Conference vmszoo::rc

    I always balance my low wing planes upside down.  You want the CG
    to be below the point you are supporting the plane so that it hangs
    like a pendulum in a stable position.  Then you want to get it to
    sit level when in balance.  I made a balance stand using two dowels
    with pencil erasers stuck on the ends.  Now, I can set the plane
    on the stands and really balance it well.  Sure beats trying to
    hold it with fingers and guess where the balance point is.
    
    Charlie
    

    Interesting concept Charlie, I'll have to try that when I finish
    building my Super Chipmunk!  I never really thought about the weight
    being suspended...makes sense!
    
    Ken

    Don't forget to do a left-right balance too.  I usually try
    to balance the wing and stabilizer both before and after
    I cover them, then balance the whole plane by holding just
    the tip of the nose and the end of the tail.
    
    Glenn

    re .3
    
    The side to side balance should be performed with the engine / muffler
    in place.  I do it before covering but with everything in the fuse.
    I then tie a thin wire around the prop shaft and to the tail wheel
    and suspend the plane upside down.  I add washers in the wing tips
    or remove material in the tip centers to balance.  Covering should
    not adversly affect the side to side balance, but it's a good idea
    to check afterward.  If you wait until after you cover the wing,
    it's hard to do the balancing.  I have found that the muffler hanging
    out the right side almost always causes the right side to be heavy.
    Now, I try to use the heavier balsa pieces on the left wing panel
    to compensate for this.  I think one of my kits suggested this.
    
    Charlie
    

re Note 547.4   
>>    I then tie a thin wire around the prop shaft and to the tail wheel
    
    I had missed this when Charlie first wrote it, but I later read about
    the technique somewhere and tried it on a high wing plane and on a
    shoulder wing.  It is described again in the current (2/90) issue of
    R/C Report with a diagram on page 12.  The technique is very easy and
    very accurate --- more so in both ways than the eraser/finger way. 
    
    To briefly reiterate, suspend the plane in a horizontal orientation
    from a single hook in the ceiling by two or more delicate fishing
    lines.  Suspend a plumb bob from exactly the same point of the same
    hook.  When everything is still, the plumb bob will point through the
    CG no matter what the orientation.  If the plane is horizontal,
    the plumb bob points to the balance point in both dimensions.
    
    Where the fishing leader is fastened to the plane will affect the 
    ease of use and accuracy, but not the validity of the technique. 

    In addition to balancing fore and aft, and side to side, there is also 
    the vertical position of the center of gravity/mass to consider.  In 
    reading a construction article for a high wing, old-timer type plane, 
    I noted that the designer stressed mounting the receiver and battery 
    high in the fuselage to avoid the "pendulum" effect.  

    He mentioned, but did not explain, that a low center of gravity had 
    negative effects.  I believe he was concerned about spin stability.  
    He even asked the question "Why do all those electric flyers mount the
    batteries on the bottom of the fuselage?"  

    At that point I became confused, and more than a little interested,
    since I will be flying electrics.  I thought that the pendulum effect 
    was beneficial to stability, and was the reason that most high wing 
    planes are quite stable.  At least when flying right side up!

    I'm going to order one of the books on model aerodynamics reccomended 
    elsewhere in this conference, but meanwhile, can someone explain why 
    a low center of gravity would be bad in a trainer or mild sport plane?

I agree with you Bob, it would seem that a low cg would be the
best, precisely because the "pendulum effect" is the reason why
high wing trainers are used.

However, I can speculate that it is not beneficial if you have a
high wing ship with ailerons.  These tend to be so stable that
some of the effect of the ailerons is lost, to the point that
adverse yaw (the drag of the upturned aileron causes a yaw in a
direction opposite to the turn) is a serious problem.  

All of this is based on my vast experience flying exactly <0>
planes with ailerons -- if you discount the time the club
president let me have the stick on his plane and I immediately
put it into a death spiral at 50 feet while trying to do a point
roll!

    re: .6, .7
    
    A few things to keep in mind here with respect to high wing oldtimer
    types: they generally have deeper than average(compared to modern
    high wing trainers) vertical fuselage dimensions. The pendulum effect
    of a significant weight set low in the fuse. will be magnified because
    of the greater moment arm acting between the weight and the center
    of lift, which is located in the horizontal plane of the wing. Any
    manouvers involving roll and yaw must be initiated by only the rudder
    and wing dihedral, remember we're talking no ailerons. The weight
    mounted low would resist the roll/yaw forces, but once set into
    motion would have a tendency to "keep going" resulting in a slow/sloppy
    recovery when the rudder returned to neutral. This is usually what
    is meant by the pendulum effect, sort of a wallowing around neutral.
    
    Bob, if you're putting an electric power setup in a modern high
    wing trainer, the vertical displacement of the mass won't be enough
    to notice, ailerons or not . 
    
    The motor batteries are mounted low, even in electric old timers,
    for strenght/construction considerations. It's easier and stronger
    to anchor the batteries down on the floor, than to mount them higher
    up in the cabin amidst the open stick structure of the typical old
    timer.
    
    As far as ailerons and adverse yaw on high wingers are concerned
    as long as you have little or no dihedral in the wing, ailerons
    will work fine assuming proper differential in the throws are observed.
    Granted, a high wing design isn't going to be transformed into a
    snap rolling fiend just by adding ailerons. The inherent stability
    will fight the roll forces to some extent. I guess that's why you
    don't see too many high wing pattern ships- but bipes are another
    matter.
    
    terry
    

    I know of three ways to find the CG of a plane --- the classic 
    fingers-under-the-wings technique, the pencils-under-the-wings
    variation, and the hang-it-from-the-ceiling method described in
    547.5 and the 2/90 issue of RC Report.  The finger method is the
    fastest and least accurate --- useless for roll balance but handy 
    at the field.  My pencil eraser device sits in a corner, never to
    waste my time again.  The hanging method is fast and accurate and
    very easy.  In one measurement the CG is located both fore and aft 
    and left to right with a accuracy/reproducibility of 1/8 inch.
    
