RC Aircraft - Styrofoam Vulcan Bomber

 

 

RC Foamie Vulcan Bomber

It is reasonably easy and low cost to make a RC plane from Styrofoam.  Foam costs almost nothing - about Euro 2.50 for a block of 5x50x150 cm - and it is available in various thicknesses, 5, 8, 10 and 20 cm being common.

 

I am building a delta wing plane that resembles an Avro Vulcan bomber, with a 1.3 m wingspan, and a Ducted Fan Engine inside the fuselage. From a safety point of view, nothing beats a foamy with a fully enclosed motor.

 

There are two kinds of styrofoam, Expanded Polystyrene (the stuff used for exterior insulation of a home) and Extruded Polystyrene (More smooth and easier to sand, but heavier and used for roof insulation). Both kinds will cut super smooth with a hot wire and it is the same plastic, so you can just as well use the cheaper and lighter white beady stuff, since if you tune your cutter temperature properly, then you don't need to use sand paper.

 

For reference, Polystyrene melts somewhere between 170 and 280 Celsius - it depends on the foam and the air pressure where you live. Therefore, you need an adjustable power supply and twiddle it to find the optimal current for your wire. I found that a steel guitar B-String starts to cut very slowly at 2 Ampere and cuts slowly and effortlessly at 2.5 Ampere. If you set it too hot, then it will make a too wide cut and if you set it too cold, then it will stick and the wire will flex and may even break if you force it. If it glows red hot, then it will just make a mess and break - the right working temperature does not glow.

 

A hot wire cutter is quite easy to make.  Buy a packet of cheap strings at a music store, screw some wood together and put a rubber bungee cord, or a spring from a chest expander on the back to keep the wire taught.  Musicians are artists after all and they also like to make things - if you tell the guy at the store that you need guitar B-string steel wire for a hot wire styrofoam cutter to build a radio control model plane, then he will understand and give you a packet of the cheapest strings.  Make the bow at least wide enough to cut the width of a block of foam - 60 cm works for me.  I glued little bolts into the ends of the wood to attach the string and pieces of copper wire, to solder the power wires to.  

The trick is to cut very, very slooowly, while sliding the wire over templates (Melamine, Bakelite, Masonite... something thin and stiff), stuck to the foam with three or four sewing pins.  With my bench power supply, a 60 cm guitar B-string works well at 5 V and 2.5 A. Therefore a big 5V USB wall wart may work for you, but it may not really be up to snuff and the magic smoke inside may escape. If you are like me and has a big bin full of old wall warts, then maybe you can just try them all until you find one that works well enough.

Toothpick trick: Stick three or four toothpicks into the leading and trailing edges, to guide the hot wire in and out of the foam block in a straight line.

You can smooth ridges in the foam with a fine sanding block. Do it outside, or the missus may want to divorce you. Holes and rough spots can be filled with light weight wall board spackle such as Tmel - when you pick it up, the container feels empty!

The best glue for foam is Bostic Mamut high hold construction glue.  UHU makes a cyanoacrylate superglue that works.  Any white wood glue also works. Test your glue on scrap foam as the wrong glue will dissolve the foam.

For wing spars, I prefer carbon arrow shafts, since they are very light and stiff. A pack of twelve costs about 12 Euro. Simply cut teeth in one end with a triangle file and stick it in a drill to slowly make a perfect fit hole.  Then glue a shorter one in place with PVA wood glue. 

You could use 8 or 10 mm wooden dowels as lower cost spars.  The process is the same - sharpen a longer one and slowly drill a hole with it, then glue a shorter one in. Wood adds a little weight though. Note that a spar does not have to run the full length of the wing.  The bending force is mostly at the root.

Only put glue on the last 10 cm or so, not the whole spar, or it may get stuck before you can push it in all the way!

