One of the themes in my RC Blimp projects is building them smaller and smaller. First, because I like small things, but also to save on helium.
However, using lightweight RC gear usually raises the build cost. So in this project, I explored hacking a cheap toy RC car (scale 1/64) for the RC gear. It turned out to be fairly simple. One thing was less so: dealing with the very thin gauge copper wire these micro cars use for their magnetic steering actuator. However, for a workshop, I worked out at easier alternative, as shown in the last step.
The cheapest version uses an 11" latex balloon (a common party balloon), well inflated with helium to carry about 10g. The propellers are very simple and made from scratch (See step 5).
The result is small blimp that is suitable to fly at home or in somewhat larger halls. It can even fight some minor air drafts, but as for any small blimp, forget about flying outdoors.
The weight of the blimp is trimmed till it sinks slowly when no power is applied. The main motor pushes it forwards and upwards simultaneously. Dosing the forwards command allows flying at more or less the same level. Steering left and right is done with a tail motor. The controls on these cheap and tiny RC cars is on-off, not proportional. But actually that works better for the blimp than for the cars, as with the blimp you can use its inertia to your advantage.
Flight times easily exceed the driving time of the original Mini Racer used, reaching over 15 minutes.
Here is a video of the blimp flying:
Step 1: Parts, Tools and Supplies
Make sure the balloon is well inflated to 10 - 11" inch. Usually latex balloons are not fully inflated to their nominal size, to minimize the risk of them bursting during transport, like in a hot car. But if you take care on your trip home, you should be OK. The balloon should be inflated to carry 9g at least and some lift capacity to spare will keep you flying longer. In high altitude area’s you will need a larger balloon, try a 12" or larger one.
As said, you can get a prepped and helium inflated balloon at the balloon shop, but you can also rent a helium tank or buy a one-way canister. When using a one-way helium canister, take in account the helium sold in those is less pure (typically +- 85 %), diminishing the lift capacity accordingly. It still works but you will need a good 11" diameter. Pre-inflating with air (and emptying the balloon again) really helps to achieve a large enough size. Also buying some Hi-Float is worthwhile. The balloon will lose its spare lift capacity of a couple of grams in a matter of hours, a day at the most, but without a Hi-Float treatment those times will easily be halved. A longer lasting, but more expensive alternative is using foil balloons (also shown in the last but one step).
Step 2: The Mini Racer
These cheap and tiny RC Cars come under a number of names. Mine was a "Mini Racer". I found very similar "Tin Can" RC cars online in 27MHz, 35MHz and 40MHz and 49MHz. They do not come without any narrower bands. So you can only fly 4 at a time, by making sure they are on a different frequency. In most countries only some of the available frequencies will be legal, but it seems the frequency bans are often not enforced for these very low power toy RC systems. In Belgium 35 MHz is reserved for flying models but we’re turning the Mini Racer is one anyway.
Before dismantling the Mini Racer, charge it as described in its manual and take it for a test drive to make sure everything works.
This range is actually rather short (about 10m), but that is fine for a small "living room" flyer. I found on rc-cam.com that at least on some types, you can actually boost the range using a longer antenna and by doubling the transmitter’s voltage (add extra batteries). It seems those transmitters can handle up to 5V, but take care not to use that configuration for charging the flight battery. I didn't test this myself..
Step 3: Dismantling
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Remove the bottom screws at the back and the front to remove the body. When taking further apart, make sure you do not pull on any wires. In particular first check and note where the two thin copper wires, coming from the steering, connect to the circuit board. These wires wire break real easy during further dismantling. I had to solder them back in place.
Before going for the rest of the screws, pry out the LEDs (two white at the front and depending on the model one or two red ones at the back). At this stage I kept on the LEDs. In my first build I later removed them to save weight, but the difference is only about 0.3g, so I recommend not to remove the lights unless you really need to lose a fraction of a gram in weight. After all some lights on your blimp are kind of cool.
Remove the screws holding the steering mechanism and the motor. Remove steering parts from the chassis except for the coil. Keep the two tiny magnets, they can be useful in other projects.
Unscrew the back motor mount. Push the motor from its plastic mount.
Push the switch out from the bottom, for example with the screwdriver. ?
Tape in the circuit board, protecting the wire attachments, but keeping the switch and charging socket accessible. Test if things still work (including the charging).
Step 4: The Tail Motor
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The steering actuator could be used to make a rudder. I actually found a description of a project doing that when hacking the Mini Racer to use in a model airplane. But, as to my experience a steering fin doesn’t work that well with slow moving blimps, I went for a tail rotor. I wasn’t sure the electronics would hold when replacing the actuator coil with a small motor, but that turned out fine.
The coil is glued to the chassis, but you can break it loose. At this stage I wanted to unwind the actuator coil for about 15 cm to make lightweight leads to the tail motor. But the windings turned out to be glued in place quit firmly. Only after soaking the coil in nail polish remover I could peel off a suitable length before breaking the wire. Put it in a small cup with nail polish remover or a similar solvent, making sure the cup is resistant to the solvent. It is best to cover it to minimize evaporation and ventilate the room. Let it soak for a couple of hours.
Of course you can use some other lightweight wire. If you want to keep the budget down you can recycle some from a discarded mouse or USB cable for example.
If the motor comes with a vibration weight on its axle, you will need to remove it, being careful not to damage the axle. I used this trick: I take the axle in between some pair of scissors or thin cutting pliers (don’t do this with your best scissors) and rest this loosely on some support (I used a bench vice). With a pin I tap the axle out of the weight.
