When a challenge is set to make something big or small (Mikeasaurus' This Month's Challenge: Big and Small) it is obvious for me to go for something small, as I love things tiny. And as there is one thing I tend to build smaller than most others, I went for the smallest RC airship ever. The idea of building a really tiny RC blimp had been lingering in my mind, but now a deadline was set. Obviously I also entered this Ible in the Remote Control Contest
As volume - and therefore buoyancy - decrease to the third power, building a small airship means achieving extremely low mass. This starts with superlight propulsion and controls. For my sub micro blimp I built propulsion and controls amounting to about 10g and for my nano air swimmer this was 6g. In this project I got it to 1.7g for the version without fin. I'm actually curious if the result really is world's smallest RC airship. If you happen to know of any smaller RC airship, please let me know. At least I wanted to share how I approached the build and exchange ideas on small blimp projects.
Check this short video on the blimp flying in different configurations:
After an overview of gear and materials used, I refer to an inspiring thread in Step 2. As finding the smallest, or rather lightest, RC gear was the first challenge, I built up this Ible's steps around each of the different components, discussing their selection and use. This way you can not only learn how I built the blimp, but also how I made the choices and came to a configuration with one main thruster for moving forwards and upwards at the same time, in combination with a tail rotor.
Building and testing was spread over a couple of days, but it could be done in within one day. The build requires handling tiny parts and trimming off weight by less than a tenth of a gram. For me the hardest part was the precision soldering under a magnifying glass. However, in each step I did not only add tips for some further weight reductions, but also options for a slightly heavier, but more "plug-and-play" alternative.
Obviously this blimp is strictly for indoor use only. It is quite forgiving when flying. Even if you hit an obstacle or a draft sends it off, you can just keep flying afterwards. If you do pop the balloon, the light gear should survive the drop to the floor (It did in my case).
Many thanks for the votes.
Note: English is not my native language and feel free to point out any errors or strange word uses.
Step 1: Gear, Materials and Tools
But here is summary list with recommended parts (and the alternatives I used)
- a DelTang ultra micro receiver for low resistance actuators like a Rx51-M (I used a Rx43-D)
- any DSM2 compatible transmitter like a so called lp4dsm2 transmitter (I used a DX5e transmitter)
- a 10mAh 1s LiPo battery (I also used a 8mAh one, barely lighter)
- connectors to connect the battery to the receiver: with the Rx51-M you need a 5mm bahoma-connector (I used male and female lightweight Molex 1.25 pitch battery connectors with leads
- a charger capable of charging at 10mA (this will also work for the 8mAh battery)
- 2 0.3g DC motors
- 2 Plantraco 32mm “butterfly” propellers
- about 60 cm of 0.1 mm diameter enamel wire
As I already had the transmitter and a charger for the battery, the cost of the gear used about 90 EUR.
Further materials are:
- a latex balloon with a net lift capacity of 2g or more
- some Hi-Float to treat the balloon for longer helium retention
- some helium (about 5l)
- a couple of grams of putty as ballast.
- some cellophane tape (sellotape, scotch tape),
- 6.5 cm of 0.5 mm diameter carbon rod
- for the optional stabilizer fin: another 15 cm of 0.5 mm diameter carbon rod and some light paper or foil (the lightest you can get) and non-stick (baking) paper.
The tools used are:
- sharp scissors
- a precision soldering iron
- a magnifying glass, e.g. on a "third hand" soldering aid
- cutting pliers
- a scale, accurate to 0.1g or better, comes in handy
Step 2: A Note on the Size of Airships:
As mentioned in this thread, the claims on the lightest airplanes, helicopters and such are well documented on the net, but little can be found on airships. The smallest one known to me is this wonderful build described in this thread. At 4.6 g total mass it is even lighter than Plantraco's experiment also described this thread, having a 3.8g gondola and a latex balloon I estimate to add at least 1.2g.
Actually in all these comparisons the mass of the helium is never taken in account. At 0.18g/l at norm conditions it does contribute to the mass significantly.