To fit the wings to the fuselage, I put four pieces of aluminum tube in the fuselage and two short spars in each of the wings - keep them staggered. The spars slide into the tubes and then I drill little holes through for retaining wires - simple. First dry fit everything together and align it - only then glue the tubes in place to make up for any skew, since drilling holes is never perfect. When drilling a spar hole, you could start drilling at the wing tip, since the root is much thicker, there is more room for error.

In retrospect, next time, I would make the wings from two sheets of foam and then sandwich the spars in grooves, in between. That will be much easier and will ensure perfect spar placement. When I started this build, I did not know how well Mamut glue works and thought a single block of foam would be best.

 

Small pieces - wing tips, fins, elevons - can be strengthened and mounted with bamboo skewers and toothpicks.  Put a skewer in a drill to make a hole, then wipe it with wood glue and stick it back in. Toothpicks, you can just glue and press in.

The elevons should be about 12% of the wing area, which brings us to the problem of hinges.  For small planes, I glue small PVC tabs into the split ends of small pieces of skewers, so that I can push three hinges into the wing and flap with some white glue.  The simplest method is to stiffen pieces of nylon rope with super glue and then stick them into the foam with more glue. There are endless ways to make light weight plastic hinges. 

Another way is to simply glue the flaps on with Mamut or silicone grout - use masking tape on the top surface to hold them together and run a 2 mm bead down the groove to form a continuous hinge. Mamut works as a hinge, but silicone RTV is more flexible.

This is much easier to do than it sounds: I cut a V, stick tape on the top side, then open it up and apply glue on both faces, then put it down flat and work the glue down into the V with the back end of a skewer. The resulting bead is the size of the skewer.

For this bigger plane, I ended up going back to the Bronze Age and made copper wire hinges by wrapping 1 mm wire around a suitably sized jewel screw driver.  The advantage is no friction and no play.

The vertical fin should be about 10% of the wing area and the rudder about 25% of that. The rudder is mainly used for take-off and landing. During normal flight I only use the elevons.

I make some bends in the wire hinge ends to give the glue a better grip, cut slots in the foam and work Mamut glue in there with a Starbucks stir stick, wiggle the hinge in and wait a day for the glue to cure. Do put some masking tape where the glue should not go, so you don’t get a stuck hinge.

Of course, there are cheap nylon hinges available at the hobby shops, but what is the phun in that?

You can make a wire control horn the same way, but do use stiff music wire for this! I made control horns from popcicle sticks: Drill a hole for the pushrod wire, put a drop of super glue on to harden it, then drill the hole open again.

Speaking of smoke, polystyrene smoke has the same toxicity as wood smoke. So, it won't kill you, but rather do your styrofoam cutting on the patio/garage/shed, not in the kitchen.

Polyurethane smoke however, can kill you - so absolutely don't attempt to cut upholstery/mattress foam with a hot wire. For mattress foam, you need an electric steak/turkey carving knife.

 

Finishing a foamie can be a bit difficult. Some people cover the foam with Monokote or Oralite low temperature iron on covering film available from www.rcworld.co.uk but it costs an arm and a leg. You could also use wrapping paper, glued with diluted white glue.  Packaging tape is much cheaper and available in multiple colours if you search around on the wild wild web, but will likely be a bit wrinkled.  

Otherwise, you could give your foamy a light coat of exterior grade PVA (latex) house paint, or acrylic arts and crafts paint if you want more fancy colours.

Stability

I recommend adding a flight controller (Gyro Stabilizer) to any RC plane.  It just makes it so much more of a pleasure to fly if the plane is mostly stable.  I am using a low cost MAT-NX3 flight controller and it works wonders.  Set the transmitter to a normal plane mode and then set the controller to Delta mode.

 

Motor and Battery

I am using a 70 mm diameter Electric Ducted Fan, which can generate more than 1.5 kg force.  

 

This plane should fly fine on a 50 mm EDF, but if you are going to do something for fun, then it is worth overdoing it! 

