The FPVThing: A fun twin which can be used for FPV

by MikeRobey | November 13, 2017 | (2 Ratings) Posted in Projects

Introduction

I had been doing some FPV flying using a cheap 600mm wingspan wing from Banggood and a tiny camera/vtx (Eachine TX02). It was quite successful but I felt the need for something bigger with a rudder and landing gear so I decided to design my own FPV ship.  It had to be a twin to leave the nose free of propellors for a nice clear camera view. It also had to be a simple build. I wasn't sure how I would go landing on wheels while flying FPV so I figured I might go through a few airframes while getting the hang of things. Other than that I also thought a twin might be fun to fly line of sight as well. I couldn't think of a decent name for it and I kept referring to it as the FPV Thing and eventually the name stuck. 


Design Goals

As stated previously the build needed to be fairly straight forward so that replacements could be easily and quickly constructed. I wanted it to be fairly agile for normal non-FPV flying but stable for FPV.  I elected to go for a mid wing design so that rolls would be axial and I needed to design the fuselage so that there was room for the flight battery, an FPV camera and VTX as well as an action camera.  To get the thing to balance the flight battery ended up in the nose which meant I had to come up with a removable nose cone. I used magnets to keep in on. I wasn't sure how landing while flying FPV would go so I used the same undercarriage mount as with my Simplicity design. If I decide belly landings are a better idea then it can be removed and the resulting hole in the fuselage belly patched up. I positioned the motors as close the the fuselage as I could to make the wiring to the esc's as short as possible.  That turned out to be a bit daft and I could have (should have?) placed the motors more outboard.

The above photo shows the flight battery strapped in. The white section sticking out of the front of the fuselage is a 3D printed frame that holds two magnets and is shaped to fit the inside of the nose cone (visible on the table below the model).  The VTX power lead is also visible. It plugs onto the balance lead of the flight battery.


The FPVThing Maiden

When connecting the esc's to the receiver I did some research and found that if the esc's have a linear bec then you do not have to pull the power wire on of the the esc's.  My esc's had linear becs so I didn't pull the power wire on one of them. This turned out to be a mistake.  On the day of the first maiden attempt the left hand motor was producing a lot less thrust than the right hand motor. Upon realising this I should have given up on the maiden but I racked in full right rudder and kept going.  The poor thing staggered off the ground but there wasn't enough power and, combined with the drag from the full right rudder, the airplane couldn't accelerate and so the inevitable happened and it stalled and went in.  Fortunately there was very little damage which was easily repaired. I pulled the power wire from one esc and had full thrust from both motors. Lesson learnt.


The day of the second maiden came around a week later.  Still had a strong swing to the left at the beginning of the takeoff roll but, as I increased power, the swing went away. Once airborne the FPVThing proved responsive and agile. Power did not provide unlimited vertical but it would zoom up a long way before running out of steam so more than enough power.  The controls were pretty harmonised so it was a joy to throw around.  My landing approach was way to fast but she slowed down and dropped gently to the ground anyway.  I think the swing was caused by the esc's not being correctly calibrated. I had calibrated them before but I did both of them at once. I did get beeps from both esc's but I am thinking maybe only one ended up calibrated correctly.  I have since calibrated each esc individually so I don't expect the swing to be there on the next flight. If it is then I will program differential thrust on my transmitter and that should sort it out.  The FPVThing is a riot to fly line of sight. I have not yet tried flying it via FPV. I need a nice, no wind, sunny day for that.  Check out the maiden at:




Specifications

  • Wingspan: 1100mm 

  • Length: 890 mm

  • Flying Weight: 1200 gms

  • Empty Weight: 1020 gms

  • Motor: 2 x Turnigy Aerodrive DST1000

  • ESC: 2 x Turnigy Plush 18 amp

  • Prop: 2 x 8 x 4.5 slow fly

  • Battery: 2200 mAh, 3S Turnigy Multistar 

  • Max Watts: 2 x 123 watts = 246 watts

  • Max Amps: 2 x 11.5 amps = 23 amps 

  • Max Thrust: 2 x 600 grams = 1200 grams

  • Max C: 10.45

  • Flight Time at Full Throttle: 5.75 minutes


3D Printed Parts

There are a number of 3D printed parts. All could be substituted with parts made from more traditional materials.


Control Horns

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

Nose Cone

When printing a raft needs to be used to keep it from peeling off the print bed while printing is in progress. It would be possible to do a nose in foam board similar to the FT Guinea if 3D printing is not an option for you.

The previously mentioned frame that fits into the front fuselage is shown below. A foam board substitute is possible.

Battery Tray

Simply glued into the front fuselage.  A piece of lite ply would be a suitable substitute.

Wheels

The wheels are 80mm in diameter. The tyres were printed using flexible filament(TPU purchased from sainsmart.com) with 5% infill.  The result is tyres which are a bit squishy and have some flex for landing. The hub was printed using ABS and 15% infill.