Some of these small motors come with a square rubber sleeve. You can slide that right off.
To solder the thin copper wires I first make sure the ends have some solder on them. Apply plenty of heat and a good blob of solder to make sure the insulating enamel layer is removed by the high temperature. Don’t worry about the polarity when soldering to the tail motor, as this is sorted out when finishing the prop.
The insulation on the thin copper wire will probably be damaged by the solvent. So it is a good idea to no longer cross the wires. I kept them apart and at the same time protected them with tape, also covering the soldered connections.
Step 5: Making Propellers
The propellers are made from strips cut out of thin plastic sheet. The plastic used in cups containing butter, cheese or similar products is very suitable. I started out with two 25mm by 5 mm strips, leaving some room for trimming the propellers later. For these small blimps such rough propellers work well enough, but of course you can experiment what dimensions and shapes work best. For example, I learned that the propeller on the smaller tail motor, needed to be smaller to work well (I ended up with a total diameter of about 15 mm).
With a pin, a small hole is made in the middle of each strip. Some corrugated cardboard as a backing helps. If your pin should is thicker than the motor axle find another pin or take care not to push it completely through the plastic. The hole should keep a tight fit on the motor axle. So make a small hole and test fit each axle (the pinion gear on the main motor should be removed). Take care not to damage the motor and not to sting yourself with the axle.
Once the strip is on the axle, check the direction it spins for each command. Keep that in mind when twisting the strip ends each in opposite direction to form a rough propeller shape.
Do this for both propellers.
Step 6: Assembly and Test Flying
I cut a bamboo skewer even thinner and glued it perpendicular to the tail motor.
The main propulsion motor is taped to the circuit board in an angle of roughly 45°, making sure the propeller spins freely. I actually accidentally taped over the switch, but was still able to operate it.
The circuit board and the tail motor’s skewer are taped to the balloon. ?Add a little non hardening clay to trim the weight of the blimp till it just slow sinks when no power is applied. ? You’re ready to fly (You might need to recharge first).
You can tune the main propeller's angle and the weight for a slower or faster cruising speed. A propeller pointing more downwards will give less thrust left for the forwards movement, which is better suited to flying in smaller rooms. Adding some weight might be needed to be able to give thrust about half of the time and achieve a reasonably smooth level flight.
Step 7: Further Improvements
In this project I tried to keep this build as cheap and as simple as possible. Obviously there is room for improvement. Some improvements can be done with recycled material, others at a cost of a couple of $.
The first one is using other balloons, latex or foil. I found some blimp shaped latex ones on eBay, lifting over 30g when used in full length. The cost of a helium fill is higher, but the spare lift capacity will make it last a week, instead of a day.
For even longer lasting fills go for foil balloons. You can adapt their shape with tape or, with some practice, make new heat seals as shown in my Hummingblimp Ible.
The 90 cm (40") long letters "I" foil balloons are a good size for this propulsion concept. They lift 13 to 17g depending on the helium quality. However there are two types: originally I had "sans serif" letters I, i.e. plain rectangles (used as a basis for the blimp shown in the first picture). But lately online suppliers, without updating the picture on their site, started to deliver "serif" style letters I, with protrusions at the end (as shown in the last but one picture). So if you want a plain rectangular shape, double check with your supplier.
When using longer balloons, it turned out the tail motor needed to be at the very back end to provide a long steering lever and get a decent manoeuvrability. As the main propulsion needs to remain in the middle of the balloon (both for weight distribution and a stable flight under thrust), the leads from the circuit board to the steering motor need to be lengthened. They should stretch about 60% of the length of the balloon.
I started replacing the thin copper wire with 1/10 mm enamelled copper wire (slightly thicker than the original). But for a sturdier solution I used 0,09 mm2 wire from a discarded computer mouse USB cable. I twisted the two wires, together with the twine that is also in the USB cable and soaked this this assembly in superglue to make it stiff by itself. That way the skewer could be omitted. I also kept the black sleeve on the motor when gluing it in place, allowing the motor to be repositioned.
The charging connecter can be replaced by a lighter solution, or be omitted completely when using clamps to charge the single cell.
If you have a gram or more to spare in lift capacity, you can add fins cut from foam trays. This makes flying straight out easier, which is particularly handy in larger rooms or halls.
In larger halls, using readymade propellers is also worthwhile. I used these from HobbyKing with great success. Again the tail propeller was shortened.
Step 8: Workshop
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For a workshop I needed an easier and more robust way to connect the tail steering motor to the circuit board. This is what worked very well in a workshop with 7 young people (14 years old in average): ? After opening up the Mini Racer car and before taking anything apart, a scrap piece of circuit board is inserted under the thin copper wires. There should be no copper layer on top, as nothing should be soldered to the piece, it just serves to protect the plastic underneath when soldering, Keeping the wires well apart the long leads are soldered to each copper wire. A good blob of solder ensures isolating enamel is burned and good electrical contact is made. After that, a small drop of low temp hot melt glue is added to keep the soldered ends from touching each other. They should not be glued to the scrap piece of circuit board. After cutting the thin copper wires running to steering mechanism, the soldered connections are glued on top of the circuit board (making sure they do not touch any metal parts directly).
However this workshop did show the other wires (running to the main motor, switch, battery and charging socket) are the next vulnerable link. In a couple of builds one of these wires broke of at their soldered connections. So for a next workshop I plan not to remove the chassis, but only shorten it at the back and the front. A small test shows this does add about 2g to the build, so at least a 12 ? latex balloon will be needed. However for workshops, I’m planning to continue to use the 40" letter I balloons