Conference vmszoo::rc

Oh goody, a chance to sound like a hotshot on a subject I know
nearly nothing about!

Side thrust, usually right, is given to ensure straight tracking,
particularly during the takeoff roll; as such its not needed
unless the plane shows a marked tendency to turn right.  Usually,
about 2 degrees, maybe 1/8 inch offset from the center line is
about correct -- I've found this to be true for everything from
the EZB indoor models to my PT40.

Down thrust is more important, it is to keep the model from
"zooming" under power.  You want enough to make the glide of the
model the same under power (with the throttle at idle) and with
engine off --- actually you want it to be the same up to about
crusing power.  Normally, about 2 or 3 degrees is right, anywhere
from 1/8 inch to 1/4 inch.  Trainers have more, pattern ships
have none or nearly so, though trends have changed lately.

Down thrust is especially well taught if you fly free flight
rubber, where you get a very distinct zooming action with full
winds and insufficient downthrust.  A couple of times having a
model buzz at you after launch is enough for a lifetime of
putting in downthrust.

                We are talking religion again!
        
                I agree  that right and down thrust are necessary on free
        flight models.   This  is  because  they  have no variable flight
        controls.  As far as I am concerned, the rudder stick is there to
        keep you tracking on takeoff  and  it  is  desirable  to have the
        throttle  used  as a altitude control.    To  level  out  at  any
        altitude and speed you have a trim lever.
        
                I say this because I enjoy RC  as the result of having to
        understand flight dynamics and if you try to get the plane to fly
        like a free flighter you don't learn anything!
        
        Anker

    Maybe I'm lazy, and maybe I'm just ignorant, but I fail to see the
    advantage of flying a flat bottom airfoil with zero downthrust.  In any
    case, I'm eager to learn though I may remain lazy to the end.
    
    Rather than hold varying amounts of down elevator to compensate for the
    current wind/power setting, it seems more logical to balance the
    increased speed with appropriate downthrust.  I suspect that the
    downthrust should automatically balance the changes in lift generated 
    by changes in thrust.  
    
    My Amptique is overpowered with a geared 15, but it assumed a nice
    power off glide with the elevator at zero deflection, and climbed at 
    a steadily increasing angle under full power.  The plans called for 
    three degrees of downthrust, but people who had flown the amptique 
    reccomended more, so I flew with five degrees of downthrust.   I've
    since made a shim to try eight degrees next time out.  BTW, no side
    thrust was called for, or seemed to be needed.

    Frank,
    
    You say you're using the long shaft geared system with the Speed
    600 motor. I assume this is the gearbox that bolts to the front
    of the motor with the ~4" shaft coupled to the motor shaft with
    a cylindrical collar and two set screws. That will be a fine setup
    for your S.L. Personally I feel that a 7 cell 900 mah SCR pack works
    best but a 1200, 6 cell will be adequate.
    
    As for down and side thrust, you need them. 4-5 degrees down, 3-4
    degrees right thrust is a good starting point. Even with downthrust
    expect some climbing at launch with neutral elevator. This is the
    safest setup. Use the trim to set it where you like it for launch,
    then run in some up trim when you cut power. Trying to set up an
    electric sailplane to achieve 0-0 attitudes strictly with thrust
    lines is a waste of time. The whole point of having that prop in
    the first place, is to get to altitude as quickly as possible so
    that you can shut it off and get back to a real mans' way of flying;^).
                                             
    Terry
    

    
                < What about pusher prop sailplanes ? >
    
    
          I see the need for "downthrust" with tractor powered
      sailplanes and have even experienced the ballooning or
      zooming effect without it, on E powered sailplanes. But
      is downthrust required on a pusher set-up?. And would that 
      downthrust be reversed depending on the location of said
      motor?. example : located above CG, behind, before or does
      that matter?.
    
    kevin p

    Kevin,
    
    I can't answer regarding whether downthrust might be needed on a pusher
    but, if it is, you'd still angle the thrust line down.  Think of the CG
    as the fulcrum of a scale; with a tractor, downthrust pulls the nose down
    ...with a pusher, downthrust pushes the tail up, thereby lowering the nose.
						 __
				|      |        / |\	   	       
      	         \|/		|______|__(o/--/  | \	   	       
      | |        00	       <|  ~~~  ____ 04 ---- | --------------------
    |_|_|        (O>o		|\)____/___|\_____|_/	   Adios amigos, Al
      |     \__(O_\_	        |	  |___/	 o	   (The Desert Rat)

    No exact values, but a bit of theory...
    
