REFLECTOR: sparrow strainer (old post)

[Alexander Balic]

Pat,
The angle of the strainer to the line of flight is not really relevant
because at the location of the sparrow strainer, the airflow is following
the "elevator" (really flap), not the airflow at the leading edge of the
canard, so it will stay relatively at the same angle of attack to the
localized flow regardless of both aircraft angle of attack, and flap
position. The reason that this airfoil is useful, is because at higher
speeds, the flap sees a higher flow velocity, and tries to move upwards into
the low pressure area behind the canard ( which would require stick UP/flap
down to correct) so the sparrow strainer is inverted to help push the flap
down, away from the tendency to float up into the low pressure area - I
think that the problem that we are having here is that we think of the
"elevator" as an "elevator" when it is actually a lift enhancing device for
the canard, and therefore should be referred to as a flap, but since moving
it results in pitch change, I can not really argue with the definition, but
I think that we all must really think about what is going on aerodynamically
to understand the purpose of the  trim system.

Alex

-----Original Message-----
From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On
Behalf Of Pat Shea
Sent: Thursday, May 22, 2003 5:08 PM
To: reflector@tvbf.org
Subject: Re: REFLECTOR: sparrow strainer (old post)


Hmmm,

 If the strainer only works as a levered airfoil to
dampen pitch deviations, then I'm puzzled why it
wouldn't be rigged to have a neutral angle of attack
(relative to the line of flight) when the elevator is
in its typical cruise position. That would mean the
strainer would need to have neutral-to-positive
incidence relative to the elevator. With negative
relative incidence, the strainer will actually
increase stick/spring forces the faster you go
(essentially like a trim tab bent the wrong way). The
inverted airfoil shape only adds to this this effect.
Obviously the factory does it this way for a reason -
maybe they are looking to lighten trim loads at lower
speeds. BTW, my templates leave the strainer incidence
open to a little interpretation. I guess it's not
critical since builders' are running them from
positive 20 degrees to negative 20 degrees...
 Funny, but sometimes it seems people are
hypercritical about some pretty minor stuff, yet we
can't even get a consensus on proper control surface
balancing or sparrow strainer incidence. Maybe it's
just me...

Pat

--- SlvEgl99@aol.com wrote:
> I agree, the strainer has a negative angle of
> incidence as compared to the
> elevator. I simply made the airfoil as the templates
> showed. My plane was
> constructed at the Velocity Service Center, so am
> confident that it is correct.
>
> The autopilot sometimes does a bit of "seeking" in
> altitude, and in
> discussions some have recommended two sparrow
> strainers (one on each side) as a
> possible cure. I have not done that yet, but make
> sure the plane is stablilzed and
> trimmed before turning on altitude hold.
>
> Bob Wood
> 658SE
>



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