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From: Brent Regan <Brent@regandesigns.com>
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Subject: Re: Balancing Elevators
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Bill states:
<<Having two elevators with grossing different weights hanging on the 
back of the plane' even with added weight in the center, I find very 
disturbing.>>

Don't be disturbed Bill, slightly uncomfortable would be better and I 
may be able to tell you why. Hinged control surfaces need to be 
balanced. By that I mean that the center of gravity needs to be on the 
hinge line so that the inertia of the surface does not produce a 
positive feedback under gust conditions that could lead to flutter. In 
the case of the elevator you do not want the inertia (CG) behind the 
hinge line. If it was then an upward acceleration of the tail would 
produce a torque that would deflect the elevator down and add force to 
whatever was accelerating the tail in the first place.

Counterweights are added at moment arms to provide balancing torque to 
move the CG to the centerline. The longer the arm the less weight is 
needed. BUT the longer the arm the more flexible it is and the lower the 
spring rate of the assembly.  Imagine that you balanced the elevator 
using a weight at the end of a length of welding rod. You can achieve a 
static balance this way BUT if the tail accelerates upward then the rod 
will bend and the elevator will deflect downward. It is therefore 
important that the assembly is balanced AND stiff.

The ailerons have the best configuration for stiffness as the weight is 
distributed along their length and well coupled into the structure. 
Unfortunately the weight is closely coupled so a lot of weight is needed 
and weight is bad.

Getting back to the condition where the elevators are not balanced 
individually but balanced as an assembly, the result of this is that the 
balancing torque must be transmitted through the hinge / control rod 
brackets. This is fine if the forces are small. As the forces increase, 
or as counterweights are added at this point, the stresses through these 
parts increases and the potential for a failure from a crack or loose 
fastener increases. If all the elevator counterweight  was hung from a 
bolted assembly then flutter possibly could be induced if the bolts were 
simply loose and not even failed. I cant say if the elevator center 
hinge weldment has been reinforced since I built my airplane but I can 
say that I would not add a counterweight arm to mine. But that is just 
my gut opinion.

Several years back there was a group that was trying to build an 
supersonic experimental jet. The prototype crashed because a 
counterweight on the elevator came off, the resulting flutter failed the 
elevator that caused the nose to pitch up into a 25 G pullup. The right 
wing folded and the main gear failed and was ejected out of the bottom 
of the airplane. The gear was found a half mile down range from the 
fuselage. Some bright light had attached the elevator counterweight with 
pop rivets.

For want of a nail the shoe was lost. For want of a shoe the horse was 
lost. For want of a horse a battle was lost, for want of the battle the 
war was lost. For want of a war the King was lost.

We trust our lives to bolted assemblies every day but a wise man does 
this as infrequently as necessary. IMHO individually balanced elevators 
are the way to go. Nothing to break, nothing to fall off and nothing to 
forget (counterweight wise).  

Regards
Brent Regan



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Bill states:<br>
&lt;&lt;<font face="Helvetica, Arial, sans-serif" color="#000000"><font
 size="3" family="SCRIPT" lang="0">Having two elevators with grossing different
weights hanging on the back of the plane' even with added weight in the center,
I find very disturbing.</font></font>&gt;&gt;<br>
<br>
Don't&nbsp;be disturbed Bill, slightly uncomfortable would be better and I may
be able to tell you why. Hinged control surfaces need to be balanced. By
that I mean that the center of gravity needs to be on the hinge line so that
the inertia of the surface does not produce a positive feedback under gust
conditions that could lead to flutter. In the case of the elevator you do
not want the inertia (CG) behind the hinge line. If it was then an upward
acceleration of the tail would produce a torque that would deflect the elevator
down and add force to whatever was accelerating the tail in the first place.
<br>
<br>
Counterweights are added at moment arms to provide balancing torque to move
the CG to the centerline. The longer the arm the less weight is needed. BUT
the longer the arm the more flexible it is and the lower the spring rate
of the assembly. &nbsp;Imagine that you balanced the elevator using a weight at
the end of a length of welding rod. You can achieve a static balance this
way BUT if the tail accelerates upward then the rod will bend and the elevator
will deflect downward. It is therefore important that the assembly is balanced
AND stiff.<br>
<br>
The ailerons have the best configuration for stiffness as the weight is distributed
along their length and well coupled into the structure. Unfortunately the
weight is closely coupled so a lot of weight is needed and weight is bad.<br>
<br>
Getting back to the condition where the elevators are not balanced individually
but balanced as an assembly, the result of this is that the balancing torque
must be transmitted through the hinge / control rod brackets. This is fine
if the forces are small. As the forces increase, or as counterweights are
added at this point, the stresses through these parts increases and the potential
for a failure from a crack or loose fastener increases. If all the elevator
counterweight &nbsp;was hung from a bolted assembly then flutter possibly could
be induced if the bolts were simply loose and not even failed. I cant say
if the elevator center hinge weldment has been reinforced since I built my
airplane but I can say that I would not add a counterweight arm to mine.
But that is just my gut opinion.<br>
<br>
Several years back there was a group that was trying to build an supersonic
experimental jet. The prototype crashed because a counterweight on the elevator
came off, the resulting flutter failed the elevator that caused the nose
to pitch up into a 25 G pullup. The right wing folded and the main gear failed
and was ejected out of the bottom of the airplane. The gear was found a half
mile down range from the fuselage. Some bright light had attached the elevator
counterweight with pop rivets.<br>
<br>
For want of a nail the shoe was lost. For want of a shoe the horse was lost.
For want of a horse a battle was lost, for want of the battle the war was
lost. For want of a war the King was lost.<br>
<br>
We trust our lives to bolted assemblies every day but a wise man does this
as infrequently as necessary. IMHO individually balanced elevators are the
way to go. Nothing to break, nothing to fall off and nothing to forget (counterweight
wise). &nbsp;<br>
<br>
Regards<br>
Brent Regan<br>
<br>
<br>
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