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Conference vmszoo::rc

Title:Welcome To The Radio Control Conference
Notice:dir's in 11, who's who in 4, sales in 6, auctions 19
Moderator:VMSSG::FRIEDRICHS
Created:Tue Jan 13 1987
Last Modified:Thu Jun 05 1997
Last Successful Update:Fri Jun 06 1997
Number of topics:1706
Total number of notes:27193

1045.0. "power pods imply different trim settings" by ELMAGO::TTOMBAUGH (High Plains Drifter) Tue Apr 17 1990 16:26

    re .560
    
    Kevin, what Kay says in .559 about stalling is a good point that
    I overlooked.
    When a motor is mounted on a pylon, it usually needs a positive
    thrust line (pointed upwards a few degrees in relation to the
    horizontal line of the fuselage in level flight). This is opposite
    to the normal negative thrust line when the motor is mounted in
    the nose.
    
    If you were flying with a neutral thrust line, then you probably
    needed a lot of up trim to hold the nose up under power, as the
    motor acting on the lever formed by the pylon vertical distance,
    tried to pull the nose down. When power was cut, then as Kay says,
    instantly too much up trim and a stall. Another example where
    a speed controller can be useful.
    
    Also, flying without a folding prop on a small airplane with such
    a high drag configuration, is resigning yourself to rather mediocre
    performance under the best of conditions. If you're really hell
    bent on electrifying a 650, try mounting the motor in the nose.
    Everything will work much better, but the installlation will be
    a !!#^&*
    
    Terry
T.RTitleUserPersonal
Name		DateLines
1045.1No power NO controlSALEM::PISTEYMon Apr 16 1990 12:5723
    
    
                      No power NO CONTROL???
    
    
       My weekend was fairly good, I was able to fly sunday aft ,
      when the rain stopped but, I seem to be having a proble with
      my olypic 650 with astro geared 05 (mounted on a pod above wing)
      11 x 7.5 prop. Th problem I had was under power she flies good
      but as soon as i cut power a sudden death spiral occurs??. This
      happened every time I cut power. I was able to recover the first
      two times and land under power (difficult, but she flies so slow)
      the third time I was only 100ft up and no way I could save it.
      (I now have the grand canyon of the east out back). Damage was
    heavy, but just to the wing. I belive I have it balanced properly
      but I know that I get two different balance points, dependeing
      on the method I use. 1/ from the wing tips (polyhedral wing I
    think) suspended by one loop of wire on a hook. OR 2/ the pencil
      eraser method, supported under the wing. the eraser method I like
      best, and is what I used. Any suggestions??.
    
    
      Kevin
1045.2Aero-doctor in the house?ELMAGO::TTOMBAUGHHigh Plains DrifterMon Apr 16 1990 16:0032
    re .555
    
    Well, this is a new one. Kevin, your pencil eraser balance method
    is correct, but exactly where are you balanced, expressed as a %
    of wing root chord? Also knowing the following would be helpful:
    
    1. How much motor thrust line offset do you have? Do you have to
    use any right rudder to hold it straight under power?
    
    2. Are you using a folding prop? I assume you are.
    
    3. Using a speed controller or just full on/off?
    
    4. Is the "death spiral" a nose-down steep spiral or more like a
    flat spin where the fuselage stays nearly parallel to the ground?
    Does it always spiral in the same direction? Which direction?
    
    Off the top of my head it sounds like you may have a combination
    center of gravity and center of pressure problem. With the CG just
    a little too far aft, the aero drag against the motor and prop,
    which are far above the normal center of pressure, with thrust removed,
    maybe be just enough to push it beyond stabiity limits.
    
    Less likely, and you would have already deduced it, is that rudder
    and elev. deflection necessary to hold straight and level flight
    under power, is too much for the unpowered phase, causing it to
    spiral in before you can correct the trim.
    
    We'll be waiting to hear more about this little rascal.
    
    Terry
1045.3answers to Terry's questionsSALEM::PISTEYTue Apr 17 1990 12:3134
    
                Reply to questions in .556
    
      Terry, here are the answers as best I know.
    
      1. how much motor thrust line offset?----- I would guess at 2-3
    deg 
         did I use any right rudder to to hold it straight under power?--
         I think I remember applying some left rudder trim and then
    with "no hands on" she was flying straight.
    
      2. folding prop? (oops sorry but no , I need a replacement for
    the one I broke, and kinda made my own)
    
      3. Speed controller or on/off? -- No speed controller just on/off
    
      4. Is the "death spiral" nose-down or like a flat spin (pancaking?)
         It was a nose down, scary fast type of spiral, almost as if
         I had stalled but did not snap roll, It was also a right spin
         or clockwise as viewed from above. 
    
    
      As far as balance goes I used the cg location on the plans. These
      are RCM plans . I think maybe that the added drag of the motor
      on a pod above the wing might have added to the problem. It was
      flying fairley slow. As another bit, I also noticed that the rudder
      "golden rod" was disattached to the fuse so that the rudder would
       not return to "zero" or the same position each time, but this
       might have happened in the crash. 
    
      
      Thanks for the questions, I have something to look at now.
    
      Kevin
1045.4great flying day!SALEM::PISTEYMon Apr 23 1990 13:4318
    
    
                         Good day for gliding!!!
    
    
      I believe I have my "death spiral" problem worked out.. I made
    5 very succesfull flights sunday with NO crashes. What may have
    been my problem was probably too much throw on my control surfaces
    and not properly balanced. After adding about 4-5 oz of lead up
    front in the oly to balance (using the suspension and plumbob) I
    noticed a much more stable response. I even landed at my feet once!!.
    
      I did also add a folding prop but am a little leery of mounting
    the motor up front as I do tend to plow the dirt when I land. 
    
      I hope everyone had as good a day as I did!!
    
      kevin