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  #1  
Old 09-13-2019, 02:23 PM
LyleM LyleM is offline
 
Join Date: Jan 2008
Location: Milan, MI
Posts: 65
Default Rudder balance

The instructions talk about the importance of elevator balance but donít say anything about the rudder balance. When I Lay the rudder and vertical stabilizer horizontal, the rudder is much heavier than itís counter weight. Is that normal? Or how close should the rudder weight be to itís counter weight? Thanks
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  #2  
Old 09-20-2019, 10:38 AM
FlyGood FlyGood is offline
 
Join Date: Dec 2009
Location: League City, TX
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Take this with a grain of salt - I am not a controls expert, but from my long experience I can repeat what I've learned.

Surfaces that are primarily horizontal need to be balanced. The behavior of these surfaces (elevators and ailerons) is a product of aerodynamic loads, control system friction and free-play, and vertical gust loads. If a horizontal control surface is not balanced, a vertical gust creates a control deflection which, typically, is dynamically unstable.

Imagine, for illustration, that all the weight of an elevator is behind the hinge line. An upward acceleration of the airplane due to a gust will cause an elevator DOWN deflection due to the inertia of the mass behind the hinge line. This, in turn, causes a nose down pitch, creating a negative vertical acceleration of the airplane which will cause an upward elevator deflection. The cycle will repeat. Depending on the other factors - primarily true airspeed - the elevator will oscillate up and down and, with the right conditions, do so with such frequency and force that it can destroy itself.

Ailerons, even though they are "balancing" each other because they are interconnected, still need to be balanced at the hinge line. Control system slop, cable stretch, or push-rod bowing under load, gives a bit of independence to the ailerons, allowing them to suffer the same dynamic instability due to vertical gusts, if not balanced at the hinge line.

For the rudder, the gust loads are much less than the horizontal surfaces, so the deflections due to gusts is a much smaller problem. Some airplanes don't have any mass balance at all for the rudder, but some mass balance can be beneficial. However, I think the problem is small enough that perfect balance is not required.

As I said at the beginning, I'm not an expert. I'm hoping someone that is will chime in and correct the things I have misstated. I'd like to learn more!
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  #3  
Old 09-20-2019, 12:13 PM
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F1Boss F1Boss is offline
 
Join Date: Aug 2006
Location: Taylor Texas
Posts: 769
Default Thumb rules

I researched this as much as I could - I'm aiming for high altitude flight and hoping to set some records (supercharged TCM 550N up front). TAS will be ~300KTS if I'm correct with my engine calculations. So, the rudder is something I looked at - closely. All else is balanced as designed.

What I found was "70%" is the starting point. So, using std methods like you did on the elevs, weigh the furthest point of the TE (rudder), then add counterweight until the TE weighs 30% of the initial weight.

Example: if the TE has 800G on it at the start, it should weigh 240G (30%) after completing the counterweight.

I hope you balanced the elevators at 100% - the rudder process is almost the same.

As is normal, paint will affect the balance. It is likely you will re-balance after final paint.
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  #4  
Old 09-20-2019, 12:30 PM
F1R F1R is offline
 
Join Date: Nov 2010
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Quote:
Originally Posted by F1Boss View Post
I researched this as much as I could - I'm aiming for high altitude flight and hoping to set some records (supercharged TCM 550N up front). TAS will be ~300KTS if I'm correct with my engine calculations. So, the rudder is something I looked at - closely. All else is balanced as designed.
300Kts speed increases as a cube root of the HP added
(10 kts added should only require an extra 1000 HP)

Hey Boss, that must be quite a blower on the continental.
Where can we get a seat on the ground to watch?
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