Serene: A 1.4m Foam Board Glider

By

Mike Robey

Introduction

I designed a 1.4m 3 channel foam board glider called Serenity roughly12 months ago. Given the limitations  of foam board I was very happy with the result.  Ever since the idea of designing a 4 channel glider has been in the back of my mind. I finally gave in and Serene is the result.  Basically its the Serenity with an aileron equipped wing.  Serene is fairly responsive and does not bite. Its a predictable flyer with reasonable agility. Its gliding characteristics are limited by its foam board construction (i.e. 5mm thick wing trailing edges and 5mm thick tail feathers) so it isn't going to win any glider competitions but it is fun to fly and glide.  I am not a serious glider guider. Serene is intended for anyone who wants a simple, cheap glider to fool around with on a nice sunny day.

Design Goals

The idea was to use the same fuselage and tail feathers as Serenity and replace the polyhedral wing with an aileron equipped wing with no dihedral.  Serenity used a thick wing section and I decided to reduce the thickness a bit as I wanted Serene to be a bit faster on the glide. As with Serenity the fuselage is split in the middle to allow it to be constructed from Flitetest sized foam board sheets. In an effort to reduce the drag, I used poster board fillets to fill the rear between the rear edge of the wing bottom and the wing trailing edge. Negligible weight increase for a reasonable decrease in parasite drag.  I chose a motor which was never going to give me anything close to unlimited vertical simply because it was in my spares box and not likely to be used on another project.  I handicapped it further  by using a prop which was smaller than recommended. I did this to avoid waiting for a new pair of folding props. I figured there would be enough power and I would increase the prop size in the long term.

The Maiden

The maiden occurred on a hot day with strong gusty winds. Because of the orientation of the flying site it is necessary to launch downwind as the alternative is to launch directly into the sun heading straight for a car park.  Given the power to weight ratio of most model planes these days it normally isn't an issue.  However the strong gusty winds were not helping and Serene took a few seconds to get to a good climbing speed. However it was simply a matter of maintaining a slow climb until she got the bit between her teeth and then got some height under her.  It was hard to get a feel for handling in the conditions.  All I could really deduce was that Serene had good control response in the gusty conditions. When I cut the power the prop blades didn't fold indicating I had been unsuccessful at setting the brake on  the esc.  The second flight occurred a week later in much calmer conditions. Launch was much more positive getting a reasonable climb from the get go. Handling was good. Power was adequate but not fantastic. Loops from level flight were possible but a steep climb out was not.  My second attempt to set the brake on the esc was a bust so I still had a wind milling propeller when gliding.  After an aeon of googling I finally managed to successfully set the brake ready for the third flight.  Check out the video of the third flight with the brake (finally!!) set:

Specs

• Wingspan: 1420 mm 
• Length: 960 mm
• Flying Weight: 1181 gm
• Empty Weight: 972 gm
• Motor: Turnigy Parkflyer 480-850kv
• ESC: Turnigy Plush 30 amp esc
• Prop: 10 x 6 folding
• Battery: 2200 mAh, 3S Turnigy Multistar 
• Max Watts: 180 watts
• Max Amps: 14 amps 
• Max C: 7
• Flight Time at Full Throttle: 9. 5 minutes
  
  

3D Printed Parts

There are a number of 3D printed parts. All of them could be made by alternative methods if you do not have access to a 3D printer.

Control Horns

Pretty straight forward. You can print these or use whatever control horns you have handy.

Motor Mount

The motor mount has 2 degrees of down thrust and 2 degrees of right thrust built into it. The holes are spaced for the Turnigy motor I used but the Sketchup file could be modified for other hole spacing.  A more traditional lite ply motor mount using washers to induce the down and right thrust could be used instead.  If 3D printing then make sure you use 100% infill.

Nose Cone

Not overly imaginative but matches up to the spinner used.  I attach it to the motor mount using UHU Por.  UHU Por allows easier removal if access to the motor is required at some point.  If no 3D printer then a balsa alternative could be fashioned or it could be left off entirely.

Canopy

Canopy is straight forward.  The plans include a foam board alternative.

The 3D printed pieces in the image below are used to re-enforce the fuselage sides where to barbecue skewer that holds the canopy on goes through. Lite ply could be used instead.

Wingtips

These were a simple attempt to reduce drag and turbulence at the wing tip.  Some simple round tips fashioned from soft balsa could be used instead or the wing tips could simply be closed off with wing tape.

The Build

The following build guide assumes a basic familiarity with FT style builds. If you are new to FT building then watch one of Flitetest's excellent build videos before proceeding. The build guide for the FT-Spitfire is probably the closest to the way my designs tend to go together..

Starting with the fuselage construction begins by installing the 3D printed hole re-enforcement pieces which re-enforce the fuselage where the barbecue skewer holding the canopy on goes through. This ensures the hole will not wear over time and cause the canopy to break free. The first step is to mark the hole by placing the canopy in position and making the hole position on the forward fuselage side. This should be done for both sides.

The hole is the created by gently pushing the barbecue skewer through the fuselage side.

The re-enforcement piece is the placed onto the fuselage side with the holes aligned. The cutout for it is then marked out and the foam carefully removed.  Test fit and trim as required to get a precise hole alignment. Repeat for the other side.

The next step is to glue the re-enforcing pieces in place ensuring that each hole is free of glue. Now for something completely different...The wings.

Wing construction commences with gluing the wing spar sides.  These are B folds. I had originally intended to use a 3D printed joiner (seen in the top right of the image above) but decided to use a wooden wing spar instead. While this ensured a very strong wing it did have some unintended consequences as we will see later in the build.

