Oshkosh/Vibration, or what I learned on my summer vacation:
First, all 550 Continental crankshaft separations and cracks have been traced to prop strikes or likely prop strikes. We are back on flight status with 15SC.
Les Doud (Applications Engineer) of Hartzell, spoke at Jeff Edwards Forum and spent 30+ minutes with me later discussing our Vibrations. The most interesting point was the weight of a metal 3 blade prop causes it to act as a gyroscope. The engine and all moving parts can create a harmonic more easily as a result. Lighter weight composite props have much less gyro effect and therefore are inherently smoother. Our stiff composite airframes are more susceptible than an aluminum one with “every rivet acting as a damper”. I previously thought the main reason was composite blades had less resonance than metal.
After all the obvious sources have been checked and fixed, the best diagnosis tool is a in flight Spectrum Analysis and Plot with the top of the line DynaVibe (tool cost $3000+). At least that is what the DynaVibe rep. said in a forum and Les agreed. Mount the sensor tightly on something solid like a spar cap, horizontal and parallel to the spar (not on anything like the panel dust cover). Determining increased first, second or third order vibrations narrows down the source. Les thought in my case of unpredictable vibration, the free iPhone ap “Vibration” could be accurate enough to find the major shake.
Les advised checking prop orientation for parallel alignment of one of the blades with #6 cylinder at Top Dead Center on compression. This is best for smoothness.
Paul Snyder, an Engineer with Lord Mounts, thinks he may be able to help. He cited the Mooney Ovation with a vibration problem that was improved by a single stiffer mount on the left front. Early Cirrus with 550’s and 3 blade Hartzell’s vibrated, later versions have 6 mounts with 4 near the front to better balance the static weight of the prop and engine. He may have suggestions when he knows the part number of the brown (silicone) mounts supplied to me by Lancair.
Oshkosh is a excellent resource for all things aviation. You can button hole the engineer/designer/owner and get a lot closer to answers. I was impressed with their willingness to help us and the lack of defensiveness on their part.
Oh yeah, the weather, just about perfect, in the 70’s most of the time. The LOBO/Lancair Banquet was great with a full house, Alberto’s Mars Curiosity talk was amazing and the new location at Best Western Waterfront Hotel is a big improvement in banquet room and food/service quality. You shoudda been there!
Steve Colwell aka “Claudette’s husband”