Control Surface Feel
 

Control Surface Feel.

The effects of control surface trailing edge arrangements on aircraft pitch trim.

I have some experience of an aircraft that is very challenging to trim in pitch. This aircraft has a very direct pitch control circuit, only one short direct rod from the stick to the elevator horn. The airframe is very stiff. All of the bearings on this circuit are low friction roller bearings. There is also almost no brake out (stiction) force, although the springs themselves are quite heavy. There is no discernible play in the system.

The trim arrangement is by means of a spring balance, one side of the system is controlled manually and the other side by a small servo. This arrangement allows a course manual control with a fine adjustment from a servo. It also somewhat complies with CS23 as an alternative pitch control circuit.

The trailing edge of the elevator closes to a very sharp edge (almost razor).

In still air the aircraft will trim, sometimes with good stability, but other times it will prove very difficult to trim and will need constant attention.

I have read with interest some information in Darrol Stintons excellent flying qualities book about the effects of different trailing edges.

I did wonder if the trailing edge arrangement had any influence on total system gain.

Could this group offer any further insight? I would like to lower the trim sensitivity. I do realise that the problem is more likely to be caused by the spring balance system but I am curious about trailing edge effects.

Any illumination would be welcome.


Thank You.

Charlie

If I recall correctly, the Fairchild FH-227B had a wedge-shaped bit riveted to the trailing edge of the elevator for, as I was told at the Fairchild factory many years ago, to enhance elevator feel and to prevent the onset of flutter, if the larger propellors were fitted...those being required for increased MTOW.
This aircraft had a higher Vmo than the other F.27's, so additional elevator flutter protection was felt necessary.

Hope this helps.

Take a close look at the tail feathers on a Cessna 208 (Caravan) particularly the rudder, they added a piece almost the length of the control surface to deal with some of the problems you state. :}