REFLECTOR: Pitch trim and pitch stability.

[Andy Millin]

I just love your explanations.

 

Thanks for the time, the education and for being on the list.

 

Andy

 

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Laurence Coen
Sent: Sunday, November 22, 2009 2:49 PM
To: reflector16
Subject: REFLECTOR: Pitch trim and pitch stability.

 

As the subject suggests, pitch trim and pitch stability are not the same
thing in spite of the fact they do interact.  All certified aircraft have
some form of pitch trim and must exhibit pitch stability.  If we look at the
history of the Velocity it started as what we now call the "Standard" with a
180 HP Lycoming. The horsepower on that same airframe has increased over
time but the trim system remained the same.  Is it any wonder that a faster
plane starts to run out of down trim?  There have been other major
modifications like the "173" or "Long Wing", the "XL Series and the RG
option.  These are factory options and don't begin to address the creativity
of the builders.  The spring trim system that we use is probably the
simplest design of any trim system.  It pushes or pulls on the elevator
control with a spring loaded force so we don't have to.

 

There are two basic areas that builders are struggling with.  First is
having more than enough up trim and not enough down trim.  A change in the
spring torque tube attach to give more down trim and less up trim is likely
the simplest solution.  A more difficult problem is lack of trim range.
This is the situation if you have enough up trim, there isn't enough down
trim and visa versa. The increased flight envelope from minimum to maximum
speed is likely the culprit here.  A longer linear actuator or a stiffer
spring could help.  I don't like the idea of reducing the incidence angle of
the canard unless it was set wrong in the first place.  The unintended
consequences are reduced pitch stability, increased stall speed and rotate
speed.

 

Now let's talk about pitch stability.  In a conventional aircraft,  the main
wing pushes up and the horizontal stabilizer pushes down.  When you reduce
power the plane slows causing less up on the wing and less down on the tail
and the nose pitches down.  Speed up. nose up, slow down, nose down.  That's
pitch stability, not pitch trim.  I used a conventional aircraft as an
example because it's simple to see how it works.  Not so clear with a
canard.  The wing and the canard both make positive lift both of which
increase with an increase in speed.  No pitch up thus no inherent pitch
stability as in a conventional aircraft.  This is overcome by setting the
canard angle of attack about six degrees greater than the main wing.  The
greater angle of attack causes the lift of the canard to change more rapidly
than the main wing giving us pitch stability.  The sparrow strainer was not
added to somehow set pitch trim speed but to increase pitch stability.  The
normal configuration has it supplying a downward aerodynamic force on the
back edge of the elevator proportional to speed.  Increasing speed causes
the nose to pitch up and the reduction of force when you slow allows the
trim spring to raise the elevator allowing pitch down.  Inverting the
sparrow strainer will give you more down trim but will actually give you
less pitch stability than no sparrow strainer at all.

 

If you got this far, you're a brave soul.

 

Larry Coen

N136LC

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