Never balanced a prop. How critical is it and how many of you do it? What is the best methods and best manufactures of balancers out there? And just how well balanced are most props form the factory? I have an OS .46 FX with about 20 gallons an 5 crashes on it, a few crashes I had to dig dirt out of the heat sinks in the head. Several props, never balanced one, BTW they were all master air screw. And the engine still runs good. Is this just a trademark of OS engines and or master air screw props or just pure luck that this things still running like a top? Thanks..... Tony

Tony: You have brought up a subject that is a can of worms. Balancing props is a must. Not all props may need balancing, but is important to check. There are a lot of products on the market that do a great job of showing if the prop is balanced, but there is a great deal of argument on where to remove material from a prop which by the way is a rotating airfoil. As a retired aircraft mechanic from Horizon Airlines in Portland, Oregon, we never removed any material from the front face of a prop. You will find a discussion by many that I am full of B.S., but the fact remains that the shape of the front of the prop is designed to develope the lift(forward thrust) and not the back of the prop. I expect a lot of poison arrows to be flying my way for saying this, but dig deep in aeronautical engineering and the truth will come out. I suspect the best way to balance a prop is to add some white, or yellow paint to the tips in an amount that brings the prop into balance. This will also give you a visual idea where the prop circle is when the engine is running. I have been told that the only props in the market place are the Bolly line from Australia which are sold by Dar Zeelon, and others.
Whoever answers your question, will no doubt give you a thread or two to read about the subject. Just keep in mind that the prop is shaped just like the wing on your model. I hope my info helps you with your flying.

Rich S.

G'day Mate,
I balance all my props, I'm using 16" to 18" generally, but at the moment, I'm setting up a 60 size Diablo, with OS 61FX, & about 15 mins ago I balanced the 13x6 APC that I'm running on it, the post from Aerorich is dead right.
I use a Dubro balancer, & I usually only remove a very small amount from the back of the heavy blade, until it will sit horizontal, without falling either way, then I balance the hub, this is usually the part MOST out of balance, I CA a small piece of Velcro to the light side of the hub, then CA microballons or similar on to that until it will sit anywhere I put it, without falling to either side.
An unbalanced prop &/or spinner will shake the crap out of your radio gear, & bearings, in your motor, as well as cause fuel foaming, & the bigger the prop the bigger the problem becomes.

Thanks AERORICH nicely said. Iv been in the hobby for a long time and Im also a pilot. Currently employed by the Air force as a avionics technician on the F-16. I know aeronautics and airfoils, but i didnt mean to bring up bernoulli's principle. Your absolutely right about not deforming the shape of a prop and I will back you up on any argument there. Im not confused about where I should remove or add weight to a prop. I just want some opinion on how often and how many people actually do it.

The prop balancer with the lowest friction would be the magnetic one from Top Flite or maybe its Great Planes? However, this balancer starts to be marginal on props bigger than the ones you are using. The magnets are not strong enough and the arbor will fall from between the magnets.

http://www2.towerhobbies.com/cgi-bin...&I=LXHY61&P=ML

I have three balancers. One magnetic, and two High Point balancers that at one time were sold by Robart.

http://bennettbuilt.com/page9.htm

The Dubro is also good.

http://www2.towerhobbies.com/cgi-bin...&I=LXD712&P=ML

The little jobs that you hold between your index finger and thumb are not very accurate but are better than nothing.

http://www2.towerhobbies.com/cgi-bin...?&I=LXK247&P=7

On the subject of props being in balance, MAS props seem to get the most complaints about the hole being off center. The Windsor prop balancer addresses this with a prop balancer arbor that allows you to reposition the hole.

http://www.masterairscrew.com/GoCart...68&RootID=1057

Props can be out of balance at the hub 90 degrees or somewhere in between that and the tips. When you start balancing a prop, you start with the tips and end with balancing the sides of the hub.

SPINNERS
Balancing spinners is as important as the propellers. Only the Dubro and some High Point balancers allow you to balance spinners

I also balance, or check the balance of all my props.

But I would not go any further than an inexpensive finger-tip balancer.


A very large majority among us run single cylinder engines, which almost by definition, cannot be perfectly balanced, even for primary order vibration, unless an elaborate, counter-rotating, concentric mechanism is used.

And that would triple the weight and the price of the engine...


So, balancing is achieving the lesser combination of up-down (reciprocating mass) and side-to-side (rotating counter-balance) vibration... If the reciprocation mass is fully counter-balanced by the crankshaft counter-weight, strong side-to-side vibration will result. If, on the other hand, no counter-weight is used, the up-down vibration will be very strong.

One type increases on account of the other type decreasing.

---------------------------------

If your engines is a horizontally opposed engine, a 90° V engine, or a turbine; then by all means, balance the prop as best you can.

I balance all my props.

