Freesteel Blog » 2018 » April

Tuesday, April 24th, 2018 at 2:05 pm - - Kayak Dive

Some old pictures of kayak diving in the Isle of Arran from 2016 that I had failed to blog about at the time are below.

The weekend before last I took a fellow DoESLiverpool-er diving in the Menai Strait, which was okay. The weather was gorgeous, but the viz was average and there were No Fish (except one butterfish I caught in my hand at the start of the dive).

Here’s us shinnying back up the sponge encrusted telephone cable back towards the Northwest shore after experiencing the current. We stayed underwater for another 20 minutes under the shelter of the main bridge pillar, but didn’t find the encrusted admiralty anchor that is reputed to be there.

I’m not sold on the idea of these side-mounted cylinders. Looked more like front mounted and dragging on the ground to me.

The weekend after that one (ie last weekend) I bagged an available place on his club trip to Eyemouth with some deep Nitrox type dives. I don’t like deep diving, but was persuaded to go anyway. It was awful. The algae had come out, the water visibility was about 2 metres, and at 40metres down it was as black as the night.

The first deep dive was to the U74. I reached it, then got freaked out by the strobe that had been tied to the shotline to “help” us find the way back, stirred up a cloud of the silt on the metal, and took the first available option to leave, passing all the other divers coming down the rope.

I have very little tolerance for danger when the fear-to-fun ratio is too high.

Second dive was just to 12m in Pettico Wick (normally a shore dive in bad weather), supposedly to the peanut wreck. I found some bits of metal, but mostly blank boulders. But then things really cheered up when I stumbled across the rock called Wick Gaunt and dropped into a slot behind it (I think on the West side) which turned into quite a nice cave.

The other divers were not amused and didn’t follow me in. I couldn’t find the other entrance when I swam around to the other side. This would be a great place with some more visibility — and done as a kayak dive.

I will be back, having obtained a copy of the 32 page A5 booklet detailing 18 dives in St Abbs and Eyemouth from the dive centre — a booklet which is not mentioned anywhere on the marine reserve website or available from Amazon. I intend to spend about a week there when I can, because it looks like the last time we gave it a go was eleven years ago in 2007.

I tried to persuade the other divers on the trip of the benefits of kayak diving, but they thought it was a bit dangerous. That’s because a 37m horrorshow dive in poor visibility with not enough lights to a section of shingly the seabed somewhere in the vicinity of the Glanmire wreck (the shot had been pulled off it) is considered A-okay.

No it wasn’t.

As we quit early, Al and I stopped off at the Kagyu Samye Ling Monastery and Tibetan Centre near Lockerbie for some quiet moments of meditation in the temple where we overlapped into a session that involved people coming in and doing a bit of chanting.

Noisy lever-latch door handles on sprung swing doors at the temple entrance. Really? This has got to be one place it’s worth investing in some subtle silent autosealing door technology. But instead they’ve blown their tech budget on motorizing all the prayer wheels around the stupa.

We took a herbal tea, and I noticed a poster about their new off-shore retreat on Holy Isle, also in Scotland.

It turned out this was where I had a nice weekend kayak diving around this particular island with Becka in August 2016 and never blogged about it.

I had liaised with some fellow from Arran COAST who had campaigned for the Lamlash Bay no take zone to get some dive site information. (They’ve not yet produced a book like the St Abbs people have.)

Unfortunately the viz wasn’t very good, and we had quite a lot of current on our attempt at a deeper dive in the south channel.

He was kind enough to take these pics of us struggling on the surface to get kitted up in the tidal stream going past our kayaks that was pulling the buoy underwater.

We did also land and climb the peak on the island and discovered a dead data logger in the heather.

Just checking on the tech, you know.

Thursday, April 12th, 2018 at 4:29 pm - - Flightlogger, Hang-glide

Okay, so that last flying day at Meduno wasn’t very adventurous on the scale of the top pilots, but I was extremely pleased with it; I did just as well as anyone else in our xtc-paragliding (hang-gliding week) group and felt perfectly up with it.

Often you come down disappointed, and can watch everyone else from the landing field going higher and further and having more fun, and you’re down wholly because of your lack the skill and competence. But this wasn’t one of those days.

Here is the page of everyone’s tracklogs.

I was particularly happy with the part of the flight where I maintained my altitude over the flat lands at about 700m for 11 minutes before finally the air currents strengthened enough to carry me up. I had a sense of calm and flow rather than panic and disappointment this time.

It doesn’t look particularly low in the picture, but it felt like it.

I thought it was rising air from a pig farm I could see below and towards the dry river bed (because it smelled as such) but it couldn’t be as this as it was about 700m cross wind. I had consistently the wrong idea of the wind direction. It shows that even with totally mistaken ideas, I was still able to stay with the weakly rising air.

At one point I was passed high over a rifle range. The pops of the guns were like tap-taps on my breastbone.

I overflew the takeoff at the end of the day and took a photo of this cute pink training glider on the ramp beside the wood pile in the car park.

