I forgot to mention in my last post about the RC Timer carbon 6x3x3 props that I needed to balance them to get them to run smooth and quiet. In my haste and excitement to bench test them, I didn't do this and the howl and vibration initially was not good. Having never balanced a 3 bladed prop before, I had to do some research on You Tube and while doing that, I realized I had never written an article on this important aspect of balancing props for max performance, efficiency and to some extent safety. Certainly I am not an expert and a quick search on You Tube will turn ups lots of "how to" videos showing different balancers and different techniques. For fully balancing two bladed props including the hub, this is probably one of the best videos out there.
So back to my personal experiences on why it is important to balance a prop. As the gentleman in the video explains, it provides much better power and efficiency as you don't have vibration and other problems robbing you of power. It also reduces the amount of amps your motor draws which not only gives you more efficiency and endurance, but helps prolong the life of your motors, ESCs and batteries. It is kind of like buying a really nice sports car and putting bald, cheap tires on it, all that power wasted by a prop not working to it's max efficiency.
From a safety standpoint, I have made the mistake of putting a very unbalanced prop on a 2700 Kv motor at the field and after two rather noisy, inefficient flights, found that the vibration had caused the foam behind the wooden motor mount to crack quite badly. Luckily the motor didn't come free from the plane or the results would have been rather catastrophic :/ So a good lesson learned there, I now balance all new props and also ensure that the props I have in my flight box are balanced in the event I need to replace one.
I currently have two balancers, one inexpensive one I picked up online years ago and the Dubro tru spin the gentleman is using in the video above courtesy of the kind generosity of my NAMC partner Dave Messina :) You can find the Dubro tru spin online on Ebay and other places for between about $20-30 USD and knockoffs from China for even less. The smaller one or one like it is also available for $5-10 all over the internet. In the second 3 bladed prop balancing video below, the gentleman is using a DIY balancer that allows his larger prop to spin freely, so I'm sure with some ingenuity, you could make one yourself and if you already have one of the balancers shown on the right use the spindle and cone assembly from that. To balance a 3 bladed prop, you do need a balancer that will allow the prop to spin completely freely, you won't be able to balance a 3 bladed prop with the smaller balancer on the right.
Before balancing, I also inspect the prop very closely and trim off any excess bits of material that might be there from the manufacturing process either with a very sharp Exacto type knife or a bit of find sand paper. This will help your prop overall be smoother and more balanced as it spins.
I use 220 grit sandpaper, I wouldn't use anything more coarse than that regardless of the prop material as you want to remove small amounts without leaving big grooves and scratches in the prop. After each sanding, I use a piece of paper towel to remove any residue that might have been left on the blade before returning the prop to the balancer to check it. Sometimes it only takes a tiny bit of residue left on the blade to convince you the blade is balanced or not and you could keep sanding and end up putting it out of balance and then you keep "chasing your tail" to balance the prop correctly. Here is a picture of my 220 grit sandpaper and the paper towel after balancing one prop, you can see there is a lot of residue on the paper towel after wiping the blade.
Another note from another video I watched is even if the prop seems completely balanced, the wheels or magnets on the balancer do create a tiny bit of resistance that might keep the prop from showing whether it is balanced or not. The trick I learned was to gently tap on the table on both sides of the base of the balancer and sometimes it would cause the prop to tilt slightly, so a little more sanding (maybe a swipe or two) on the heavy blade would then fix that and the blade would be as close to perfectly balanced as I could get it, no matter how much I tapped the table.
In this video, you can see the DIY balancer the gentleman is using. He has chosen to use small bits of tape placed on the blades for balance. I have actually tried this before with props on park jet motors and it works for awhile, but almost every time, the tape comes off either from high RPM, moisture or heat of the sun and then my prop was out of balance in the air. I know lots of folks do it with slower RPM props and motors and it works quite well. By very carefully sanding, I find I actually lighten the prop and once balanced unless it gets knicked or something, it should remain balanced for a very, very long time and I never have to worry about the tape coming off.