After syncing the carbs I was less than pleased with the amount of vibration present, especially at low RPM. This being the case, I decided to deviate from my previous plan and do the DynaVibe deal before first flight. I had planned to put this off until after Phase 1 flight test, as many people do, and fly the plane over to Akron, CO to a shop where others I know had done this with satisfactory results.
The Mothership recommends doing it before first flight, however. A member of my local EAA chapter 648, Rick Hall, owns the most expensive version of the DynaVibe balancer and offers it, along with his expertise, free of charge to chapter members (he did accept a bottle of Beefeater gin).
Using this device requires attaching three things to the engine: an accelerometer, preferably as close to the engine centerline and as near the prop as possible (shown here nearest the prop hub with the single black wire), a piece of special reflective tape to the back of one prop blade, and a laser pulse counter (the yellow device to the right). The laser and reflective tape measure prop RPM. The tach in the airplane shows engine RPM.
The engine is run at low RPM (2500 in the case of a Rotax 912), then up to cruise power (5200 with the airplane static, depending on how you pitched the prop) where the engine will live most of its life. The DynaVibe then gives an angular location for the temporary stick-on balance weight and the mass of the weight. The DynaVibe assumes the weight will be on the perimeter of the rear bulkhead for the spinner. If for some reason the weight must be attached at a different radial location, a simple calculation allows the new mass value to be determined (r1 * m1 = r2 * m2).
I tied the tail to Rick's pickup truck since I'd had the airplane jump the chocks on a previous full-power run-up. When that happened I feared I had set the prop pitch too fine, thereby producing too much static thrust. My fears were allayed when I saw the correct static RPM (5200) at rull power. On one full-power run I did throw off one of the stick-on weights, heard it hit something over the roar of the engine, but could never find it.
This process stretched over two days (due to starting mid-afternoon) but could have been done in two hours if I had been prepared. I was not familiar with where and how the brackets for the accelerometer and laser attached to the engine. Turns out I needed two M8 bolts which I didn't have. A trip to the aviation aisle at a nearby Home Depot solved that problem but wasted time.
The picture at right shows the temporary stick-on weights on the aft face of the spinner bulkhead. It was necessary to remove the paint to keep them attached at high RPM.
Van's warns against succumbing to the temptation to attach the permanent weights to the existing screws which attach the spinner to the bulkhead. Instead, a new hole should be drilled in the bulkhead face and bolts and washers should be used to match the weight of the temporary ones. Being an anal engineer, I calculated the weight of the aluminum removed by the drill (0.1 grams) and ground the required metal off the washers to get the matching weight. The required weight was 13.1 grams (yeah, I know that grams is a unit of mass, not weight, but irritating though it may be they express weight in grams in Europe where the Rotax is made).
I was initially worried that the existing nut plate or rivets would interfere with the bolt head or nut of the permanent weight, but it turned out to not be a problem.
Shown first is the aft face of the spinner bulkhead, then below it the forward face. I used large diameter washers for the AN3 bolt to keep the stack short. Ideally, the DynaVibe would be used again to check the final balance with the temporary weights but this wasn't done. I'm confident it's good.
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