Conference vmszoo::rc

    
    
    	Last night my buddy and I used my new High Point Balancer 
    for the first time. Let me tell you that this is an amazing item.
    We highpointed my main and tail rotor heads for my Junior 50. I
    was amazed at how little a descrepancy in balance could be found.
    While we were at this another buddy came over and weighted his blades
    for his new cobra. He's using superior blades and has added weight
    to work on autos. We got the blades within 1/4 of a grain difference.
    The end to end balance between blades was almost perfect. We couldn't
    measure the difference. Well for the hell of it we Highpointed his
    head along with the blade honding bolts installed and then installed
    the blades. Were we amazed. THe rotor spun like it had one pound
    of weight difference on the heavy blade. We just touched the end
    of the blade with 320 paper and found that we had take to much off.
    Now that is what I call accurate. When I balanced my tail rotor
    I had to take off a corner of the center yoke of the head to get
    it to balance. If you haven't seen one of these work you will be
    amazed. So, we had to do it. We tried to highpoint a prop. After
    about an hour of work we were down. Boy was that an experience.
    	The 27.00 that they ask for this unit is an investment in GOLD.
    Well worth anyone getting. This is one product that works well
    and lives up to the claims of the manufacturer.
    
    						Tom

    Tom, 
    
    Interestingly enough, I just learned about a new [to me] phenomena
    while reading a blurb about prop-balancing just this morning.  The
    phenomena is known as "moisture migration" and here's what it could
    mean to us:  If you've taken the time to carefully balance a prop
    and then place it in a plastic bag or such and hang it on the wall,
    in time the moisture in the wood will migrate to the bottom of the
    prop and throw it out of balance again.  The article went on to
    say that wood props for full-scale aircraft are always stored hori-
    zontally to negate this effect.  It concluded that, unless you
    immediately install a freshly balanced prop on the aircraft, it
    should always be stored in a horizontal attitude.  Amazin' eh?
    
    Adios,	Al

    RE:426.0
    
    Oops, er, Tom,
    when I talked to you before on the phone did I forget to mention
    the high point balencer? It is indeed a must tool for a copter pilot.
    As you probably found out, the only hard part is getting both rotor
    blades perfectly straight. If they aren't in alignment with each
    other it will throw the high point off. A friend of mine made up
    a little device to take the guess work out of getting the rotors
    straight. What he did was to take two dowel rods and attach them
    together on one end with a bolt. On the free ends he attached two
    short pieces of dowel that have been milled out length wise to fit
    onto the flybar rods. now measure in from the leading edge of the
    each rotor blade to the mounting bolt. At the tip of each blade
    measure in the distance you prieviously measured and make a mark.
    The length of the long dowels should be set up so that when the
    short dowels are sitting on the flybar the bolt holding the long
    dowels together should be at the tip of the rotor blade. Picture
    it as being an equalateral triangle. The flybar makes up the base
    of the triangle and the two long dowels make up the sides. Now move
    the rotor blade so that the mark you made is lined up with the bolt
    and the blade should be straight.
                                                                 
    
    Dan Eaton

Hi Al, 

Prop Balancing is very simple but it requires a lot of care. If you remove too
much material or remove it from the wrong place it can weaken the structure and
you end up with a lethal weapon. At all times try to avoid modifying the hub. 

Firstly arm yourself with a sharp flat 2nd cut file about 8" long, a range of
sand paper's, and a small block of wood about 1" x 4" x 3/4" to use as a sanding
block. 

You have purchased probably the best prop balancer on the market, however you
must treat it as a precision instrument; always keep it clean and dry, and
store it carefully. Never take the balancer to the flying field, balance your
props at home and take a selection of props to the field. 

Before you put the prop on the balancer spindle check the hole in the prop is
clean and the balancer spindle is going to fit in the center and be
perpendicular to the prop. If the prop is not true to the spindle you get false
results. De-burr the prop removing any sharp edges and obvious imperfections in
the finish. Unless you are going to correct the pitch of the prop avoid
touching the back of the prop, that is the side away from you when you are in
front of the model. All material removed for balancing is taken from the front
or sides of the prop. 

