Retractable Landing Gear

Retractable Landing Gear

by HilldaFlyer

Having retracts on a warbird is just the thing! 

This retractable landing gear is designed so you can add them to a plane that already you have already built. That’s right, you can add this retract design to your favorite warbird, even after a few belly lands. You just need a way to snake the retract servo wire to the fuselage, assuming that is where your receiver is located. It is really easy if your wing is removable.

The article below provides several designs with alternative setups with the final version being built based on the experience of the first prototypes (at the bottom), the foundation principle of K-I-S-S (keep it simple stupid) and were inspired by adopting the goals of the Flying Penguin when he built the Swappable Landing Gear. http://flitetest.com/articles/flying-penguin-rc-removable-langing-gear#sthash.0jTpjsJv.dpuf

He stated the design should be

1. Simple and low cost. Check ✓
2. Removable and interchangeable between different designs. Probably not easily removed.
3. Capable of being installed on planes already built. Check. ✓
4. Be strong, but also repairable and replaceable. Check ✓

3 out of 4 ain’t bad.

I included the prototype builds with the thought that they may provide you with ideas of how to build your own.

This is so exciting, so let’s get crackin’...

Materials:

• 2x 9g Metal Gear Servos (Hobby King)
• Plywood, 1/16 3 ply, (Balsa USA)
• Plywood 3/16” thick (I used flooring underlayment (prepared to 3/16 thick).
• Servo Screws, Micro Fastners http://www.microfasteners.com/index.cfm
• Clothes Hanger (wife’s closet)
• Sand Paper (60 grit from local hardware store)
• Drill and bits (1.5 mm & 3/32)
• Nylon Arm 2.8 x 6 x 7.5 (OR7-00103x10) or thicker: Nylon Arm 2.8 x 5 x 6 OR7-00105x10) (Hobby King)
• Poster board (Dollar Tree)
• Hot Glue (Walmart)
• Wood Glue (Walmart)
• Servo Speed Regulator, optional (Hobby King Product ID: TSSR-3)

First, to keep the retract system thin, I dispensed with the front wheel steering mechanism used in the first prototypes (below) and built the hinge mechanism out of a piece of plywood and Nylon Arms. First, a sneak peak at the prototype #3 and then the real build of the most awesome retracts…

Here we go!

A) Nylon arms mounted to 3/16 inch plywood with servo screws, pre-drilled 1.5 mm.

B) Shiny clothes hanger (don’t tell your wife you took one).

C) 9g metal gear servo.

Glued to ⅛ inch 3 ply plywood (not labeled).

Constructing the retract mechanism

Cut ⅛ inch 3 ply into a rectangle 4.5 x 8.5 cm with the top grain running the length. Mark a line 1.0 cm from the top.

Cut the 3/16 inch thick 3 ply 1.5 x 5 cm. If you use flooring underlayment, cut it so the top grain is perpendicular to the length of the rectangle. Here comes the cheapo in me… to get 3/16 inch ply, I prepared it from standard flooring underlayment (I had some laying around after I remodeled my bathroom). The underlayment is originally 27/128 of an inch thick, but the two outside to surfaces peel off pretty easy, so I removed them with a razor blade and then sanded with some 60 grit sandpaper. You don’t have to use the razor, the sandpaper takes off the top layers pretty quick. To the 1.5 x 5 cm rectangle, cut a rectangle out of one corner 0.75 x 1.5 cm.

Drill the servo arm with a hole to fit the hanger wire (I used 5/16 to fit my wife’s hanger). I drilled the third hole from the mount.

Bend the hanger in a “Z” curve so that the shaft aligns with the center of servo screw after the end is put through the hole in the arm. The hangers are free, so practice until you get this right, it is important so the servo doesn’t bind while turning. Then cut off the excess end of the servo arm.

Prepare the 3/16 plywood to receive the nylon arms. I aligned the one to the outside edge (right) and push up till the round part of the nylon arm bumps into the edge of the plywood. Drill the 1.5 mm hole (this is the size I use for servo screws, but you may have other fasteners to use) but don’t screw the nylon arm down, this was just to get the hole in the right place. 

Mark the hanger with a line 3.5 cm from the Z bend. A little shorter is OK, but a little longer will make your landing gear loose.

Slide two nylon arms onto the hanger wire up to the Z bend and then make a 90 degree “L” bend at the mark that is in line with the Z bend. Aha! you forgot to slide the nylon arms onto the wire before bending the wire… so did I, three times in a row, so I had to acquire another clothes hanger. 

At 8 mm from the last “L” bend, bend the wire about 45 degrees off of the Z bend. This will allow the wire to lay flush with the bottom of the wing. The end product should look like this:

Lay the wire beside the mounting platform and screw in the one servo screw into the pre-drilled hole (on the right) ½ way. Pull the left nylon arm to the left  until it bumps into the Z bend, it should be pretty close to the notch. Drill the 1.5 mm hole for the second nylon arm and screw the servo screw in ½ way.

Glue this platform to the base plywood at the 1.0 cm mark (use wood glue and clamp).

