Flight Time
What impacts flight time? Battery capacity, quad weight, and the efficiency of the system components.
- Long flight time (>10 minutes)
- High capacity battery
- Weight reduction
- High efficiency motors and electronics
Tackling the battery seems simple enough. I want this to be a 3 cell (3S) system. That means that the voltage when fully charged should be around 12.6V. This is good because I won't have to worry about messing with a voltage regulator for electronics that can only accept up to 13V. A 4S system would be ~16.8V when fully charged.
Capacity vs Weight
Now the capacity. Well as capacity goes up, so does weight. There is obviously a trade-off here. The heavier your quad the more thrust you need and therefor the more power you use, resulting in you battery dying faster. It isn't quite that simple though. It really depends on how much more capacity you are getting for a larger battery vs the added weight. Not quite sure how to quantify this yet. I guess I should start by calculating the weight of my system with the components I have at the moment even though they are likely to change.
| Quad Weight | Weight (g) |
| Frame and hardware | 165 |
| Motors | 76 |
| Battery | 155 |
| Video Tx | 18 |
| ESC | 12.6 |
| Radio Rx | 9.8 |
| Fllight Controller | 5.7 |
| Camera | 33 |
| Props | 19.6 |
| extras | 100 |
| Total weight | 594.7 |
| thrust per motor needed | 297.35 |
Assuming that this is close the total weight of the quad minus the battery is 439.7g. So that is the base weight.
I have been using the guides on Hovership.com as a starting point for what I should probably be looking at for components with respect to size, weight, capacity, etc. This is because I plan on yusing the MHQ V2 printable frame available on thingiverse. They suggest that for a 3S system the battery should be 1300mah - 1800mah, and the smaller battery should be used if you plan on using a gopro or other camera for recording. I don't plan on doing this at the moment so I will go for a battery around 1300mah. I might use a little larger one if it will extend the flight time significantly.
Until I know what the other electronics are, and the motors especially it is going to be hard to pick out a battery. I need to be able to look at all the loads and create a power budget so I can calculate the average flight time. So tabling this for now.
Update
Actual Weight of Quad
| Parts | Weight (g) |
| Frame | 138 |
| Motors/ESC | 100 |
| Power distribution board and battery lead | 14.5 |
| Step Down Reg | 1.3 |
| Liquid Electrical Tape | 1 |
| Zip Ties | 5 |
| Propellers 8 pairs | 52 |
| 3S Battery (more than 1 recommended) | 155 |
| Battery Connector (5 pack) | 0 |
| Lost Model, Battery Alarm, Lost Signal | 7 |
| Charger | 0 |
| Flight controller | 5.7 |
| Radio transmitter mode 2 | 0 |
| Trainer Cable | 0 |
| Case | 0 |
| Radio receiver | 9.8 |
| Video Transmitter | 16 |
| Video Receiver | 0 |
| Video Camera | 80 |
| Flight Googles | 0 |
| Hardware | 44 |
| Total Weight | 629.3 |
with 10% safety weight is equal to 693g
Thrust per motor needed (g) = (693 x 2)/4 = 346 g
Lets just say that each motor needs 350g of thrust.
The motors I chose (EMAX MT2204 2300KV) produce 440g of thrust with a 3S system and 6 inch props according to this blog post, see picture below.
So according to all of this I am about 100g under the max output of the motors I bought. I should have no problems getting this little quad to zip around.
Weight Reduction
Weight reduction will happen along the way and at the end of component selection.
High Efficiency Motors and Electronics
So again determining the correct motor is dependent on other factors, principally weight. The thrust from the motor required to get the quad in the air and perform well can be calculated by the following formula. Thrust per Motor (in grams) = (Weight of quad x 2) / 4
I am starting to see that this could get really in depth fast, so I am going to take a different approach. I know that I want >10 min flight time while reducing costs for this first quad as much as possible. So picking a motor that can provide at least 400g of thrust should certainly be good enough (based on the weight of the system I spec'd out quickly). I'll start with that.
The MHQ V2 frame requires M3 screws with 16 x 19 mm spacing, so my motors will have to conform to this. Hovership also recommends that the motors weigh < 21g and is a 1900 - 2300kv.
I found this page about picking out motors, which seems very useful.
http://blog.oscarliang.net/how-to-choose-motor-and-propeller-for-quadcopter/
Motor Options
I looked at quite a few motors and the hard thing was finding a motor that put out enough thrust, was well known, and used 16x19 M3 mounting. I really don't think I know enough to pick a motor that no one has reviewed, so I am going with the Cobra 2204. Oscar Liang wrote about this motor for a build he did, and he seemed to really like it. The SunnySky motors and the Cobra motors are both very popular, so they are hard to buy. However since many more people seem to go with the SunnySky I want to be a little different than the crowd.
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