Introduction: The Hummingblimp
Second Prize in the
Make It Move Challenge
Check it out in this video:
As most of my projects involve something that moves I just had to partipate in the Make It Move Challenge". Lately I have been experimenting with building RC-blimps based on the balloons sold by Deutschen Zeppelin-Reederei GmbH. These balloons are about 120 cm long and 25 cm in diameter. Filled with helium these balloons lift a mere 20g. But modern micro RC-gear brings this within reach of anyone with some basic experience in building remote controlled models.
A classic gondola setup resulted in a blimp that is quite quick for its size and very agile. But at high power a lot of roll back and forth occurs. Positioning the main thrusters higher, closer to the centre of mass should remedy that. But instead of putting the thrusters on the sides (as in Zeppelin airships rather than blimps), I tried a single propeller at the stern, inspired by Daniel Geerys awesome Hyperblimp. As the complete controls and propulsion are at the stern, an even stricter weight limit is set for these parts (under half of the lift capacity, i.e. 10 g). For this I came up with a setup around the Spektrum AR6400L Ultra Micro Receiver with integrated servo's. The battery goes on the bow.
The result is a lively indoor blimp suitable for spaces the size of a class room or larger. Controlling it in a smaller class room takes some practice (Im still on it). But it doesnt matter if you hit anything, you can just keep flying. Obviously you should avoid all hot, sharp or fragile objects in the room.
You do not need any flying skills, but this project does require a willingness to work with tiny, flimsy components. Some minor soldering is needed to extend the battery leads.
Many thanks for all of your votes in the Make It Move Challenge.
Step 1: Materials:
The “Zeppelin NT” blimp balloon I used is no longer available, sadly. You can try a 40" letter I or number 1 balloon, but these are shorter and tend to give about 10 to 20% less lift.
99%+ Helium (have your balloon inflated at your local balloon shop, after preparing it as shown in step 2).
An ultra micro receiver with two servos and ESC (Electronic Speed Control) operating on a single cell rechargeable Lithium battery (1s Lipo). The Spektrum AR6400L brings al this, integrated on a tiny print at a mere 3,9 g.
A matching transmitter, I used a Spektrum DX6I, but a even the simplest, like the DX5E or the type that comes with RTF RC planes would work too. As long as it is compatible with the receiver.
A DC brushed motor under 4 grams (3 grams is better) and matching propeller. I used a motor from a micro servo and tested some propellers I had lying arround, but you can get both from Plantraco. They have great tiny motors and propellers. I would recommend the 57x20 prop for 1mm Shaft with the 7mm 3.3 Ohm motor with with nanoconnector.
A single cell lipo rechargeable battery 125 mAh, 4 g.
2.6 m of 0.14 mm2 wire and connectors matching battery and receiver.
Some scrap balsa, 1 and 4mm thick, and Tyvek (or other micro plane construction and hinge materials).
Some 3mm extruded polystyrene sheet (like Depron or Climaplan).
Two times a couple of cm of steel wire 0.8 mm diameter (or even thinner).
About 10 cm of light plastic tube, e.g. use with air filled latex balloons.
Superglue (regular or thin), cellotape, double sided tape.
Tools: soldering iron with adjustable temperature or simple soldering iron and a clothing iron, scissors, hobby knife, working surface.
Step 2: Preparing the Envelope or Balloon
The balloon is used backwards. What is the bow on the Zeppelin NT is the stern on the Hummingblimp and vice-versa. This is because the highest buoyancy and therefore the most volume is needed at the stern.
The Zeppelin NT balloons come with one tail fin and a gondola to be inflated. As these contribute more to the weight than to the buoyancy, they are removed for this project.
When inflating the balloon, the tail fin and gondola are filled through a couple of short interruptions in the seam. So before inflating, these interruptions are hot sealed. I use a soldering iron with adjustable temperature, set at its lowest temperature (about 200 °C) and with a broad tip (actually a tip broadened by a drop of solidified solder). I put some leather underneath the ballon when applying the seam. Leather resist the heat (at least for the short time needed) and allows to apply some pressure smoothly when making the seam.
Alternatively, you can use a clothes iron, but in that case I would cut of the fin and gondola first. You should do some tests to determine the correct temperature setting (a strong seam, but no wrinkling). On a clothing iron I used in similar projects i found the best setting to be just under "Wool".
