Know How...

Aug 20th 2015

Know How... 157

DIY Swamp Cooler, BeerLift, & TrackingPoint Hack

Build your own AC, Beer lifting quad, and the TrackingPoint gun hack from BlackHat
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Category: Help & How To

How an AC works and how to build your own simple version for staying cool this summer, Beer lifting quad copter, and the TrackingPoint gun hack from BlackHat.

Let's talk about Air Conditioning

To talk about AC, we first need to make clear what "hot" and "cold" really are.
-- "Hot" is really just energy in the process of being transfered from one body to another -- From the body that has more energy, to the one that has less
-- In any given system, this transfer of energy will take place until you reach maximum entropy: the energy is as evenly distributed among all the bodies in a system as is possible.
** Also, in any given system, the energy in that system will stay constant (unless that energy is somehow moved away from that system)

So... if we have air blowing in our face, and it feel "hot", it just means that the molecules in that air contain more energy than the molecules in our face, and energy is being transfered from the air to our faces.

Conversely, if that air feels "cold" it means that the molecules in that air contain less energy that the molecules in our face, and energy is being transfered from our faces into the air.

Now that we have that sorted, let's talk about HOW we make that blowing air hot or cold.

To make air feel hot, we simply add energy to it:
* A furnace burns gas or uses electricity to heat up coils
* Air is blown over the element being heated by the gas or electricity
* That air "absorbs" heat from the element
-- TA DA! (Hot Air)

But to make cold air, the process isn't so simple:
* We can't just add energy into air to make it cold. We need a way to take energy away.
* And remember... the energy in a given system will stay constant unless that energy is somehow moved away from that system
* The answer is to build a system that will absorb heat from one system, and release it into another.
** And THAT is what an air conditioner is designed to do!

Let's talk about a simple 2-stage AC: It has 4 parts
1. Compressor
2. Condensor Coil
3. Expansion Valve
4. Evaporator Coil

1. Compressor
* The Compressor takes a gas within the system and pressurizes it.
* Important to note: If you have a gas that has energy level "x" at its regular volume, and you pressurize it to 3x it original pressure in the same volume, then that volume MUST contain 3x the amount of energy. (Again, in a closed system)
* So... if I have 1 cubic meter of gas at 50 degrees, and I pressurize it to 3x it original pressure, then that volume of gas (at 3x pressure) should be 150 degrees.

Super Side Note: This is the "Ideal Gas Law"
((P1*V1)/T1) = ((P2*V2)/T2)
* (Initial Pressure * Initial Volume)/Initial Temperature = (Final Pressure * Final Volume)/Final Temperature

** Ok... so now we have a bunch of HOT pressurized gas, but we want COLD gas.

2. Condensor Coil
* The condensor coil takes the now pressurized gas and gives it contact (through a conductive surface that allows the closed system to retain the gas, while conducting the energy of that closed system) with an outside system.
* We know that energy (heat) will want to flow from a body of high energy, to a body of lower energy
* If the system in which that condensor coil is of a lower energy state than the system contained within the coil, then energy (heat) will flow out of the gas, and into that outside system.
* Given enough time, the temperature of the gas in the coil will equalize with the temperature of the outside system.

** Just for giggles, lets say that our pressurized gas loses enough energy to drop its temperature by half.
** (Also, just for the smart kids in the class, the compressor is constantly running so that the system re-pressurizes to 3x even as the drop in temperature causes a drop in pressure.)

3. Expansion Valve
* Let's run the compression of the gas in reverse: We have 3x pressure in a volume of air that is now at 75 degrees. It now goes through an expansion valve that allows the pressure to drop to 1x.
* Just as compressing the gas raised its temperature, expanding the gas will drop its temperature.
* If the 3x pressurized gas was at 75 degrees, then allowing it to expand into 3x the orginal volume will allow the gas to drop to 25 degrees.

** Woot! Now we have a bunch of REALLY cold gas!

4. Evaporator Coil
* We run that cold gas through the evaporator coil.
* Air is blown over that evaporator coil.
* The gold gas absorbs energy from the air being blown over the evaporator coil.
* The blown air now has a lower energy state than it had previously
* It blows on our face... and we love it.

** Now the system repeats! Compress, Exchange, Expand, Exchange.

Build a Swamp Cooler

Parts List
1. Honeywell TurboForce Fan HT-90 $24.00
-or - 120mm Fan $6.99
2. Home Depot 5-gal bucket $2.97
3. 5-gal Bucket Liner $6.00
4. Flow Vents

Tools List
1. Marker
2. Cutting Tool
3. Glue/Tape

1. Mark a circle around the fan on the bucket lid.
2. Mark circles around the outlets
3. Use the cutting to cut a slightly smaller circle in the lid
4. Use the cutting tool to cut out the outlets
5. Insert the foam liner into the bucket
6. Use the cutting tool to cut outlet holes into the liner
7. Set the foam lid for the liner and close the lid on the plastic bucket
8. Use the cutting tool to cut through the foam lid
9. Secure fan to plastic lid
10. Secure vents to outlets

Building a Beer Lifting Quad

Weight: One of the easier ways to increase cargo capacity is to reduce weight.
1. Lighter ESCs that are closer to the max expected draw of the motor.
- We were using 30amp RED series RTFQ ESCs: 32 grams
- We're switching to 20amp HUG RTFQ ESCs: 5 grams

** This saves us 108grams

2. Frame:
- We were using a Diatone frame that weighed 102 grams
- We're dumping the legs and cage to get that weight down to 78 grams

** Saves us 24grams

1. Prop
- We want the largest, most agressive prop that we can use on this frame (x250)
- The Diatone frame can fit a 6" prop.

2. Motor
- We want a motor than can turn a 6" prop with a 4s battery.
- The RTFQ 2208 can pull ~300watts and supports 4s batteries
- This should max out our 20 amp ESCs while turning a 6x4.5 prop

** But let's find out how much thrust we can generate with a single 2208 and a 6x4.5 prop

Power System
1. Battery
- We want a light, low-capacity, high-discharge battery
- We only need to lift for 10 seconds, but we DO need a lot of current

2. FC Power
- Our ESCs don't have BECs, so we need to either use a seperate BEC, or we need to use a single ESC that has a BEC.

The Build
1. How Many Motors?
- We can go with a straight quadcopter, which should give us ~ of lift, or we can opt to use more motors
- However, since we're stiring up already turbulent air, we're not going to get the full thrust we would get if the motor was turning a prop in clean air
- We need to balance the increase in weight and system complexity with the amount of thrust it gives us.

2. The Cargo Area
- We don't want the cargo to swing wildly, so we need a way to secure the cargo to the craft.

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