Pulley System for "Mad" Elevator
This past summer a number of slope fanatics successfully recreated the famous "Mad" elevator setup popularized a decade ago by Benoit Paysant-Le Roux (AKA BPLR) one of the world’s greatest R/C pilots and progenitor of the Madslide 3D glider. (see video below).
The most obvious element of a Mad elevator is the extreme throw: 75° to 90° each way... and you need all of that if you want to do flips. BPLR used full-flying stab with a pull-pull system, similar to what is used on powered 3D craft, to get the large elevator throws. Others, myself included, have experimented with push-pull setups, similar to what we see on indoor 3D planes. Both these systems work well; but have their problems as you reach 180° of total throw (you want to do flips upright and inverted... right?). Geometry starts to work against you at the extreme ends of the throw.
I recently started experimenting with pulleys to see if I could get the full 180° of rotation (or more )... without any binding or slack issues. The pulleys have an even pull at any rotation AND it doesn't matter that one pulley is larger than the other. This second point is important since most servos only rotate 90° to 120°.
I started out by computing the size of the pulleys. I wanted 180° or elevator rotation from my 120° of servo rotation. A little geometry helped be figure that the servo pulley radius had to be 1.5 times the size of the elevator pulley. (180°/120°=1.5) The sizes I used was 3/4" and 1 1/8".
I then made some circles of plywood to those dimensions. I marked the centers and circumference on 1/8" ply (centers) and 1/32" (flanges). I cut the flanges a 1/8" larger. I cut them roughly with scissors/tin snips; and then rounded them out with sandpaper. The center of the stab pulley was enlarged to just fit over my stab spar (carbon tube) and glued into place. The servo pulley was screwed and glued to a medium size round servo "horn."
I used 50# test fly line backing (a Spectra blend) for my cable. Since I already had a Golden Rod tube in my fuse I threaded the cable through the tube around the stab pulley one and half times (through the "lock" holes, see diagram below); back through the tube (no twists) to the servo pulley. Each cable end cable wraps around the pulley about 270° (opposite directions) to a notch in the flange and then tied to a small screw. The screw will allow you to tighten/loosen the cable and make a elevator center adjustment.
This probably sounds more complicated than it is. Just look at the photos and video and I am sure you can figure it out.
The pulley system worked well; but I later made a larger servo pulley (1.25") so I could get 200° of throw.
Doug Montgomery had put together a pulley kit that he cuts with his laser cutter from G-10 and 1/16" plywood. (See pictures below) You can contact Doug here at RC Groups (dougmontgomery).
I think is important to make the pulleys big enough and have a good non-stretchy cable to keep any slop from creeping in.
I used a good quality digital servo for max resolution and to help centering. This really helps the low rates.
I added some weight to the LE of the elevator to make it more balanced... weight-wise. (This seems to minimize flutter issues.)
I want to experiment with finding the perfect aerodynamic center for the stab... 25% seems to be a good starting point; but I wonder if that can be refined.
The cable does 1 1/2 turns around the AMT pulley AND out and back in two small holes drilled in the flange. I put a drop of glue on that spot as well. This locks the cable in place.
I am flying with 90% expo at the Mad rates. However even with that much expo I can over-control the stall recoveries... or when I am scratching for lift. I have had to learn to have a very soft touch. I often flip to my NORMAL or THERMAL flight phases when things get dicey. ￼
Video of pulley system:
Videos of the mad elevator on my Le Fish Lilght:
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