Many companies recommend doing Dynamic Prop Balancing, Whirlwind, McCauley, Sensenich and even Power Flow Exhaust Systems. When it comes to Power Fly, the time length of their warrantee is cut in half if you do not do a dynamic prop balance.
The next question was: How do I know if I need prop balancing? If you haven?t done prop balancing - Then you need it. Do you put tires on your car without balancing them? Same is true for a prop. They both spin and the slightest unbalanced weight becomes more and more of an unbalanced force the faster they spin.
There are two types of balancing that should be done to props ? Static and Dynamic.
Static balancing: Is, or should be done by the prop manufacture or overhaul shop. Let?s talk fixed pitch and constant speed props here? Fixed Pitch: This is done by placing a precision arbor through the center hole of the prop. Then the prop and arbor are placed on a low friction balance known as ?knives?. The prop is dresses so no mater what position the blades are places the prop does not move. At this point the prop is Statically Balanced. Constant Speed: This is a bit trickier, since there is no through hole to put an arbor through. A flange mounted arbor with a counter weight length is bolted to the prop. Then the prop is placed on a different design set of ?knives?, the prop is dressed until the same balance results are obtained as with the fixed pitch prop. At this point the static balancing is done.
Dynamic balancing: Is done on the plane, the engine is run to the very top speed that it can achieve while on the ground. So, if you have a constant speed prop you do it at the flattest pitch, you also lean to achieve the highest RPM. Why? The slightest amount of static imbalance it amplified as the rotational speed increases. When on the ground you can only reach what is called a Static RPM; in the air the plane moves forward and the ?Air Load? on the prop decreases, this is know as unloading the prop and then you will see the higher RPM?s 2400, 2500, 2600 and 2700 RPM.
Balancing: Or the lack thereof, is the displacement of a rotating device (PROP and ENGINE) off the proposed circular track. This displacement is measured in units known as IPS (Inches Per Second). A small, a very small amount of weight being swung around at speed can create a very large displacement. And as the RPM increases so does the IPS; even though the weight has not changed. The effect of the weight has changed due to RPM.
How much vibration is too much? Whether you feel the vibration or not is NOT the way you determine if you should or need balancing, you must use the equipment. How sensitive are your toes or your fingers? Not sensitive enough to replace the equipment! Your plane will feel it, your instruments will feel it and you will feel the effects of it with Noise, Stress, lack of Concentration and Irritability. Just like sitting next to a crying baby on a long road trip. Solid-state instruments will take longer to show the effects but they do. The needle type instruments will show vibration with the needle bouncing around and in a fairly short period of time the jeweled bearings will fail. These instruments are: Attitude Indicator, Directional Gyro, Altimeter, VSI and the VOR needle; all expensive equipment.
A question was brought up: I put on about 55 hours on my plane and prop without dynamic balancing, did I do any damage? No, not yet. But my answer is considering what I have seen as normal or average amounts of vibration. Should you get it balanced? You bet ya! And you are playing a betting game? You are betting that no damage will be done. I?m betting it will be. Vibration Wins! You loo$e.
When balancing the prop, a reading of 0.07 IPS is considered EXCELLENT.
On the last job I did we started off with a reading of 0.35 IPS and finished with a reading of 0.02 IPS. Sometimes you can reach 0.00 other times you cannot. There are quite a few variables in balancing? Prop, Engine and all the associate attached equipment to the engine and plane. One job drove me crazy, I just could not get the vibration down. Then I LQQKed at the Starter Ring Gear? It was wobbling? It was 0.043? out of flat. And that was while it was still on the engine. I pulled the starter ring gear and put it on a surface plate, there it was 0.067? out of flat. The fellow had a real good friend with a spare starter ring gear and as if a genie granted a wish, the wobble and most of the vibration went away.
So, Flight, if you have any question, please ask. And if you are interested in getting a Dynamic Balance, I am located in NJ.
Clean side up, dirty side down,
Test Engineer, Mechanical Engineer & Electrical Engineer.