Retractable Landing Gear

by HilldaFlyer | May 4, 2014 | (31) Posted in Projects

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) 






***********************************

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

Prototype #1 Materials:

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:


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




COMMENTS

Flynn on May 31, 2014
Nice job! I do like the way you have minimal cutting on the spar.
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Yogenh on June 6, 2014
Love it you did a great job. You came up with a way not to spend money!!
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scottie on June 1, 2014
I've fitted retracts to as Spitfire wing,it wasn't easy and truthfully something I'm not going to do again in a hurry :) but this is brilliant to be honest,simplicity is first class.
Nice job indeed.

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jamesomega on June 11, 2014
All of these designs are brilliant, thank you for documenting the process and sharing!
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jtuttle on June 2, 2014
Awesome engineering, However I'd like to point out that the pivot point of the Spitfire landing gear is inside the tire radius. The wheels should retract to the outside like the B-24, Meserschmitt 109.
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HilldaFlyer on June 2, 2014
Yup - it is and that was my dumb mistake. I actually built the retracts for the flying penguin's P51 - but I had an extra set of wings for the spitfire laying there and without thinking about the orientation, I just plugged them in. After publishing the article I was looking at them and saying, somethings not right. Then it dawned on me. I'm going to try plumbing them in with the correct orientation.
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jtuttle on June 7, 2014
I don't mean to sound critical, I plan to get started on a set of this design for my new Multi-Motor that's loosely based on the Flite Test swappable concept. (Hint) it looks a little like a C-130.
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HilldaFlyer on June 10, 2014
Great, let us know how it turns out. I'm working on beefing up the wire and getting the wheel more forward of the CG.
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jtuttle on October 4, 2014
The four motor foamy is finished, I decided on fixed gear, but the plane is awsome. I have pictures and a short video of motor test on my FB page. just look for the guy holding up the 'Deuces Wild'
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Cooter3590 on February 5, 2016
I'd like to see your plane design. You have a link to the picture?
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HilldaFlyer on February 7, 2016
The plane is none other than the FT Spitfire
http://flitetest.com/articles/ft-spitfire-build


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raphino on June 1, 2014
Cool idea! How is you system holding the landings? Had a similar idea on my mind how to construct some retracts, but I had always some concerns on how the servo is handling the beating from landing since u got a unfavorable leverage on the servoarm.
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HilldaFlyer on June 1, 2014
Good Question. If you noticed the plane has no motor or servos... I will have to crash a plane or buy more parts to complete the plane build. I have simulated landings by attaching a battery and dropping the plane from 1 and 2 feet. The wire needs to be a bit thicker to survive a harsher landing. I know it isn't the right answer, but the best I can tell you now. My biggest concern after building it (and posting) is the fact that the wheels are so close to the CG that any friction (grass field) will nose-over the plane. I've been thinking about ways to get the wheels more forward but it would demand mounting the retract system tilted off from flush. I thought that most of the force is placed on the nylon arms. I will follow up with more info after I fly and land a few times.
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raphino on June 1, 2014
Thanks for the answer! Yeah I sea that especially when u got e good 90° angle to the base the force should be minimized alot but some moving out of the perfect 90° because of the material strengh. You could also experiment with replacing the pushrod from the servo arm by kinda spring formed pushrod. Anyway I will follow when u got some data out of test flight!
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Hookdriver on June 11, 2014
Just because they are made from dollar tree foam doesn't mean these little airframes cant have retracts. Well done on a super simple and effective design. Anyone can poke holes in any design...but There are many production airframes out there with retracts that wont work this well. Thank you so much for sharing.
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Gise y Mimu... on October 23, 2014
Hi! Im Gisela from Argentina. Im here with my cousin and questions...

Whats the angle of the servo in this picture?
https://lh4.googleusercontent.com/gYshoAqgSkrJ2QYiQVLt3YAl-Uzkc5SpAg414TGjieqmUXncYKGl5lRgrsj5RSAjIOjzi8Zeg3pzQ38rNABCHnG9rFhqflTAcmPjGU3ACUS3M0FDMqe37QtC9bdyL7dYnw

the servo arm is in the top of it position when the gear is up?

Thanks!

Gi & Mimu...
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HilldaFlyer on October 24, 2014
The servo arm is 45 degree counter clockwise from straight up. The rotation of the servo arm spans 90 degrees total, it travels from 45 degrees on the left of center to 45 degrees to the right of center. Hope that helps.
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flyboy10111 on November 23, 2015
Hi All
Hope you can help as I am looking to install the retracts in the Mustang cant seem to open any of the pictures on the post by Hilldaflyer could you repost to see if it helps
Thanks
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HilldaFlyer on November 24, 2015
That was really weird. They were not there a second ago and they reappeared when I looked at it again. Post a comment again if you can't see the photos.
HilldaFlyer
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HilldaFlyer on November 24, 2015
Sorry about that - I broke the links - I think they're back now.
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RCMasterx on November 26, 2015
Looks great!

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HilldaFlyer on November 26, 2015
Thanks - If you try this, you should use music wire or something stronger than a hanger. They bend too easily.
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dharkless on December 3, 2015
This is a great idea and well thought out. I will definitely use it at some time in the future.
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Ethanator10000 on May 23, 2016
I like it! I will probably do this when build a spitfire. One thing I would do though is swap the left and right gear in your picture so that they open inwards and towards the fuselage just to make it look a bit more scale.
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HilldaFlyer on May 24, 2016
Thanks, Comment June 2 2014 pointed out that I had the retracts going the wrong way. I wasn't thinking and was just after the retracts. This works, in principle, but what I have discovered is that they need to be tilted forward to get the wheels in front of the CG line... or the plane just flips over on grass. I don't have a paved landing strip, so small gear is an issue for scale.
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kilroy07 on March 31, 2018
What an elegant/simple solution!
Do you mind if I try and develop a 3D Printed design based on this concept?
(I'll post a link once I've finished.)
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HilldaFlyer on March 31, 2018
Hi Lilroy07 - That is a great idea - You should really do that! I just got a 3D printer. The one drawback to this design was the use of wimpy wire. My only concern with 3D printing this part is the stress it has to endure on impact landings. Layer adhesion may prove to be a weakness... but let's try. When you do that design, start a forum thread and I'll follow along. Others can print, install, test and then we'll have a solution to easy retracts.
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smiling albert on March 22, 2019
Hi
I am a relative newbie from the UK and am very impressed with this .
I assume this excellent idea is for big warbirds ,i.e. 40 inch wingspan.
Do you think it might be possible to do this on 30 inch warbirds such as
https://www.foamflying.com/rc-prop-planes.html
Particularly the Hellcat !
Thanks
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HilldaFlyer on March 23, 2019
I appreciate your interest. I was a newbie when I did this. The idea is OK, but the performance was really poor. Need to use much thicker gauge wire to make it strong enough to withstand a grass field and hard landings. I have not reworked this design because bending thicker wire is extremely difficult to get the correct angle at the right spot. The idea is to isolate the forces of the model from the servo.
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Retractable Landing Gear