Of course you can “weld” your own balloon completely from foil, but personally, I never managed to reach the same gas tightness as machine sealed balloons. Even when making long seams on commercial balloons, keeping the self sealing valve, the gas tightness is noticeably affected. On the other hand the short hand made seams do not seem to make a difference and the balloon remains to keep for one to several weeks.
Step 3: Yaw (left-right Steering)
The steering is set up, literally around the receiver-servo unit. A tiny balsa beam forms a back bone to which the receiver/servo unit is attached with double sided tape. Attachment is done only when the hinges and controls are finished after intermediate test fitting.
Each hinge is made out of Tyvek, glued on balsa and soaked with superglue to reinforce. After the glue has set the hinge is loosened up by moving back and forth a couple of times.
Each control horn is made out of a 1mm thick balsa triangle, strenghtened by soaking with superglue. The lever length of the horn should be close to half the servo travel.
Step 4: Pitch (up-down Steering)
The hinge for the pitch is made in a similar way, on the other side of the reciever/servo unit. First a 1,5 cm balsa "beam" is glued in a "T" to the main beam. It is important to put the pivot point high enough to make room for the motor connector and to make sure the control rod can move freely when "giving down".
Here the "moving part" of the hinge is chosen larger as it will actually become the fixed support to be attached to the blimp. Three balsa "beams", are glued to this part, to be attached to the inflated balloon. They are stuck on the balloon with some double sided tape, one by one. This is done by removing the protective layer on the second piece of double sided tape only after the first is stuck in place. T The third one is done proceeding the same way.
Step 5: The Leads
The battery leads are extended to cover the length of the balloon, with about 10 cm extra. For this I extended a “Ultra-Micro Connector -to BEC connector” conversion lead, as the battery was already converted to a BEC connector. Of course you can keep to Ultra-Micro Connectors and forget about the BEC connector. But as weight is less critical at the bow I prefer the larger BEC connectors.
The existing leads are cut about half way and two 130 cm 0,14mm2 leads are solderded in. Each soldered connecton is seperatly insulated with some cellotape (You can use some small shrink tubing if you prefer).
It might be possible that main leads thinner and lighter than 0,14mm2 would work too. However a test with very thin wire (I guess under 0,05mm2) gave a voltage drop that made the receiver cut the power far too early (integrated Lipo low voltage protection function).
Determining the lightest, suitable leads would need further testing. Obviously you should keep an eye on the quality of the connections to avoid to large a voltage drop over them too.
The battery leads follow the bottom seam of the balloon from stern to bow and are kept in place with some scarcely used cellotape. They actually lower the center of mass, keeping the blimp upright.
Step 6: Trimming and Fins
The weight of blimp is trimmed to have it slowly sink to the floor when no power is applied. At the same time it has to be balanced to hang more or less horizontal.
I first put the battery and some ballast putty at the bow, but after adding the fins I had to restore the balance and ended putting the battery out in front of the bow. Adding weight at the bow was not possible anymore as the lift capacity of the balloon was already reached. So bringing forward the center of mass meant moving forward the weight already present. The putty was replaced by a light plastic tube (as used with air filled balloons) to make that possible.
I first tested without fins and found the yaw (up-down rotation) of the blimp difficult to control. The pitch (left- right rotation) was wel suited for small spaces. So, as I had hardly had any buoyancy to spare, I only put on horizontal fins.
The fins are made out of 3 mm extrude polystyrene sheet. They are glued on a "foot" to be attached to the blimp with double sided tape. You can use special glue for polystyrene foam or use a tiny amount of regular superglue. Most superglue will "eat" polystyrene foam, but very small amounts do work on the extruded type.
Trimming of the radio control was basic: choosing the direction of the servo and trimming the neutral position.
Step 7: Lessons Learned
1. A single tail rotor in a directional setup is suited to fly in quite small spaces like a classroom.
2. Saving weight is absolutely critical at the back. I plan to try a lighter motor and maybe redo the balsa construction and hinges. This way the need to position the battery out of the bow, on a stick, should be avoided.
3. Only after that, I would test thinner, lighter battery leads. As these contribute more or less evenly to the weight they do not contribute directly to the balance, but it could help to make room for some ballast and/or some helium loss over time.
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