(Having to do this again as Thingiverse deleted my first description stating "Token Expired". Want a refund for 2 hours on my life :( )
I recently got the Flitetest Spitfire https://store.flitetest.com/ft-spitfire-speed-build-kit/ and am loving it but I'm missing something from my build.... retractable landing gear.
Now it's near impossible to add real retracts to this because I'd be weakening the support beam that runs the length of the wing so I've designed a 3D printable kit that you can attach below the wing. As an added bonus I've included designs to add tail gear that you can attach to your rudder servo and steer the plane while on the runway. This all should be usable on other planes but be aware I designed this for the FT Spitfire so your mileage may vary. Below are 2 video's of the gear in action so you can see what to expect.
If you wish to edit this I have included the .SKP file (Google Sketchup) so you may edit this without converting the .STL
I have landed and take off with these however it should be noted the card layer on the foamboard may tear if the landing or surface is a bit rough. Print wise the more infil used the more versatile the gear will be. So far I've had to re-glue the gear to the wing once due to a bad landing. Maybe a wider base to spread load might be an idea.
Other than a 3D printer and a plane to put this on you'll need:
- Super Glue and/or Hot Glue
- 2x 9g Servo
- 2x Servo extension leads (200mm or more)
- Servo Splitter (needed if you can't place each servo on it's own output on the receiver / FC)
- Servo reverser (needed if you required the splitter)
- Push Rod (for the tail wheel)
Included is a variety of files, you can choose to print them all in one go, as the assemblable sections or completely individually.
All parts should be done with 50% infill and a wall thickness of 1.2mm or more. Ideally the tail gear leg will want printing with more infill to give the pivot strength.
All parts are oriented in the most ideal position for best print quality. If you are having issues printing you may want to rotate them but some will be harder to remove support material.
To make support material easy to remove an angle of 70 degrees or more should be used at least in your slicer namely to avoid issues removing support from the servo housing.
After all parts are printed and all support, brim and stringing has been removed test that the wheels fit in on their axles and that the retractable gear leg and pivot brace can twist. Please use the images in this thingiverse page for reference on assembly. There are exploded and assembled images for reference.
Having now familiarized yourself with the design with the included images we'll start with the retractable gear. You may want to do just one for now but if you're in a pinch for time go for both just ensure you're not mixing the parts. Take the retractable gear and a large wheel. try rotating it again on the axle, if it's able to spin freely take the wheel cap and place some glue on the thin rod. Slide that into the hole in the axle and press fit it. You may want to rotate the wheel as the glue sets to ensure any excess doesn't lock the wheels in place too.
While we wait for that to dry take your servo horn and glue it into the matching indentation on the retractable leg. Where the servo attaches must be facing out and not inside the gear. Try to get the centre of the pivot point on the horn aligned to the pivot point of the retractable leg else it may not function properly later on.
While the glue is setting place the servo in the servo enclosure, if printed correctly and to the correct size they should snugly fit inside. The servo must have the pivot point closest to the edge rather than the centre of the servo mount so that when retracted the gear doesn't cover the enclosure (see images if unsure) The cable must slide through the slot on the flat face so it can be routed through the wing later. You don't strictly need to glue this in if you press fit this on the wing later however you may glue this if you find this is looser than it should be.
Turn on your servo's to get them to centre themselves, skipping this step may lead to the servo not having enough throw to fully retract/extend the gear.
Assuming your glue is set from earlier (wait if not) you can place the 2 assemblies together so that the retractable leg is in the extended position (down) and perpendicular to the servo housing. This will be so it by default is down. Take your servo screw and lock the horn you glued earlier onto the servo.
Place the pivot brace onto the assembly and twist it about, the flat surfaces should be level with each other and can be tested by placing on a flat surface so it's facing upside-down. The stopper on the pivot brace should be against the flat face of the landing gear so that when extended it can't go any further, placing this on the wrong side will result in the gear not working correctly. Check the reference images in case you are unsure.