Then I tried to narrate part of my glide down to landing to the camera, which doesn’t work at all with my full face helmet.

One of the folks on the hill was SashaZ whose long blogpost about surfskis is what caused me to book my Tarifa trip with Becka.

Here are some other pics from previous days.

We had some long drives there and back in someone else’s car. Becka spent the whole time at SpeleoCamp caving, and so this shouldn’t count as a hang-gliding holiday.

Oh, I might as well put down my notion of the physics of flight here, while I have it worked out. It goes like this:

A heavier than air object with a mass of 100kg wishes to avoid accelerating downwards to the ground under a gravitational force amounting to 10 metres per second per second.

As each second that passes there is 100×10 = 1000 kg m/s of momentum that must be accounted for by blowing a volume air downwards at a speed k m/s.

Suppose the craft encloses a horizontal area a square metres within which it blows the air downwards at k m/s. In one second this would be ak cubic metres, which, with a density of about 1 kg per cubic metres, is ak kilograms, sent downwards with a momentum of ak2 kg m/s.

If the area a was circular, then you could cover it with a circular propellor like a helicopter, and maintain your altitude by blowing the air at sqrt(1000/a) metres per second downwards to counteract the gravity.

But imagine the shape of a is rectangular, and instead of a rotating blade, the blade moves horizontally on rails of length v and has a width w. This is somewhat like a wing with a span w flying at a velocity v.

My glider has a wingspan of the order of 10m, and an airspeed of 16 m/s, so the air needs to be blown downwards at a speed of sqrt(1000/(10*16)) = 10/4 = 2.5 m/s.

The kinetic energy embodied in this is 1/2 * mv2 = 0.5*160*2.5*2.5*2.5 = 1250 Joules/second.

If I weigh 100kg I can generate 1250 Joules from potential energy if I sink at 1.2 m/s — which is about the rate that my glider sinks on a steady glide.

This is a story of what needs to happen to the air to keep you up, not how it is done with aerofoils, vortices, induced drag or any stuff like that. And it also suggests that our lovely gliders have already hit certain limits of what they could physically achieve for their size and speed.

One way to get them to go up will be to add an electric motor to give you that extra to get off the ground, or to find a thermal when you’re going down.

That ad says they have 24 Ah in their 57.8V battery, which equates to 24*57.8*60*60=5Megajoules. This can maintain a horizontal flight for 27 minutes, which means it’s at the rate of 3000 Watts. That’s about a 50% conversion rate from the battery to powered energy, which is plausible.

It also gives a “max summit height” of 750m, which is a budget of 6660 Joules per metre. I need to give it 1000 Joules per metre in potential energy, so suppose my climb rate is k m/s then it will take me 750/k seconds to get up there, consuming 3000*750/k + 750*1000 = 5Megajoules which computes to a climb rate of 0.53 m/s over 23 minutes.

I can’t afford this stuff. I should be happy with the massive amount that I’ve already got.

Thursday, April 5th, 2018 at 10:02 pm - - Flightlogger, Hang-glide

I’ve been deeply not keeping up with blogging on this Slovenia hang-gliding trip. Telegram and Twitter seem to take the wind out of such activities. So maybe this thing is for mainly technical reports. There are a lot of dead blogs out there that only have such things. This blog was started for technical content, and then I began putting all my own activities into it.

I’ve been working on this technical thing to do with gliding and tracklogs for so long without any breakthrough that I finally decided I had to start reporting negative results.

My latest failure was attempting to use a Hough transform to derive wind speed and direction from the 2second interval GPS sample point of a glider flying around in the air mass.

There are many made up algorithms for doing this, but I wanted something mathematical. This time I based it on the assumption that the glider is mostly flying at a constant speed, so that changes in its GPS/ground speed were entirely due to flying with or against the wind. In particular, given three consecutive positions p0, p1, p2 with td seconds between them, then the correct wind velocity w would satisfy the following equation:

|p1 - p0 - wtd| = |p2 - p1 - wtd|

There is no unique solution for w in this equation; the solutions all lie along a line. So if we add some spread and combine the probability fields of solutions for every sequence of three points in the track, then the peak probability will be the best guess at the wind direction.

It’s all explained here in this jupyter notebook.

After so many failures, I’m much pleased with this result. The actual wind was blowing towards the northeast, and the bad guesses are when the glider was on glide and not doing any circles.

That was from a four hour mega flight all round the three ridges near Gorzia where at one point I got lifted smoothly one thousand metres into the blue sky at the rate of 5m/s. I could see from the capital city inland to the container ships on the Adriatic.

Here’s a picture after landing from a lesser flight today where the clouds were pretty low on the ridge.

I need to grab some self-portraits from the other folks some point real soon of me taking off, and me landing quite properly on my feet. I’m starting to hanker after a new glider, one that’s sleeker and goes faster. This one’s beginning to feel sluggish all of a sudden. I can’t afford anything else now, and it would be quite naughty. And after my spectacular failure of an XC last week on Bradwell, I don’t deserve an upgrade.