## What’s the Vector, Victor?

Posted in Film, mathematics, The Universe and Stuff with tags , , , on February 18, 2020 by telescoper

Following on from Sunday’s post about the trials and tribulations caused by Storm Dennis, here is a clip of a plane (an Airbus 380)  landing at Heathrow airport on Saturday.

There are other clips of this same event on Youtube and some of them describe this landing as `dangerous’. Although it undoubtedly involved skill and concentration by the pilot it’s not actually dangerous. Aircrew are trained to land in windy weather like this, and it’s fairly routine. My plane to Dublin (an Airbus 320) landed like this on Saturday evening and, although the pilot got a well-deserved round of applause on landing, nobody was ever really at risk.

As it happens, this week I start teaching vector algebra to my first-year Engineering students, so the weekend’s weather events have  given me a good illustration of vector addition. The plane has to have a velocity vector relative to the air such that the sum of it and the wind vector adds to a resultant vector directed along the runway. Lots of people seem to think this is just guesswork but it isn’t. It’s applied mathematics.

This is in principle simple as long as the crosswind is steady, but obviously the pilot needs to be alert to gusting and make adjustments along the way. When the plane has slowed down enough to land in normal conditions, the wind over the wings is still causing a bit of extra lift. You can see that in the last moments before touchdown this aircraft is gliding because of this effect. I’m told that because of this, in windy conditions planes usually descend at a steeper angle than usual.

The interesting bit for me is that the plane touches down in such a way that its body is at an angle to the runway. As soon as it has landed it has to correct this and point along the runway. I think this is done with the rudder rather than the undercarriage, but I don’t know. Perhaps any experienced pilots that happen to be reading this could give more details through the comments box?

P.S. The title of this post is a reference to the film Airplane!

## Weekend Weather and Travel Updates

Posted in Biographical, Cardiff with tags , , , on February 16, 2020 by telescoper

I got back from London to Dublin last night, rather later than planned courtesy of Storm Dennis. I made my way to Heathrow more in hope than expectation, as there seemed to be a significant probability that my flight would be cancelled (as several were), but in the end it was only delayed by about an hour. It was a bit wobbly coming in to land, but the weather in Dublin wasn’t all that bad last night so there wasn’t the level of drama I expected. I did miss the last Hopper bus, though, and had to take a taxi to Maynooth.

There was another bit of disruption earlier in the day when I discovered that there were no Piccadilly or District line trains between Hyde Park Corner and Acton Town (in either direction), which makes it impossible to get to Heathrow directly by Tube. That gave me an opportunity to explore the London Underground to find a way through. I had plenty of time so I thought it would be fun. In the end I chose the Central Line to Ealing Broadway then a quick couple of stops on the District Line to Acton Town where there were trains to Mornington Crescent Heathrow Airport.

Dublin is to leeward when bad weather comes in from the Atlantic, so doesn’t get the worst of it. This seems to have been the case for Storm Dennis. Wales hasn’t been so lucky and I’ve spent the morning following messages on social media from friends and former colleagues about flooding along the Rover Taff. At one point there were 85 active flood warnings in Wales:

When I bought my house in Pontcanna I signed up for Flood Alerts, and I got one of them too. Here are some pictures of the area taken this morning:

This grim scene is not far from my house, but it’s not as scary as it looks. The River Taff is flanked by steep embankments as it passes into Cardiff and these provide strong flood defences. There is also a tidal barrage across Cardiff Bay which prevents tidewater coming up the River Taff while floodwater is trying to come down it, which is the usual recipe for a flood. The whole system is designed so that before these are breached, water floods out further North over Pontcanna Fields (a wide flat area that is part of the natural flood plain of the Taff that doesn’t have houses on it). A huge amount of water can be `parked’ in this area until the spate subsides. This happened before when I lived in Cardiff in 2009 and it’s nothing to worry too much about. It’s not good for people wanting to play rugby on the playing fields though!

After the terrible floods of 1979 Cardiff built very strong flood defences, but the same is not true for smaller towns up in the Welsh valleys. It seems that Pontypridd has been particularly badly hit this time. There seems to be no political will to spend money protecting such places, which in my view would be far more worthwhile than building HS2.

This all reminds me of the time when I first moved in to my house in Pontcanna over a decade ago. Late one rainy evening the phone rang and it was an automated flood warning. I responded by doing everything the message told me not to do: I put on my coat and went to the river to see what was happening. It’s only a ten minute walk from my house to the embankment. When I got there, I found a crowd of about a dozen people watching. The river was only about a metre below the level of the embankment as it roared its way down to Cardiff Bay carrying along tree trunks, car tyres and various other items of rubbish. It was quite impressive, but I didn’t watch for long as it was very cold and wet.

I’m told that the worst part of the 1979 flood was not the surface water, but the fact that the drainage system couldn’t cope and sewage backed up into the houses and streets. That must have been horrible as well as causing a serious health hazard.

Update: I heard last night that the level of the Taff on Sunday was 80cm higher than it was during the 1979 floods.

Anyway, I was glad I did get back last night as today I am giving a talk at the Joint Congress of University Astronomical Societies. I’d better get on and write it!