 

The high power is great to zip away from a hand launch, but it draws an enormous amount of electric current, so it requires an 80A ESC and a 6s 22V LiPo battery.  A small 33Wh battery can run this motor for about 2 minutes at half throttle. So if you want to do high speed flying, then you will need a big battery, or multiple battery packs and 100A Schottky diodes to combine them, or just ensure that the battery is easy to swap out.

Servos

The way I built the plane, I had to reverse everything.  All the servos and even the throttle were upside down!

 

Servo settings are done in the RC Transmitter. The menus may be a bit cryptic, so get a big mug of coffee and a candy bar...

Preflight Test

Start up:

• Pull the throttle back to minimum and turn the transmitter on.
• Plug the plane battery in to turn the plane systems on.
• Wait for the ESC beeps to stop.
• With the gyro flight controller, the servos should come alive after a few seconds - krr, krr, krr... 

Engine test:

• Hold the plane firmly, then check that the engine starts and move the throttle smoothly up and back down to confirm that the engine works. 

Gyro test:

• Lift the right wing up - the right elevon should also lift up (to push the wing back down).
• Lift the left wing up - the left elevon should also lift up (to push the wing back down).
• Yaw the plane left/right - the rudder should deflect  right/left (to push the tail back in line).

Transmitter Control Test:

• Move the right stick up/down - both elevons should move together down/up.
• Move the right stick left/right - the elevons should move oppositely to roll the plane left/right.
• Move the left stick left/right - the rudder should move left/right.  

All Right Fellas, Lets Goooooooo!

A foamy is allergic to ground, so for best results, find a (cowless!) meadow with tall grass to use as your aerodrome.

 

Trim the elevator so it is about 5 mm up at the back, so the gyro can get and keep the plane in the air when you let go of the controls.

 

Set a two or three minute Bingo timer, adjust the throttle to 3/4 power, powerfully toss the plane slightly up into a strong(!) 20 km/h wind, grab the transmitter and fly away on one wing and a prayer!

After glueing the plane foam pieces and splinters back together a few times, I decided to launch it with a rubber bungee instead.  A bungee is more repeatable and good for low wind.  One needs about 4 to 5 times more pull than the weight of the plane for a rapid fire bungee launch.  That means at least 4 strands of 6 mm surgical tubing - about 40 meters total (or 10 meters of 10 mm tubing, if you can find it) - 3 meters soft light weight nylon rope for the plane hookup and foot release pin and a slanted board (1.5 meter) to use as a launch ramp - to get it up and over the grass. 

 

In effect, you want the heavy rubber to snake across the grass so that the plane only has to lift the two or three meters of nylon to get going - it needs high forward speed for the control surfaces to work and it needs to get to that speed before it can decide to roll over.  For the rope, I used two packs of boot laces that were lying around. My first attempt belly flopped into the grass, so I added more boot lace.  Instead of using tent pegs as anchors (which could pull out of soft ground and hit you in the teeth), I use a couple of 20 kg dumbbells to hold the bungee and board. The plane should take off low over the grass so that if something goes wrong it will skitter to a stop without damage.  

 

Immediately after releasing the plane with your foot, push the throttle forward and hope the gyro will do its thing, then you can pull up once it went over the relaxed bungee and it is hopefully detached - it is much is too fast to actually see what is going on.  If it doesn't detach, then you will have a one time use lawn dart...  

 

There are lots of bungee launch videos on Jootoob.  Obviously they only show successful launches!

European Red Tape

European Red Tape is legendary and one usually needs a laptop computer, one or two mobile phones and a small printer to get anything official done.  One mobile phone dedicated to Google Translate helps a lot for those times when a government web site stubbornly reverts to Slovenćina, no matter how many times you select Anglicky.

 

This size toy plane will weigh just over 1 kg (EU Open Category A3), so there are some crazy regulations to comply with. Some of the below links are for Slovakia only, but once you know what to look for, Google can easily find the relevant web sites for your country.

