Balancing Propellers For RC Planes
On my website: www.rookiercflyer.com I discussed how to select propellers for rc planes. Once you have your propeller it needs to be balanced before you fit it to your model and attempt to fly with it.
An out of balance propeller produces excessive vibration that can travel through the entire airframe and, if bad enough, can affect the handling of the model. It can endanger the structural integrity of your model, loosen nuts and bolts and at its worst cause the engine or motor to part company with your plane.
Propellers that are badly out of balance can self destruct by shedding a blade (or blades) which in turn can cause the power plant to be ripped from the model. I know this because it has happened to me!
Why Is Balancing Necessary?
All synthetic materials that are used in the manufacture of propellers for model aircraft can vary in density throughout the mix. Wooden propellers, because they are made from a natural material will vary in density throughout their length.
To prove this just take a length of medium density balsa wood and try cutting through it across the grain with a sharp blade. You will feel the change in material density as variation in resistance at different points across the cut line. This is due to changes in the grain of the wood.
Wooden propellers are machined from solid wood and, again, because of variations in wood density throughout their blades, each blade if identical in profile will be different in weight. Even the hub can weigh more on one side than the other.
Because synthetic propellers are mass produced in moulds, minute differences in material density or mould inaccuracies can cause one blade to be heavier than one or more of the others. Such a propeller spinning at several thousand revolutions per minute will develop considerable vibration because of the imbalance of the centrifugal forces produced by each blade.
Not only this but a good deal of power is lost, either in the form of current in electric models or fuel in Glow/Nitro models. The effect of this is to shorten your flight times.
What is a Centrifugal Force?
The best way to explain this phenomenon is to imagine each blade of your propeller is a ball with the weight of each blade at the end of a piece of string. If we spin these weights round at the speed of the average model power unit the balls will rotate around the central axis point that represents the motor shaft.
Since the balls are traveling in a circular path, an outside force must be acting on them to keep them moving in a circle instead of flying outward. That force is the string which is pulling the balls back toward the axis, acting as what we call centripetal forces.
Centrifugal force is actually not a real force. If the centripetal forces that pull the balls toward the centre stop working (i.e. the string breaks), then the balls’ inertia takes over and sends them travelling in a straight path. If centrifugal forces were real forces and the string broke, the balls would move straight away from the centre at the point where the string let go its hold. This does not happen, however, the balls follow their paths of inertia and move in a straight line that is a tangent to the circular path (as shown in the diagram).
Many people think that if a propeller is balanced so that the blades remain horizontal when positioned that way then the job is done. This is definitely not the case! Sure, it will be much better than one that continuously drops a heavy blade but it is still not totally correctly balanced.
The first objective is to check that the hub is correctly balanced. This is done by setting the propeller in the vertical position on an accurate balancer. I say “accurate” here because the accuracy of the balancer is the true measure of the final balance of your propeller.
It is not unusual for the hub to be heavier on one side than the other, irrespective of how the blades are balanced. Take at least two checks and correct the imbalance by filing away a little of the hub material on the heavy side or, alternatively, add a very small quantity of epoxy resin to the indent in the back of the hub on the lightest side. Don’t worry if you add too much, you can always use a small drill to remove some of the set epoxy. The propeller should not move from the vertical position you place it in if the hub is correctly balanced.
The next stage is to balance the propeller horizontally. To do this place the propeller on the balancer with the blades extending outward in the horizontal position. If correctly balanced the blades should remain in the selected position. Double check by rotating the propeller through 180 degrees. A heavy blade should be corrected by gradually removing small amounts of material from the front face of the blade without changing the aerofoil shape of the blade.
Progress slowly whilst continuously rechecking. Once you reach a situation where the blades remain in the position you place them irrespective of the angle, do a final recheck of the hub balance, making any fine alterations to ensure a perfect balance.
Choice Of Balancer
There are numerous models on the market varying in price from a few pounds or dollars to really sophisticated types the will represent a considerable financial investment.
Here is a small selection of prop balancers demonstrating the variety and range of available. The unfortunate thing about the first three is that they will only help you balance the blades (horizontally). They are not tall enough to enable you to carry out the hub balance check we discussed above.
The deluxe balancer is completely flexible and will enable you to test both horizontally and vertically. Not only this but you can also check the balance of spinners. The adjustable height is useful for checking propellers with longer blades. But should you need to check propellers with blades longer than the height extensions provided, you can arrange the balancer so that the prop sits outside the supports. You can stand it on the edge of a table with the blades able to rotate beyond the edge of the table.
Here is a video provided by the distributors of this model that explains the full benefit range of the product:
If, having seen the above video, you would like to purchase this product please click on either of the following links:
Trouble free flying is the ultimate target for any radio control airplane flyer so anything we can do to eliminate problems and anything that reduces the efficiency of our models is worth committing to. As I mentioned earlier in this post, vibration is a killer and can result in catastrophic end results.
Both glow/nitro engines and especially gas engines produce more than ideal levels of vibration on their own. The last thing you want to be doing is to add to this by fitting an out of balance propeller.
Because the electric motors fitted to rc planes are virtually vibration free, the airframes tend to be more lightly constructed to save both weight and cost of materials. Because of this it is totally undesirable to introduce unnecessary vibration by fitting an unbalanced propeller.
So I will close this post by stating once again that it is essential to balance all propellers for rc planes.
Don’t forget that this is just one of a series of informational posts under the umbrella of my website www.rookiercflyer.com aimed at helping newcomers to our hobby gain knowledge and improve their progression.
Please feel free to share the link to this post with anyone you think would find it useful.
See you soon.