Finish screwing the servo screws. With the landing gear laying flat on the work surface, the Z bend should lay at an angle 45 degree to the work bench.

Center the servo with the arm perpendicular to the base plywood using a servo tester or Tx/Rx. Get it as close as possible to center. Rotate the servo arm 45 degrees from center counter clockwise (the best way is to use a servo tester and rotate it all the way counter clockwise). Insert the Z bend into the servo arm. If it doesn’t exactly match, twist the rod between the Z bend and L bend with two pair of pliers to get the servo arm hole to align while the wire is laying flat on the work surface and the servo is laying flat on the plywood. 

Glue the servo to the mounting plywood so that the edge of it contacts the notched mounting platform. If you use hot glue, the servo will get loose or come off in the event that you need to remove the retract unit from the plane with a hair drier (explained below). For a non-hot glue option, I used a dab of shoe goo. Notice that I cut off the servo mounting tabs (the servo was used in a previous application), but this is not necessary.

Now build a mirror image of the retract system for the other wing. The second one took me 15 minutes. The hardest part was getting the “Z” bend the right height to match the hole in the servo arm.

This completes assembling the mechanical part. Now, let’s mount these puppies on the plane. 

For the next section, I have used the bottom surface of a FT Spitfire wing as an example. It has the spar mounted (or lines where the spar goes), but I left the top surface of the wing off so I could show you both the inside and outside of the wing. Just pretend the wing is already built.

First, created a template to outline the hole that needs to be cut into the bottom surface of the wing. After cutting the template out (hashed part), I slipped it over the servo and mounting platform just to make sure it fits snugly.

The body of the plane (FT Spitfire) is about 6 cm wide, so I marked a line 4 cm from the root (center edge) of the wing to give me place for the rubber band (3 cm for ½ fuselage and 1 cm rubber band). I drew in the placement of the spar on the outside of the wing’s bottom surface so that I know where the wheel I should not be placed.

Leave about 2 to 4 mm gap between the fuselage and I used a toothpick to puncture a hole in the wing where the wheel should go. 

Draw a line around the wheel using the toothpick to hold it in place.

Place the retract system with the wire into the wheel toothpick hole. 

Align the mechanical part so the wing spar lies over the bar between the two nylon arms (not shown).

With a pencil, reach under the plywood surface and mark where the servo touches the bottom surface of the wing.

Lay the template onto the wing’s surface using the lines made from the servo as placement guides.

Draw the rectangle of what should be cut out. In the photo below, I also outlined the placement of the template, but it is not necessary to do so.

Cut the rectangular hole. Also, cut a notch for the servo wires and servo mounting tabs if you didn’t cut them off.

To get the plywood face to mount flush, it is necessary to cut a small notch in the spar to allow the rod to pass under it. 

Cut out the wheel hole

Install the retract system and sand the wheel hole bigger until it fits.

Attach the retract system to the wing with hot glue. In the event of a broken wing or mechanical failure, you can heat up the wood of the retract system, loosen the glue and remove the retractable landing gear from the plane’s wing. So in that sense, it is removable for repairs. 4 out of 4!

Now…. Let’s quickly do it on a fully completed wing… FT Spitfire.

I repeated the process of installing the retracts on a completed FT Spitfire wing. It took about 15 minutes. Here are the pictures.

Draw lines on wings indicating the location of the wing spar. Punch wheel placement holes in wing with a toothpick. I put these wheels too close to the body of the plane, oops! There wasn’t enough room for the rubber band.

Locate retract system using wheel hole as reference. Looking under the plywood facing, line up the wing spar so it passes between the nylon arms. 

Take a pen or pencil and outline the servo. Overlay the template using the servo lines as a guide and draw the cut-out rectangle (in pencil below).

Cut out the rectangle, but be careful not to cut the aileron servo wires (they’re in the wing somewhere). To keep the aileron servo wires from tangling with the retract system, I attached it to the spar with a small piece of poster board. Rip out the cut foam rectangle. 

Cut a notch in the spar for the bar and another notch for the servo wire.

Cut wheel hole and sand until it is big enough for the wheel to pass without contacting the wing.

Pass wires to hole in top surface of wing.

Use hot glue to attach retracts to the plane wing making sure to align the wheel with the hole while the glue hardens. Make fine adjustments by bending the wire. 

For added flash, add landing gear covers made from poster board. When mounting the covers onto the wire with hot glue, give the poster board a little bend to wrap it around the wire while it sets.

To get the landing gear to retract and deploy at more “scale” speeds, you can use programmable setting on some upper end transmitters or you can purchase a servo regulator (Turnigy 3 Channel Servo Speed/Direction Regulator) 

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Below are the prototypes that preceded and helped formulated the design above.

