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Aileron mass balance

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VideoFlyer(at)aol.com
Guest

Posted: Fri Apr 17, 2009 11:53 am Post subject: Aileron mass balance

Would adding some mass to the aileron bell crank (6W10-1) inside the wing  serve to balance the aileron?  It would move horizontally at a right angle  to the direction of aileron travel, but it seems to me that it would accomplish  the same thing.  There's plenty of room between ribs seven and eight, so a  fairly long moment arm could be used, lessening the total weight.  I'm  certainly not an engineer, and maybe my theory is totally wrong, but .....if I'm  right, this seems like it could be a fairly easy retrofit.

  

 Dave Harms

 N618PZ  601XL/Corvair



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  [quote][b]

notsew_evets(at)frontiern
Guest

Posted: Fri Apr 17, 2009 12:06 pm Post subject: Aileron mass balance

The control surface must be balanced not the linkage.....

 I think I read you post correctly..

 [quote]   ---

Thruster87

Joined: 16 Apr 2008
Posts: 193
Location: Australia

Posted: Fri Apr 17, 2009 1:10 pm Post subject: Re: Aileron mass balance

The De Havilland DH-3 Drover has mass balance weights attached to the control rods.Check the pic at the following Cheers                 http://www.hars.org.au/fleet/drover/index.html

notsew_evets(at)frontiern
Guest

Posted: Fri Apr 17, 2009 2:44 pm Post subject: Aileron mass balance

We can see mass balance weights below the ailerons.



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hansriet

Joined: 09 Feb 2007
Posts: 93

Posted: Fri Apr 17, 2009 2:49 pm Post subject: Re: Aileron mass balance

The thought is not totally crazy though.

I can imagine that springs attached to the bellcrank can have a damping effect on any oscillations of the ailerons. The engineers on this list can now jump on me and tell me why that would not work.

notsew_evets(at)frontiern
Guest

Posted: Fri Apr 17, 2009 3:07 pm Post subject: Aileron mass balance

The problem needs to be stopped at the source.

Not down the line of connection.

I guess you could use a lead control stick in your hand and it would feel 

like there is no flutter.

A car shock absorber would help also.  But these will not prevent the cause 

which is aerodynamic.

Aerodynamic?  when not flying through the air ( Static on the ground) you 

probably will not have flutter.  Just when air travels over the unbalanced 

airfoil....

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grs-pms(at)comcast.net
Guest

Posted: Fri Apr 17, 2009 3:16 pm Post subject: Aileron mass balance

Here is the first response from an engineer:  Springs don't provide damping. 

Quite the opposite.  Damping is provided by elements or devices that absorb 

energy, converting it to heat.  Shock absorbers are also known as dampers. 

The oil in an air-oil landing gear strut is forced thru an orifice to 

provide damping.  Friction dampers were used on old-time race cars and some 

autogiro and helicopter rotor heads.  Even the atmosphere will provide some 

damping, which is why sensitivity to flutter varies with density altitude.



A spring will store energy and then give virtually all of it back, sometimes 

when you least desire it.



George

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jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Fri Apr 17, 2009 3:25 pm Post subject: Re: Aileron mass balance

Dave,

         If you balance with weights at the inboard end of the cable you are setting up for a resonant coupling, which will increase vibrations not damp them. As someone posted earlier, the correct solution is lever weights attached at mid span of each aileron, like the Avid flyer AD required. I was visiting a fiend with a MK4 Avid yesterday, sorry i didnt take a photo for you all, it had a 4" wide plate rivited across the underside of the aileron, spar to trailing edge, with a 3/4" tube welded on at an angle such that it faces forward approx. level, and 5" of lead tube attached over the end. This end was cut at a 45 degree face so it would tilt downwards in an airstream. Dimensions only approximate.  Such an attachment should be made to meet the rules:



The design rules applying to European 601XL is CAP482 British Civil Airworthiness Requirements section S - small light airplanes, section D, design and construction, S 6569 



"Mass balance 



The supporting structure and the attachment of concentrated mass balance weights used on control surfaces must be designed for: 



a) 24 g normal to the plane of the control surface; 



b) 12 g fore and aft; and 



c) 12 g parallel to the hinge line"

I'd wait for ZAC to supply the fix, they already have for Europe models although I would have thought the Europe models already had been flight tested to induce flutter as required under BCAR-S and DS 10141e



Ralph

_________________
Ralph - CH701 / 2200a

hansriet

Joined: 09 Feb 2007
Posts: 93

Posted: Fri Apr 17, 2009 3:26 pm Post subject: Re: Aileron mass balance

Thanks for the explanation George. Does the principle of trying to dampen the excitation make any sense though? Or is that not at all a way to avoid a resonance in the system. Or am I totally off and is flutter something completely different?

grs-pms(at)comcast.net
Guest

Posted: Fri Apr 17, 2009 7:18 pm Post subject: Aileron mass balance

The initial excitation comes from something like a gust, vibration from the 

engine or prop, abrupt deflection of the controls, stuff like that.  I'm not 

sure such inputs can be damped.  In fact, in flutter testing, one technique 

is to bump or "pulse" the stick to try to excite the control surface 

concerned.



Flutter is a form of harmonic motion.  Think of a weight hanging on a 

spring, or a of pendulum, or a kid on a playground swing..  All these 

systems have what is called a "natural frequency".  Once displaced from 

their resting position they will make a certain number of oscillations in 

each given interval of time.  Kinetic energy (the push on the swing) is put 

in and converted to potential energy (in the form of  gravity or spring 

tension in the above examples).  Then the energy is returned.  In a 

frictionless world the motion, once excited, would continue forever.  The 

amplitude of the motion and the "period" or time to return to the initial 

position would be constant.  Moreover, if additional energy is put in at the 

right point in the cycle, the amplitude will increase.  Think of "pumping" a 

swing to make it go higher.



For a moving control surface, energy is stored by deflection of the 

supporting structure:  in the case of an aileron, by the twisting and 

bending of the wing, like storing energy in a giant spring.  A certain 

amount of damping comes from friction within the structure and a certain 

amount from the friction of the airstream, but at some value of airspeed the 

energy input from the airstream exceeds the energy dissipated by damping and 

the amplitude of the motion increases with each cycle.  Soon the deflection 

of the supporting structure exceeds the amount of deformation the structure 

can resist.  This can happen in a fraction of a second, leaving no 

opportunity to recognize the danger and take corrective action.



It isn't practical to make control surfaces absolutely flutter-free, so the 

practical solution is to somehow raise the natural frequency of the system 

beyond the point where it will be reached before the airplane's limits are 

reached or exceeded.  Mass balancing the surface is one way to do this. 

Increasing the stiffness of the wing by going to heavier gage material or by 

increasing the depth of the structure would be another.  Wing torsional 

stiffness is reduced by the presence of large cutouts, like wheel wells or 

access openings not adequately reinforced.



Control cables store energy as they are stretched.  Their natural frequency 

increases with tension, like the strings on a musical instrument, so proper 

tensioning becomes important.



I recognize that my college courses in vibration and flutter are some 50 

years behind me.  My professional work was in aircraft structural design and 

airplane configuration, rather than in flutter and vibration.  I welcome 

corrections from anyone who has worked in that field.  Also, note that I am 

not claiming that flutter is the cause of the 601XL crashes that we are all 

concerned about, or that mass-balancing the ailerons will be the solution.



George

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