servo question

[Troy Newman]

Very good stuff....


I learned a little too....

I subtrim the servos to get the pushrods square to the arms....This is the
way I was taught to do it.

The built expo thing will be radically changed when using the radios expo
function so I can see that one going away for all practical purposes...

But yes with the servo arms square to the pushrod....This is putting the
neutral point in the worst place on the servos rotation for power. You are
giving the force applied from the other side the max moment arm to push the
servo against its will....But at neutral control throw how much force is
being applied to the servo.....very little would be my guess compared to
when the servo is deflecting.

The one time that ailerons for example are neutral and have huge forces is
in a pull or push. The forces of pushing or pulling a corner even a wide
radius will cause the ailerons to deflect....This is where the digital
servos EXCEL...they hold the ailerons tighter to the neutral position by the
fact of their increased holding power....So your loop segments are
straighter...because as the ailerons blow-back the question is are they
going to move the same amount....?

The answer to that is probably not....and you know they are "trimmed" at the
neutral position. So if you can just hold them at neutral what ever it takes
it will help the looping segments.....

Yes this power of the servo comes at a little cost the battery packs need to
be a little larger...The power in the servo comes from somewhere and its the
battery pack.

These forces in flight are another reason to have very stiff control
surfaces and linkages.....If the aileron can flex they will...An no matter
how good of servo you install it won't help...Wood varies and the two
ailerons if allowed excessive flex will not flex the same as each other....
so minimize the flexing by making the surfaces as rigid as possible...

You will get more consistency in your flying....Just like running a voltage
regulator...consistent voltage means same power and speed on the servos
throughout the flight and from flight to flight....The name of our game is
consistency...If the model flies the same everytime then you can really
practice and learn to fly the maneuvers....If the model is changing under
different conditions all the time,  all your practice time is spent learning
the model and it faults and flaws under that days conditions.....Makes the
advancement of your flying skills take even more fuel burning than needed.

A quote from a Famous Pattern flyer....."Make the model do the work....tune
the model so that it does it for you and makes the flying as easy as
possible."

TN
----- Original Message ----- 
From: "JOddino" <JOddino at socal.rr.com>
To: <discussion at nsrca.org>
Sent: Sunday, August 03, 2003 4:11 PM
Subject: Re: servo question


> Hi Troy,
> The reason I asked this question is that I heard a guy claiming his
neutrals
> were more positive if the servo horn was perpendicular to the pushrod at
> neutral.  I see you recognize the system becomes stiffer the closer you
get
> to the servo arm being parallel to the pushrod.  That is why we used 180
> degree servos on landing gear.  So the servo arm perpendicular to the
> pushrod doesn't improve holding power at neutral.
>
> In the old days before computer radios we used to offset the servo
neutrals
> on purpose to get more up than down ailerons for example.  Now we can set
it
> anywhere we want so the angle isn't critical from that point.
>
> As far as expo (non-linearities) caused by linkages, it is insignificant
> (with reasonable linkages) compared to the expo we add.  Forget about
linear
> distances, look at angles.  The change in lift is a function of the angle
of
> attack, an angle.  The angle of attack is a function of the surface
> deflection, an angle.  The surface deflection is a function of the servo
> rotation, an angle.  Therefore the airplane responds to the servo angle
not
> the linear change of pushrod distance.  It takes a special linkage to get
a
> linear relationship between servo angle and control surface angle but even
a
> setup that looks bad really isn't too bad after we adjust end points and
add
> expo.  It seems to me it is more important to make sure the angles and
push
> rod lengths are the same (speaking of two elevator servos or two servos
> coupled to one surface)  rather than worrying about a non-linearity that
is
> difficult to measure.  It seems to me it is much easier to set the servos
> arms  90 degrees to the case than to try to figure out the angle that will
> make them 90 degrees to the pushrod.  In fact I'm not sure how you'd do it
> other than by cutting and trying.  I normally strive for both,
perpendicular
> to the case and the pushrod. However, I'm building a new plane with the
> servos on their sides in the wing and stab where I can't have both so I'll
> have to look at this a little closer.  I'm still betting it is
> insignificant.
> Jim
> ----- Original Message ----- 
> From: "Troy Newman" <troy_newman at msn.com>
> To: <discussion at nsrca.org>
> Sent: Friday, August 01, 2003 5:02 PM
> Subject: Re: servo question
>
>
> > Oh come on Jim....
> >
> > For the rest of the guys...The servo moves in a  rotational motion...So
> the
> > linear distance traveled by a pushrod attached to the arm or wheel will
be
> > greatest right off of center and will reduce the amount of linear travel
> of
> > the rod as it rotates around. A linkage not setup this will have a funny
> > expo like knee in it...But it will be as the arm passes through the
square
> > position so it would be like adding expo on only one side of the servos
> > travel...and reverse expo on the other side of travel.......
> >
> > Also it could be way up high on the travel of in a funny middle
spot...and
> > you will get more throw one side than the other....
> >
> > This sq to the pushrod also gives the servo best mechanical advantage
> > through the range...as the servo gets to the endpoints say full travel
up
> > elevator..the highest force is being applied the surface...and the servo
> arm
> > is rotated in such a way to reduce the moment arm on the servo by the
> > pushrod pushing back.....The arm travels in an arc and the arm gets
closer
> > to the center of the servo as it rotates off of "center"....This moment
> arm
> > is leverage for the pushrod to pushback on the servo.....So if you
reduce
> > this distance when the forces are higher then it will make the servo
have
> a
> > mechanically better shot and holding its position.
> >
> > did I get it right Jim?
> > anything to add Jim?...I'm by far not the expert on this stuff...
> >
> > Ok Still TMI....
> >
> > TN
> >
> >
> >
> >
>
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