That clip only has the first part of the bit. Here's the transcript of the full bit, which plays out throughout the episode.

ARTHUR: Right. Yes. Sorry. Sorry, Mum. I-I’m just so excited about the trip.
CAROLYN (pulling the plug out of the plughole): Arthur! You’ve been on hundreds of trips. Hasn’t the novelty worn off a little?
ARTHUR: No, never! It’s just always exciting! That amazing moment when twelve tons of metal leaves the earth – and no-one knows why!
CAROLYN: Yes, we do.
ARTHUR: Yeah, but … you know, not really. I mean, we know you need wings and engines and a … sticky-up bit on the end for some reason, but it’s not like we actually know why a plane stays in the air.
CAROLYN: No, no, Arthur, we really do. We-we do, we do know that.
ARTHUR: Oh! How, then?
CAROLYN: Well … er, because … Will you give me that towel?
(The dog yaps as Carolyn rubs her down.)
CAROLYN (soppy high-pitched voice): Okay, okay, good doggie! Keep still!
(She chuckles fondly, then switches back to her normal voice.)
CAROLYN: Because there are four forces acting on the plane, and so long as two of them are bigger than the other two, the plane flies.
ARTHUR (smiling): … Mum, I don’t mind that no-one knows.
CAROLYN: But we do! We do! That’s it! What I said: that’s how.
ARTHUR: Well, what are the four forces, then?
CAROLYN: Yes! Well, I will tell you what they are. Lift … weight … er …
ARTHUR: Up and down?
CAROLYN: No, no, no, no, no. Tho-Those are up and down. No, it’s lift, weight …
ARTHUR: Left and right.
CAROLYN: No, no, no, no. Lift, weight …
ARTHUR (tentatively): En…gines?
CAROLYN: No, no … well, yes, yes, yes, sort of. Um, thrust, thrust. Lift, weight, thrust and …
ARTHUR: Time.
CAROLYN: Drag. Lift, weight, thrust and drag. So, the weight and drag are overcome because the engines give the plane thrust, and the wings give it lift. And that’s how a plane flies.
ARTHUR: How do the wings give it lift?
CAROLYN: What?
ARTHUR: The wings are really heavy. How does bolting two ginormous lumps of metal to a ginormous lump of metal give it lift?
CAROLYN: Well, because they are wings. Well, they’re like birds’ wings.
ARTHUR: Yeah, but birds’ wings flap. Ours don’t flap. They’ve got flaps, but I once watched the flaps all the way to Stockholm and, take it from me, they are seriously mis-named. So-so why does having wings make a plane leave the runway?
(Carolyn draws in a breath. At that moment the doorbell rings.)
CAROLYN (relieved): Ah, they’re here! Now go and wait in the car with them. I need to clean my teeth.
ARTHUR: Yeah, but how do the wings …
CAROLYN (interrupting): Answer the door!

DOUGLAS: It seems so. And now it’s back to the boring old plane flying.
(Footsteps as the men leave the cargo hold.)
ARTHUR: Oh, yes. About that. Um, I wanted to ask you something, Skipper. Mum was telling me this morning that planes fly because they’ve got wings.
DOUGLAS: Is there anything that woman doesn’t know?
ARTHUR: But she didn’t really explain – why do wings lift us up?
DOUGLAS: Ah, well. Essentially …
MARTIN: Uh, Douglas, he asked me. Listen carefully, Arthur. The wing is curved on top but flat on the bottom. When it meets the air, it splits it in two. The air that goes over the top has further to go, so it has to go faster to keep up with the air underneath. That reduces the pressure above the wing, giving us lift.
ARTHUR: Ah, fantastic! Thanks, Skipper! I totally get it now.
MARTIN: You’re welcome.
ARTHUR: Except … why does it have to?
MARTIN: Why does what what?
ARTHUR: Why does the air on the top have to keep up with the air on the bottom? Why don’t they just split up?
(Everyone stops walking. There’s a long pause.)
DOUGLAS: … For the sake of the kids?

DOUGLAS (talking over him): Arthur, you were asking why the air over the wing has to keep up with the air underneath.
ARTHUR: Ooh, yes. Do you know?
DOUGLAS: Indeed I do. Attend: the air is not passing over the wing; the wing is passing through the air, so the curved upper side stretches the air forced over it apart, reducing pressure, producing lift. The lift pushes up; the weight pushes down – so as long as the lift is more than the weight, up we go. And that, my friend, is how an aeroplane flies.
ARTHUR: Got it! Right, yes! Cracking! I completely get it now.
DOUGLAS: Good. You see, it’s actually quite easy to grasp when it’s explained properly by someone who understands …
ARTHUR: So that’s why planes can’t fly upside down.
DOUGLAS: Er, yes they can.
ARTHUR: Can they?
DOUGLAS: Well, of course they can. Haven’t you seen the Red Arrows?
ARTHUR: But … doesn’t that mean the curved side of the wing is on the bottom, so the lift is pushing down as well as the weight? How does that work?
MARTIN (smugly): Yes, Douglas. How does that work?