Back in engineering school years ago, we learned of static and dynamic balance. We learned that two dimensional objects (very small depth compared to length or diameter) could be static balanced. Three dimensional objects (large depth compared to length or diameter; IE. crankshaft) require dynamic balance. Now I read here and other places about props that have blades and hubs balanced. I don't understand what they're talking about.

Is there someone here who can set me straigtht with a good, technical explanation of balance. Thanks

You are right about static and dynamic balance.
A statically balanced propeller will probably be very close to being dynamically balanced as well.

Balancing a spinner using a static balancer is more of a hit or miss affair. Think of it this way:
An unbalanced spinner has a certain diameter and depth. Let's say that you deliberately made a spinner unbalanced by epoxying a leadshot on the inside of the spinner about halfway between the back plate and the front of the spinner.
Now try balancing the spinner by securely attaching extra leadshots to the back-plate and eventually you would end up with a statically balanced spinner. (Of course this is only a thought experiment, its a very bad idea to attach anything to a rotating prop or spinner!)
If you tried using this spinner it would vibrate badly.
The reason is that it is dynamically unbalanced. The centripetal force created by the lead shot inside the spinner and centripetal force created by the lead shots attached to the backplate create radial forces that are applied at different axial positions. The moments created by these forces will not cancel out and will cause the spinner to wobble if it were free to move about at will. Because it is firmly attached to the engine, it will insted try to make the engine wobble, i.e. vibrate.

The lesson is:
- A statically balanced object is not necessarily dynamically balanced.
- Thin objects such as propellers (large diameter with little axial thickness) that are statically balanced are also close to dynamically balanced.
- For thick objects objects (large axial dimension compared to diameter) static balance is a necessary, but not sufficient condition. It really needs to be dynamically balanced as well.

/Red B.

I don't contend to be an expert or engineer on the matter but have balanced a few props and flywheels. In layman's terms...

I have a High Point brand static balancer from B&B but the Dubro balancer is nice too. Windsor prop makes a nice one but it is dedicated to props with a certain hole size. The Top Flite Balancer is fine for small props under 12".

Anyway, as you start balancing a prop by your favorite method of removing material from the heavy blade or painting the light blade, you will eventually find that the prop may balance with one side of the hub down but not with this side up?

As the differential weight between the two blades gets to a value near or below the imbalance of the hub, you will find yourself removing material from the hub instead of the prop blade. Due to not being able to determine the actual vector of the imbalance, you will find yourself chasing the heavy area until the prop or flywheel comes into complete balance? This all tends to just go in one direction unless you over correct.

The Windsor balancer has the arbors fashioned into a clamp. You move these arbors about the prop until you find the balance point. Then you drill a new (larger) hole through the hub using a part of the hub clamping device as your drill guide. This new hole will be at the static balance point of the prop. With this sytem, you are not removing material from a blade or from one side of the hub.

Enjoy,

Jim

Both replies are right on from a technical viewpoint. As a practical matter, I have been flying powered models since 1947 (control line in those days). I usually just put the propeller on as it comes from the store (of course, I smooth the sharp edges on molded propellers). I've never had a noticeable problem with vibration due to imbalance. When I first started flying R/C I would carefully balance the prop and noticed no practical difference. I do check a new prop for gross imbalance and have discarded some (mostly wooden props). A single cylinder, two stroke cycle engine has enough inherent vibration so that any additional due to a slightly out of balance propeller would be hard to detect without instrumentation.

I totally agree with LouWon this matter. I have balanced maybe 3 props in my lifetime (Flying over 40 years) and that was more out of boredome than anything else.

HOWEVER... If you fly extreme speed planes where you are running extreme RPM, THEN balance becomes VERY necessary.

LouW. I am with you on this. I have a good friend who balances his props to the finest degree. However, I use APCs mostly and I size the hole for the Crankshaft and slap them on. I have had the same OS 70 4-Stroke for 7 years now and it is still kickin' strong. Maybe I'm lucky but I take care of my motors.

I balance my props on a magnetic balancer and write BAL on one blade with a marker. Makes me feel virtuous. I did get two or three APC 11 x 5's which had one blade noticeably thicker than the other. They needed considerable balancing.

Jim

People with the big props like >16" do more balancing. but the Menz and some other brands are reasonably close anyway?

Enjoy,

Jim

I do not agree -not to be contrary but------------
On single cyl engines - balance does make a difference, as does eliminating tip flex from any prop
APC is pretty dang good-but they do -and I don't know how miss on some .
On th tiny electric APC props -(8x3.8 SF) a rebalance does show marked improvement on some motors
th balance may only be 1" of invisible tape.
Typically tho the unbalance is not along th blade - but due to the centre being oriented incorrectly - vertically .

These sometimes require careful work along the hub -or a rebore (file off center then bore then resize).
On my big engines (running props up to34") -I really make certain the prop is dead on.
The unbalance in some singles is so bad that the prop can be used to slightly correct things -but I don't use those engines .

Thanks for all the good discussion. I'm still not clear on the issue mentioned by w8ye repeated below.

Anyway, as you start balancing a prop by your favorite method of removing material from the heavy blade or painting the light blade, you will eventually find that the prop may balance with one side of the hub down but not with this side up?

