REFLECTOR: Aerodynamic trim

[Dave Philipsen]

Ok, so I think I'm starting to understand this a little better.  The way 
I'm seeing it, the elevator is pretty sensitive.  Without the trim 
spring, you might tend to be constantly adjusting/correcting pitch and 
if you let go of the stick and accidentally bumped it you might be 
facing a pretty drastic pitch change which is undesirable.  Perhaps the 
sparrow strainer is really on there for prevention of flutter.  It would 
seem that it just keeps the elevator loaded with some pressure so that 
it doesn't want to 'flap in the wind'.  So, does it really matter which 
way the loading is imposed on the elevator?  As long as there is some 
pressure there, maybe it's ok either way.  I guess I could come up with 
one reason you might want the sparrow upside-down: Let's say your on 
final approach and you're ten feet above the runway.  A 15 mph gust of 
wind blows straight at you head on.  If the sparrow's upside-down, the 
plane might pitch up just a bit.  If it's right-side-up, maybe the plane 
will pitch down enough to make your nosewheel bounce on the runway.  
Just thinking aloud here.

If the sparrow strainer is really there just to keep the elevator from 
oscillating, could you do the same thing by mounting some sort of small 
shock absorber to the elevator linkage?  Has anyone tried flying without 
the sparrow strainer (like the Cozys and Log-Ezes do)?  Is there a 
significant difference?

I just thought of another possible reason that the factory advises to 
put the sparrow on upside-down:  If you start a descent and your 
airspeed increases, the sparrow would pitch the airplane down a little 
more which would increase airspeed a little more which would pitch it 
down a little more.....well, you can see where we're going!  Down, and 
fairly fast.




John Dibble wrote:
> Pat Shea wrote:
>
>   
>> 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.
>>     
>
> Actually, Duane swing claims that without either, very little stick pressure is
> required and that is what causes instability.
>
>
>
>   
>> 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.
>>     
>
> I would say it works against you at all 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...
>>     
>
> I see no change in stability with it inverted.
>
> John
>
>   
>> 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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-- 
Dave Philipsen
STDFG
N171SP

soon-to-be N83DP