Spinners may be balanced statically with some of the tools mentioned, but not dynamically. So, I use my spinners as received.

oooooooo, thats a whole can of worms right there. Bernoulli's principal was worked out on a fluid and not in an airmass (although they are almost the same in theory, there is enough evidence of differences to disprove that the theory is relevent to aerodynamics) I have this discusion with students on a regular basis. It makes them research a little harder.
Just being a stirrer and a disscusion starter.
have a read of A.C. Kermode's Mechanic of flight.
As for the prop balancing, I start at the rear of the tip for the blades and move to the hub. for a really bad one, I will drill a few shallow depresions into the hub, then bring the weight back up if I have over done it with a epoxy micro ballon mix. then I usually take it out and dork it into the ground and break it.

Hi Tony The first thing I do is remove price and warning tags. Yes I have seen some flyers leave these on. I carry 80 grit sandpaper in my flight box . I lightly sand just the edges of both blades down too hub to get rid of excess flashing and sharp edges. I use the finger tip balancer ($5) out of the wind and check balance. The heavy blade I sand only the tip edge and not worry about hub , because it is so close to center of rotation. Some guys will tell you to put heavy blade oppisite of top dead center and not balance. Oh make sure you ream out hole before you balance. I only do this at field when needed because I own different makes of engines and prereamed props only to find out the engine needed metric instead of sae size. I want to keep this simple and not make a ritual out of this. On wood I only sand heavy tip and no sides.

I'll agree with that. There's a point of diminishing returns for the effort put into balancing a prop given that any remaining prop vibration would be lost in the noise of engine vibration. To balance, I always start by sanding the back of the blade across the whole width as best as possible starting at the tip because that's where it'll have the most affect. I tend to leave the top surface alone but it doesn't really worry me giving that a light sand because the aerodynamic shape of most props isn't exactly ideal anyway. Put it this way, you'll affect the aerodynamics of the airfoil a lot more by sanding off the moulding flash.

I've never NOT balanced a prop. An OOB prop can damage your engine, cost performance, and even shake your airplane apart.

Most composite props are pretty well in balance from the factory. Wooden props can be perfectly in balance, or so OOB that they tilt your workbench. ALL props should be checked.

I use the little Dubro teeter balancer. It works well, needs no setup, and is easy to use. If a prop gently swings down to the heavy side, I balance it. If the heavy blade slams down to the workbench, I take the prop back and exchange it.

I do NOT recommend sanding any prop, especially a composite prop, to balance it. I spray a light coat of clear poly-u on the front of the light blade until the prop balances. Use light coats and let dry between each one. Drying can be accellerated by putting the prop in a 200 degree oven or using a covering heat gun on it.

Dr.1

Brian,


I use the same with my props.

With the Bolly Clubman props, the removal of the LE and TE with a 'box cutter', no further balancing is required.

I saw a Kavan plastic prop that was curved... The blades were not exactly 180° apart...
All I could do is to heat it and hope it hardens straight, or just to throw it away.

Dar, I'll have to throw in a plug for the Bollys, I have 6 of them now and all are spot on perfect as they arrived. I have two balancers, a Robart and a simple four wheeled balancer from Hobby-Lobby. I check all props on both.

This little guy is the most sensitive and I assume the most accurate.

Depending on just how slowly the prop moves while I'm in the zen-like process of prop balancing, this is normally where I quit, whether it's the blades that I'm working on (sanding the heavy side - starting with 80 grit and finishing with 320 - from the back) or the hub/hole position (by adding CyA sped up with kicker to the high side). While I used to take the balancing to the Nth degree, I have found that a very slowly rotating prop is not an issue worth worrying about.

Dave,


Thank you for your positive remark on the Bolly props.
I checked those that I sent - there was no need to balance them.

About that four-wheel balancer... To accurately balance a prop, it is necessary to verify each of the four wheels is itself perfectly balanced.

This kind of adds four more 'degrees of freedom' to the balancing act... It is a good thing it must only be done once.

A magnetic balancer must have its hub in perfect balance...


I would more easily trust a finger-tip balancer.

I have been using Dubro high-point balancer and like it. It can balance props (all sizes) and spiners with good precision. Unbalanced props cause bad vibrations, which will demage the airframe, servos and most importantly the receiver. I've seen plane crashing due to this.

A prop hitting the running way durng landing can be badly out balanced and should be rebalanced right away.

--qc

I totally agree on always balancing. To me the argument that single cylinder engines are always going to vibrate doesn't hold water. I know that they are never in perfect dynamic balance, but how is running an out of balance prop going to help, unless the orientation of the prop is somehow magiaclly perfect to counteract a dynamic imbalance in that particular engine? It;s not going to help if the prop is out. IMHO balance the prop carefully and eliminate that variable from the equation! The day I became fanatical about balancing props is the day all my engines started running more smoothly and the aircraft certainly appreciated it too.

I always sand flash off the leading edge and tips of new props, using wet 600 grit paper. Most airfoils don't work better with flash on the LE apex.

I have to disagree a bit on the composite prop statement though - I have found pretty noticable imbalances in a large percentage of the props I've purchased, mind you I use APC almost exclusively. I recently sifted through about 30 pylon props plus an assortment of 9-6, 9-7, 8-8, 8-9, and maybe 4-5 were pretty good out of the package. Most needed one tip or the other tweaked, but some were very out of balance in the other axis, IOW one side of the hub was heavy so they would settle hard in one orientation, but with the blades at or near horizontal. Yes btw I was indexing off the molded holes - I would never even think about using the pre-drilled through hole as they come! I always ream oversize and bush the prop hub in the rear molded section. The "accuracy" of APC's drilled through holes is somewhat of a joke in my books these days and I've grumbled about it ad nauseum on RCU.

