The Foo Fighter

By

Mike Robey

Introduction

Yes there was the rock band but the name of this model didn't come from them. It came from the term used by allied pilots for an unidentified aircraft.  The inspiration for the design came from a recent Flitetest video where Josh and Alex threw a couple of Multiplex Dog Fighters around (well actually, quite literally at each other). I decided to design something along the same lines.  The idea was top come up with a simple to build and fun to fly model with WW2 fighter looks.  The end result flies a lot like the FT-Spitfire but with faster aileron response.  I used a lot of 3D printed parts but I have provided alternate plans for a traditional FT style fuselage with power pod etc.  I haven't provided an alternative to the canopy but I imagine the poster board FT-P51 canopy could be made to fit.  The result is a lot of fun to fly.  The build did involve a few unexpected issues but they have been resolved so now it would only take a few hours to knock up another one (not counting the time to print the 3D printed parts).

Design Goals

I wanted to come up with a bit of a mongrel. I had already designed a 3D printable model for a P51 canopy so I decided to use that. The wings are sort of BF-109G – ish and the tail feathers are Spitfire-ish and the rest is whatever seemed like a good idea.  The wing section is, once again, derived from the Bloody Baron wing section by Mr Sponholz. It has proved so successful in the past that it simply didn't make sense not to use it in this model.  I intended to 3D print the fuselage front deck and nose but I also wanted to provide a FT style fuselage with power pod for people who wanted to go that route. That meant keeping the fuselage width to 60mm so the standard FT powerpod would fit. Once I had finished the initial design I designed an alternate fuselage which uses the FT powerpod.  I haven't built a fuselage using those plans but I am confident they should be fine.  I loved the FT-Spitfire to bits (quite literally as I flew mine until it started to crumble) but I always felt it could do with a faster aileron response so I made the ailerons on the Foo Fighter larger.  All of the FT-Spitfires that I have built have had no down or right thrust and they all flew fine so my Foo Fighter doesn't have any either.  It may not look it but I spent quite a bit of time figuring out the colour scheme.  I admit that had more to do with using the colours that I had wing tape for than it did trying to come up with a stunning colour scheme.  All in all I think the Foo Fighter ticks the boxes I wanted ticked. Its an easy build and a lot of fun to fly and, like the FT-Spitfire, it doesn't bite.

The Maiden

The maiden was uneventful and the Foo Fighter performed almost as expected.  Holding the model just behind the trailing edge I throttled up and then pushed her off. The Foo Fighter easily escaped my grip and clawed its way upwards.  The roll rate is brisk and there is plenty of power with the setup used (Flitetest C pack).  I needed a tiny bit of left aileron trim to get her flying hands free. Aileron response was crisp as is elevator.  I did a few hammerheads which told me the rudder response is fine.  My normal approach to setting a model up for a maiden is to have as much control deflection as possible without straining the servos and incorporate a fair amount of expo just in case (I usually start at 42% - I mean it is the meaning of life, the universe and everything so why not?).  These settings felt fine.  I know I have a keeper when the timer expires in what feels like seconds instead of minutes and that is what happened on the Foo Fighter maiden. It felt like 30 seconds but was actually 5 minutes. The Foo Fighter came in, slowed down and gently dropped to the grass.  Dan  Sponholz's wing section once again provided me with a model that has a wide speed envelope. I had two more flights that day and they were both like the first. I am very happy with my Foo Fighter.

Check out the flight video at:

On subsequent flights I have noticed a need for some right thrust. I am not going to bother with that with mine. I simply dialled in some right rudder trim to compensate. Not a perfect solution but does the job for me. Now the only time I really notice is on launch where she kicks to the left a tad (watch the video and you will see).  Somebody did suggest that a hand launch at half throttle would possibly eliminate that. If I was building another I would build in a couple of degrees of right thrust.

Specs

• Wingspan: 1100 mm 
• Length: 825 mm (not counting spinner)
• Flying Weight: 845 gm
• Empty Weight: 636 gm
• Motor: Emax GT2215/09
• ESC: Turnigy Plush 30 amp
• Prop: 10 x 4.7 Slow Flyer
• Battery: 2200 mAh, 3S Turnigy Multistar 
• Max Watts: 212 watts
• Max Amps: 21 amps 
• Max Thrust: 1056 grams
• Max C: 9.5
• Flight Time at Full Throttle: 6.5 minutes

3D Printed Parts

There are a number of 3D printed parts. As stated previously using the plans for the Flitetest/powerpod style fuselage would avoid most of them 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.

