Lift, drag and angle of attack — transcript

00:00:00.400 --> 00:00:04.658 Lift, drag and angle of attack: how every wing and propeller blade works.

00:00:06.900 --> 00:00:09.177 An aerofoil is shaped to make lift.

00:00:09.177 --> 00:00:14.554 The chord line runs from the leading edge to the trailing edge. Camber is the curve.

00:00:17.600 --> 00:00:21.046 Relative airflow comes from opposite your direction of travel.

00:00:21.046 --> 00:00:24.000 Between it and the chord line is the angle of attack.

00:00:26.900 --> 00:00:33.640 Three ideas explain lift. Bernoulli: faster air over the top means lower pressure.

00:00:33.640 --> 00:00:39.649 Coanda: the flow follows the curve and turns down. Newton: push air down,

00:00:39.649 --> 00:00:41.842 and the wing is pushed up.

00:00:42.192 --> 00:00:47.234 A propeller blade is a rotating wing. It throws air down, and the drone goes up.

00:00:50.192 --> 00:00:55.858 More angle of attack makes more lift, and more drag, up to the critical angle.

00:00:55.858 --> 00:01:00.878 Past it, the airflow breaks away. Lift drops sharply and drag climbs.

00:01:00.878 --> 00:01:02.026 That's a stall.

00:01:04.192 --> 00:01:09.013 Double the airspeed, and you get four times the lift. So the slower you fly,

00:01:09.013 --> 00:01:14.022 the more angle of attack you need. Slow enough, and you hit the stalling angle:

00:01:14.022 --> 00:01:15.712 that's the stalling speed.

00:01:19.792 --> 00:01:24.020 A slow, nose-high fixed-wing stalls and drops, with the motor still running.

00:01:24.020 --> 00:01:27.315 Recover by lowering the nose to reduce the angle of attack.

00:01:30.692 --> 00:01:34.689 Quick check. What happens past the critical angle of attack?

00:01:35.892 --> 00:01:38.217 Lift drops sharply, and drag rises.

00:01:39.692 --> 00:01:44.553 Now drag. Parasite drag is everything not making lift: form drag,

00:01:44.553 --> 00:01:47.278 skin friction and interference drag.

00:01:47.278 --> 00:01:51.108 Twice the speed means four times the parasite drag.

00:01:51.458 --> 00:01:56.575 Induced drag is the price of lift. Air spills around the tips into vortices.

00:01:56.575 --> 00:01:59.764 It's worst when slow, and fades as speed rises.

00:02:02.458 --> 00:02:06.305 Add them up. The bottom of the curve is the minimum drag speed:

00:02:06.305 --> 00:02:09.972 best range in still air. Best endurance needs minimum power,

00:02:09.972 --> 00:02:11.836 which comes at a slower speed.

00:02:15.458 --> 00:02:21.038 On a multirotor, every blade is an aerofoil. Fly fast or climb hard, and drag rises,

00:02:21.038 --> 00:02:23.007 so the battery drains faster.

00:02:25.258 --> 00:02:27.435 On a fixed-wing, guard your airspeed.

00:02:27.435 --> 00:02:30.587 Extra weight and steep turns raise the stalling speed.

00:02:32.458 --> 00:02:38.306 Recap: angle of attack makes lift, until the stall. Induced drag rules when slow,

00:02:38.306 --> 00:02:42.515 parasite drag when fast, and minimum drag sits in between.

00:02:42.865 --> 00:02:46.104 Final check. Which drag is greatest at low airspeed?

00:02:48.065 --> 00:02:49.345 Induced drag.