Rudder counterbalance ?

[Gray E Fowler]

It is all about the natural frequency of both the aero surface and the 
structure. Those who have seen tail failures in the Impact talk about the 
rudder flutter, but it could be induced totally by the fuselage. If the 
fuse is flex twisting, then how is the servo supposed to hold the rudder 
in position? It cannot and the rudder flutters and the results are known. 
Our planes are low speed and our aero surfaces are really light weight and 
relatively stiff, creating a high natural frequency for the control 
surface. Opposite that is a fuselage, with a distance of about 40 inches 
from the wing to the tail, with all the fiber at 0/90 degrees creating a 
much lower natural frequency. This is where the problem starts, and the 
first indication of this is when a former in the tail fixes a problem.
Relating full scale stuff to our planes may make for interesting 
conversation, but what we do the best  and what they do the best are not 
neccesarily the same thing.  I never implied that the full scale guys do a 
poor job, but I would venture to state that a full scale 
aerodynamic/stucture engineers first attempt at designing a pattern plane 
would in essence "suck".
Spend your weight budget on a stiffer airframe. Not only will your aero 
surfaces NOT flutter, but you plane will fly better.



Gray Fowler
Principal Chemical Engineer
Composites Engineering




"Bill Southwell" <bnbsouthwell at avsia.com>
Sent by: discussion-request at nsrca.org
04/28/2005 11:41 AM
Please respond to discussion

 
        To:     <discussion at nsrca.org>
        cc: 
        Subject:        RE: Rudder counterbalance ?


If a counter balance is a "trick" Then full scale aerodynamic/structure 
engineers have been doing a poor job for years! I would say it is a cure 
not a patch or trick. If you need to stiffen your structure and or control 
system you will most likely add weight to the aft end of the airplane. If 
you "spend" the same weight to add mass ahead of the hinge line and the 
same effort aft of the hinge line to remove weight you make flutter 
impossible regardless of your stiff control system or lack of it. Dick 
Hanson proved this long ago. He also showed less stress on servos and a 
lighter plane overall. If an ounce or two on a under weight pattern ship 
cost the airframe and gear with in it, it is a rather false economy.  If 
you eliminate the possibility of divergence via surface balance you will 
not have flutter occur. If you have a component fail in a stiff  ( but 
unbalanced) system and the surface gets "loose" then you have the flutter 
get you?if its balanced it can't.  Of course none of us have any thing 
fail on us do we :>) 
 
Regards
Bill 
 

From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org] On Behalf Of Gray E Fowler
Sent: Thursday, April 28, 2005 10:09 AM
To: discussion at nsrca.org
Subject: RE: Rudder counterbalance ?
 

"Why not mass balance for flutter elimination. When most full scale 
aircraft
get painted they have to check balance of the surface ( ie Piper
Cherokee's). 

Bill " 


Because most spend hundreds of dollars removing ounces of weight. 

Flutter is natural frequency-speed driven. If you have flutter it is 
usually an indication of a serious stiffness problem, either in the set up 
or the structure itself. Instead of counter balances and otehr tricks, 
just increase the stiffness. That will be the biggest bang for the weight 
buck.



Gray Fowler
Principal Chemical Engineer
Composites Engineering

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.f3a.us/pipermail/nsrca-discussion/attachments/20050428/6db96df2/attachment.html