The wing spar  is then hot glued to the top of the wing as shown above.  It is located between the two rear most creases in the wing top. The shape of the wing means we need a spacer between the rear edge of the wing bottom and the wing top. Pop sticks are a perfect fit so they are hot glued onto the top of the rear edge of the wing bottom as shown in the image above.

The process is repeated for the other wing half and any pop stick extending past the wings is then trimmed off.

The two wing halves are aligned and taped together.

The wing is flipped over and hot glue applied to the join before pushing down and holding flat against the build table while the hot glue sets. Don't forget to wipe away any excess glue with a piece of foam board scrap.

The tape is then cut away from the hole for the servo leads.

The next step is to glue the wooden spar in place. My spar was chosen using the scientific technique of picking the lump of wood in the hardware store that looks like the best fit. It is entirely possible that the wooden spar is not needed however it makes me feel better having it.

The wing is then glued and folded in the traditional Flitetest manner (i.e. apply glue to spar top and leading edge, fold over and hold in place for about 5 minutes then apply glue to the rear of the wing bottom and hold down for another 5 minutes).

There is nothing worse than gluing the wing and then finding its come apart later so I like to weigh down the top surface and leave it for a few hours. Absolute overkill but much better than building the wing a second time.

Now its time to return to the fuselage. Construction commences with gluing the front and rear halves of the fuselage together. Apply the glue to the edge, position the pieces and then slide them around as the glue sets so the fuselage doesn't end up stuck to the build table.

Now we are in familiar Flitetest territory.  First one side is folded and glued ensuring its perpendicular to the fuselage bottom. These are B folds. I am never sure why but I very rarely end up using an A fold.

The process is then repeated for the other side.

Next step is to glue the rear fuselage bottom the the sides. Its a long piece so make sure you have plenty of hot glue and apply the glue as quickly as you can.

Now the fuselage rear top is glued in place. This is best done in two steps. Cut and crease the top piece about half way along and then glue the rear part in place.

Then glue the front part in place.

The next step is to glue the doublers together. These go over where the rear fuselage joins the front fuselage.

I used UHU Por instead of hot glue to give me more time to get each doubler in place.

The bottom front fuselage piece is glued in place next.

Followed by the front fuselage top piece.

The canopy is test fitted. Mind you I am not entirely sure what I could have done about it if it didn't fit. I guess its a good thing it did fit.

Because I covered the glider in wing tape the next step was to apply wing tape to all of the fuselage joins. I cut strips of wing tape about 10 or 12mm think for this purpose.

The slot for the vertical stabiliser is then cut free.  If this is cut free before fuselage construction then the rear of the fuselage has too much flex and will be really hard to glue together with everything in the proper place.

At this point the fuselage is done.

The next step is to bolt the motor to the motor mount and poke the wires through the hole in the motor mount before connecting them to the esc. Make sure you have the down and side thrust oriented correctly (in the image above the motor mount is upside down).

Back to the wings where poster board is used to fill the gap between the rear edge of the wing bottom and the wing trailing edge. Small bits of foam board were cut to size and glued against the rear edge of the wing bottom before gluing the poster board in place.

We do the same thing for the ailerons only its a tad more tricky. First we cut and glue foam board pieces as shown above. These will act as support for the poster board.  The piece that fits onto the aileron front edge has to have a chamfered edge to allow the aileron full movement.

Next the poster board is glued in place and the process repeated for the other side.

The above image shows my boo boo with my wing design.  When I placed the cutout for the aileron servos I forgot to allow for the thickness of the wing spar.

I corrected this by re-orienting the servo and patching the hole. The plans have been corrected so this is not an issue for you if you decide you need Serene in your life.

The above image shows my precarious method for positioning the air frame while I glue each wing tip in place. Never use hot glue when gluing 3D printed parts as it will not hold. I used UHU Por.

I got carried away with getting the thing put together and forgot to take pictures.  So prior to this the wing is slotted into the fuselage and glued in place and the tail feathers attached in the usual Flitetest manner.  Do the wing first so that it can be used to eye the tail feathers and ensure the tail plane is parallel to the wing and the vertical stabiliser is perpendicular to the wing. The motor mount has been glued on using epoxy glue for strength and the nose cone glued on using UHU Por. The servos have been installed and the control horns hot glued in place. The receiver is placed inside the fuselage just behind the wing. This was done by cutting a rectangular hole in the fuselage bottom, doing the installation and then replacing the piece cut out and using sticky tape to keep it in place.  Wow! That's a lot of photos to forget to take. I do apologise.  

The CG is 60mm from the leading edge (as indicated on the wing plans) which I achieved b y placing the battery as far forward as it would go. If you build using Flitetest foam board you should not need to have the battery as far forward.

The above image shows the port wing tip glued in place.

Data Files

The pdf files for the plans (tiled A4 and Letter as well as full sheets) can be found here: PdfFiles.zip.

The dxf files for the plans can be found here: dxfFiles.zip.

The Sketchup files for the 3D printed parts can be found here: SketchupFiles.zip.

The STL files for the 3D printed parts can be found here: STLFiles.zip.

Conclusion

I was happy with the result and happy with the way Serene flies. I wasn't happy with trying to figure out how to turn the brake on with a Turnigy Plush esc but you know what they say, a pure stubborn streak will always get you there even when your ability to read and process fiddly instructions fails you (actually did anybody really say that? I guess I just did!).  Serene is a happy addition to my hanger and a nice flying, cheap and simple motorised glider.