Legal issues:

https://drone-laws.com/drone-laws-in-slovakia/

You can get a hobby pilot license (The basic A1/A3 is all you need for self built planes and it is very easy - read the materials, answer the questions, done) for about 50 Euro:

https://www.droneflightacademy.eu/uk/

You can register as a UAV operator for about 17 Euro - OR you can join a RC flying club, if you can find one (find a model shop and ask them - chances are that the owner also runs the club).  The English version of the web site is not the same as the Slovak version.  Registration is only in Slovak, so you may need Google translate in Lens mode on a phone:

https://letectvo.nsat.sk/bezpilotne-letectvo/registracia-prevadzkovatelov-uas/

The Slovensko Mobile App is required for authentication for the above. Look at the top right to select English/Slovenćina.  At the bottom of the below web page are links to help with installation and activation - even a video, in Slovak of course...  This authentication app is also needed to access certain EU web sites in Brussels, so it is worth installing it.  You also need to buy an USB ID card reader for about twenty bucks at NAY Elektrodom, to read your ID card on your computer and get a code to enter in the phone app. 

https://ep.slovensko.sk/en/title 

 

Of course convincing a smart card reader to work with an Apple Macbook requires a secret magical incantation and an offering of a hecatomb (100 oxen!) to the computer gods.  In my case, the card has to be upside down in the Axagon reader (despite the user guide pictures to the contrary) and I have to restart my Macbook with it all plugged in for it to actually work.  The ID card also needed to be configured at a Police Station with a PIN, which the system will ask for at startup - multiple times - don't ask me why.  It all therefore takes rather longer than the proclaimed 10 minutes to get it all sorted - took me about 3 months.  Patience, young padawan.:

https://ep.slovensko.sk/titulna-stranka

The easier and cheaper method is to download the paper form, pay 35 Euro at a Post Office and mail the form, proof of payment stamp and copy of ID or passport to them!

 

The laws changed and you now also need a Remote Identification Broadcast Module for your toy UAV. For example Holyton FAA Compliant Remote Identification Broadcast Module for $30 on Amazon and you need the serial number of the module for the operator registration, so get it first.  American made versions of this little Chinese widget costs ten times more.

 

Insurance is required in most countries for about 19 Euro, so get this before trying to register as an operator above:

https://www.coverdrone.com/eu/

 

There is an airspace map here that shows the flight restrictions when you click on a place: 

https://gis.lps.sk/vfrm/index.html

 

In general, around airports, you may not approach closer than 6.5 km or fly your toy higher than 30 m and you may not fly closer than 100 m to people, buildings or roads.  Slovakia is a small country and doesn't have big parks.  So, the sensible and safe thing to do, is to find an open spot in a forest, or an old abandoned quarry, or a friendly farmer's meadow and fly there, just watch out for startled running deer...

Technology Readiness Levels

This is a toy project, but here is some information to real systems engineering, the so called Technology Readiness Levels, as developed by NASA and adopted by the EU.

• TRL-1. Basic principles observed and reported
• TRL-2. Technology concept and/or application formulated
• TRL-3. Analytical and experimental critical function and/or characteristic proof of concept
• TRL-4. Component and/or process validation in laboratory environment - Alpha
• TRL-5. Component and/or process validation in relevant environment - Beta
• TRL-6. System/process model or prototype demonstration in a relevant environment - Beta
• TRL-7. System/process prototype demonstration in an operational environment - Integrated pilot
• TRL-8. Actual system/process completed and qualified through test and demonstration - Pre-commercial demonstration 
• TRL-9. Actual system flight proven in its environment

The nine levels as defined by NASA: https://www.nasa.gov/directorates/somd/space-communications-navigation-program/technology-readiness-levels/

 

A readiness calculator by the European Space Agency: https://trlcalculator.esa.int/

 

So where am I with this Vulcan toy? Level 6, I would say, since I have actually managed to fly it for a minute or two. It has now been glued back together many times and I am already thinking of building a new one - it is too heavy - I need to use much less of everything and more wing area...

 

Numbers 

When all else fails, one has to look at the numbers!  