Now put the prop on the balancer and determine the heaviest point; if you look
carefully at the heaviest side you will probably see the profile of the
aerofoil is thicker at some point, that is the position to start removing
material with the file being careful to maintain a basic Clark "Y" aerofoil.
Remember when removing material the greater the distance from the hub of the
prop the greater the effect. 

Balancing on the short axis is hard as the imbalance is usually caused by
incorrect positioning of the hole with respect to the hub. Some minor
correction can be achieved by removal of material from the lowest point of both
blades. The High Point balancer is particularly sensitive so you can normally
tolerate the "short axis" imbalance.

Once the prop is basically balanced use the sanding block and sand papers to
get final balance and a smooth finish. I finish wood props with 400-500 grade
paper and then lightly coat with some CA before finally sanding and rechecking.
On glass or carbon props I finish with wet 400 grade wet and dry paper. With
the wood props it is important to re-seal the prop with the CA to prevent fuel
and moisture changing the balance again. Some people use varnish which looks
good but it takes time to dry. CA is functionally as good but a lot quicker. 

Finally make sure you maintain the profile of the prop or you loose efficiency
and can even generate dynamic imbalance (ie. one blade has more lift than the
other). The prop is a lifting section and produces forward motion on the model
from the lifting section as well as the angle of attack of each blade.

The only way to learn is to try it yourself and develop your skills by
experience but don't take necessary risks. 

Any questions ��. 

John. 

    John, thanks for taking the time to write such a comprehensive text
    on balancing. I strongly beleive that the time taken to precisely
    balance a prop pays big dividends in increased power and reduced
    fatigue on airframe, receiver and servos. I don't have means at
    my disposal to quantify the clear inbalance I see on the balancer
    but I wouldn't be surprised if it was a gram+. Given that an ounce
    imbalance on a 15" tire exceeds 50 lbs. at 60mph, it is clear that
    at the rpm's modern model engines are capable of that the resultant
    force from even fractions of a gram imbalance could easily reach
    suprising figures.
    
    So much for my self justifying rationale. You bring up a good point
    by suggesting that the short axis imbalance is more likely due to
    to some displacement from center of the shaft hole. If that be true,
    it's probably the primary cause for the tip to tip imbalance also.
    Typical density gradients of wood along this long axis may be also
    be another variable but one that you can do little about. I had
    reamed out all three prop's shaft holes to fit the larger shaft my
    engine took prior to checking the balance. I would have liked to
    see what the balance was on the factory stock hole before doing
    that. Could using the commonly available reamer for this purpose
    cause this problem or make it worse? Would a different type reamer
    or a higher quality, than the hobby shop variety be better? If the
    unreamed hole is off, that could be corrected by drilling the
    larger hole required, properly centered, using a drill press and
    a side cutting bit like a Forstner. Well, I think it's worth a
    shot anyway. I'll get a few more props (I'm new to RC and I KNOW
    I'll be using them up) and focus my attention on the hole 
    question. I'll let you know how it turns out.
    
    Al
    I checked 

All your points are valid Al but let me confuse things further. If the prop can
be drilled in a way that gives prop imbalance, the hole can also be drilled in
such a way as to give prop pitch error; if the hole is drilled at an angle you
may have one blade with a pitch of say 5 3/4" while the other may be 6 1/4".
The only way to correct this type of error is with a pitch gauge. 

I use as Fox multi sized reamer which has been modified by me to ream a slight
taper in the prop. This ensures that the prop is a firm fit, not sloppy on the
shaft. I always ream before I adjust the pitch or balance the prop. 

I have found the biggest cause of prop imbalance is through manufacturing
inaccuracy of the individual blades, rather than hole inaccuracy or material
density variation. 

Good luck 

John. 

    What is the " standard clark "Y" airfoil" ??????
    
    bob
    

    Bob,
    
    Basically, the Clark-Y is a flat-bottom airfoil with a slight
    undercamber just behind the leading edge.  All flat-bottom airfoils
    tend to be [erroneously] referred to as Clark-Y's but, without the
    slight undercamber, they're not _true_ Clark-Y's.
    
    Adios,	Al