CTE

[Jeff Hughes]

Nice work! The other factor in the feel of the plane is air temp 
(density). A 50 degree swing in temperature can make quite a 
difference in how a plane feels flying. I know in the early spring 
when i'm flying the plane feels much more locked in than mid summer 
when it is 90 degrees. Also, throttle changes slightly, (more power at 
the same setting when it is cold), so if you fly to a throttle 
setting, at higher temperatures, the plane will fly slower and need 
more trim.



> OK, I'll take a shot at it.
> 
> Using the coefficient of thermal expansion that Gray suggested for 
> fiberglass of 6.5E-6 in/in F with a 50 degree F temperature swing 
and 
> a 40" equivalent fuselage length (really pushrod length since we're 
> assuming the pushrod is carbon fiber with a coefficient of thermal 
> expansion of zero) I get the same differential expansion of 0.013" 
> that Jeff did.  Using a large MK horn (~1-1/4" height) and a 1/2" 
> thick elevator leading edge yields a hinge point to pushrod 
> attachment point distance of 1.5".  Taking the arcsin(0.013/1.5) = 
> 0.49 degree elevator deflection!
> 
> Guys, half of a degree trim change on an elevator is a lot!
> 
> I can't state it in terms of how many clicks of trim that is (that 
> would depend on how your [digital] trim sensitivity is set), but I 
> can tell you that most of my planes are set up with +/- 10 to 12 
> degrees of elevator throw.  When using my laser system to set up the 
> surface deflections using the radios endpoint adjustment the best 
> resolution I see at the surface is ~0.075 to 0.1 degree.  The laser 
> system is accurate to somewhat better than 0.05 degree (1/20th of a 
> degree), but the scale can only be easily interpolated to that.  
That 
> equates to ~ 5 to 7 bits of command resolution to trim out (assuming 
> that you can set your radio's digital trims to output one bit per 
> trim input).
> 
> Disclaimer:  YMMV
> 
> Let's digress a bit.
> 
> [Note to Robert Gainey:  The "Geek Alert" is now in effect!]
> 
> Why is the servos resolution worse than the command resolution?  If 
> you set your ATV to 150% and your servo moves +/- 45 degrees your 
> servo has a max resolution of 90 degrees/1024 bits = 0.088 
> degrees/bit.  So we're really servo limited.
> 
> Lets look at this another way.  If you have a 10 bit resolution 
> system (1024 steps) and you are using 2/3rd's of your resolution (on 
> a JR that'd be equivalent to leaving your trace rate set at 100%) to 
> drive your surfaces +/- 15 degrees the minimum theoretical surface 
> deflection you could command is 0.044 degrees/bit.  But as shown 
> above your servos are only good to ~ 0.09 degrees at the servo 
output 
> wheel, or about half of the command resolution.  Again, we're 
limited 
> by the servo resolution.
> 
> Jim Oddino, you out there?  Wanna check my numbers?
> 
> Jerry
> 
> 
> >Well, easier said than done! I thought I'd just measure the linear 
motion,
> >but there doesn't seem to be a reproduceable method. Probably have 
to go to
> >one of those pointer methods to scale up the motion to get an 
accurate
> >measurement.
> >
> >
> >----- Original Message -----
> >From: Jeff Hughes <jhughes at hsonline.net>
> >To: <discussion at nsrca.org>; <discussion at nsrca.org>
> >Sent: Monday, December 01, 2003 12:01 PM
> >Subject: Re: CTE
> >
> >
> >>  Easy enough to check. Tonight I'll measure one click on a typical
> >>  futaba set up. (I know it will also depends on arm orientation 
(servo
> >>  motion is rotational converted to linear) and arm length). But at