Wing 

• Wing root: 0.5 m
• Span: 1.2 m
• Tips: 0.1 m
• Area: 0.36 m2

Elevon

• 12% of 0.18 m2 = 6 x 36 cm

Fin

• 10% of wing = 0.03 m2

Rudder

25% of 0.03 m2 = 3.75 x 20 cm

 

Centre of Gravity

• CG Calculator: http://fwcg.3dzone.dk
• 20% CG = 263 mm
• 15% CG = 246 mm 

Weight

• Tail fin: 12
• Hinges x6: 13.5
• Wing: 262
• Rx: 10
• Gyro: 7
• Servos x3: 111
• EDF Motor 70 mm: 172
• 6S Battery: 254
• Fuselage: 55
• Spars: 102
• Aluminium tubes: 17.4
• Wires: 11
• Total: 1.028 kg 

I need to put this foam brick on a diet.  The wings, servos and spars are quite heavy.  If I use 9g (actually 11 gram) servos, cut the wings and fuse full of holes and cover it with paper and use carbon fibre spars, then I may be able to trim up to 250 g of fat.  The weight reduction would be worth a try and make it easier to launch by reducing the minimum flying (stall) speed.

Typical Stall Speed

• W = L = Cl x 1/2 x R x V2 x S
• V = sqrt(2 x W / Cl x R x S)

• g = 9.81 m/s2
• Cl = 1.5 typical
• Air = 1.2 kg/m3

• Vs = sqrt ( 2 x 1.028 x 9.81 / 1.5 x 1.2 x 0.36) = 5.6 m/s = 20 km/h

So, with this foam brick, one needs a lot of wind and an athletic javelin throw, to take off! 

 

Which explains my general lack of success getting safely of the ground and having to glue the pieces back together almost every time.

 

Landing it is likewise quite spectacular - very tall grass is required to do more than one flight... 

If the weight can be halved (or if the wing can be doubled):

• 500 g = 3.8 m/s = 14 km/h

Therefore one really needs a strong bungee or a big Trebuchet to launch this 1 kg foam brick...

Most of the weight of a foamy is in the propulsion system.  A light weight 250 g plane is possible if one would use a 40 mm EDF (30 g) and a 3S or 4S battery (160 g), with carbon fibre spars and a very thin wing.  This size/weight of plane can be easily hand launched in very low/no wind.  

• 250 g = 2.75 m/s = 10 km/h 

There are numerous delta wing designs like that on Jootoob.

Bungee 

According to the RC forum gods, one needs about 5 times the weight in rubber force to launch a flying wing EDF plane. 

• 10 mm bungee cord stretched to 5 kg force

This is fairly typical for a RC wing launcher. 

The problem with a rubber bungee is that it starts out strong and becomes weaker very rapidly as the rubber contracts (and hits you in the teeth).   So a bungee is a brute force launcher - there is nothing subtle about it.

Trebuchet

If you drop a weight 1.5 meters, then it will hit your foot at about 5.5 m/s (and you may then jump around at about 10 m/s).

• A Trebuchet with a weight arm of 1.5 m and a throw arm of about 2 m plus 1.6 m of rope will reach about 20 km/h 

I don't think anyone tried a Trebuchet launcher yet! 

Note that a falling weight launcher doesn't have to use the long swinging arms of a Trebuchet.  The same can be accomplished very compactly using two pulleys of different diameter - a small one for the weight and the bigger one for the plane rope. 

The advantage of a falling weight launcher is that you get constant acceleration (until the weight hits your toe).  It is compact and easy to adjust by changing the height and weight.

Conclusion

I have worked on heavy (500 kg) observation UAVs and this foam brick can indeed fly if the wind is just right for take-off, but considering the usual wind and the size of the cow meadows outside our village, I need to try a very light weight design, aiming for 250 g with smaller engine, battery and servos, since this is supposed to be a toy, not a siege engine!

La voila!

 

Herman