    I prefer to at least nearly trim out the plane with downthrust and
    sidethrust. You may call me lazy for not working with the trims, I
    prefer to think about it as a safety issue. I don't want to find me in
    big trouble if I decide to go around after a bad landing approach,
    apply full throttle and find the plane completely out of trim, near the
    ground, nose up, likely to stall. IMHO, thrust offset is mainly
    important near the ground at takeoff or go-around, and I like to have
    it set up so that I at least am not too surprised about what the plane
    does. Once the nose is up and airspeed low, control surfaces often
    don't help much.
    
    I can't remember values, but the suggestions already given are
    probabaly good points to start at.
    
    A bit of theory (for pushers etc.). Considering a stable flight
    situation, you can look at your plane 'statically'. One of the most
    important laws of statics is: Sum of all forces and sum of all
    momentums is zero. For powered horizontal flight this means: Lift
    equals weight, thrust equals drag. For your glider, you must draw a
    vector diagram. Drag lies in flight path (path at glide angle), lift is
    perpendicular. If you draw a parallelogram, weight compensates both of
    these. Simplified, you can apply all the forces in one point (center of
    gravity) and don't have any momentums around this point.
    
    Now, on flat bottom or similar wings, if you increase airspeed, the
    point where the lift applies wanders forward. This gives you a momentum
    around the c.g., and the nose goes up. You can compensate by down
    elevator (lift of elevator equalling nose up momentum). But if the
    increase of airspeed is caused by the motor, you can give the motor
    some downthrust and create a moment around the c.g. that is
    proportional to thrust and nerly proportional to airspeed. The lever
    arm of this force is perpendicular to thrust axis through the c.g.
    So... if you have a pusher, it depends very much where it sits. If it's
    behind the c.g., it should push up to get the nose down. If any (push
    or pull setup) is not near the fuses centerline, but, say, on a pylon,
    you already have a considerable lever arm, and you might even want to
    give a puller upthrust in order not to increase the lever too much and
    tuck the nose under whenever you apply throttle.
    
    Sidethrust is used to compensate for the motor torque. Look at it as if
    the air wanted to 'hold your prop' and it will try to roll the plane.
    With sidethrust, your plane wants to turn in the other direction, and
    both forces/momentums should balance.
    
    Not so easy to explain technical things in a foreign language. I hope
    you could understand.
    
    Important is: Symmetrical airfoils don't have much wandering of the
    point where the lift applies, that's why they don't need up/downthrust
    as long as the motor is nearly in line with the wing. The more your
    lift wanders, the more downthrust you need.
    
    Sidethrust depends on your motor torque. High speed engines on pattern
    planes usually have low torque and don't need sidethrust. But on an
    electric - especially with a geared motor to increase torque - you
    should try to compensate for the torque as much as you can. There were
    enough crashes already with planes on final approach where the torque
    of switching the motor on put the plane into an uncontrollable
    situation and into the ground (strong reason for using a controller
    instead of a switch). Start with the values already suggested and
    finetune as your ship needs it.
    
    Hope this helps.
    
    Have a nice weekend with lots of successful flights!
    
    Regrads,
               Hartmut

    
    
        AL,
        Hartmut,
    
                Thanks for the info on "thrust lines" . With
         the last two replies I started thinkin (big mistake)
         and it seems logical at first, but then when you add in
         all the other forces... uh oh. 
    
        Hartmut, 
    
                Your technical description certainly was understandable
        I only had to read it one line at a time , visualize, 
        remember what has happened to me in the past, and then
               *I get confused*
    
        Really thanks , gives me some rework on my current modofied
        E powered sailplane but meybe you all saved me a crash.
    
       kevin p

    Full size (i.e. 1 to 1 scale) use down thrust and (depending on
    engine rotation direction) left or right offset to minimize the
    effect of the engines thrust on the controlability of the plane.
        
    If you are running a pusher you use up thrust, not down.  My
    experience.
    
