I’ll start out by saying that 95% of my total airplanes have retracts, and it’s definitely true that I’ve had my share of issues. For the root causes here’s what I’ve found so far.
1. Mounting: A lot of retracts are screwed into a small plastic pocket that is barely glued into the surrounding foam. In many cases, this leads to the gear being pulled out on the first landing or sometimes even taxi. If done properly, a clever balance in the amount of glue application or structural design allows the gear to break away in the event of a very bad landing or crash. Unfortunately, most manufacturers either under or over do it on the glue. This is generally more so in foam models, and less common in balsa and composite. Also, any interference or parts of the gear touching the wheel wells could possibly trigger the current protection, and stop deployment.
2. Field conditions and pilot error: If you hit too hard, they’re going to break. Either that, or punch up through the wing. The model also has to be suited to the field. No small EDFs with tiny wheels in long bumpy grass. Trust me, it doesn’t end well. Sand and dusty conditions will also greatly reduce the life of a retract, especially in fore-aft deploying gear, where it gets kicked up into the retract by the wheels.
3. Strut design: Most gear either have no strut, or a poorly designed strut. The straight wire (no coil spring) transfers all the landing loads straight to the retract trunnion and jack screw. This might be ok on a paved runway if you are great at landings, but on grass, somethings sure to give. The straight “oleo” type strut (vertical compression only) looks scale and does a great job of taking vertical stresses down a notch. However, rearward loading and bending moments are transferred straight to the retract. On bumpy grass, those loads take a toll, and something will probably wear or bend. The trailing link strut helps disapate the rearward loads as well as vertical, and is best on grass, but is generally not as Scale looking with a few exceptions.
4. Retract Design: For most sub-$500 Models, they all use more or less the same retract design. A motor driving a jack screw pulling a slider that moves the trunnion and looks it up/down. Generally, this is encased in a plastic housing. The design is optimized for a fore-aft rearward deploying gear, and puts the loads through the strongest part of the housing. However, for a forward deploying gear, all of the load is on the jackscrew. This can bend the part even slightly, and cause failures later down the road. Gear folding horizontally put all the bending loads into the thinnest parts of the plastic frame, and can cause cracking. Rotating retracts (like a Corsair) become even more complex, by applying the torsional loads into a small pin prone to bending. More expensive gear like Eflite or Robart are engineered better, but are only available for huge models.
One of the advances that I have loved to see in gear is a combination of increased scale fidelity and durability. The main gear of the Freewing 90mm F-16 are beautiful and can really take beating. They incorporate the same folding locking arms as the full scale to better distribute loads. The new FMS Tigercat is the same way.
All this said, I’ll keep using them and fixing them because I’m a hopeless scale nut who loves seeing the gear go up after takeoff. Hopefully this shed some light on some of the more common failure modes.