                   ______________j________ ceiling with small cup hook
                                /|\
                               / | \
                              /  |  \
                             /   |   \
                            /    |    \ fishing line with loops at ends
                           /     |     \     (no knots or loops at hook,
                          /      |      \    but twisted twice over hook
                         /       |plumb  \   to avoid the plane tilting)
                        /        | line   \ __   
                       /         V         / |  loops around tail and 
                      )_____======________/  |     around prop shaft
                      oo__________________=====                    
                      (     OO             o

    The plumb bob line terminates in my hand and is lowered until the bob 
    is just about to touch the plane.  That point is marked on a bit of tape.

    Dumb question follows...8-)
    
    When balancing a low-wing aircraft upside down which is better: to have
    the nose slightly down or slightly up ?
    
    Confused-E.

I would think you'd prefer to have it nose heavy which would push the nose 
down/low. (until the repeal the law of gravity 8^)

    Low wing planes are "supposed" to be balanced upside down. That said,
    nose down is still nose heavy and nose up is still tail heavy. I would
    first balance according to the plans and see what I got. If it was 
    "slightly" nose heavy, I'd leave it and see how it behaves. It's always
    better to be a little nose heavy  than tail heavy. From there you can
    start working the CG back until it's where you want it. If you balance
    according to the plans and it comes out level (spot on), I'd leave it
    there also.

    What I have done is to move the CG forward about 1/4 - 1/3 " from where
    it has been for a good while now. The spec for the CG is 3" +- 1/4"
    from the LE, but when I looked it was approx 3.25". Now its about 2.75"
    or so. I am going to try it tonight and see if it any flies better. In a
    180 degree stall turn I hope that with the CG forward that it will have
    less of a tendancy to flop over. I think that over the last few months
    I did add some heavier servos (mini->std) rearward of the CG and may
    have made it a little tail heavy. It sort of flies in a tail heavy
    manner aka stalls happen a little to easily. 
    
    Testing the trimming last night indicated that the CG was OK for
    upright and inverted flight. 180 Stalls were iffy. Humpy bunps were or
    vertical 1/2 rolls indicated that the lateral balance was very
    sensitive. 
    
    Back to the flying field !
    
    Thanks.
    
    E.

    How about Level?  Most planes can be balanced by moving the Ni-Cad
    around.  I agree that CG forward is better than CG back but there is
    only one really correct CG that can be found by flying.  If there is a
    question as to where the CG should be, then start on the conservative,
    forward side but don't leave it there unless you like the flight
    characteristics.  I prefer to have the CG where the plane tracks well
    but stil will snap and spin with enough control throw.  I've flown some
    planes with the CG too far forward and they won't snap roll and don't
    spin well.  They also tend to run out of elevator on landings because
    they require more and more up elevator to keep the nose level as you
    slow down.  I've flown some planes with an AFT CG that were downright
    uncontrolable.  They hunted up and down in pitch and were impossible to
    fly well.  This setup will eventually bite you when you get it too slow
    on takeoff or landing.
    
    CHarlie
    

    Level would be a good compromise. I remember balancing it when I
    originally built the plane and it was near level but not quite and as
    the previous notes say I was unsure about the nose up or down. I would
    guess that I am now nearer the original balance point. As you know it
    is a little more difficult to adjust the CG when the servo tray etc is
    glued in place. 
    
    Since building it maybe a year and a half ago I have added epoxy to
    glue the U/C plate on again (maybe twice), some more on the tail when
    the horizontal feathers came slightly adrift and replaced the mini
    servo on the throttle to a std one. The original CG would have not
    moved all that much. And its difficult to see it going backward as it
    appears to have. Taking moments and positioning of what was added
    changed I would have guessed it hadn't moved much.
    
    Well I tested the new CG last night and I was pleased with my results.
    The 180 stall turns were predictable in that they didn't fall over. I
    checked the normal and inverted flight and I needed less up than before
    in the inverted position to keep it level. The swinging of the tail
    after a vertical 1/2 roll seemed to be lessened. Overall I think that
    the CG is better positioned than before. Takeoff was as to be expected
    - a little less steep with the same amount of elevator.
    
    I noticed that the ailerons were a little more sensitive than before
    especially when getting the wings level for a vertical end manoeuver. I
    think that I will dial in some expo to reduce this so as to get level
    entry, essential for vertical manoeuvers. That said I would have
    expected the elevator to be more sensitive but this was not the case -
    presumably because I didn't move the CG too far away. Well I can refine
    the throws, expo's tonight after work with another flying session !
    
    So after all the comments and hints here I believe I now have a plane
    that is back to its original form or at least it is nearly there ! 
    
    Now its off to the Nationals this weekend  and see if I can do better
    than is the regional competitions (masters class). Right the plane wont
    cut the schedule near as well as a pattern ship but it should do better
    than it is doing. I have been placing around 5th in the competitions so
    far, which I guess is OK. But then 'she that must be obeyed' is glad to
    see less of those tropies come in  8-).
    
    
    
    Thanks and regards,
    
    Alt-E.

    Eric,
    	You need UP for inverted level flight????  Most ships need a tad of
    DOWN to fly level inverted.  My Conquest needs just a little down
    elevator for inverted flight.  It would be ideal to need no down
    elevator inverted but I've never flown a ship that didn't need some. 
    Changing incidences is too difficult without adjustment capability
    built in so I've never experimented with that.
    
    Charlie
    

    OOoops, I meant DOWN and not UP.
    
    Eric.