My Pico Blimp is still a lot heavier than the lightest RC micro planes under 0,5g. But then in manned flight too, the smallest blimps tend to be larger than the smallest airplanes too. One could think of strapping a 0.5g micro plane under a balloon, but not only would this be a very expensive solution, the control surfaces would be too small for the large volume and low speeds. Still, a lot of inspiration can come from these micro planes to build small blimps.
The build of a Pico Blimp is still significantly simpler than for an ultra light plane. The parts are also tiny, but fot example alignment is far less critical. And flying is rightout easy, even in the smallest rooms.
Step 3: Battery and Charger
20mAh LiPo cells are available from a number of micro flying specialist shops, but they weigh a whopping 0.8g.
I was happy to find 10 and 8mAh cells at only 0.35 and 0.325g. They are available from Micro Radio Flier and MikroAntriebe.
I added some short leads with female Molex 1.25 pitch battery connectors. This connector is commonly used on batteries for Minium Kyosho, Blade and Vapor micro fliers, but I used an extra lightweight version as available from a.o. Aether Sciences. I used a precision soldering iron thin soldering wire and worked fast and decisively in order not to overheat the battery. I put solder about halfway the battery strips. I also pre-tinned the stripped wire ends before soldering them on. Then I cut of the excess strips ends, taking care not to short circuit the battery by touching both strips at the same time with the pliers. Finally I covered the bare strips with some tape, also covering part of the leads and battery to strengthen the assembly somewhat.
Rated at 10C, these batteries are not meant to deliver more than 80 to 100mA, so I was curious to find out how well they would perform. As described in the further steps, three motors proved to be too much, making the battery voltage drop too much and making the receiver cut off. But with two motors these batteries give 4 to 7 minutes of flying time.
Having no 20 mAh batteries at hand, I also experimented with the 10 and the 8 mAh batteries soldered in parallel (see last image). Flight time was well over 10 minutes with two motors. But with three motors the flying still came to an end quickly as soon as the three motors were used close to full power simultaneously.
Note on the charger: LiPo batteries should typically be charged at 1C, this means with a charging current equal to the mA value of their capacity (in other words, taking one hour to charge them). I used an Eggy Universal Charger from Aether Sciences with a minimum charging current of 10mA. I expect no problem charging the 8mAh battery at 10 mA. Actually I charged these batteries a couple times at about 60 mA (by accident) and it did not destroy them. I could however not fully charge them and their lifetime might be shortened.
Further lowering the mass: The 0.35 and 0.325g cited above are the masses when trimming the battery strips and sides. As delivered they are both close to 0.4g. I did not dare to trim the sides yet. I also did not shorten the strips on the 8mAh one to much yet, as soldering on short strips was rather hard on the 10 mAh one.
Plug-and-play alternative: order your battery with connectors already mounted. A number of specialist shops will deliver the 20mAh cells with 5 mm Bahoma magnetic connectors (also see Step 5 on the receiver).
Step 4: The Motors
I did not only use this type motor for main propulsion, but also for a tail rotor. I always prefer a tail rotor over a rudder, as it gives unsurpassed manoeuvrability, which is obviously important for living room flying. It also works better than twin motor propulsion, as the lever is larger. With 0.3g motors and a 0,055g prop the low mass is also hard to beat with an actuator+rudder assembly (although probably not impossible, I admit).
If you receive your motors with their excentric weights still attached here’s a trick to remove them: take each of them loosely in a pair of scissors between the motor and the weight and put this on some open support (e.g. a sturdy cup), with the weight on top. Tape the scissors so they can not open. Slide some hobby knife in the weight’s slit holding the motor axle. With a moderate blow of a hammer on the knife, push the axle out of the weight.
The tail motor gets a pair of 25cm long enamel wires. The main propulsion only needs a couple of cm. To solder the enamel wire to the motors I first put the wire end in a fresh and hot drop of solder. After the wires are soldered on the motor, I wrapped some tape around. The intention was that this also serves as stress relief for the wires. It was still fragile, but it was useable.
