The critical parts of flight testing are behind me, and I'm happy to report nothing dramatic happened, unless you count the euphoria of being shot through the air by something I built. All things considered, everything went quite smoothly. But that doesn't mean there weren't issues. Even with careful building and triple-checking everything, the transition from static airplane to flying airplane surfaced all kinds of things. One thing I noticed about myself during this phase was that I had become hyper-vigilant. When you spend thousands of hours with an airplane, you notice every little thing. A sound that might go unnoticed in a rental becomes a concern in your own build. A gauge reading that shows a minimal anomaly starts to look suspicious. More than once, I reached out to A&Ps, experienced builders, and Bearhawk veterans about something that was bothering me, only to hear "that's pretty normal" or "I wouldn't worry about that." Their perspective helped me calibrate what actually needed attention. It's probably good to err on the cautious side, but constantly chasing down non-issues also has pitfalls. Several mechanics and pilots pointed out that this is normal for most airplane. Getting into reliable service takes flying, careful ground inspection, and a process of elimination. This doesn't just apply to homebuilts but to factory-new certified aircraft as well. This is probably related to the nature of low volume production of complex systems. ![[Flight Testing Debrief 1-1767124523121.webp]] ## Ground Operations Surprises **Transponder silent on the ground.** Before the first taxi, I noticed the GTX 45R wasn't transmitting. Spent a while troubleshooting before discovering the G3X inhibits transponder replies when it detects the aircraft is on the ground. There's a ground testing mode in the settings. Felt a bit silly once I found it. **Fouled spark plugs from ground runs.** I ended up doing quite a few engine ground runs. I simply felt the need to get a good feel for the engine. The next time I do this, I'll probably do far fewer. Unfortunately, this is also exactly how you foul plugs in a Lycoming. Learned that one firsthand. I replaced all eight before the first flight (I'll refurbish them later) and now lean aggressively during any ground operation. Interestingly you can tell from the G3X engine logs exactly which plugs are fouled. **Taxi speed over-indication.** The G3X showed 10 knots during what felt like walking pace. The G3X has a zero IAS calibration feature that solves this. After calibration, the G3X and analog Winter ASIs agree almost perfectly, which is satisfying. **Tailwheel stinger rotational play.** The stinger developed some rotational slop after taxi tests. I had already applied Loctite 660 along the stinger to take out lateral play. Adding plenty of Loctite 660 to the retaining bolt helped with the rotational play as well. I'm watching this for now and might consider using tapered bolts later. ![[Flight Testing Debrief 1-1767123304181.webp]] ## First Flight Distractions **Squelch cutoff set too low.** The radio picked up background noise during the first climb-out. Usually not an issue, but on first flight it's very distracting. A squelch adjustment on the Trig TY-91 fixed it, but every second of distraction on first climb is suboptimal. I should have caught this during ground checks. **CO warning in the climb.** The GD-40 alarmed shortly after takeoff, showing around 200 ppm. Got my attention. Traced it to the landing gear strut interface with the fuselage—not enough sealing where the strut passes through. Swelling tape brought readings under 20 ppm. I highly recommend an active CO warning device. I probably would have had a bad day if I hadn't caught this. **Pulsating prop RPM.** The constant-speed prop hunted during the first few flights, oscillating 50–100 RPM. This was one of the issues that seemed like a big problem at the time. What if the governor wasn't properly calibrated to the prop and needed replacement? The solution turned out to be stupidly simple. I just needed to do much more extensive prop cycling (10-15x), including holding at high pitch for several seconds each cycle. I had learned never to hold the prop at high pitch/low RPM during startup, and this was so ingrained in me that I didn't adjust the procedure when putting a new engine and prop into operation. ![[Flight Testing Debrief 1-1767118676120.webp]] ## Engine and Systems **Oil pressure warnings.** The G3X kept flagging high oil pressure. Even after adjusting the oil pressure relief set screw and changing from Aeroshell 100 to Aeroshell 80 (the OAT had already dropped quite a bit), the pressure still seemed high. After much head-scratching, the solution was stupidly simple again: I had mistakenly set the limits too narrow. Everything was fine all along. A local A&P called it a "too much data" problem. **CHTs running high in climbs.** Still working on this one. Sustained climbs above 75% power push CHTs toward 420°F. Not over limits, but higher than I'd like, especially during break-in. Experimenting with climb speeds and thinking about cowl modifications. As the engine got more broken in and the OAT came down, this problem solved itself—at least until next summer. **Cold weather starting difficulties.