## Shopping for Electronics

I started shopping around online for radio gear, servos, motors and propellers recently. The whole thing can get a little confusing and over-whelming trying to match all these different components together, from different suppliers, and trying find stuff small enough and *light* enough..

Then I ran across a posting in a forum, on either RCGroups.com or AirshipModeler.com, that mentioned some R/C gear by “Plantraco.” So I headed over to MicroFlight.com to see what was available.

So I think I have decided to go with the Plantraco “**Deluxe Starter Set #3**” from **MicroFlight.com** for $100. I also want to buy 2 more motors (I want to run 3 total), but I also need to figure out how, *and if*, I can wire up 3 motors with this system.. I don’t want to overload anything. I’ll keep you posted on what I find.

Anyway.. This system looks great, and everything is *really* light. The components it comes with, in addition the transmitter (which also charges the batteries!), are

Micro9 3 Channel PlugnPlay 0.9g Receiver ($54.99)

This little receiver weights just 0.9 grams. They say only 0.5 grams if you remove the connectors (*but I think the solder welds needed might negate any benefit* to removing them). It’s also pretty dang small too, at 14mm x 16.5mm x 5mm.

You can connect 1 motor and 2 actuators to it, via these tiny plug-in connectors on the board. A tiny 3.7 volt LiPo battery is easily attached via 2 magnet connections.

MiniAct Magnetic 1.1g Actuators ($14.99/ea)

These 2 little actuators only weigh 1.1 grams each. Servos weigh quite a lot more, and my airship doesn’t need the range of a servo. All I need to do is kick the rudder to one side or the other every once in a while, and the same with the elevator.

I’ve been trying to figure out how to link things up so I can just use 2 actuators (1 for rudder and 1 for elevator). But due to the tail gas bag hogging the inside space all the way to the back I may end up using 4 actuators (1 for each fin control surface), in which case I may opt for the smaller 0.4 gram actuator instead.

LP90-FR Bahoma Cell with 10mm Bahoma ($12.99/ea)

This tiny battery is just 14.5mm x 23mm x 4.8mm, and weighs 3.0 grams. It’s a 3.7v single-cell LiPo battery, and it connects to the receiver via two small magnetic connectors.

The kit I’m gonna order comes with 2 of these, and they get charged on the transmitter.

7mm x 16.5mm 2.9g Motor ($4.29)

This small electric (brushed) motor is 7mm x 16.5mm and weighs 2.9 grams. It connects directly to the receiver via a *nanoconnector* socket. When used with the 65mm propeller (below) it should generate ~11 grams of thrust. I plan to run 3 of these on the airship, of which one in the rear may be steerable.

65mm 0.05g Propeller ($3.25/ea)

Small 65mm (~2½ in) propeller with a 1mm bore, which should fit the 7mm motor (above) perfectly.

Three of these with the above motors should generate around ~33 grams of thrust total. Being a lighter-than-air craft, I *think* this should get the airship moving.. I will be testing them on the test section once I finish building it to see how they do.

So at this point it is looking like the entire propulsion and control system might weigh only **14.95 grams total!** There will of course be a few additional grams of carbon rod control links and several feet of wire, but the whole thing should still remain very light. 🙂

*† Note: This post is not an ad, and I haven’t bought anything from them just yet. So I am not even sure if this is even good stuff at this point. I will post more information and photos once I have ordered my stuff from them.*.

## Total Airship Lift Capacity

I have now calculated the airship’s lifting capacity for all 4 of it’s gas bags in previous posts. There will be one in the nose, two in the middle, and one in the tail. So here is what I have calculated just to recap.

**Note**: There will be almost no space between the gas bags within the finished airship. I’ve added a couple inches between them in the diagram (right) for illustration only.

Volume: 2.4469 ft^{3}

Lift Capacity: **69 grams**

Middle (Fore)

Volume: 5.4002 ft^{3}

Lift Capacity: **152.2856 grams**

Middle (Aft)

Volume: 5.4002 ft^{3}

Lift Capacity: **152.2856 grams**

Volume: 4.3412 ft^{3}

Lift Capacity: **122.42 grams**

So it looks like the airship will have a **total gas volume of 17.5885 ft ^{3}** and a

**lift capacity of 495.9912 grams**. Google says there are around 453.5924 grams in a pound. So the airship’s helium volume should,

*hopefully*, be able to

**lift around 1.09 pounds**. At this point it seems to be more than enough, but we’ll see as I work through the rest of the frame and equipment weight calculations..

## Calculating Gas Bag Volume / Tail

I decided it would be a good idea to calculate the lift capacity for the whole ship before continuing the build work, and have now moved on to the tail gas bag. Once again, there will only be a single gas bag in the tail, but I have drawn it in ring segments (Fig. 1) to make calculating the volume easier. We will calculate the volume of each ring segment (Fig. 2), then add them all together, to get the total volume of the aft (tail) gas bag.

*Since Sketchup will not draw smaller increments than 1/16″, so once again I have again rounded up 1/32″ (radius) in a couple of places. The calculations should still be fairly accurate though.*

We will be using the same equation we used to calculate the volume of the nose gas bag to calculate the volume of the tail segments. There is also an online calculator that can also make this easier, but for now we will work the equations manually to illustrate the process.

