Linkage Design Plots


Linkage Design provides three plots for each for the four linkage types that help the designer to visualize what is going on with the mechanism. The plots are approximately the same for all four linkage types.

The two groups of plots shown here represent two different cases. The upper group shows typical output for an elevator or other symmetrical-throw surface. The lower group shows a carefully designed flap linkage.

Main Plot

The main plot shows the surface position as a function of the servo arm position (blue line). It also shows the "gearing ratio" over the full throw (green line). This is an indication of the leverage the servo has on the control surface, with a low value indicating better leverage.

Because of the automatic scaling feature, the gearing ratio scale exaggerates the variation in the green line here - for this setup the gearing ratio is effectively constant. Compare this plot with the one for flaps below.

Servo Torque and Pushrod Force

The smaller two plots relate the servo torque and pushrod force to the control surface position.

Compare these to the plots below for a flap linkage.

Main Plot for a Flap Linkage

This plot shows a flap linkage that uses 80° of servo travel to provide 69° of flap deflection. The gearing ratio drops from 1.60 at zero flap deflection (where the loads are low) all the way to 0.38 at full deflection where loads are high. This reduces the load on the servo and reduces the chance of breaking the servo gears when the deflected flap hits something.

Servo and Pushrod Loads for a Flap Linkage

The servo torque is plotted here against flap travel. The geometry is such that the maximum torque occurs at about 45° deflection (not full deflection) because of the improved leverage of the servo at full deflection. Experimentation with key variables showed that this setup provided the lowest maximum servo torque.

Pushrod force is also plotted against flap position.

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