With the wires connected to the motors, I first wanted to measure the current drawn with and without prop and at stall (also see the step on the propellers). I learned the motors draw around 70 mA with little difference for different propellers. Even without load they still draw about 35 mA. This means a heavy load for the tiny batteries used. Three motors did indeed prove too much. So I decided to go for a configuration with one main propulsion motor in an inclined position, offering forwards and upwards movement.
A 6,5cm 0.5mm carbon rod is glued to the motor to be serving as tail rotor.
Note on the power drawn from the battery: Trimming propellers for lower power consumption proved to be hard at this scale. But limiting the maximum rate on the transmitter could open the possibility of using three motors and offering independent vertical and horizontal movement. I should test that another time.
Further lowering the mass:
- The carbon rod carrying the tail rotor could be shortened some more.
- The motor's contacts and their plastic support could be shortened to win a couple of hundreds of a gram :-).
- The lightest motors that I know of are the Daniel Baird Micro Brushless Motor.These marvels of miniature handwork go down to 0.235g. They are however not self starting, which makes them less suitable for steering on RC blimps. One could however be used for main propulsion however. If a low enough idle running is possible they could even be used in a twin motor steering configuration.
Plug-and-play alternative:Aether Sciences offers slightly heavier 0.48g motors with enamel wire leads and can deliver them with attached plugs fitting ultra micro receivers.
Step 5: Receiver and Transmitter
Several of the DT receiver family have actuator outputs that can handle 400 mA. This is more than enough, but lighter receivers' actuator outputs only handle about 25 mA, so connecting a motor would destroy them.
I choose an Rx43-D, with both servo and actuator outputs and a double ESC, suitable for other projects too. It weighs 0.35g. I ordered it at Aether Sciences with a light weighth 1.25 mm pitch Molex male battery connector mounted, for 47.90 EUR. With this battery connector the mass is 0.41g.
After carefully checking the manual to find out which is which, the motor wires are soldered to the corresponding pads. The main motor goes to the throttle output and the tail rotor to the aileron output. This requires a precision soldering iron and very thin soldering wire. Personally, I also needed a magnifying glass.
As putting too much heat in the receiver should be avoided. For this, I tinned the pads with a minimum of solder, avoiding any short circuits and after that I soldered the already tinned wire ends to the pads.
I kept to simply way of covering the receiver and strengthening the wire attachments with tape, as it's easily corrected when prototyping.
The main propulsion motor too is attached with tape. This is strong enough and also offers resilience whenever the balloon pops and things drop to the floor.
Note on the transmitter: Obviously, the receiver determines the type of transmitter, in this case a Spektrum DSM2 compatible transmitter. The possibility of Dual Rate proved to be handy, as the tail rotor is on the one hand more powerful than needed, and on the other hand limiting the current drawn is worthwhile, taking in account the maximum current rate of the battery.
Further lowering the mass: a DT Rx51 would be the perfect fit for the control concept used in this project and weighs only 0.24g.
If you do go for an actuator controlled rudder instead of a tail rotor you could go for a 0.075 receiver available from Micro Flier Radio.
Plug-and-play alternative: The DT receivers can be ordered with female 1.27 pitch round section connectors as common for use with actuators. The DT Rx5... series can also be ordered with 5mm Bahoma magnetic battery connectors. The Rx51MC still only weighs 0,42g.
Step 6: The Propellers
I experimented with cutting propellers to a smaller diameter. I did this simply on sight, as balancing is not critical on this scale. It probably ruined the efficiency, but I think I lessened the difference in performance between the two directions of rotation, which is desired for the tail rotor. And it gets rid of yet another couple of hundreds of a gram...
Further lowering the mass: The lightest propellers are probably the custom made ones available at Micro Flier Radio, going down to 0.025g.
Plug-and-play alternative: Aether Sciences offers 30mm propellers fitting the 0.7 mm shaft of their 0.48g motors. They way 0.1g a piece.
