Question of balance

A subject that draws many questions within our hobby concerns propellers. How to balance them correctly, how to test the balance, the difference between static and dynamic balance, and how to accurately enlarge the shaft hole. It’s all good, valid stuff, so let’s dive straight in.

I’ve lost count of the number of times I’ve asked the question, “Are you sure the propeller’s properly balanced?” when a caller asks for my input regarding a vibrating engine problem. Almost as many times the answer is that it sits perfectly horizontal on their Super Whizzo balancer. Well, sorry to say that the propeller is out of balance nevertheless, the indication being that it’s heavy on one side or the shaft hole is off-centre (which also puts it heavier on one side). For me, the better balancers for all prop sizes are the high-point style. That said, for props of up to 12 or 13” diameter the magnetic balancer is fine, although due to height and weight limitations of the magnetic bar, this unit can’t support larger props.

Balancing a propeller is a very precise process that has to be carried out under ideal conditions, i.e. on a solid, flat bench or machine table (the table on a pedestal drill, for example). How on earth could you expect to check the balance with a ‘double pointed shaft and two cones’ type balancer held between finger and thumb? With one of those I can show you any balance position you like, just by the mere twist of my wrist. The high-point balancer has two sets of intersecting (usually metal) wheels, which have very sharp edges to reduce friction. On the intersection of these wheels sits a hard, freely-rotating metal shaft, upon which the prop is set with the aid of two metal cones, one fixed, the other adjustable. I set my balancer up with the shaft on top of one set of wheels and under the other set, as this stops large propellers from tipping over the front of the balancer (see pic).

It’s imperative that the cones on the shaft are tight against the propeller hub, as any wobble will give a false reading. What’s more, when balancing large propellers it’s a good idea to clamp the balancer to the bench or place a substantial weight on the rear of its base. Set the shaft on / in the wheels and give the prop a gentle touch to get it spinning slowly (chances are it’s already showing an imbalance by the way it’s sitting). Let it rotate one or two turns, and note where it stops. Give it another touch and again, note where it stops. If it stops in the same position or if it sits horizontally or vertically, then it’s out of balance. If it stops anywhere randomly, then it’s balanced.

Before removing any material, remove the prop from the balancer and very carefully measure the hole position - it must be absolutely central in the length and width of the propeller. Another very important check is to make sure that it’s square to the hub. An out-of-square hole will cause tracking problems, producing severe vibration, and if it’s out-of-square across the hub then you’ll see pitch variations as the propeller rotates, the oscillation action sending ripples of vibration that will be a real problem in many ways.

BALANCING AND DYNAMIC UPSET

What we’ve just discussed is static balance, i.e. the balance of the propeller in a static position. Dynamic balancing is better, this being the process that’s carried out when having new tyres fitted to your car. Here, the complete wheel is balanced whilst being rotated, the machine indicating imbalance stations around the wheel that are addressed by fixing balance weights. With a propeller there’s another consideration – the length of the blade. If you were to set up a single-blade prop you’d have a counterweight to balance the single blade, and that counterweight would need to be on the hub to prevent a dynamic imbalance. With a two- (or more) blade propeller, the blades must be in balance over their length, i.e., halfway along the blade (for example) they must all be equal in weight and dimension. The blades are first marked at equal stations - points along the blades - and each is compared at the same point on the opposite blade. How many station marks depends on the degree of accuracy you’re looking for. Checking the weight at different stations is quite an order. With wooden props it’s not a great issue as material selection plays a big part in ensuring the weight is even. With composite props it’s another matter as, being moulded, the introduction of air can be a problem, as can the varying amounts of fibres and resins. Fortunately it’s extremely rare for modern propellers to exhibit a dimensional or weight imbalance, this due to the accuracy of the manufacturing process. However, it can happen so keep it in mind for that rare time when a perfectly (static) balanced propeller still causes a lot of vibration. The proof of this is to use another prop and compare the vibration factor.

