Return-Path: Received: from imo22.mx.aol.com ([198.81.17.66]) by truman.olsusa.com (Post.Office MTA v3.5.1 release 219 ID# 0-52269U2500L250S0V35) with ESMTP id com for ; Wed, 13 Jan 1999 22:36:12 -0500 Received: from RWolf99@aol.com by imo22.mx.aol.com (IMOv18.1) id 6EDJa01440; Wed, 13 Jan 1999 22:34:23 +1900 (EST) From: RWolf99@aol.com Message-ID: <6910ecf1.369d65bf@aol.com> Date: Wed, 13 Jan 1999 22:34:23 EST To: steve@raf.com, SWolf99@aol.com, MbClapp@aol.com, mattknapp@alum.mit.edu, lancair.list@olsusa.com Subject: Dynamic Propeller Balancing X-Mailing-List: lancair.list@olsusa.com Mime-Version: 1.0 <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << Lancair Builders' Mail List >> <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> >> I had the propeller dynamically balanced on my Cessna 150 today. The guy attached a reflector tape to the back of one blade. He then duct-taped an optical sensor to the cowling, looking at the tape. This allows him to determine the location of the prop blade relative to the vibration sensor, which he called a "velocity sensor" (I think it was actually an accelerometer) bolted onto the engine. To attach this sensor to the engine, he used the excess thread length of one of the studs on the engine case up near the prop flange. A fixture gets attached to that stud without needing to remove the nut. After all that, he gets a measured disturbance correlated to the "time since blade passage", which if you have a constant RPM, gives you disturbance correlated to propellor angle. Both sensors fed into a gizmo a little smaller than a laptop computer which he laid on the passenger seat. He then ran the engine while the aircraft was tied down. The disturbance is measured in "inches per second", hereinafter called IPS. Less is better. The initial run had 0.14 IPS. That's not bad, he says. He said that many folks don't even try to correct below 0.10, and some outfits don't even worry if it's as high as 0.20 IPS. He says he's generally not able to get below 0.05 IPS. I wanted the airplane to be as smooth as it was before the prop was repainted. (note to y'all -- the leading edge of the prop was getting a little raunchy. They stripped off all the paint, made the leading edge totally smooth, and repainted it at the annual inspection.) It wasn't too bad, but it vibrated enough that the writing at the bottom of an approach plate was unreadable. Up near the clip it was readable. So it was basically not possible to fly an approach without one hand on the chart, turning a normally three-handed operation into a four-handed one. Not good. When all was said and done, the prop balanced out at 0.03 IPS. This was accomplished by adding 3.3 grams to the spinner backing plate. This was the combined mass of a half-inch #8 bolt, a small metal stop nut, and one #8 washer. That's all. In this business you measure fasteners to one-tenth of a gram. It was truly astounding how such a small mass, located only at the spinner edge, could make such a difference. And to think I used to fly with bird poop on only one blade! The whole process took 90 minutes and cost $150. It's incredibly smooth now. It was worth every penny. By the way, a tachometer calibration is a by-product. Mine was reading 90 RPM low. No wonder I get book speed on a 1972 machine! - Rob Wolf rwolf99@aol.com