Step 7: The Balloon
Inflated with helium to their maximum size they start with a lift capacity of 4.7g (with the inlet rim counted as ballast). Inflated to a safer size and as much of the inlet and rim removed, they still carry a good 3g. Treated with Hi-Float I could fly for the better part of the day. With the mass afterwards checking in on 1.2g, the mass of the Hi-Float proved to be measurable. I did use the Hi-Float sparingly and it was worthwhile.
To obtain a more airship like shape I also tried half of the length of a Qualatex Q646 large diameter modelling balloon. This quality balloon is however a lot heavier at 6,8g (including inlet and rim). Also a longer shape means more surface and therefore more mass for the same volume.
In order to inflate only half of it, without putting a heavy knot in it, I used a cardboard tube, cut to about 20 cm, for both pre-inflation with air as for filling with helium. It was also important to get the Hi-float deep enough before spreading it on the inside by rubbing the outside of the pre-inflated balloon. I ended up with a 50 cm long balloon, carrying only carried 1.9g when fresh.
Further lowering the mass: Alan Sherwood managed to make a 4.6 litre Mylar balloon with a proper mass of only 1.5g, which is quite amazing. Commercial foil balloons tend to be significantly heavier than latex balloons, but this one isn't. It offers possibilities for even lighter balloons arround 3.5 litre. Mylar balloons also need less pressure to hold their shape, giving more lift for the same volume
Plug-and-play alternative: The long balloon did fly better fly without fin, so I'm planning on some more experiments with smaller (mainly shorter and lighter) "airship" balloons.
Step 8: The Fin
One fin turned out to amount to 0.2g.
Further lowering the mass: Obviously a lighter foil should be used. Also the fin can be made smaller, as it turned out to stabilize flight very well. One could also try and span a foil between a single carbon rod and the balloon.
Plug-and-play alternative: A piece of thin EPP (expanded polypropylene) or Depron would be an easier alternative. Or simply using a longitudinal balloon makes a fin less needed. Flying straight forward should be stable enough for living room flying.
Step 9: Assembly and Flying
The battery is put in front of the receiver. Putty is added to trim both the buoyancy and the blimp keeping level. Actually, it is best to reposition the parts in such a way that all the ballast putty is in the middle. This way, when removing putty when the balloon looses lift and buoyancy needs to be corrected, the blimp remains level. This proved to be tricky sometimes, but a less than perfectly level blimp flies too.
As usual for RC blimps, the buoyancy is trimmed in such a way the blimp slowly sinks when powered down.
To fly, you power up the main motor till the blimp rises and throttle down a little to keep to the desired altitude. So the forward speed is determined by the power needed to get the blimp up. Trimming the buoyancy and main prop angle will set the characteristic cruising speed of the blimp.
You could put the main motor on a reversing output, adding a backwards/downwards function. But with the tail rotor you can make a turn almost on the spot, so reversing is not really needed. And obviously powering down means going downwards. On the other hand, the better fine tuning of the power with the throttle channel did prove useful to control the flying altitude.
The version without fins was flyable, but tricky to control. Mainly flying in a straight line was hard. With the fin added it flew as a charm.
In the 3 gram I also did not count the ballast, as at the end of the balloons life al ballast was removed so I at that time I had a flying RC blimp with a total mass that, after removing the helium by cutting the balloon, proved to be 3g. The version with the fin and tape to keep the fin in place came to 3.3g.
Actually, the helium itself is not taken in account in this total mass as it is hard to measure). With 0.18g/l at norm conditions and rough estimate of 4 litre (compressed to 3l by the balloon), this does add 0.72g.
Further lowering the mass: Using ordinary cellophane tape, is not the lightest way to attach things. Using small drops of superglue is worth a try.
Plug-and-play alternative: Putting the battery, receiver, main motor, ballast putty, tail rotor and fin all on a long carbon rod would make it easier to attach to the balloon. A 0.5mm diameter rod might prove too fragile though.