Luckily, there’s one form of dynamic imbalance over which we have good control, and that’s tracking the blade tips. If the blades oscillate when the propeller rotates, the resulting vibration is going to shake the model and, sooner or later, cause a structural failure. Check this balance with the prop fitted (ready for use) on the engine in the model. Start by taping a thin dowel (or similar) on one wing such that one end is just touching the inside tip of the prop. Remove the glow plug from the engine (to allow it to turn over smoothly) and carefully rotate the propeller until the opposite blade comes to the dowel. If it misses or hits by more than a millimetre, the propeller is going to oscillate and cause vibration. Measure how much it’s out, divide that amount by the distance from the centre of the hub to the dowel and that’s how much you need to file off the far side of the prop hub to correct the problem (this won’t be very much). If the blade hits the dowel then file the opposite side of the hub and vice versa.

Looking at the common definition: ‘Dynamic Balance (aeronautics). The state of equilibrium in which centrifugal forces, due to a rotating mass (e.g. a propeller), do not produce force in the shaft and so vibration is reduced.’

HOW TO 

As we can change the tracking balance for dynamic requirements we have complete control over static balance and that, once achieved, will assist greatly with the required dynamic balance. Keeping in mind what I said about the general quality of modern propellers, we must consider them to be dynamically balanced as far as shape and thickness is concerned. If, however, we find a static imbalance and to rectify this start re-shaping, shaving or sanding one blade, what do we then achieve? Dynamic imbalance, indeed all the good work of the manufacturer will have been destroyed. How, then, can we achieve a static balance? Well, we can add weight to the hub, i.e. the nearest section of the propeller to the shaft. Before we go any further, mind, it’s most important to check that the shaft hole is correctly centred. If not, it must be rectified by filling it with a thick mix of epoxy and micro balloons or glass powder. For wooden propellers, glue in a section of hard dowel. Accurately mark the centre position for the new hole and drill through with a drill of no more than 4mm diameter. Then, take a metal drill bit guide – made from thick metal, with a hole the same size as the drill you’re going to use – and clamp it tightly to the propeller, centrally located over the 4mm hole. Now drill slowly and carefully in a pedestal drill (or my propeller drilling jig described later) as this is a very exacting way to do the job. Don’t try to simply open the 4mm hole with the larger drill as it will certainly wander due to different material densities, and that will put you back where you started, or worse.

Okay, let’s put on a bit of weight. For this job we’ll use a 4mm drill (or thereabouts – size isn’t critical), some Liquid Gravity from Deluxe Materials or small split shot fishing sinkers, a drop or two of Roket cyano, some soft paper tissue, a black felt pen and some sticky tape (any type will do). On the propeller balancer, establish the lighter blade and mark this with the pen, thereby ensuring that you’re working on the correct blade. In the side of the hub of the lighter blade, drill a hole to near full depth and fill this (not quite completely) with Liquid Gravity, jam it in with a bit of tissue and seal with a short length of sticky tape. This, incidentally, is the procedure to follow right through the job, hereinafter called the ‘seal’. Test the propeller on the balancer and if you need to remove some weight then do so carefully. In truth, it’s more than likely that you’ll need to repeat the process, i.e. drill, fill and seal. Leave about 4mm between the holes and drill as many as required until that perfect balance is achieved. When you have perfect random position balancing, lay the prop flat on the bench – holes uppermost – and remove the sticky tape and tissue from one hole only. Drop in some cyano and a spray of accelerator to set it off quickly. Repeat for any other holes. The glue’s weight will be close enough to equal that of the removed tape and tissue, so the balance will be maintained.

For propellers that have a groove in the back, drill down into the groove and pack a bit of tissue both sides of the hole so you can fill up to the back face in the groove. If you get to a real fine point where the prop’s almost balanced, drill a few holes in the heavy side of the blade.

OTHER CONSIDERATIONS

I’ve often heard (and read) about dipping a propeller tip in paint or spraying one blade with lacquer as a way of adding weight to a lighter blade. This can cause a few problems if, like me, you like to paint both tips (mine are white or yellow) for safety reasons. If you use the painted tip method for balancing then you’ll need to use the same paint (to prevent any potential paint bubbling or lifting) and add a second or third coat to the lighter blade tip. The results will take as long as it takes for the paint to dry, and as the carrier liquid within the paint evaporates to let the paint dry, the weight reduces. What if one coat isn’t enough? How many coats do you use? I can imagine a well-unbalanced propeller with a blobby-painted tip on one blade! This, of course, will be ugly, dynamically out of balance and there’ll be fair chance that lumps of dried paint will be flung off at high speed. As to the lacquer spray, okay on a wooden prop if it’s lacquered, otherwise you end up with one shiny blade and one dull blade. And if the prop’s already coated, it would be handy to know what coating was used? Your lacquer might, for example, attack it, wherein the whole lot will have to be removed. And again, what if one coat isn’t enough? 16 coats of lacquer resulting in a weird-looking prop? Believe me, the prop would have to be so close to balanced as damn it in the first place as a coat of dried lacquer weighs three fifths of nothing.

