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Prop balence, is it important?


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When we look at the situation of a microlight prop with a diameter of 180cm, turning at 1800rpm (6400rpm engine speed through a 3.47 gearbox), a 1gram out-of-balance at a prop tip becomes a force of 3.263 kilograms, hammering everything into oblivion at a rate of 30 hits per second. A 20gram out-of-balance situation at the tip generates a massive 65 kilograms of force. This much imbalance is not unknown, even on some big-name, expensive imported props.

Rotax specifies the maximum acceptable out-of-balance situation as 1.2 grams, acting at 78cm from the centre of the prop. This generates 3.393kilograms of force at 1800rpm. A 1.2g out-of-balance is not much - an ordinary sheet of A4 laser printing paper weighs 5 grams...

Static versus dynamic balancing

A car wheel is balanced both statically and dynamically. Static balancing is achieved by mounting the wheel on a very free-turning bearing, giving it a slight spin by hand, and marking the bottom after it comes to rest. A weight is attached to the lighter side, and the process is repeated until the wheel appears to be balanced. Such static balancing does not necessarily imply that the wheel will be vibrationless at speed. Consider the following: if the inside (say left side, as fitted on the car) of the wheel has a heavy spot on the rim, and the outside (say right side of the wheel) has an equally heavy spot diagonally opposite the first, the wheel will be in static balance, but will nevertheless be very wobbly at speed. This situation can only be fixed by dynamic balancing. This means that sensitive accelerometers are used to measure the direction and magnitude of any out-of-balance forces, so that weights can be fixed to the rim in the correct positions.

When a wheel is very narrow, like a motorcycle front wheel, dynamic balancing is not necessary. Excellent balancing can be achieved by static means. If you have a set of knife-edges to rest the axle on, it is quite possible to do an excellent balancing job in your kitchen.

So why do the experts talk about the need for dynamic balancing of a prop? It is quite true that a prop is so narrow that there is no compelling reason for dynamic balancing, as long as the hole in the prop is concentric with the mounting bolt circle. Static balancing is normally done with the prop supported on a shaft through the hole in the prop, but when the prop is mounted on the gearbox of the engine, it is mounted with 6 or 8 drive bolts. If the drive bolt circle and the hole are not PERFECTLY concentric, then of course static balance will bear no relation to balance when the prop is eventually mounted on the gearbox. A mismatch of a fraction of a millimeter may be enough to make the prop appear to be unbalanced.

In the case of non-concentric mountings, there are only two solutions. Solution number one is dynamic balancing of the prop, as mounted on the engine and gearbox, and number two is to statically balance with a jig that is mounted via the bolt circle, and not via the central hole. Since the first is prohibitively expensive due to the few facilities equipped (or willing) to do this on a microlight, the second is the way to go when the normal static jig does not appear capable of achieving a smooth running prop.

The manufacturer of a prop has the option of lightening a blade by sanding or filing of the surface. It is also often done by adjusting the length and size of the leading edge sheathing. On wooden props the addition of a bit of varnish can be sufficient. The user has fewer options - balancing a used prop is usually done by drilling a small (2mm or so diameter) hole in the end of the prop and inserting a weight of some kind. This weight may be a short piece of wire or something similar, but it MUST be securely epoxied in place. The hole is also sealed with epoxy. If you do this, please keep in mind that the maximum tip speed of a prop approaches the speed of a .38 revolver bullet. Should that weight come out it can easily maim or kill a bystander. If you do not have experience of fixing weights to a prop, get advice from the manufacturer.

Tracking

The blades of a correctly tracked prop should follow exactly one behind the other in their path around the driving shaft. An edge-on view of a prop disc when the engine is running at maximum revs should show that the prop tips track within a few millimeters of each other. This is the ideal situation, which is not often seen in real life, the reason being that the prop tips are much more flexible than the inner part of the blades, and are easily distorted by tip vortices and turbulent eddies. If you need to correct for tip tracking errors at full throttle, painting each tip a different colour makes it possible to see which tip goes where.

Due to tip distortion and variation in tip shape (sometimes introduced by the manufacturer during the prop balancing process), tracking is often done, not at the tip, but at the 75% position. This is also the area where most of the prop's thrust comes from. Tracking is then done by measuring the position of the leading edge of the blade from a fixed position on the airframe. All blades should be within a very few millimeters of the same distance. Interestingly enough, some manufacturers insist that their props be shimmed or adjusted until they track very accurately, while other equally respected suppliers are of the opinion that tracking errors of up to 15mm or so are quite acceptable.

Tracking errors are usually corrected by fitting shims under the hub mounting bolts.

Pitching

Setting all blades of a prop to the same pitch is important for smooth running. On a two-blade wooden prop the user can do very little to improve matters - the manufacturer either got it right or he got it slightly wrong. It must be said that most prop-makers do get it right. On a ground-adjustable multiblader the onus is on the user to get it right. To minimise vibration it is necessary to get the pitch of all blades to within less than half a degree of each other. The most expensive and complicated prop setting gauges are not necessarily the best, or the easiest, to work with. I know of one very simple gauge (from an unnamed eastern bloc country), giving excellent, accurate results.

Why does blade-to-blade pitch variation cause vibration? Consider the thrust contribution of each blade - total thrust from a microlight prop is usually at least 120kg. This means that on a three-blader each blade contributes 40kg of thrust. If the pitch of one blade is more than that of the other two, that blade will contribute more than its share of thrust, which will be provided as a rotating pull on the prop. Let us say that the difference in thrust is 5kg, which implies that this blade will push 5kg more than the other blades, and this thrust is centred at approximately the 75% position. This force rotates at 1800rpm, pushing the engine/gearbox/prop combination out of line 30 times per second. Think what this does to the gearbox bearings...

What is the correct pitch for a ground-adjustable prop? The answer is usually: that pitch value that allows the engine to run at maximum allowable rpm at full throttle cruise. If the engine overspeeds, the prop needs more pitch (or larger diameter) and if it never achieves maximum rpm, the pitch should be decreased. It is essential to pitch the prop correctly, since a very large part of the total thrust is generated by the last 10% of engine speed.

I would like to thank the local propeller manufacturers with whom I have had interesting and instructive discussions. Pieter de Necker especially has contributed much to my understanding of the practical realities of the art of prop making. Nonetheless, if there are errors in the above, they are all entirely of my own making.

Courtesy of

Jan Coetzee

jan-elz@iafrica.com

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So, the answer to your question is yes, prop balance is important. :)

Pitch matching of a two blade prop may be adjusted by shimming, just as tracking is adjusted.

For low airspeed machines such as paramotors, correct prop pitch may be set on the ground, without the need for rpm measurements at cruising speed.

The reasoning is that the speed at which air enters the prop is always very low relative to the speed at which it leaves, so the prop is operating under very similar conditions on the ground and in flight.

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