If the prop is horizontal in the balancer and doesn't move, one should be able to rotate it 180 degr., and it should remain horizontal. If the prop is rotated to vertical and stays, good. If it wants to rotate, there must be some weight difference in the hub. That difference didn't show with the prop horizontal because it is directly above or below the pivot point. The side of the imbalance will be indicated by the direction the prop rotates from vertical. Make sense? Whatever it is, I think it must be too small to worry about. i'm going with Lou W's advice.

Any static unbalance is magnified when the engine runs -
Depending on your idea of good enough - you may choose to leave it alone
This is not an "airfoil opinion" thing - this is real , demonstrable physics.
For me - I want the prop perfect or I toss it -or- I may make a newer prop from larger prop - but unless it turns out balanced and with no blade tracking problems and no tip waver -- I won't use it.
A 22" prop running at 8000 is not a toy-and a 26" at 7000 really has a lot of loads working on it.
Even the electrics - My new Hackers and tiny rewinds show real power losses running crummy props -so they get balanced on a magnetic field pivot balancer . (almost zero friction.)

I can understand then need for tight tolerances. When my Heli is off by just a little I am after it for hours trying to dial it in just right to eliminate all vibrations caused by any unbalanced items but I also haven't noticed the benefit of really getting my smaller props dead on as far a balance.

Hopkimf,

Your description of determining if the hub is out of balance is fine with me. Its probably better than the way I described it?

Enjoy,

Jim

Balancing props may be a waste of time on small single cylinder engines. Most of them are not completely balanced internally, so having the heavy blade on the counterbalance side of the crankshaft may make the engine run smoother.

The story > I am at a Formula One race, on the line warming up the engine and adjusting the needle valve during a racing heat. My engine (ST X-40) is running at around 21.5 K when it throws part of a blade. The engine picks up to 22.5 K and is running very smooth. So I race it. Engine ran great, viberation normal, and I cut 4 seconds off the best time that particular airplane had ever ran for a heat time. The blade that had come apart was half gone. It was a grain flaw in the wood that caused the blade at the hub to be full width, while the blade at the tip was only a sharp point. This prop was way out of balance, yet on the engine ran smooth.

On an electric, it is a good idea to balance, but on single cylinder internal combustion? Maybe not.

Any rotating object runs smoother when in Balance. Dynamic balance will show an improved indicator of imbalance IF one could negate the effect of air resistance on a pair of blades that may be of slightly different blade lead. (Perform in a Vacuum) At 15,000 rpm one should strive for .03 oz inch of balance.(.03 oz per inch from rotating center) This measurement can be done by applying a dab of clay weighing .03 oz located 1" from rotating center.
Clay can be used as an add-on to balance rather than the old sand paper. Of course once one has obtained good balance then remove prop material as one removes clay. Hub imbalance is easiest to correct by simply drilling (paralell to prop centerline) and fill with short lengths of screws..ex. 4-40.
Here is where the clay is most helpful. Cut screw length to suit wt of clay blob.
Present balance stand can be made better by buying more powerful magnets. (Pair for $2 at Rural King or Farm and Fleet) I made my own stand too to accomodate larger props. Existing balncers can be improved by carefully repointing arbor to eliminate flat spot on end. NEVER be satisfied with prop stopping horizontally. Hub will be out of balance.

If you had been flying RC back in those days you would have had a problem with the relays both in the tube rcvr and the rubber band escapements...both very sensetive to vibrations. That is where I became an advocate of BALANCE.
Your comments re engines being inherently 'out of balance'... I would agree but engine mfgrs make a real effort to balance by applying a Bob weight to the crank pin (usually 1/2 of the weight of the reciprocating weight: connecting rod, wrist pin and piston assy). The crankshaft with this bob weight attached is then dynamically balanced electronically to .03 oz inches. Thes figures vary with mfgr. Glen

Absolutely -
single gassers require accurately balanced props for performance to be maximized .
If one can't note any difference in power/vibration with balanced /unbalanced props -----there is something else adding to the problem.
The dynamic unbalance of any good singler cyl gasser I have tested on my flexing board test stand, peaks between 2700 and 3100 rpm .
If you still have a "whole lot of shakin goin on "- your airframe/engine/prop setup has other problems.
the Volvo wagon has a HO Mustang v8-that is real fun --

I would agree with what you say and add that the electrics are perhaps even more critical than gassers for balance. Interesting RPM range for maximum out of balance. What size engines have you tested on your Flex Board? Glen

25cc singles to 220 cc twins
the singles ALL have this 300 rpm shake -which is nothing more than where the designers have counterbalanced and futzed with timing to get thru the inherant dynamic unbalance.
In the pic- note that the 3/8 thich board is held at top and bottom -then the engine is mounted along one side -so the board can flex sideways .
On twins I mount them centrally on a 1/4" board top to bottom atachment

I also can attach using a long board across the sides
This allows me to watch the harmonics .
PS-the Volvo wagon has a HO V8 Mustang conversion- and no harmonics

I would pay for perfectly prebalanced props for may pattern and speed planes. Does anyone offer this service?

Check this site http://64.6.252.92/ So far as I can tell props I got from Supercool were perfectly balanced on arrival.

Jim