With the gear assembled you must position this on the plane wing so that the wheels are in front of the centre of mass and parallel. I have found 80mm from the front of the wing to the front of the servo housing and 20mm spacing between the fuselage and the servo housing worked for me. As long as you're happy with the placement and works for you then trace around the servo housing onto the wing.
Having traced out where you want to put the gear measure where your servo connector pokes through the slot and mark in the wing where that will be inside where you traced on the wing earlier. The hole only needs to be big enough to fit the connector through although you may want it slightly bigger to use wire later to pull this connector through the wing and into the fuselage. If your wings are stuck on this might be difficult although is possible.
If you have attached your wings to the fuselage don't panic, I did too and although this bit is a complete pain it's not impossible. You'll want some flexible wire to thread through the nose of the plane and into that hole between the wings where the other servo leads come through. When in try to get that wire to reach all the way down to the tip of the wing so that you can hold bot ends. If your wings are removable this is dead easy for you (Oh how I envy you)
When you have something coming out of both wing tips and the central hole in the wing to route wires in the fuselage start by cutting the holes for the servo connector to come through where you traced back in step 9. Don't cut out the entirety of where you traced the servo connector, just the small bit for the connector.
Now with the small hole try to wiggle the wire or whatever you used earlier until you can see it passing the hole you made. Pull the wire through the hole so that one end is now coming out this hole and the other is through the fuselage.
If you're using servo extenders attach one end of them to the wire at the hole end and gently pull it back through with the wire; Else use your servo connector amusing it's long enough. Ensure your knot is good enough too, last thing you want is it falling off and having to do step 10 again! When through and you have access to both ends tape down the connectors at the fuselage or plug them into your receiver / FC so you don't accidentally pull them back through. If you used extenders glue the servo connector to the connector so it doesn't detach in the wing and thread it through leaving you enough to apply glue.
Lets glue the landing gear on now. Messing this up may lead to trying again being messy so make sure you're in a position to apply glue and attach it with time to spare. Do these one at a time too so you have time to adjust the gear as it sets. Apply glue to the flat side of the servo housing and press that onto the wing where you drew an outline. you should be able to pull the cable through as you go to make the housing flush with the wing. Wiggle it into position then hold it in place while the glue sets.
Once set or enough that it won't slide easily apply glue to the flat surface of the pivot brace and and slide that onto the landing gear pivot while pressed onto the wing. At this point try rotating the gear to make sure it can move in the state you have glued it, if not adjust the pivot mount before it sets until it's all the way on the leg and able to move.
If successful the gear should happily raise and lower without too much effort and not flex too much thanks to the pivot brace. at this point you can adjust your trim and servo direction to fit your needs. If you are using a splitter and your servo's don't quite match up you can use a servo trimmer in line with one to adjust the difference.
Moving onto the tail we'll start by test fitting the wheel in the tail leg. It should spin freely in between the fork but if no try fling down any excess material. Test fit your pivot plate in the gap near the tail of your plane, it should fit snugly.
Before we fit the wheel into the fork lets put the leg into the plate. If you brake the leg doing this you'll have less to reprint later. Carefully try and pinch the pivot point and push it through the plate so that the flat side is against the flat part of the leg. The extruded bit shall fit inside the gap in your fuselage later.
Fit the axle into the fork with the wheel in and glue the cap in like you did with the retractable gear earlier. Spinning the wheel here while your glue sets will help prevent it locking up.
Take your push rod and bend one end so you can link it onto the rudder servo horn. Measuring or guessing correctly bend the other end so that it will mean that the wheel is in line with the rudder when fit in place. You may want to take your time with this bit.
- Finally with the push rod linked through one of the small holes in the tail gear on the extended plate and the servo horn you can put glue along the topside of the flange on the pivot plate and press it into the spitfires tail portion.
Congrats, you're done and you have some inexpensive retractable landing gear for your foam-board FT Spitfire!
Happy flying and enjoy, feel free to comment and share your experience with the gear.