Is your engine vibrating during flight ?
Chances are your propeller needs to be re-balanced.
Common causes of imbalance in propellers:
Propellers suffer wear and tear that may cause them to go out of balance. Damaged propellers either by hitting an object or the abrasion by sand blast that are repaired, almost certainly will be out of balance, incorrect Storage may also cause imbalances to develop.
Occasionally even a newly delivered propeller may be out of balance, or you may have done some modifications to your propeller that require re-balancing, also the addition of protective lacquers and leading edge materials may require the propeller be re-balanced.
There are two types of balancing of Propellers including other spinning objects !
Static Balancing: Each blade needs to weigh the same, and Centre of Gravity (CG) should be at the same radius.
Dynamic Balancing: Even if static balanced, one may have it's CG closer to one side of the prop (forward or backward) than the other blade.
The thicker the spinning object is (along the axis of spin) and the greater its rotational energy, the more critical Dynamic Balancing becomes. Dynamic Balancing requires very sophisticated laboratory or workshop equipment. Crankshafts and flywheels MUST be very carefully and accurately balanced both static, and dynamically. Fortunately for us, propellers are relatively thin objects and spin at much lower RPM than than crankshafts, so we shall concentrate only on static balancing.
All you need to do on any average propeller is to ensure that the entire propeller with all its blades fitted will not be heavier on one side than the other. If you removed your drive belt and allowed the prop-hub to rotate freely, there should be no tendency for the prop to rotate to one specific position each time, if it does it could be the Propeller, Re-drive pulley or more commonly the combination of prop Plate, Bolts & Washer causing the imbalance, to quickly check the latter, measure from the ground or note the position where the prop Stops, then mark with a texter or pencil where the prop is mounted onto the re-drive pulley, then rotate the prop plate one or two bolt holes in either direction whilst keeping the bolts & washers in the same position in prop plate, then re-tighten & re-test, if the prop returns to the same position as you earlier noted, the imbalance is in either the re-drive pulley or the propeller as you have already eliminated the prop-plate, bolts & washers, so now without removing the prop-plate and bolts from the propeller rotate the prop & bolts either way, if the propeller returns to the same location as before you will need to be check the Propeller.
There are two simple techniques for checking the balance of propellers. The first is real straightforward and serves most purposes, the second a bit more accurate.
The First and easiest method is to get a roller-bearing that fits snugly into your propeller's central hole, you can add a few turns of insulation tape evenly around a bearing that is slightly too small.
Next, you need to find a long bolt that fits snuggly into the centre of this roller-bearing.
Slide the bearing over the bolt until it stops against the bolt-head. Then fit a nut onto the bolt, and turn it until it comes up against the bearing hub, holding it in place against the bolt-head, but allowing the bearing to turn freely.
Now position the bolt protruding horizontally in a table-vice, with the bearing in a vertical position, now place the propeller carefully over the bearing, with one blade level. This blade's tip is the "Start Position". Now let go and see if that blade tends to rise or fall. Rotate the blade until the next blade is in the start position and again, note if it rises or falls. If there are more than two blades, repeat with each.
You will soon clearly see if one side is lighter than the other.
The Second and more accurate technique, you can purchase this type of balancing jig for $25 or follow the instructions below to manufacture one yourself, it is relatively simple to make but you will need to very accurate during manufacturing, this jig consists of only three basic parts as follows;
- Strong mono-filament fishing line, about 1mm or thicker in diameter and 60cm or longer (to reach from your ceiling or hang point to about elbow level.
- A 8mm diameter steel bolt, about 50mm in length or a bit less. You need to Very accurately drill a hole through the centre of the bolt, along its entire length. NB: This hole should exit at the very centre of the bolt-tip (the other end is not to critical). The diameter of this hole should closely match the fishing line thickness you use, so that the line can be passed through the centre of the bolt with a knot at the bolt head, and the line length protruding from the bolt-tip. NB: There should also be no lateral movement of the line in the hole, otherwise the line will not be in the very centre & give you inaccurate balancing.
- A "plug" of nylon, wood or aluminium, turned in a lathe, that fits snugly into the central hole on your prop, usually 1 Inch diameter. It must have a round lip on one side, to prevent it from passing through the hole. It should protrude about 35~40mm into the prop centre.
