Gents,

 

Chris is right.  In other words:

 

Interesting.  Think about a balance beam scale used by that Justice lady.  First, if the weights were above the pivot point, the scale/system would be unstable.  Notice that the weights hang much below the pivot making the system stable and, should it be perfect, the beam would stay in whatever position if the weights were equal.  But, such scales have a very slight built in bias to only be in balance when the beam is level.

 

Let's think about an LNC2 aileron that is hinged on top (pivot) - note that both the aft and fwd weight center is below the pivot point and is thus stable when balanced to a "level" position.  The fore and aft weighting is not symmetrical, thus changing from the level position may have an unknown result because the weight CGs and thus its arms may not change in the same ratio as they were in the level balance,  And, then we attach the push rod that throws off the level balance also.

 

The elevator on a small tail LNC2 is also hinged at the top and the weights are below the pivot (not very far), thus it's stable.  When displaced, the horn weight distribution CG can alter the fore/aft arm relationship.  Also, the addition of the push rod certainly changes the balance that was done at level and without push rods attached. 

 

The MKII tail is interesting since the center hinge may have as much fore and aft weight above the pivot as below - thus, not very stable and perhaps would stay in any displaced position (depending on friction and balance) until the push rod is attached. 

 

The rudder - hmmmmm.... The recommendation was to add X weight to the horn and half round lead bars to the leading edge.  Then, If I remember correctly, balance it to at least 80% (whatever that meant).  When mounted, exactly what is the angled pivot for the rudder and at what pitch orientation of the fuselage?  After all, when the rudder is mounted, most of the fwd weight is above the pivot points.   What happens to the balance when the rudder cables are attached?  What about those that installed the highly curved tubing as in the plans and introduced a tremendous amount of friction in the rudder system?  Note that those of us that straightened the rudder cable tubing have very little friction in the system.

 

Finally, the control rods and cables actually provide a stabilizing effect whereas an unbalanced horizontal control surface can induce flutter because the bias may exacerbate the action/reaction of induced vibration.  

 

Balance the control surfaces in whatever way and to whatever degree the construction manual calls for and do not exceed Vne.

 

Grayhawk

 

Consider the vertical position of the CG of the surface relative to the point from which it is being supported during balancing.  Assume a surface that is perfectly symmetric in the z direction.  If you remove a hinge pin (1/4" bolt on the elevator, for example) and hang the the surface by wire run through the hinge hole, the CG is below the support point and a balanced surface will tend to return to level.  If the same surface was balanced by setting it down on hinge pins (360 rudder, for example), the CG would now be above the support point and the surface will be difficult to keep level.  If truly balanced on the hinge center line, it will stay in any position. 

In reality the vertical CG position is not always known.  If the surface will not stay in position, look at which direction it wants to fall when released from a level position.  If the surface does attain a stable position it is easy to see which end is heavier. 

 

Chris Zavatson

N91CZ

360std

www.N91CZ.net

 

From: Jim Nordin <panelmaker@earthlink.net>
To: lml@lancaironline.net
Sent: Tuesday, February 21, 2012 8:30 AM
Subject: [LML] 360 rudder balance

Hummmm … correct me if I’m wrong.

Balance of control surfaces is attained when the component (aileron or elevator for example) hung at the center of rotation and given a perturbation (trailing edge pushed down) minimally returns to the level condition or better a small nose down condition. Any other condition (leading edge high above level, trailing edge below level) warrants adding weight to make the nose (leading edge) to settle below level.

Nose heavy is the point here. Any other condition may result in flutter.

Jim

From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Charles Brown
Sent: Tuesday, February 21, 2012 8:02 AM
To: lml@lancaironline.net
Subject: [LML] Re: 360 rudder balance

 

They should stay in any position where you let them go.  They should not return to level.  If they are out of balance, they will have some preferred orientation (as you say, will move up or down).

 

Charley Brown

MS Aero/Astro Engineering

 

On Feb 20, 2012, at 6:51 AM, Bill Bradburry wrote:

I have a question about balancing the flight surfaces.  If they are in balance, should they return to level from where ever you move them to, or should they just stay where you put them.

 

I assume that if they are out of balance they will either move up or down from level depending on whether you need to add or remove weight.

 

Bill B