Mad Stab: PulleysEdit


Pulley designs by Doug Montgomery


Pulley on the elevator servo


elevator pulley


The oval pulley

One of the problems with any pull-pull linkage is that you can develop slack in the cables... especially if your horn geometry is not perfect. This is not usually a problem with a rudder linkage; however with this can cause flutter and control issues (double centering) in the mad stab.

Dawson Henderson, an avid slope flyer living in Flagstaff, AZ, has experimented with a pull/pull setup utilizing pulleys instead of horns. He discovered that the round pulleys would give a nice linear slack-free pull throughout the full range of servo and stab movement. Furthermore by making the pulley on the servo side larger than on the stab side, he could easily get a full 180° or more of stab throw. (Most analog servos move only in a range of 90° to 120°. Many digital servos can be programed to move 180°)

To the right is a picture of a laser cut pulley kit created by Doug Montgomery of Phoenix, AZ. These pulleys are made from thin fiberglass sheet with wood ply cores.

Below that image are several pictures of Henderson's homemade servo and elevator pulleys. There are small screws on the servo pulley to adjust the tension of the cable and provide a centering adjustment to the stab. The cable is made from 30 pound test fly line backing (Spectra).

A recent variation of the pulley system uses one or more oval (ellipse) shaped pulleys instead of round pulleys. This oval shape induces some EXPO into the movement. The effect is that small movements of the servo produce small movements of the elevator; while larger movements give corresponding larger elevator movements. This is allows good control for normal flying; but still gives you full 180° elevator for flips.

Mad Stab using a new oval pulley

Mad Stab using a new oval pulley

Below is a video showing a Mad Stab setup with and oval servo pulley and circular elevator pulleys on a Le Fish light.

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