[NSRCA-discussion] Guys I need help...

[Randall Bearden]

On 12/27/06 4:08 PM, "gseeling" <gseeling at smartfella.com> wrote:

> e mail dave guerin, you can get him @piedmont models
> ----- Original Message -----
> From: "John Konneker" <jlkonn at hotmail.com>
> To: <nsrca-discussion at lists.nsrca.org>
> Sent: Wednesday, December 27, 2006 9:10 AM
> Subject: [NSRCA-discussion] Guys I need help...
> 
> 
>> I am working on my Focus II.
>> I have the wing adjusters for one wing installed.
>> In order to minimize the chances of a less than level table to affect the
>> incidence I decided to -0- the plane to itself.  The fuse sides at the
>> canopy are supposed to be level with the "water line" so I put a level on
>> it
>> to check...bubble at -0-.  I then double checked against the fin bed and
>> it
>> showed level.  I put a Robart incidence meter on the wing, set the panel
>> to
>> +1/4 deg and glued the wing adjusters on that side.
>> This morning I was getting ready to do the other wing panel when I decided
>> to do some measuring.
>> Here is where I ran in to the unexplainable...
>> I measured the wing leading edge to the table top and then the trailing
>> edge
>> to the table top.
>> I show approx. .15 inches positive!
>> I measured the leading and trailing edge to the piin stripe down the fuse
>> side (this stripe appears close to the water line) same positve incidence
>> shown.
>> I then rechecked with my incidence meter and got +1/4 deg with the fuse
>> level...I then "reversed" the incidence meter to check to see if swapping
>> ends had any effect...very little...maybe 1/8 degree...I can't read it
>> that
>> close.
>> I then traced the leading and trailing edges on the fuse side with a felt
>> tip pin and rechecked my measurements to the table top and pin
>> stripe...same.
>> I am at a loss.
>> How can I seem to be off this much?
>> I could just cry....
>> Any ideas?
>> Thanks!
>> JLK
>> 
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> 
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. You are not off that much.  Your water line is the reference or thrust
line and set to 0 degrees.  Good so far.  Your wind incidence is is measured
to the thrust line but the real height measurement is the length of the
hypotenuse the triangle you created from the center of the wing tube to the
leading tip edge raised 1/4 degree from the adjacent side.  You must
remember the further the distance from the center of the wing tube and the
leading edge is the higher the leading edge will be at the same angle.

If you have the measurements of the length of the Hypotenuse (center of wing
tube to the leading edge) and the angle of incidence you can accurately
calculate the length of the opposite side using basic trigonometry.  It can
be calculated using the following formula; the height of the leading edge
above the thrust line = the length from the wing tube center to the leading
edge times the SIN incidence angle)

    SIN incidence = the length of the wing tube center and the leading edge
    a = the height of the leading edge above the thrust line
    A = incidence angle

    a = c * sinA

Please consider this when you are making measurements.  Unless the wing tube
is exactly centered on the wind and you compensate for the taper of the
wings at the measurement point, then the distances  from the zero line above
and below the zero thrust line will be different.  The incidence meter will
accurately display the wings angle of attack relative to the thrust line
regardless of the wing tube position and that is the more important
measurement when setting up the airplane.

When you  recheck the thrust line and verify the wing incidence angle is
still at +1/4 degree then you are fine and unless you really like doing the
math enjoy finishing your build.

I need to get back to my building table.....

Randall Bearden