One last trick for the unwary. If the prop’s to be used on, say, a petrol engine that has a number of caphead bolts to retain it on the prop drive hub, drill the holes for the bolts before doing the balancing act. You wouldn’t want to be drilling through a load of lead shot and glue later on when drilling the holes for the prop retaining bolts, this having forgotten that you balanced the prop earlier with a load of fine buckshot.

REAMER

For small shaft holes, stepped propeller reamers can do quite a good job if used correctly. Reaming by hand is a last resort as far as I’m concerned, but it will do the job if done carefully. To obtain the best result, use a lightweight hammer to tap the cross handle out of the reamer, and use the reamer in a pedestal drill. Have a care here as this could cause the propeller to spin around, so it’s best to clamp or use a drill vice for the job. That said, for the absolute best, most accurate and safest method for any size propeller, I recommend (drum roll and fanfare…) the WOO Propeller Drilling Jig.

WOO PDJ

With my model engineer’s hat on, there are some projects that I do to the absolute finest degree and finish, however as a practical modeller it’s often the case that any material that’ll do the job adequately is the correct material. And so it is with my drilling jig. The base must be accurate, as must be the securing bolt holes, but the rest of it can be made up using material to hand as long as it’s a form of angle iron (I used Dexion, which is light and rigid). If using plain angle iron you’ll have to do some marking out and drilling but not a lot. You’ll need two 6 x 80mm long coach bolts (mushroom head with a square section underneath) plus washers and wing nuts, a block of hardwood and a length of angle iron (you can use a flat steel bar, no less than 3mm thick).

My wooden base is dead square hardwood measuring 55 x 55 x 160mm. An easy way of obtaining such a block is an offcut of 50 x 50mm or 50 x 75mm DAR (Dressed All Round) hardwood. If you have a machine vice for your drill then you’ll hold the wood base in that. If not, use another two pieces of angle to bolt onto each side, for the purpose of bolting or clamping the jig to the drill table. Using my jig for reference – you can vary the wood measurements – I drilled two 20mm diameter holes 6mm deep using a spade drill (Speedborer) in the base, 75mm apart, as a recess for the heads of the carriage bolts. At each drilling (and setting), drill the large hole then continue right through the block with a 6mm drill. Using simple spacers, fit the bolts up through the base, fit the washers, spacers and butterfly nuts, then tighten firmly to set the square section into the wood. If you’re using bits of Dexion-type angle then you’d have placed the bolts in the block to match the holes in the angle. If, on the other hand, you’re using blank angle or bar, drill two holes to match the bolt centres and make them a generous clearance on the bolt diameter. Drill a hole in the centre of those two holes and make it up to about 16mm diameter. If you don’t have a drill that size, file the hole or grind it with a hand tool to around 16mm noting that it doesn’t have to be perfectly circular. Drill a 12mm (1/2”) hole 20mm deep in the centre of the wood block for clearance on the drilling bits.

Mount it in the vice or on the table, and centre it up so that a drill will go down in the middle of the 16mm hole. You need a length of metal rod in the chuck that’s a neat fit in the existing hole of the propeller (a drill bit can be used). Slide the propeller under the top angle, bring the chuck down and slide the metal rod through the propeller. Move the propeller around until the blades are against the jig bolts, then tighten the wing bolts. Remove the metal rod and substitute it with a drill of the size you need. Drill at slow speed through the hub, and that’s it, done and dusted, and a hole spot-on in the centre and square to the hub.

If your model is still doing the shimmy-shakes having applied the above then it’s not the propeller at fault, so you may need to think about that cheap plastic engine mount or thin firewall and rectify as necessary.

X
Login using Modelflying Login