Tail Skid

I elected to go for the plain tail skid which has served me well on many of my models.

MotorMount

The motor mount is designed specifically for the motors I used. However the Sketchup file could easily be modified to suit a different motor or a piece of lite ply could be used. If using the 3D printed part make sure you print with 100 percent infill for strength.  The motor mount has 2 degrees of down thrust built in which felt about right on the maiden. I used contra rotating props so no side thrust was built in.

Cowling

These simply glue onto the motor mount. I used UHU Por which makes them easier to remove if I need to get at the motors for any reason.

Motor Pod Covers

These cover the space between the motor pod sides top and bottom. Poster board would be a suitable alternative.

ESC Vents

The esc's ended up in the wings between the motor pods and the fuselage. Its very tight space wise so I had to cut holes in the bottom of the wing to make room for the esc's. I used these vent covers to cover the holes and provide some cooling.

Undercarriage Mount

I used the exact same setup as used on my simplicity design along with the same undercarriage legs from Hobbyking (https://hobbyking.com/en_us/alloy-struts-12cm-high-19g.html). Print with 100% infill for strength. A Piece of plywood could be used as a substitute.

FPV Camera/VTX Mount

I designed this to fit what I had which was an old Immersionrc vtx and a cheap camera from Banggood. You could adapt what I have to suit your needs or come up with your own rig.  The FPVThing has a nice flat reasonably sized are on the nose top for you to place your FPV gear.

Action Camera Mount

Again designed for the camera I have. The bottom is curved to match the curve of the fuselage top. You could use this or do your own thing to suit your camera.

The Build

There were Three unexpected areas of pain in the build.  The first was I decided to try a wavy trim colour scheme. It turned out okay but it was an awfully fiddly thing to get right and a few times I started to think I would be cutting out new pieces to replace ones I was on the verge of ruining but I managed it in the end.  I elected to use wing tape and that is what made the curvy trim difficult. My advice on that is go for a simpler colour scheme!  The second was the wiring for the motors and the placement of the esc which did not go as planned. However I have worked it all out now so also not a problem for future FPVThing builders. I'll explain it as we go.  The final problem was realising the battery had to go right in the nose and, even though my solution is pretty obvious in hindsight, it took me quite a while to figure out.  Again its all worked out so not an issue on future FPVThing builds.  Finally 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 this one.


Okay as always I started with the fuselage.  Any wing tape covering or painting is best done before assembly so I cut the parts out and did that first.

The above image shows the fuselage parts.  The fuselage top (located at the bottom of the image) had the paper removed from the inside because it needed to curve quite a bit.

I started by folding and gluing the sides ( B-Folds). I glued the front halves of the sides first making sure the sides were perpendicular to the bottom.

Next the front deck was glued in place. This is where the FPV gear will go.

I then glued the rear bottom of the fuselage to the rear sides. The glue was applied and then the sides pushed against the bottom which was pressed against the build table to ensure and good joint. When pressing the fuselage was slid back and forth a bit so that any excess glue didn't stick the fuselage to the build table.

Next step was to glue the rear bulkhead in place. Care was required when positioning this. If its too far forward it will be in the way of the wing opening.

The tail plane was placed in its location on the fuselage and then the fuselage top was positioned by letting it butt up against the tail plane.

Glue was applied to the rear of the fuselage top and it was glued to the fuselage.

Then glue was applied to the front of the fuselage top which was then glued to the fuselage.  Pressure was kept on for a good five minutes so the front of the fuselage top does not start to come away from the fuselage sides. If you look carefully in the above image you can see I didn't hold it in place long enough and the top has come away a little. I had to apply more glue and pressure to fix the problem.

When the glue was dry I opened up the wing slots. That's it for the fuselage at the moment. Now we move onto the wing.

The first step in constructing the wing was to screw to motor bases onto the motor mounts.

The above image shows the motor wiring.  I didn't but its a good idea to bind your receiver and calibrate your esc's now before everything is hard to get at in the model.  I had so much fun doing this afterwards. Its important to use identical esc's and make sure they are both programmed the same.

The wing is a typical FT style wing. I am using The Bloody Baron wing section courtesy of Dan Sponholz.  I have had great success with that section and see no reason to go for anything else.

As usual the two wing halves were taped together and then glue applied to the joint before pressing the wing flat onto the build table. The rear hole is for the aileron servo wires and the front hole is for the esc leads. When I was putting it together I realised that  these holes need to be on the bottom of the wing and not the top. I have modified the plans to reflect this. Note also that you can just make out the positions for the motor pod sides. I marked these in pencil to make it easier to position the motor pod sides later.

The wing tops were creased and folded in the usual FT manner.

The spars were then folded and glued. Its important to remove any excess glue from the spars so that they will lay flat against the wing bottom when they are glued in place..

