MCR4S Elevator Sploiler

MCR4S Elevator Spoiler

Minor elevator buffet is continuous in flight with my MCR4S and efforts to smooth out the upper surface of the fin resulted in random sever pitch problems. It is noted that Dynaero have made two previous modifications to the elevators, one was to reinforce the mounting brackets and one to reinforce the trim tab hinge where cracking occurred along the hinge edge. Also the NZ aircraft encountered severe wear in the trim tab lever system after only 40 hours. All this indicated turbulence behind fin horn.

On an effort to determine what was happening, a Go Pro camera was fitted below the fin directed to the underside of the elevator, which had a series of wool tufts taped in the field of view. A flight was conducted recording cruise, slow cruise and approach speeds, after which the camera results were inspected.

Attached are the video results as a Quicktime movie. Download file.

Inspection immediately showed a widening fan of turbulence extending behind the fin horn. Assuming that the result is the same on the opposite side, the following diagram traced from the Go Pro images approximates the extent and direction of this airflow. The top surface could be expected to have similar reduced effect with less of the fin horn extending above the elevators.

Elevator Vortex

The fan angle for the turbulence reaches around 60° wide and can extend to some 90° of the width of the trim tab. Reverse flow occurs at the rear of the fin, the distance from the fin for this and for the turbulence is dependent upon the airspeed.

From this it is clear that the fin horn is acting as a very efficient spoiler and that the buffet and previous problems could be explained as being due to this turbulence and redirected airflow. It is hard to believe that any other aircraft would be designed with such an elementary fault and not have corrected it in the test phase of design. It is customary for the leading edge of top mounted elevators to be mounted forwards of the fin, not behind as in this design.

Continuous buffeting on the most critical flying surface on the aircraft is not acceptable design feature in any professional circles. Its rectification would have to be removal of the fin horn and introducing a faired surface in its place.

I have asked Dynaero to prepare and test such modification, so far without reply.

Barry Wrendford.