So when balancing, my advice is watch carefully how you are indexing the prop onto the balancer shaft - it must be the same method used to index the prop on the crankshaft. That way off-center through holes won't add a new factor to the balance situation.

MJD

1) acceleration (vibration) felt by the engine from the unbalanced blades of the prop = w^2 x r

Not to piss anyone off that hates math but if you look at the above equation, you will want to balance your props. When you double your RPM's (w), you quadruple the acceleration (vibration) that the unbalanced prop puts on your engine's prop shaft. When you double the size of your prop you double the acceleration that the unbalanced prop puts on the prop shaft (if you are at the same RPM's, bigger prop's are usually lower RPM's). If you triple your RPM's the acceleration goes up by 9 times. Quadruple your RPM's and the acceration goes up by 16 times. That's significant IMO.

So regardless of the prop size the formula tells you that when you increase the RPM's, the unbalanced prop wreaks havoc on the engine and airframe.

Note to scientists and mathematicians: Yes I know "w" is not really RPM's but I'm trying to keep it simple here. w is radians/second, but the concept is the same. Double your radians/second and it doubles your RPM's.

2) Use Lustercoat and spray the light tip until it balances. Add epoxy to the light side of the hub if it's off.

3) The Dubro balancer is great for big props. The magnetic one is good for small ones (Top flight?). I have and use both. I think the magnetic one is more sensitive and better for the small props (14 inches or less).

I have both, and I love the magnetic balancer for everything up to .61 size props. If anything, you can get too fanatical with it due to the sensitivity, but with practice you pretty much know when to quit as well. It rules for 1/2A props - with all my small high rpm props I spend the time to get them as close as I can. I once made the mistake of running a combat .049 on a test stand with new unbalanced prop in the mid-20's - once. Bad idea, not very kind to the poor engine at all, and I had to apologize to it before it would run again.

MJD

using mostly wood zingers in the 11 to 13 inch range i do use a magnetic prop balancer they are pretty close if not I just spray the light blade with some clear lustre coat now you have to wait for the solvent to evaporate just a minute or two and recheck it might need an other short shot I can get a perfect balance no sanding if over done just give the other blade a short spray quick easy
martin

Hey there Hobbsy you have my balancer, or should I say, the very same kind I use. I have been using this balancer for sometime now, and have not detected any bad vibrations when handling the models when the engine is operating after balancing the props. I am going to check to see if the wheels are not out of balance as Dar pointed out. That situation snuck past me. I really appreciate everyones input on prop balancing. I do not feel so alone now knowing that everyone on this thread feels balancing is important.

Rich S.

Great thread guys thanks! In my origenal post I mintioned that iv never ballanced a prop.... well I lide. "Iv never used a comercail ballancer". IV used something as simple and jack leged as a pincle to get a heavy blade to drop. A round picle not the hex shaped, or any small smoth dowl, for 46 - 60 c.i. like I hn fbood luck with my engines. Good thread though guys keep it coming.... I just g



nd I allways look for an openi vcbfvput it in every thread. " Is it just me or = engines the sh*t. Yah theres bad threads on them out there but mine sure has ate some dirt 50-100 MPH +. The remote neddle valve was broke off and I wired it back on with michanics wire, atleast two gallons with a tuned pe pulling 18,000 rpm

Great thread guys thanks! In my original post I mentioned that iv never balanced a prop.... well I lied. "Iv never used a commercial balancer". IV used something as simple and jack legged as a pencil to get a heavy blade to drop. A round pencil not the hex shaped, or any small smooth dowel, for 46 - 60 c.i. And like I said iv had good luck with my engines. Good thread though guys keep it coming.... I just gotta say, and I all ways look for an opening to put this in every thread. " Is it just me or r those OS engines the sh*t". Yah theres bad threads on them out there but mine sure has ate some dirt 50-100 MPH +. The remote needle valve was broke off and I wired it back with mechanics wire, at least two gallons with a tuned pipe pulling 18,000 rpm, still running strong and starts easy every time, man what a tank. Yah over priced, but ill pay an extra 50 bucks for this kind dependability. Im not an OS sells rep BTW... maybe i should be though uh. lol thanks agen guys. T

Holly crap where did that double post come from? OOOOPS sorry rcu

When I was doing slot cars back in the late 60s, we sometimes rewound our motors. I used two single edge razor blades with the backs stuck into a piece of balsa. Worked well. As long as the balancer allows free, smooth movement of the object being balanced, it will work.

As for OS engines...My first engine was a Picco .45 (a mean little Italian engine with lots of power, lots of weight, and lots of compression. My second was the old model Royal (average in every way). Then I got a Webra and an OS. I've never gone back on either. They're both great engines. I've also used ST and love them, too. I've heard good reports about TT Pro and others, but for me, the OS, Webra, and ST are it.

Dr.1