CTE
vicenterc at comcast.net
vicenterc at comcast.net
Tue Dec 2 04:10:31 AKST 2003
Fact: On both my Focus and the Hydeout the trim changed from warmer to colder days. Probably around 3-4 clicks.
Vicente Bortone
> 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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