REFLECTOR: Pitch trim and pitch stability.

Scott Derrick scott at tnstaafl.net
Mon Nov 23 10:23:35 CST 2009 

No, with no change in trim and an increase in power you want it to
climb, not go faster!  If it didn't you would have a nasty tiger by the
tail!

Scott

-------- Original Message --------
Subject: Re: REFLECTOR: Pitch trim and pitch stability.
From: <aminetech at bluefrog.com>
To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
Date: 11/23/2009 07:53 AM

> 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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