REFLECTOR: Aerodynamic trim

[Pat Shea]

It doesn't seem reasonable to suggest an airframe
design change to fix a trim adjustment without at
least indicating such or explaining the affects of the
change - whether positive or negative. You're implying
that the sparrow strainer is an aerodynamic trim -
which it's not by the factory's design. Sure it can be
used that way, and some do and like the way it works,
but at least disclose that it's a departure from the
plans which will alter the way the plane flies.  

Adjusting the incidence of the canard would adjust the
trim too, but no-one would suggest that...

Here is a post I previously sent on the sparrow
strainer's function:

--- Pat Shea <xl340hp at yahoo.com> wrote:

There seems to be some confusion on this issue. The
fact that Steve completely altered the design and
function of of his "sparrow strainer" and turned it
into an "aerodynamic trim tab" isn't helping, but I'm
glad it's working for him.

For everyone else using it in it's original design
configuration...

Probably the easiest way to analyze the function of
the "sparrow strainer" is to take it and the trim
spring out of the picture. In that scenario, at high
cruise speed the pilot would need to press forward
(hard) on the stick to maintain level flight. Now add
on the sparrow strainer which will be pulling down on
the trailing edge of the elevator with its inverted
airfoil and negative incidence relative to the
elevator, and the pilot has to push forward even
harder. Yes, it's actually working against you at high
cruise speeds. The pitch trim spring simply replaces
this force required by the pilot. Therefore, the pitch
trim spring has to work harder at high cruise speeds
with the sparrow strainer installed.

Here comes the purpose...

Since the pitch trim spring is working harder, it is
further "pre-loaded." This gives the stick a heavier
(or stiffer, or whatever...) feel in pitch at high
cruise speeds. This higher pre-loading of the pitch
trim spring also makes the elevator (and therefore the
plane) less susceptible to pitch deviations from
turbulence, etc.  

What's all this mean? If you are prefer heavier stick
forces at cruise and a little more stability, use the
sparrow strainer. If you prefer lighter stick forces
at cruise and are willing to forgo a little stability,
take it off or even turn it into an aerodynamic
trim...   

Pat


--- John Dibble <aminetech at bluefrog.com> wrote:

> Dave,
> 
> I had the same problem with my SERG.  I reversed the
> strainer and now I can descend at
> 200 kn/1000 fpm and still have more trim available. 
> Reversing the strainer is a lot
> easier than changing the trim spring.



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