CTE
Jerry Budd
jbudd at QNET.COM
Mon Dec 1 21:06:53 AKST 2003
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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