Inside a multirotor — transcript

00:00:00.400 --> 00:00:04.092 Inside a multirotor: airframe choices, motor mixing,

00:00:04.092 --> 00:00:06.669 and what happens when a motor fails.

00:00:07.019 --> 00:00:11.704 A multirotor uses three or more rotors, each driven by its own motor.

00:00:11.704 --> 00:00:16.189 Quads are the most common. Hexacopters and octocopters add motors,

00:00:16.189 --> 00:00:18.330 for heavy lifts and redundancy.

00:00:18.680 --> 00:00:22.050 But every extra motor adds weight, cost and power draw.

00:00:23.180 --> 00:00:29.188 In a plus layout, one arm points forward. In an X, the nose sits between two arms.

00:00:29.188 --> 00:00:31.069 There's also the H-frame.

00:00:33.680 --> 00:00:39.774 Fixed-wings lift with the wing. Tails vary: conventional, an H-tail with twin fins,

00:00:39.774 --> 00:00:42.458 a V-tail, or none, on a flying wing.

00:00:42.808 --> 00:00:47.905 The same battery that flies a quad for 25 minutes might fly a fixed-wing for 60 to 90.

00:00:47.905 --> 00:00:49.077 But it can't hover.

00:00:52.308 --> 00:00:57.520 A VTOL hybrid takes off and lands like a multirotor, then cruises on its wing.

00:00:57.520 --> 00:00:58.708 No runway needed.

00:01:00.808 --> 00:01:06.189 Quick check. What's the trade-off when you choose a fixed-wing over a multirotor?

00:01:06.908 --> 00:01:09.128 Longer endurance, but it can't hover.

00:01:09.808 --> 00:01:11.616 You never fly the motors directly.

00:01:11.616 --> 00:01:15.492 The flight controller turns each stick command into separate motor speeds.

00:01:15.492 --> 00:01:16.939 That's called motor mixing.

00:01:19.008 --> 00:01:23.090 Climb, and all four speed up together. Move, and the change is shared across them,

00:01:23.090 --> 00:01:24.467 hundreds of times a second.

00:01:28.808 --> 00:01:33.640 Inside, a gyroscope senses rotation and an accelerometer senses tilt.

00:01:33.990 --> 00:01:39.447 When a gust tips the drone, the control loop spots it and corrects the motors,

00:01:39.447 --> 00:01:41.174 often before you notice.

00:01:43.590 --> 00:01:47.903 Hover and forward flight are different airflow problems. In a hover,

00:01:47.903 --> 00:01:50.591 the air moves in a mostly vertical column.

00:01:51.590 --> 00:01:57.207 In forward flight, the rotors meet cleaner air, and the frame and payload add drag.

00:01:57.590 --> 00:02:02.632 Now, lose a motor. On a quad, one failure usually means an unrecoverable crash.

00:02:03.590 --> 00:02:07.580 A hexacopter can often fly on, with reduced performance.

00:02:07.580 --> 00:02:11.009 An octocopter can often absorb one, or even two.

00:02:11.359 --> 00:02:14.232 If it flies on, it's degraded: land straight away.

00:02:17.759 --> 00:02:21.442 Flying over water or at height? Choose motor redundancy,

00:02:21.442 --> 00:02:24.995 and know your aircraft's failure modes before you fly.

00:02:26.259 --> 00:02:31.288 Recap. More rotors buy redundancy, at a cost in weight. Wings buy endurance,

00:02:31.288 --> 00:02:36.839 but lose the hover. And the flight controller turns every command into motor speeds.

00:02:37.759 --> 00:02:42.670 Final check. One motor fails on a quadcopter. What's the most likely outcome?

00:02:43.059 --> 00:02:45.828 An unrecoverable crash. A quad has no spare motor.