ORIGINAL: YellowAircraft
John,
Thank you. Does 'mass balanced' mean ''balanced on the pivot'', because I've been around FS aircraft that have control surfaces described as 'mass-balanced' when they're not balanced on the pivot. While it's common in FS aircraft to have 'mass-balancing', I've yet to personally encounter a single aircraft with a full-flying tail that's balanced on the pivot.... and that's with me at the airport at least once a week, every week of my life since I was 25. Now, what I have encountered every time I've encountered a plane with a full-flying tail is a drooping trailing edge (when power is off). Not a single exception so far.
Hi Shaun,
Yes you are correct. Mass balancing means that the surface is weighted at the leading edge so that it balances on the pivot. The reason for doing this is simple. If the tail surface encounters some turbulence that forces it up or down until it reaches it's elastic limit, and if it is balanced then it should arrive at it's elastic limit with little or no twisting moment. With the turbulence removed, the surface (if it is stiff enough i.e. correctly designed) will snap back past the nominal postion and bend in the opposite way, but to a lesser extent, and after a few cycles it will dampen out and become stable again. However. if the surface is not balanced on the pivot then when the surface reaches it's elastic limit, the side with the higher mass will continue on and impose a twisting moment on the surface. Normally an unbalanced surface is MUCH heavier at the trailing edge.
So as well as reaching the elastic limit, we now have a twisted surface which will now impose a large opposite force on the surface, drving it in the opposite direction with significant force. The surface now is driven to it's opposite elastic limit and the process repeats, it does not dampen out. In short we have aeroelastic flutter. This can quickly cause some kind of failure (in seconds or less) by either overstressing the structure itself or the linkages or the hinges can fail.
As I understand it a perfectly balance surface has a tendency to "float" around neutral and have a rather soft feel, so they are left a little bit tail heavy to overcome this problem. I hope that this helps,
John