** As temperatures dropped, starting went from stubborn to nothing resembling combustion. Found two issues: a small primer system leak at a fitting, and my manual primer pump being smaller than on similar aircraft, so the usual 3–4 pumps weren't enough. Tightened the fittings and changed the procedure to 6–8 pumps with a 30-60s wait. I'm still refining the procedure. The coldest I've started the engine so far was 1°C. I did 8 pumps, a few cranks, then 3 more pumps. This started the engine with a total of 4-5 crank rotations. **Fuel leakage at T-fittings.** Post-flight inspections showed faint blue residue on some fittings—no drips, just evidence of minor seepage. This one surprised me but was easily fixed. Retightened all suspect connections. ![[Flight Testing Debrief 1-1767122845950.webp]] ## Flight Characteristics **Slight yaw to the right.** Above 90 KIAS, I need left rudder to stay coordinated and on heading. I wasn't sure how to correct this at first because it wasn't clear to me what caused it. I settled on using a Gurney flap on the rudder. I was surprised how small it needed to be to correct the right yawing tendency. Note that you can just tape these on for testing. My final install is also just taped on, this time with high-quality double-sided tape (3M 468MP). **Manual trim sensitivity.** Pitch trim is touchy, even on its least aggressive trim tab lever setting. Although, I'm getting used to it and it's not a critical issue. If I were building from scratch, I might consider more mechanical disadvantage in the system. **Autopilot overshoot.** The GMC 507 was overshooting heading and altitude captures. Default gains were too conservative for how the Bearhawk handles. Bank gain to 1.5 and pitch gain to 2.0 fixed it. For the autopilot to handle turns well and regain coordinated flight, the rudder trim (Gurney flap) is crucial. Before adding the rudder trim, I was concerned the two-servo autopilot might not be able to fly the Bearhawk well. Luckily, another seemingly big problem that wasn't one. ![[Flight Testing Debrief 1-1767114354054.webp]] ## Airframe Issues **Transponder intermittent dropouts.** ATC reported occasional dropouts. Tracked it to a poorly crimped antenna connector—one of those frustrating intermittent ones. I also have no idea how this could have happened since I tested all my connectors meticulously. There must have been only one tiny wire strand in there that shorted the antenna under physical strain or vibration. In the end, I was happy it was a quick fix and not a send-in-the-transponder-wait-a-couple-months ordeal. **Spinner bulkhead lock nut failure.** This one is strange. Found one spinner lock nut had backed off during a post-flight inspection, with minor play in the spinner bulkhead. Consulted Hartzell and replaced all spinner nuts. I'm very glad I caught this one early. Best case scenario of a broken spinner would have been a few months of lead time for a new one, for the low price of 50 hours of avgas. **Moderate vibration.** There was vibration across the speed range that didn't feel right, or was it normal? The problem was I couldn't tell what amount of vibration is normal. Balanced the propeller dynamically. Significant improvement. Probably better to do this as early as possible for every new prop install. **Aft side windows contacting fuselage.** At speed, the windows deflected inward enough to touch the cross tube. Added padding. **Landing gear cutout clearance.** Fuselage cutouts for the main gear struts needed enlarging on the aft side. Same for the left tank drain. **Wider gear stance on grass.** Main gear seems to spread a bit wider than expected on wet grass. This must be caused by the gear tracking inward on grippy surfaces but not on more slippery ones. Still within limits, but monitoring. **Tailwheel shimmy.** Shimmy developed on hard surfaces. Tightening control cables helped. Then I actually read the tailwheel manual in detail and increased friction on the swivel bolt and roll axle. I haven't had shimmy since. This was another black hole of concern that wasn't really a problem at all. When you search the internet for help on this, it sounds like a never-ending problem. It wasn't. ![[Flight Testing Debrief 1-1767123049175.webp]] ## Where Things Stand Flight testing is a strange mix of exhilaration and tension. Every issue could be a potential showstopper, something that grounds you for weeks while you wait for parts. But one by one, each problem found a solution, usually simpler than I'd anticipated. One unexpected reward of this phase has been the conversations. Troubleshooting led to rewarding hangar talk with A&Ps, experienced builders, avionics specialists, and fellow airplane owners. You find the people who love this stuff really quickly. I learned more in these few months than I had in years of flying. Engine break-in has been its own challenge. Flight testing at high power settings feels counterintuitive when a slight nose-down pitch can push you toward Vne. Meanwhile, the Bearhawk is also a fantastic slow flyer, and many of the prescribed tests call for lower power anyway. I found it takes real discipline to stick to the full throttle regime. The next borescope will show how badly I glazed up my cylinders. Fingers crossed it's not too bad. I'm glad I'm seeing a steady increase in confidence with this aircraft. The kinks are worked out, and the XHawk is starting to feel like a dependable machine. And most importantly, it's just fun to fly. <iframe src="https://www.youtube.com/embed/xcUTxVqoY3k?si=jAIgbe_hxhBdwdF1" frameborder="0" allowfullscreen style="width: 100%; aspect-ratio: 16/9;"></iframe>