> >>  least it will be a ball park.
> >  >
> >>
> >  > > ----- Original Message -----
> >>  > From: "george kennie" <geobet at gis.net>
> >>  > To: <discussion at nsrca.org>
> >>  > Sent: Monday, December 01, 2003 1:11 PM
> >>  > Subject: Re: CTE
> >>  >
> >>  >
> >>  > > If a digital servo moves one increment of it's total 
resolution
> >>  with each
> >>  > > click of the trim lever, then .013 is equal to about 39 
clicks of
> >>  your
> >>  > > trim. Maybe enough to do a big loop?
> >>  > > Probably no where near the realityof the actual, but I'll 
still
> >>  bet there
> >>  > > are a lot of clicks involved to move your elevator .013 ( I'm
> >>  betting more
> >>  > > than 10 and probably close to 20). I'm assuming, of course, 
that
> >>  you have
> >>  > > your trim rate adjusted to it's smallest value.
> >>  > > G.
> >>  > >
> >>  > > Jeff Hughes wrote:
> >>  > >
> >>  > > > I stayed out of this thread earlier. But if you run the 
numbers
> >>  for a
> >>  > > > 50 degree temp swing, your only talking 0.013" of 
differential. I
> >>  > > > doubt if someone is noticing that. Probably the horsepower
> >>  difference
> >>  > > > over a 50 degree swing makes more difference in how the 
plane is
> >>  > > > flying.
> >>  > > >
> >>  > > > > Nat
> >>  > > > >
> >  > > > > > I was gone last week....The CTE for balsa will change 
with the
> >>  > > > density of
> >>  > > > > the balsa it self, but as you have noted, humidity plays 
a
> >>  larger
> >>  > > > part
> >>  > > > > here in expansion and contraction. This is not a factor 
with
> >>  the
> >>  > > > composite
> >>  > > > > pushrods.
> >>  > > > >
> >>  > > > > I am not as inclined as Jason was to read all those 
emails
> >>  while I
> >>  > > > was
> >>  > > > > gone, but in short CTE is not so much a problem as is 
the mis
> >>  > > > matched CTE,
> >>  > > > > in this case the carbon fiber pushrod vs. the fiberglass
> >>  fuselage. I
> >>  > > > have
> >>  > > > > not run the calculations but anyone interested assume 
the CF
> >>  rod to
> >>  > > > have a
> >>  > > > > CTE of zero, then the fuselage to have a CTE of 6.5 X 10-
6
> >>  in/in F.
> >>  > > > I
> >>  > > > > would imagine that if you set your trim at 100F then 
flew in
> >>  > > > December at
> >>  > > > > 50F you may have one or two clicks of trim to 
compensate.  The
> >>  > > > original
> >>  > > > > post asked about storage down to zero F. In reality the 
servo
> >>  arm or
> >>  > > > > elevator would just move with the cold contraction.
> >>  > > > >
> >>  > > > >
> >>  > > > >
> >>  > > > > Gray Fowler
> >>  > > > > Principal Chemical Engineer
> >>  > > > > Composites Engineering
> >>  > > > >
> >>  > > > >
> >>  > > > >
> >>  > > > >
> >>  > > > > "Nat Penton" <natpenton at centurytel.net>
> >>  > > > > Sent by: discussion-request at nsrca.org
> >>  > > > > 11/24/2003 09:24 PM
> >>  > > > > Please respond to discussion
> >>  > > > >
> >>  > > > >
> >>  > > > >         To:     <discussion at nsrca.org>
> >>  > > > >         cc:
> >>  > > > >         Subject:        CTE
> >>  > > > >
> >>  > > > >
> >>  > > > > Gray what is the CTI for balsa ? I have carbon push 
rods /
> >>  balsa
> >>  > > > fuse and
> >>  > > > > my elev trim moves around considerably. I suppose it is
> >>  because of
> >>  > > > > humidity variation between the shop and outdoors ?? The
> >>  pushrods are
> >>  > > > 50"
> >  > > > > > long.      NatPenton
> 
> -- 
> ___________
> Jerry Budd
> mailto:jbudd at qnet.com
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> 

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