Prototype #1 Materials:

• 1x Front Wheel Steering Arm & Mount Set 40mm, Hobby King Product ID: 07-01002x5
• 1x 4 mm Carbon Tube, Radical RC, http://www.radicalrc.com/category/Carbon-Rod-Hollow-410
• 2x 9g metal gear servos (Hobby King)
• Plywood (Balsa USA)
• 4x Screws, Micro Fastners http://www.microfasteners.com/index.cfm
• Servo Screws, Micro Fastners http://www.microfasteners.com/index.cfm
• Clothes Hanger
• Rotary cutting tool
• Sand Paper
• Drill and bits
• 3/16” Plywood (flooring underlayment)
• Prototype #1

Prototype #1 was inspired by the design used David Windestål’s in his tricopter yaw mechanism. Insert the 4 mm carbon fiber tube into the front wheel steering arm base. It is a tight fit, sharpen the end of the carbon tube and it will slide in with a little force. Cut off the excess.

The steering arm has a brass internal collar and a set screw. Since the rod hole in the steering arm is about twice as large as the coat hanger diameter, I made a shim out of a 1 cm long 4 mm carbon fiber rod. 

Cut a slot the length for the set screw.

Using a rotary cutting tool, flatten one side of the steering arm and the outside of a servo arm (sandpaper or a file can be used, but it will take a lot of sanding to remove that much of the brass fitting). Flatten the side of the steering arm that will not interfere with set screw operation. Align the center of the servo arm and center of the steering arm and glue in place. Drill a hole through both arms and screw together with servo screws. 

Bend the clothes hanger in an “L” and slide it through, cut off the excess, tighten the set screw and mount the servo.

The mechanism can be mounted on plywood with the servo secured. 

Okay, it worked… but that was a lot of work to get the prototype completed. The flattening of the two arms, aligning and drilling was pretty difficult. On the second and third trials, while cutting the steering arm, the brass internal collar go too hot and melted the steering arm. On the fourth try, the holes didn’t align so the servo wanted to lift off the plywood when rotating. When I took the prototype to the plane wing I noticed something else, it is a little thicker than the wing and the length of this retract unit from end to end is about ⅓ the wing’s width leaving no room for the spar. Hmmmm…. a wing with no spar… that might be a problem. After thinking a bit, I decided to rotate the servo 90 degrees and connect the servo to the steering arm with a connecting rod… 

Prototype #2

Because prototype #1 was so difficult to get the servo arm and steering arm cut flat and aligned (after three attempts) I decided it was too complicated without precise cutting and aligning mechanisms that the project wouldn’t be good for do it yourselfers (like me who was unable to replicate the first prototype).

Prototype #2 Materials:

• 1x Front Wheel Steering Arm & Mount Set 40mm, Hobby King Product ID: 07-01002x5 http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=8432
• 1x 4 mm Carbon Tube, Radical RC, http://www.radicalrc.com/category/Carbon-Rod-Hollow-410
• 2x 9g Servos Hobby King
• Plywood, Balso USA
• 4x Screws, Micro Fastners http://www.microfasteners.com/index.cfm
• Servo Screws, Micro Fastners http://www.microfasteners.com/index.cfm
• 1x .039 music wire, Radical RC, http://www.radicalrc.com/category/Music-Wire-414
• Clothes Hanger
• Rotary cutting tool
• Sand Paper
• Drill and bits

Prototype #2 was a slight variation using a connecting rod to connect the servo to the steering arm connected to the retract wire. The preparation of the steering base and the steering arm shim was the same, but it was not necessary to flatten the steering arm or the servo arm. A hole had to be drilled in the steering arm at the same distance as the third hole on the servo arm and the rest of the arm removed. The connecting music wire made it easy to mount the steering mechanism next to the servo. That took minutes compared to hours on prototype #1.

I really liked this design, so I thought it was not time to put it into a wing. The first wing I tried was the FT-Spitfire. When I tried to lay this retract unit in the wing, it was too thick, so I had to start chopping holes in the wing, but with the plywood on the bottom wing surface, both the steering arm and servo arm would be sticking through the top of the wing. That would look really stupid. Then I thought about mounting it (the plywood side) on the inside of the top wing surface and cut holes in the bottom wing surface because it would already have holes for the wire and wheel.

So I mounted it between two airfoil bends (B and C, right where the spar lays). 

The parts of the wing surface that were not impacted by the retract unit were covered to give me and idea of what would be protruding. I cut out another wing and made a template of the holes. I moved the spar further back in the wing so it ran right in front of the 2nd airfoil fold (Fold C). I made a “split spar” design hoping to reinforce the gap in the spar. The servo on the plywood was still a bit thicker than the wing, as you can tell by the stress bends in the lower wing surface. I also bent the landing gear arm 45 degrees so when retracted, would lay flush with the outside of the wing (arrow). 

When the wing is folded, the arms of the retracts extend about 1 cm below the wing surface. For this prototype I cut the holes a little big, so the blue tape covers gaps where I shouldn’t have cut.

Well, that would work, but it is not very elegant. The steering gear might be used by larger models and would be ideal for larger gauge wire landing gear. Probably really sturdy overkill for a parkflyer. Then I thought about ways to make it thin, simple and able to be added to wings already built…. go to the top to find out the rest.

HilldaFlyer - May 2014