V = (pi * h / 12)(d^{2} + db + b^{2})

Each segment of the aft (tail) gas bag, with the exception of the tail cone at the end, is a “frustum of a cone” shape (*a cone with the top cut off*). And each segment is separated by the frame ring where the bevel/angle happens.

We will work the calculations from left (largest) to right (smallest) *as pictured above*, denoting each as section **A**, **B**, **C**, **D**, **E** and **F** (cone). *I am again using only 4 decimal points for everything, but if you notice an error in my math (it happens) don’t be shy and let me know!*

Section A

V = (3.1459 * 6 / 12)(21.625^{2} + 21.625 * 22.25 + 22.25^{2})

V = 1.570795 * 1,443.8594 = 2,268.0071 in^{3} (*or* **1.3125 ft ^{3}**)

Section B

V = (3.1459 * 6 / 12)(19.875^{2} + 19.875 * 21.625 + 21.625^{2})

V = 1.570795 * 1,292.4531 = 2,030.1789 in^{3} (*or* **1.1749 ft ^{3}**)

Section C

V = (3.1459 * 6 / 12)(17.0^{2} + 17.0 * 19.875 + 19.875^{2})

V = 1.570795 * 1,021.8906 = 1,605.1806 in^{3} (*or* **0.9289 ft ^{3}**)

Section D

V = (3.1459 * 6 / 12)(12.875^{2} + 12.875 * 17.0 + 17.0^{2})

V = 1.570795 * 673.6406 = 1,058.1513 in^{3} (*or* **0.6124 ft ^{3}**)

Section E

V = (3.1459 * 6 / 12)(6.75^{2} + 6.75 * 12.875 + 12.875^{2})

V = 1.570795 * 298.2344 = 468.4651 in^{3} (*or* **0.2711 ft ^{3}**)

Section F (Tail cone)

V = ((pi * r^{2}) * h) / 3

V = ((3.14159 * 3.375^{2}) * 6.0) / 3 = 71.5693 in^{3} (*or* **0.0414 ft ^{3}**)

So it looks like the aft (tail) gas bag should have a **total volume** of approximately **4.3412 ft ^{3}**. Since a cubic foot of helium can lift around 28.2 grams, this should give the aft gas bag a total

**lift capacity**of around

**122.42 grams**.

I have not yet figured out what the frame, and tail fins, will weigh though. This is probably enough to lift them though, since a good portion of the tail will be built from 1/16″ balsa sticks. But if not, there is some excess lift capacity in the two middle sections to compensate for it. I will finish calculating the whole frame weight in a future update just to be sure.

## Calculating Gas Bag Volume / Nose

I decided it would be a good idea to calculate the lift capacity for the whole ship before continuing the build work, starting with the forward gas bag in the nose. There will only be one single gas bag in the nose, but I have drawn it in ring segments (Fig. 1) to make calculating the volume easier. We will first calculate the volume of each ring section (Fig. 2), then add them all together, to get the volume of the forward (nose) gas bag.

*Since Sketchup will not draw smaller increments than 1/16″ I have rounded up 1/32″ (radius) in a couple of places. There is enough space in between the rings for the gas bag to expand a little that the calculated volume should still be okay.*

Like many, I’m no expert with geometry. And some formulas I found are a little too math nerd for me. But I found a very useful online calculator that makes it easy to calculate the volume and surface area of the frustum of a cone. The volume calculation is based on the following formula:

V = (pi * h / 12)(d^{2} + db + b^{2})

Each segment of the forward gas bag, with the exception of the cone at the end, is a “frustum of a cone” shape (*a cone with the top cut off*). And each segment is separated by the frame ring where the bevel/angle happens.

We will work the calculations from left (largest) to right (smallest) *as pictured above*, denoting each as section **A**, **B**, **C** and **D** (cone). *I am only using up to 4 decimal points for everything, but if you notice an error in my math (it happens) don’t be shy and let me know!*

Section A

V = (3.14159 * 6 / 12)(20.75^{2} + 20.75 * 22.25 + 22.25^{2})

V = 1.570795 * 1,387.3125 = 2,179.1835 in^{3} (*or* **1.2611 ft ^{3}**)

Section B

V = (3.14159 * 6 / 12)(18.0^{2} + 18.0 * 20.75 + 20.75^{2})

V = 1.570795 * 793.3125 = 1,246.1313 in^{3} (*or* **0.7211 ft ^{3}**)

Section C

V = (3.14159 * 6 / 12)(6.875^{2} + 6.875 * 18.0 + 18.0^{2})

V = 1.570795 * 495.0156 = 777.568 in^{3} (*or* **0.445 ft ^{3}**)

Section D (Nose cone)

V = ((pi * r^{2}) * h) / 3

V = ((3.14159 * 3.4375^{2}) * 2.75) / 3 = 34.0288 in^{3} (*or* **0.0197 ft ^{3}**)

So from what I gather so far it looks like the forward (nose) gas bag should have a **total volume** of approximately **2.4469 ft ^{3}**. Since a cubic foot of helium can lift around 28.2 grams, this should give the forward gas bag a total

**lift capacity**of around

**69 grams**.

This may not be enough to lift the frame of the nose (*I have not calculated that yet*), but there will be some excess lifting capacity left over in the middle sections that can compensate for this, if needed. I will most likely also build most of the nose section from smaller 1/16″ balsa sticks too, so that should help keep the frame weight down.