The ideal wing mounting system should have low drag, stand up to any launch or flight loads and give, with impact and inertial loads, in a crash. Nylon wing mounting bolts can come close to meeting this ideal. Plug-in wings with rod shaped joiners do not meet the impact and inertial load objective. Rubber bands, if used in sufficient quantities to resist winch zoom launching, do not meet the impact and inertial load objective and, in any case do not meet the low drag objective. Nylon screws at the trailing edge with an indexing pin at the leading edge do not meet the impact and inertial load objectives either.
Nylon bolts which are counter sunk to be flush with the wing surface have low drag. Nylon bolts which are just big enough to take launch loads and flight loads meet the second objective. Nylon bolts which are small, close together and properly supported in the wing and fuselage will shear in a crash with minimum damage to the structure (if there isn't a bulkhead directly in the path of the wing). By placing the screws closer together the force on the wing tip to shear the bolts is reduced and the smaller the tip force the smaller or less likely the damage.
I have used nylon screws for wing mounting in dozens of R/C sailplane models of various sizes over the last ten years and never had a screw break or pull out in th air. Over those ten year's I have gradually reduced the size of screws. My flying buddies will tell you that I have a very heavy foot on the pedal. They will also tell you about some of my spectacular crashes and the surprise at how little damage resulted. Based on that experience, here are my recommendations for screw size and spacing:
| Model | Screw | Number | Spacing (max) |
|---|---|---|---|
| Handlaunch | 6-32 | 1 | 1/2 inch |
| 2-Meter | 8-32 | 2 | 1 inch |
| Unlimited | 10-24 (OR 32) | 2 | 1-1/2 inches |
| F3B | 1/4-20 | 2 | 1-1/2 inches |
| Cross-Country | 1/4-20 | 2 | 2 inches |
For handlaunch two 4-40 nylon screws are big enough but I have not been able to find a source lately.
The threads in the fuselage may be provided by a T-nut, plywood or, a slug of filled epoxy. The minimum number of threads which engage must be at least 6. The grain of the plywood must be perpendicular to the screw and the threads must be reinforced with CA and then chased with a tap. The filled epoxy must be the thin, slow setting kind and the filler must be high strength (such as chopped glass, flox or, silica gel). The fuselage mounting must transfer the loads into the rest of the structure so that the screws shear before anything else gives.
The threads must be chased with a tap and lubricated until the screw turns freely. A dry teflon spray lubricant works well in this application. This will facilitate removal of a sheared screw. A sheared screw may be removed by pressing the point of a No. 11 blade into the end of the screw and using the blade as a screwdriver. If that doesn't provide enough torque then the No. 11 blade may be used to cut a slot in the top of the screw stub for a small screwdriver blade.
The wing must have a block to take the compression load of the tightened nylon screws. The block should be incorporated into the wing spar structure and the screws located near the thickest part of the wing. The blocks may be made of a hardwood, plywood or filled epoxy. The block in the wing must be tied into the wing structure so that the screws will shear before anything else gives. The block should be carefully countersunk so that the screw heads are flush. Any gaps may be covered with a small piece of tape.
Two piece wings work well with this system. Just locate a nylon screw near each wing root and the screws will also keep the wing halves from separating on their joiner rods.
Maintenance is simple. When the nylon begins to turn yellow, the screws should be replaced. The screws should be inspected after each flying session or hard landing. If they show signs of distortion where the wing meets the fuselage they should be replaced. If the screws start to turn brittle they can be restored by boiling them in water.
I will be glad to answer any questions about nylon wing mounting (except the cause of my crashes). I can be reached on the internet at ocwilson@sunline.net