    Will

    
    
      Will,
    
          What effect would that have on the stabilizer or the
      pitch of the plane as in my set-up the prop wash will angle
      down on the stab. Would it not cause the stab to go down thus
      rising the nose (stalling very fast). ??. What I did go and try
      was to set the thrust line level initially, test fly 2 flights
      and trimmed it out. Then I added a bit of down thrust, as in
      if the prop wash was coming at the stab from underneath it.
      Guess what? . the nose dropped very little and only a minimum
      of retriming was needed. I might try a little up thrust, but
      this works and I don't seem to have to retrim for powered
      VS unpowered glides. 
    
      kevin p

    
    Time to tap into the collective experience once again 8^)
    
    I'm (still!) building a Flair Puppeteer (A Sopwith Pup
    look-not-entirely-unlike). I'm using a .70 four stroke engine, which is
    at the top end of the recommended power range. The plans mention down
    and side thrust should be added for larger engines, but don't mention
    any specific figures.
    
    From reading the stuff in here about the subject, I've decided to add
    right and down thrust, and I'm guessing at 2 degrees of each. 
    
    Does anyone have any comments or advice on this?
    
    Thanks
    
    Nigel
    

>    From reading the stuff in here about the subject, I've decided to add
>    right and down thrust, and I'm guessing at 2 degrees of each. 
>    
>    Does anyone have any comments or advice on this?

Sounds like a good start to me.
You'll now right away if 2 degrees is correct for the down thrust
but the right thrust is a little more subtle.  Many folks
never put any right thrust in.  I like to use the commercial
thrust plates.  It makes it easier to change.  The biggest
hassle is when you change thrust angle you change alignment
of the prop shaft coming out of the cowl.  So it sure is nice
if the designer of the plane recommends a specific amount of
thrust and if you use the same engine.

Bye          --+--
Kay R. Fisher  |
---------------O---------------
################################################################################

I, the Great Duffer, have not put side thrust in my planes for several
years now.  Its just a case of learning to take off without it, and it
does save those ugly off-center cowls.

    
    Thanks for the inputs. I've been looking through the plans again, and
    I'm going to have to build a box on the back of the firewall to take
    the engine mount. This is because the Laser I'm using is a bit longer
    than the OS shown in the plan, and it's not suitable for mounting by
    the backplate screws.
    
    This means that changing the thrust angles is going to be a real pain.
    I'm beginning to lean towards 2 degrees downthrust and no side thrust.
    I wish someone would take the decision for me! 8^)
    
    Of course there's the problem of building a nice accurate box to get
    the angles right anyway......
    
    I suspect the final decision will be made when I check the angles on
    the finished article (wince!).
    
    Thanks again.
    
    Nigel
    

    
    Nigel,
    
    I would not recommend leaving the descision as it turns out. Some
    points to consider : With a fully symetrical airfoil I'd recommend no
    downtrust. You only need downtrust when the airfoil generates lots of
    lift to compensate for the different amount of lift at different speeds
    aka power output. 2% on a half symetrical and 3% or above on Clarky or 
    similar.
    Right thrust is needed more when using a high torque engine (4-Stroke)
    of approx. 2%
    If you use a whining 2-stroke 0.5% -1 % is sufficiant.
    If your plane is a fully_symetrical_airfoil - every_day_plane for
    non_precision_aerobatics use 0% - 0%.   
    
    	Bernd
    

    Most pattern ships run 2-3 degrees of right thrust and no down thrust. 
    I would put 2 degrees of right in and offset the engine on the firewall
    so that the spinner is centered on the nose.  Then you can add or
    delete some of the side thrust after test flying.  Any less than 2
    degrees will be too little.
    
    Charlie
    

    True for precision aerobatics and high torque long stroke. 
    
    
    Bernd
    

    
    Ah.....
    
    The plane is a biplane "floater". It's designed to look like a Sopwith
    Pup, and I'm intending to fly it in a "scale-like" way. This means
    moderate aerobatics, and low slow passes. The aerofoil is flat
    bottomed. I guess it's beginning to look like back to 2 degrees down 
    and right. I can offset the engine to get the prop-nut in the centre of 
    the cowling.
    
    Thanks guys!
    
    Nigel.

    Nigel,
    	You will be glad you added down thrust if the airfoil is flat
    bottom.  They always fly better with downthrust.  Otherwise the sucker
    will climb like crazy at full throttle and you will never be really
    able to trim it for level flight.
    
    Charlie