Bearing Noise By James Goss      I am not referring to mechanical noise from a bearing in this article; instead it is in reference to electrical noise generated by the bearing. Here is something for you to think about. The other day one of my friends got in touch with me about his Postage Stamp fun fly plane he has built. It is build exactly like mine, but with an OS .32 ball bearing helicopter engine instead of a bushing engine like I was using. I have the little OS .46 LA engine on my Postage Stamp and it flies smooth and has plenty of power for vertical pull from a hover. The problem he has deals with his plane being very jerky while flying at any speed. Since his plane is identical to mine and also others that fly well, we thought his radio system might be the instigator and he was going to change out his receiver and antenna system. He was using a base mounted antenna like he uses on his helicopters. This was about a week ago and I am waiting the results.      Since then I have build another Postage Stamp and decided to use a little more power up front. This time I am using a Thunder Tiger .36 ball bearing engine. It really flies the plane well and does great maneuvers, but I did notice a difference in the handling of the plane. It was a little jerky in the air and that reminded me of what my friend had said about his stamp. I landed the plane and checked everything out, it all looked fine. I continued and had a couple of good flights with several vertical landings. I can see that more power means more fun with this plane.      On the way home I got to thinking about the random jerking situation and zeroed in on the two types of engines. A bushing engine compared to a ball bearing engine. Now we all know that metal to metal rubbing will generate rf interference for our radios and give us glitches. I started wondering if the steel balls in a ball bearing could roll without any rubbing action between them, the spacers, and the housing of the bearing. At one time or another some sliding instead of rolling of the balls would have to occur and produce a difference of potential between the two objects due to friction. I know ball bearings get hot because of friction while under a load so it only makes sense they will generate small voltages and the electrostatic field that is associated with a voltage. This could explain some of our unexplainable glitches we get from time to time. We blame the wind most of the time while in reality it may be our engine generating the signal.      I decided to look a little deeper into this topic so I setup the following experiment in my shop. I connected a small ball bearing to one end of a ¼ inch dowel 36 inches long and the other end of the dowel to a dremel tool for rotating the bearing at high speed. The outer race of the bearing was anchored to my workbench so only the inner race could rotate. For a sensor I used a small iron core coil with several thousand turns of 36 awg copper magnet wire connected to my oscilloscope. The first run resulted in too much line noise being generated by the drive motor and being picked up by the scope. I used the 36-inch wood dowel to separate the bearing from the motor for just that reason. I then connected the scope through an isolation transformer to cut down on some of the noise pickup and then got a useable signal. There is evidence that electrical noise can be generated by a ball bearing when rotated at high speed. This bearing under test was a new bearing. I imagine all bearings will exhibit this action and the older the bearing the larger the signal amplitude would become. As ball bearings wear with use the balls become out of round so I would think more friction would occur. So this is something to think about and another plus for the bushing engine.     I understand about wavelength multiples and realize the bearing will not act as an antenna as well as a piece of music wire 16 inches long connecting the throttle horn to its servo and being allowed to rub the horn. The wire would radiate much more signal interference than the bearing, but still there has got to be some radiation from the bearing. Of course as I mentioned in my article “Grounding”, if we bond the engine to the negative reference line of the battery it would help eliminate any electrical noise generated by the bearing or any other part of the engine.      I was wondering why the bearing noise has showed up more on the Postage Stamp planes than it has on others. It could be that this plane is very sensitive to any command it receives where other planes flight characteristics are not altered. Having a flat airfoil and a ¼ inch thick wing means when you move a control surface the plane responds instantly. So it may be this relationship that created a jerky in-flight situation. Remember that this type plane wants to fall out of the sky anyway so it doesn't need much encouragement to alter its flight path. Still this is the most fun plane I have ever flown no matter what engine is up front.