REFLECTOR: Pitch trim and pitch stability.

Laurence Coen lwcoen at hotmail.com
Mon Nov 23 10:19:47 CST 2009


John,

If you can in fact go faster by adding power without adjusting the trim you 
have neutral pitch trim stability.  I bet you do have to adjust your trim 
for takeoff then again for cruise and once more for approach.  You do this 
to adjust the speed that the plane flies.  Your problem is not enough down 
trim and by fixing it by inverting the sparrow strainer you have sacrificed 
stability.  It's a fix but I wouldn't call it a GOOD fix.

Larry Coen
N136LC

--------------------------------------------------
From: <aminetech at bluefrog.com>
Sent: Monday, November 23, 2009 8:53 AM
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Subject: Re: REFLECTOR: Pitch trim and pitch stability.

> In that case I definitely don't want trim stability, because when I add 
> power I want to go faster.
>
> I'll have to check my pitch stability with the sparrow strainer inverted, 
> but I haven't noticed any issues in normal flying conditions.
>
> John
>
> --- scott at tnstaafl.net wrote:
>
> From: Scott Derrick <scott at tnstaafl.net>
> To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
> Subject: Re: REFLECTOR: Pitch trim and pitch stability.
> Date: Sun, 22 Nov 2009 17:09:07 -0700
>
> Hmmm...
>
> AS I understand it there is a difference between  pitch  stability and
> trim stability.
>
> Trim stability is,  you set up in flight at a certain speed so the
> aircraft flies hands off with out changing altitude.  Now increase
> power, the plane starts to climb(nose up), but maintains the speed set
> by the trim,  decrease power and the plane descends(nose down) to
> maintain the set trim speed.  Stable trim stability...  If the speed
> changes with power you have negative trim stability.
>
> Pitch stability is where  you  have the  plane trimmed for level flight,
> now push forward on the stick the plane noses over and starts to descend
> and pick up speed, release the stick,the plane should then climb to
> reduce the speed to the trimmed airspeed, it will overshoot, then nose
> over and pick up speed overshooting and then nose up, this will repeat.
> If the oscillations reduce with each cycle, narrowing in on that
> previously set trim speed, you have positive pitch stability, if the
> oscillations  stay the same, neither getting smaller or larger you have
> neutral pitch  stability, if the  oscillations keep get larger  and
> larger(bad) you have negative pitch stability.
>
> my 0.02$ worth.  I've probably got it goofed up in some way! :-)
>
> Scott
>
> -------- Original Message --------
> Subject: REFLECTOR: Pitch trim and pitch stability.
> From: Laurence Coen <lwcoen at hotmail.com>
> To: reflector16 <reflector at tvbf.org>
> Date: 11/22/2009 12:48 PM
>> 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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>
>
>
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