.

The diagnostic tell was clean: the ADAHRS ground test passed with the cowl off and failed with the cowl on. N997CZ already uses heavier-than-standard pins, but I’ll be checking the lower vertical pins specifically — it’s a low-cost check worth doing early.

Garmin Service Bulletin SB 2144 — Both My Units Are Affected

Garmin issued SB 2144 specifically addressing GSU 25 units susceptible to “acoustic noise energy” — their language for vibration-induced sensor degradation. The bulletin was traced back to COVID-era supply chain issues that resulted in different MEMS components being used in a production run of units. Both of my GSU 25Cs fall within the affected serial number range.

Some builders had both units replaced under SB 2144 and reported clean results. However, others found the hardware upgrade alone wasn’t sufficient — it took the cowl pin fix or a mount relocation on top of the new units. The SB appears to be a necessary step, but not necessarily sufficient on its own.

Van’s Service Bulletin SB-00028 — Rubber Isolators

Van’s issued SB-00028 (currently RV-12 specific) directing owners to assess ADAHRS performance and, where issues are found, install rubber isolators on the GSU mounts. This directly contradicts Garmin’s guidance to mount to “the stiffest part of the airframe.” The community notes the irony, and the flight experience of multiple builders suggests Van’s got it right.

The Stiff vs. Soft Mount Debate

The intuition for stiff mounting is that it keeps the sensor stable relative to the airframe. But that logic breaks down when the airframe itself is vibrating: a stiff connection transmits vibration energy directly into the sensor. The natural frequency of a mounted system is fn = (1/2π) × √(k/m). You want fn well below the engine’s excitation frequencies — which means lower stiffness k and higher mass m. Soft rubber isolators reduce k; adding mass to the isolated platform reduces fn further.

A useful analogy: mechanical vibration systems map directly to RLC electrical circuits — mass to inductance, spring stiffness to 1/capacitance, damping to resistance. A low-pass filter attenuating high-frequency noise is doing the same job as a soft, heavy vibration isolator. The back-of-GDU solution likely works through both added mass and reduced stiffness in the mount path, arriving at isolation inadvertently.

Mechanical vs. electrical vibration isolation analogy
Mechanical mass-spring-damper system and its RLC circuit electrical analog — the math is identical, so filter design intuition translates directly to mount design.
Softer spring and heavier mass lower the resonant frequency
Softer spring (lower k) and heavier mass (higher m) both push fn down — the goal is to get the resonant frequency well below the engine’s excitation band.

Even Aft Mounts May Need Additional Reinforcement

One builder who relocated to Van’s designated aft ADAHRS mount (behind the baggage compartment) still had significant vibration issues during Phase 1. The eventual solution was adding substantial metal reinforcement to the mount structure. Their full troubleshooting documentation is in a dedicated VAF thread: GSU-25 Remote Mount Vibration Issue — worth reading before deciding on a relocation strategy.

What Garmin Tech Support Confirmed

CAN Bus Dropouts Confirmed in Flights 1–3

Garmin confirmed intermittent CAN bus dropout signatures in the first three flight logs. What causes the 999% hard pegs specifically has not been established — the dropouts are a confirmed concurrent issue, but attributing those particular readings to the bus alone would be premature. Garmin’s guidance: fix the CAN bus first, get clean logs, then evaluate what vibration problems remain.

CAN Bus Length: ~87 Feet vs. 66-Foot Maximum

Garmin’s maximum recommended CAN bus length is 20 meters (66 feet). My estimate puts the total run in N997CZ at ~87 feet — about 21% over spec. Beyond that length, signal reflections and attenuation degrade bus integrity, especially with non-spec cable.

Wrong Wire: Standard Twisted Pair Instead of 120Ω Controlled-Impedance

The CAN bus requires 120Ω characteristic impedance. Standard aircraft twisted pair (~$1/ft) doesn’t reliably hit that spec, causing signal reflections that look like dropouts at the receiver. Garmin specifically recommends Carlisle IT P/N CAN24TST120(CIT) or GigaFlight P/N GF120-24CANB-1 (~$7/ft). Estimated rewire cost: $420–$560 in cable for a 60–80 ft run.

Two Problems, a Ranked Action Plan

Problem 1 — Fix First (per Garmin): CAN Bus. Wrong wire spec, over-length run, confirmed dropouts. Resolve this completely before evaluating what vibration problems remain.

Problem 2 — Address After CAN Bus Is Clean: Vibration. The tumbling horizon and escalating deviation pattern across five flights make it clear something is wrong — the clean-CAN-bus baseline will tell us how much of it is vibration.

  1. Measure CAN bus run length precisely before ordering wire.
  2. Rewire CAN bus with Carlisle IT CAN24TST120(CIT) or GigaFlight GF120-24CANB-1. Verify 120Ω termination at both ends.
  3. Re-fly and pull fresh logs — establish a known-good CAN bus baseline.
  4. Dynamic prop balance — highest-yield vibration fix, lowest cost. Also explore prop re-clocking.
  5. Contact Garmin re: SB 2144 — both GSU 25Cs are in the affected serial number range.
  6. Inspect lower cowl vertical hinge pins for any play. Replace with oversized pins if flex is detectable.
  7. Evaluate GSU mount options — rubber isolators on current sub-panel location, or relocation to GDU back-panel or aft Van’s mount with reinforcement.
  8. Post-fix flight test — five clean flights, deviation at baseline, horizon that stays put.

I’m cautiously optimistic that this is all solvable — the VAF community has collectively worked through every one of these failure modes, and there are clear paths forward. But I want to earn that optimism with data, not just a plan.

If you’ve dealt with ADAHRS deviation on your own build, or have experience with the SB 2144 units, I’d love to hear from you in the comments or over on the VAF thread. This community has already saved me multiple trips down dead ends and I’m grateful for every one of those replies.

N997CZ is a homebuilt experimental aircraft (Van’s RV-10). This post documents personal experience during Phase 1 flight testing. Nothing here constitutes maintenance or airworthiness advice. Consult your avionics manufacturer and A&P/IA for guidance specific to your aircraft.