The spars were then glued onto the wing bottoms. Trial fitting is important to ensure that the tabs fit firmly into the slots and that the two spar halves slide together smoothly.

In the normal FT manner, glue was applied to the leading edge and spar top and then the wing was folded over and held down for at least five minutes.

Glue was then applied to the rear upper surface of the wing bottom which was then pressed down and held in place for 5 minutes to ensure the glue was absolutely cooled and solid.

I removed the wing tape from the area where the engine pods will go so that the pod sides are glued to the wing and not the wing tape.

The next step was to cut the holes for the wires that run between the motor and the esc.

It was then I made a tragic blunder.  I glued the motor pod sides in place. Its important to make sure they are vertical and in the right place. Have you worked out what I forgot about?  Think about the fact that the wing is not yet installed in the fuselage.  As I gazed happily at my handy work the sad truth slowly dawned on me...

I realised that I could not slide the wing through the wing slots in the fuselage with both motor pods in place. I sighed. I groaned. I cut away the pod sides from one side and made some new ones up.  On the one hand making a part for the second time because of a mistake is about as much fun as having your teeth drilled without an anesthetic. However on the other hand imagine how much fun it would have been if I had not noticed my mistake until after the motors were in place.

The next step was to slide the wing in place. Once sure it is dead centre and straight, I applied hot glue to each join top and bottom, wiping away the excess glue as I did. I checked the alignment by measuring the distance from fuselage to wingtip, left and right and front and rear. The remains of my scrapped motor pod sides can be seen on the build table. Sigh...

Tranquility descended upon me as my tragic error was erased from the universe when I glued the new pod sides in place. Now its as if it never happened.

Next was the fun job of threading the wires through the wing and fuselage.

It was at this point I realised that the esc's would have to be located inside the wing leading edge between each motor pod and the fuselage. The cavity is not big enough so I had to cut holes in the bottom of the wing where the esc's were placed.

The above image shows the wiring in place and the holes in the wing bottoms. Note the use of the hole for the undercarriage mount while getting everything threaded through. Once I worked out where the esc's were going to be placed and realised the esc wires had to come out of a hole in the wing bottom it was easy, especially with the holes for the esc's in the wing bottom. It took hours to figure out how things should be placed and minutes to get it done. This is a good time to make sure your motors are turning the right way. I used contra rotating props however I am not sure that is really needed. I had them to hand so i thought why the heck not?

The next step was to check the motor mount alignment and trim the front edges of the motor pod sides to ensure the motor mounts end up vertical and with no unintended side thrust. I next applied epoxy glue and waited for the whole lot to dry. I slid a piece of metal into the undercarriage mount hole and used a clamp to clamp the piece of metal to the build table top. This ensured everything was going to stay right where it should while the glue dried.

The next step was to glue the vertical stabiliser to the horizontal stabiliser ensuring they were perpendicular to each other.

Then I glued the tail feathers to the fuselage making sure the horizontal stabiliser was parallel with the wing.

The next step was to glue the motor pod covers on top and bottom.

I then removed the wing tape and paper from where the undercarriage mount was to be glued on. I wanted a really, really solid bond between the mount and the fuselage. After all, some of those FPV landings might be a tad on the hard side.

I next bolted the legs onto the mount and glued the mount in place. I used UHU Por to give it a little bit of flex but still maintain a strong join. Some time around this stage of construction I fed in the servo wires, glued the servos and control horns in place and hooked everything up.

Next the tail skid was glued in place with epoxy.

The above image shows the esc covers glued in place. I cut a rectangular opening in the fuselage bottom behind the wing and placed the receiver there with some velcro. The rectangular piece was the  fixed back in place with some sticky tape.

I used cable ties to hold my FPV camera mount in place and glued the action camera mount to the top of the fuselage.

The final step was to glue some magnets into the nose cone mount frame and then glue it to the fuselage. Two rectangular pieces of metal were glued into the correct place in the nose such that they line up with the magnets and then I was finally done.

The cg is on the wing spar and the control throws are as much as I could get without hearing the servos straining.


Data Files

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

The pdf files (whole, tiled A4 and tiled letter) can be found here: PdfFiles.zip.

The Sketchup files for the 3D printable parts can be found here: SketchupFilesPartA.zip. and here: SketchupFilesPartB.zip.

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


Conclusion

Fitting the esc's and the wiring turned into a bit of a nightmare but now that I have worked it out it should be straight forward for anyone else.  If I accidentally crash my FPVThing it would not take me long to build a replacement. Flies great and even if you don't want an FPV ship is still a fun twin which can provide plenty of power and maneuverability on a 3S 2200 battery. Its an easy build and a great flier. I am certainly happy with the result.

COMMENTS

grifflyer on November 15, 2017
WOW! that looks great I love the color scheme, and the article is very descriptive and well written

:Thumbs Up:
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MikeRobey on November 15, 2017
Thaks. Glad you like it.
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