Nose and Front Deck

The above image shows the pieces for the front deck and the nose.  The two rear most pieces of the front deck are so I can print them in different colours. The rearmost piece was printed in white and painted orange and the other pieces, except for the battery hatch cover, were printed in blue.  The battery hatch was hinged to the nose piece using Blendherm tape.

Motor Mount

If using the Flitetest fuselage then this piece is not used and a powerpod is used instead. The mount is sized for the Emax GT2215/09 motor (Flitetest C power pack). If doing the 3D printed nose version then the mount is glued with the lug on top butting against the top of the fuselage. I scratch up the paper on the fuselage sides to ensure the mount ends up glued to the foam as well as the paper. I used epoxy glue for strength.

Battery Tray

This piece is glued up against the inside of the fuselage top such that the high part butts against the motor mount. Velcro is glued to the angled part and that is where the battery sits. A Velcro strap is then used to make absolutely sure the battery isn't going anywhere in flight.  Lite ply could be used instead.

Exhausts

What would a faux WW2 fighter be without some cool exhausts? Not sure if these are cool but its what I came up with.

Canopy

The canopy is printed in two parts to make printing it easier. CA or any other kind of superglue is used to join the two pieces together.  If building the Flitetest style version then try using the FT Mustang canopy.

Prop Spinner

The prop spinner isn't my design. This is the same one I used in my PC6 design where I took a design by Pintokitkat (see his FT article) and scaled his 52mm spinner down to 40mm.  I have seen enough 3D printed spinners explode on youtube videos to be nervous every time I use one of these but, so far anyway, they have been fine. I always print these in the highest resolution possible to keep the print as accurate as possible. Otherwise use whatever 40mm spinner you like but please do use a spinner as it really finishes off the looks of the thing.

Aileron Servo Covers

I noticed that the servos were copping a bit of a beating on landings so I added these servo covers to fix the problem.  They are simply glued in place with UHU Por.

UHU-Por allows the covers to be easily removed if a servo needs replacing.

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 this one.

As always we start with the fuselage. An observant person might spot the fact that I used the wrong colour wing tape on the turtle deck. I only noticed after I had glued it in place. It was so much fun to remove it, cut a new one, cover it in orange wing tape and then attach it!  Also in the above picture the 3D printed front deck is 25mm short. I had to design and print another the right size. Oh well if designing these things was straight forward it wouldn't be half the fun would it?

Construction commences with a fuselage side (doesn't matter which) that is folded and glued to the fuselage bottom ensuring that the bottom is perpendicular to the side. This is a B fold.

Then the other side is folded and glued. Both sides are B folds.

The next step is to glue the fuselage top to the fuselage sides. Again B folds. The hot glue is applied and the top placed in position. Then the fuselage is flipped over so the top is flat against the build table and held down against the build table while sliding the fuselage back and forth a bit to prevent it being glued to the build table. This ensures that nice flat joins result.

Glue is then applied to the rear fuselage bottom and the same process as for the fuselage top applied (i.e. position, flip press, slide).

If using wing tape, thin strips of wing tape are applied to cover the joins.

The next step is to glue the rear cockpit bulkhead in place.

And then the remaining bulkhead is glued in place at the location indicated on the plans.

Now the the fuselage has been glued together the wing slots (partially cut when cutting the parts out as per normal FT construction) are now opened up.

Wing slots all ready to receive the wing when it has been completed.

Oh dear look what I went and did. The other mistake I made was not realising that the blue wing tape is going to be much darker when applied over the orange wing tape than over a plain white surface.  If it was supposed to be blue then I would have covered the turtle deck in orange and then blue to make the colours match. However it was supposed to be orange so...

I made a new one. (apologies for the blurry photo).

After trimming the turtle deck to size, blue wing tape was used to cover the overlap.

Next the canopy is glued in place. Prior to this the two canopy halves are glued together and a piece of foam board glued to the bottom to make the thin 3D printed part a bit stiffer. I used some filler to cover the join.  The canopy is painted up and is then ready to be glued in place as shown above.

The 3d printed front deck pieces are glued together next.

The front deck is glued in place next. I used UHU Por glue.  Line the front of the deck up with the fuselage front so you have a nice, no gap, join when you glue the nose piece on. If you have a gap at the other end a little filler and paint is used to correct the problem. In my build I did not actually glue it at this point but if I built another this is when I would glue it.

