SERVO TAB OPERATION Aircraft to Relieve Servos of Some of the Load of Operating TAILPLANE ELEVATOR TAB Control Surfaces

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SERVO TAB OPERATION Aircraft to Relieve Servos of Some of the Load of Operating TAILPLANE ELEVATOR TAB Control Surfaces

Servo tabs are used on model SERVO TAB OPERATION aircraft to relieve servos of some of the load of operating TAILPLANE ELEVATOR TAB control surfaces. They are fixed as shown in fig.1 direct to the 1. fixed airframe. The movement of the surface itself operates the tab, which moves in the 100% (1:1) linkage Fixed link opposite direction to the surface providing an automatic balance. Elevator pivot Tab pivot B Fig.2 shows how the system 2. works in the airflow, the red A upward arrow being the lift Airflow generated by the main surface and the small dark blue arrow the effect of the tab. This small force is multiplied by the lever arm at which it operates from the elevator hinge more than doubling its effectiveness. This balancing force is effectively subtracted from the red arrow 3. so that the servo only had to work to the amount roughly indicated by the light red arrow. The compensating effect of the 200% (1:2) linkage tab can be further increased by changing the moment arm at which the link operates at the tab. Seen in figs.3 and 4, halve 4. this arm to give a 200% ratio rather than the 100% shown above. Any ratio can be chosen but there will come a point at which turbulence and drag will occur. Servo tabs can be employed on any control surface - not just elevators and be equally useful. Here is a Frise aileron with about 130% ratio used. I have Frise aileron shown top hinging for the tab just for variety and to show one of the limitations. Any saving of effort from a servo has a similar saving in electrical energy which not only helps in flight times and safety margins, but means that in stead of buying a more powerful (and more expensive) servo for a large control surface a cheap ‘standard’ servo can ofte n be used. I have used a single standard servo for control surfaces of around 200 square inches without problem - but then a light airframe and slow, scale airspeed also helps a great deal in this respect. Are servo tabs worth the bother? I’m sure of it! Robin Fowler.

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