prop balancing

[tinaroel]

Hi, I did a search on prop balancing and didn't find any answer to my question...anyways I have the tools to balance my prop properly but how will you know when a prop is properly balanced???? :confused:

[freddy warbird]

With electric props like APC, I just laterally balance them. Basically I position the prop @ 3:00 And 9:00 And then position it 180 degrees opposite and recheck.I will also rotate the centering cones as I re-check the balance. I look to see that the prop won't move and prop blades are equal with each other @ and @ 180 degrees. But with my big gas ships, I balance the blades and the hub. This is more involved and can be done with any prop, but I have found it only necessary on the big boys. Balancing propellers is important for a vibration free spinning prop. It also is good for the bearings in the electric or gas motor/ engine. Plus the motor/engine won't work as hard being vibration free. It is also a good idea to run a spinner. Believe it or not the prop will loose about 4% efficiency without one.

[tinaroel]

Well if you position the prop at 3:00 and 9:00 that means its parrallel to the ground yes? and that means if the prop is balanced properly it should stay at the 3:00 - 9:00 position yes?

[blizzardwarrior7]

Yes...and Yes..

How ever a prop doesnt lose 4% efficiency without a spinner....

[freddy warbird]

Well if you position the prop at 3:00 and 9:00 that means its parrallel to the ground yes? and that means if the prop is balanced properly it should stay at the 3:00 - 9:00 position yes?
You also want to rotate the prop. 180 degrees and recheck.

Here is some text from Model Sport Aviator

You know that all propellers must be balanced, right? Unbalanced propellers cause
excessive vibration, resulting in three major problems. First, the engine’s bearings wear quickly.
Second, the onboard radio components, especially servos, suffer excessive wear and can fail
early. Whenever a servo quits in flight, much of the fun of flying RC models is diminished.
Third, an unbalanced propeller will cause a 3%-4% rpm loss. An engine that would have
turned a balanced propeller at 11,000 rpm turns an unbalanced propeller at only 10,600 rpm.
Climb rate and aerobatic performance are reduced.

SPINNERs BOLLY WEB SITE:As can be seen from the diagrams, a large diameter spinner or fuselage doesn't have as large an effect on propeller efficiency as expected. Using an exposed propeller nut will reduce propeller performance, instead a short blunt spinner flaring into the fuselage should be used.

Here is a comprehensive guide to prop balancinghttp://www.masportaviator.com/ah.asp?CatID=2&ID=37

[freddy warbird]

Yes...and Yes..

How ever a prop doesnt lose 4% efficiency without a spinner....

Thats interesting. I read in a resent article of a model magazine that a lot of drag is produced by the inner area of the prop.That the actual working part of the blade is much further out. Choosnig the correct size spinner is important for good efficiency as well as aerodymanics. And running with just a nut one could notice as much as a 450 rpm drop or 4% loss. Even the Bolly manufacturig CO. is saying something Similar. :confused:


[abenn]

Very interesting. Must go get some small spinners to cover my bare nuts.

[ivanc]

There's some inconsistency between the text and pictures. Looking at the pictures the only case where we have increased prop efficiency is the 8-th one - bare prop in front of a body @ 33% of prop diameter. The text above says "Using an exposed propeller nut will reduce propeller performance, instead a short blunt spinner flaring into the fuselage should be used".

In fact, the hub of a prop doesn't produce thrust, instead it creates drag. The higher the speed, the more drag from the hub. If you have a fast plane, you'd like to use a spinner to reduce this drag. On slow flying and 3D planes it really doesn't matter whether you use a spinner or not. OTOH I have to do a static experiment (plane on the ground, engine spinning prop) with and without a spinner - I wonder if a spinner would get some more rpm out of the engine.

[ctdahle]

Ivan, and others. If you measure with an accurate tach, you will find that in most cases, there is indeed a decrease in RPM without a spinner. However, this is not the only consideration.

For beginners at the "learn to land well, so I'm breaking lots of props" phase of training, I say ditch the spinner because it is taking you lots of extra time to remove it and replace it everytime you break a propeller. Also, youv'e probably already broken the spinner at this stage and there is not a lot of point in buying a new one until you have passed it.

However, once you are at the "I can land and take off really well so by golly watch me do a lomecevak" stage, get that spinner back on because it is also known as a crush zone and might save your crankcase when you pull back on the stick to pull out of low inverted pass.

But this thread is about prop balancing.

Did you ever notice that when they balance your tires, they spin them and the machine tells the mechanic to put 6 ounces of lead at 3 o'clock and then add 2 more ounces at 9 o'clock? Why didn't they just put 4 ounces at 3? They may put weights in two or three different places on the rim just to get it perfect.

I've owned balancers with ball bearings, razor blades and magnetic suspensions. Flip around the mounting cones, get a bit of grease on the wheels, have a bit of un-noticed grit in the hub and your efforts at balancing can take a balanced prop completely out of whack. Balance your prop carefully as you can on your razorblade balancer and then give it to your buddy to check on his magnetic suspension balancer, and half of the time he'll say your perfectly balanced prop is out of balance. You'll say that about his too. Balance on two different balancers and you'll get two different results.

Once you've balanced with care and precision, put the prop on your plane. Is the prop shaft absolutely straight? Is the motor perfectly balanced? The last time you pulled the drive washer off, did you get it back squarely on the collet? The 1 hole, 2 stroke motor is inherently unbalanced already and your perfectly balanced prop isn't going to change that. The truth is that if you could dynamically balance the prop by perfectly lapping it to the prop shaft and stratigically placing weight where it needed to be to create a perfectly running motor, you'd probably still be out of balance because one prop blade was pulling harder than the other.

After a very good flyer explained all this to me, I threw away my balancers and now I do what he did. When I am prop shopping I take the prop and spin it on the shaft of a screwdriver. If the same blade stops down three spins in a row, I put it back on the shelf and pick another.

If this test shows it to be unbalanced, it is too far out of whack for me to correct. If it is balanced according to the screwdriver test, then even if it is off, I can't accurately do anything to improve it.

[ivanc]

Did you ever notice that when they balance your tires, they spin them and the machine tells the mechanic to put 6 ounces of lead at 3 o'clock and then add 2 more ounces at 9 o'clock? Why didn't they just put 4 ounces at 3? They may put weights in two or three different places on the rim just to get it perfect.One of the weights is on the outside of the rim (relative to the vehicle), the other one is on the inside. A properly balanced tire should have maximum one weight on one side of the rim. If it has two or more - the guy has absolutely no clue what he's doing.

Back on topic - it's quite rare to buy a prop that is way off balance and even such props can be balanced. Before balancing the prop, wedge the hub of the prop - cut all excessive material that came out of the mold. This will ensure proper prop mounting on the balancer and crankshaft. Do it AFTER reaming the prop to the right size (matching the crankshaft).