Now accurately drill and tap a hole in the very centre of this plug, to fit the bolt.
Screw the bolt into the plug, from the side with the lip.
How to Balance using this method: Place the plug into your prop's central hole, until the lip is against the prop hub.
The line protruding from the other side of the prop should be hung from your ceiling or a nail above a doorway etc. so that the prop is hanging at a comfortable height. Now the bolt-head should be at the very bottom, pointing upward with the tip somewhere inside the prop and the line continuing upward toward your ceiling/hang point.
How this system differs from the first method above, is that the prop is now hanging with its hub in a horizontal plane, not vertically as with the bearing.
You can now check the balance of a two blade prop not only along the line of the blades, but also side to side (or edge to edge). If you have more than two blades, you can instantly see which side is the heavier. Furthermore, the sensitivity of the system can be adjusted by turning the bolt and adjusting how far it protrudes into the prop. The tip of the bolt is the fulcrum on which everything balances. If the tip is much higher than the propeller's centre of gravity (approx half its thickness), then the system is less sensitive and will only tilt slightly toward the heavier side. As you adjust the bolt-tip lower and closer to the CG, it will become more sensitive and indicate smaller imbalances. If you un screw the bolt too far, the fulcrum may be under the CG, and the prop will constantly roll-over in any direction.
With this system, if you have a two blade propeller, you may find that not only is one blade heavier, but it may be heavier on one side (the blade points down, and is rolled over to one side). You need to remove material on that side, or add weight on the other blade, on the lighter edge.
You can either remove material from the heavier side or add material to the lighter side, whichever is more practical for you. Be careful not to change the overall shape and profile of the propeller.
If slightly out of balance, simply apply a bit of lacquer (or paint) to the lighter side until correctly balanced. Apply a very thin layer of lacquer evenly over a large surface, preferably the entire blade. Once the paint has dried, re-check the balance.
Use similar paint (or varnish) to the original. If clear paint, you can use a two pack type paint, where you mix two parts, such as Glatex 8 or Duco equivalent K2 type.
You can either brush the paint on carefully, or spray it on. Aerosol spray cans may be used, or a spray gun. I recommend you use a miniature air-brush designed for hobby work. These are designed to run off aerosol cans of pressurised air which are expensive on such large jobs, so if you don't have a compressor you can improvise by attached an air-hose with a car tyre valve-clip taken from an old tyre pump to your spare wheel from your car, pumped to about 7 bar (100 PSI) will supplies enough air to completely paint one prop, or re-balance 2 or 3. Re-inflate the tyre when it gets down to 1.5 BAR (22PSI).
Apply a layer of paint, wait for it to dry completely, then re-check the balance and apply more paint if and where it is required. While checking for balance with either of the two systems, ensure there is no wind or drafts in your workshop or area to affect the prop. If you open a window to facilitate paint-drying, close it again when you re-check the balance.
A well balanced propeller will run smoothly, with noticeably less vibration, hence less pilot stress and aircraft fatigue. It may not be possible to eliminate all vibration, especially on single-cylinder engines as it may not be the propeller vibrating, it may be your crankshaft, flywheel, magneto stator, a drive pulley, or the drive belt etc. All of these (except the belt), can be re-balanced by any motorcycle crankshaft-balancing workshop. They may even be able to dynamically balance your propeller, for extra smooth operation.
IMPORTANT: Storage of Propellers, if a wood propeller is stored in a vertical position for a long time, it WILL eventually go out of balance! This may happen while still fitted to your motor, so turn to propeller to the horizontal position if you leave it on the motor during long term storage.
I am not sure if this is because the moisture in the wood (there is always a small amount) gravitates downward due to its weight, or if it rises due to capillary action, either way your fine prop loses its balance and starts doing a little dance in flight.
It is common to store a prop on a wall peg or long nail, do this in the horizontal position, but rotate it 180 degrees every few weeks.
The worst way is to store a prop standing against a wall like a rifle, or hanging vertically in a propeller protective bag. The best way is to have the prop lie flat on a level surface, such as a storage shelf sitting on some thick cotton underlay.