Conference vmszoo::rc

    Jim,
    	507's and 517's are the same except that the 517's have ball
    bearings.  507's will do the job in that plane.  Use one per aileron,
    one on rudder, and 1 or two on elevator.  Two on elevator gives you a
    backup if one fails and you don't have to worry about having enough
    servo torque.  I think one would do it on the elevator torque wise
    though.
    
    Charlie
    

    Charlie,
    
           Why is the elevator servo more prone to failure.  Is this the
    control surface that experiences the greatest load?

    
       Jim,
    
          The load can be significant on the elevator on larger planes, but 
    just as important is that there is no "backup" for the elevator. If your
    rudder, throttle, or one aileron servo (use two) goes, you can probably 
    get the plane back down in one piece, but if the elevator servo lets go,
    you are coming down NOW! The redundancy of two elevator servos is nice
    insurance. 
          The other alternative is to go with larger 1/4 scale servos on
    the elevator. This will tend to keep the flight loads imposed well
    within the design specs of the servo. I have talked to people who fly
    the larger planes, and there are definately two schools of thought. One
    group likes the ONE 1/4 scale servo, and the other group likes the
    redundancy of two servos.
    
         Its kind of like the argument used with full size airplanes. Are you
    better having one LARGE engine (which is easier to manage/monitor, but
    presents a single failure point), or two smaller engines (which offer
    redundancy, but are more difficult to manage).
    
           
                                                     Dan

    Those are good points, Dan.  We lost a towplane this year due to elevator
    servo failure.  The servo did not fail due to flight loads, however.
    It was a cheap, Royal servo which went to full down elevator when it
    failed.
    
    Our typical towplanes represent an investment of $750 - $1000, so
    if spending an additional $30 for a better servo or adding redundancy
    prevents a crash, it is money well spent.
    
    I think I will follow this for the elevator on my towplane.
    
    Thanks for the info.

    Jim,
    	Dan answered your question the way I would.  I have landed with a
    failed throttle servo, a failed aileron servo, but not a failed
    elevator servo.  Most servo failures don't go to full deflection so
    having two leaves you with elevator control.  I would never use a
    Royal servo on a critical control surface.  I used two on throttle and
    they both failed in less than two seasons.  They were not good even
    when they worked.  Deadband was huge.  
    	The 507 servo should be just as reliable as the 517, but it will
    wear and develop more slop.  For a slow tow plane, this will not be a
    problem - and you can buy the bearing kits to upgrade them if you want
    to.  
    
    Charlie
    

    The 507 is very reliable but the output shaft will wear out in about
    100 flights and get sloppy. All plain bearing servos wear like this in
    power planes.
    
    I agree with Charlie on the elevator.I have lost three planes when a
    linkage or a connector failed on the elevator. I have never lost a
    plane when any of the other primary controls failed.
    
    
    E. - who's next pattern plane will have 2 elevator servos on two
    channels. :-)

    Just a few weeks ago I was flying a club members trainer and pop goes
    the elevator servo. And despite all I managed to land it, using the
    throttle to maintain attitude. Nevertheless I wasn't going to be doing
    straight and level flight ! It was a trainer with a high dihedral and
    the engine just about managed to control the airplane. I was surprised
    that I managed to control it with the engine.  If it had been any other
    type of plane DOWN would have been the only item on the menu.  Since
    then he has replaced and made sure of the connection. 
    
    Eric.

    Eric,
    	You're right - you'll only get away with that with a flat bottom
    wing trainer where speed greatly affects pitch.  With a symmetrical
    wing plane, no such luck.  You also have to be lucky enough that the
    balance is such that it wants to fly pretty level with no force on the
    elevators if the linkage came off.  Consider yourself very lucky that
    the plane was manageable.
    
    Charlie
    

    I just swapped a guy a 7-cell battery charger for 4 Futaba S-148
    servos.
    
    Do you guys feel it is worthwhile to buy the ball bearing conversion
    sets.
    
    A set of four is $27 from Tower.
    

    Jim, at $27 for four, that's about $7 per servo.  Add that to the price
    of the servo and see if the total is more or less than the model that
    has the ball bearing in it.
    
    For JR I buy the $15 model 507, add a ball bearing which I get for
    about $5 each, making the total $20.  The equivalent 517 servo runs
    about $30 if memory serves correct, so there's a net saving.
    
    Regards,
    Joe

    I've run sport planes for years with standard servos.  Ball bearings
    are nice but not really necessary.  $27 for four bearings is a rip-off
    unless that includes cases also.  The JR servos don't require a case
    change to add the bearing because they use a plastic bushing in place
    of the bearing.  You just pull that bushing out and add the bearing.  I
    think the Futaba bearing kits include the case cover.
    
    Charlie