Prop balancing
 

I was just balancing a well known maker's 22x10 prop. I know to sand the back of the blades to get it right in that axis. What about causing a dynamic imbalance? After the blades are in balance the hub may be heavy on one side of the other. On the prop I was just messing with it was way off. The amount is equal to an inch of lead solder on the hub. I have seen instructions that say to use velcro and CA. Frankly that is really ugly may come off. So, on full sized they are balanced with lead put in holes drilled in the hub then secured with epoxy. The lead is installed on the same axis as the crank shaft, not radially. So, what is wrong with doing it that way on our props? The hole needed around 3/32 of an inch. It seems to me that a tiny hole like that that is subsequently filled with epoxy could not compramise the srength. These holes are much smaller than the ones used to secure a prop that uses multiple small screws instead of one big prop nut. There would only be one or two tiny holes, not , say six, larger ones like are often used to mount props.

RE: Prop balancing
 

Go for it and tell us your findings?

RE: Prop balancing
 

It worked great. Nothing bad happened. The engine has never run so smoothly. I found this video that explains it.

http://www.wonderhowto.com/how-to-ba...rplane-169165/

RE: Prop balancing
 

props have FIVE balance parameters:
1) Blade angle and thickness
2) blade weight lengthwise (balance)
3) prop balance cross-wise
4) hole offset
5) blade shape

ANY of these can be off in a poorly made prop, and result in severe in-service vibration.

If in balancing you alter the bladethickness out of whack, you will introduce some kind of aerodynamical imbalance. (Airfoils of different thickness produce non-equal lift.) Best way is to drill a small hole in the light blade, and tap a small lead ball in, secure with glue. (gunshot)

RE: Prop balancing
 

Sounds like we are on the same page. I did a prop today without any sanding. It is a nice looking well finished prop but not an expensive one. I put it on the balancer and found where it came to rest. I marked a vertical line that radiated out from the hole toward the cieling on the prop face under where the hub goes. That marked the light side. Just like the video I posted. I put a hole that just fit my solder 80% through the hub area of the prop. It will be hidden by the propwasher. I cut and tried my solder until I got it right. I put it in the hole and tapped it inside. A drop of CA sealed the deal.

The prop I played with yesterday does have different pitch a the tips, possibly 5-degrees. It runs smooth but not perfect.

I am thinking the business about gas engines being "shakers" might have to do with props sometimes. Being a sport flyer, and a cheap-skate I buy cheap props. My engines shake. The 3D guys run simmilar engines but they are butter smooth. They run expensive props and probably know how to balance them. I think I just improved my game.

RE: Prop balancing
 

Cheap props ALWAYS need all kinds of balancing, whilst expensive props are factory balanced by expert personnel in the production process. I have seen the cheapos out of whack on all 5 accounts I mentioned in my previous post. These cheap props are CNC machined without manual finishing and balancing. After the cnc operation there are many forces liberated in the wood that distort the propeller, which consequently does not fit the production jigs any more. Result: a cheap shaker that reduces the life of your airplane by failing electronics and failing frame integrity. 
Gassers in fact shake less than glow engines if they have the same horsepower. BUT, large engines shake more than small engines due to the larger torque pulses.

RE: Prop balancing
 

PE, Thanks for the data about props. I did not know all the things that can effect the balance. Best Regards Capt,nhttp://www.rcuniverse.com/forum/js/f.../thumbs_up.gif

RE: Prop balancing
 

I don't sand props to balance them nowadays.
I use clear spray lacquer on the back of the light blade (where it will have the least effect on aerodynamic imbalance).

For the hubs, I use small holes with Lead shot epoxied in.

I saw both those tips here on RCU a few years ago.

However on any engine I care about, I only use decent props (generally the more expensive brands) and if they are so far out of whack that they will need much more than 2 or 3 coats of lacquer or 3 or 4 pieces of shot - I don't use the prop. For "Top End" brands of props, I check them as soon as I get them (I use masking tape to hold on weights to get an idea of the balance required) and if they are badly out (it has only ever happened to me on one occassion), I send it back.

One other check I do is I put the prop on my balancer (home made setup) and check the blade tracking before I do anything else. If the blade track is out of whack, no matter what you do, that prop is not going to perform well.