Page 133 - Build Your Own Quadcopter_ Power Up Your Designs with the Parallax Elev-8

112 Bu il d Y o ur O w n Q u a d c o p t e r

Figure 5.25 Chart of power versus propeller r/min.

propeller at 7000 r/min. 150 W would mean that approximately 12.3 A would be needed at
a 12.2-V battery-supply voltage. I would estimate that 7000 r/min would be the maximum
rotational speed for the particular propellers used in the Elev-8. The following calculation
shows you how to determine the maximum flight time using 100% power:
One motor/propeller combination at 7000 r/min = 12.2 A
Four motor/propellers at 7000 r/min = 12.2 × 4 = 48.8 A
Using the 3S LiPo battery discussed in Chapter 3 = 4200 mAh = 4.2 Ah
Maximum time at 7000 r/min = 4.2/48.8 × 60 min = 5.16 min

Wow! Only about 5 minutes at maximum power is a startling fact for quadcopter
operations. Operating times can be extended by using higher-capacity batteries, but that
comes at a cost of reducing the effective payload capacity, since using bigger batteries means
heavier batteries. The other preferred way of extending operating times is to operate at
much lower r/min settings. I created Table 5.4 to show estimated operating times and thrust
versus r/min for the battery described above.
Table 5.4 shows how the propeller speed affects both time and the creation of thrust. It is
definitely a tradeoff that you have to consider constantly while you are operating the

Power Voltage Amperage Time Thrust Total Thrust
r/min (w) (V DC) (A) (min) (grams) (grams)
0 0 12.56 0.02 n/a n/a n/a
1140 3 12.56 0.24 263.8 50 200
3450 20 12.53 1.60 39.5 168 672
4710 51 12.44 4.10 15.4 343 1372
6030 103 12.35 8.34 7.6 631 2524
7000 150 12.20 12.3 5.1 860 3440

Table 5.4 Thrust and Time versus r/min