The fuselage is then put aside and wing construction commenced. As shown in the above image things don't get any more classic Flitetest than this. This wing goes together in exactly the same way as the FT-Spitfire.  The only difference being I used a different wing section and I positioned the servos so the control horns can go on the centre of the ailerons and not towards the outer edge. Note the dihedral gauge on the bottom right corner of the image.

First step is to fold and glue the spars for each side. If you look closely you can see where I screwed up on the spar design.

The spars are then  glued to the wing bottom. Test fit first as some trimming might be required.  My error should now be obvious. I have the dihedral on the spar going the wrong way. The plans have since been corrected.

After that the spar for the other wing half is glued in. The incorrect dihedral angle is even more obvious in this image.

Each wing half was then folded and glued.  Glue is applied to the inside leading edge and the top of the spar and then the wing top is folded over and held down for a good 5 minutes. Then glue is applied to the rear end of the wing bottom and then the wing top is held down for another five minutes.  The process is repeated for the other side. It was only at this point that I actually noticed my error with the dihedral angle on the wing spar.

I corrected my mistake by cutting off the offending wing spar ends and cutting a foam board brace.

The brace was then glued behind the wing spar on one wing prior to gluing the wings together.

Hot glue was then applied to one wing root before the wing halves were pushed together. One wing is pushed down flat against the build table and the dihedral gauge is popped under the other wing tip. The result is 2.5 degrees of dihedral on each side. As can be seen in the above image I had a bit of a gap on the wing top join so I filled it with hot glue.

Leave for a good 30 minutes to ensure everything is well and truly set before removing the dihedral gauge.

Apply tape around the wing join.

Then the tape is cut away from the servo lead slot.  Next install the servos and y lead and feed it out of the servo lead slot in the wing root.  Squash the lead back into the servo lead slot so it lies flat against the wing in order to allow the wing to be positioned by sliding it through the wing slot on the fuselage.

The next step is to slot the wing into the fuselage and position ready for gluing.  Using a metre rule the wing needs to be positioned such that it is central.  This was achieved by matching the distance between the wing root and the wing tip on both sides and similarly for the trailing edges. Once you are happy apply hot glue to both wing roots top and bottom.  After gluing the wings the tail feathers were attached in the normal FT way, using the wing to ensure the tail plane and vertical stabiliser are lined up correctly. Using a wire or whatever else you think will do the job, tease the aileron servo Y lead out of the wing root.

The next step was to bolt the motor onto the motor mount.

Next the motor mount is glued in place with epoxy and the nose piece glued on with UHU Por.

The exhausts are glued on next. Its important to ensure they are in the same place on each side. Mostly nobody will notice if you get one out of position but you will know and it will niggle at you every time you fly the thing.

In my build I had to make cutouts in the rear edge of the wing bottom to ensure full and free movement of the control horn. Test fit and cut your slots to fit whatever control horn/servo combo you are using.

Controls horns are then glued in place. I cut a rectangular opening in the fuselage just behind the wing root. To complete the electronic installation I fed the servo leads out of this opening and connected them up to the receiver and then pushed the receiver back through the hole and located it on the inside of the fuselage side with Velcro.  The next step was to adjust the battery location to achieve the desired CG.  Once the battery position was decided the battery tray was epoxied in place. In mine that meant the battery was as far forward as it could go. If building from Flitetest foam board then your battery will probably be a little more rearward. The CG is right on top of the wing spar.

And then I was ready for the maiden!

Post Maiden Modifications

As previously stated I noticed the aileron servos were having a rough time on landing so I added these 3D printed servo lead covers to fix the problem. An alternative fix would be to have the servos protrude from the upper wing surface in which case the covers are not needed.

Data Files

The pdf files for the plans (Tiled A4, Tiled Letter and non-tiled) can be found here: pdfFiles.zip.

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

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

THe sketchup files for the 3D printed parts can be found here:  SketchupFilesA.zip 

and here: SketchupFilesB.zip 

and here: SketchupFilesC.zip.

Conclusion

The goal of this project was to design and build a simple but cool looking fighter with a mongrel background. I think it turned out rather well. Flies great and is a simple build. If I ever wreck this one via dumb thumbs I will definitely build another.  A nice simple, easy to fly model that has plenty of power on the Flitetest C power pack.